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Chillers are used in buildings to provide chilled water for use in air conditioning or other cooling applications. In some cases, waste heat from chillers is also used as a heat source for space heating or water heating in the building.

Click on a topic of interest below for more information about specific space cooling technologies.



Absorption Chillers

Use heat from gas-fired burners or waste heat from steam generating processes to produce chilled water.

General

The absorption cycle uses a heat-driven concentration difference to move refrigerant vapors (usually water) from the evaporator to the condenser. The high concentration side of the cycle absorbs refrigerant vapors (which, of course, dilutes that material). Heat is then used to drive off these refrigerant vapors thereby increasing the concentration again. Lithium bromide is the most common absorbent used in commercial cooling equipment, with water used as the refrigerant. Smaller absorption chillers sometimes use water as the absorbent and ammonia as the refrigerant. As you can probably guess, the absorption chiller must operate at very low pressures (about l/l00th of normal atmospheric pressure) for the water to vaporize at a cold enough temperature (e.g., at ~ 40°F) to produce 44°F chilled water.

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The simplified diagram here illustrates the overall flow path. Starting with the evaporator, water at about 40°F is evaporating off the chilled water tubes, thereby bringing the temperature down from the 54°F being returned from the air handlers to the required 44°F chilled water supply temperature. One ton of cooling evaporates about 12 pounds of water per hour in this step. This water vapor is absorbed by the concentrated lithium bromide solution due to its hygroscopic characteristics. The heat of vaporization and the heat of solution are removed using cooling water at this step. The solution is then pumped to the concentrator at a higher pressure where heat is applied (using steam or hot water) to drive off the water and thereby re-concentrate the lithium bromide. The water driven off by the heat input step is then condensed (using cooling tower water), collected, and then flashed to the required low temperature (40°F in our illustration) to complete the cycle. Since water is moving the heat from the evaporator to the condenser, it serves as the refrigerant in this cycle. There are also absorption chillers in use (e.g. in motor homes) that use ammonia as the refrigerant in the same cycle. The absorbent is the material that is used to maintain the concentration difference in the machine. Most commercial absorption chillers use lithium bromide. Lithium bromide has a very high affinity for water, is relatively inexpensive and non-toxic. However, it can be highly corrosive and disposal is closely controlled. Water of course is extremely low cost and safety simply isn't an issue.

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Absorption chillers are available in two types:

1. Single Effect (Stage) Units using low pressure (20 psig or less) as the driving force. These units typically have a COP of 0.7 and require about 18pph per ton of 9 psig steam at the generator flange (after control valve) at ARI standard rating conditions.

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2. Double Effect (2-Stage) Units are available as gas-fired (either direct gas firing, or hot exhaust gas from a gas-turbine or engine) or steam-driven with high pressure steam (40 to 140 psig). These units typically have a COP of 1.0 to 1.2. Steam driven units require about 9 to 10 pph per ton of 114 psig input steam at ARI standard rating conditions. Gas-fired units require an input of about 10,000 to 12,000 BTUH HHV per ton of cooling at ARI standard rating conditions. To achieve this improved performance they have a second generator in the cycle and require a higher temperature energy source.

Absorption chillers - maintenance considerations

Properly designed and installed absorption chillers can function without full time attendants. The machine can be started and brought on line with simple time clocks or energy management systems. Non-condensables are automatically purged and the operator can schedule normal routine maintenance. Obviously, local building codes may dictate that a full time operator is, or is not, required. This, in turn, is often a function of the size of the equipment, steam pressure, etc. Always consult local codes when considering these issues.

There are three primary maintenance areas: mechanical components, heat transfer components, and controls. The following segments discuss mechanical and heat transfer maintenance areas.

One manufacturer's absorption chillers has a single motor/multiple pump configuration for refrigerant and solution flow and a purge unit. Other manufacturers use individual hermetic solution and refrigerant pumps cooled and lubricated by the pumped solution. Another uses open motors with a shaft seal.

Pictured here are two hermetic, refrigerant cooled and lubricated pump assemblies. The hermetically sealed motor drives the solution and refrigerant pump impellers. In this multiple pump arrangement, motor coolant and lubrication is by the fluid being pumped. Hermetic pump designs eliminate the need for external shaft seals Ð a maintenance item and potential source of air leakage.

The life, performance, and cooling capacity of absorption equipment hinges on keeping heat transfer surfaces free of scale and sludge. Even a thin coating of scale can significantly reduce capacity. Therefore, cooling tower water chemistry is critical, and failure to properly treat this water could void manufacturer warranties.

Scale deposits are best removed chemically. Sludge is best removed mechanically, usually by removing the headers and loosening the deposits with a stiff bristle brush. The loosened material is then flushed from the tubes with clear water.

When the electric motor and pump bearings fail, one design permits replacement of pump parts without removing the lithium bromide solution from the machine. The first step is closing the hand valves in the lubrication circuit, disconnecting the electrical supply, and removing the motor. The pump shaft seal maintains machine vacuum. Major pump repairs are accommodated by charging the machine with nitrogen to atmospheric pressure. Once complete, the machine is evacuated, and pump parts removed and repaired or replaced. Other designs require a more complicated replacement procedure.

Pump maintenance begins with the magnetic strainer which must be cleaned 2 weeks after the initial startup and at the mid-point in the cooling season. Shaft seals should be examined for wear at three year intervals.

In the case of seasonal or prolonged shutdown, refrigerant may migrate from the evaporator to the absorption chiller causing a low refrigerant level in the evaporator pan and piping. Since refrigerant is used to lubricate pump and motor bearings, lubrication from an auxiliary source must be provided during the startup phase of operation. Once an operating charge of refrigerant has been recovered from the solution, the machine may be returned to normal operation.

This auxiliary circuit is usually established by connecting city water to the external connections of the pump lubrication piping. In all cases, follow the manufacturer's recommended procedures.

All absorption chillers must be purged of non-condensable gases to maintain performance. The three methods used are steam jet, solution jet (or "motorless purge"), or a vacuum pump, with the vacuum pump being by far the most common.

Non-condensable gases migrate to the area of lowest pressure in the absorption chiller (the evaporator) where a small portion of the vapor is extracted and condensed in the purge unit using cooling water. Non-condensable are then evacuated by the vacuum pump. In normal operation, the purge system should operate about one hour a week. The vacuum pump oil level should be observed, maintained, and changed as necessary. Oil purge pump motor bearings should be inspected and replaced, and the belt adjusted as needed. In addition, the vacuum pump should be flooded with oil during seasonal shutdown to prevent internal corrosion.

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Purging of non-condensables can be accomplished using a "motorless purge" as shown here. Where motorless purging is used, an optional vacuum pump can also be used for evacuation.

In all cases, the operator should log purge operation and monitor purge operation trends. Increasing purge operation signals increasing in-leakage of air and moisture.


Applications

Single stage steam absorption chillers

Provide chilled water for cooling when low pressure steam, cooling tower (or other water for heat rejection), and electric power is available.

Two stage absorption chillers

Provide chilled water for cooling when high pressure steam, high temperature hot water (HTHW) or natural gas, as well as electric power and cooling tower (or other water for heat rejection) is available.

Waste heat fired absorption chillers

Provide chilled water for cooling when clean, hot exhaust gas, cooling tower (or other water for heat rejection), and electric power is available.

Best applications

Single stage steam absorption chillers

When steam in the 12 to 20 psig range from a process or other steam use is available at little or no cost (i.e. steam would otherwise be wasted).

Two stage absorption chillers

When steam in the 40 to 140 psig range from a process or other steam use is available at little or no cost (i.e. steam would otherwise be wasted), When natural gas is available at low cost relative to the cost of electric power,

When the heating load can not be readily served by an existing boiler and it can be served from this chiller/heater, thus avoiding adding a boiler or where space is not available for a boiler.

When adequate electric power is not readily available for added and needed cooling capacity,

When emergency cooling capacity is needed and stand-by generation capacity is not available to operate electric cooling. (Consider adding emergency generation capacity, which may be lower in cost than absorption cooling capacity).

Waste heat fired absorption chillers

Where exhaust from a gas turbine provides cooling for the intake air to improve turbine performance in hot weather,

Where cooling is required and clean exhaust gas is available, emitted from an industrial process such as those related to printing, drying or baking.

Possible applications

Single stage steam absorption chillers

When steam in the 12 to 20 psig range from a process or other steam use is available at a reasonable cost or where boilers must be operated for other reasons and the user is looking for other steam uses to adequately load the boiler.

Two stage absorption chillers

When steam in the 40 to 140 psig range from a process or other steam use is available at a reasonable cost or where boilers must be operated for other reasons and the user is looking for other steam uses to adequately load the boiler,

Replacement for existing inefficient single stage steam chiller without an electrical service upgrade.

Waste heat fired absorption chillers

Where clean exhaust gas is available and there are cooling requirements.

Waste heat fired absorption chillers

skilled operating personnel will not on duty during system operation,

operations are planned to use absorption chiller as a peak shaving unit. Absorption chillers require added to time and effort to bring on- and take off-line. Operators tend to end up using absorption as a base chiller and peak with the electric chiller, thereby defeating the purpose, and actually adding to, rather than saving, operating cost.

Extended operation at 30% and less of design capacity is likely.

Technology types (resource)

Two stage absorption chillers

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The energy efficiency of absorption can be improved by recovering some of the heat normally rejected to the cooling tower circuit. A two-stage or two-effect absorption chiller accomplishes this by taking vapors driven off by heating the first stage concentrator (or generator) to drive off more water in a second stage. Many absorption chiller manufacturers offer this higher efficiency alternative.

Notice that two separate shells are used. The smaller is the first stage concentrator. The second shell is essentially the single stage absorption chiller from before, containing the concentrator, condenser, evaporator, and absorption chiller. The temperatures, pressures, and solution concentrations within the larger shell are similar to the single-stage absorption chiller as well.

Steam at pressures typically in the l25 - 150 psig range is brought into the stainless steel tubes of the first stage concentrator causing the solution there to boil. The pressure at which boiling occurs and the pressure of the released refrigerant vapor is approximately 5 psig (20 psia). The partially concentrated solution from this first stage flows through the high temperature heat exchanger where it is cooled by the lower temperature dilute solution returning from the concentrator. This concentrate then passes into the lower pressure second stage concentrator where the vapors from the first stage take it to the final desired concentration levels. This second stage operates at a pressure of 0.1 atmosphere (1.5 psia).

The reuse of the vapors from the first stage generator makes this machine more efficient than single stage absorption chillers, typically by about 30%. Two-stage absorption chillers are typically driven by high-pressure (60 to 130 psig) steam, direct-fired with natural gas or #2 fuel oil, or using hot exhaust gas from combustion engines.

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Steam-fired 2-stage absorption chillers

Steam at pressures typically in the l25 - 150 psig range is brought into the stainless steel tubes of the first stage concentrator causing the solution there to boil. The pressure at which boiling occurs and the pressure of the released refrigerant vapor is approximately 5 psig (20 psia). The partially concentrated solution from this first stage flows through the high temperature heat exchanger where it is cooled by the lower temperature dilute solution returning from the concentrator. This concentrate then passes into the lower pressure second stage concentrator where the vapors from the first stage take it to the final desired concentration levels. This second stage operates at a pressure of 0.1 atmosphere (1.5 psia).

The reuse of the vapors from the first stage generator makes this machine more efficient than single stage absorption chillers, typically by about 30%.

Direct-fired absorption chillers

Direct-fired absorption chillers utilize a burner as the heat input for the absorption cooling cycle. Most operate either on natural gas or No. 2 fuel oil. Since the heat input is at a very high temperature, they achieve a very high efficiency for the absorption cycle...something approaching 12,000 Btu of fuel input for each ton hour of cooling output. The absorption cycle itself is virtually identical to that of the two-stage steam absorption chillers. However, unlike most steam absorption chillers, the direct-fired absorption chiller lends itself fairly readily to "chiller-heater" applications where both cooling and heating are achieved in the same unit. This can result in a smaller footprint for the boiler room in some situations.

Advantages

Where a boiler can be eliminated by the dual heating and cooling capability of this machine, the cost and space savings can be a significant. In addition, steam is not required, which can be important in situations where local codes require licensed boiler operators for steam-driven units but permit unmanned operation of direct-fired absorption chillers.

Disadvantages

Direct-fired absorption chillers require a stack to vent combustion products. This is not necessary in a steam-fired unit. In addition, the first cost of direct-fired units are higher than steam driven units. Maintenance costs on the heat rejection circuit tend to be higher due to more rapid scaling. Also be careful to check warranted life of absorption chiller heat transfer surfaces (especially the generator section) and the refrigerant and solution pumps. All absorption chillers use electric power to operate these pumps, the condenser water pumps and cooling tower fans. They also use more water as they must reject more heat and require larger cooling towers.

Absorption chillers are more difficult than electric chillers to put on-line (start up) and to take off-line (shut down) as they require a dilution cycle. All of these issues should be addressed in discussions with manufacturers, designers and mechanical contractors.

Waste heat fired absorption chillers

Most absorption chillers use either steam or fuel (natural gas, propane) for heat input. But, waste heat from process, reciprocating engine, gas turbine, or a cogeneration system can also be used in the absorption process. The exhaust should have a minimum temperature of about 550 F and a maximum of 1,500 F. The most common application is using the exhaust from a gas turbine to provide cooling for the intake air or other cooling requirements. The available cooling is a function of the exhaust gas temperature and mass flow rate, using this formula:

Chilling capacity in tons = m x (Tg - 375) / 40,950

Where m = mass flow rate in pound per hour

Tg = exhaust gas inlet temp (F) to absorption chiller

40,950 = conversion factor

More detail

Waste steam from a cogeneration system obviously produces the same level of cooling as boiler generated steam, Low pressure waste steam sources (say 14 psig) typically require 18-20 pounds of steam per hour to produce one ton of cooling in a single-stage absorption chiller. That performance improves to 10-12 pounds per ton-hour of steam when steam pressures are in the 50 to 130 psig range and used in a 2-stage (double effect) absorption chiller.

Steam absorption chillers are nominally rated as follows:

  • Single stage: 9 psig at generator flange
  • Two stage: 114 psig steam input pressure.

Capacity ratings are decreased as steam pressure drops below nominal. For example, a nominal 100-ton unit's capacity will drop to 84 tons with 78.5 psig steam.

Direct-fired absorption chillers can often be modified to accept hot air or exhaust from a gas turbine or engine. Performance is almost totally dependent upon air temperature, For example, waste heat air temperatures °F or higher offer performance similar to direct-fired absorption chillers where every 13,000 Btu of heat recovered produces one ton of cooling. When calculating heat recovery, remember to assume waste heat leaving the absorption chiller at 375° to 400°F (this means the absorption chiller will not reclaim all of the waste heat potential).

For exhaust gas heat recovery

Chilling capacity (tons) = m x (Tg - 375)

40,950

Heating capacity (BTUH) = m x (Tg - 375) x 0.257

where m = exhaust gas flow rate in pounds per hour

Tg = exhaust gas inlet temperature in °F

40,950 = cooling constant representing average gas specific heat, interconnect efficiency, cooling COP and the conversion from BTUH to tons

0.257 = heating constant representing average gas specific heat and the interconnect efficiency

375 = minimum temperature of exhaust gas leaving chiller in °F.


Centrifugal & Screw Packaged Chillers

Packaged for easy installation, these units are used in larger buildings. They are the most efficient and offer the lowest weight, height and footprint of any chiller alternative.

General

Centrifugal, and to a lesser extent screw, packaged chillers are the heart of most central systems for large buildings. They consist of the centrifugal or screw compressor-motor assembly, water-cooled condenser, insulated liquid cooler, expansion device, interconnecting refrigerant piping, lube system, oil and refrigerant charge, control panel and wiring, auxiliaries, and in some cases the compressor motor-starter. They are made in both hermetic and open types, and with single- and multi-stage centrifugal designs. They are manufactured, factory assembled and tested, charged, and shipped in one assembly up to about 2,000 tons capacity; in large sizes they are factory disassemble in major pieces for shipment and installation. Installation consists of piping supply and return chilled water or brine piping, and cooling water piping, power wiring and interconnection of external controls, evacuation and charging when necessary, check-out and startup.

NOTE: Screw compressors are also sometimes referred to as ahelical rotary compressors.

Advantages

  • Factory packaged for ease of proper installation
  • Most efficient chilling package - low kW per ton (centrifugals at 0.50 and less available, with screws at somewhat higher kW per ton)
  • Several major reliable suppliers, each with a service network of trained technicians
  • Available from 100 to 10,000 tons capacity in a single centrifugal chiller, with screws available in a lower tonnage range - check suppliers)
  • Lowest weight, height and footprint of any alternative
  • Use environmentally acceptable refrigerants
  • Have excellent and step-less part load characteristics
  • Are available in some sizes in dual compressor models for even more efficient part load operation
  • Chillers are relatively easy to operate with their modern controls and designs

Disadvantages

  • More costly in smaller sizes than other types of chiller packages
  • Centrifugal chillers are available only in water-cooled models
  • Screw compressor chillers are somewhat noisier than other designs
  • Screw compressor chillers are somewhat less efficient than centrifugals
  • Applications

    Centrifugal and screw packaged chillers are typically applied in single and multiple units to provide chilled water for air conditioning large buildings and complexes, and in district cooling systems. They are also used in heat pump form for both heating and cooling. They are also used to chill brine in industrial processes.

    Best applications

    Centrifugal chillers are usually the best selection among alternatives when used to provide chilled water for cooling large buildings with automated operation.

    Technology types (resource)

    See also:

    Efficiency

    Centrifugal chillers are the most efficient chilling packages available. Recent design improvements result in a low kW per ton (centrifugals are available at 0.50 and less kW/ton, with screws at somewhat higher kW per ton). Higher kW per ton can also be supplied at lower first costs; these may or may not be prudent depending on the present and anticipated cost of power (both demand and energy).

    Contact us for a detailed list of manufacturers for this equipment.


    Reciprocating & Scroll Packaged Chillers

    Packaged for easy installation, these chiller units are typically used in smaller buildings and offer lower installation costs.

    General

    Reciprocating, and to a lesser extent scroll, packaged chillers are the heart of most central systems for medium sized buildings. They consist of one or more reciprocating or scroll compressor-motor assembly, air- or water-cooled condenser (and condenser air fan(s)), insulated liquid cooler, expansion device, interconnecting refrigerant piping, oil and refrigerant charge, control panel and wiring, auxiliaries, and the compressor motor-starter. They are made in both full- and semi-hermetic and open types. They are manufactured, factory assembled and tested, charged, and shipped in one assembly up to about 2,00 tons capacity. Installation consists of piping supply and return chilled water piping, (and cooling water piping where applicable,) power wiring and interconnection of external controls, evacuation and charging when necessary, check-out and startup. Units are also available without any condenser for field piping to a separate remote air-cooled or evaporative condenser.

    Advantages

    • Factory packaged for ease of proper installation
    • Many reliable suppliers, most with a service network of trained technicians or through factory approved service providers
    • Reciprocating models available from many manufacturers in capacities up to 200 tons and larger in incremental steps (i.e. 20, 25, 30, etc)
    • Scroll chillers are available from a lesser number of manufacturers and more limited models (20 to 60 tons - check suppliers)
    • Scroll chillers have improved part load efficiencies
    • Use environmentally acceptable refrigerants
    • Various condensing options available, including air-cooled and for low ambient operation
    • Chillers are relatively easy to operate with their modern controls and designs

    Disadvantages

    • Reciprocating models: Part load capacity is stepped - cylinder unloading, compressor on/off of multiple compressor units
    • Central plant systems tend to be more costly than unitary systems

    Links to more detail

    Applications

    Reciprocating and scroll packaged chillers are typically applied in single and multiple units to provide chilled water for air conditioning small to medium sized buildings using central system designs.

    Best applications

    High quality installations requiring central chilled/hot water systems to provide temperature and humidity control of multiple spaces.

    Retrofit applications

    Many models will fit through a 30 inch door for ease of installation at minimum cost of structural modifications.

    Technology types (resource)

    Reciprocating compressor packages have been the mainstay of this segment, and many advances and improvements have been made over the years. They are in wide-spread use and trained service technicians are available almost everywhere.

    Scroll compressor packages are the latest advancement in positive displacement compressors and indications are they may offer better reliability and improved efficiency. The network of trained service technicians is in the process of development and may not be available in all locations.

    Efficiency

    Older water cooled chillers are in the 0.82 to 1.0 kW per ton range, while newer high efficiency models range from 0.78 to 0.85 kW per ton at ARI conditions.

    Contact us for a detailed list of manufacturers for this equipment.


    Natural Gas - Engine Driven Chillers

    These are similar to their electric counterparts but use compressors driven by natural gas engines.

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    General

    While the majority of the packaged chillers sold and installed are electric drive, efforts by the gas industry have resulted in several manufacturers offering packaged natural gas engine driven chillers using reciprocating, screw and centrifugal chillers. These chillers are essentially the same as electric driven counterparts except open drive compressors are used. These in turn are matched with natural gas engines and often optional heat recovery heat exchangers. Most chillers use R-22 refrigerant.

    Advantages

    • Modulation of both engine speed and compressor unloading provide good part load operation
    • Can reduce demand charges if properly operated
    • Heat recovery options for producing hot water are available
    • Both water- and air-cooled models are available

    Disadvantages

    • Require high electric parasitics to operate (pumps - chilled water, tower, lube, etc. and fans)
    • Require periodic and added maintenance time and cost, with engine maintenance performed by a different trade that normally services chillers
    • Requires added water use due to higher heat rejection; may require larger tower and pumps Higher weight and space requirements
    • Emissions may require permitting and emission reduction controls
    • While lean-burn low-emission engines are available, they tend to have higher emissions at part load - a condition at which most chillers operate a large portion of the time
    • Engines require major overhaul or rebuild after a period of time
    • When heat recovery is used, provision must be made to reject unwanted heat whenever it can not be put to use
    • Engine noise control must be considered in design and added cost sound enclosure may be required
    • Units are more expensive than conventional electric chillers
    • The engine is only operated when cooling is needed.

    Applications

    Can be applied almost anywhere a comparable electric chiller is used, if space and a supply of low-cost gas is available and added weight can be handled by the structure.

    Best applications

  • Where insufficient power is available to operate an all-electric chiller.
  • Where demand charges are high and cost of gas is low
  • Technology types (resource)

    There are two philosophies in the application of engines to chillers. The first is to connect the engine and compressor, directly or through gearing. This is what is described above, and what usually comes to mind when the phrase "engine driven chiller" is used.

    The second questions why an engine is installed and only used when cooling is required, which is usually during only several months a year and then at partial load most of the time. Two alternatives can be considered. One connects both a generator and the compressor to an engine; this tends to be both more expensive to purchase, and complicated to install and operate.

    The second approach is to install a conventional high efficiency electric chiller and an engine-driven generator sufficient in size to power the chiller. The power generated is used to drive the chiller and its auxiliaries. When all the output power is not required, which is during most of the year, the excess power is used for other purposes in the building. This makes sense when the customer wishes to diversify fuel use.

    Another consideration is that most users do not realize the commitment they are making when they install gas engine drives. They take the minimum maintenance of electric motors for granted and tend to expect the same of engines. This is not so.

    Internal combustion engines require periodic scheduled shutdown for routine maintenance (lube oil and spark-plug changes, etc) , plus top and major overhauls at various points in their lifetime. These "time between overhaul" periods can range from 3,600 hours for light high-speed engines used in small gensets to 15,000 - 20,000 or higher run-hours for heavier slow speed industrial grade engines. Set-asides of downtime and dollars must be made for these procedures

    Efficiency

    High efficiency gas engine-driven chillers have COP's at full load of 1.2 to 1.7 with part load up to 2.2. Typical integrated part load values (IPLV) at ARI conditions range from 1.6 to 2.0. Water-cooled models with engine heat recovery usefully used, can gain an additional 0.5 COP.

    Contact us for a detailed list of manufacturers for this equipment.


    Centrifugal Compressors

    Typically found in large chiller units, these are the most efficient systems and use one or more rotating impellers to compress refrigerant.

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    General

    Centrifugal compressors use one or more rotating impeller to increase the refrigerant vapor pressure from the chiller evaporator enough to make it condense in the condenser. Unlike the positive displacement, reciprocating, scroll or screw compressors, the centrifugal compressor uses the combination of rotational speed (RPM), and tip speed to produce this pressure difference. The refrigerant vapors from the chiller evaporator are commonly pre-rotated using variable inlet guide vanes. The consequent swirling action provides extended part-load capacity and improved efficiency. The vapors then enter the centrifugal compressor along the axis of rotation. The vapor passageways in the centrifugal compressor are bounded by vanes extending form the compressor hub, which may be shrouded for flow-path efficiency. The combination of rotational speed and wheel diameter combine to create the tip speed necessary to accelerate the refrigerant vapor to the high pressure discharge where they move on to the chiller condenser. Due to their very high vapor-flow capacity characteristics, centrifugal compressors dominate the 200 ton and larger chiller market, where they are the least costly and most efficient cooling compressor design. Centrifugals are most commonly driven by electric motors, but can also be driven by steam turbines and gas engines.

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    Depending on the manufacturer's design, centrifugal compressors used in water chiller packages may be 1-, 2-, or 3-stages and use a semi-hermetic motor or an open motor with shaft seal.

    Advantages

    Due to their very high vapor-flow capacity characteristics, centrifugal compressors dominate the 200 ton and larger chiller market, where they are the least costly and most efficient cooling compressor design.

    More detail

    Packaged water cooled centrifugal compressors are available in sizes ranging from 85 tons to over 5,000 tons. Larger sizes, typically those 1,200 to 1,500 tons and larger are shipped in sub-assemblies. Smaller sizes are shipped as a factory-assembled package. While some smaller air-cooled centrifugal models are manufactured, they are largely exported to the Middle East and other arid areas where water is simply not available for HVAC condensing use, even in cooling towers.

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    The centrifugal compressors mentioned here will be using HCFC-123, HCFC-22 and HFC-134a. This usually calls for semi-hermetic designs, with single or multi-stage impellers. Two manufacturers (Carrier and McQuay) offer semi-hermetic gear driven models. Trane offers multi-stage direct drive semi-hermetic units. York offers an integrated open-drive geared design.

    Chillers using ammonia as the refrigerant are not generally available with centrifugal compressors. Only open drive screw or reciprocating compressors are compatible with ammonia, largely because of its corrosive characteristics and reactions with copper.

    The selection of single stage, multi-stage, open or hermetic designs is largely a function of individual manufacturer preference and the application. For example, centrifugal compressors are limited in their compression ratio per impeller. Therefore, applications calling for high temperature lifts (such as with ice thermal storage) may require multi-stage designs.

    Power requirements for centrifugal chillers are the lowest of all chiller types currently available, and efficiencies have been improving even further over the years as a result of improved impeller designs, better unit configurations, enhanced heat transfer surfaces, and the increased utility emphasis on reducing energy requirements.

    At ARI standard rating conditions centrifugal chiller's performance at full design capacity ranges from 0.53 kW per ton or lower to 0.68 kW per ton. This performance includes the semi-hermetic refrigerant cooled or open type compressor motors.

    Open drive chiller power requirements are sometimes rated in shaft brake horsepower (bhp). To convert from bhp to electric input in kW, the efficiency of the motor must be considered (which is usually between 90 and 95 percent for centrifugal machines). For example, a rating of 1,000 bhp at 93 % motor efficiency would translate to 802 kilowatt input.

    (1,000bhp x 0.746 kW/bhp) = 80.2 kW input

    93% Motor efficiency

    Centrifugals chillers 200 tons and larger cost less to install than reciprocating chillers (available up to the 175 to 200 ton range) and the same or slightly less than screw chillers in most all sizes. Centrifugals offer the advantages of high efficiency, infinitely variable capacity control (down to about 10 percent of full load), they're lighter (which reduces floor loadings) and they take up much less space for a given tonnage.

    First cost of centrifugal chiller packages generally start higher than recips under 200 tons, and then cost less in the larger sizes. More definitive costs are shown in the Compare segment.


    Screw Compressors

    Typically found in mid-size chiller units, these highly efficient systems use one or two rotating screws to compress refrigerant.

    General

    Helical rotary (or screw) compressors are positive displacement machines. Two types are used - single-screw and twin-screw. A twin-screw compressor consists of accurately matched rotors (one male and one female) that mesh closely when rotating within a close tolerance common housing. One rotor is driven while the other turns in a counter-rotating motion.

    A single-screw compressor uses a single main screw rotor meshing with two gate rotors with matching teeth. The main screw is driven by the prime mover, typically an electric motor. The gate rotors may be metal or a composite material. The screw-like grooves gather vapors from the intake port, trap them in the pockets between the grooves and compressor housing, and force them to the discharge port along the meshing point path. This action raises the trapped gas pressure to the discharge pressure. If the power input is adequate and pressure differential between outlet and inlet pressures is within the design range of the machine, the screw compressor delivers the appropriate refrigerant gas volume.

    Notice that the refrigerant gas enters and exits the compressor through ports; not valves like reciprocating compressors. Compressors of this type are called ported compressors for this reason. The mating rotors are rotating at such close tolerances, they require cooling and lubrication. This may be provided by forcing oil into the compressor at strategic points. The oil also acts as a seal for rotor-to-rotor and rotor-to-housing clearances.

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    The oil is entrained by the flowing refrigerant gas, leaves the compressor, and is recovered by an oil separator for reuse (after cooling and filtering). Since the oil sump is on the high pressure side of the system, a mechanical pump is not required for oil circulation. The compressive action of the screw itself provide the necessary pressure differential.

    In other designs, subcooled liquid refrigerant injection (instead of oil) cools and seals the compressor. The use of liquid refrigerant eliminates oil management problems as there are no oil separators or oil recovery systems. The system is sealed, cooled and lubricated with liquid refrigerant which also attenuates the noise. Capacity is controlled with two slide valves.

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    Since the screw compressor is most often driven by a constant speed electric motor and the screw compressor is a positive displacement machine, the natural tendency is to move a fixed volume of refrigerant gas. This would make refrigeration capacity control difficult. The design uses a slide valve that opens to vent some gas back to the suction port, reducing both the net gas flow and power input.

    Several manufacturers offer packaged water chillers using helical rotary or "screw" compressors. Water-cooled units range in size from 50 tons to over 1200 tons. They normally use HCFC-22 and HFC134a as refrigerants in space cooling designs and ammonia in process refrigeration (particularly food processing). In the smaller sizes, they compete with reciprocating chillers. In larger sizes they compete with centrifugals. Screw compressors usually employ hermetic or semi-hermetic designs for higher efficiency, minimum leakage, ease of service, and volume production reasons.

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    Air- and evaporatively-cooled models can be used from about 60 to 350 tons, and can use open-drives. Chillers using ammonia always use open type compressors, typically with direct-coupled electric motors. The selection of open or hermetic design depends on the application, refrigerant, and the manufacturer.

    Technology types (resource)

    Two types are used - single-screw and twin-screw. A twin-screw compressor consists of accurately matched rotors (one male and one female) that mesh closely when rotating within a close tolerance common housing. One rotor is driven while the other turns in a counter-rotating motion.

    A single-screw compressor uses a single main screw rotor meshing with two gate rotors with matching teeth. The main screw is driven by the prime mover, typically an electric motor. The gate rotors may be metal or a composite material. The screw-like grooves gather vapors from the intake port, trap them in the pockets between the grooves and compressor housing, and force them to the discharge port along the meshing point path. This action raises the trapped gas pressure to the discharge pressure. If the power input is adequate and pressure differential between outlet and inlet pressures is within the design range of the machine, the screw compressor delivers the appropriate refrigerant gas volume.

    Contact us for a detailed list of manufacturers for this equipment.


    Reciprocating Compressors

    Typically found in smaller chiller units, these use pistons and intake and exhaust valves to compress refrigerant. Only refrigerants that operate as a vapor can be used.

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    Most cooling systems in use today rely on reciprocating piston-type compressors. Reciprocating compressors are manufactured in three types:

    1. Hermetic - compressor-motor assembly contained in a welded steel case, typically used in household refrigerators, residential air conditioners, smaller commercial air conditioning and refrigeration units.
    2. Semi-hermetic - compressor-motor assembly contained in a casting with no penetration by a rotating shaft and with gasketed cover plates for access to key parts such as valves and connecting rods.
    3. Open - compressor only with shaft seal and external shaft for coupling connection to belt - or direct-drive using as electric motor or natural gas engine. These are largely used for ammonia refrigeration applications as hermetic designs cannot be used with ammonia refrigerant, and for engine-driven units.

    As the piston nears the bottom of its stroke within the cylinder, the intake valve opens and the refrigerant vapor enters. As the piston rises, the increased pressure closes the intake valve. Then as the piston nears the top of its stroke, the exhaust valve opens permitting the vapor at the higher pressure to exit. Reciprocating compressor capacity is a function of the bore and stroke of the piston-cylinder configuration as well as the speed of the machine, and the clearance tolerances. Compressor capacity is also related to the compression ratio.

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    The mechanical design is rugged and reliable but has one significant limitation. Reciprocating compressors are designed to handle vapors, not liquids. When liquid enters the cylinder on the intake stroke, it tends to damage the valves on the compression stroke and possibly the compressor itself. This is why chillers incorporate liquid-to-suction heat exchangers, which assure some level of vapor superheat at the compressor suction. Capacity is controlled by multiple staging of smaller compressors or in large multiple cylinder reciprocating compressors by unloading banks of cylinders on the compressor. This tends to make the machine most efficient at full load. Therefore, for maximum efficiency recips should generally be operated at full load. This is the reason small compressors are cycled on and off in most residential and small commercial applications.

    The mechanical design is rugged and reliable but has one significant limitation. Reciprocating compressors are designed to handle vapors, not liquids. When liquid enters the cylinder on the intake stroke, it tends to damage the valves on the compression stroke and possibly the compressor itself. This is why chillers incorporate liquid-to-suction heat exchangers, which assure some level of vapor superheat at the compressor suction.

    More Detail

    Due to better valve designs and configurations that reduce pressure losses, power requirements for reciprocating chillers have been improving over the years. Overall mechanical and compression efficiencies vary with the compression ratio, but are generally in the 72 to 78% range including the hermetic-type refrigerant-cooled 1,750 rpm motor. Compression ratio is computed by dividing the absolute discharge pressure by the suction pressure both measured in psia.

    At ARI Standard rating conditions (44°F leaving chilled water, 85°F entering condenser water), typical chillers operate around 40°F evaporating and 100°F condensing temperatures equivalent to pressures. A modern reciprocating compressor has an energy efficiency ratio (EER) of about 15, equal to 0.79 kW per ton. However, in air-cooled conditions the condensing pressure is likely to run up to a 130°F temperature corresponding to pressure, with EERs ranging from about 10.4 up to 11.3, which equate to 1.15 to 1.06 kW per ton.

    Assembled into chiller packages in the 20 to 200-plus ton capacities, air-cooled units will typically have EERs ranging from 9.0 to 10.9, equal to 1.33 to 1.10 kW per ton with an average of about 1.22 kW per ton. Similar water-cooled chiller packages will have EERs ranging from 13.1 to as high as 15.8, which equates to 0.92 to 0.76 kW per ton with an average of about 0.82 kW per ton.

    Manufacturers continue to develop more efficient models. In some cases, scroll compressors are being used, in place of reciprocating.

    While they are the least efficient of the chiller package options, reciprocating or scroll compressor chillers have a definite first cost advantage in the smaller chiller sizes. The first cost of reciprocating chiller packages is the lowest of the various electric chiller options, certainly when expressed in $ per ton. The compressors are competitively priced since they are used in many different chiller models. Plus, many more reciprocating chillers are produced than larger centrifugal and screw type chillers. These economies of scale result in a lower unit cost, especially for models up to about 200 tons.

    Compare - Installed Costs - Chillers

    For example, an air-cooled chiller serving a hospital operating room suite that operates year-round could well have a lower annual electric cost than a comparable water-cooled unit, due to the large number of operating hours the unit will be operating at part-load and low-ambient temperature conditions. Only a careful energy use analysis of each application performed by a qualified professional can identify the most economical equipment choice.

    Maintenance costs must also be factored in. Here are some typical mid-1995 $ per ton annual values.

    Chiller Type 20 Tons 50 Tons 75 Tons 100 Tons 150 Tons 200 Tons
    Water Cooled $79 $67 $58 $51 $40 $35
    Air Cooled $70 $50 $45 $43 $35 $31

    If the chiller is driven by a natural gas engine, the added maintenance costs of the engine must also be included. Typically this amounts to about $0.012 per ton per operating hour.

    Reciprocating chiller emissions fall into two major categories: direct (or on-site), and indirect (or emissions resulting from the production of the energy used to operate the equipment).

    Direct on-site emissions are confined to the release of refrigerant due to leaks or servicing. Federal law now mandates no intentional release. It is the responsibility of the user and service agency to minimize leaks and service release. Good preventive maintenance practices are imperative. Other factors affecting emissions include chiller age, application (whether it's a single package or split system), compressor type (open compressors with shaft seals tend to leak more than hermetic designs).

    A typical semi-hermetic type chiller might lose about 3 to 5 percent of its charge annually. With the refrigerant charge running about 3 pounds per ton, the emission of refrigerant might total about 0.12 pounds per ton per year. An open chiller might lose 5 to 7 percent, and thus emit about 0.18 pounds per ton-year. To allow for less than ideal conditions, a conservative estimate of emissions might be 0.25 pounds per ton-year.

    Natural Gas engine-driven reciprocating chillers must use open-type compressors. In addition to the same refrigerant emissions as an electric chiller, they also produce emissions from the combustion of the natural gas. Also, the leakage of natural gas into the atmosphere although small, is believed to contribute comparable greenhouse gases as refrigerant leakage.

    These emissions can be estimated, based on the annual gas consumption. Typical gas engine driven chillers use about 9,300 Btu per ton-hour of natural gas (on a HHV basis). Using the annual ton-hours of cooling, the emissions of CO2 and the criteria gases can be estimated using these relative values of pounds per million Btu of fuel burned. The emissions of all gases other than NOx are relatively constant throughout the loading range. NOx emissions will vary considerably, depending on the annual load profile.

    On-site emissions in pounds per million Btu of Natural Gas burned
    CO2 CO NOx SOx VCC Particulates*
    118 20% 44% 0 .31 .00%

    *Particulates are 10 microns or less. Volatile organic compounds (VOC) includes hydrocarbons (HC).

    While so-called "lean-burn engines" emit less NOX than conventional engines at full load, they emit more at part load conditions. Since chillers operate largely at part load, the added expense of a lean-burn engine is usually not justifiable.

    Indirect emissions occur at the power plants generating the electricity used to power chillers. Remember that comparing different chillers (for example, electric versus gas) must include the effect of the chiller and system auxiliary energy consumption - not just the chiller's power use. These emissions can be estimated from the annual power consumption in kWh and the local electric utility's emission data.

    Most utilities know their typical emissions of the various gases andparticulates on a "per kWh" basis.


    Scroll Compressors

    This recent design uses one stationary and one orbiting scroll to compress refrigerant. Being more efficient, these will eventually replace most reciprocating compressors.

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    The scroll compressor uses one stationary and one orbiting scroll to compress refrigerant gas vapors from the evaporator to the condenser of the refrigerant path. The upper scroll is stationary and contains the refrigerant gas discharge port. The lower scroll is driven by an electric motor shaft assembly imparting an eccentric or orbiting motion to the driven scroll. That is, the rotation of the motor shaft causes the scroll to orbit - not rotate - about the shaft center.

    This orbiting motion gathers refrigerant vapors at the perimeter, pockets the refrigerant gas, and compresses it as the orbiting proceeds. The trapped pocket works progressively toward the center of the stationary scroll and leaves through the discharge port. Study this time lapse series carefully to see how the trapped gases are progressively compressed as they proceed toward the discharge port.

    Scroll compressors are a relatively recent compressor development and will eventually replace reciprocating compressors in many cooling system applications, where they often achieve higher efficiency and better part-load performance and operating characteristics.

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    Advantages

    Scroll compressors are a relatively recent compressor development and will eventually replace reciprocating compressors in many cooling system applications, where they often achieve higher efficiency and better part-load performance and operating characteristics.


    Mechanical Drives

    A variety of electric motor, gas turbine, reciprocating engine, and steam turbine alternatives are available to drive chiller compressors.

    Chiller compressors can be driven by electric motors, reciprocating engines, gas turbines, or steam turbines. The selection of alternative drive technologies rests primarily on the issues of first cost and operating cost, as well as any fuel diversity and power reliability criteria. While there are other issues involved in the selection process, including CFC phaseout and other refrigerant-related issues, the selection between the alternatives just mentioned will probably not be driven by CFCs. In other words, a refrigerant that might be applicable for a chiller driven by a reciprocating engine would also work for an electric motor drive. A discussion of these criteria can be found elsewhere in this digital reference library.

    While mechanical drives other than electric motors are also discussed, the primary alternatives presented will be reciprocating engines in the 100-500 ton range and steam turbines which are typically much larger.

    Gas turbine-driven chillers are seldom seriously considered for three reasons:

  • The limited number of gas turbine sizes available
  • Their economic reliance on heat recovery and
  • Their relatively poor on-peak performance during hot weather.
  • Technology types (resource)

    Electric motors

    The electric motor is far and away the most common chiller compressor drive. Most of these are fixed speed motors (typically 1,800 or 3,600 rpm). Since compressor power requirements are proportional to the difference between evaporator and condenser pressures and refrigerant flow requirements, motor loads vary accordingly. Load variations are handled by cylinder unloading or multiple compressor staging for reciprocating units, slide vane capacity control in screw compressors, and inlet guide vanes (and infrequently hot gas bypass) for centrifugal compressors.

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    In cases where the ability to change compressor speed may offer a better way to modulate compressor capacity and/or performance, a variable speed electric motor should be considered. This approach is seldom utilized in new chiller installations since chiller manufacturers can now build in excellent modulation control. Variable speed motors have been more often used in retrofit applications. One word of caution: always consult the chiller manufacturer for warranty and performance verification before accepting the claims of anyone wishing to modify an existing chiller in this way.

    Steam turbines (back pressure & condensing)

    Steam turbines, reciprocating engines, and sometimes gas turbines are used to drive chiller compressors. The most common applications are very large (over 1500 tons) steam turbine-driven centrifugal chillers used in cogeneration applications for large hospitals or industrial cooling. In situations where electrical demand charges are high (say over $25 per kW per month) or where a demand ratchet could make an electric-driven chiller too expensive to operate for a few months a year, steam turbine-driven chillers are often specified.

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    Why not reciprocating engines or gas turbines? Steam turbines use the existing boiler system so they don't have to worry about fuel supply or air emissions. Since the steam needs of the site are usually dictated by the colder months, the existing steam generating capacity is often more than adequate to support cooling. Plus, the operating and maintenance characteristics of steam turbine-driven chillers are much better than reciprocating engines or gas turbine-driven equipment. Finally, where existing boiler capacity is adequate, steam turbine-driven chillers cost less than reciprocating engines or gas turbines. There are two basic steam turbine designs: back pressure and condensing. These indicate whether steam leaving the turbine goes on into the steam distribution system to satisfy process or heating requirements (this is "back pressure"), or whether the steam leaving the turbine goes straight to a dedicated steam condenser where it is rejected via a cooling tower or river water. Logically, condensing steam turbines are more expensive and less efficient than the back pressure designs.

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    When site steam requirements are reasonably steady and in excess of the steam flows necessary to drive the chiller, the back pressure design makes the most sense. Where this is not true, and power costs would be high for an electric-driven machine, a condensing steam turbine may be the most cost effective alternative. In many cases where steam turbines are considered, rather than apply them to a chiller operating relatively lower hours a year, the turbine is typically used to drive a generator to take maximum advantage of its power generating capabilities.

    Selecting a chiller design like this requires careful consideration of site-specific conditions. Steam turbine driven chillers represent a complex design in any situation. It is wise to consult with qualified design professionals and reputable equipment manufacturers before making a final decision.

    Reciprocating engines

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    Reciprocating engines are usually selected to drive chillers in the smaller range — 100 to 500 tons. The compressor (usually a screw or centrifugal model) is usually directly coupled to the engine drive shaft. Engines are often considered where the site can use the energy in the hot water and/or hot air exhausts produced. Roughly one-third (or less) of the total fuel input is converted to compression power. Therefore, the economics of reciprocating engine drives usually depend on the cost-effectiveness of heat recovery. Engine jacket water (which can reach temperatures as high as 220°F) is easily recovered and also represents about one-third of the fuel input. The heat in the engine exhaust represents the remaining third of fuel input, but this heat is generally not fully recoverable.

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    Engine-driven chiller cost effectiveness can best be determined using a cautious, conservative assessment by a professional that considers these three factors:

    1. Heat recovery that reflects actual site-specific heating efficiencies and needs,
    2. Conservative annual heating requirements, and
    3. Realistic operating and maintenance costs (which are typically higher than any other mechanically driven chiller alternative).
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    Once realistic heat recovery estimates have been factored into the equation, the only other major issue is that of O&M expense. Here, the Gas Research Institute uses $0.01 per ton-hour more than an electric-driven chiller design. While your costs could be different, a figure of $0.01 to 0.12 per ton per operating hour represents a reasonable first cut estimate. Always rely on qualified design professionals and reputable equipment manufacturers for installed cost, operating cost, and performance estimates.

    Gas turbine designs

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    Gas turbines are seldom selected to drive chiller compressors because the efficiency of the cogeneration system using a gas turbine relies heavily on recovering the engine's waste heat. Most sites simply don't have a use for all the waste heat. In cases where the heat can be used, the gas turbine is typically used to drive a generator to take maximum advantage of its power generating capabilities. The main problems associated with using gas turbines as chiller drives include:

    1. Gas turbine power levels (and the resulting chilled water production) are significantly reduced (~ 25-35%) at high ambient temperature levels. This means that at the very time the site needs maximum power to drive a chiller compressor, the gas turbine is least capable of delivering it. One solution might be to use some of the chilled water production to cool gas turbine inlet air, but this also reduces net chilled water production.
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    2. Operating and maintenance procedures are relatively sophisticated. The engines must be protected against inlet dust, contaminants, frosting, or damage from foreign objects. When placed in the hands of qualified, experienced personnel, and run continuously, gas turbines have recorded extremely high annual availability and low maintenance costs. Unfortunately, chillers seldom run continuously.
    3. If the gas turbine is fueled with natural gas, gas pressures have to be higher than with any other mechanical driver — typically 300 - 400 psig for the gas turbine. These pressures aren't always available from suppliers, and therefore require a supplemental gas compressor. Since this gas compressor is relatively unreliable, a "spare" is usually added in the system design, making it an expensive design attribute. Coupled with the power used to compress the natural gas fuel input, this compressor becomes a significant element in the cost-effectiveness equation.
    4. Careful matching of the turbine and compressor, both available in limited size increments is essential. Starting and stopping torques are specially important. These requirements typically increase the chiller cost not economically supportable.

    This doesn't mean that the gas turbine is a necessarily bad choice for a mechanical drive application, it just highlights the primary concerns the designer and owner should consider in evaluating the alternatives. Therefore, it would be prudent to rely on qualified design professionals and reputable equipment manufacturers for gas turbine installed cost, operating characteristics, and site-specific performance estimates.


    Evaporative Cooling

    Reduce the energy consumed by mechanical cooling equipment by using the cooling effects of evaporating water.

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    Evaporative cooling supply air can reduce the energy consumed by mechanical cooling equipment. The two general types of evaporative cooling are direct and indirect systems. The effectiveness of either of these methods is directly dependent on the low wet bulb temperature in the supply air stream. This is why these systems are popular in desert climates. In some applications, the two types are combined as shown here.

    Applications

    The effectiveness of either of these methods is directly dependent on the low wet bulb temperature in the supply air stream. This is why these systems are popular in desert climates.

    Technology types (resource)

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    Evaporative cooling - direct

    Direct evaporative cooling introduces water directly into the supply airstream (usually with a spray or some sort of wetted media). As the water absorbs heat from the air, it evaporates. While this process lowers the dry bulb temperature of the supply airstream, it also increases its wet bulb temperature by raising the air moisture content.

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    While an evaporative cooling system can effectively reduce the required capacity of the mechanical cooling equipment, it usually does not eliminate the need for a conventional cooling coil (except in certain arid regions of the country). Additional static pressure typically around 0.2 to 0.3 inches water column is required by the air handling system whenever evaporative coils are used in conjunction with a conventional cooling coil.

    Evaporative cooling - indirect

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    Indirect evaporative cooling uses an additional waterside coil to lower supply air temperature. The added coil is placed ahead of the conventional cooling coil in the supply airstream, and is piped to a cooling tower where the evaporative process occurs. Because evaporation occurs elsewhere, this method of "precooling" does not add moisture to the supply air, but is less effective than direct evaporative cooling. That is, it will not cool air to as low a temperature at the same outside air wet bulb.

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    Free Cooling Effect

    Produce chilled water without operating the chillers.

    "Free Cooling" is the production of chilled water without operating the chillers. Free cooling is not really free as the chilled and tower water pumps and the tower fan(s) must operate.

    The heat removed from the building by the chilled water coils is rejected by one of these alternatives.

    • Refrigeration Migration
    • Strainer Cycle
    • Plate and Frame Heat Exchanger

    Technology types (resource)

    Refrigeration migration

    One method for reducing the energy consumption of a centrifugal water chiller is to add a refrigerant-migration free cooling cycle. This type of free cooling is based on the principle that refrigerant migrates to the coldest point in a refrigeration circuit.

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    When water returning from the cooling tower is colder than the chilled water, refrigerant pressure within the condenser is lower than that in the evaporator. This pressure differential drives the refrigerant vapor "boiled off" in the evaporator to the condenser, where it liquifies and flows by gravity back to the evaporator. As long as the proper pressure difference exists between the evaporator and condenser, refrigerant flow and the consequent free cooling continues.

    Under favorable conditions, refrigerant-migration free cooling can provide as much as 40 percent of the chiller's design tonnage if the chiller is designed appropriately. Since the chiller and free cooling cycle cannot operate simultaneously, free cooling of this type can only be used when the cooling capacity of the tower water is sufficient to meet the entire building load.

    Little, if any, free cooling capacity is available when the ambient wet bulb temperature is above 50°F. Accessories such as chilled water pumps, condenser water pumps and cooling tower fans continue to operate in the conventional manner while the chiller operates in the free cooling mode. The energy cost savings realized from free cooling operation results from the compressor's inactivity during this cycle. The cooling tower must be designed for winter operation.

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    Strainer cycle

    Like other methods of free cooling, the addition of a strainer-cycle waterside economizer is intended to reduce water chiller energy consumption. This particular method uses cooling tower water to satisfy the building's cooling load. Whenever ambient wet bulb temperature is low enough, cooling tower water is "valved" around the chiller directly into the chilled water loop. The cooling tower water typically passes through a filter (or strainer) before entering the chilled water circuit. This is why it is commonly referred to as "strainer cycle."

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    Pumping cooling tower water throughout the entire chilled water loop increases the risk of pipe corrosion and air handler coil plugging. This risk can be mitigated through more costly water treatment. Strainer cycle economics are limited since free cooling is only available when the cooling load can be satisfied with cooling tower water. The cooling tower must be designed for winter operation.

    Plate and frame heat exchanger

    One method for reducing water chiller energy consumption is to add free cooling. The method shown here uses a plate and-frame waterside economizer that pre-cools the chilled water before it enters the chiller's evaporator. When the ambient wet bulb temperature is low enough, the heat exchanger allows the transfer of heat from the return chilled water to the water returning from the cooling tower. Lowering the temperature of the water entering the evaporator reduces both chiller loading and energy consumption.

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    The plate-and-frame method of free cooling requires an additional heat exchanger. This adds to the initial cost of the system and increases pumping costs due to the added pressure loss. Free cooling is only of significant value when the ambient wet bulb temperature is lower than the design return chilled water by about 10°F. The cooling tower must be designed for winter operation, and the water entering the chiller condenser must be maintained within the manufacturer's specified temperature limits while the chiller is operating.

    Note that plate-and-frame free cooling can be accomplished with a variety of piping arrangements, depending on the operational characteristics desired. The schematic illustrated here shows just one method of piping that can be used to permit simultaneous free cooling and mechanical cooling.

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    Compare technologies for this integral element in building cooling solutions.



    Braising Pans

    Even heating, insulated bottom, cooks in tilted position, better heat transfer, fast recovery.

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    The braising pan is perhaps the most versatile piece of commercial cooking equipment available.

    The braising pan is also known as a tilting skillet, fry pan, and braiser (as well as many other names). It can braise, boil, simmer, griddle cook, fry, steam, thaw, poach, blanch, heat canned foods, act as a proof box or oven, and store hot bakery products. This flexibility is valuable in a commercial kitchen, where labor and floor space are limited and a menu item can be prepared entirely in this single pan. Cooking with a braising pan, a food operation can realize a 50% or greater labor savings over conventional top or stock pot methods (mostly because of reduced cleaning requirements). The value of a braising pan is even higher in new kitchens where it can substitute for numerous other pieces of kitchen cookware.

    The pan can be tilted a few degrees to drain fat away from food as it cooks, such as in griddling or braising meats. Boiling about an inch of water in a covered braising pan can be used to steam food held in special perforated pans or racks. Proofing can be done similarly by using hot instead of boiling water.

    Braising pan types

    There are three types of braising pans: table models, floor models (mounted on a set of open legs or a cabinet base), and wall-mounted units. The cooking capacity of a braising pan is rated by its manufacturer. Table models range from 10 to 15 gallons. Floor models typically range from 19 to 40 gallons.

    Comparing electric vs. gas braising pans

    There are many factors to consider when selecting a braising pan: initial cost, food preparation productivity, ease of operation, heat generation in the kitchen, and whether electricity or gas is used as the energy source. However, consider that energy only accounts for 3 to 5 percent of a typical food service establishment's total costs. Therefore, while one fuel may be less expensive in a BTU to BTU comparison, the best choice in cooking equipment is the one that minimizes total operating costs, not just energy costs. Features that reduce labor costs or result in higher food product yield will nearly always outweigh any energy considerations. Make sure that you include all of these factors in any equipment evaluation.

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    Therefore, when comparing gas an electric models, compare equipment that is similar in all ways except the energy source.

    Advantages of electric braising pans

    Electric and gas braising pans have virtually the same preheating capabilities, with both reaching a cooking temperature of 300°F in about 10 minutes. However, electric braising pans have several advantages over gas models:

    An electric braising pan unit costs an average of 20 to 25% less than similar gas models.

    Electric braising pans use less energy than their gas equivalents. The average efficiency of electric models is about 80%, while gas model efficiency is just over 50%. This higher efficiency translates into less heat into the kitchen, which lowers cooling requirements from the HVAC system.

    Electric braising pans are much easier to clean and maintain than gas models.

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    Braising pan components

    A braising pan looks like a large flat griddle with 7- to 9-inch side walls. It is typically made of stainless steel over aluminum block, or a steel griddle base. On gas heated units, aluminum baffles are added to the bottom to promote even heating.

    All units are equipped with both a hinged lid and a tilting mechanism. The lid or cover holds heat in the pan. Tilting mechanisms for braising pans come in three types: manual, hand crank, and electric. The hand crank with a self-locking worm gear is the most popular. The tilting mechanism tilts past 90 degrees so an operator can pour foods out of the pan and clean the unit easily. The pouring side of the pan usually has a notched spout.

    The cover should fit tightly and be counterbalanced with springs so it doesn't shut on an operator's hand. Lifting handles typically run the length of the pan front (but an operator should also be able to raise the cover from the side to avoid a blast of steam on their hand). Most covers are available with a condensate drip shield and a vent.

    Controls for the braising pan include a power off switch and a 100° to 450°F thermostat. Some units include a 60 minute timer and buzzer.

    With the exception of 15-gallon models, braising pan units are generally rectangular. One manufacturer produces a round, 15-gallon model. Also, some models contain infrared coils in the pan cover to accommodate special tasks such as baking and top browning.

    Accessories

    Braising pans may offer useful accessories that add versatility and labor savings, including:

    • Hot and cold water spray hoses.
    • Food receptor pan supports, hinged to facilitate tilting. Casters for greater mobility.
    • Pan racks that hold 12 by 20 inch steaming pans.
    • An electronic ignition on gas units.
    • Food strainers that slip on and off the pouring spout.
    • Steamer racks, pasta baskets, and poaching pans.
    • A drain valve and hose.

    Skittles (combi-pans)

    A skittle, sometimes called a combi-pan, performs the functions of seven pieces of kitchen equipment. A skittle can serve as a steamer, a skillet, a griddle, a fryer, a kettle, a roaster, and a holding cabinet.

    The value of such a versatile unit is easy to see. Commercial kitchens are growing more and more complex. Kitchen space is expensive, and demands for more flexible menus and quicker preparation strains both staff and equipment. Manufacturers have responded by creating the skittle, which can completely replace serve as a backup for several pieces of cooking equipment. The skittle's flexibility makes it ideal for smaller food service establishments without room for multiple pieces of equipment. Skittles are available in gas, electric, and high performance electric.

    Perhaps the skittle's greatest value is as a steamer. This is because the skittle is the only steamer not requiring a boiler, thus eliminating a major maintenance problem. This means lower maintenance costs and no descaling or deliming.

    As a skillet, a skittle provides even heating, excellent heat retention, and quick recovery. As a griddle, it offers the advantage of tilting, which allows grease to be drained off even while cooking. The skittle is also excellent for shallow or deep-fat frying. Cooking oils can be drained safely off into a container for filtering or storage. As a kettle, the skittle can be used to prepare soups, sauces, rice, and other foods, with capacity ranging from 7 to 40 gallons. As a roaster, the skittle prepares food in dry heat or in combination with steam, making it ideal for roasting meats, baking potatoes, or reheating prepared foods in 14 cubic feet of oven space. Skittles can also be used as holding cabinets because they have capsule lids that preserve the moisture content of food during holding.


    Broilers

    Reduced shrinkage, improved heat transfer, increased production, less spatter, less effluent.

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    Broilers (Overview of All Types)

    Broilers provide an alternative method for cooking flavorful, nourishing, and healthful foods. Broilers are used to cook a wide variety of foods by a process that usually takes 3 to 6 minutes. Products commonly prepared with broilers include steak, poultry, seafood, hamburgers, pizza, and ethnic dishes.

    Some types of broilers are used to "finish off" items like toasted breads, cheese sauces, and hot sandwiches. Depending on the broiler type, these food items may be cooked in metal pans, glass casseroles, or directly on the surface of broiler grates or conveyor belts.

    In the 1950s, only about 10% of the nation's food service establishments featured a broiler. Today, one third are equipped with broilers.

    Broiler Types

    Four major types of broilers are available.

    Click on the desired type below for more information.

    Standard over-fired broilers are heavy-duty units designed to cook large quantities of food by exposing it to radiant energy. This energy is emitted by heating radiants above the grid. Infrared broilers work much like standard broilers, but they operate at higher temperatures (up to 1,600°F) to produce high intensity infrared radiation. Infrared broilers have very fast preheat compared to standard over-fired broilers.

    Temperature and cooking time is controlled by moving the grid up or down through several grid positions. The grid is spring-loaded or counter balanced for convenient up or down adjustment. The grid also rolls in and out for easy loading and is removable for fast clean-up. Each deck of the broiler typically has separate temperature controls, usually with high-low or high-medium-low settings. This varies depending on the manufacturer and the type of fuel powering the broiler.

    Drip shields are located below the grids and move with the grid to collect grease and food particles. V-shaped channels deposit the liquid residue in a drip pan for disposal.

    Over-fired broilers are usually installed on stainless steel counters and are available as single and double-decked modular units. They can also be mounted on a one-pan oven base, convection oven base, or a storage cabinet base. In fact, some combinations of single gas broiler decks have storage cabinets below and a finishing oven above the broiler deck.

    The waste heat generated by gas burners is sometimes used in the finishing oven cavity. This is an effective method of saving energy by recycling heat that would otherwise go unused.

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    Under-fired broilers or charbroilers are typically medium- to heavy-duty units. They have the ability to cook large quantities of food by exposure to radiant energy produced by heating elements located below the grid. Charbroilers are available in countertop, cabinet base, or stainless steel frame models.

    Charbroilers cook food much like an outdoor barbecue grill. Food is placed on a cast iron grate above the heat source, and cooking occurs primarily from radiant heat and conduction by the grate. The energy source may be electricity, gas, wood, or charcoal. As the food cooks, fats or marinades drip onto the coals or ceramics producing smoke. The smoke produces the characteristic charred flavor, while the hot grates create the strip marks that are typical on charbroiled foods.

    There are two types of under-fired charbroilers. One type allows the radiant heat source to heat a radiant to a cherry red color. The radiant, in turn, broils the food product. The other type of charbroiler uses a heating source above or below to heat lava rocks or ceramic briquettes. The rocks or briquettes distribute the heat more evenly than the heat source alone. Some manufacturers use both methods to increase efficiency and reduce preheat times.

    The broiler grate is adjustable to both level and tilted positions. Typically, the charbroiler is designed for the rear two-thirds of the grate to be hotter than the front section. Many models also have grease troughs fastened to each blade in the top grates to channel excess fat runoff and reduce flaming. Excess residual fat drains into a large grease drawer in a cool zone for disposal.

    A charbroiler, like an open range-top burner, consumes energy at a constant rate, which depends on the temperature control setting. Because the charbroiler has a significant thermal mass of heating material that requires preheating and retains heat, the unit cannot be turned "on" and "off" quickly on demand.

    Maintenance costs for a charbroiler are typically higher that any other broiler types. This is partly because the heating radiants below the open cooking grates are exposed to any materials falling through the radiants.

    Back-shelf broilers, salamanders, and cheese-melters are often used to supplement existing broilers. These light-duty units "finish off" partially broiled products and browned foods that are not normally broiled for the complete cooking cycle. These small units are capable of broiling other foods as effectively as larger standard units, but not as quickly.

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    Small amounts of food are "finished off," melted, or broiled by exposing the food to radiant energy from the heating radiants located above the grid or rack. Back-shelf and salamander units are always single-deck broilers. Some salamander/cheese melter units are loaded and unloaded from one side, while others are equipped with pass-through capabilities. The units may be mounted on steel wall back-splashes above the ranges, mounted on 4 to 6 inch legs and placed on counters, or simply wall-mounted above prep stations.

    Infrared models work like standard models, but the infrared radiants operate at considerably higher temperatures, increasing their heating capability and shortening preheat times. Back-shelf radiants are located above the cooking grids or racks. Each radiant has a separate temperature control with high-medium-low settings. In addition, the grid moves up or down through several levels. As with over-fired broilers, the grid of a salamander is spring-loaded or counterbalanced for easy operation. Drip shields are located below the grids or racks to collect grease and food particles. The grease and other liquid residue collects in a drip pan.

    Some units have switches that turn on 100% heat when food is placed on the rack and then automatically lower the heat to a standby temperature in between cooking jobs.

    Conveyor broilers combine the principles of over-fired broilers and under-fired broilers using a stainless steel belt to convey and consistently cook large quantities of food between two sets of heating radiants. One radiant is located above the food and one below . Each conveyor broiler may have one or more broil belts. IN multiple belt units, the speed of each belt is regulated with a separate digital speed control so different foods can be cooked simultaneously.

    Conveyor broilers can bake, broil, heat and melt a variety of food items faster and with less labor than other broiler types. The production capacity of a conveyor broiler depends on operating temperatures and the characteristics of the food being cooked, such as composition, diameter, and thickness of the food product. Fresh or frozen hamburgers, steaks, pork chops, hot dogs, sausage, bacon, chicken, fish, or any product that can fit in single-serving oven-safe cookware can be prepared using conveyor broilers.

    The heating radiants of the conveyor broiler, which operate at temperatures up to 1,600°F, are controlled by on and off switches. The speed of each stainless belt is controlled by a variable speed belt control. Cooking time is a function of the intensity of the heat source and belt speed.

    Conveyor broilers can be free-standing floor models or countertop models and can be flow through or front load-front return operation.


    Fryers

    Improved heat transfer, fast recovery, greater oil life, long service life, improved food quality.

    Fryer types

    Used in about 85% of food service establishments, fryers are an extremely popular commercial cooking appliance. A fryer is designed to cook chicken, fish, breaded vegetables, specialized pastries, French-fried potatoes, and other foods.

    Two major types of fryers are available:

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    Conventional open fryers — The most common fryer type is the open, deep-fat fryer. These come in many sizes ranging from counter top models to large stand-alone units with multiple frypots. The fryers have a variety of optional features such as automatic controls, filtration systems, and accessories for holding cooked food.

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    Pressure fryers — These units have a special lid that keeps vapors inside the fry vessel. The vessel captures steam from the cooking food, increasing pressure inside the unit to prevent additional moisture from being released from the food. This seals in juices, improving food taste and reducing the oil absorbed by the food. The also produces shorter cooking cycles, making pressure fryers more productive than open fryers. Pressure fryers are especially popular for cooking fried chicken.

    Besides these two major fryer types, specialty fryers are also available for special needs. One example is the doughnut fryer, which has a wide, shallow frypot designed for cooking doughnuts and other fried pastries. Another example is the convection fryer, which is an open vessel design that improves cooking by circulating hot oil around food in much the same way as a convection oven circulates hot air.

    Comparing electric vs. gas fryers

    There are many factors to consider when selecting a fryer: initial cost, food preparation productivity, ease of operation, heat generation in the kitchen, and whether electricity or gas is used as the energy source. However, consider that energy only accounts for 3 to 5 percent of a typical food service establishment's total costs. Therefore, while one fuel may be less expensive in a BTU to BTU comparison, the best choice in cooking equipment is the one that minimizes total operating costs, not just energy costs. Features that reduce labor costs or result in higher food product yield will nearly always outweigh any energy considerations. Make sure that you include all of these factors in any equipment evaluation.

    Therefore, when comparing gas an electric models, compare equipment that is similar in all ways except the energy source.

    Advantages of electric fryers

    In general, electric frying equipment offers these advantages:

    • The electric heating elements operate at lower temperatures, which saves energy, reduces fat breakdown, and uses less fat. Gas burners can create hot spots in the fryer, which breaks down the oil prematurely.
    • Electric fryers add less heat to the kitchen because they are more energy efficient.
    • Electric units require less maintenance and require less ventilation.
    • Electric units have faster preheat and recovery times than gas units.
    • Electric induction units are now available that use magnetic induction coils to heat the oil. Some electric fryer manufacturers are also using lower watt-density elements to improve efficiency and achieve longer oil life.

    Energy and money saving tips

    Here are a few common-sense operating tips that save money with a fryer.

    • Turn the fryer off or down to an idling temperature during slack periods when the unit is not in use.
    • Operate the fryer at the proper temperature, 325° to 350°F. Excessive temperatures waste energy and often result in improperly cooked food.
    • Do not load the fryer baskets beyond the manufacturer's recommended capacity. This is usually one-half to two-thirds full. Overloading results in poor food quality.
    • Check fat levels frequently. Low fat levels can cause premature oil breakdown.
    • Drain and strain the oil frequently. This saves oil and preserves food quality.
    • Keep the units clean and properly maintained.
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    Fryer components

    Frypot

    The most common fryer is the open vat fryer. The portion of the fryer that contains the oil is called the frypot (also called the fry kettle, vat or fat container). The frypot is usually rectangular and ranges from 14 to 18 inches long by 18 inches wide and 18 inches deep. Wire baskets containing uncooked food are lowered into the frypot for cooking. Next to the frypot are supports that hold the wire baskets while cooked food drains excess oil back into the frypot. Some units have a removable frypot while others have frypots that are fixed in place.

    Some frypots are split into two sections so the operator can cook two different kinds of foods without transferring taste. In addition, the operator can turn off one side of the unit during slow periods. This saves energy costs and prolongs oil life.

    Most fryers have a 1- to 3-inch separation between the frypot and the outer housing or cabinet. Some units have insulated frypots, while others have an insulated cabinet. The use of insulation reduces energy costs and heating up of the kitchen.

    Heat source

    Electric units have heating elements submerged in the bottom of the frypot. These are either fixed in position or hinged to the main structure of the fryer. Hinged units can be lifted out for easy cleaning.

    Gas units have burners located outside the frypot. Some more advanced units have fire tubes that extend through the frypot in order to transfer more heat to the oil. These fire tubes often contain baffles to improve heat transfer and reduce the amount of heat wasted by escaping up the flue.

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    Cold zone

    Most fryers have a cold zone, which is a small section of the frypot bottom extending below the heat source. The oil in this section is intentionally cooler than the oil in the cooking zone. When particles of food, batter, and breading escape from the basket, they sink to the bottom and collect in the cold zone and stop cooking, which prevents oil breakdown and lengthens cooking oil life. This design also creates a natural convective flow of oil throughout the frypot so cooler oil continuously recirculates with hot oil. Allowing the oil to cool in this way further reduces breakdown.

    Controls

    Nearly all fryers have a thermostat to maintain the temperature of the frypot. This control is either located on the front panel or above and behind the frypot. Some units also have a timer that alerts the operator when the food has cooked for a preset amount of time. More sophisticated models have elaborate automatic controls, such as an automatic basket lift, that reduce labor requirements and more closely monitor the cooking process. Some units can even be programmed so an operator only needs to specify the food type, such as French fries, and the unit automatically controls the cooking time and temperature. This reduces training costs and improves product quality.

    Better fryers include automatic filtration equipment that reduces the labor requirements for daily cleaning.

    Fryer operation tips and issues

    General operation

    Most fryers take between 5 and 15 minutes to reach full operating temperature. Typically, a signal light stays on when the temperature is below the set temperature point. Operators set the thermostat to the desired temperature and wait till this light turns off, indicating the fryer is ready.

    Many fryers have timers as well as thermostat controls. Operators need to know both the proper temperature and cooking time for each food product. For consistency of quality, these settings must be maintained. To address this need, some units have devices that automatically raise and lower baskets into the fryer at specified times, taking responsibility away from the operator. This saves labor costs and ensures more consistent quality.

    Fry baskets should be loaded to at least one-half of their capacity but never more than two-thirds, because food does not cook properly if overloaded. After loading, a basket is lowered into the fat and the timer started. For automatic units, the baskets are attached to the automatic elevator supports and with the press of a button the frying process begins.

    At the end of the recommended cooking time, the baskets are lifted out of the oil bath and hung on basket supports for draining. Automatic units are programmed to do this without operator assistance.

    During slack periods, the fryer should be turned off or its temperature turned to a 200 degree standby setting. This saves energy and increases the life of the fat.

    Finally, all units should have a safety thermostat to warn the operator when the temperature exceeds 400°F. Some models have a warning light that turns on or flashes when the unit overheats. If this occurs, the unit should be turned off and allowed to cool. If the unit overheats again, it should be serviced.

    Fryer preparation

    The cooking medium for all fryers is oil (also called shortening, frying compound, or fat), which is heated to about 350°F. The oil is typically vegetable or animal fat purchased in solid or liquid form. Top grade commercial shortening with a high smoke point and resistance to breakdown results in better tasting food and longer fat life.

    Most fryers have a marker in the fry vessel that shows the proper shortening level. In units without a marker, shortening should cover the heating elements by at least one inch .

    If you use liquid shortening, fill the kettle to the proper level and set the thermostat to the desired temperature.

    For solid shortening, first pack the shortening solidly around the cooled heating elements. Next, set the thermostat to 250°F to let the solid fat melt slowly. Continue to add fat and wait for it to melt until it reaches the proper level before turning the thermostat up to the desired cooking temperature.

    Performance

    The quality of the final food product largely depends on the quality of the oil that is used. Flavors develop in the oil transferred to the foods being cooked. Also, oil is expensive, ranging from 30 to 75 cents per pound. Since a single fryer's oil capacity can range from 28 to 110 pounds, the cost for replacing used oil can be significant.

    Food particles eventually degrade oil. Particles continue to cook long after the food is removed from the fryer, and can eventually burn, leaving a bitter taste in the oil. To minimize this problem, most fryers have a cold zone at the bottom of the fryer where food particles collect. The temperature in this zone is lower than the cooking zone, so food particles stop cooking. However, the fryer operator should still frequently filter the oil to remove excess food particles and prolong the life of the oil.

    Excess temperature can also destroy cooking oil. If the fryer's temperature exceeds 400°F, the oil will begin to break down and develop a bad taste. Thermostat overrides and hot spots along burner tubes in gas fryers are frequent culprits.

    Cooking temperature also greatly affects the quality of the final food product. Cooking at too high a temperature may overcook the outside of food while leaving the interior portion partially uncooked. However, cooking at too low a temperature causes food to absorb more oil, which makes it soggy and adds to food preparation costs.

    For more information about the benefits of electric fryers vs. gas, please contact us for a copy of an EPRI performance or ventilation report.


    Griddles

    Uniform heating, no hot or cold spots, high production capacity, fast recovery, faster cleaning.

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    The griddle is the workhorse of the fast food industry. Nearly every commercial cooking operation uses some type of griddle.

    A griddle is simply a flat metal plate that cooks food by conducting heat directly from the griddle surface to the food product. A thin layer of cooking oil or grease from the cooked item usually separates the food from the griddle surface to keep the food from sticking. Griddles are used to cook a variety of foods including: bacon, eggs, chicken, hamburgers and steak. Some also like to use the hot griddle surface to heat food in a small pan, like melting butter.

    Some griddles are equipped with a platen placed a few inches above the griddle surface to provide additional cooking from above. This add-on cooks the top surface of the food by exposing it to radiant heat energy, cooking the food faster and sealing in the juices for improved taste and reduced shrinkage.

    Griddle types

    Two major types of griddles are available: single-sidedand double-sided. Single-sided griddles cook food on the bottom only. Double-sided griddles cook food on both sides simultaneously.

    Single-sided griddles

    A single-sided griddle can be installed as:

    • a built-in unit
    • part of a range or cooking center
    • a free-standing unit that sits on tubular steel legs
    • a portable unit mounted on a stainless steel mobile stand.
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    Heavy duty griddles are usually free-standing. The cooking surface typically range from 30 to 36 inches deep and up to 72 inches wide. Often, two or more free-standing units are installed side-by-side, or back-to-back.

    Counter-top griddles are small, free-standing units, normally located on a countertop or in a counter base. They range from 15 to 24 inches deep and from 15 to 72 inches wide.

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    Double-sided griddles

    Double-sided griddles heat food from both the top and bottom. They have a large bottom griddle plate and at least one platen on top. Platens press on the food, "sandwiching" it between two hot pieces of metal. This allows food to be cooked on both sides simultaneously. Stop devices on the platen keep the food from being crushed. A counter-balanced lift holds each platen in place when raised.

    A double-sided, non-contact griddle has a plate on the bottom and at least one platen on top. However, a non-contact griddle's top platens do not actually contact the food. The "hood" stays about one inch above the food, heating like a broiler. The heat source may be a gas burner, conventional electric elements, quartz lights, or a ceramic infrared burner.

    A double-sided griddle cooks food very quickly. For example, 8 to 12 hamburgers cook in roughly 3 minutes. The double-sided, non-contact design eliminates the need to turn food for uniform cooking, which can reduce labor costs.

    Comparing electric vs. gas griddles

    There are many factors to consider when selecting a griddle: initial cost, food preparation productivity, ease of operation, heat generation in the kitchen, and whether electricity or gas is used as the energy source. However, consider that energy only accounts for 3 to 5 percent of a typical food service establishment's total costs. Therefore, while one fuel may be less expensive in a BTU to BTU comparison, the best choice in cooking equipment is the one that minimizes total operating costs, not just energy costs. Features that reduce labor costs or result in higher food product yield will nearly always outweigh any energy considerations. Make sure that you include all of these factors in any equipment evaluation.

    Therefore, when comparing gas an electric models, compare equipment that is similar in all ways except the energy source.

    Advantages of electric griddles

    In general, electric griddles offer these advantages :

    • More uniform temperature across the surface of the griddle, which makes electric griddles easier to operate and produces consistent food quality.
    • Thinner griddle plates that use less energy and about half the time to preheat.
    • More efficient operation with less heat loss into the kitchen, lowering kitchen cooling costs and reducing maintenance.

    A recent new electric technology is the induction griddle on which the griddle surface is heated by a magnetic field inducing electric currents across the griddle plate. This produces a more uniform surface temperature and brings the griddle surface up to cooking temperature very quickly, saving money, rejecting less heat to the kitchen, and producing a more consistent food product.

    Energy usage

    Single-sided electric griddles normally consume 3 to 25 kW of power. The average preheat time can range from 7 to 20 minutes depending on the plate configuration and BTU input. Energy consumption for gas single-sided griddles normally approaches 20,000 to 30,000 BTUs per 12-inch section, with preheat times of 15 to 23 minutes. Again, these figures depend on plate configuration and BTU input.

    A low energy input figure generally implies slow pre-heat and recovery time. Typical kW consumption for the electric double-sided griddle ranges from 21 to 35 kW, with a preheat interval of about 18 minutes. Typical ratings for gas powered double-sided griddles range from 90,000 to 140,000 BTUs, with preheat times of 18 to 23 minutes.

    Energy and money saving tips

    Here are a few common-sense operating tips that save money:

    • Heat only the griddle sections necessary for a task.
    • Pre-heat only until the griddle surface has achieved the correct cooking temperature.
    • Set the temperature for each section no higher than that required to cook the food.
    • Turn the griddle down or off during slow production times.
    • Use pre-cooked foods and avoid frozen products where possible.
    • Use a cover while cooking where it will not adversely affect the cooking process.
    • Scrape the cooking surface between production intervals. Cleaning some types of griddle surfaces requires special tools. Consult the manufacturer or owner's manual for details.
    • Clean the griddle frequently, and always re-season the griddle afterwards.
    • Inspect each section of the griddle periodically, searching for hot or cold spots.
    • On gas units, make sure each gas flame burns blue and adjust the gas-to-air ratio when necessary.
    • It takes 77 BTUs to heat a pound of ground beef from 40°F Fahrenheit to 140°F, but 196 BTUs are used to heat the same pound of beef from 0°F to 140°F. Therefore, simply thawing food before cooking can increase energy savings.

    Heat loss issues

    Griddles are among the largest energy users in food service, so energy efficient operation is an important way of reducing operating costs. Most of a griddle's operating costs arise from heat loss from the bottom, the top, and the four edges of the cooking surface. In addition, cooking surface losses are increased due to the relatively small quantities of food typically cooked on the large surface during most of the day.

    Heat lost from a griddle warms the kitchen, which makes workers uncomfortable unless the cooling system removes the excess heat. These losses can therefore add greatly to overall cooling costs, which is an important factor favoring electric griddles over gas units. Even if a kitchens is not air-conditioned, so cooling costs are not an issue, worker productivity and morale suffer as room temperatures rises, increasing costs through lower worker performance and increased turnover.

    Many higher quality griddles are designed for improved energy efficiency, partly through the use of newly developed griddle plate surfacing. These improved surfaces restrict the griddle's normally excessive radiation of energy. In full-load cooking tests, griddle efficiency ranges all the way from 31% to 71% depending on model. Griddle inefficiency is most evident in light-load cooking operations, where efficiency ranges from 13% to 50%.

    Griddle components

    Components

    Griddles come in many sizes and may be freestanding or built into a range body with ovens below. Generally, the griddle surface is divided into 12-inch sections, each with its own heating unit and control mechanism. This design lets different sections operate at different temperatures, so the chef can cook different kinds of food at the same time.

    All griddles have at least one thermostat dial that controls the cooking temperature. Some griddles also have surface temperature indicator lights that are typically located on the control panel. Gas griddles have slotted vents for each burner for the intake of combustion air.

    Griddles normally have a metal splash guard surrounding all but the front of the cooking surface. The splash guard keeps food from sliding off and minimizes grease splatter. A grease trough, usually running along each side of the griddle plate, drains grease and residual food particles, depositing these wastes into a collecting pan. Grease troughs may also be located on the front or back of the griddle. Some griddles have a slightly tilted griddle plate that causes grease to run off. These units also usually produce less smoke while cooking.

    The cooking surface of a single-sided griddle is called a plate and its design dictates the performance of the griddle. High quality plates distribute heat uniformly across the griddle. The most common griddle plate is made of flat steel or cast iron and ranges in thickness from one-half to one inch.

    Griddle surfaces are usually smooth and flat, but some types of griddles have ribbed or grooved surfaces. Grooved surfaces are designed to emboss food with charred grid marks, similar to broiled and grilled foods.

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    Ribbed surfaces cook somewhat slower than flat surfaces because only the parts of the food that touch the raised edges of the grooves are exposed to full heat. For this reason, manufacturers usually install a grooved surface on only a single section of the griddle, with remaining sections equipped with a flat plate for total direct-contact cooking.

    Griddle operation tips and issues

    General operation

    Griddles can operate between 200° and 550°F, but cooking temperatures normally fall between 225° and 375°F. Most units preheat to their thermostatically controlled cooking temperature in 15 to 30 minutes.

    Griddles are usually turned on at the beginning of the cooking day and left on all day. This arrangement wastes significant energy if the unit is only used occasionally. This practice is common because griddles take a relatively long time to preheat; it can be impractical to turn off the unit when its not being used. In addition, food service operators like to have the griddle cooking capacity in reserve and so they will rarely turn it off until the end of the cooking period.

    Performance

    Griddle surfaces often develop hot spots and cold spots. Hot spots usually occur near the heat source while cold zones occur in areas farthest from the heat source. Food cooks faster in hot zones and may be difficult to control because of the higher heat. Some griddles develop a cold zone around the perimeter, about two inches wide, which is not useful for cooking but can be used to keep cooked food warm.

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    An experienced chef knows where hot and cold zones are and can adjust the cooking approach accordingly. However, most griddle operators, especially in fast food restaurants, are not this experienced. They fail to adjust cooking times to account for hot zones and cold zones, cooking everything for the same amount of time. This results in inconsistent quality, with some food under-cooked and some over-cooked.

    Good griddle design can minimize changes in surface temperature across the griddle and help maintain consistent food quality. These units also reduce the amount of training needed for new griddle operators.

    Maintenance

    Griddle surfaces should be cleaned regularly. A clean griddle surface offers more uniform heat distribution and operates more efficiently. A clean griddle also prevents the bitter taste of charred food in the final food product.

    Griddle operators should:

    Scrape excess food and fat particles from the surface with a flexible spatula, grill brick, or other device after each cooking load.

    Clean and wipe out grease troughs, remove any stuck-on food, and clean the surface with a soft cloth, rubbing with the grain of the metal while the surface is still warm. This should be done at least once a day and more frequently during heavy cooking loads.

    The platen on a two-sided griddle can often be much harder to clean. Some models have stainless steel platens that make cleaning easy. Others have a special coating like Teflon to prevent food from sticking. A few models use disposable non-stick paper to prevent sticking.

    For more information about the benefits of electric griddles vs. gas, please contact us for a copy of an EPRI performance or ventilation report.

    Ventilation study

    As part of a larger study to identify optimal designs for commercial kitchen appliances, researchers tested one electric griddle and one gas griddle in operation with two hood types: an exhaust-only, wall-mounted canopy hood and a custom-engineered backshelf hood.

    These tests revealed the following:

    • The cooking capture and containment (C&C) flow rate under a canopy hood for the electric griddle is 241 scfm/lf, 13% lower than for the gas griddle, 40% lower than the 400 scfm/lf building code value, and 7% lower than the 260 scfm/lf Underwriters Laboratories (UL) listing.
    • The cooking C&C flow rate under a custom-engineered backshelf hood for the electric griddle is 100 scfm/lf, 9% lower than for the gas griddle, 67% lower than the 304 scfm/lf building code value, and 26% lower than the 136 scfm/lf UL listing.
    • The idle C&C flow rate under a canopy hood was 26% and 32% less, respectively, than the cooking C&C flow rate for gas and electric griddles, and was 0.5% and 22% less, respectively, under the backshelf hood.
    • At the cooking C&C flow rate, the electric and gas griddles required about 60% lower flow under the backshelf hood than under the canopy hood. These results indicate that custom-engineered backshelf hoods can operate with exhaust flows about 65% below code values, and that electric griddles with both hood types require about 10% less exhaust than gas units. Designers should apply site-specific data when evaluating equipment options.

    Background

    To help electric utilities and the food service industry minimize commercial kitchen exhaust hood operating costs, EPRI is undertaking a series of tests to determine the exhaust requirement for a wide range of food service equipment and ventilation hoods. The exhaust requirement is the air flow needed to capture and contain cooking products and heat. Findings compare actual exhaust requirements with building code and UL levels. The ventilation tests described here examined electric and gas griddles operating under a wall-mounted canopy hood and under a custom-engineered backshelf hood using American Society for Testing of Materials (ASTM) standard method production conditions.

    Test Equipment and Conditions

    Both griddles measured 28 in by 3 ft. The electric griddle was rated at 17.1 kW and the gas griddle at 90,000 Btu/h.

    The canopy hood, an exhaust-only, wall-mounted type, was 5-ft wide by 4-ft deep and UL listed at 260 scfm/lf for cooking operation. The backshelf hood, a custom-engineered, exhaust-only type, was 3.4- ft wide by 3.5-ft deep by 5-ft high and was UL listed at 136 scfm/lf for cooking operation. Both hoods had three nominal 20-in by 20-in standard baffle filters.

    For each test, researchers positioned the griddle under the hood in accordance with ASTM F1275-95 and performed the tests using ASTM F1704-96. The temperature of the griddle was set to a calibrated 375°F.

    The project team evaluated C&C with visualization techniques aided by a smoke generator. They ran each test a minimum of three times in a consecutive series to attain statistical certainty as prescribed in ASTM F1704-96.

    Results

    Figure 1 shows C&C flow rates for electric and gas griddles operating under both canopy and custom backshelf hoods, as well as flow requirements under two specification options. Operating under a canopy hood, the electric griddle's measured cooking C&C flow rate is 241 scfm/lf, 40% lower than the rate required by building codes and 7% lower than that listed by UL. The idle C&C flow rate is 165 scfm/lf, 32% lower than the cooking rate. The gas griddle's measured cooking C&C flow rate is 276 scfm/lf, 31% lower than the rate required by building codes and 6% higher than that listed by UL. The idle C&C flow rate is 203 scfm/lf, 27% lower than the cooking rate.

    Operating under a custom-engineered backshelf hood, the electric griddle's measured cooking C&C flow rate is 100 scfm/lf, 67% lower than the rate required by building codes and 26% lower than that listed by UL. The idle C&C flow rate is 78 scfm/lf, 22% lower than the cooking rate.

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    The gas griddle's measured cooking C&C flow rate is 110 scfm/lf, 64% lower than the rate required by building codes and 19% lower than that listed by UL. The idle C&C flow rate is 109 scfm/lf, 0.5% lower than the cooking rate.

    References

    • Commercial Kitchen Ventilation Performance Report, Electric Griddle Under Canopy Hood, EPRI TR-106493-V4, July 1996.
    • Commercial Kitchen Ventilation Performance Report, Gas Griddle Under Canopy Hood, EPRI TR-106493- V3, July 1996.
    • Commercial Kitchen Ventilation Performance Report, Electric Griddle Under Custom Engineered Backshelf Hood, EPRI TR-106493-V6, July 1996.
    • Commercial Kitchen Ventilation Performance Report, Gas Griddle Under Custom Engineered Backshelf Hood, EPRI TR-106493- V5, July 1996.
    • Too Much Hot Air: Reexamining Commercial Kitchen Ventilation Systems, EPRI TB-105709, October 1995.
    • Minimum Energy Ventilation for Fast Food Restaurant Kitchens, EPRI TR-106671, July 1996.
    • Standard Test Method for Performance of Commercial Kitchen Ventilation Systems, ASTM F1704-96.
    • Standard Test Method for the Performance of Griddles, ASTM F1275-95.

    Ovens

    Even heating, uniformity in baking & roasting, Fast recovery, Calibration consistency, Better heat retention.

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    Oven cooking is as ancient as civilization, as old as the baking of bread. Today, oven cooking is the most common food preparation method, and ovens are one of the most widely used kitchen appliances. Even the smallest establishment usually has a microwave to heat appetizers or sandwiches, and large facilities may have a conveyorized bake oven for high volume production.

    Ovens are available in numerous sizes and designs. Some are designed for specialized food preparation tasks, while others are meant for a range of cooking applications. One recent design, called a Flash Bake oven, uses a combination of high intensity visible light and radiant heat to increase production speed and improve food preparation quality. There are also many "cook and hold" ovens that improve preparation consistency and product quality and cook foods at lower temperatures to increase nutritional value and reduce energy consumption.

    Ovens are often the largest consumers of energy in a food service kitchen. Oven design and construction quality, as well as fuel type, affect the amount of energy lost to the kitchen as heat. In typical electric ovens, only 40 to 60% of the source energy is used to cook; in gas ovens, this is only 10 to 30%. The remaining source energy escapes into the kitchen, making staff uncomfortable and adding to cooling costs.

    Some newer oven technologies increase speed and energy efficiency and reduce food-portion weight loss, making oven operation more economical. These improvements usually involve greater initial cost, but may actually reduce overall costs over time.

    Oven types

    There are a wide range of oven types. The most common types are described below.

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    All-purpose ovens

    All purpose ovens are used for baking, roasting, cooking pizza, and many other combined cooking tasks. Each section of an all-purpose oven adapts easily to a modular lineup.

    All-purpose oven production capacity varies by model. Independent controls and heating element banks are usually located at either the top or bottom of the oven unit. These let operators perform special tasks, such as custom browning, with balanced or unbalanced heat.

    Some standard oven decks are air cushioned to improve heat diffusion. Others have a removable core-plate that optimizes heat diffusion and holding.

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    Convection ovens

    Convection ovens circulate air in the oven cavity using a fan. This air movement speeds cooking by increasing heat transfer to the food. Convection ovens are ideal for low-temperature slow roasting, and many feature a slow "cook and hold" setting. Slow roasting meats at low temperature reduces shrinkage (and thus food costs) and tends to produce food of higher quality. Electric convection ovens are often preferred because they have smaller losses in oven humidity during cooking.

    The air flow through the oven chamber allows convection ovens to cook large loads and multiple racks effectively. Modern units have oven chambers insulated on all six-sides, providing peak energy efficiency. Solid state thermostats precisely control temperature, with cooking times digitally displayed for easy monitoring.

    Most electric convection ovens preheat to a typical operating temperature of 350°F within six to ten minutes. Comparable gas ovens are generally slightly slower coming to temperature. Both types offer optional non-stick or stainless steel liner panels that are removable for speedy cleanup.

    An optional heat-keeper recirculation system can save energy costs with gas convection ovens by re-using heated air that would normally be wasted. In these gas models, a power burner is provided for maximum energy efficiency.

    Convection ovens are not ideal for every oven application. They tend to dry products out during cooking, which may deteriorate food quality, especially with pastries. In these cases, a traditional oven is better.

    Half-size convection ovens

    Half-size convection ovens are a good choice when a full size oven for a given commercial cooking application would result in significant energy losses. In these cases, the smaller size of the half-size oven reduces losses while still meeting food production requirements.

    Combination ovens

    A combination oven combines the features of a convection oven and an atmospheric steamer. These ovens use the combination of oven and steamer cooking methods to maximize quality and speed. Steam injection is especially desirable in producing high-quality, golden-brown, crusty breads. Quality is further enhanced by the forced air distribution.

    This multipurpose oven offers a variety of cooking methods:

    • Hot air convection (some with water injection for high moisture)
    • Convection steam
    • A combination of convection, steam, and hot air circulation, and a cook-and-hold feature

    Enclosed tubular convection heating elements produce heat that circulates by a small blower motor. Most units have an extra large observation window to monitor cooking and a timer to track cooking time. Time and temperature controls with digital displays help operators track the cooking process. Some units also offer memory programming for multiple recipes with cooking cycles. The steam boiler can be heated electrically or by gas.

    A roll-in floor model combination oven and steamer has a cooking capacity of dozens of cafeteria pans and bun pans.

    Proofing ovens

    Many breads and pastries require a high humidity environment for optimal yeast action and product baking. To meet this need, special ovens exist with enhanced humidity control for the "proofing" stage in baking. These ovens can also be used to hold cooked food for extended periods of time.

    Most holding ovens surround food with hot air to keep it warm. This causes moisture to evaporate, which shrinks food, reduces visual appeal, and deteriorates flavors, texture, and consistency. However, food will not release moisture and dry out if the air around it is kept saturated. Most proofing ovens have been perfected to the point that they can keep some foods moist and others crisp in the same oven enclosure.

    Steam injection ovens

    Steam injection ovens are essentially standard convection ovens that can produce and inject steam. This steam injection desirable in producing high-quality, golden-brown, crusty breads. Quality is further enhanced by the forced air distribution.

    Electric rotary ovens

    Electric rotary ovens are ideal for supermarkets, delis, and convenience stores. Large glass doors help build purchase-point interest by allowing the product to be viewed during cooking.

    These ovens cook with a combination of convection and radiant heat and often incorporate an air circulation system that allows the unit to act as a warmer. This system maintains high humidity to keep the food contents juicy. In addition, the self-basting cooking action of these units enhances browning .

    Electric rotary ovens are equipped with digital timers and controls simplifying operation. They also usually come with multiple racks. The large removable trays and racks are easy to clean, which reduces labor costs.

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    Microwave ovens

    Microwave ovens cook by heating water and chemical molecules in food with short-wave radio energy like that used in radar and television. The frequency most commonly used in the microwave oven is 2,450 megahertz. Microwave ovens consume the least amount of energy and are highly space efficient.

    One microwave oven advantage is quick de-frosting, heating, and cooking of foods. The ability to defrost and warm foods in a matter of seconds makes these units popular with food service facilities wanting high menu variety for a large volume of customers. These establishments often pre-cook foods and refrigerate them until peak serving periods and then quickly heat portions during peak time periods.

    A disadvantage of the microwave, however, is it cooks foods from the inside out, as opposed to outside in as with most cooking systems. This typically does not provide the surface browning desired in many cooking applications. This can be solved by transferring the food to another type of oven for final browning.

    Cooking capabilities differ only minimally among different microwave oven models. Microwave ovens are available in an array of sizes and with a number of features. Top and bottom, or bottom-only energy feeding systems are available. Each type has rotating wave guides to minimize "hot spots" common to residential style units.

    Flash bake ovens

    A Flash Bake oven uses a combination of intense visible light and infrared energy to cook food rapidly. The visible light penetrates the food to provide heating while the infrared energy cooks the food surface to achieve the desired browning. Microprocessor control makes these units flexible and intelligent in their operation, and can produce superior quality for fish, meat, vegetables, breads, and many other types of foods.

    The primary benefits of Flash Bake technology are its speed and energy efficiency. The shortened cooking time also has the advantage of producing more nutritious and better tasting food. The Flash Bake oven was designed to cook relatively flat, thin foods. Pizza, nachos, quesadillas, and other foods with similar geometry are ideal for this technology.

    The primary disadvantage is one of perception: the oven simply appears to be too small to be a serious food preparation device. However, the unit's high speed and excellent performance have been proven in many establishments. These establishments have found the unit highly cost effective. Perceptions should change as the benefits of this oven are demonstrated to commercial food service professionals.

    Another disadvantage of the Flash Bake is that the increased heat transfer rate must be balanced against possible surface overheating. This can be minimized by operator training and advanced computer controls.

    New oven technologies

    Gas and electric oven manufacturers continually improve oven insulation and controls, heat transfer effectiveness, and heat recovery. These improvements yield higher efficiency and shorter preheat times. Many newer designs also maintain a more uniform temperature in the oven zones.

    For example, conduction ovens circulate a heat transfer fluid through plates to provide more accurate and uniform heating. Also, Flash Bake technology is especially effective in the preparing trendy foods, such as quesadillas and pizzas.

    Comparing electric vs. gas ovens

    There are many factors to consider when selecting an oven: initial cost, food preparation productivity, ease of operation, heat generation in the kitchen, and whether electricity or gas is used as the energy source. However, consider that energy only accounts for 3 to 5 percent of a typical food service establishment's total costs. Therefore, while one fuel may be less expensive in a BTU to BTU comparison, the best choice in cooking equipment is the one that minimizes total operating costs, not just energy costs. Features that reduce labor costs or result in higher food product yield will nearly always outweigh any energy considerations. Make sure that you include all of these factors in any equipment evaluation.

    Therefore, when comparing gas an electric models, compare equipment that is similar in all ways except the energy source.

    Advantages of electric ovens

    In general, electric ovens offers these advantages:

    • Electric units are more efficient, adding less heat to the kitchen that must be removed by the kitchen cooling system.
    • Electric units require less maintenance, require less ventilation, and are more portable.
    • Electric ovens, especially those with electronic controls, deliver more consistent run quality and require less operator supervision. They are also considered to be cleaner and more flexible (especially where maintaining oven humidity levels is important). Kitchen design and modification may also be simplified because venting may be unnecessary.

    Energy and money saving tips

    • Here are a few common-sense operating tips that save money with a oven.
    • The efficiency of ovens depends upon how well they are constructed, and insulation levels and quality are significant factors. Consider this in purchasing decisions, since some inexpensive ovens have little-to-no insulation in the oven door and will cost more to operate.
    • Ovens consume considerable energy when left on, even if no food is being cooked. Energy is lost through the oven walls and leakage around the door opening. These losses can be a significant operating expense, so turn all oven equipment off or lower temperatures during non-operating intervals. This saves energy, reduces cost, and increases oven life.
    • When a food service production does not call for a full sized oven, consider a half-size oven that may operate at much lower cost.

    Oven components

    An oven is composed of a box-like enclosure, heating elements, and controls. The enclosure ranges from a counter-top size to larger free-standing and floor model units. Ovens usually have a hinged door at the front or side (conveyer ovens have openings on two sides), and include adjustable racks or trays to hold food . The quality and amount of insulation and presence of an air curtain (to retain oven heat when the door is opened) all affect energy efficiency and uniformity of heating.

    In standard electric ovens, electric heating elements may be at the top, bottom, or sides of the oven. Gas units use gas combustion chambers. Microwave designs provide heating energy by channeling electromagnetic waves into the oven and rotating the food items to assure uniform heating. Flash Bake ovens use a combination of high intensity light plus infrared radiant energy for extremely rapid heating. In some special oven designs, steam is used to shorten cooking times and improve certain food preparation. Yeast-raised breads and pastries are often baked in humidity-controlled proofing ovens.

    Deck ovens and conveyer ovens use convection as a heat transfer medium, but are named for the special large heated deck on which food is placed during cooking. These are commonly used for roasting, baking, and cooking pizza.

    Oven controls indicate desired oven temperature. Certain designs also provide "cook and hold" cycles that extend holding time and improve the quality of food.

    Oven operation tips and issues

    Since ovens are so common, most people are familiar with their operation. The oven is first preheated to the desired cooking temperature. Next, food to be cooked in the oven is usually placed in containers of metal or glass or on metal pans. The food is then heated at a specified temperature for a certain period of time. The time required depends on the size and shape of the food items heated and the rate of acceptable heat transfer to those items. For example, thin items like pizza heat much rapidly than large items like whole turkeys, and a stuffed turkey takes significantly longer to cook than an unstuffed turkey.

    Some foods require changes in oven temperature during cooking, especially where surface browning is desired. For example, a recipe may require extended initial baking at 325°F, and the last few minutes at 425°F.

    The most common oven-cooking process surrounds the item being cooked in hot air. However, air is a relatively poor heat transfer agent, especially compared to the heat transferred by a griddle or immersion in hot oil. The air heat-transfer can be accelerated by circulating or blowing the hot air around the food being cooked.

    Apart from air, some oven designs use high intensity and infrared light, microwave energy, or steam. Each design has a special niche in the preparation of foods. No one oven design is ideal for all food preparation, so many modern ovens incorporate a combination of these technologies.

    For more information about the benefits of electric convection ovens vs. gas, please contact us for a copy of an EPRI performance report.