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Compression Cooling

Reference Note 32

How Compression Cooling WorksRefrigerantsThe Components of a Compression Cooling

SystemSystem LayoutsThe Effect of Operating Temperatures on COP

■■■■■ Maximum Theoretical COP■■■■■ The COP of an Ideal Machine with Real Refrigerants■■■■■ The COP of Real Systems with Real Refrigerants

System Temperature Differential is Larger thanCooling Load Temperature Differential

The Effect of Operating Temperatures on CapacityThe Compressor and System Efficiency

■■■■■ The Refrigerant■■■■■ Efficiency of Gas Flow in the Compressor■■■■■ Interstage Heat Transfer■■■■■ Fixed or Variable Compression Ratio■■■■■ Efficiency of Output Modulation■■■■■ Limitations in Reducing Condenser Temperature

Types of Compressors and Throttling Methods■■■■■ Centrifugal Compressors■■■■■ Reciprocating Compressors■■■■■ Screw Compressors■■■■■ Scroll Compressors■■■■■ Other Compressor Types

Compressor Drivers■■■■■ Internal vs. External Motors■■■■■ Motor Efficiency■■■■■ Other Types of Drivers

Evaporators■■■■■ Flooded Evaporators■■■■■ Liquid Overfeed Evaporators■■■■■ Evaporators with Expansion Valves■■■■■ How Evaporator Superheat Reduces COP

Energy Recovery from Refrigerant ExpansionEvaporator and Condenser Heat Transfer

■■■■■ Fouling FactorParallel and Series Evaporator CircuitsCondensate Subcooling

The great majority of cooling equipment worldwideoperates by compression. This Note is an overview ofcompression cooling systems, emphasizing the aspectsof design and equipment selection that affect systemefficiency.

In principle, you could select a system with thehighest efficiency simply by using manufacturers’efficiency quotations. However, this is not a prudentapproach. The efficiency of a cooling system isdetermined by a variety of factors involving cost, safety,space requirements, maintenance, and other issues. Youneed to consider all of these to achieve a good system.

There is fierce competition among manufacturerswith different designs, and each touts its own advantageswhile being silent about its own disadvantages. ThisNote will help you to navigate through the competingclaims by understanding the principles of compressioncooling. It will enable you to ask the right questionswhen selecting new equipment, and it will give you asolid foundation for making efficiency improvementsto existing cooling systems.

How Compression Cooling WorksAll contemporary space cooling and process cooling

equipment exploits the fact that a liquid absorbs heatwhen it evaporates. (There are a few exceptions, whichare limited to unusual applications.) A liquid used forcooling is called a “refrigerant.” The energy absorbedby the refrigerant in changing from a liquid to a vaporis called its “latent heat.” When this heat is drawn froma body, the body is cooled. For a good demonstration

of this, swab some alcohol on your arm. The wet spotwill chill your skin until it all evaporates.

You can create cooling without any machinery ifyou have enough liquid to evaporate. Water has beenused for cooling since before the dawn of humanity.However, it is not very satisfactory as a coolant underambient conditions because it evaporates too slowly.There are many liquids that provide a large evaporativecooling effect, but they are not cheap enough to be usedon a once-through basis.

Mechanical cooling can use more effectiverefrigerants and recycle them indefinitely. The processof doing this is called a “cooling cycle.” The outer shellor casing of the cooling equipment serves as a pressurevessel, isolating the refrigerant from air and atmosphericpressure. This allows the cooling cycle to operate underconditions that provide the greatest cooling capacity andefficiency from the refrigerant.

The most common kind of cooling equipment usesa compression cooling cycle. After the refrigerant hasbeen evaporated by heat from the cooling load, the vaporis compressed. This raises the temperature of the gaswell above ambient temperature, so that the heat in thegas can be removed by cooling it with air or water atambient temperature. Removing the heat causes thecompressed gas to condense back to a warm liquid.

The warm refrigerant liquid from the condenser ismetered into the cooling area (evaporator) by a flowcontrol device of some kind. The pressure in theevaporator is determined by the suction of thecompressor and by the rate of evaporation. Because the

1300 11. REFERENCE NOTES

ENERGY EFFICIENCY MANUAL

evaporator pressure is lower than the condenser pressure,a small portion of the liquid refrigerant “flashes” intovapor when it passes through the control device. Thisflashing cools the remaining liquid to the temperatureof the evaporator. The liquid refrigerant is now readyto absorb heat from the cooling load, repeating the cycle.

RefrigerantsNowadays, selecting the refrigerant may be the first

choice you make in selecting or designing a coolingsystem. The choice of refrigerant limits the system’sefficiency and affects the design of all the systemhardware. Safety concerns with some refrigerantsdictate where the equipment can be located, and howthe surrounding space must be designed. Refrigerantselection has recently become more important andcomplex because of environmental concerns. To learnmore about refrigerants and how to select them, seeReference Note 34, Refrigerants.

The Components of a Compression Cooling SystemAll compression cooling systems have five basic

elements. As illustrated by the typical system inFigure 1, they are:

• the compressor. On its discharge side, thecompressor increases the pressure of the evaporatedrefrigerant vapor to raise its temperature. On thesuction side, the compressor lowers the pressure ofthe liquid refrigerant, causing the refrigerant toevaporate more rapidly, and at a lower temperature.In most systems, cooling capacity is regulated byvarying the output of the compressor. There areseveral major types of compressors, discussedbelow.

• the evaporator, which is a heat exchanger on thesuction side of the compressor. This is where theliquid refrigerant evaporates to do its work. If themachine is designed to cool air directly, as inFigure 1, the evaporator is an air coil. If themachine is designed to cool liquid, the evaporatortypically is a tube-and-shell heat exchanger.(A cooling machine that is designed to delivercooling by means of a liquid is commonly called a“liquid chiller,” or simply a “chiller.”)

• the condenser, which is a heat exchanger on thedischarge side of the compressor. The high-pressurerefrigerant gas from the compressor is cooled inthe condenser so that it returns to a liquid state. If

Carrier Corporation

Fig. 1 The elements of a compression cooling system In this system, the evaporator is the coil in the fan-coil unit. An expansion valve, installed on the side of the fan-coil unit, serves as a refrigerant metering device.The condenser is an air coil on the outside of the building. The system does not have a distinct refrigerantaccumulator, but the bottom of the condenser coil and the refrigerant liquid pipe serve this function.

1301NOTE 32. COMPRESSION COOLING


the cooling medium is air, the condenser is an aircoil. If the cooling medium is water, the condenseris usually a tube-and-shell heat exchanger, orsometimes a coaxial tube heat exchanger.

• an accumulator or receiver, which is a reservoirthat holds the liquid refrigerant until it is needed.Many cooling systems do not have a separateaccumulator, instead depending on the volume ofthe condenser, evaporator, and/or refrigerant linesto hold the liquid refrigerant. For example,packaged water chillers typically use the evaporatorshell as the liquid storage vessel. In Figure 1, thebottom of the condenser coil and the liquiddischarge line act as the accumulator.

• a device or combination of devices for separatingthe high-pressure side of the system (condenser)from the low-pressure side (evaporator) and, insome systems, for metering the flow of refrigerantinto the evaporator. The device keeps the hotrefrigerant gas from blasting through the condenser

coil, and it maintains the discharge pressure that isnecessary for condensing the refrigerant. In mostliquid chillers, a simple orifice or float valvebetween the condenser and the evaporator servesthis purpose. If the evaporator is partially wetted,rather than flooded, the flow of refrigerant into theevaporator is controlled by a metering device calledan expansion valve. Most air coils and some water-cooled evaporators use expansion valves. A recentdevelopment is the use of a small turbine to act asa metering device. This design recovers energyfrom the pressure difference that exists between thecondenser and evaporator.A compression cooling system may also have

accessory devices or specialized features, such ascrankcase heaters, purge units, hot gas bypass circuits,valves for controlling the flow of refrigerant to differentparts of a coil, etc. Most of these accessories are specificto particular types, models, or system designs.

Trane Company

Fig. 2 Packaged water chiller The machine packages all the primary elements of a cooling system together.This model has a three-stage centrifugal compressor. The cooling output of the machine is controlled bythrottling the flow of gas into the compressor with an inlet vortex damper. The evaporator, at bottom, is floodedwith refrigerant. Tubes for chilling water are immersed in the refrigerant. The refrigerant evaporates under thereduced pressure of the compressor suction, chilling the water. The vapor is compressed to a hot gas by thecompressor, and is discharged to the condenser in the rear. Cool water from a cooling tower flows through thetubes in the condenser, absorbing heat from the hot gas and condensing it. The warm refrigerant liquid dropsdown into the evaporator through an orifice. Upon exposure to the lower pressure in the evaporator, a portionof the warm refrigerant liquid evaporates, cooling the remaining liquid to the saturation temperature thatcorresponds to the evaporator pressure. The cycle continues as long as the compressor runs.



System LayoutsThe five elements of a compression cooling system

can be grouped in a variety of ways. You can findvirtually all possible combinations in practice.

For example, Figure 1 shows a commonarrangement where the compressor is located in thebasement, an air-cooled condenser and accumulator isinstalled on the roof, and air coils with their expansionvalves are installed on each floor.

In the popular “split system,” the compressor ispackaged with the condenser outside the building, andliquid refrigerant is piped to air coils or to a water chillingevaporator inside the building. This keeps the noisiestequipment outside the building.

Large water chillers generally include all theircomponents in a compact package that is assembled atthe factory. Figure 2 shows a typical packaged waterchiller. Such packaging is becoming increasinglycommon in smaller systems as well. On the other hand,a cooling system may distribute its componentsthroughout the facility.

If a system distributes its cooling by means of chilledwater, this is called “hydronic” distribution. If liquidrefrigerant is evaporated inside an air cooling coil, thisis called “direct expansion” or “DX.”

Larger systems generally use water for condensercooling, because this allows the condensing temperatureto be reduced by evaporative cooling of the water in acooling tower. Smaller systems, and systems used inlocations where the supply of water is limited, may rejectheat directly to the outside air through an air coil that iscalled an “air-cooled condenser.” A compressor that ispackaged with an air-cooled condenser is called an “air-cooled condensing unit.” An intermediate approach isspraying water over a condensing coil that is cooled byair flow. This arrangement is called an “evaporativecondenser.”

The Effect of Operating Temperatures on COPThe amount of power needed by the compressor

depends on the difference in pressure between thecompressor inlet (from the evaporator) and outlet (tothe condenser). You know from basic chemistry thatthe temperature of a gas is proportional to its pressure.Therefore, we can express the power needed to compressthe refrigerant vapor in terms of the difference betweenthe evaporator and condenser temperatures.

This is a useful way of looking at compressor power,because the evaporator and condenser temperatures aredetermined largely by factors that are under the controlof the equipment manufacturer, the system designer, andthe plant operator. These factors are discussed below.But first, we need to understand the relationship betweensystem temperatures and COP. If you need anexplanation of COP, see Reference Note 31, HowCooling Efficiency is Expressed.

■■■■■ Maximum Theoretical COPIn 1824, the French engineer Sadi Carnot developed

an analysis of compression cycles that allows theirtheoretical maximum efficiency to be expressed in termsof temperature alone. Carnot found that the maximumpossible COP of a compression cooling machine isdetermined by this remarkably simple formula:

In this formula, the temperatures are absolute. Youcan use any system of absolute temperature units in thisformula because the units cancel. (The Rankine absolutescale equals Fahrenheit plus 460°F. The Kelvin absolutescale equals Celsius plus 273°C.)

The lowest temperature in the system occurs at thecompressor inlet. The highest temperature occurs atthe compressor discharge. These temperatures are notfixed by the compressor. Instead, they are imposed onthe compressor by the cooling application, by the

Ideal COP =lowest temperature

(highest temperature) - (lowest temperature)

Fig. 3 The effect of system temperatures on COP The topcurve shows the theoretical maximum COP that is possible,which is determined solely by the system temperatures. Thesecond curve shows the efficiency loss related to a highlyefficient real refrigerant. The bottom two curves show theCOP’s of real cooling machines with the same refrigerant.

0

5

10

15

20

25

30

60 70 80 90 100 110 120

Condensin g Temperature (°F)

Coe

ffici

ent o

f Per

form

ance

Ideal machine, ideal refrigerantIdeal machine, R-22Reciprocating machine, R-22Screw machine, R-22

Evaporator Temperature: 40°F



temperature of the environment to which the heat isrejected, and by many system design factors.

This formula reveals the crucial role of theevaporator-to-condenser temperature differential insystem efficiency. Cooling equipment moves heat fromone place to another. The energy input to a coolingsystem is needed to overcome the “resistance” of thetemperature differential. This viewpoint is evoked bythe term “heat pump,” which is used when a coolingmachine is turned around to make it a heating devicerather than a cooling device.

In Figure 3, the top curve shows the maximumtheoretical COP’s that can be achieved with an idealcooling machine and an ideal refrigerant. This curve iscalculated using Carnot’s formula, assuming a coolingtemperature of 40°F and a typical range of condensingtemperatures.■■■■■ The COP of an Ideal Machine with

Real RefrigerantsIn Figure 3, the second curve shows the COP’s that

would be achieved by an ideal cooling machine usingHCFC-22 as a refrigerant. The curve assumes a coolingtemperature of 40°F and a typical range of condensingtemperatures. HCFC-22 (originally called Freon-22) isthe most common refrigerant at present, and it is one ofthe most efficient.

This curve shows that some real refrigerants canapproach the theoretical maximum COP. However, thereis wide variation in efficiency among differentrefrigerants. The theoretical COP’s of the most popularrefrigerants are tabulated in Reference Note 34,Refrigerants.■■■■■ The COP of Real Systems with Real Refrigerants

In Figure 3, the bottom two curves shows themanufacturer’s COP ratings for two typical chillers, bothusing HCFC-22 refrigerant. As with the other curves,the cooling temperature is 40°F.

These two curves show that the COP’s of realmachines are considerably lower than the theoreticalmaximum, even accounting for limitations in therefrigerants. The losses are caused by mechanicalinefficiencies in the compressor, by inefficiencies in thecompression process, by energy waste in the expansionof refrigerant when it returns to the evaporator, and byheat transfer losses within the system. There is potentialfor improvement in all these areas.

The two curves also show that there is a considerabledifference in COP between screw compressor machinesand reciprocating machines. The COP of a goodcentrifugal machine would be somewhat higher than theCOP of the screw machine.

Condensing temperatures below 75°F are not shownfor the two real machines, because various designfeatures keep them from operating with lowercondensing temperatures.

The System Temperature Differential is Much Largerthan the Cooling Load Temperature Differential

The temperature differential between evaporator andcondenser is generally much higher than the temperaturedifferential of the actual cooling load. This is a matterof great concern, because the temperature differentialis the underlying theoretical factor that limits COP. Asa realistic example of how the temperature differentialbuilds up, consider the example of an air-cooled waterchiller used for air conditioning. The temperaturesthroughout the system are depicted in Figure 4.

An air-cooled system was selected for this exampleto avoid the complications of temperature changes incooling towers, which are really supplementary coolingsystems. Other thermal complications, especiallyevaporator superheat, condensate subcooling, andinterstage heat exchange, are discussed below. Theillustration ignores small losses, such as the rise in chilledwater temperature in the distribution system.

Fig. 4 The compressor temperature differential is muchhigher than the cooling load temperature differentialIn this example, the outside-to-inside temperature differentialis only 25 degrees. Yet, the temperature differential onthe compressor, which determines the system energyrequirement, is 81 degrees! Of the excess, the evaporatorside, including hydronics, accounts for 9 degrees; thecondenser side accounts for 20 degrees; and, the airdistribution system accounts for 27 degrees.



In this example, the original 25°F temperaturedifferential created by the weather becomes an 81°Fdifferential at the compressor! This has the effect ofreducing the theoretical COP from 21.4 to 6.2, anenormous loss of efficiency. The COP of the actualchiller is reduced by roughly the same ratio.

Figure 4 shows that the compressor dischargetemperature is determined primarily by the outside airtemperature. However, the compressor suctiontemperature is much lower than the inside airtemperature. This is mostly because of heat transferlosses in the cooling system and in the air handlingsystem. Indeed, most of the temperature loss in thisexample occurs in the air handling systems.

This example illustrates an important point, namely,that cooling system efficiency is determined largely byfactors outside the design of the cooling equipment.Much of the waste in a typical cooling system occursbecause the compressor must operate at a highertemperature (or pressure) differential than necessary.The struggle to achieve better cooling efficiency islargely a war against temperature differential that isfought in many parts of the system.

Reducing the temperature differential in air handlingsystems is accomplished by various Measures inSection 4. Increasing chilled water temperature andlowering condenser temperature is accomplished inSubsection 2.2. Reducing the temperature differentialsinside the cooling system itself is a matter of selectingthe best design options, which are explained below.

The greatest potential for reducing the systemtemperature differentials occurs during off-peak periods,when the system temperature differentials may be muchlarger than needed to satisfy the load.

The Effect of Operating Temperatureson System Capacity

The cooling system must overcome a temperaturedifferential that resists the flow of heat. Reducing thetemperature differential increases the cooling capacityof the system. This effect is dramatic. Figure 5 showsthe loss of capacity for two typical machines, one witha reciprocating compressor and the other with acentrifugal compressor.

Conversely, by reducing the system temperaturedifferentials that occur under peak load conditions, youcan reduce the nominal capacity of the equipment thatyou need to buy for new installations. You do this byspending more money for larger heat exchangers, andby designing air distribution systems for more efficientflow. This extra cost is partially offset because theincreased efficiency allows smaller compressors tosatisfy the load.

The change in cooling capacity with temperaturedifferential varies substantially among different typesof compressors. Centrifugal compressors suffer morecapacity loss than other types. This is because the gasis driven through the compressor only by its owncentrifugal force. As the condenser temperatureincreases, this force is counteracted by the greaterpressure in the condenser.

In contrast, positive displacement compressors pushthe gas through the machine without regard to thetemperature differential. The cooling capacity ofcompressors with a fixed compression ratio, such asscrew compressors, is affected least. This is becausethe refrigerant gas is isolated from the discharge pressureduring the compression process.

The Compressor and System EfficiencyIn a compression cooling machine, most of the input

energy goes to compressing the refrigerant gas to makeit liquify. Therefore, the design of the compressor andother aspects of the compression process play a largerole in the system’s efficiency. The following are themajor factors that determine the efficiency of thecompression process.

Fig. 5 The effect of system temperatures on capacityIncreasing the temperature differential across a coolingmachine reduces its capacity. The effect is important in typicalapplications. It varies with the type of compressor.

60

65

70

75

80

85

90

95

100

105

75 85 95 105 115

Condensing Temperature (°F)

Cap

acity

(as

Per

cent

of M

axim

um)

ReciprocatingCentrifugal

Evaporator Temperature: 44°F



■■■■■ The RefrigerantUntil about 1990, it was possible to read a cooling

equipment catalog without finding any mention of therefrigerant used by the equipment. That situation hasreversed because of environmental concerns aboutrefrigerants, so that the choice of refrigerant may be thefirst consideration in selecting a compressor. Asdiscussed previously, the choice of refrigerant limits thepotential system efficiency.

With respect to the system hardware, the refrigerantaffects the system pressures, the types of metals thatcan be used, the handling of lubricants inside and outsidethe compressor, safety features, and other equipmentselection considerations. See Reference Note 34 formore about these issues.■■■■■ Efficiency of Gas Flow in the Compressor

Refrigerant vapor has mass, so it has kinetic energy.If the flow through the compressor is turbulent, a portionof this energy is wasted by converting it to heat energy.Reciprocating compressors unavoidably generate a largeamount of turbulence as a result of the oscillating pistonmotion. In contrast, the other major types ofcompressors have smoother gas flow.

In order to achieve ideal thermodynamic efficiency,a compressor would have to provide the minimum powerat each incremental stage of compression. No type ofcompressor is perfect in this regard. For example,efficient compression is achieved in a centrifugalcompressor by careful attention to the shape of theimpeller and diffuser, but this shape achieves maximumefficiency only at one refrigerant flow rate.

Any leakage within a compressor wastes compressorenergy by allowing compressed gas to re-expanduselessly. Leakage occurs around the piston rings of areciprocating compressor, around the impeller of acentrifugal compressor, between the scrolls of a scrollcompressor, and between the rotors and casing of a screwcompressor. Minimizing leakage is a challenge in alltypes of compressors.

All types of positive-displacement compressorssuffer re-expansion as a result of “clearance volume”,which is the volume of gas left to re-expand at the endof the compression process. This problem is most severewith reciprocating compressors, which leave asignificant amount of compressed gas in the cylinder atthe top of the piston stroke. After the exhaust valvecloses, the remaining gas expands in the first part of thesuction stroke. The energy consumed to compress thisgas is wasted.■■■■■ Interstage Heat Transfer

When a gas is compressed, the heat of compressioncreates an expansion force that opposes the compressionand increases compressor power. In an ideal compressor,the heat of compression would be removed as the gas iscompressed. (If you are interested in the theory, this

corresponds to the isothermal compression phase of theCarnot cycle.) Unfortunately, no conventionalcompressor design allows the gas to be cooledcontinuously during compression.

If the compressor has more than one stage, heat canbe removed between the stages. Unfortunately, theinterstage cooling cannot be done with ambient air ordomestic water, because the heat exchange processoccurs at low temperature. The compression processstarts with a cold refrigerant gas and never gets veryhot. The cooling is usually done by taking some liquidrefrigerant from the condenser at high pressure, flashingit at a reduced pressure to get cool vapor, and injectingthis vapor into the compressor between the stages. Themass of the added refrigerant also increases the coolingcapacity.

The general technique of lowering gas temperatureand/or increasing mass flow between stages is called an“economizer.” (The same name is given to efficiencymeasures in other types of equipment, includingabsorption chillers, boilers, and air handling units). The

Carrier Corporation

Fig. 6 Interstage heat transfer in a screw compressorsystem A portion of the liquid refrigerant from the condenseris flashed into gas, which is introduced part way through thecompression process. This lowers the average temperaturerise in relation to the mass of gas that is compressed, whichlowers the average work of compression.



specific technique of using refrigerant flash gas for thispurpose is called “flash intercooling.”

With compressors that convey an isolated volumeof gas through the compression process, as occurs inscrew and scroll compressors, ports can be built intothe compressor to inject gas for flash intercooling.Figure 6 shows how an economizer operates with ascrew compressor chiller.

Interstage heat transfer produces a significantefficiency gain only if the compressor has a fairly highcompression ratio. In this case, the heat of compressionadds substantially to the gas pressure, increasing thework required from the compressor.■■■■■ Fixed or Variable Compression Ratio

If the compressor discharge is open to the condenseras the compression is taking place, then the dischargepressure never exceeds the pressure needed to condensethe refrigerant (except for a small expansion lossbetween the compressor and condenser). The leadingexamples of such compressors are the centrifugal andreciprocating types. In such compressors, thecompression ratio adjusts itself to the needs of thesystem.

On the other hand, some compressors work bytrapping a volume of gas and then compressing thisamount independently of the condenser pressure. Thegas is released to the condenser only near the end of thecompression process. This is true of some screwcompressors and most scroll compressors, and of someother types, such as vane compressors.

If the compressor discharge pressure exceeds thecondenser pressure, then a fraction of the compressionenergy is wasted in compressing the gas more thannecessary. On the other hand, if the compressordischarge pressure is lower than the desired condenserpressure, the condenser is not able to condense the gasat higher coolant temperatures. Therefore, systems usingcompressors with fixed compression ratios always erron the side of excess discharge pressure, which wastesenergy at reduced loads.

The compression ratio corresponds to a ratio oftemperatures between the evaporator and the condenser.Therefore, energy conservation measures that attemptto improve efficiency by raising the evaporatortemperature or lowering the condenser temperature areless effective with compressors having fixedcompression ratios.■■■■■ Efficiency of Output Modulation

In all common types of cooling machines, themethod used to throttle output is an integral part of thecompressor, and may account for much of its efficiencylimitations. Some compressors that are efficient at highload become inefficient at low loads. Some compressorscannot operate at all below some minimum load.

In typical applications, cooling systems operate overa wide range of loads, with full-load operation being anexception that occurs mostly on start-up. Many oldercooling plants were designed at a time whenmanufacturers did not stress part-load efficiency, andthese commonly provide major opportunities forimproving efficiency by substituting machines thatrespond more efficiently to partial loads.

Losses in throttling output are most severe incentrifugal compressors. In the past, economies of scaleled to decisions to cool a facility with one or two largecentrifugal machines, but this configuration is wastefulat low loads. To increase part-load efficiency, you needto use a larger number of smaller machines, or to usedifferent types of compressors, perhaps in combinationwith centrifugal machines. Improving part-loadefficiency in an existing plant may require installing anadditional, smaller machine to serve low loads.(Measure 2.8.1 recommends this technique.)

Reciprocating compressors retain their efficiencyfairly well at lower loads, although part-load efficiencyvaries among different models. Smaller reciprocatingcompressors usually operate in an on-off mode, so theysuffer efficiency loss only during the start of each cycle,while refrigerant flow in the system is stabilizing.

Scroll compressors usually operate in an on-offmode, so the efficiency of capacity reduction is not afactor. Other methods of modulation are beingdeveloped for scroll compressors, as discussed below.The efficiency performance of modulating scrollcompressors is not yet well documented.■■■■■ Limitations in Reducing Condenser Temperature

The previous discussion emphasized the importanceof keeping condenser temperature as low as possible.With some compressors, the condensing temperaturemust be kept above a certain level. In such cases, thecondenser temperature is kept artificially high bychoking off the air or water that cools the condenser.This wastes energy in applications where the systemmay operate during cooler weather.

There are several reasons why a system may requirean elevated condenser temperature. In some cases, thecompressor is the limiting factor, because highcondensing temperature may be needed to preventmigration of lubricants away from the compressor. Morecommonly, high condensing temperature may be neededfor reasons not related to the compressor, such asmaintaining adequate pressure for operation of anexpansion valve or capillary refrigerant distributors.Other reasons are given in Measure 2.2.2.

Types of Compressors and Throttling MethodsThe following are the efficiency characteristics of

the most common types of cooling compressors. Oneof the most important efficiency characteristics is themethod used by the compressor to reduce its output.



■■■■■ Centrifugal CompressorsCentrifugal compressors are the most efficient type

when they are operating near full load. Their efficiencyadvantage is greatest in large sizes, and they offerconsiderable economy of scale, so they dominate themarket for large chillers. They are able to use a widerange of refrigerants efficiently, so they will probablycontinue to be the dominant type in large sizes. Thepeak efficiency of centrifugal compressors has improveddramatically and progressively since about 1980, andseveral major improvements to part-load efficiency havebeen made in that time.

Centrifugal compressors have a single major movingpart, an impeller that compresses the refrigerant gas bycentrifugal force. The gas is given kinetic energy as itflows through the impeller. This kinetic energy is notuseful in itself, so it must be converted to pressureenergy. This is done by allowing the gas to slow downsmoothly in a stationary diffuser surrounding theimpeller.

Figure 7 is a detailed drawing of a centrifugalcompressor that has two impellers on the same shaft,for two stages of compression. The impeller speed isincreased by using a gear train between the motor andthe impellers.

Figure 2 shows a centrifugal compressor that hasthree impellers, for three stages of compression. Theimpellers are driven directly by an electric motor, whichlimits the rotation speed. Three stages are needed toachieve the needed condensing pressure with theparticular refrigerant that is used in this machine. Usinga large impeller diameter also compensates for the lowrotation speed.

Figure 8 shows a centrifugal chiller in which thecompressor has only a single, relatively small impeller.The motor speed is limited by the line frequency, sothis machine uses gears to increase the impeller speed.

High energy efficiency in a centrifugal compressorrequires efficient gas flow through the impeller and thediffuser. Unfortunately, the design of the impeller and

Carrier Corporation

Fig. 7 Centrifugal compressor with internal motor This model has two impellers on a common shaft, providing two stages ofcompression. To further increase compression, the impeller speed is increased by gears. Typical of most centrifugal compressors,the output of the machine is adjusted by throttling the flow of gas into the first stage using pre-rotation inlet vanes.



diffuser can be optimized for only one gas flow rate. Tominimize efficiency loss at reduced loads, centrifugalcompressors typically throttle output with pre-rotationvanes (inlet guide vanes) located at the inlet to theimpeller(s). This method is efficient down to about halfload, but the efficiency of this method decays rapidlybelow half load.

Older centrifugal machines are not able to reduceload much below 50%. This is because of “surge” inthe impeller. As the flow through the impeller is chokedoff, the gas does not acquire enough energy to overcomethe discharge pressure. Flow drops abruptly at this point,and an oscillation begins as the gas flutters back andforth in the impeller. Efficiency drops abruptly, and theresulting vibration can damage the machine.

Many older centrifugal machines deal with low loadsby creating a false load on the system, such as by usinghot gas bypass. This wastes the portion of the coolingoutput that is not required.

To improve both turndown range and low-loadefficiency, some machines use two separate centrifugalcompressors. The two compressors share a commoncondenser shell and a common evaporator shell.Figure 9 shows this approach used with a large chiller.

Another approach is to use variable-speed drives incombination with inlet guide vanes. This may allowthe compressor to throttle down to about 20% of fullload, or less, without false loading. Changing theimpeller speed causes a departure from optimumperformance, so efficiency still declines badly at lowloads.

A compressor that uses a variable-speed drivereduces its output in the range between full load andapproximately half load by slowing the impeller speed.At lower loads, the impeller cannot be slowed further,because the discharge pressure would become too lowto condense the refrigerant. Below the minimum loadprovided by the variable-speed drive, inlet guide vanesare used to provide further capacity reduction.

Carrier Corporation

Fig. 8 Packaged centrifugal water chiller Details and accessories vary from one model to another, and from one manufacturerto another. Here, the flash chamber provides cool gas for motor cooling and oil cooling. This compressor has only a single stage,so an economizer is not possible.



Unlike positive-displacement compressors, whichare not subject to surge, the minimum load of acentrifugal compressor is sensitive to the evaporator andcondensing temperatures. Raising the evaporatortemperature and lowering the condenser temperaturereduce the tendency to surge, and hence reduce theminimum load. The minimum load of an individualcentrifugal machine varies widely, depending on thesetemperatures.

It is not advisable to turn a centrifugal compressoron and off to deal with low loads. Most centrifugalcompressors use journal bearings, which are notpositively lubricated until the shaft reaches a certainspeed. Refrigerant flow within the machine may requireseveral minutes to stabilize after each start-up. The typesof motors used to drive centrifugal machines are morelikely to overheat with frequent start cycles. And, thegears in gear-driven compressors are subjected to highstress on start-up.

The pressure output of a centrifugal compressordepends on the diameter and speed of the impeller(s).Refrigerants that have low condensing pressures can becompressed with a centrifugal machine that is drivendirectly from a 50 Hz or 60 Hz motor. Gases that requirehigher pressures require the compressor to have speedincreasing gears, or several stages of compression(several impellers), or both.

For a given refrigerant, the equipment designer canchoose to increase the number of stages of compression,while reducing the impeller speed and diameter. Usingseveral stages allows an economizer (describedpreviously) to be used between each stage. Thetheoretical efficiency improvement becomes smallerwith each additional stage, and flow losses accumulateat the inlet and outlet of each impeller, so there is anoptimum number of stages for each refrigerant.Increasing the number of stages increases cost andcomplexity.

Increasing the impeller speed by using gears incurslosses in the gears and in the additional bearings. Italso eliminates the opportunity of exploiting interstageheat transfer.

■■■■■ Reciprocating CompressorsReciprocating compressors are the oldest type, and

they continue to be the dominant type in the small andmedium size range. Figure 10 shows a typical unit.

In principle, reciprocating compressors can bedesigned for virtually any refrigerant. In practice, theyare most economical for refrigerants that operate atmedium and high pressures, because the higher gasdensities provide more refrigeration effect for a machineof a given size.

The maximum efficiency of reciprocatingcompressors is lower than that of centrifugal and screwcompressors. Efficiency is reduced by clearance volume(the compressed gas volume that is left at the top of thepiston stroke), throttling losses at the intake anddischarge valves, abrupt changes in gas flow, andfriction. Also, reciprocating compressors tend to be usedwith systems having expansion valves, which aresomewhat less efficient than the fluid metering methodsused in larger systems. Lower efficiency also resultsfrom the smaller sizes of reciprocating units, becausemotor losses and friction account for a larger fractionof energy input in smaller systems.

For most air conditioning applications, reciprocatingcompressors can easily compress the refrigerant gas ina single stage, so there is no opportunity for improvingthe theoretical efficiency of compression by using aneconomizer (described previously). However, multiplestages of compression are commonly used in low-temperature refrigeration, and compression efficiencycan be improved by using economizers in thoseapplications.

Reciprocating compressors suffer less efficiencyloss at partial loads than other types, and they mayactually have a higher absolute efficiency at low loads

Trane Company

Fig. 10 Reciprocating compressor Capacity is controlledby disabling individual cylinders, which can be done in severalways, or by varying the speed. The large springs allow thecylinder heads to lift if liquid refrigerant enters the cylinders,avoiding destruction. This type of enclosure, with an internalmotor and bolted access plates, is called “semi-hermetic.”

Trane Company

Fig. 9 A large water chiller with two centrifugalcompressors This is one way of maintaining efficiency atreduced cooling loads.



than the other types. Smaller reciprocating compressorscontrol output by turning on and off. This eliminatesall part-load losses, except for a short period ofinefficient operation when the machine starts.

Larger multi-cylinder reciprocating compressorscommonly reduce output by disabling (“unloading”)individual cylinders. When the load falls to the pointthat even one cylinder provides too much capacity, themachine turns off.

Several methods of cylinder unloading are used, andthey differ in efficiency. The most common is holdingopen the intake valves of the unloaded cylinders. Thiseliminates most of the work of compression, but a smallamount of power is still wasted in pumping refrigerantgas to-and-fro through the unloaded cylinders. Anothermethod is blocking gas flow to the unloaded cylinders,which is called “suction cutoff.”

If a multi-cylinder reciprocating compressor canreduce its capacity by more than 50%, a significantamount of additional complication is required to providestable refrigerant flow through expansion valves, toavoid coil frosting, to achieve proper return of lubricatingoil to the compressor, etc. For this reason, the trend inlarger reciprocating systems is to use multiplecompressors with independent refrigerant circuits, witheach compressor unloading no more than 50% (i.e., intwo steps of capacity).

Variable-speed drives can be used with reciprocatingcompressors, eliminating the complications of cylinderunloading. This method has not become popular yet,probably because the attention of designers is nowfocussed on newer types of compressors. Automotiveair conditioning compressors, which are driven directlyby the engine, operate successfully over a wide rangeof speeds.■■■■■ Screw Compressors

Screw compressors, sometimes called “helicalrotary” compressors, compress refrigerant by trappingit in the “threads” of a rotating screw-shaped rotor.There are two types of screw compressors, twin-screwand single-screw. At present, the twin-screw version isthe most common.

In the twin-screw compressor, the refrigerant ispinched between the lobes (raised portions) and flutes(recessed portions) of two parallel mating screw rotors.Figure 11 is a cutaway drawing of a typical twin-screwcompressor.

Figure 12 is a cutaway drawing of a chiller that usesa twin-screw compressor.

In single-screw compressors, the refrigerant ispinched between the threads of a single helical rotorand one or two star-shaped rotors (called “gaterotors”)that rotate at right angles to the main rotor and mesh

Carrier Corporation

Fig. 11 Twin-screw compressor The two helical rotors mesh, squeezing gas from the rear toward the front. The capacitycontrol slide valve, 9, is a movable part of the housing that surrounds the rotors. By sliding away from the rest of the housing, itcreates a gap of variable size that disables the adjacent portion of the rotors.



Trane Company

Fig. 12 Chiller with twin-screw compressor Refrigerant gas enters the near end of the screw from the evaporator, in the lowerrear. After a short passage through the rotors, the compressed gas is discharged into the condenser. The valves under thecondenser flash liquid refrigerant for motor cooling, and perhaps, for an economizer.

with the helix, like pinions being driven by a worm gear.The only purpose of the gaterotors is to block flowthrough the threads. They are idlers, and play no part inmoving the gas.

Screw compressors have been used for a long timeas low-pressure air compressors and enginesuperchargers, but leakage between the screws has madethese units too inefficient for refrigeration work until

the 1980’s. At that time, improved machiningtechnology made it possible to achieve the closetolerances that are necessary for good efficiency, andimproved designs reduced other efficiency losses. Now,the best screw compressors are significantly moreefficient than reciprocating compressors at all loads, andthey may be more efficient than centrifugal compressorsat low loads.



Screw compressors have increasingly taken overfrom reciprocating compressors of medium sizes andlarge sizes, and they have even entered the size domainof centrifugal machines.

Screw compressors are applicable to refrigerants thathave higher condensing pressures, such as HCFC-22and ammonia. They are especially compact. Thepractical upper limit of screw compressor size isdetermined by competition from centrifugal machines,which offer greater efficiency in larger sizes. However,it may turn out that screw compressors are better able toexploit the efficiency potential of their refrigerants,especially in applications with high temperaturedifferentials (such as making ice), so screw compressorsmay prove to have no economical upper size limit. Thelower limit of size is determined by price competitionfrom reciprocating compressors and other types.

A major distinction between screw compressors iswhether the unit operates with or without oil injectioninto the rotors. Oil is used to cool the refrigerant as it iscompressed, reducing the work of compression andimproving cycle efficiency. It also seals the leakagepaths, allowing the compressor to operate efficiently atlower speeds. The oil typically takes up less than onepercent of the compression volume.

In twin-screw compressors, the oil serves as alubricant between the rotors, allowing one rotor to drivethe other by direct contact. Rotor wear is small in thisdrive arrangement because the shape of the rotors isdesigned to mesh them with a rolling contact.

The oil is cooled in an oil cooler to get rid of theheat of compression. Cooling may be provided by anyavailable cooling medium, including condenser water,

Carrier Corporation

Fig. 13 What is all that stuff? Screw compressor cooling machines have some large accessories. Note the size of the oilseparator, 14. Other models have equally large suction filters to protect the narrow clearances of the rotors from debris in therefrigerant system.



ambient air, refrigerant, or chilled water. Cooling theoil with refrigerant or chilled water reduces the overallsystem efficiency, so avoid these methods of oil cooling.

Oil-injected compressors also require an oilseparator to remove oil from refrigerant after it leavesthe compressor. The oil separator presents someresistance to the flow of refrigerant gas on the dischargeside, so it adds an efficiency penalty. Also, it typicallyis larger than the compressor itself. Figure 13 shows ascrew compressor chiller with all its accessories,including the large oil separator.

Some screw compressors have another largeaccessory, a suction filter. This protects the compressor,which has very close tolerances, from any debris thattravels through the refrigerant system, such as pieces ofsolder, motor insulation, etc.

In twin-screw compressors without oil injection,clearance between the two rotors is maintained bydriving them through meshed gears. This increases cost,size, and maintenance requirements. Dry compressorsmust operate at high speed to minimize leakage of gasbetween the rotors, which increases noise. Since thereis no oil to seal the gaps, more elaborate mechanicalseals are required. On the positive side, no oil separatoror oil cooler is needed.

Single-screw compressors may also operate with orwithout oil injection. In machines that do not inject oil,liquid refrigerant may be injected into the rotor,achieving both sealing and improved mass flow. Thesecompressors can operate with minimal lubricationbecause the gaterotors are made of a compliant plasticmaterial that provides tight sealing against the mainrotor.

A variety of methods are used to control the outputof screw compressors. There are major efficiencydifferences among the different methods. The mostcommon is a slide valve that forms a portion of thehousing that surrounds the screws. When the valve slidesaway from the rest of the housing, it creates a gap thatdeactivates the adjacent portion of the rotors.

Using a variable-speed drive is another method ofcapacity control. It is limited to oil-injectedcompressors, because slowing the speed of a drycompressor would allow excessive internal leakage.

There are other methods of reducing capacity, suchas suction throttling, that are inherently less efficientthan the previous two.

Screw compressors can achieve a much betterturndown ratio than centrifugal compressors, typicallydown to about 10% of full load. Screw machines toleratefrequent on-off cycling for low-load operation, as withreciprocating machines. This avoids the temptation tocreate false loads to keep the machine running.

The basic screw compressor has a fixed compressionratio, which has efficiency limitations discussed

previously. To minimize this problem, some screwcompressors have a variable compression ratio. This isachieved with a variable discharge port that taps thecompressed gas from different areas near the dischargeend of the rotor(s). The most common form is a slidevalve. The mechanism for this slide valve is sometimescombined with the inlet slide valve that controlscompressor capacity.

The efficiency of single-screw and twin-screwcompressors can be improved by flashing some liquidrefrigerant and injecting the cool refrigerant gas intothe rotor at a point downstream of the primary suctionport. This feature is called an “economizer,” and it isdescribed above.

Don’t confuse the economizer process with injectingliquid refrigerant into oil-injected compressors to coolthe oil. The latter function has an efficiency penalty,because the oil heat adds to the cooling load.

Trane Company

Fig. 14 Scroll compressor The compressor itself occupiesthe upper third of the case. The rest is the motor.



■■■■■ Scroll CompressorsThe scroll compressor is an old invention that has

finally come to market. Figure 14 shows a typical unit.The gas is compressed between two scroll-shaped

vanes, shown in Figure 15. One of the vanes is fixed,and the other moves within it. The moving vane doesnot rotate, but its center revolves with respect to thecenter of the fixed vane, as shown in Figure 16. Thismotion squeezes the refrigerant gas along a spiral path,from the outside of the vanes toward the center, wherethe discharge port is located. The compressor has onlytwo moving parts, the moving vane and a shaft with anoff-center crank to drive the moving vane.

Scroll compressors have only recently becomepractical, because close machining tolerances are neededto prevent leakage between the vanes, and between thevanes and the casing. A variety of designs are used toachieve good sealing, including oil flooding andpressure-loaded sliding contacts between the scrolls.

Scroll compressors promise good efficiency becauseof their smooth gas flow. Unlike the gas flow in areciprocating compressor, the gas moves through a screwcompressor in only one direction, from the inlet porttoward the outlet port. The absence of reversing gasflow greatly reduces the variation of motor torque duringthe compression cycle, as shown in Figure 17. Thispromises reduced vibration and noise.

Scroll compressors have a fixed compression ratio,which has efficiency limitations discussed previously.If they prove to be satisfactory, they seem likely tobecome prominent at the low end of the system sizerange.

Like other small compressors, scroll compressorsusually control output by turning on and off. Variablecapacity can be achieved by using several compressors.Electronic variable-speed drives have been used on somescroll compressors, but this method of capacity controlis expensive in small sizes. Another method is to openports that bleed gas from the early (outer) portion of thecompression cycle, but this method wastes some energy.■■■■■ Other Compressor Types

Mankind has devoted a great deal of ingenuity tofinding ways of compressing gases, resulting in manycompressor designs. The most efficient types approachthe maximum achievable efficiency, but still leave roomfor significant improvement. Greater efficiency, reducedcost, reduced maintenance, and lower noise are allfactors that may cause a new type of compressor to burston the scene.

At present, there are no major contenders to displacethe previous types, but this could change quickly, aswitness the rapid commercialization of screwcompressors and scroll compressors after years ofdormancy. The best advice is to check the latestdevelopments before selecting a compressor, but makesure that a model is well proven before buying it. Thepotential advantages of novel types are too small tojustify reliability risks.

Vane compressors and trochoid compressors aretypes that have been used in various smaller refrigerationapplications, such as residential refrigerators and airconditioners, but not yet in larger cooling systems. Theyare appealing because of their apparent simplicity, theircompactness, and their ability to operate quietly. Themajor practical problem with both has been sealing.

Vane compressors use a rotor that is locatedeccentrically in a chamber. Sliding vanes in the rotorcreate spaces between the rotor and the chamber wall.The volume of these spaces changes as the rotor rotates.Refrigerant gas is drawn into the spaces in the half ofthe rotation where the size of the spaces is increasing.The gas is compressed as the rotor turns, and isdischarged at the point where the spaces are smallest.

Trane Company

Fig. 15 The parts of a scroll compressor The top scroll isfixed. The bottom scroll moves within the fixed scroll. It doesnot rotate, but moves with an eccentric motion that squeezesthe refrigerant gas from the outside of the spiral toward thecenter, as shown in Figure 16. The slot on the side of thelower scroll engages a fixed pin that keeps it from rotating. Inback is the drive shaft for the moving scroll, with acounterbalanced eccentric drive at the bottom. Theperformance of the compressor is dependent on very accuratemachining of the scrolls.



Trochoidal compressors are a general type in whicha rotor rolls along the inside of a chamber, compressingthe gas into pockets formed between the rotor and thechamber wall. (“Trochoid” is the name of a curve thatis traced by the perimeter of a rolling circle.) The bestknown example of a trochoidal device is the Wankelrotary engine, a variant of which has been made into acommercial compressor.

Compressor DriversAll the energy input to a compression cooling system

goes into the compressor driver. This may be an electricmotor, a reciprocating engine, a gas turbine, or othermachine. Your selection of the compressor driver is acritical factor in efficiency and operation. The followingare the most common choices to be made.■■■■■ Internal vs. External Motors

If the compressor is driven by an electric motor thatis located outside the compressor housing, a shaft mustpenetrate the compressor housing. This type ofinstallation is called an “open drive.” Figure 18 showsa typical open drive.

The motor may also be installed inside thecompressor casing, along with all the other moving parts.

Trane Company

Fig. 16 How a scroll compressor moves The dark scroll is fixed. The light scroll moves within it. The movingscroll does not rotate, but its center moves in a small circle. The effect is to squeeze gas from the outsidetoward the inside along a spiral path.

Trane Company

Fig. 17 Torque characteristics of scroll and reciprocatingcompressors The scroll compressor has less torquevibration, largely because the gas does not have to reversedirection in going through compression.



If the compressor and motor is totally encapsulatedwithin a welded housing, the compressor is called“hermetic.” True hermetic compressors are found onlyin smaller sizes. Figure 19 shows a typical example.

If the motor or other components are accessiblethrough bolted access plates, the compressor is properlycalled “semi-hermetic,” although it is common to callsuch compressors “hermetic” also. The compressorshown in Figure 10 is semi-hermetic.

Are open or hermetic compressors better? There isno definite answer. Both types work well within theirappropriate applications. However, increasing concernabout the environmental hazard of certain refrigerantshas shifted the balance toward hermetic machines, atleast for machines that use harmful refrigerants. Theadvantages of hermetic systems are:

• reduced refrigerant leakage. Installing the motorinside the compressor housing eliminates the shaftpenetration, which is the main source of refrigerantloss with open drive compressors. Low leakage isan important advantage if the refrigerant hasobjectionable environmental properties.

• motor longevity. Refrigerant cooling promises longmotor life because temperatures are lower, and thecoolant is clean. However, all compressionrefrigerants have some solvent behavior, and theytend to be absorbed by the organic materials usedfor motor insulation. This may create unexpectedproblems with newer refrigerants.

Hermetic motors have these disadvantages:• reduced cycle efficiency and/or increased

condenser size. The motor of a hermeticcompressor may be cooled by cold suction gas fromthe evaporator, or by liquid refrigerant from the

condenser. In both cases, the motor heat goes intothe compression cycle.For example, the efficiency of the large motors usedwith centrifugal chillers ranges from 92% to 96%.The remaining percentage becomes motor heat,which reduces the COP of the chiller by about oneor two percent.

• difficult access to the motor for maintenance.Getting to the motor requires pumping therefrigerant out of the system, and removing theaccess covers.

• need to clean out the entire refrigerant system inthe event of a motor burnout. The electric arcingthat occurs in an insulation burnout createscorrosive materials from most refrigerants. Thisrequires cleaning out the entire refrigerant side ofthe cooling system.

• possible inability to retrofit variable-speed drives.If a hermetic compressor was originally designedto operate at a constant speed, it may not be possibleto retrofit an electronic variable-speed motor drive.This is because the full motor speed may be neededfor proper bearing lubrication and motor cooling.

WESINC

Fig. 18 Compressor with open drive An importantdisadvantage of this arrangement is leakage of refrigerantaround the drive shaft. On the other hand, the arrangementhas several advantages.

Trane Company

Fig. 19 Hermetic compressor This arrangement virtuallyeliminates refrigerant leakage. However, it is not practical torepair this type of unit at the site.



■■■■■ Motor EfficiencyMotor efficiency is generally not an optional factor

with hermetic compressors, where the motors are custommade for the machine. Motors of higher efficiency maybe an option with open-drive compressors.

Controlling the compressor output with an electronicvariable-frequency motor drive keeps the motoroperating more efficiently at partial loads, although thereis a small efficiency penalty at high loads. For moreabout this, see Reference Note 36, Variable-SpeedMotors and Drives.■■■■■ Other Types of Drivers

The great majority of compression cooling machinesare driven by electric motors, but there are circumstancesin which a different source of shaft power may beappropriate. The first refrigeration compressors weredriven by reciprocating steam engines. Moderncompressors are sometimes driven by diesel engines,natural gas reciprocating engines, gas turbines, andsteam turbines.

A decision whether to choose such a drive requiresdetailed knowledge of the engineering, operatingpractices, and economics of the drive in question. Theremay be a temptation to use non-motor drives for“obvious” but illogical reasons. For example, a largeurban hospital uses a condensing steam turbine to drivea chiller, apparently because the facility was designedwith a high-pressure steam supply for other reasons. Thesteam turbine drive in this facility is much moreexpensive to operate than an electric motor would havebeen.

In general, electric motors combined with variable-frequency drives provide better part-load performancethan other types of drives, along with lowest first costand minimal maintenance requirements. If you areinterested in a reciprocating engine drive, you have tofind out whether the compressor is vulnerable to thetorsional vibrations of the engine drive. This requires asophisticated analysis of the rotating components by thecompressor manufacturer.

Packaged engine-driven cooling machines areavailable. These are sold primarily as a means ofreducing electrical demand during peak load periods.

EvaporatorsThe design of the evaporator can have a significant

effect on system efficiency. We will cover the mostcommon types of evaporators. Then, we will discussevaporator superheat, which is the most important factorin evaporator efficiency.■■■■■ Flooded Evaporators

Flooded evaporators are used in a large fraction ofpackaged water chillers, as shown in Figure 2. Chilledwater is cooled in tubes that are completely submerged

in liquid refrigerant. This provides complete utilizationof the tube surface. The large free surface of the liquidrefrigerant is directly exposed to the compressor suctionso there is virtually no pressure loss between theevaporation area and the compressor suction.

This arrangement is simple as well as efficient.Liquid refrigerant from the condenser is simply drainedback to the evaporator, typically using an orifice or floatvalve to isolate the evaporator from the pressure of thecondenser.

The only significant disadvantage of floodedevaporators is that they require a larger quantity ofrefrigerant than evaporators where the refrigerantevaporates inside the tubes.■■■■■ Liquid Overfeed Evaporators

In liquid overfeed systems, refrigerant iscontinuously pumped, or flows by gravity, through anynumber of evaporators. The evaporators can take anyform needed by the application, such as an air coil or aplate ice maker. The name “liquid overfeed” derivesfrom the fact that more liquid refrigerant is fed to eachevaporator than evaporates, the excess liquid flowingback to a receiver. The receiver is open to the compressorsuction, so the refrigerant liquid is kept at thetemperature corresponding to the compressor suctionpressure. Thus, each evaporator behaves almost like aflooded evaporator.

The system is flexible, in that it can serve anynumber and variety of evaporators. The only limitationis that all evaporators must operate at the same pressure,and hence, at the same temperature. Liquid overfeedhas generally the same efficiency advantages as aflooded evaporator, except for a minimal energy loss inrefrigerant pumping and piping pressure loss. There isno pressure difference between the evaporator receiversand the evaporators, except for any gravity head thatmay exist as a result of differences between the heightsof the evaporators. As a result, the pumping power isminimal.

In addition to the low-pressure receiver for eachevaporator, the system generally has a single high-pressure accumulator to allow for fluctuations in thequantity of refrigerant in the evaporators.

Since each evaporator in a liquid overfeed systemis kept flooded with refrigerant, there is no way ofcontrolling the evaporating temperature individually. Tocontrol the cooling output of each evaporator, it isnecessary to throttle the flow of the cooled mediumthrough the evaporator, or to bypass a portion of thecooled medium around the evaporator.

If only a single evaporator is needed, as in a centralchiller system, liquid overfeed offers no significantadvantage over a flooded evaporator, and it would beunnecessarily complex.



■■■■■ Evaporators with Expansion ValvesAn expansion valve is an automatic valve for

metering the flow of refrigerant into the evaporator.Figure 1 shows where an expansion valve is installed inthe system.

Unlike flooded evaporators and liquid overfeedevaporators, the flow of refrigerant into evaporators thatuse expansion valves is restricted.

The name “expansion valve” is a misnomer. Anexpansion valve is actually a liquid refrigerant meteringdevice. The only expansion that occurs at the valve itselfis a relatively small fraction of the refrigerant that flashesbecause of the drop in pressure upon entering theevaporator. Most of refrigerant evaporates as a resultof absorbing heat inside the evaporator.

Most air coil (DX) evaporators use expansion valves.Many smaller water chilling evaporators use them.Expansion valves limit the amount of refrigerant usedin the evaporator to the minimum needed to satisfy thecooling load. This reduces the evaporator size and therefrigerant charge. Expansion valves are the mosteconomical way of serving multiple evaporators in asingle system. Compared to liquid overfeed, the pipingis smaller and simpler. The main disadvantage ofexpansion valves is that they reduce the system COP,for reasons discussed below.

Until recently, virtually all expansion valves wereset to maintain a constant difference between thetemperature of the vapor leaving the evaporator and thetemperature of the liquid entering the evaporator. Thistemperature difference is called “superheat” because itrepresents sensible heating of the vapor above itssaturation temperature after all the liquid is evaporated.Typical superheat ranges from 5°F to 10°F. Using anexpansion valve forces the vapor outlet end of the coilto be dry, so this portion of the coil provides onlyminimal cooling capacity.

Direct-expansion heat exchangers, such as air coils,are commonly divided into many separate refrigerantcircuits to increase the effective cooling area. In orderto use all the surface most effectively, the refrigerantmust be distributed evenly to all the circuits. To do this,thin distributor tubes are installed between the expansionvalve and the individual circuits. The distributor tubesall have the same resistance to flow, so refrigerant isdistributed uniformly.

An important energy conservation principle iskeeping the condenser temperature as low as possible.Unfortunately, using an expansion valve limits thistechnique during cooler weather, when it is mostvaluable. The reason is that the condenser pressure mustbe kept high enough to drive refrigerant through theexpansion valve and distributor tubes.

In small cooling equipment, the function of theexpansion valve may be performed solely by thedistributor tubes, in which case the internal diameter ofthe distributor tubes is carefully selected so that theyprovide the proper amount of resistance to isolate theevaporator from the condenser pressure. In suchapplications, the distributor tubes are called “capillarytubes.” This method cannot accurately control superheat,or prevent liquid refrigerant from getting all the waythrough the evaporator. Therefore, it may requireadditional features to protect against liquid entering thecompressor, such as using an accumulator at thecompressor suction.

“Electronic” expansion valves are an improvementthat became widespread during the 1990’s. An electronicexpansion valve adapts to changing load conditions,minimizing the superheat at all times. Figure 20 showsan electronic expansion valve.■■■■■ How Evaporator Superheat Reduces COP

In an ideal cooling machine, the liquid refrigerantin the evaporator would be cooled down to thecompressor suction temperature. This condition is nearlymet in flooded evaporators and in liquid overfeedevaporators. This is because the liquid refrigerant isexposed directly to the compressor suction.

However, if an expansion valve is used to controlthe flow of refrigerant through an evaporator, all therefrigerant must evaporate before it leaves theevaporator. To ensure that this happens, the evaporator

Trane Company

Fig. 20 An “electronic” expansion valve This expansionvalve minimizes superheat under all load conditions bychanging its throttling behavior. The valve receives inputsignals from a microcomputer that senses system conditions.



is oversized in relation to the refrigerant flow. As therefrigerant continues to travel through the evaporatorafter it has evaporated, it picks up sensible heat,increasing its temperature. The rise in vapor temperatureabove the evaporation (saturation) temperature is called“superheat.”

Superheat reduces the system COP. To understandthis, consider the example of a direct-expansion air coilevaporator that is operating with a constant cooling load.Assume that the superheat is adjustable. Initially, thesuperheat is set at zero, so the coil is completely filledwith evaporating refrigerant. Then, increase thesuperheat setting. The immediate effect of this is torestrict the flow of refrigerant into the coil. Since thequantity of refrigerant must be constant to satisfy thecooling load, there is a rise in the air temperature. Thesystem controls sense the rise in temperature, andincreases the compressor power input to compensate.The greater compressor suction restores the flow ofrefrigerant through the expansion valve to its formervalue (less a small amount that accounts for the sensiblecooling of the air by the superheated refrigerant vapor).

The increase in compressor power can be expressedin terms of the temperature difference across thecompressor. In the previous example, the compressorpower increased as the suction pressure dropped,requiring evaporation to occur at a lower temperature.As we discussed earlier, the COP of a cooling systemsuffers if the temperature differential between theevaporator and the condenser increases.

It is a common belief that superheat improves theefficiency of the machine because the absorption of

sensible heat by the refrigerant provides additionalcooling effect. This is not true. The sensible cooling isminor in relation to the increased compressor powerrequired by the lower suction pressure.

Energy Recovery from Refrigerant ExpansionThe refrigerant in the condenser is at high pressure

and the refrigerant in the evaporator is at low pressure.When refrigerant is released into the evaporator, thereduction of pressure causes a fraction of the refrigerantto flash into vapor. In most large cooling machines,flow from the condenser to the evaporator is regulatedby a float valve, a simple orifice, or an expansion valve.With any of these devices, refrigerant flashing is a freeexpansion process.

Free expansion wastes most of the energy that wasrequired to compress the flashed refrigerant initially,because the expansion does no useful work. The flowof liquid refrigerant across the pressure difference alsocomprises a waste of energy, but this is a minor factorbecause the volume of the liquid is small.

Some new chiller designs recover this lost energy.The most direct approach is to pass the refrigerant fromcondenser to evaporator through a turbine. Figure 21shows a machine that uses this approach.

The liquid refrigerant enters the turbine from thecondenser. Inside the turbine, the liquid tends to flashinto vapor, but it also tends to re-condense because it islosing energy. The turbine is designed to minimize theenergy left in the refrigerant. The turbine, which isdriven by the pressure differential and the expansion ofthe flash gas, helps to run the compressor.

Carrier Corporation

Fig. 21 Energy recovery from refrigerant expansion The condenser is at much higher pressure than the evaporator. Energyis recovered by passing refrigerant through a turbine on its way from the condenser to the evaporator. The turbine helps the motorto drive the compressor. In this machine, the turbine wheel is on the left end of the motor shaft, and the compressor is on the right.The turbine refrigerant flow is entirely independent of the compressor flow.



(A similar idea is used in steam plants, wherepressure reducing turbines are used in place of pressurereducing valves to get useful work.)

Since energy has been removed from the refrigerantbefore it enters the evaporator, it has more ability toabsorb heat from the load. It also has less superheat,which reduces the power needed by the compressor tocompress the gas.

This method can be used with flooded evaporatorsand liquid overfeed systems. It has little or no potentialwith expansion valve systems and with air coils that usecapillary tubes or other types of flow restrictors todistribute refrigerant within the coils. This is becausethese devices require a significant pressure differentialto operate properly.

Evaporator and Condenser Heat TransferTemperature differentials that occur in the

evaporator and the condenser are important factors inthe amount of work that the compressor must do, becausethese temperature differentials translate to an increasedpressure difference across the compressor. Typicalevaporator and condenser temperature drops are shownin Figure 4. The temperature differential is proportionalto three factors: the heat transfer coefficient of thesurfaces, the area of the heat transfer surfaces, and therate of heat flow, or cooling load.

The evaporator and condenser temperaturedifferentials are determined largely when the coolingmachine is selected. They are a compromise betweenefficiency and first cost that the engineer makes in theselection process. Larger heat exchangers improve heattransfer, but heat exchangers are a major part of thesystem cost. The efficiency benefit is subject to a severelimitation of diminishing returns as the temperaturedifferentials become small. For example, reducing thetemperature differential from 4°F to 2°F requiresdoubling the heat transfer surface. This makes it veryexpensive to reduce the last few degrees of temperaturedifferential.

Heat transfer is not limited primarily by theconductivity of the metal, but by heat transfer at theinterfaces between the metal, liquid, and gas. Therefrigerant side of evaporators involves two heat transferprocesses, from the gas to the liquid film on the tubesurface, and from the liquid film to the metal. Wherewater is used in evaporators and condensers, heat transferis reduced by fouling from the water. This is especiallysevere with open cooling towers.

With refrigerant-to-water evaporators andcondensers, minimum water flow rates are required tomaintain adequate turbulence for good heat transfer. Invariable-flow systems designed to reduce pump power,a separate loop must be provided to maintain a minimumwater velocity through the heat exchanger.

Major improvements in the heat transfer coefficientsof evaporators and condensers have been made sincethe 1970’s. These improvements are achieved primarilyby giving the tube surfaces a texture that increasesturbulence, enhancing contact between the differentmediums.

Heat transfer in air coils involves air velocity, finspacing, number of rows of coils, and refrigerantcircuiting. Air-cooled (DX) evaporators may requireseparate coil sections to allow operation at partial loads.Distribution of refrigerant within a DX evaporatorrequires elaborate distributors, and is difficult tooptimize.

Heat transfer efficiency improves at partial loadsbecause the heat transfer surface is then larger in relationto the amount of heat to be transferred.■■■■■ Fouling Factor

When a system uses water for heat transfer in eitherthe evaporator or condenser, a “fouling factor” is usedin the rating conditions to account for fouling of thetube surfaces by dirt in the water. The fouling factor isa measure of thermal resistance, like the R-value usedto rate insulation. The units are the same, for example,square feet-°F-hour per BTU in U.S. practice.

In the U.S., a fouling factor of 0.00025 is commonlyassumed in rating large cooling machines. Experiencehas shown that this figure is appropriate for bothevaporators and condensers. It is assumed that thefouling factor is kept from rising above this value bythe scrubbing action of the water flow. For thisassumption to be valid, the equipment must be designedwith appropriate water velocities to maintain turbulentflow at the tube surfaces. Also, if the condenser useswater from an open cooling tower, you need to maintainan appropriate water treatment program for the towerwater. The rating also assumes that tube surfaces arecleaned at appropriate intervals.

When comparing cooling equipment specifications,be careful to check the fouling factor that is actuallybeing quoted by the manufacturer in its efficiency andcapacity ratings. Although a fouling factor of 0.00025is a standard value, a manufacturer may fudgecomparisons with competing equipment by usingdifferent fouling factors.

The refrigerant sides of evaporators and condensersare kept clean by the solvent and detergent propertiesof the refrigerant materials. (Halocarbon refrigerantswere widely used for cleaning purposes, before theywere banned for environmental reasons. Ammonia isstill a common cleaning agent.) Therefore, no foulingfactor is needed for the refrigerant sides.

Parallel and Series Evaporator CircuitsIn most facilities that have multiple water chillers,

the evaporators are piped in parallel. This allows thechillers to be operated in any combination. However, it



is possible to improve efficiency at high loads, at leastin principle, by connecting the evaporators so that thechilled water flows through them in series.

To understand this, let’s look at an example.Consider a facility that has two chillers of equal capacity,each operating at full load on a peak-load day. There isa large spread between the chilled water supply andreturn temperatures, for example, supply at 40°F andreturn at 52°F. If the chillers were operating in parallel,each would have to chill the water to 40°F. However, ifthe evaporators are connected in series, one of themachines can operate with a higher evaporatortemperature. In this example, all the return chilled wateris cooled in the first machine from 52°F to 46°F, and inthe second machine from 46°F to 40°F. The COP of thefirst machine is increased substantially, while the COPof the second machine is reduced by a small amount.The average COP of the two machines might beincreased by perhaps five percent under full load.

The efficiency advantage disappears below abouthalf load, because one of the chillers is turned off ineither the parallel or series arrangement. In most comfortcooling applications, the cooling load is less than halfthe peak design load for a majority of the time. Thus,the opportunity of exploiting a series connection islimited. However, if there are more than two chillers inthe plant, the number of efficient operating hours maybe extended by combining series and parallelconnections.

The efficiency advantage of a series connection alsodrops rapidly below full load because the evaporatortemperature of the chillers in a parallel connection canbe raised to follow the load.

The main disadvantage of a series connection is thatthe chilled water must always flow through theevaporators of both chillers, even if one chiller is turnedoff. This wastes pump power. To minimize the pumppower, the evaporators of series-connected chillers areusually designed for lower resistance, which wastessome efficiency and capacity in the individual chillers.For a given plant cost, these factors may eliminate anyefficiency advantage that the series connection offers.

In principle, any number of chillers could beconnected in series, but practical considerations limitthe number to two.

Series-connected chillers should be similar in size,because the temperature drop across the evaporator ofeach machine will be proportional to its relative capacity.Some simple mathematics shows that the efficiency

advantage diminishes if the temperature drops acrossthe chillers are not divided almost equally.

To maximize the benefit of this configuration, thechillers should be controlled so that they share the loadwhile both are running, and that one of the chillers isturned off at the appropriate load, typically at half ofthe cooling load or somewhat less. Inefficient loadsharing between series-connected chillers wastes moreenergy than inefficient load sharing among parallel-connected chillers, which is another reason to be cautiousabout this configuration.

Condensate SubcoolingIn an ideal cooling machine, the liquid refrigerant

in the condenser would fall to the same temperature asthe cooling water or air. Because heat transfer isimperfect, the condensed refrigerant is actuallysomewhat warmer than the cooling medium. Thisreduces efficiency by holding the compressor dischargetemperature higher, increasing compressor power.

A condenser may cool the refrigerant below thetemperature at which it condenses on the condensersurface. This is called “condenser subcooling.” It is acheaper alternative to more efficient condenser heatexchanger design. Subcooling provides some additionalsensible cooling of the refrigerant, but sensible coolingis a minor part of the cooling potential of the refrigerant.Note that condenser subcooling does not significantlyreduce the compressor discharge pressure.

Subcooling can be done in several ways. Mostcommonly, the refrigerant is routed through the coldestpart of the condenser on the way out to the evaporator.Most condensers are tube-and-shell units, with thecooling water running in the tubes. To achievesubcooling of the condensed refrigerant, the water isbrought into the condenser through the bottom tubes,and the condensate is allowed to pool around these lowertubes before flowing out of the condenser.

Condensate subcooling can be combined with aneconomizer. To do this, a portion of the condensate isrouted through a separate vessel that is intermediate inpressure between the condenser and the evaporator. Aportion of the condensate is flashed into vapor, coolingthe condensate, and the cool vapor is injected into anintermediate stage of the compressor, as describedpreviously.

The term “condensate subcooling” is also used formethods of improving the overall cycle efficiency ofmulti-stage compressors. This involves more theory thanwe need to discuss here.

compression cooling - energy institute press · • the condenser, which is a heat exchanger on the...

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