41 ASHRAE Jou rna l J a n u a r y 2 0 0 0

A SHRAE JOURNAL

TTTTT o start, lets define what we mean by low temperature refrigeration.The term low temperature refers to the cooler side of a general tem-perature range. In air conditioning, chilled water or glycol at 32F (0C)would be considered low. In industrial refrigeration, blast freezing at 50F to60F (45C to 50C) is considered low. In the realm of cryogenics, tem-peratures approaching 460F or 0R (273C or 0 K) are suitably desig-nated low. But none of these belong in the category discussed in this article.

Low temperature refrigeration is therange of temperatures falling below whatis normally considered industrial refrig-eration and above the temperatures as-sociated with the field of cryogenics. Thetemperature range of this classification isfrom 58F to 148F (50C to100C). This range of temperatures in-cludes applications for food, pharmaceu-tical, and chemical processing. It is gen-erally used in the petroleum and chemicalindustries as laboratory environmentalchambers and thermal storage equipment.

The size and type of equipment usedvaries from small pre-packaged low tem-perature environmental chambers of onlya few horsepower (kW) to large custom-designed and often field-erected systemsranging to several hundred horsepower(kW). These larger units are by natureone-of-a-kind, designed and built for aspecific purpose, for a specific duty at aprecisely controlled low temperature con-dition. The large units use a combinationof one, two or more refrigerants in opencascade arrangements to obtain the de-sired low temperature.

Manufacturers of the small packagedenvironmental chambers typically use anautocascade cycle. They maintain propri-etary control over the processes and thespecific refrigerants used in their systems.

The compressors are generally stock her-metic compressors available from the com-mercial refrigeration trade. The refrigerantmixtures, however, are proprietary as arethe combinations and balancing of the heatexchangers required to obtain the final lowtemperature in an efficient manner.

Custom-designed equipment, both en-vironmental chambers and field-erectedsystems, begins where the auto cascadesystems tend to reach their maximumaround 10 hp (7.5 kW). Autocascade sys-tems are not suitable for liquid cooling,so when there is need for a low tempera-ture secondary cooling fluid (brine), theequipment must be custom-designed.

The engineering, design and field erec-tion of the custom-designed equipmentis generally done by people from the in-dustrial refrigeration market area. How-ever, the need for custom-designedequipment in the low temperature rangeis infrequent, so experience and expertisewithin organizations that accept work inthis area tends to accumulate slowly.

This article brings together current in-formation and experience available on thesystems, designs, refrigerants, secondarycoolants, practical recommendations,and cautions pertaining to low tempera-ture systems. This information will beuseful to suppliers, purchasers, design-

ers and end-users of low temperature re-frigeration systems.

System TypesSeveral types of systems are used to

achieve low temperatures, depending onthe actual temperature desired and thespecific compressors used. The large liftor pressure difference from the evapo-rating temperature to the condensingtemperature is one of the biggest engi-neering problems for these low tempera-ture systems.

Single-stage economized screw compressorsystem. This is the simplest low tempera-ture system. It is useful down to about 60F (50C). A single-stage system us-ing a reciprocating compressor wouldhave excessive discharge temperatures.A screw compressor can attain low tem-peratures in single-stage compressionbecause the discharge temperature iscontrolled by the amount and tempera-ture of the oil flooding the compressor.The screw compressor also has a rela-tively flat volumetric efficiency curve,permitting a volumetric efficiency of 80%or more at a 20:1 compression ratio.

Figure 1 shows the relationship of volu-metric efficiency (VE) and compressionratio for both screw and reciprocating com-pressors. The chart shows that recipro-cating compressors perform poorly be-yond a ratio of 10:1. They would also op-erate at excessive discharge and oil tem-peratures.

A screw compressor with an econo-mizer subcools the liquid refrigerant.This subcooling significantly increases

Rudy Stegmann, P.E., is president of The Enthalpy Ex-change, Williamsburg, Va. He is a member of SSPC 15,Safety Code for Mechanical Refrigeration, ASHRAE Tech-nical Committee (TC) 8.1, Positive Displacement Com-pressors and TC 10.4, Ultra-Low Temperature Systemsand Cryogenics.

About the Author

Low Temperature RefrigerationBy Rudy Stegmann, P.E.Member ASHRAE

The following article was published in ASHRAE Journal, January 2000. Copyright 2000 American Society of Heating, Refrigerating and Air-ConditioningEngineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form withoutpermission of ASHRAE.

42 ASHRAE Jou rna l J a n u a r y 2 0 0 0

compressor capacity. It also improves the overall operatingefficiency and comes close to that of a two-stage system.

Two-stage single refrigerant system. The two-stage single refrig-erant system can use either screw or reciprocating compres-sors. To determine the overall compression ratio of a two-stagesystem, multiply the compression ratios of each stage. Thecompression ratio of each stage will be in a range suitable forreciprocating compressors, and their discharge temperatureswill be moderate. The minimum horsepower requirement for atwo-stage system operating at a given set of conditions occurswhen the compression ratios of each stage are about the same.

Two-stage systems include interstage cooling thatdesuperheats the discharge gas leaving the low-stage (booster)compressor. Inter-stage cooling also cools the liquid refriger-ant to a temperature near the interstage temperature. Coolingthe liquid refrigerant increases system capacity.

The temperature limits for a two-stage system depend onthe specific refrigerants chosen. Some of those limits include:

Refrigerant Minimum low temperatureHCFC-22 90F/70C

R-507 90F/70CR-717 60F/50C

Two-circuit cascade system. The two-circuit cascade system isthe most common and is suitable for the entire low temperaturerefrigeration range. It has two separate refrigerant circuitsahigh temperature circuit and a low temperature circuit. They arecoupled thermally at the condenser of the low temperature cir-cuit. The evaporator of the high temperature circuit is the con-denser for the low temperature circuit. The high temperaturecircuit uses a standard refrigerant like that found in a single-stage system. The low temperature circuit contains a refriger-ant suitable to obtain the desired low temperature.

Standard refrigerants cannot operate at very low temperaturesbecause their saturation pressure at the low temperature is toolow. If the saturation pressure is less than about 21 in. Hg vac/4psia (28 kPa), very little refrigerant vapor is drawn into the com-pressor. Vapor density is also extremely low at these pressures,so the mass flow of refrigerant through the system is very low.

Refrigerants used for the low temperature circuit of cascadesystems generally have a saturation pressure at the low tem-perature condition above atmospheric pressure to help keep airfrom being drawn into the system. The higher pressure cascaderefrigerant, because of its density, will require a much smallercompressor to provide the needed system capacity than if astandard refrigerant were used.

Three-circuit cascade system. To obtain temperatures around150F (100C), the standard high temperature circuit plus twocascade circuits may be required. The evaporator for the highertemperature cascade refrigerant would be the condenser for thelower temperature cascade refrigerant. The system then has anadditional step of temperature reduction to obtain the final de-sired low temperature. Very few of these systems exist.

Autocascade systems. Low temperature conditions similar tothose obtained with large custom-designed and field-erected cas-cade systems may be achieved by an autocascade system.

Autocascade systems are complete, self-contained systems inwhich multiple stages of cascade cooling occur simultaneously.This is accomplished by means of several steps of vapor/liquidseparation and adiabatic expansion of the different refrigerantscontained in the refrigerant charge. The low temperature may beachieved with a single-stage compressor in conjunction with theappropriate mixture of two or more refrigerants and a series ofcounterflow heat exchangers.

Autocascade systems are typically much smaller than cus-tom-designed equipment, and they use standard hermetic com-pressors ranging up to about 10 hp (7.5 kW). These systemshave a surprisingly low compression ratio and a high volumet-ric efficiency. Heat exchanger design and system chemistry arecomplex. The compressor displacement is large for the unitslow temperature capacity. The refrigerant compositions and thesensitive component arrangements are proprietary.

The components of an autocascade refrigeration systembasically include a compressor, a condenser (water- or air-cooled), a mixture of refrigerants with varying boiling points,and a series of heat exchangers. Figure 2 shows a schematicdiagram of a simple single-stage autocascade system using atwo-refrigerant mixture.

In this case, the higher boiling point refrigerant is liquified inthe condenser (3) but the lower boiling point refrigerant re-mains as a vapor. These are separated in vessel (5). The lowerboiling point refrigerant vapor proceeds to the cascade con-denser (7). In the cascade condenser, the high boiling pointrefrigerant passes through the expansion device (9) and flashesto a vapor. The heat to vaporize the high temperature refriger-ant liquid condenses the low boiling point refrigerant vapor.The low boiling point liquid refrigerant proceeds through asecond expansion device (10) and into the low temperatureevaporator (11) where it cools the air blown across the coil toobtain the desired low temperature. Both refrigerants return tothe compressor via the suction lines (12).

This simple system leads one to the idea that lower tempera-tures can be achieved efficiently if there were more refrigerantsin the mixture and the cascade process repeated several timesprior to reaching the low temperature evaporator. Figure 3 showsa single-stage compressor using a refrigerant mixture composedof four refrigerants with descending boiling points to providefour cascade stages in a more complex autocascade system.

Figure 1: Typical volumetric efficiency vs. compression ratio for industrial open-drive refrigeration compressors.

J a n u a r y 2 0 0 0 ASHRAE Jou rna l 43

Refrigeration

The condensation and subsequent expansion of one refrig-erant provides the cooling required to condense the next lowertemperature refrigerant in the heat exchanger downstream. Theprocess continues until the lowest temperature refrigerant isliquified. This refrigerant boils in the low temperature evapora-tor providing the desired cooling effect.

The most common use for autocascade systems is low tem-perature environmental chambers. The chambers with theirautocascade systems are provided as complete packagedunits. The component selection and refrigerant mixtures areproprietary, so little can be said here about specific designs.Equipment manufacturers and suppliers of these systems canprovide details on their designs.

System SelectionMany factors need to be considered in selecting the type of

system and its components for low temperature applications:1.The temperature required for the project. Temperature re-

quirements will influence the refrigerant selections, materials ofconstruction, insulation, compressor selections, heat exchang-ers, and vessels. These factors are discussed individually later.

2. The fluid to be cooled. In general, air is the medium in directcontact with the cooling coil. However, secondary liquid cool-ants are sometimes used to achieve the desired temperature.

3. Pull-down and operating conditions such as whether theduty is for batch cooling or a continuous operation.

RefrigerantsThe refrigerants for single-stage systems, two-stage sys-

tems, and the high side of the cascade system are those com-monly used in the industrial refrigeration field. Table 1 indi-cates the refrigerants and their saturated conditions at theirminimum temperatures.

There are fewer low temperature cascade refrigerants tochoose from now that CFCs are no longer commercially avail-able. Two refrigerants, both HFCs, are currently in use. Theseare shown in Table 2.

R-508b is an azeotrope of 46% HFC-23 and 54% HFC-116.The pressure-temperature relationships are quite similar to thoseof HFC-23, but the capacity and efficiency are much better thanHFC-23. The discharge temperature of R-508b is much lowerthan that of HFC-23. R-508b is the preferred refrigerant for lowtemperature applications at this time.

Some hydrocarbon refrigerants have good low temperaturecharacteristics, but they are all flammable. As such, they are notnormally used in industrial situations. Refineries, however, re-quire low temperature refrigeration. They are typically preparedto handle flammable gases in their processes and may requestthe use of a specific hydrocarbon gas for their systems. Whenflammable refrigerants are used, electrical components must berated for hazardous (classified) environments.

Choosing the refrigerants for a low temperature system re-quires a study to determine the optimum balance for efficiencyand coverage of the full range of specified temperatures. Thestudy will include choosing the interstage temperature andthe cascade condensing temperature. Varying these conditionsaffects both compressor size and operating horsepower. Per-

haps the best place to begin this analysis is to determine therefrigerant pressures where the compression ratios for eachstage are approximately equal. This arrangement usually re-sults in the lowest power input, although interstage coolingaffects horsepower and capacity. These days, with computer-ized compressor rating data readily available, it is relativelyeasy to make several selections to find an economical balancethat satisfies the load with a good match of available compres-sors.

The cascade refrigerants HFC-23 and R-508b have saturatedpressures in the range of 600 psig (4000 kPa) when the liquidrefrigerant is warmed to room temperature. This condition wouldrequire that all components in the low temperature circuit besuitable for this high pressure. This is economically impractical.

To permit the use of these refrigerants, realizing that thesystem from time to time will be warmed to room temperature,a fade-out vessel is installed on the cascade circuit. A fade-out vessel is simply an empty pressure vessel that is open tothe cascade refrigerant. When the system is shut down andthe temperature rises, the combined volume of the fade-outvessel and the remainder of the system is large enough forall of the liquid refrigerant to expand to a vapor withoutexceeding a reasonable limiting pressure.

Figure 2: Simple autocascade refrigeration system (from 1998 ASHRAE Hand-bookRefrigeration, Chap. 39).

Figure 3: Four-stage autocascade system (from 1998 ASHRAE HandbookRe-frigeration, Chap. 39).

44 ASHRAE Jou rna l J a n u a r y 2 0 0 0

Table 2: Low temperature cascade refrigerants.

Table 1: Refrigerants for single-stage and two-stage systems and the high sideof the cascade system.

For example, the total volume of the fade-out vessel andthe cascade system can be sized so that at a 120F (49C)equalization temperature, the cascade refrigerant has enoughroom to expand to a vapor at a pressure no higher than 200 psig(1400 kPa). Any other suitable equalization temperature andpressure limit could be selected. The equalization tempera-ture should be the highest temperature the refrigerant is likelyto reach during the shut down. The design pressure is a bal-ance between vessel size and design pressure. The lower thedesign pressure, the larger the required vessel.

Calculating the required volume (Vc) requires knowingthe total refrigerant charge (R) and the specific volume ofthe expanded refrigerant vapor (Vr) at the desired equaliza-

tion temperature. Since the entire refrigerant charge will bea vapor, it will be superheated. Figure 4 is a P-H diagram forR-508b and can be used to illustrate this calculation for asystem with 250 lbs (113 kg) of refrigerant and a designlimiting pressure of 200 psia (1380 kPa). At the normal op-erating temperature of 140F (96C) evaporating and37F (38C) condensing, the pressures are 10 psia (70 kPa)in the evaporator and 130 psia (896 kPa) in the condenser.If liquid and vapor were allowed to coexist in a closed ves-sel, the temperature could not exceed 12F (24C) beforethe pressure would reach the design limit of 200 psia (1380kPa). Providing additional volume in a fade out vessel forthe vapor to expand until all the liquid has evaporated al-lows the pressure to remain below the design limit of 200psia (1380 kPa):

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J a n u a r y 2 0 0 0 ASHRAE Jou rna l 45

Refrigeration

Vc = R VrVc = 250 lbs 0.30 ft3/lb (from Figure 4)Vc = 75 ft3

CompressorsThe compressors for low temperature systems are either re-

ciprocating or oil-flooded screw compressors. They may behermetic or semi-hermetic compressors that will operate withHFC refrigerants. The smaller sizes may be obtained from com-mercial refrigeration sources.

Larger systems will use compressors obtained from industrialrefrigeration sources. These will be open-drive compressors andmay be either reciprocating or oil-flooded screws. The industrialcompressors may be used with ammonia (R-717), HFCs, and hy-drocarbon refrigerants.

The high temperature circuit is typically a standard industrialrefrigeration system whether single- or two-stage. It is desirableto take advantage of liquid subcooling or interstage liquid cool-ing to obtain efficiency and minimum compressor sizing.

The compressors and their components must be compatiblewith the chosen refrigerant. In the case of the low temperaturecascade compressor, it is a good idea to contact the compres-sor manufacturer directly to obtain rating information for thecomplete duty expected. Since there is little call for this informa-tion, the data might be an estimate and not based on actualtests. Therefore, designers should be conservative when se-lecting the cascade compressor, so there will be sufficient ca-pacity when the system is in service.

Reciprocating compressors have limits of operation basedon maximum oil and discharge temperatures. Their capacity andvolumetric efficiency go down as their compression ratio in-creases. Be aware of these factors and operate the compressorwithin the manufacturers specifications.

Screw compressors have fewer limits of operation becausethey are oil flooded. Discharge temperatures can be controlledwith the amount of oil fed to the compressor. The oil feed to ascrew compressor means the unit must have adequate oil sepa-ration equipment to prevent oil carryover.

The compressor manufacturer might recommend a minimumtemperature of 50F (45C) for suction gas returning to thecompressor. If necessary, a suction line heat exchanger can be

included to use heat from the liquid line to superheat the suc-tion gas to the desired minimum temperature. As an addedbenefit, a liquid/suction heat exchanger increases capacity bysubcooling the liquid refrigerant. If the suction line heat ex-changer does not raise the suction temperature enough, a smallamount of hot discharge vapor may be introduced into the suc-tion to achieve the 50F (45C) minimum suction gas tem-perature.

If superheat is added to the suction vapor, calculate the ac-tual volume of gas flow required at the superheated tempera-ture and select the compressor for that volume flow. Do not usethe simple tonnage rating typically presented in the rating data.

Evaporators, condensers, and miscellaneous system vessels. Someevaporators, condensers and miscellaneous system vesselsmay be selected from standard available equipment. However,the low temperature items may have to be designed specificallyfor the system.

Oil management. Compressor lubricants should be those rec-

Table 4: Property variation with temperature decrease.

Table 3: Temperature range of construction materials for low temperature sys-tems.

Figure 4: P-H diagram for R-508b.

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46 ASHRAE Jou rna l J a n u a r y 2 0 0 0

ommended by the compressor manufac-turer. They should also be compatiblewith the refrigerants and have a suffi-ciently low pour point to permit recov-ery from the low side of the system at allexpected temperature levels. HFC refrig-erants will generally use a polyolestersynthetic lubricant. For ammonia refrig-erants, a hydrotreated parafinic oil is rec-ommended.

All compressors should have coalesc-ing oil separators sufficient to limit oilcarryover to no more than 5 ppm so oilwill not concentrate in the evaporators.Oil in the evaporator prevents proper heattransfer and impedes oil recovery.

Most oil recovery is accomplished di-rectly by the refrigerant flow through theevaporator coil and suction line. The re-frigerant tends to sweep the oil along. Inflooded ammonia systems, oil is recoveredthrough the use of oil pots that are locatedbelow the refrigerant level. The heavier oildrains into the oil pots and may be recov-ered either manually or automatically.

Materials considerations. The materialschosen for use in construction of the sys-tem must be based on ASME Code re-quirements with consideration given forthe low temperature. Special materialsand/or treatment may be required. Plaincarbon steel passes through a transitionzone where it changes from ductile to brittlebehavior. The transition temperature de-pends on several factors, including com-position and geometry. As the tempera-ture goes down, it becomes more impor-tant to verify that the materials involvedwill retain their ductility and necessarystrength. Carbon steel is suitable to 20F(30C) but may be used to 50F (45C)when the coincident low pressure associ-ated with the temperature is considered.An alternative is to use a grade of carbonsteel such as SA-333, which remains duc-tile at low temperature, or one of the stain-less steels. Table 3 indicates the tempera-ture range of these materials.

Field-erected systems must be pipedin accordance with the Refrigeration Pip-ing Code ASME B31.5. This code speci-fies acceptable materials and other detailssuch as pressure ratings, methods ofwelding, and attachments.

For larger industrial-sized systemsthe piping is generally of welded steelconstruction. Smaller, custom designed

factory packaged units quite often usecopper tubing when compatible withthe refrigerants. Copper tubing is suit-able for use at the low temperatures be-cause copper does not lose its ductilitythe way carbon steel does. Joints maybe made with silver solder. Investigatethe properties of the filler metal to de-

termine a grade of silver solder suitablefor the temperatures involved.

When using standard compressors inthe low temperature applications, thecompressor manufacturer might not con-sider the materials of the compressor andthe suction connection suitable for lowtemperature applications. However, most

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J a n u a r y 2 0 0 0 ASHRAE Jou rna l 47

Refrigeration

Table 5: Common low temperature secondary coolants.

Table 6: Low temperature insulation components.

compressors and these components aremade of cast iron, and the ASME B31.5Piping Code permits the use of cast ironto 150F (100C).

Heat Transfer CharacteristicsThe heat transfer characteristics of the

high temperature circuit of the cascadesystem are well known since they are thesame as those of the industrial refrigera-tion field. However, in the low tempera-ture cascade circuit, the heat transfer char-acteristics are significantly different dueto the lower temperatures and propertiesof the refrigerants which are less known.Data from laboratory tests and field ob-servations are scarce for low temperatureheat transfer coefficients. For these rea-sons, it is well to be cautious and conser-vative when selecting heat exchangers forthese low temperature conditions. Chap-ter 4 of the 1997 ASHRAE HandbookFundamentals may assist the designer inobtaining reasonable information fromequipment manufacturers regarding theirspecific ratings and quotations.

Some of these expected changes inproperties, as the temperature becomeslower, are shown in Table 4.

Secondary CoolantsMany low temperature field-erected

systems cool a secondary heat transferfluid to cool the final product. It isoften difficult to find a suitable liquidto perform as a low temperature second-ary coolant. Some desirable propertiesand attributes of the secondarycoolant are:

Properties: high specific heat, low viscosity, high liquiddensity, high thermal conductivity.

Attributes: non-toxic, non-flammable, environmentallystable, compatible with standard engineering materials, non-corrosive, low vapor pressure.

The viscosity and freezing point of some secondary cool-ants are the primary barriers to their use. Although non-flamma-bility is a desirable attribute, most of the secondary coolantspractical for low temperature applications are flammable. De-sign consideration for using these fluids must include safetyfeatures to contain the flammable fluids within their heat trans-fer systems as well as the necessary alarm devices to be acti-vated in the event of a leak.

Some secondary coolants for low temperature systems:Acetone, ethanol, and methanol have reasonably low viscosi-

ties, high thermal conductivity, and high specific heats inthe range of 0.45 to 0.55 Btu/lbF (2.16 to 2.65 kJ/kgK).They are flammable.

Diethylbenzene is a synthetic aromatic fluid with a freezingpoint slightly below 100F (73C). This fluid is limited touse in the higher end of the low temperature range.

d-Limonene is a terpene oil extracted from orange and lemonpeels. It is economically available as a food grade fluid, butsince it is a light oil, it also is flammable. It has a relativelyconstant low viscosity throughout the low temperature range.d-Limonene is not recommended for use with certain plasticand rubber materials since it can be corrosive. There are reportsof a gradual increase in viscosity with time which may requireperiodic replacement of the coolant.

Hydrofluoroether is a new fluid, available but expensive. It isnon-toxic, non-flammable, and covers most of the low tempera-ture range. Its viscosity increases dramatically with tempera-ture decrease, and it may not be suitable at the very low tem-perature end of the range due to pumping costs and laminarflow in heat exchangers.

Polydimethylsiloxane, commonly known as silicon oil, is envi-ronmentally friendly and non-toxic; however it is flammable. It

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48 ASHRAE Jou rna l J a n u a r y 2 0 0 0

Two Cascade SystemsCascade System 1 (Cascade System 1 (Cascade System 1 (Cascade System 1 (Cascade System 1 (FFFFFigures 5igures 5igures 5igures 5igures 5 and and and and and 66666)))))Date: 1990Cascade temperature:

80F (62F) evap./105F (41C) cond.High temperature circuit:

HTC = HCFC-22 at 35F/105F (37C/41C)HTC = two 350 hp (167 kW) economized single-stagescrew compressors

Low temperature circuit:LTC = HFC-23 at 85F/20F (65C/29C)LTC = one 250 hp (187 kW) single-stage screwcompressor

Lubricant: HTC & LTC = akylbenzene ISO vg 68HFC-23 cascade condenser: HCFC-22 DX shell and tubeHFC-23 cascade evaporator:

HFC-23 flooded shell and tubeLTC secondary coolant: methylene chloride entering at

50F (46F)/leaving at 70F (57C)Note:1) The HTC single-stage economized screw compressor

units.2) The HFC-23 condenser/HCFC-22 evaporator.3) The HFC-23 flooded chiller.4) The HFC-23 chiller automatic oil recovery unit.5) The HFC-23 suction line heat exchanger.6) The two large HFC-23 expansion (fade-out) tanks.

Cascade System 2 (Figure 7)Date: 1994Cascade temperature: 80F (62F) evap./75F (24C)

cond.High temperature circuit:

HTC = HCFC-22 at 10F/75F (23C/24C)HTC = one single-stage 10 hp (7.5 kW) hermeticreciprocating compressor

Low temperature circuit:LTC = HFC-23 at 80F/0F (62C/18C)LTC = one single-stage 6.5 hp (5 kW) hermeticreciprocating compressor

Lubricant:LTC & HTC = alkylbenzene ISO vg 68

HFC-23 cascade condenser: HCFC-22 DX plate heatexchanger

HFC-23 cascade evaporator: HFC-23 DX plate heatexchanger

LTC secondary coolant: d-Limonene entering at 50F(46C)/leaving at 70F (57C)

Note:1) The HTC has a single hermetic recip compressor.2) The HFC-23 / HCFC-22 condenser/evaporator is a plate

heat exchanger.3) The HFC-23 evaporator is also a plate heat exchanger.4) All piping, including the LTC, is copper.5) Several flex sections were added to minimize vibration

and thermal stress.6) The HFC-23 suction line heat exchanger.7) The large HFC-23 expansion (fade-out) tank located

Figure 5: General schematic for Cascade System 1.

Figure 7: Cascade System 2.

Figure 6: Equipment layout for Cascade System 1.

under the recip compressors.8) The small size of the 16.5 hp (12 kW) factory packaged

unit compared to the 950 hp (708 kW) field-erectedSystem 1.

J a n u a r y 2 0 0 0 ASHRAE Jou rna l 49

Refrigeration

can be used through the entire low temperature range. How-ever, at low temperatures, the high viscosity of the fluid re-sults in laminar flow and poor heat transfer characteristics.

Table 5 presents common secondary coolants and some typi-cal information.

Several other fluids, less frequently chosen, can be used aslow temperature secondary coolants. Some of these are: halo-carbon refrigerants (HFCs), hydrocarbon fluids (propane, bu-tane, pentane), and methyl ethyl ketone (MEK).

Any fluid chosen must be thoroughly researched regardingthe complete specifications of the fluid throughout the expectedrange of application (including stand-by shutdown) to assuresafe, satisfactory performance.

None of the secondary coolants mentioned here is fully suit-able throughout the entire low temperature range. Each has itsown range of application and should be applied accordingly.

InsulationThe insulation on low temperature refrigeration systems is

critical because the low temperature piping is generally at roomtemperature and not located within a cold space. The insulationmust be thick enough to prevent moisture condensation on the

outside of the insulation and must prevent moisture from en-tering the insulation system.

The insulation should be multi-layered so the insulation canmove as the pipe expands and contracts with temperaturechanges. The outer ply has sealed joints, and the inner plies areto be allowed to slide. Table 6 presents the components of alow temperature refrigerated pipe insulation system and indi-cates the functions of each component.

ConclusionsLow temperature cascade systems, especially those custom

designed and/or field-erected, present the designer with a num-ber of engineering choices. This article highlights some impor-tant factors in the design of low temperature cascade systemsand will give the designer a direction and an awareness of theareas of concern. Designers can refer to Chapter 39 of the 1998ASHRAE HandbookRefrigeration for additional information.

The appendix contains information on two industrial lowtemperature cascade systems that the author designed. Thefirst system is a large field erected design with a total of 950 hp(710 kW). The second system is a smaller unit that was pre-packaged and has a total of 16.5 hp (12 kW).

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