75872167 Copeland Refrigeration Manual Part 2 Refrigeration System Components

8/10/2019 75872167 Copeland Refrigeration Manual Part 2 Refrigeration System Components

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1967 Emerson Climate Technologies, Inc.All rights reserved.

This is the second of a series of publications comprising the Emerson Climate Technologies,

Inc. Refrigeration Manual, and follows Part 1, Fundamentals of Refrigeration.

The information included on refrigeration components is general in nature and is intended

only to give a brief description of their operation. Detailed information as to specic products

is available from manufacturers of components and accessories.


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1

Part IIREFRIGERATION SYSTEM COMPONENTS

Section 4. COMPRESSORS

Reciprocating Compressors .............................4-1Open Type Compressors ..................................4-2

Accessible-Hermetic Motor-Compressors ........4-2Welded Hermetic Motor-Compressors ............. 4-2Compressor Speed...........................................4-2Basic Compressor Operation ...........................4-4Suction and Discharge Valves .......................... 4-4Compressor Displacement ...............................4-4Clearance Volume ............................................4-4Lubrication ........................................................4-5Dry Air Holding Charge ..................................... 4-6Compressor Cooling .........................................4-6

Compressor Capacity .......................................4-6Two Stage Compressors .................................. 4-6Compressors with Unloaders ......................... 4-7Tandem Compressors ......................................4-7

Section 5. CONDENSERS

Air Cooled Condensers ....................................5-1Water Cooled Condensers ...............................5-2Evaporative Condensers ..................................5-4Condenser Capacity .........................................5-5Condensing Temperature ................................. 5-5Non-Condensable Gases .................................5-5Condensing Temperature Difference ................5-6

Section 6. EVAPORATORS

Types of Evaporators........................................6-1Blower Coil Construction ..................................6-1Pressure Drop and Other Factors

in Evaporator Design .................................6-2Evaporator Capacity .........................................6-2Temperature Difference and

Dehumidication ........................................6-2Defrosting of Blower Coils ................................6-3

Section 7. CONTROL DEVICES, REFRIGERANT

Thermostatic Expansion Valves .......................7- 1Other Types of Expansion Valves .....................7- 2Distributors .......................................................7- 2Capillary Tubes .................................................7- 2Float Valves ......................................................7- 8Solenoid Valves ................................................ 7- 8Crankcase Pressure Regulating Valves ...........7- 9Evaporator Pressure Regulating Valve ............. 7- 9Hot Gas Bypass Valves .................................... 7- 9Reversing Valves ..............................................7-10Check Valves ....................................................7-10Manual Shut-Off Valves .................................... 7-11

Compressor Service Valves .............................7-11Schrader Type Valve ........................................ 7-11Pressure Relief Valves .....................................7-12Fusible Plugs ....................................................7-12Water Regulating Valves ..................................7-12

Section 8. CONTROL DEVICES, ELECTRICALControl Differential ............................................8-1Line Voltage and Low Voltage Controls ............8-1Low Pressure and High Pressure Controls ......8-1Condenser Fan Cycling Control .......................8-2Thermostats......................................................8-2Oil Pressure Safety Control ..............................8-2

Time Clocks ......................................................8-2Relays...............................................................8-3Time Delay Relay .............................................8-3Transformers ....................................................8-3


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Section 9. MOTORS

Motor Temperature ...........................................9-1Open Type Motors and Belt Drives ...................9-1Hermetic Motors ...............................................9-2Nameplate Amperage .......................................9-2Voltage and Frequency.....................................9-3Three Phase Motors .........................................9-3Single Phase Motors ........................................ 9-3Split Phase Motors ...........................................9-3Capacitor Start-Induction Run Motors

(CSIR) .......................................................9-4Capacitor Start-Capacitor Run Motors

(CSR) ........................................................9-4Permanent Split Capacitor Motors (PSC) .........9-5Dual Voltage Motors .........................................9-5Two Phase Motors ............................................9-6

Section 10. STARTING EQUIPMENT AND MOTORPROTECTORS

Contactors and Starters....................................10-1Capacitors ........................................................10-1Start Capacitors ................................................10-2Run Capacitors .................................................10-2Reduced Voltage Starting .................................10-3

Motor Protection ...............................................10-8Internal Inherent Line Break Protector.............. 10-8External Inherent Protector...............................10-9Internal Thermostats ......................................... 10-9External Thermostats .......................................10-9Current Sensitive Protectors............................. 10-9Thermotector ....................................................10-9Solid State Protectors .......................................10-9Fuses and Circuit Breakers .............................. 10-9Effect of Unbalanced Voltage and Current

on Three Phase Motor Protection .............10-10

Section 11. ACCESSORIES

Receivers..........................................................11-1Heat Exchangers ..............................................11-1Suction Accumulators .......................................11-1Oil Separators...................................................11-2Dehydrators ......................................................11-2Suction Line Filters ...........................................11-2Vibration Eliminators .........................................11-2Strainers ........................................................... 11-3Sight Glass and Moisture Indicators .................11-3Discharge Mufers............................................11-3Crankcase Heaters ...........................................11-3Refrigeration Gauges .......................................11-4


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SECTION 4COMPRESSORS

RECIPROCATING COMPRESSORSThe design of the reciprocating compressor is somewhatsimilar to a modern automotive engine, with a pistondriven from a crankshaft making alternate suctionand compression strokes in a cylinder equipped withsuction and discharge valves. Since the reciprocatingcompressor is a positive displacement pump, it is suitablefor small displacement volumes, and is quite efcient athigh condensing pressures and high compression ratios.Other advantages are its adaptability to a number ofdifferent refrigerants, the fact that liquid refrigerant maybe easily run through connecting piping because of thehigh pressure created by the compressor, its durability,

basic simplicity of design, and relatively low cost.

An exploded view of a typical Copelametic accessible-hermetic motor-compressor is shown in Figure 10.

The compressor has two functions in the compressionrefrigeration cycle. First it removes the refrigerant vaporfrom the evaporator and reduces the pressure in theevaporator to a point where the desired evaporatingtemperature can be maintained. Second, the compressorraises the pressure of the refrigerant vapor to a levelhigh enough so that the saturation temperature is higherthan the temperature of the cooling medium availablefor condensing the refrigerant vapor.

There are three basic types of compressors; reciprocating,rotary, and centrifugal. Centrifugal compressors arewidely used in large central air conditioning systems,and rotary compressors are used in the domestic

refrigerator eld, but the overwhelming majority ofcompressors used in the smaller horsepower sizesfor commercial, domestic, and industrial applicationsare reciprocating, and this manual will cover onlyreciprocating compressors.

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OPEN TYPE COMPRESSORS

Early models of refrigeration compressors were of theso-called open type, with the pistons and cylinders sealedwithin a crankcase, and a crankshaft extending throughthe body for an external power source. A shaft sealaround the crankshaft prevented the loss of refrigerantand oil from the body.

Although at one time open type compressors were widelyused, they have many inherent disadvantages such asgreater weight, higher cost, larger size, vulnerability toseal failures, difcult shaft alignment, excessive noise,and short life of belts or direct drive components. Asa result, the open type compressor has been largelyreplaced with the accessible-hermetic and hermetictype motor-compressor in most applications, and theuse of open type compressors continues to declineexcept for specialized applications such as automobileair conditioning.

ACCESSIBLE-HERMETIC MOTOR-COMPRESSORS

The accessible-hermetic motor-compressor design waspioneered by Emerson Climate Technologies, Inc. andis widely used in the popular Copelametic models.The compressor is driven by an electric motor mounteddirectly on the compressor crankshaft, with both the motorand the compressor working parts hermetically sealedwithin a common enclosure. The troublesome shaft seal

is eliminated, motors can be sized specically for theload to be handled, and the resulting design is compact,economical, efcient, and basically maintenance free.

Removable heads, stator covers, bottom plates, andhousing covers allow access for easy eld repairs inthe event of compressor damage.

WELDED HERMETIC MOTOR-COMPRESSORS

In an effort to further decrease size and cost, the weldedhermetic motor-compressor has been developed, andis widely used in small horsepower unitary equipment.

As in the case of the accessible-hermetic motor-compressor an electric motor is mounted directly on thecompressor crankshaft, but the body is a formed metalshell hermetically sealed by welding. No internal eldrepairs can be performed on this type of compressorsince the only means of access is by cutting open thecompressor shell.

COMPRESSOR SPEEDEarly models of compressors were designed for relatively

slow speed operation, well below 1,000 RPM. In orderto utilize standard 4 pole electric motors, accessible-hermetic and hermetic motor-compressors introducedoperation at 1,750 RPM (1,450 RPM on 50 cycle). Theincreasing demand for lighter weight and more compactair conditioning equipment has been instrumental in thedevelopment of hermetic motor-compressors equippedwith 2 pole motors operating at 3,500 RPM (2,900 RPMon 50 cycle).

Specialized applications such as aircraft, automotive, ormilitary air conditioning equipment utilize even higherspeed compressors, but for the normal commercialand domestic application, the existing 60 cycle electric

power supply will generally limit compressor speeds tothe presently available 1,750 and 3,500 RPM.

Higher compressor speeds introduce lubrication andlife problems, and these factors as well as cost, sizeand weight must be considered in compressor designand application.

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CROSS-SECTIONALVIEWO

FCOPELAMETICMOTOR-COMPR

ESSOR


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BASIC COMPRESSOR OPERATION

A cross-sectional view of a typical Copelametic motor-compressor is shown in Figure 13. Following is a briefdescription of its operation.

As the piston moves downward on the suction stroke,pressure is reduced in the cylinder. When the pressurefalls below that in the compressor suction line, thepressure differential causes the suction valves toopen and forces the refrigerant vapor to ow into thecylinder.

As the piston reaches the bottom of its stroke andstarts upward on the compression stroke, pressure isdeveloped in the cylinder, forcing the suction valvesclosed. The pressure in the cylinder continues to riseas the piston moves upward, compressing the vaportrapped in the cylinder. When the pressure in thecylinder exceeds the pressure existing in the compressordischarge line, the discharge valves are forced open,and the compressed gas ows into the discharge lineand on into the condenser.

When the piston starts downward, the reduction inpressure allows the discharge valves to close becauseof the higher pressure in the condenser and dischargeline, and the cycle is repeated.

For every revolution of the crankshaft, there is both asuction and compression stroke of each piston, so in1,750 RPM motor-compressors there are 1,750 completecompression and suction cycles in each cylinder eachminute, and in 3,500 RPM motor-compressors, 3,500complete cycles each minute.

SUCTION AND DISCHARGE VALVES

Since the parts of the compressor most apt to requireservice are the suction and discharge valves, onCopelametic compressors these valves are mountedon a valve plate which can be removed for easy serviceor replacement. A typical valve plate is shown in Figure10, part number 11.

Most reciprocating compressor valves are of the reedtype, and must seat properly to avoid leakage. The leastbit of foreign material or corrosion under the valve willcause leakage and the utmost care must be used inprotecting the compressor against contamination.

COMPRESSOR DISPLACEMENT

The displacement of a reciprocating compressor is thevolume displaced by the pistons. Emerson Climate

Technologies, Inc. publishes the displacement of acompressor in terms of cubic feet per hour, but somemanufacturers rate their compressors in terms ofcubic inch displacement per revolution, or in cubic feetper minute. For comparative purposes, compressordisplacement may be calculated by the followingformulas:

DISPLACEMENT

CFM = x D x L x RPM x N4 x 1728

CFH = x D x L x RPM x N x 604 x 1728

Cu. In./Rev. = x D x L x N4

CONVERSION FACTORS

1750 RPM 3500 RPMCFH = 60 x CFM 60 x CFMCFH = 60.78 x 121.5 x

Cu. In./Rev. Cu. In./Rev.CFM = 1.013 x 2.025 x

Cu. In./Rev. Cu. In./Rev.Cu. In./Rev. = .01645 x CFH .00823 x CFHCFM = Cubic feet per minuteCFH = Cubic feet per hourCu. In./Rev. = Cu bi c in ch di sp la ce me nt pe rrevolution= 3.1416

D = Cylinder bore, inchesL = Length of stroke, inchesN = Number of cylindersRPM = Revolutions per minute1728 = Cubic inches per cubic foot

D = Area of a circle4

CLEARANCE VOLUME

As mentioned previously, the volumetric efciency ofa compressor will vary with compressor design. If thevalves seat properly, the most important factor affectingcompressor efciency is clearance volume.

At the completion of the compression stroke, there stillremains some clearance space which is essential ifthe piston is not to hit the valve plate. There is also agreat deal more space in the discharge valve ports inthe valve plate, since the discharge valves are on top ofthe valve plate. This residual space which is unswept bythe piston at the end of the stroke is termed clearancevolume, and remains lled with hot, compressed gasat the end of the compression stroke.

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When the piston starts down on the suction stroke, theresidual high pressure gas expands and its pressureis reduced. No vapor from the suction line can enterthe cylinder until the pressure in the cylinder has beenreduced below the suction line pressure. Thus, the rstpart of the suction stroke is actually lost from a capacitystandpoint, and as the compression ratio increases, agreater percentage of the suction stroke is occupied bythe residual gas.

With high suction pressures, the compression ratio islow and clearance volume is not critical from a capacitystandpoint. Additional clearance volume is also helpfulin reducing the compressor noise level. Since lowergas velocities through the discharge ports reduce bothwear and operating power requirements, on Copelandbrand air conditioning compressors, valve plates aredesigned with greater clearance volume by increasingthe diameter of the discharge ports.

On low temperature applications, it is often necessaryto reduce the clearance volume to obtain the desiredcapacity. Low temperature valve plates having smallerdischarge port sizes to reduce the clearance volume areused on low temperature Copelametic compressors.

LUBRICATION

An adequate supply of oil must be maintained in thecrankcase at all times to insure continuous lubrication.The normal oil level should be maintained at or slightlyabove the center of the sight class.

On all Copelametic compressors 5 H.P. and larger insize, and on 3 H.P. NR models, compressor lubricationis provided by means of a positive displacement oilpump. The pump is mounted on the bearing housing,and is driven from a slot in the crankshaft into which theat end of the oil pump drive shaft is tted.

Oil is forced through a hole in the crankshaft to thecompressor bearings and connecting rods. A springloaded ball check valve serves as a pressure reliefdevice, allowing oil to bypass directly to the compressorcrankcase if the oil pressure rises above its setting.

Since the oil pump intake is connected directly to thecompressor crankcase, the oil pump inlet pressurewill always be crankcase pressure, and the oil pumpoutlet pressure will be the sum of crankcase pressureplus oil pump pressure. Therefore, the net oil pumppressure is always the pump outlet pressure minus thecrankcase pressure. When the compressor is operatingwith the suction pressure in a vacuum, the crankcasepressure is negative and must be added to the pumpoutlet pressure to determine the net oil pump pressure.

A typical compound gauge is calibrated in inches ofmercury for vacuum readings, and 2 inches of mercuryare approximately equal to 1 psi.

For example:Pump Net Oil

Crankcase Outlet PumpPressure Pressure Pressure50 psig 90 psig 40 psi

8 vacuum 36 psig 40 psi(equivalent to a reading of minus 4 psig)

In normal operation, the net oil pressure will varydepending on the size of the compressor, the temperatureand viscosity of the oil, and the amount of clearance inthe compressor bearings. Net oil pressures of 30 to 40 psiare normal, but adequate lubrication will be maintainedat pressures down to 10 psi. The bypass valve is setat the factory to prevent the net pump pressure fromexceeding 60 psi.

The oil pump may be operated in either direction, thereversing action being accomplished by a friction platewhich shifts the inlet and outlet ports. After prolongedoperation in one direction, wear, corrosion, varnishformation, or burrs may develop on the reversing plate,and this can prevent the pump from reversing. Therefore,on installations where compressors have been inservice for some time, care must be taken to maintainthe original phasing of the motor if for any reason theelectrical connections are disturbed.

The presence of liquid refrigerant in the crankcase canmaterially affect the operation of the oil pump. Violentfoaming on start up can result in the loss of oil fromthe crankcase, and a resulting loss of oil pressure untiloil returns to the crankcase. If liquid refrigerant or arefrigerant rich mixture of oil and refrigerant is drawninto the oil pump, the resulting ash gas may resultin large variations and possibly a loss of oil pressure.Crankcase pressure may vary from suction pressuresince liquid refrigerant in the crankcase can pressurizethe crankcase for short intervals, and the oil pressuresafety switch low pressure connection shouldalways be connected to the crankcase.

During a rapid pull-down of the refrigerant evaporatingtemperature, the amount of refrigerant in solution in thecrankcase oil will be reduced, and may cause ash gas atthe oil pump. During this period the oil pump must pumpboth the ash gas and oil, and as a result the oil pressuremay decrease temporarily. This will merely cause the oilpump to bypass less oil, and so long as the oil pressureremains above 9 psi, adequate lubrication

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will be maintained. As soon as a stabilized condition isreached, and liquid refrigerant is no longer reaching thepump, the oil pressure will return to normal.

DRY AIR HOLDING CHARGE

All Copeland brand compressors are thoroughlydehydrated at the factory, and are shipped with a dry airholding charge. The pressure inside a factory processedcompressor is a guarantee that the compressor is leaktight, and the interior is absolutely dry. When installed,the compressor must be evacuated to remove the airfrom the system.

COMPRESSOR COOLING

Air cooled compressors require an adequate owof cooling air over the compressor body to preventthe compressor from overheating. The air ow fromthe fan must be discharged directly on the motor-compressor. Air drawn through a compartment in whichthe compressor is located usually will not cool thecompressor adequately.

Water cooled compressors are provided with a waterjacket or wrapped with a copper water coil, and watermust be circulated through the cooling circuit when thecompressor is in operation.

Refrigerant cooled motor-compressors are designedso that suction gas ows around and through themotor for cooling. At evaporating temperatures below0 F. additional motor cooling by means of air ow isnecessary since the decreasing density of the refrigerantgas reduces its cooling ability.

COMPRESSOR CAPACITY

Capacity data is available from the manufacturer oneach model of compressor for the refrigerants with whichthe compressor can be used. This data may be in theform of curves or in tabular form, and lists the BTU/hr.capacity at various saturated suction and dischargetemperatures.

It is difcult to estimate compressor capacities accuratelyon the basis of displacement and compression ratiobecause of design differences between different models,but occasionally these factors can be valuable inestimating the comparative performance of compressorson the same application.

TWO STAGE COMPRESSORSBecause of the high compression ratios encounteredin ultra-low temperature applications, two stage

compressors have been developed for increasedefciency when evaporating temperatures are in the-30 F. to -80 F. range.

Two stage compressors are divided internally into low(or rst) and high (or second) stages. On Copelametictwo stage compressors now in production, the ratio oflow stage to high stage displacement is 2 to 1. The threecylinder models have two cylinders on the low stage andone on the high, while the six cylinder models have fourcylinders on the low and two on the high.

The suction gas enters the low stage cylinders directlyfrom the suction line, and is discharged into the interstagemanifold at interstage pressure. Since the interstagedischarge vapor has a relatively high temperature,liquid refrigerant must be metered into the interstagemanifold by the desuperheating expansion valve toprovide adequate motor cooling and prevent excessivetemperatures during second stage compression. Thedischarge of the low stage enters the motor chamber

and crankcase, so the crankcase is at interstagepressure.

Desuperheated refrigerant vapor at interstage pressureenters the suction ports of the high stage cylinders, andis then discharged to the condenser at the condensingpressure.

See Figures 6 and 7 on pages 3-6 and 3-7 of Part Ifor typical two stage systems.

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TANDEM COMPRESSORSIt is often desirable to interconnect two compressors on asingle refrigeration system as a means of varying capacityaccording to the system requirement. This immediatelyintroduces lubrication problems, for unless the pressuresin the two crankcases are equalized, the oil will leavethe crankcase having the highest pressure.

In order to solve the troublesome problems of oilequalization and vibration of connecting oil lines whileobtaining the advantage of interconnected compressors,the tandem compressor was developed.

Basically this consists of two individual compressorswith an interconnecting housing replacing the individualstator covers. Since each compressor may be operatedindividually, the tandem provides simple, foolproofcapacity reduction with maximum power savings, andgreatly simplies system control.

The tandem offers a much greater factor of safety thana single compressor, and allows staggered starting toreduce inrush current requirements. In the event offailure of one of the compressors, emergency operationof the remaining compressor may be continued untilreplacement of the inoperative motor-compressor. Inorder to provide maximum protection for the system inthe event of the failure of one compressor, a suction linelter should always be provided in the suction line ofa tandem compressor, and an adequately sized liquidline lter-drier should be provided in the liquid line.

COMPRESSORS WITH UNLOADERS

In order to provide a means of changing compressorcapacity under uctuating load conditions, largercompressors are frequently equipped with unloaders.Unloaders on reciprocating compressors are of twogeneral types. In the rst, suction valves on one or morecylinders are held open by some mechanical means inresponse to a pressure control device. With the suctionvalves open, refrigerant vapor is forced back into thesuction chamber during the compression stroke, andthe cylinder performs no pumping action.

A second means of unloading is to bypass a portion ofthe discharge gas into the compressor suction chamber.Care must be taken to avoid excessive dischargetemperatures when this is done.

Copelametic compressors with unloaders have abypass valve so arranged that discharge gas from anunloaded cylinder is returned to the suction chamber.During the unloaded operation, the unloaded cylinderis sealed from the discharge pressure created by theloaded cylinders. Since both suction and dischargepressures on the unloaded cylinder are approximatelythe same, the piston and cylinder do no work otherthan pumping vapor through the bypass circuit, andthe problem of cylinder overheating while unloaded ispractically eliminated. Because of the decreased volumeof suction vapor returning to the compressor from thesystem and available for motor cooling, the operating

range of unloaded compressors must be restricted,and operation beyond established limits can causecompressor overheating.

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The condenser is basically a heat exchanger where theheat absorbed by the refrigerant during the evaporatingprocess is given off to the condensing medium. Asmentioned previously, the heat given off by the condenseris always greater than the heat absorbed during theevaporating process because of the heat of compression.

As heat is given off by the high temperature highpressure vapor, its temperature falls to the saturationpoint and the vapor condenses to a liquid, hence thename condenser.

AIR COOLED CONDENSERS

The most commonly used condenser is of tube and

external n construction, which dissipates heat tothe ambient air. Except for very small domestic units,which depend on gravity air circulation, heat transfer isefciently accomplished by forcing large quantities ofair through a compact condenser assembly. A typicalrefrigeration condensing unit equipped with an air cooledcondenser is shown in Figure 16.

Air cooled condensers are easy to install, inexpensiveto maintain, require no water, and there is no danger offreezing in cold weather. However, an adequate supplyof fresh air is necessary, and the fan may create noiseproblems in large installations. In very hot regions, therelatively high temperature of the ambient air may resultin high condensing pressures, but if the condenser

surface is amply sized, air cooled condensers can beused satisfactorily in all climatic regions. They have beenused very successfully for many years in hot and dryareas where water is scarce. Because of the increasingscarcity of water in densely populated areas, the useof air cooled condensers will undoubtedly increase inthe future.

When space permits, condensers may be made with asingle row of tubing, but in order to achieve compact size,condensers are normally constructed with a relativelysmall face area and several rows of tubing in depth. Asthe air is forced through the condenser, it absorbs heatand the air temperature rises. Therefore, the efciency

of each succeeding row in the coil decreases, althoughcoils up to eight rows in depth are frequently used.

Draw-through fans, which pull the air through thecondenser, result in a more uniform air ow throughthe condenser than the blow-through type. Since evenair distribution will increase the condenser efciency,draw-through type fans are normally preferred.

Most air cooled refrigeration systems which areoperated in low ambient temperatures are susceptibleto damage due to abnormally low head pressure, unlessadequate means of maintaining normal head pressureare provided. This is true, especially with refrigerated

truck units parked outdoors or in unheated garages, roofmounted refrigeration or air conditioning systems, or anysystem exposed to low outside ambient temperatures.The capacity of refrigerant control devices (expansionvalves, capillary tubes, etc.) is dependent upon thepressure difference across the device. Since they areselected for the desired capacity with normal operatingpressures, abnormally low head pressure reducingthe pressure difference across the expansion valve orcapillary tube, may result in insufcient refrigerant ow.This can cause erratic refrigerant feed to the evaporator,and may result in frosting of the evaporator coil on airconditioning applications. The lower refrigerant velocity,and possibly lower evaporator pressure, permits oil to

settle out and trap in the evaporator, sometimes causingshortage of oil in the compressor crankcase.

Several proprietary systems are available employingthe principle of partially ooding the condenser withliquid refrigerant to reduce condensing capacity. Someof these systems result in very stable condensingpressures, but usually they require a large increasein the refrigerant charge which may cause problemsin system performance. Controlling the condenser air

SECTION 5CONDENSERS

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ow by means of louvers is also an effective means ofcondensing pressure control. Cycling the condenser fanis a simple but less effective means of control.

WATER COOLED CONDENSERS

When adequate low cost condensing water is available,water cooled condensers are often desirable becauseof the lower condensing pressures and better headpressure control is possible. Water, particularly fromunderground sources, is frequently much colder thandaytime air temperatures. If evaporative cooling towersare used, the condensing water can be cooled to a pointclosely approaching the ambient wet bulb temperature.This allows the continuous recirculation of condensingwater and reduces water consumption to a minimum.

Because of waters excellent heat transfer characteristics,water cooled condensers can be quite compact. Severaldifferent types of construction are used including shelland coil, shell and tube, and tube within a tube styles.Normally the cooling water is run through tubing or coilswithin a sealed shell into which the hot gas is dischargedfrom the compressor. As the refrigerant condenses itcan be fed out the refrigerant liquid line, thus makingthe use of a separate receiver unnecessary. A watercooled condensing unit equipped with a shell and tubecondenser is shown in Figure 17.

A pressure or temperature sensitive modulating watercontrol valve can be used to maintain condensingpressures within the desired range by increasing ordecreasing the rate of water ow as necessary.

Cooling water circuits in compressors with water jacketsand in water cooled condensers may be either seriesor parallel as required by the particular application. Theuse of parallel circuits results in a lower pressure dropthrough the circuit, and may be necessary when thetemperature of the cooling water is such that the watertemperature rise must be held to a minimum.Occasionally condensers may be damaged by excessivewater velocities or cavitation on the water side ofthe condenser tubes. In order to prevent operatingdifculties, care should be taken to follow the installationrecommendations as outlined below:

1. Water velocities through the condenser should notexceed 7 feet per second. Higher velocities can resultin impingement corrosion. This is a condition in whichprogressive erosion of the tube can occur due to the highwater velocity washing away the inner oxidized surfaceof the tube at points where excessive turbulence mayoccur. This can originate with a minute imperfection onthe tube inner surface, but it becomes progressivelyworse as the pitting increases.

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Figure No.18 illustrates the type of circuiting normally used on all standard condensing units using city water supply. All water

cooled condensing units are shipped from the factory with the connections as shown above, and water connections must be

modied in the eld if parallel circuits are desired.

Figure No.19 illustrates a condenser with parallel circuits connected to a motor-compressor with a straight-through circuit. This

type of circuiting is frequently used when the condensing water is cooled by a water tower . The straight-through compressor

circuit would be used when connecting a motor-compressor wrapped with an external water coil.

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discharge connection with a high vertical drop couldresult in cavitation in a manner similar to a pump on

the outlet of the condenser.

EVAPORATIVE CONDENSERS

Evaporative condensers are frequently used wherelower condensing temperatures are desired than areobtainable with air cooled condensers, and where theavailable water supply may not be adequate for heavywater usage. The hot refrigerant vapor is piped througha spray chamber where it is cooled by evaporation of thewater coming in contact with the refrigerant tubing.

Water which is exposed to air ow in a spray chamberwill evaporate rapidly. Latent heat required for the

evaporating process is obtained by a reduction in sensibleheat and, therefore, a reduction in the temperature ofthe water remaining. An evaporative spray chambercan reduce the water temperature to a point closelyapproaching the wet bulb temperature of the air.

Wet bulb temperature is a term used in air conditioning todescribe the lowest temperature that can be obtained bythe evaporating process. The term wet bulb temperatureis derived from the fact that a common mercury bulbthermometer exposed to the ambient air

Figure No.20 shows parallel circuits in both water cooled condenser and the motor-compressor water jacket. Each water jacket

circuit is connected in series with one circuit of the split condenser. This type of water circuiting is used when a minimum of

water pressure drop is required.

In order to maintain water velocities at an acceptablelevel, parallel circuiting of the condenser may be

necessary when high water ow is required.2. If a water circulating pump is used, install so that thecondenser is fed from the discharge side of the pump. Ifthe pump were on the discharge side of the condenser,the condenser would have a slight vacuum in the watersystem, and therefore the water would be much nearerits boiling point. A combination of a localized hot spot inthe condenser together with a localized velocity increasethat might reduce pressures even lower, could result intriggering a cavitation condition.

Cavitation is basically a condition where a uctuatingcombination of pressure and temperature can cause

instantaneous boiling or ashing of water into vapor, withthe subsequent collapse of the bubbles as the conditionsvary. This can result in very rapid erosion and destructionof the water tube. Maintaining a positive pressure in thecondenser will prevent this condition.

3. If the condenser is installed more than 5 feet higherthan the outlet drain point of the condenser, a vacuumbreaker or open vent line should be provided to preventthe discharge line from creating a partial vacuumcondition in the condenser water system. An unvented

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indicates the dry bulb or ambient temperature, while if awick wetted with water is placed around the mercury bulband the thermometer is exposed to rapid air movement,the temperature indicated by the thermometer will be thewet bulb temperature. The difference between the drybulb and wet bulb readings is determined by the rate ofevaporation from the wet surface of the wick, and thisin turn is proportional to the moisture content or vaporpressure of the air. The wet bulb temperature is alwayslower than the dry bulb temperature, and for a given drybulb, the less the moisture content of the air, the lowerthe wet bulb temperature will be.

Since the cooling is accomplished by evaporation of thewater, water consumption is only a fraction of that usedin conventional water cooled applications in which thewater once used is discharged to a drain. Evaporativecondensing is therefore widely used in hot, arid regionsof the world.

Corrosion, scale formation, and the danger of freezingare problems that must be solved with both evaporativeand water cooled condensers. With both cooling towersand evaporative condensers, a bleed to a drain must beprovided to prevent the concentration of contaminantsin the cooling water.

CONDENSER CAPACITY

The heat transfer capacity of a condenser dependsupon several factors:

1. Surface area of the condenser .

2. Temperature difference between the cooling mediumand the refrigerant gas.

3. Velocity of the refrigerant gas in the condenser tubes.Within the normal commercial operating range, thegreater the velocity, the better the heat transfer factor,and the greater the capacity.

4. Rate of ow of the cooling medium over or throughthe condenser. Heat transfer increases with velocityfor both air and water, and in the case of air, it also

increases with density.

5. Material of which the condenser is made. Since heattransfer differs with different materials, more efcientmetals will increase the capacity.

6. Cleanliness of the heat transfer surface. Dirt, scale,or corrosion can reduce the heat transfer rate.

For a given condenser, the physical characteristicsare xed, and the primary variable is the temperature

difference between the refrigerant gas and thecondensing medium.

CONDENSING TEMPERATURE

The condensing temperature is the temperature at whichthe refrigerant gas is condensing from a vapor to a liquid.This should not be confused with the temperature ofthe cooling medium, since the condensing temperaturemust always be higher in order for heat transfer to takeplace.

In order to condense the refrigerant vapor owing intothe condenser, heat must ow from the condenser at thesame rate at which heat is introduced by the refrigerantgas entering the condenser. As mentioned previously,the only way in which the capacity of the condensercan be increased under a given set of conditions is byan increase in the temperature difference through thecondenser walls.

Since a reciprocating compressor is a positivedisplacement machine, the pressure in the condenserwill continue to increase until such time as thetemperature difference between the cooling medium andthe refrigerant condensing temperature is sufcientlygreat to transfer the necessary amount of heat. Witha large condenser, this temperature difference may bevery small. With a small condenser or in the event airor water ow to the condenser has been blocked, thenecessary temperature difference may be very large.This can result in dangerously high pressures, and it isessential that the condenser is operating properly anytime a refrigeration unit is in operation.

The condensing temperature and therefore thecondensing pressure is determined by the capacityof the condenser, the temperature of the coolingmedium, and the heat content of the refrigerant gasbeing discharged from the compressor, which in turn isdetermined by the volume, density and temperature ofthe gas discharged.

NON-CONDENSABLE GASES

Air is primarily composed of nitrogen and oxygen, andboth elements remain in gaseous form at all temperaturesand pressures encountered in commercial refrigerationand air conditioning systems. Therefore, although thesegases can be liqueed under extremely high pressuresand extremely low temperatures, they may be consideredas non-condensable in a refrigeration system.

Scientists have discovered that one of the basic laws ofnature is the fact that in a combination of gases, eachgas exerts its own pressure independently of others,

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and the total pressure existing in a system is the totalof all the gaseous pressures present. A second basiccharacteristic of a gas is that if the space in which it isenclosed remains constant, so that it cannot expand,its pressure will vary directly with the temperature.Therefore, if air is sealed in a system with refrigerant,the nitrogen and oxygen will each add their pressureto the system pressure, and this will increase as thetemperature rises.

Since the air is non-condensable, it will usually trapin the top of the condenser and the receiver. Duringoperation the compressor discharge pressure will be acombination of the refrigerant condensing pressure plusthe pressure exerted by the nitrogen and oxygen. Theamount of pressure above normal condensing pressurethat may result will depend on the amount of trapped air,but it can easily reach 40 to 50 psig or more. Any time asystem is running with abnormally high head pressure,air in the system is a prime suspect .

CONDENSING TEMPERATURE DIFFERENCE

A condenser is normally selected for a system by sizing itto handle the compressor load at a desired temperaturedifference between the condensing temperature andthe expected temperature of the cooling medium.Most air cooled condensers are selected to operate ontemperature differences (commonly called TD) of 20F. to 30 F. at design conditions, but higher and lowerTDs are sometimes used on specialized applications.Standard production air cooled condensing units areoften designed with one condenser for a wide rangeof applications. In order to cover as wide a range aspossible, the TD at high suction pressures may be from30 F. to 40 F., while at low evaporating temperaturesthe TD often is no more than 4 F. to 10 F. The designcondensing temperature on water cooled units is normallydetermined by the temperature of the water supply andthe water ow rate available, and may vary from 90 F.to 120 F.

Since the condenser capacity must be greater than theevaporator capacity by the heat of compression and themotor efciency loss, the condenser manufacturer mayrate condensers in terms of evaporator capacity, or mayrecommend a factor to allow for the heat of compressionin selecting the proper condenser size.

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SECTION 6EVAPORATORS

The evaporator is that part of the low pressure side ofthe refrigeration system in which the liquid refrigerantboils or evaporates, absorbing heat as it changes into avapor. It accomplishes the actual purpose of the system,refrigeration.

TYPES OF EVAPORATORS

Evaporators are made in many different shapes andstyles to ll specic needs. The most common styleis the blower coil or forced convection evaporator inwhich the refrigerant evaporates inside of nned tubes,extracting heat from air blown through the coil by a fan.However, specic applications may use bare coils with

no ns, gravity coils with natural convection air ow,at plate surface, or other specialized types of heattransfer surface.

Direct expansion evaporators are those in which therefrigerant is fed directly into the cooling coil through ametering device such as an expansion valve or capillarytube, absorbing the heat directly through the walls ofthe evaporator from the medium to be cooled. Figure21 shows a direct expansion coil of one manufacturerprior to assembly in a blower unit.

In other types of systems, secondary refrigerants suchas chilled water or brine may be used for the actualspace or product refrigeration while the evaporator is thewater or brine chiller. A complete packaged water chiller,designed to furnish chilled water for air conditioning orother cooling applications is shown in Figure 22.

BLOWER COIL CONSTRUCTION

A typical blower coil is made up of a direct expansioncoil, mounted in a metal housing complete with a fan forforced air circulation. The coil is normally constructedof copper tubing supported in metal tube sheets, withaluminum ns on the tubing to increase heat transferefciency.

If the evaporator is quite small, there may be only onecontinuous circuit in the coil, but as the size increases,the increasing pressure drop through the longer circuit

makes it necessary to divide the evaporator into severalindividual circuits emptying into a common header. Thevarious circuits are usually fed through a distributor whichequalizes the feed in each circuit in order to maintainhigh evaporator efciency.

The spacing of ns on the refrigerant tubing will varydepending on the application. Low temperature coils mayhave as few as two ns per inch, while air conditioningcoils may have up to twelve per inch or more. In general

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if the evaporator temperature is to be below 32 F. sothat frost will accumulate, n spacings of 4 per inch orless are commonly used, although closer n spacingsare sometimes used if efcient defrost systems areavailable. In air conditioning applications, icing of thecoil is seldom a problem, and the limit on n spacingmay be dictated by the coils resistance to air ow.

Since the heat transfer efciency of the coil increaseswith an increase in the mass ow of air passing throughit, high velocities are desirable. However at face velocitiesgreater than 500 to 600 FPM, water collecting on the coilfrom condensation will be blown off into the air stream,and except for specialized applications, these velocitiesare seldom exceeded.

PRESSURE DROP AND OTHER FACTORS INEVAPORATOR DESIGN

As mentioned previously, pressure drop occurring in theevaporator results in a loss of system capacity due tothe lower pressure at the outlet of the evaporator coil.With a reduction in suction pressure, the specic volumeof the gas returning to the compressor increases, andthe weight of the refrigerant pumped by the compressordecreases.

However there are other factors which must also beconsidered in evaporator design. If the evaporator tubingis too large, refrigerant gas velocities may become solow that oil will accumulate in the tubing and will not be

returned to the compressor. The only means of assuringsatisfactory oil circulation is by maintaining adequate gasvelocities. The heat transfer ability of the tubing may alsobe greatly decreased if velocities are not sufcient toscrub the interior tubing wall, and keep it clear of an oillm. The goals of low pressure drop and high velocitiesare directly opposed, so the nal evaporator design mustbe a compromise.

Pressure drops through the evaporator of approximately1 to 2 psi are acceptable on most medium and hightemperature applications, and 1/2 to 1 psi are commonin low temperature evaporators.

EVAPORATOR CAPACITY

The factors affecting evaporator capacity are quite similarto those affecting condenser capacity.

1. Surface area or size of the evaporator .

2. Temperature difference between the evaporatingrefrigerant and the medium being cooled.

3. Velocity of gas in the evaporator tubes. In the normal

commercial range, the higher the velocity the greaterthe heat transfer rate.

4. The velocity and rate of ow over the evaporatorsurface of the medium being cooled.

5. Material used in evaporator construction.

6. The bond between the ns and tubing is quiteimportant. Without a tight bond, heat transfer will begreatly decreased.

7. Accumulation of frost on evaporator ns. Operationat temperatures below freezing with blower coils willcause the formation of ice and frost on the tubesand ns. This can both reduce the air ow over theevaporator and reduce the heat transfer rate.

8. Type of medium to be cooled. Heat ows almost vetimes more effectively from a liquid to the evaporatorthan from air .

9. Dewpoint of the entering air. If the evaporatortemperature is below the dewpoint of the enteringair, latent as well as sensible cooling will occur.

TEMPERATURE DIFFERENCE ANDDEHUMIDIFICATION

Since for a given installation, the physical characteristicsare xed, the primary variable as in the case of the

condenser, is the temperature difference between theevaporating refrigerant and the medium being cooled,commonly called the TD. For a blower coil, the colderthe refrigerant with respect to the temperature of the airentering the evaporator, the greater will be the capacityof the coil.

Temperature differences of 5 F. to 20 F. are commonlyused. Usually for best economy, the TD should be keptas low as possible, since operation of the compressorwill be more efcient at higher suction pressures.

The amount of moisture condensed out of the air isin direct relation to the temperature of the coil, and a

coil operating with too great a differential between theevaporating temperature and the entering air temperaturewill tend to produce a low humidity condition in therefrigerated space. In the storage of leafy vegetables,meats, fruits, and other similar perishable items, lowhumidity will result in excessive dehydration and damageto the product. For perishable commodities requiring avery high relative humidity (approximately 90%) a TDfrom 8 F. to 12 F. is recommended, and for relativehumidities slightly lower (approximately 80%) a TD from12 F. to 16 F. is normally adequate.

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SECTION 7CONTROL DEVICES, REFRIGERANT

In modern refrigeration practice, a wide variety ofrefrigerant control devices are used to obtain efcienteconomic operation. Small systems with manual controlor simple on-off automatic control may require only oneor two controls, but large systems with more elaborateautomatic control may have a multitude of controls,the proper operation of each being essential to thesatisfactory performance of the system.

In order to adjust a control for efcient performance,or recognize the effect of a malfunction, it is essentialthat the function, operation, and application of eachrefrigeration control be completely understood.

THERMOSTATIC EXPANSION VALVES

The most commonly used device for controlling the owof liquid refrigerant into the evaporator is the thermostaticexpansion valve. An orice in the valve meters the owinto the evaporator, the rate of ow being modulatedas required by a needle type plunger and seat, whichvaries the orice opening.

The needle is controlled by a diaphragm subject to threeforces. The evaporator pressure is exerted beneath thediaphragm tending to close the valve. The force of asuperheat spring is also exerted beneath the diaphragmin the closing direction. Opposing these two forces is

the pressure exerted by the charge in the thermal bulb,which is attached to the suction line at the outlet of theevaporator.

It is most convenient to visualize the action of thethermostatic expansion valve by considering thethermal bulb charge to be the same refrigerant as thatbeing used in the system. With the unit in operation,the refrigerant in the evaporator is evaporating at itssaturation temperature and pressure. So long as thethermal bulb is exposed to a higher temperature itwill exert a higher pressure than the refrigerant in theevaporator, and therefore the net effect of these twopressures is to open the valve. The superheat spring

pressure is a xed pressure causing the valve to closewhenever the net difference between the bulb pressureand the evaporator pressure is less than the superheatspring setting.

As the temperature of the refrigerant gas leaving theevaporator rises (an increase in superheat) the pressureexerted by the thermal bulb at the outlet of the coilincreases, and the expansion valve ow increases; asthe temperature of the leaving gas decreases (a decreasein superheat) the pressure exerted by the thermal bulbdecreases, and the expansion valve closes slightly andthe ow decreases.

With an evaporator and an expansion valve correctlysized for the load, the expansion valve feed will be quitestable at the desired superheat setting. An oversizedexpansion valve or an oversized evaporator can causeerratic feeding of the evaporator, which may result inlarge uctuations in compressor suction pressure, andpossible liquid return to the compressor.

Because of the pressure drop due to refrigerant owthrough the evaporator, the evaporating pressure at theoutlet of the evaporator coil will be lower than that at the

expansion valve. If this pressure drop is of any magnitude,a higher superheat will be required to bring the forcesacting on the valve diaphragm into equilibrium, andthe evaporator will be partially starved. To compensatefor pressure drop through the evaporator, an externalequalizer connection is often used on the expansionvalve. This introduces the evaporator outlet pressureunder the valve diaphragm, rather than the evaporatorinlet pressure, and the valve operation is then free fromany inuence due to evaporator pressure drop. Valveswith external equalizer connections are recommendedwhenever the pressure drop through the evaporator

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is above 2 1/2 psi for high temperature applications,1 1/2 psi in the medium temperature range, and 1/2psi in the low temperature range. Valves with externalequalizers must be employed when a pressure droptype of distributor is used.

Pressure limiting expansion valves are often used to limitthe power requirement of the compressor. The valve isconstructed in such a manner that it limits the suctionpressure to a given maximum value, and restricts therefrigerant feed if the suction pressure rises above thatpoint.

Gas charged pressure limiting valves have a limitedcharge, and at temperatures of the thermal bulbequivalent to its maximum operating pressure, all of theliquid charge has vaporized, and any further increasein temperature can only superheat the gas, but cannotexert additional pressure. Any increase in evaporatorpressure will then act as a closing force on the expansionvalve. The disadvantage of the gas charged valve is thepossibility of the limited charge condensing in the headof the expansion valve, if the head is colder than thethermal bulb, causing the valve to lose control of theliquid feed. With gas charged valves, the thermal bulbmust always be colder than the head of the valve, andthe gas charged valve normally is used only on hightemperature applications such as air conditioning.

Mechanical limiting valves are available, usually witha spring loaded double diaphragm type construction.If the evaporator reaches a preset pressure, thediaphragm collapses, and the valve feed is restricteduntil the pressure decreases sufciently for the springtension to restore the diaphragm to its normal operatingposition.

In order to achieve closer control for varying applications,expansion valves are available with different typesof charge in the thermal bulb, each having differentoperating characteristics. The superheat spring is alsonormally equipped with an external adjusting screw sothat it can be set for the desired superheat on a givenapplication. Before adjusting any expansion valve, theexact characteristics of the valve should be thoroughly

understood. The manufacturers catalog data must beconsulted for detailed information on a given valve.

OTHER TYPES OF EXPANSION VALVES

The automatic expansion valve is really better describedas a constant pressure expansion valve, since itmodulates its feed to maintain a constant preset pressurein the evaporator. The automatic expansion valve waswidely used at one time, but because of its tendency

to starve the evaporator on heavy loads, and ood theevaporator on light loads, it has been largely replaced bythe thermostatic expansion valve and capillary tubes.

Hand expansion valves are sometimes used when anoperator is available and manual liquid refrigerant feedis acceptable. A needle valve is adjusted as required tomaintain the desired ow.

DISTRIBUTORS

When the refrigeration load is such that large evaporatorsare required, multiple refrigerant circuits are necessaryto avoid excessive pressure drop through the evaporator.To insure uniform feed from the expansion valve to eachof the various circuits, a refrigerant distributor is normallyused. A typical distributor mounted on a direct expansioncoil is shown in Figure 21, page 6-1.

As liquid refrigerant is fed through the expansion valve,a portion of the liquid ashes into vapor in order toreduce the liquid temperature to evaporator temperature.This combination of liquid and ash gas is fed intothe distributor from the expansion valve, and is thendistributed evenly through small feeder tubes, the numberdepending on the construction of the distributor and thenumber of circuits required to provide proper refrigerantvelocity in the evaporator .

Without the distributor, the ow would separate intoseparate gas and liquid layers, resulting in the starvingof some evaporator circuits. To avoid variations in circuitfeed, extreme care must be taken to insure that tubinglengths are equal, so equal resistance is offered byeach circuit.

There are two different approaches in the design of adistributor. A high-pressure drop distributor dependson the turbulence created by an orice to achievegood distribution. A low-pressure drop distributordepends on a contour ow pattern with high velocity inthe distributor throat to give proper distribution of therefrigerant ow. Both types of distributor give satisfactoryperformance when properly applied in accordance withthe manufacturers instructions.

CAPILLARY TUBES

On small unitary equipment such as package airconditioners, domestic refrigeration equipment, andself-contained commercial refrigeration cases, capillarytubes are widely used for liquid refrigerant control. Acapillary tube is a length of tubing of small diameter withthe internal diameter held to extremely close tolerances.It is used as a xed orice to perform the same function

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CAPILLARY TUBE SELECTION R-22HIGH TEMPERATURE

45 F. evaporating temperature (Preliminary Selection Only)Final Selection Should Be Determined by Unit Test

**Length to balance unit at 45 F. evaporating,130 F. condensing, 10 F.Sub-cooling.

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CAPILLARY TUBE SELECTION R-22MEDIUM TEMPERATURE

25F, to 10F. Evaporating Temperature (Preliminary Selection Only)Selection Should Be Determined by Unit Test

**Length to balance unit with 115F. condensing, F. sub-cooling incondenser, Heat Exchanger to give 15F. sub-cooling.

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CAPILLARY TUBE SELECTION R-12MEDIUM TEMPERATURE

25F, to 10F. Evaporating Temperature (Preliminary Selection Only)Final Selection Should Be Determined by Unit Test

**Length to balance unit with 115F. condensing, 5F.sub-cooling in condenser, Heat Exchanger to give 15F.sub-cooling.

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CAPILLARY TUBE SELECTION R-22LOW TEMPERATURE

15F. to 25F. Evaporating Temperature(Preliminary Selection Only)Final Selection Should Be Determined by Unit Test

*Length to balance unit at 110F. condensing and 20F.Liquid sub-cooling (15F. in condenser, 15F in heatexchanger)

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CAPILLARY TUBE SELECTION R-502LOW TEMPERATURE

15F. to 25F. Evaporating Temperature(Preliminary Selection Only)Final Selection Should Be Determined by Unit Test

*Length to balance unit at 110F. condensing and 20F.Liquid sub-cooling (15F. in condenser, 15F in heatexchanger)

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as the expansion valve, to separate the high and lowpressure sides of the system, and meter the proper feedof liquid refrigerant.

Since there are no moving parts, it is simple and troublefree if kept free of foreign material. A capillary tube is ofvery small diameter, and absolute freedom from foreignmatter and moisture is essential, making a factory sealedunit a practical necessity.

Since the orice is xed, the rate of feed is relativelyinexible. Under conditions of constant load, and constantdischarge and suction pressures, the capillary tubeperforms very satisfactorily. However, changes in theevaporator load or uctuations in head pressure canresult in under or over feeding of the evaporator.

A major advantage of the capillary tube in some systemsis the fact that refrigerant continues to ow into theevaporator after the compressor stops operation, thusequalizing pressures on the high and low sides ofthe system. This allows the use of low starting torquemotors.

The refrigerant charge is critical in capillary tubesystems since normally there is no receiver to storeexcess refrigerant. Too much refrigerant will causehigh discharge pressures and motor overloading, andpossible liquid oodback to the compressor during theoff cycle; too little will allow vapor to enter the capillarytube causing a loss in system capacity.

Due to its basic simplicity, the elimination of the need fora receiver, and the low starting torque requirement, acapillary tube system is the least expensive of all liquidcontrol systems.

Sizing of a capillary tube is difcult to calculate accurately,and can best be determined by actual test on the system.Once determined, the proper size capillary tube canbe applied to identical systems, so it is well adapted toproduction units. Figures 24, 25, 26, 27, and 28 givetentative selection data for capillary tubes.

FLOAT VALVES

On some specialized applications, it may be desirableto operate with completely ooded systems, that is, withthe evaporator completely lled with liquid refrigerant. Atypical application might be an industrial process coolinginstallation where a brine or liquid is piped through a chillershell in which the refrigerant level is to be maintained.Special liquid level controls are available from expansionvalve manufacturers. These normally are mounted in asecondary oat chamber and modulate ow as necessary

to maintain a given liquid level. Such applications arequite specialized and the manufacturers instructionsshould be followed closely. Unless some means isprovided for positive oil return, oil may accumulate in aoat chamber causing lubrication difculties.

Commercial or domestic applications using either highside or low side oat chambers for liquid feed havebeen largely replaced by capillary tube and expansionvalve control.

SOLENOID VALVES

A solenoid valve is an electrically controlled refrigerantow control valve. It is not a modulating valve, and iseither open or closed.

The valve consists of a body, a plunger with an ironcore which seats in the valve orice, and an electricalsolenoid coil. A normally closed solenoid valve is closedwhen the coil is deenergized and the plunger is seated.When the solenoid coil is energized, the magnetic effectof the coil lifts the plunger and opens the valve. Normallyopen valves with a reverse type action are made, butare rarely used.

Solenoid valves are commonly used in refrigerantliquid and hot gas lines to stop refrigerant ow whennot desired, or to isolate individual evaporators when

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multiple evaporators are used. On large installations,large numbers of solenoid valves may be necessary forsatisfactory automatic control.

CRANKCASE PRESSURE REGULATING VALVES

This type of valve, commonly called a CPR valve ora holdback valve, limits the suction pressure at thecompressor below a preset limit to prevent overloadingof the compressor motor. The valve setting is determinedby a pressure spring, and the valve modulates fromfully open to fully closed in response to outlet pressure,closing on a rise in outlet pressure.

The crankcase pressure regulating valve should belocated in the suction line between the evaporator andthe compressor. Since the power requirement of thecompressor declines with a fall in suction pressure, theCPR valve is normally used to prevent motor overloadingon low temperature units during pulldown or defrostcycles. Use of the valve permits the application of a

larger displacement compressor without overloading agiven size motor, but pressure drop through the valvemay result in an unacceptable loss of system capacity

unless the valve is adequately sized.

EVAPORATOR PRESSURE REGULATING VALVE

On systems with multiple evaporators operating atdifferent temperatures, or on systems where theevaporating temperature cannot be allowed to fall belowa given temperature, an evaporator pressure regulatorvalve is frequently used to control the evaporatingtemperature. This valve, often called an EPR valve, acts

similarly to the crankcase pressure regulator, except thatit is responsive to inlet pressure. It should be located inthe suction line at the evaporator outlet.

An EPR valve modulates from fully open to fully closed,closing on a fall in inlet pressure, and its sole function isto prevent the evaporator pressure from falling below apredetermined value for which the regulator has beenset.

HOT GAS BYPASS VALVES

Hot gas bypass valves are used where it is desirableto modulate the compressor capacity and at the sametime prevent the suction pressure from falling toobjectionable low levels. These valves operate in the

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same fashion as crankcase pressure regulators sincethey are responsive to outlet pressure, modulate fromfully open to fully closed, and open in response to adecrease in downstream pressure. The constructionmust be suitable to withstand the high temperaturedischarge gas from the compressor.

Hot gas valves are set to maintain a desired minimumpressure by spring tension, and may be either director pilot operated. They are normally equipped with anexternal equalizer connection, which operates in thesame fashion as an external equalizer on an expansionvalve to compensate for pressure drops in the lines. Theexternal equalizer should be attached to the suction lineat the point where it is desired to control the suctionpressure.REVERSING VALVES

In recent years, usage of the heat pump principle toenable an air conditioning unit to supply both cooling andheating has become increasingly popular. Basically thisinvolves switching the functions of the evaporator andcondenser by a change in refrigerant ow as desired, sothat the indoor coil becomes the evaporator for coolingpurposes, and the condenser for heating usage. Theoutdoor coil in turn is a condenser during the coolingcycle, and an evaporator during the heating cycle.

To conveniently reverse the system operation, four-wayreversing valves have been developed. By means ofa slide action actuated by a solenoid, the connectionsfrom the compressor suction and discharge ports to theevaporator and condenser can be reversed at will.

Three-way valves are being increasingly used for hotgas defrosting. This valve enables the ow of hot gasfrom the compressor discharge valve to be shunted fromthe condenser to the evaporator for defrosting purposes,and then conveniently returned to the condenser whennormal cooling is resumed.

CHECK VALVES

It is often desirable to prevent refrigerant from reversing

its direction of ow during an off cycle, or during achange in the operating cycle. A simple spring loadedvalve such as shown in Figure 34 allows ow in onedirection only, and closes if pressures are such thatreverse ow could occur. Check valves may be usedin either liquid or gas lines, and are frequently used toprevent backow of liquid refrigerant or hot gas in lowambient condenser controls, and in reverse cycle heatpumps. Check valves used in refrigeration systemsshould be spring loaded to prevent noise and chattering

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which may be caused by pulsations in refrigerant oworiginating in the compressor.

MANUAL SHUT-OFF VALVES

Manual shut-off valves are often used so that portionsof the refrigeration system can be isolated for service orrepairs. Special valves designed for refrigeration usageare required to avoid leakage.

COMPRESSOR SERVICE VALVESCompressor suction and discharge service valves areshut-off valves with a manual operated stem. Most

service valves are equipped with a gauge port so thatthe refrigerant operating pressure may be observed.

When the valve is back-seated (the stem turned all theway out) the gauge port is closed and the valve is open.If the valve is front-seated (the stem turned all the wayin) the gauge port is open to the compressor and theline connection is closed. In order to read the pressurewhile the compressor is in operation, the valve shouldbe back-seated, and then turned in one or two turnsin order to slightly open the connection to the gaugeport. The compressor is always open to either the lineor the gauge port, or both if the valve is neither frontnor back-seated.

SCHRADER TYPE VALVE

The Schrader type valve is a recent developmentfor convenient checking of system pressures whereit is not economical, convenient, or possible to usethe compressor service valves with gauge ports. TheSchrader type valve is similar in appearance andprinciple to the air valve used on automobile or bicycletires, and must have a cap for the tting to insure leak-proof operation.

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This type of valve enables checking of the systempressure, or charging refrigerant without disturbingthe unit operation. An adaptor is necessary for thestandard servicemans gauge or hose connection to tthe Schrader type valve.

PRESSURE RELIEF VALVES

Safety relief valves are required by many localconstruction codes. Various types of relief valves areavailable, and the system requirement may be dictated bythe local code requirement. Normally code requirementsspecify that the ultimate strength of the high side partsshall be a minimum of 5 times the discharge or rupturepressure of the relief valve, and that all condensing unitswith pressure vessels exceeding 3 cubic feet intervalvolume shall be protected by a pressure relief device.Discharge may be to the atmosphere, or it may be adischarge from the high pressure side of the system tothe low pressure side.

A typical reseating type valve is shown in Figure 38.The valve opens at a preset pressure, and refrigerantis discharged until the pressure falls to the reseatingpoint.

Some Copeland brand compressors have reseatingtype pressure relief valves installed internally in thedischarge chamber which allow excessive pressuresto discharge to the suction chamber. A typical internaltype valve is shown in Figure 39.

Rupture disc type relief devices have a thin disc which isdesigned to rupture at a given relief pressure, dischargingthe refrigerant to the atmosphere.

FUSIBLE PLUGS

A fusible plug is a safety device with a metal inserthaving a specied melting point. The allowable meltingpoint is dened by code, but normally it is the saturationtemperature of the refrigerant at a pressure no greaterthan 40% of the ultimate bursting pressure of therefrigerant containing vessel, or the critical temperatureof the refrigerant, whichever is lower .

Fusible plugs are limited to units with pressure vesselsnot exceeding 3 cubic feet internal gross volume. Theyare used as a safety device in the event of re, areresponsive to temperature only and will not protectagainst excessively high pressures.

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WATER REGULATING VALVES

On water cooled condensers, a modulating waterregulating valve is normally used to economize onwater usage and to control condensing pressureswithin reasonable limits. Water valves may be eitherpressure or temperature actuated and act to throttleow as necessary.

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SECTION 8CONTROL DEVICES, ELECTRICAL

Both electrical and pneumatic controls are widely usedfor air conditioning and refrigeration system control.Pneumatic controls are primarily used on large centralsystems, while electric controls are used on applicationsof all sizes. Since electric controls are used almostexclusively in the commercial refrigeration eld, thismanual will cover only electric controls.

CONTROL DIFFERENTIAL

The basic function of most electrical control devices isto make or break an electric circuit which in turn controlsa contactor, a solenoid coil, or some other functioningpart of the system. Controls are available which may

make or break a circuit on either a rise or fall in pressureor temperature. The type of action required dependson the function of the control and the medium beingcontrolled.

The point at which a control closes a contact and makesa circuit is called the cut-in point. The point at which thecontrol opens the switch and breaks the circuit is calledthe cut-out point. The difference between the cut-in andcut-out points is known as the differential.

A very small differential maintains close control butcan cause short cycling of the compressor. A largedifferential will give a longer running cycle, but may

result in uctuations in the pressure or temperaturebeing controlled, so the nal operating differential mustbe a compromise.

The differential may be either xed or adjustable,depending on the construction of the control. Adjustmentof controls varies depending on the type and themanufacturer. On some controls, both the cut-in andcut-out points may be set at the desired points. On manypressure controls, the differential can be adjusted, andthis in turn may affect either the cut-in or the cut-outpoint.

LINE VOLTAGE AND LOW VOLTAGE CONTROLS

Line voltage controls are designed to operate on thesame voltage as that supplied to the compressor. Both110 and 220 volt controls are quite commonly used,and 440 volt controls are available but are seldomused due to the danger from high voltage at the wiringconnections.

Local codes often require low voltage controls, and acontrol circuit transformer may be used to reduce line

voltage to the control circuit voltage, usually 24 volts.

LOW PRESSURE AND HIGH PRESSURECONTROLS

A low pressure control is actuated by the refrigerantsuction pressure, and normally is used to cycle thecompressor for capacity control purposes, or as a lowlimit control. The low pressure control often is used asthe only control on small systems which can toleratesome uctuations in the temperature to be maintained.The standard low pressure control makes on a rise inpressure, and breaks on a fall in pressure.

A high pressure control senses the compressordischarge pressure, and is normally used to stop thecompressor in case of excessively high pressures.Since the allowable pressure limit varies with different

refrigerants, the proper high pressure control for therefrigerant in the system must be used. A high pressurecontrol makes on a fall in pressure and breaks on arise in pressure. Either manual reset or automatic resetcontrols are available, the choice depending on thedesired system operation.

Dual pressure controls are comprised of a low pressureand a high pressure control mounted in a single housingwith a single switch operated by either control.

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CONDENSER FAN CYCLING CONTROL

In order to maintain air cooled condensing pressuresat a satisfactory level during low ambient conditions,a condenser fan pressure control is frequently used.The control acts to break the circuit to the condenserfan on a drop in condensing pressure and makes thecircuit on a rise in condensing pressure. Since this isthe reverse of the action on a normal high pressurecontrol, this is often described as a reverse acting highpressure control.

THERMOSTATS

A thermostat acts to make or break a circuit in responseto a change in temperature. There are numerous typesof thermostats ranging from a simple bimetallic switch tomultiple switch controls operating from remote sensingbulbs. Thermostats may have a xed control point ormay have variable adjustments.

Normally a cooling thermostat will make on a rise intemperature and break on a fall in temperature, whilea heating thermostat will make on a fall in temperatureand break on a rise.

OIL PRESSURE SAFETY CONTROL

Special pressure controls have been developed toprotect the compressor against loss of oil pressure. The

control is actuated by the difference in pressure betweenthe outlet oil pressure of the oil pump and crankcasepressure. Since the inlet pressure of the oil pump isalways crankcase pressure, the net difference in thetwo pressures is the net lubrication oil pressure.

Oil pressure safety controls are available with bothadjustable and non-adjustable control settings, but thenon-adjustable type is preferred to avoid difcultiesarising from improper eld adjustment.

If the oil pressure falls below safe limits, the controlbreaks to stop the compressor. As an added renement,a time delay circuit is incorporated to delay the actionof the control for a period up to 2 minutes to allow thecompressor to establish oil pressure on start-up withoutnuisance tripping.

TIME CLOCKS

Frequently it is desirable to stop the compressoroperation for a period of time to allow defrosting. In orderto insure that this is done regularly at convenient times,a time clock can be used to either make or break wiringcircuits at preset time intervals. Clocks are available forboth 24 hour and 7 day cycles, and the defrost intervaland time of initiation and termination can be adjustedas desired.

Various types of defrost control circuits are commonlyused, such as time initiated, time terminated; timeinitiated, temperature terminated; or time initiated,pressure terminated. Normally on circuits with pressure ortemperature termination, an overriding time termination is

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provided in the event the defrost cycle for some reasonis abnormally prolonged.

RELAYS

A relay consists of a set of contacts together with amagnetic coil mechanism which controls the contactposition. The contacts may be normally open or normallyclosed when not energized, and a given relay may havefrom 1 to 5 or more sets of contacts. When the coil isenergized, the contacts reverse their action and makeor break various circuits as desired.

A relay may be used to control a large amperage load bymeans of a pilot circuit, to allow interlocking of controlson separate circuits, or for any application where remotecontrol is required.

Most relays are of the potential type, and are actuatedwhen the coil is energized with the proper voltage.

Current relays are actuated by a sufcient currentowing through the relay coil, and are normally usedwhen it is desirable to make or break a circuit when alarge change in current ow occurs. These are used insingle phase motor starting circuits, and occasionallyin safety circuits.

An impedance relay is similar to a normal potential relayexcept that the coil is wound so as to create a highresistance to current passage. When wired in parallelwith a normal relay, the high impedance (resistance)of the relay will shunt the current to the normal circuitand the impedance coil will be inoperative. If the normalcircuit is opened and the current must pass through theimpedance relay, the relay coil will be energized and theimpedance relay will operate. The voltage drop across therelay coil is so large that other magnetic coils in serieswith the impedance coil will not operate because of theresulting low voltage. Impedance relays are frequentlyused for safety lock-out circuits in the event of a motorprotector trip.

TIME DELAY RELAY

Some relays are constructed with a time delay actionso that the relay must be energized for a predeterminedlength of time before the magnetic coil can actuate thecontacts. The time delay is normally non-adjustable, butrelays are available with varying periods of delay.

This type of relay may be required for part winding startmotors; in circuits to prevent short cycling, or for otherspecialized applications.

TRANSFORMERS

A transformer is an electrical device for transferringelectrical energy from one circuit to another at adifferent voltage by means of electromagnetic induction.Transformers are frequently used in control circuits tostep voltage down from line voltage to a lower controlcircuit voltage. There are no moving parts and the actionof the transformer is determined by its coil windings.

The transformer output is limited by its size, buttransformers are available for almost any output desiredfrom a tiny alarm bell circuit to the giant transformersused on high voltage power transmission lines.

The selection of control circuit transformers can vitallyaffect the performance and life of many electricalcomponents in a refrigeration or air conditioningsystem.

An inadequate transformer supplying abnormally lowvoltage to the control circuit will result in improperoperation of contactors and/or motor starters due tochattering or sticking contacts, burned holding coils,or failure of contacts to properly close. Since any ofthese conditions can cause eventual system failure andpossible damage to the compressor, control transformersmust be properly sized.

Even though a proper size transformer has beenselected, care must be taken to avoid excessive voltagedrop in a low voltage control circuit. When using a 24volt system with a remote thermostat, wire of sufcientcurrent carrying capacity must be installed between thetransformer and the thermostat .

According to NEMA standards, a solenoid or contactormust operate satisfactorily at a minimum of

Documents

75872167 Copeland Refrigeration Manual Part 2 Refrigeration System Components