table of contents - tpc training · do occur in the compressor, they often point to problems in ......

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Table of Contents

Lesson One Introduction to Compressors............................................................3

Lesson Two Reciprocating Compressors ...........................................................21

Lesson Three Rotary, Helical, and Scroll Compressors.......................................37

Lesson Four Centrifugal Compressors................................................................51

Lesson Five Compressor Motors........................................................................63

Lesson Six Compressor Control and Protection...............................................79

Lesson Seven Compressor Maintenance, Troubleshooting, and Repair.............97

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Compressors

COMPRESSORS

Lesson One

Introduction toCompressors

43301

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1

4

Lesson

Introduction to Compressors

Function of the CompressorTypes of CompressorsOpen CompressorsHermetic CompressorsReciprocating CompressorsRotary Compressors

Helical (Screw) CompressorsScroll CompressorsCentrifugal CompressorsMultiple Compressor ApplicationsCompressor Replacement Considerations

TOPICS

After studying this Lesson, you should be able to…

• List the five kinds of air conditioning and refriger-ation compressors.

• Contrast the operation of positive-displacementand kinetic-displacement compressors.

• Explain how each kind of the five kinds of com-pressor raises the pressure of the refrigerantvapor.

• Define staging and cascading and explain whyeach is used.

• List important considerations in compressorreplacement.

OBJECTIVES

Kinetic 1.04 related to motionHermetic 1.05 airtightStaging 1.35 using multiple impellers (or com-

pressors) to produce high compression ratios

Cascade system 1.46 uses two or more com-plete refrigeration systems to obtain very lowtemperatures

KEY TECHNICAL TERMS

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Function of the Compressor

1.01 The compressor is the refrigeration system com-ponent that raises the pressure and temperature of therefrigerant vapor high enough for it to condense whenexposed to the condensing medium. In performing itsfunction, the compressor draws the cold refrigerant vaporfrom the evaporator through the suction line. Then thecompressor raises the pressure and temperature of thevapor by compressing it (reducing its volume), and forcesit out the discharge line to the condenser.

1.02 Because the compressor and condenser worktogether to condense the refrigerant vapor, the compres-sor is often considered part of the condensing unit. Thecompressor also functions with the metering device tomaintain the pressure difference between the high sideand low side of the system. Thus the compressor is a veryimportant component in the refrigeration system.

Types of Compressors

1.03 The five kinds of compressors used in refrig-eration systems are: reciprocating, rotary (vane), heli-cal (screw), scroll, and centrifugal. The reciprocating,rotary, helical, and scroll compressors are positive-dis-placement compressors. Positive-displacement com-pressors use a piston or other device to reduce the vol-ume of the refrigerant vapor in a compression cham-ber, as shown in Fig. 1-1. At one time, reciprocatingcompressors were the type most widely used in therefrigeration and air conditioning industry. However,because of their relatively high cost and low efficien-cy, they are being phased out in favor of helical, rotary,and the relatively new scroll compressors.

1.04 The centrifugal compressor is a kinetic-dis-placement compressor. It works on a different principlefrom the positive-displacement types. The word kineticrefers to motion. A kinetic-displacement compressorcompresses the refrigerant vapor by using an impeller(which is similar to a fan) to force the vapor from alarge chamber to a small tube at high speed, as shownin Fig. 1-2 on the following page. As you can see, thevolume of the chamber is not mechanically reduced asit is in a positive-displacement compressor.

1.05 Each of the five basic types of compressorscan be classified as either open or hermetic, dependingupon how it is combined with the motor that drives it.The motor on an open compressor is outside the com-pressor shell. The motor and compressor assembly ofa hermetic compressor are sealed together inside ashell or housing. Hermetic means airtight.

5

Most people who work with air conditioning or refrigeration equipment considerthe compressor the “heart” of any system. When maintaining or troubleshootinga system, the compressor is generally the focal point of any investigation. Forexample, the pressure and temperature conditions of the refrigerant entering andleaving the compressor tell you how the system is performing. When problemsdo occur in the compressor, they often point to problems in other components ofthe refrigeration system. In short, understanding compressor operation can be avery important diagnostic tool.

In this Lesson you will be introduced to the five basic types of compressors, theirmain parts, and how these parts work together to compress refrigerant vapor.You will also learn the differences between open and hermetic compressors andthe advantages, disadvantages, and common uses of each. This Lesson alsocovers the reasons for using multiple compressor systems and their basicdesign.

Chamber atfull volume

Chamber volume displaced

Compression chamber

CompressedvaporVapor

Piston

Piston

Fig. 1-1. Compression by positive displacement


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6 Lesson One

Open Compressors

1.06 The motor on an open compressor drives thecompressor either through a drive shaft and cou-plings, or through a drive belt and pulleys. Units thathave a drive shaft are called direct-drive open com-pressors. Units powered through a belt and pulleysare called belt-drive open compressors. Most belt-drive open compressors are reciprocating compres-sors. Figure 1-3 shows both types.

1.07 The chemical industry is the largest user ofopen compressors today, because chemical processingoften requires specially designed systems that mustmeet specific operating conditions. Some of these sys-tems must be very flexible, and all systems must be

easy to service to keep process lines running. Opencompressors meet these requirements. For example,you can easily change motor size on an open com-pressor to meet specific conditions. And you caninstall a motor to run on dc current or special voltages.On belt-drive types, you can change compressor speedby simply changing pulley sizes.

1.08 Open compressors have one major disadvan-tage, however. The seal around the drive shaft where itpasses through the compressor housing can wear outfrom friction caused by dirt or lack of oil. Or, the sealcan dry out if the system is shut down for long periods,such as for the winter. A worn or dry seal allows refrig-erant and oil to leak out, and moisture to enter the sys-tem. A low refrigerant charge reduces the system’scapacity, low oil can cause compressor damage, andmoisture combines with the refrigerant to form acids.

Hermetic Compressors

1.09 Unlike open compressors, hermetic compres-sors do not require a seal, because the motor is part ofthe compressor assembly and is sealed inside the com-pressor shell. The shell on a hermetic compressor iseither bolted together or welded together. Figure 1-4shows a hermetic compressor with a bolted shell. It isusually called a semi-hermetic compressor. It is boltedtogether and has gaskets between the parts of the shellto make the shell airtight. It is sometimes called a bolt-ed hermetic compressor or a serviceable hermeticcompressor. You can service these compressorsbecause you can disassemble them.

Impeller

Tube

Chamber

Fig. 1-2. Compression by kinetic displacement

Direct drive Belt drive

Fig. 1-3. Direct-drive and belt-drive open compressors


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Introduction to Compressors 7

1.10 A full hermetic compressor, like the oneshown in Fig. 1-5, is also called a sealed hermeticcompressor or welded hermetic compressor. The onlyway to service it is to cut through the shell. This fea-ture is a major disadvantage, because most shops donot have the equipment required to open and reseal afull hermetic compressor. Thus you must replace theentire unit if a full hermetic compressor breaks down.You can sometimes trade in the damaged unit to themanufacturer.

1.11 Most hermetic compressors operate by draw-ing the cold vapor from the suction line into the assem-bly over the motor windings. The cold vapor absorbsmotor heat and cools the motor. The motor heat in turnhelps vaporize any liquid refrigerant that might other-wise reach the compressor.

1.12 The motor adds very little superheat to thevapor in high-temperature refrigeration systems. Butit can add too much superheat to the refrigerant inlow-temperature systems. The result is that the dis-charge temperature becomes too high. This condi-tion causes the refrigerant oil to break down and notlubricate the compressor. The high heat can alsoburn the compressor valves and cause the formationof acid in the system if there is any air or moisture inthe system. Thus, you will often see air-cooled orwater-cooled hermetic motors used in low-tempera-ture systems.

1.13 Air-cooled hermetic motors are usually usedwhen the system has an air-cooled condenser. Eitherthe condenser fan or a separate fan blows air over thecompressor motor. Water-cooled motors are usedwhen the system has a water-cooled condenser. Thecondenser water passes through a water jacket or cool-ing tubes around the motor housing before it goes tothe condenser.

1.14 A disadvantage of hermetic compressors isthat a motor burnout can contaminate the entirerefrigeration system. A motor burnout is caused bya short circuit or overheating in the motor wind-ings. As a result, the insulation in the motor wind-ings can burn, producing moisture, acid, soot, var-nish, and hard carbon. All of these burnout productscan enter the refrigeration system, because themotor is in the system. If they do, you must cleanthem out of the system completely to preventanother breakdown.

Reciprocating Compressors

1.15 Reciprocating compressors use a piston toreduce the volume in a cylinder and compress therefrigerant vapor. There are two basic kinds: singleacting and double acting. In the single-acting type,compression takes place only on one side of the piston.Compression takes place on both sides of the piston inthe double-acting type.

Suction lineconnector

Discharge line connector

Fig. 1-4. Semi-hermetic compressor

Charging valve

Electricalconnection box

Dischargelineconnector

Suctionlineconnector

Fig. 1-5. Full-hermetic compressor


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8 Lesson One

1.16 Single-acting reciprocating compressors.The single-acting reciprocating compressor is themost common type of reciprocating compressorfound in both air conditioning and refrigerationapplications. As shown in Fig. 1-6, a rod connectsthe piston to a crankshaft, and a motor turns thecrankshaft to force the piston up and down. Valvesbetween the suction and discharge lines and thecylinder control the flow of refrigerant vapor intoand out of the cylinder.

1.17 To follow through one complete cycle of a sin-gle-acting reciprocating compressor, start with the pis-ton at the top of the cylinder in Fig. 1-6A. Both thesuction valve and discharge valve are closed as the pis-ton starts its downstroke. As the piston moves down,the volume of the cylinder increases and the pressurein the cylinder decreases because the cylinder issealed. When the pressure in the cylinder becomeslower than the suction-line pressure, the suction pres-sure opens the suction valve, as shown in Fig. 1-6B,and refrigerant vapor begins flowing into the cylinder.The piston continues to increase the volume in the

cylinder and draw in suction vapor until it reaches thebottom of its downstroke. The valve to the suction linecloses the instant the piston reaches the bottom of thecylinder and stops increasing the volume in the cylin-der. Thus the cylinder is again sealed, as shown in Fig.1-6C.

1.18 Next the piston moves up and decreases thevolume in the cylinder, compressing the refrigerantvapor and increasing its pressure. The vapor’s tem-perature also increases because of the mechanicalenergy expended by the compressor. This is calledthe heat of compression. When the vapor pressure inthe cylinder becomes higher than the vapor pressurein the discharge line, the cylinder pressure opens thedischarge valve. The piston continues upward andforces the compressed vapor out the discharge lineand into the condenser, as illustrated in Fig. 1-6D.The discharge valve closes the instant the pistonreaches the top of its upstroke and stops decreasingthe volume in the cylinder. The cycle then beginsagain.

1.19 The sizes and uses of single-acting recipro-cating compressors vary widely in both air condi-tioning and refrigeration. You will find small, one-cylinder models in water coolers, refrigerators, andwindow air conditioners. Models having up to 16cylinders are used in commercial and industrialrefrigeration and air conditioning systems. Thecylinders in multiple-cylinder compressors can bearranged in-line, in a V, a W, or a VW, as shown inFig. 1-7. They are available as direct-drive and belt-

Connectingrod

Piston

Crankshaft

A. Piston at top of cylinder

Suctionvalve

Suctionline

B. Downstroke C. Piston at bottom of cylinder D. Upstroke

Dischargeline

Dischargevalve

Fig. 1-6. The single-acting reciprocating compressor

In-line V W VW

Fig. 1-7. Cylinder arrangements in reciprocatingcompressors


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drive open types, and both full- and semi-hermetictypes. Hermetic reciprocating compressors range insize from under 1 hp up to 20 hp. The other types goup to 100 hp or more.

1.20 Double-acting reciprocating compressors.The first refrigeration compressors were double-act-ing reciprocating compressors with a horizontalcylinder. A few of these compressors are still operat-ing in industry. They have the same basic parts as thesingle-acting type, but the cylinders have suctionand discharge valves at both ends, as shown in Fig.1-8. The piston is rigidly attached to a rod that pass-es through a stuffing box at one end of the cylinder.A connecting rod links the piston rod to the crank-shaft.

1.21 As the piston moves in one direction, it com-presses the refrigerant vapor ahead of it, and drawsin refrigerant vapor behind it. When the pistonreaches the end of the stroke and reverses its direc-tion, the two sides of the piston reverse functions.

This feature gives the compressor the name “double-acting.”

1.22 Double-acting compressors are large, heavymachines requiring a strong foundation. They have along piston stroke and operate at low speed. The stuffingbox is the critical part of these compressors. The pres-sure and temperature on the box alternate from suctionconditions to discharge conditions. And, as the pistonrod slides back and forth through it, friction generates agreat deal of heat. Thus oil is pumped through the box tolubricate the seal and carry away some of the heat. Someliquid refrigerant also passes through an expansion valveand into a passage around the seal to carry away heat.

The Programmed Exercises on the next page willtell you how well you understand the material youhave just read. Before starting the exercises,remove the Reveal Key from the back of the book.Read the instructions printed on the Reveal Key.Follow these instructions as you work through theProgrammed Exercises.

Valves (4)

Piston

Stuffing box

Piston rod

Connecting rod

Fig. 1-8. Double-acting horizontal reciprocating compressor


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1-1. Name the five kinds of compressorsused in refrigeration systems.

1-2. A reciprocating compressor is an exam-ple of a(n) -displacementcompressor.

1-3. A centrifugal compressor is an exampleof a(n) -displacement com-pressor.

1-4. The chemical industry is the largestuser of compressors.

1-5. The compressor motor is sealed insidethe shell of a(n) compressor.

1-6. A disadvantage of a hermetic compres-sor is that a motor can cont-aminate the system.

1-7. What reduces the volume of the cylin-der in a reciprocating compressor?

1-8. Double-acting reciprocating compres-sors have the same basic parts as sin-gle-acting models, except that the cylin-ders have at both ends.

1-1. RECIPROCATING, ROTARY, HELI-CAL, SCROLL, AND CENTRIFUGAL

Ref: 1.03

1-2. POSITIVE

Ref: 1.03

1-3. KINETIC

Ref: 1.04

1-4. OPEN

Ref: 1.07

1-5. HERMETIC

Ref: 1.09

1-6. BURNOUT

Ref: 1.14

1-7. A PISTON

Ref: 1.15

1-8. SUCTION AND DISCHARGE VALVES

Ref: 1.20

10 Programmed Exercises



Rotary Compressors

1.23 The rotary compressor, like the reciprocatingcompressor, is a positive-displacement compressor.However, the rotary compressor uses a rotor instead ofa piston to reduce the volume in the compressionchamber. Two basic designs are available: single vaneand multiple vane.

1.24 Figure 1-9 shows an end view of a single-vanerotary compressor with the end cover plate removed.As you can see, the cylindrical rotor is offset on a shaftso that it touches the larger cylindrical compressionchamber only at one point. The shaft turns the rotor inthe chamber. The spring-loaded vane is mounted in thecompressor housing. It rides on the rotor and seals

against it to separate the intake and discharge sides ofthe rotor. A thin film of oil between the vane and rotorprovides lubrication and forms a vapor seal.

1.25 Multiple-vane rotary compressors havevanes mounted in the rotor itself instead of in thecompressor housing, as shown in Fig. 1-10. Eithersprings or centrifugal force hold the vanes againstthe wall of the compression chamber. The rotoritself never contacts the chamber wall in the multi-ple-vane design.

1.26 A single-vane rotary compressor raises thepressure of the refrigerant vapor as follows. Startingwith the rotor in the position shown in Fig. 1-11A, therotor has sealed the suction port, and the chamber is

Vane

Spring

Suctionport

Dischargevalve

A B C

Discharge Suction

Rotor

Vane

Spring

Rotor

Vane

Suction port

Drive shaft

Discharge valve

Fig. 1-9. Single-vane rotary compressor

Fig. 1-11. Compression cycle in a rotary compressor


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Fig. 1-10. Multiple-vane rotary compressor

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12 Lesson One

full of suction vapor. As the rotor turns, it compressesthe refrigerant vapor ahead of it because the volume ofthe chamber between the rotor contact point and thevane is being reduced, as shown in Fig. 1-11B. At thesame time, the suction port is uncovered, and the rotoris drawing in suction vapor behind it because the vol-ume is increasing on the back side of the rotor betweenits contact point and the vane. When the vapor pres-sure in the chamber ahead of the rotor becomes high-er than discharge line vapor pressure, the dischargevalve opens and the compressed vapor enters the dis-charge line, as shown in Fig. 1-11C. As the rotor’s con-tact point passes the vane, the cycle starts again. Themultiple-vane types operate the same way, except thatcompression takes place between the vanes.

1.27 Rotary compressors have no suction valves.Some have a check valve in the suction line to pre-vent the discharge vapor from leaking back to theevaporator when the compressor stops running. Thevapor can leak back because the rotary compressorcannot hold a seal between the vane and rotor, orbetween the vanes and chamber wall when it is notrunning.

1.28 Rotary compressors are available as direct-drive open compressors and as both full and semi-her-metic types. Most of these compressors are used inhousehold refrigerators and small air conditioners upto 5 ton capacity. Because rotary compressors requirevery close fits and clearances, very few large ones aremade. However, some large, multiple-vane rotarycompressors, up to 600 hp, are used as booster com-

pressors in low-temperature industrial systems, suchas food freezing plants.

Helical (Screw) Compressors

1.29 The helical compressor (also called a screw orrotary-screw compressor) is another kind of positive-dis-placement compressor. In this compressor, the refrigerantvapor is compressed between meshing male and femalehelical rotors. Figure 1-12 shows the compression cycleof a helical compressor. Suction vapor flows in to fill thespace between the lobes of the unmeshed rotors and thewalls of the cylindrical housing. As the rotors turn andstart to mesh, they trap the vapor in the space between thelobes, called the interlobe space. Further rotation reducesthe interlobe space, and the meshing rotors compress thevapor. Finally, the interlobe space becomes exposed tothe outlet port, and the compressed refrigerant vapor dis-charges toward the condenser.

1.30 Helical compressors have no discharge valves.Most designs do have a sliding valve under the rotorsto control compressor capacity. Helical compressors,unlike reciprocating and rotary types, operate smooth-ly and quietly. Their rotating motion makes themalmost vibration free, even when operating at highspeed and capacity.

1.31 Most helical compressors range in size from20 tons up to 1500 tons. They are not economical toproduce in small sizes. Both direct-drive open andsemi-hermetic types are used in industrial refrigerationand air conditioning.

Lobe

Groove Suction

Discharge

Fig. 1-12. Compression cycle of a helical compressor

Fig. 1-13. Spiral-shaped scrolls from a scrollcompressor


Impeller

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Scroll Compressors

1.32 Scroll compressors are a fairly new develop-ment in the air conditioning and refrigeration industry.They offer both simplicity and durability. They arecurrently being manufactured in sizes up to 15 tons.From the outside, they look like small reciprocatingcompressors. Their operaton, however, is very differ-ent.

1.33 The scroll compressor has two scrolls withspiral-shaped walls, as shown in Fig. 1-13. One scrollis fixed and the other is driven in an orbit by a devicecalled a swing link. As the orbiting scroll moves with-in the fixed scroll, trapped refrigerant is forced into aprogressively smaller volume and compressed. Scrollcompressors have no valves or moving parts otherthan the moving scroll. The swing link lets the scrollsmove apart if any liquid is present. This feature helpsprevent damage to the compressor, which can becaused by the incompressable liquid.

Centrifugal Compressors

1.34 Centrifugal compressors are kinetic-dis-placement compressors. They are members of theturbine family and produce compression using ahigh-speed impeller or rotor. An example is shown inFig. 1-14. In operation, the impeller draws therefrigerant vapor into its center from the suctionline. The high-speed rotation of the impeller forcesthe vapor to the outside of the impeller and into thesmaller volume of the diffuser tube at high speed,thus increasing the pressure.

1.35 A single-impeller compressor, like the onein Fig. 1-15, handles large volumes of vapor, but hasa low compression ratio. A single-impeller compres-sor is called a single-stage machine. Centrifugalcompressors having multiple impellers can producehigh compression ratios. The use of multipleimpellers to produce high compression ratios iscalled staging. The first (or low-stage) impeller dis-charges gas into the next impeller, and so on. Eachstage raises the compression ratio, resulting in a highcompression ratio at the discharge. It is for this rea-son that most centrifugal compressors are multi-stage.

1.36 The amount that one impeller will compress thevapor depends upon the impeller’s tip speed. The tip

speed is determined by the impeller’s diameter and rpm.Some compressors use impellers only 6 to 8 in. in diam-eter, but rotate as fast as 30,000 rpm. Other centrifugalunits use larger impellers and slower speeds.

1.37 Most centrifugal compressors are eitherdirect-drive open or semi-hermetic. Some modelshave gears between the motor and the compressor.The gears raise the speed of the impellers above thespeed of the motor. Single-stage and two-stage her-

Suction

Discharge

Motor

Diffuser

Fig. 1-14. Compression in a centrifugal compressor

Fig. 1-15. Semi-hermetic single-impeller centrifugal compressor


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14 Lesson One

metic centrifugal compressors from 50 to 3000 tonsin size are used in commercial and industrial waterchilling systems. Water is often used to cool theelectric drive motors on centrifugal water chillingsystems.

1.38 The main advantage of a centrifugal com-pressor is that it has few moving parts. It has novalves that can bend or break. There are no pistons,piston pins, or connecting rods to wear out. A cen-trifugal compressor rotates smoothly. Thus, it doesnot vibrate like a reciprocating or rotary compres-sor.

1.39 Centrifugal compressors must move a largevolume of vapor. Also, their compression ratios aregenerally lower than positive-displacement compres-sors. For these reasons, their use is limited to large,higher-temperature applications, such as commercialand industrial air conditioning.

Multiple Compressor Applications

1.40 There are two situations that require morethan one compressor in a system: systems that mustoperate at widely varying loads, and low-temperaturerefrigeration systems that must operate below -20°F(-28.9° C). Using two or more compressors to meetwidely varying loads is sometimes called parallel-compressor operation. As many compressors as nec-essary can be run to meet the cooling load require-ments. For example, the parallel compressor system

in Fig. 1-16 uses two 25 ton and one 50 ton com-pressor. Thus, this system can supply 25, 50, 75, or100 tons of refrigeration.

1.41 Low-temperature refrigeration systems usemore than one compressor because one compressorcannot raise the refrigerant vapor pressure highenough for it to condense at the temperature of mostcondensing mediums. Two general designs of multi-ple-compressor systems are used for low-tempera-ture refrigeration systems: multiple-stage compres-sion systems and cascade systems.

1.42 Multiple-stage compression systems. Figure1-17 shows a simple two-stage system using recipro-cating compressors. The first-stage compressor drawsthe refrigerant vapor from the suction line and com-presses it. Compressing the vapor also raises the vaportemperature. Thus, the second-stage compressor alsoraises the vapor temperature. If the vapor is not cooledbetween stages, it will be too hot when it leaves thesecond stage. The results will be burned valves and oilbreakdown in the second-stage compressor. For thisreason, the first-stage compressor discharges the vaporto an interstage cooler (which acts like a second con-denser) to cool the vapor from the first-stage compres-sor before it reaches the second-stage compressor.Some systems use a water-cooled second-stage com-pressor instead of an intercooler.

1.43 The cooled vapor then flows from the liquidcooler to the second-stage compressor. The second-stage

25 ton

50 ton

25 ton

Evaporator

Compressor

Fig. 1-16. Parallel-compressor system

ReceiverInterstagecooler

Evaporator

First-stagecompressor

Second-stagecompressor

Condenser

Fig. 1-17. Two-stage compression system for low-temperature refrigeration


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compressor brings the vapor up to the pressure requiredfor condensing, and discharges it into the condenser.

1.44 Two-stage refrigeration systems can cooldown to about -150°F (-101.1°C). A three-stage sys-tem will reach temperatures below -150°F (-101.1°C)and down to -200°F (-128.9°C). A three-stage systemworks like a two-stage system, but it uses three com-pressors. You must cool the vapor between each stage,like you do in a two-stage system.

1.45 There are also reciprocating compressors thatcontain two stages. Part of the cylinders make up thefirst stage, and they discharge into the other cylin-ders, or the second stage. For example, three cylin-ders could make up the first stage, and discharge intothe fourth cylinder. The cylinders in each stage canhave different cylinder diameters, or bores. The pis-tons can also have different strokes. In any case, thedischarge line of the first stage must be outside thecompressor so that the vapor can be cooled beforeentering the second stage.

1.46 Cascade system. A cascade system usestwo or three complete refrigeration systems toobtain very low temperatures. The evaporator of onesystem cools the condenser for the second system.Remember that the purpose of compressing thevapor is to raise its pressure enough so that it willcondense at the temperature of the condensingmedium. In the cascade system, one refrigerationsystem supplies a low-temperature condensing

medium for the second system. Thus the secondsystem does not have to raise the pressure of thevapor as high as it would have to without the firstsystem’s help.

1.47 Figure 1-18 shows a simple two-stage cas-cade system. The second-stage compressor com-presses the vapor from the evaporator and dis-charges it into a refrigerant-cooled condenser. Thefirst-stage system supplies the refrigeration for thesecond-stage condenser. Two-stage and three-stagecascade systems can supply refrigeration down to -200°F (-128.9°C).

Compressor Replacement Considerations

1.48 Many factors must be considered whenselecting a replacement compressor for a particularapplication. Generally, compressors have a horse-power rating. Compressors are also designed to oper-ate within specific suction and discharge pressureranges, and with specific types of refrigerant. Thesuction and discharge pressures and the type ofrefrigerant used in a system all depend upon the tem-perature range in which the system is operating.Therefore, make certain that a replacement compres-sor has the same horsepower rating, and is designedto operate at the same suction and discharge pres-sures, and with the same type of refrigerant in thesystem as the one you are replacing. When replacinghermetic compressors, also make sure the motor hasthe same electrical rating.


Liquidreceiver Evaporator

First-stagecompressor

Second-stagecompressor

Condenser

Interstage evaporatorand condenser

Liquidreceiver

Fig. 1-18. Two-stage cascade system for low-temperature refrigeration


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1.49 The best place to find the information youneed for compressor replacement is on the compres-sor nameplate (also called the serial plate). An exam-ple is shown in Fig. 1-19. For many full hermeticcompressors under 5 tons (sometimes called cancompressors), the information is stamped on a platewelded to the top or side of the compressor shell. On

open compressors, the plate is usually fastened tothe base or housing. Make sure to check all relevantnameplate information when you must select areplacement compressor. Manufacturers’ catalogsusually list some important compressor details, suchas electrical characteristics, capacity, and connectionsizes.

16 Lesson One

A H 3 0 1 F T - 0 9 7T : J0997 281254 AH5540E

V230/208HZ60 LRA 103.0V200 Hz50 PHI USA P

The months are identified as follows:

January - A February - B March - C April - D May - E June - FJuly - G August - H September - J November - L December - M

Bill ofmaterialNo.

Electrical rating(Volts — Hertz)

Phase

Serial numbers Compressor model number

Letter indicates month (see code), next 2 digits indicateday of month, following 2 digits indicate year

Fig. 1-19. Compressor nameplate information




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1-9. The two basic designs of rotary com-pressor are and .

1-10. In a multiple-vane rotary compressor,where does compression take place?

1-11. In a helical compressor, vapor istrapped and compressed in the

.

1-12. A scroll compressor contains a spiral-shaped scroll that within afixed scroll.

1-13. A centrifugal compressor impeller’s tipspeed is determined by its and .

1-14. If you have a parallel-compressor sys-tem that uses four 20-ton compressors,what refrigeration loads can this systemsupply?

1-15. In a cascade system, the first refrigera-tion system supplies a low-temperaturefor the second system.

1-16. You can find data needed for compres-sor replacement on the unit’s

.

1-9. SINGLE VANE and MULTIPLE VANE

Ref: 1.23

1-10. BETWEEN THE VANES

Ref: 1.26

1-11. INTERLOBE SPACE

Ref: 1.29

1-12. ORBITS

Ref: 1.33

1-13. DIAMETER and RPM

Ref: 1.36

1-14. 20, 40, 60, and 80 TON

Ref: 1.40

1-15. CONDENSING MEDIUM

Ref: 1.46

1-16. NAMEPLATE

Ref: 1.49

18 Programmed Exercises


PREVIEW

COPY

1-1. The refrigeration system compressor is oftenconsidered part of the

� a. condensing unit� b. control system� c. evaporator� d. metering device

1-2. Which of the following is a kinetic-displace-ment compressor?

� a. Centrifugal� b. Helical� c. Reciprocating� d. Scroll

1-3. What is the main disadvantage of an opencompressor?

� a. Difficulty in cooling� b. Inability to open shell� c. Motor burnout contamination� d. Seals can leak

1-4. In what position are the suction and dis-charge valves when the piston in a recipro-cating compressor starts its upstroke?

� a. Both closed� b. Both open� c. Discharge only open� d. Suction only open

1-5. In a single-vane rotary compressor, refriger-ant vapor is compressed between the rotorcontact point and the

� a. cylinder head� b. interlobe space� c. suction valve� d. vane

1-6. The helical compressor compresses therefrigerant vapor between the

� a. helical rotor and discharge valve� b. impeller and chamber wall� c. lobes on meshing rotors� d. rotor vanes

1-7. Which of the following best describes theoperation of a scroll compressor?

� a. A moving scroll orbits within a fixed scroll

� b. A rotating scroll forces vapor into a smaller area

� c. A swing link compresses vapor withinthe interlobe space

� d. A swing link operates a rotating vane

1-8. A two-stage centrifugal compressor has two

� a. drive shafts� b. impellers� c. motors� d. suction valves

1-9. A system using two or more complete refrig-eration systems to obtain very low tempera-tures is called a system.

� a. cascade� b. coupled� c. linked� d. multiple-stage

1-10. What is your best source of informationabout a compressor, should it need replac-ing?

� a. Catalog� b. Nameplate� c. Operator� d. Warranty

Self-Check Quiz 19

Answer the following questions by marking an “X”in the box next to the best answer.


PREVIEW

COPYAnswers to Self-Check Quiz

1-1. a. Condensing unit. Ref: 1.02

1-2. a. Centrifugal. Ref: 1.04

1-3. d. Seals can leak. Ref: 1.08

1-4. a. Both closed. Ref: 1.17, 1.18

1-5. d. Vane. Ref: 1.26

1-6. c. Lobes on meshing rotors. Ref: 1.29

1-7. a. A moving scroll orbits within a fixed scroll. Ref: 1.33

1-8. b. Impellers. Ref: 1.35

1-9. a. Cascade. Ref: 1.46

1-10. b. Nameplate. Ref: 1.49

Contributions from the following sources are appreciated:

Figure 1-4. Dunham-Bush Inc.Figure 1-5. Dunham-Bush Inc.Figure 1-13. Copeland CorporationFigure 1-15. Westinghouse Electric Corporation

The compressor is the refrigeration systemdevice that raises the pressure and temperatureof refrigerant vapor and forces it out the dis-charge line to the condenser. There are five basictypes of refrigeration system compressors: recip-rocating, rotary, helical, scroll, and centrifugal.The first four are positive-displacement compres-sors. The centrifugal compressor is a kinetic-dis-placement compressor. Each of these five typescan be classified as either open or hermetic,depending on how it is combined with the motorthat drives it.

A reciprocating compressor uses a piston toreduce the volume in a cylinder and compressrefrigerant vapor. These compressors can be sin-gle acting or double acting. A rotary compressorcan be single vane or multiple vane and uses arotor to reduce the volume in the compressionchamber. Helical compressors are sometimesreferred to as rotary-screw compressors or sim-ply as screw compressors. In these compressors,refrigerant vapor is compressed between mesh-

ing male and female rotors. Scroll compressorsuse two scrolls with spiral-shaped walls to com-press the vapor as one orbits within the other.Finally, the centrifugal compressor uses a high-speed impeller to force refrigerant vapor into asmall space, thus increasing its pressure.

Two situations commonly require the use of morethan one compressor. Systems that must operateat widely varying loads use multiple compressorsin parallel. Those that require very low tempera-tures use two or three compressors in series orthey cascade refrigeration systems. Cascade sys-tems use two or three complete refrigeration sys-tems.

If you must replace a compressor, there are sev-eral things you must keep in mind when selectingthat replacement—horsepower rating, suctionand discharge pressure ranges, and type ofrefrigerant, for example. The compressor name-plate can give you a great deal of useful informa-tion, as can manufacturers’ catalogs.

20 Lesson One

SUMMARY


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