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LESSON

4LECTURE

TYPES OF COMPRESSORS

SUB-OBJECTIVE

At the end of this lesson the t!inee "ill #e !#le to$

%& Identif' !nd st!te the diffeent t'(es !nd )ses of*o+(essos )sed in S&,&C&C& !sso*i!ted "ith !i *onditionin&

%&. E/TERNAL 0RIVER COMPRESSORS

An external drive (open) compressor is bolted together. Its crankshaft extendsthrough the crankcase. A flywheel (pulley) drives the crankshaft and belt or it

can be driven directly by an electric motor.

Fig. 44! illustrates a cross section through an open compressor. "his is afourcylinder #type compressor. An eccentric type crankshaft is used. "hepistons are fitted with rings.

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A& C!nsh!ft3 B& E**enti*& C& Conne*tin od& 0& Piston3 E& Pistonins& F& C'linde3 5& V!l2e (l!te& 6& C!nsh!ft Se!l3 I& S)*tion se2i*e2!l2e& J& E1h!)st se2i*e 2!l2e& 7& C!n*!se&

A master connecting rod is mounted on each eccentric and is connected to apiston in one bank of the 1#1. "he connecting rod which is attached to thepiston in the other bank is connected with a pin through a flange on themaster connecting rod. "hese connecting rods are somewhat shorter than themaster connecting rod and are called articulated connecting rods.

A crankshaft seal is re2uired where the crankshaft comes through thecrankcase.

8&. 6ERMETIC COMPRESSORS

"he motor in a hermetic compressor is sealed inside a dome or housing withthe compressor and is directly connected to the compressor. A crankshaft sealis not needed.

A motor rotor is usually a press fit on the compressor crankshaft. /ome motor

compressors are made with the motor at the top3 while others have the motorat the bottom and the compressor at the top.

"he unit is usually spring mounted inside the hermetic dome. "his preventsmost of the compressor vibration from being felt outside of the dome.

"he exhaust and suction lines inside the dome are made flexible. Aconnection through the dome provides means of fastening the compressorlines to the remainder of the system. "he electrical connections to the motorpass through the dome by means of an insulated leakproof seal.

"o lubricate the compressor3 the return suction gas is fed into a hollow diskmounted on the motor compressor shaft. +entrifugal force throws the oil anda li2uid refrigerant to the outer rim of the disk and flows over the motorwindings. (+entrifugal force action rotates things to pull spinning particlesaway from the center of the rotation.) %nly the vapor refrigerant remains atthe center and is drawn into the cylinders of the compressor.

A hermetic motor compressor usually re2uires an outside electrical relaystarting mechanism. Fig. 44 shows a section through such a motor.

/ome motor compressors are twospeed. "hese are popular in large systems

and in air conditioning where heat loads change.

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Fi& 4-4-8& Re*i(o*!tin he+eti* *o+(esso& Co+(esso is !t to( !nd+oto !t #otto+& Asse+#l' is +o)nted on s(ins inside do+e&

A& Moto oto3

B& Moto st!to&

C& Co+(esso *'linde

0& Co+(esso (iston

E& Conne*tin od&

F& C!nsh!ft&

5& C!n tho"&

6& Co+(esso shell&

I& 5l!ss se!led ele*ti*!l *onne*tions tho)h *o+(essodo+e&

9&. TYPES OF COMPRESSORS

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"here are four basic types of compressors in use6

!. *eciprocating (piston cylinder). *otary5. /crew type4. +entrifugal

9&% RECIPROCATIN5 COMPRESSORS

"he original energy source is usually an electric motor. Its rotary motion mustbe changed to reciprocating motion. A crank and a rod connecting the crankto the piston usually make this change. "he complete mechanism is housedin a leak proof container called a crankcase. It is very efficient. Itsconstruction resembles3 in many ways3 that of the automobile engine. Atypical external drivereciprocating compressor is shown in Fig. 445.

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7asically3 this compressor is a cylinder and a piston. Fig. 444 shows theprinciple of operation of a reciprocating compressor. In illustration ,o. !3 thepiston3 73 has moved downward in cylinder3 A3 and has moved refrigerant

vapor from the suction line3 +3 through the intake valve3 3 and into thecylinder space3 -. In illustration ,o. 3 the piston has moved upward and hascompressed the vapori8ed refrigerant into a much smaller space (clearancespace) marked 93 and has pushed the compressed vapor through valve3 F3 into the condenser.

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Fi& 4-4-4& B!si* *onst)*tion of e*i(o*!tin *o+(esso&

A. +ylinder. 7. iston. +. Intake port from suction line. &. xhaust port tocondenser. . Intake valve. F. xhaust valve. -. iston displacementindicates volume of vapor drawn into cylinder on intake stroke. 9. +learancespace at end of compression stroke. efthand illustration shows intake stroke:righthand the exhaust stroke.

4&. CYLIN0ERS

+ompressor cylinders for external drive compressors are usually made of castiron. "he cast iron must be dense enough to prevent the seepage ofrefrigerant through it. /ome nickel is usually added to give the casting thisdensity.

The #od' is ! *!stin )sin liht"eiht !llo'& C!st ion *'linde lines!e (e+!nentl' *!st into *!n*!se #od'&

/mall compressors usually have fins cast with the cylinders to provide better

air cooling larger compressors may have water ;ackets surrounding thecylinders for cooling. /ome compressors are built with cylinder liners orsleeves3 which may be replaced when worn.

'sually3 the crankcase is part of the same casting as the cylinder. "hispractice cuts down the number of ;oints that might leak. It also permits closealignment between crankshaft main bearings and cylinder. "he main bearingsare ball type. +onstruction is shown in Fig. 4.4.

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Fi& 4&4&: C)t!"!' 2ie" of s+!ll3 e1ten!l di2e3 t"o *'linde e*i(o*!tin*o+(essos

9ermetic (sealed) compressors usually have cast iron cylinders. /ome maybe of aluminum or other materials.

"his is typical of hermetic compressor cylinders shown in + in Fig. 4.4..

Another type of hermetic compressor is pictured in Fig. 4.4.=. "his is a boltedtype hermetic and can be dismantled easily for servicing.

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:&. PISTONS AN0 PISTON RIN5S

iston used in external drive compressors is usually made of cast iron3 whilein small highspeed hermetic compressors they are of diecast aluminum./maller si8es do not have piston rings.

/ince the temperature of pistons seldom goes higher than F. (!! +.)there is not much expansion of either piston or cylinder. istons may be fittedwith as little as .>>> in. (.>>

"he smaller pistons have oil grooves cut in them. Fig. 4.4.? illustrates acommon piston connecting rod assembly.

Fig. 4.4.@ illustrates a commercial type piston and connecting rod assembly."his one is fitted with piston rings.

"here are two types of piston rings. "he upper ring or rings are known ascompression rings and the lower is designed to control the oil 1flow1 past thepiston. It is an oil ring.

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Fi& 4&4&< Piston !nd *onne*tin od !sse+#l'& Note oil oo2es *)t in (iston&

iston rings are usually made of cast iron. /ome bron8e rings have beenused3 however.

*ings should be fitted to the groove as closely as possible and still allowmovement. A 4< deg. tapered or angled ring gap permits the ring to push outagainst the cylinder wall.

"his gap should be about .>>! in. (.>

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Fi& 4&4&= Co+(esso (iston !nd3 *onne*tin od !sse+#l'& Note ho"

*onne*tin od>s lo"e ?left@ end is s(lit !nd then #olted toethe3 to (o2ide#e!in to fit *!nsh!ft o)n!l.

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"here is a valve plate under the cylinder head with both the intake and

exhaust valve located in it3 Fig. 4.4.

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In hermetic systems3 the construction of the pistons and rings3 if used3 aremuch the same as those used in external drive compressors.

9owever3 since the hermetic compressors usually run at a higher speed thanexternal drive compressors3 the pistons are smaller in diameter and are madeas light as possible.

+ool refrigerant vapor from such line flows over motor windings to aid in

cooling motor. +ompressor uses /cotch yoke piston crank mechanism.+ompressor is inverted and a hori8ontal motor shaft is used.

,ote position of motor crankshaft3 piston3 cylinder and connecting rod3.

+ompressor is spring mounted within dome to minimi8e vibration. *efrigerantvapor inlet at top protects compressor from possible surge of li2uid refrigerant.

;&. CYLIN0ER 6EA0

+ylinder heads for both external drive and hermetic compressors are usually

made of cast iron.

"he head serves as a pressure plate to support and hold the valves and valveplate in position. It also provides the vapor passages into and out of thecompressor.

"he pressures of compression may amount to as much as 5>> psi (! kgBcmC)depending upon the kind of refrigerant used.

"he valve plate must3 therefore3 have good support so that there will be noleakage at the gaskets on either side of the valve.

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e!te 2!l2e lift to (ote*t *o+(esso in *!se of se2ee li)idefie!nt o oil ()+(in&

#alve plates are some times made of cast iron3 but hardened steel is alsoused3 as plates can be thinner with longer wearing valve seats.

+ompressor valves are usually made of high carbon alloy steel. "hey are3heat treated to give them the properties of spring steel and ground to aperfectly flat surface.

"he intake valve is usually kept in place by small pins or the clamping action

between the compressor head and valve plate. "he exhaust valve may beclamped in the same way.

/ome different valve designs are displayed in Fig. 4.4.!!3 4.4.! shows atypical valve plate assembly.

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Fi& 4&4&%% So+e t'(i*!l *o+(esso 2!l2e desins& A& Reed 2!l2e3 s(in*losed& B& Po((et 2!l2e3 s(in *losed& Used on so+e l!e *o+(essos& C&Reed 2!l2e& Pess)e diffeen*e ee(s 2!l2e *losed&

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A& Inlet (ot& Note e1tension to (e2ent oil fo+ entein int!e& B&E1h!)st (ot& Note "ie lo* )sed in the t"o e1h!)st 2!l2e s*e"s& C&Oil et)n to the *!n*!se&

"he valve disks or reeds must be perfectly flat. A defect of only .>>>! in.

(>.>>

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%f the two valves3 the intake gives the least trouble. "his is because it isconstantly lubricated by oil circulating with the cool refrigerant vapors. Also3 itoperates at a relatively cool temperature.

"he exhaust valve must be fitted with special care. it operates at hightemperatures and must be leak proof against a relatively high pressuredifference.

7ecause of the high vapor pressures and the high temperatures3 there is a

tendency for the heavy ends (heavy molecules of hydrocarbon oils) to collecton the valve and valve seat as carbon.

"he valves open about .>!> in. (>.>>>>! in. (.>>>>

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Fi& 4&4&%9 C!nsh!ft se!l *onst)*tion fo e1ten!l di2e *o+(essos&

%& Se!l )sed in *o++e*i!l *o+(essos&

B& Se!l )sed "ith !n !)to+o#ile !i *onditionin *o+(esso&

C& Re(l!*e+ent se!l& 0& Bello"s t'(e se!l&

&. SCOTC6 YO7E

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"he /cotch yoke mechanism has no connecting rod. "he cylinder and pistonare both 2uite long3 and even at the lower end of the stroke the piston is stillguided by the cylinder wall.

"he crankshaft pin3 also called the crank throw3 connects to the lower end ofthe piston by means of a floating bearing.

/ee Fig. 4.4.!4. "he /cotch yoke is popular in small highspeed

compressors.

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A& C!n tho" !nd S*ot*h 'oe& B& 6ollo" Piston& C& Co+#ined +otosh!ft !nd *!nsh!ft& 0& C!nsh!ft th)st #e!in& E& Inten!l +o)ntins(in& F& Oil ese2oi&

%.&. S,AS6 PLATE

A popular type of reciprocating compressor used on many automobile airconditioning systems is known as a 1swash1 plate or 1wobble1 platecompressor.

,o connecting rod is used in this type of compressor. "he cylinder andpistons are mounted as in Fig. 4.4.!

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dimensions. "his is an external drive compressor. It re2uires a seal wherethe drive shaft passes through the compressor housing.

Fi& 4&4&%:& Coss se*tion tho)h ! Ds"!shD (l!te t'(e of e*i(o*!tin*o+(esso&

A& 0i2e sh!ft& B& S"!sh (l!te& C& Piston& 0& C'linde ,!ll E& 0i2e #!ll&F& B!ll shoe& 5& V!l2e (l!te ?2!l2e not sho"n@& 6& Th)st #e!in J&Piston stoe& As di2e sh!ft !nd s"!sh (l!te e2ol2e3 do)#le end (iston

is +o2ed #!* !nd foth in *'linde&

%%&. ROTARY COMPRESSOR

"here are two basic types of rotary compressors. %ne has blades that rotatewith the shaft.

"he other has a stationary blade. "he rotary blade compressor using two

blades3 shown in Fig. 4.4.!= is typical of this type.

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"he lowpressure vapor from the suction line is drawn into the opening andfills the space behind the blade as it revolves.

As the blades revolve3 trapped vapor in the space ahead of the blade iscompressed until it can be pushed into the exhaust line to the condenser.

Fi& 4&4&%;& A ot!' #l!de *o+(esso&

7lack arrows indicate direction of rotation or rotor. *ed arrows indicaterefrigerant vapor flow.

A commercial rotary blade compressor3 using eight blades3 is pictured in Fig.4.4.!?. "he basic operation of the eightblade compressor is the same as thetwo blade.

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Fi& 4&4&%= Rot!' *o+(esso& St!tion!' #l!de o di2ide #lo* is in *ont!*t"ith ! olle ?i+(elle@

A& Identifi*!tion of (!ts B& O(e!tion&

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"hese compressors have three advantages6

!. "hey provide a large si8e opening into the suction line.. "hey provide large inlet port openings.5. "hey have a very small clearance volume.

/ince the lowside pressure may be 2uite low3 this means that the lowsidevapor will be drawn into the compressor. Fig. 4..!@ represents a stationary

blade (often called a divider block) rotary compressor. An eccentric shaftrotates an impeller in a cylinder. "his impeller constantly rubs against theouter wall of the cylinder.

As the impeller (or roller) revolves3 the blade traps 2uantities of vapor. "hevapor is compressed into a smaller and smaller space3 building up thepressure and temperature. Finally the vapor is forced through the exhaustport into the highpressure side of the system (condenser).

In rotary compressors3 check valves are usually used in the suction line toprevent the highpressure vapor and compressor oil from flowing back into the

evaporator.

%8&. SCRE, TYPE COMPRESSOR

"he screw type compressor uses a pair of special helical rotors. "hey trapand compress air as they revolve in an accurately machined compressorcylinder. "hese compressors are available in either external drive or hermeticconstruction. "hey are used in large system (> tons and up).

Fig. 4.4.! illustrates a cross section. "he two rotors are not the same shape.%ne is male3 the other female. "he male rotor3 A3 is driven by the motor. "he

female rotor3 73 meshes with and is driven by the male rotor. It has sixinterlope spaces. "he cylinder3 +3 encloses both rotors.

Fi& 4&4&%& Coss se*tion of s*e"-t'(e *o+(esso&

A& M!le oto& B& Fe+!le oto& C& C'linde& V!(oied efie!nt enetes!t one end !nd e1h!)sts !t othe end& ?St!l Refie!tion AB@

In operation3 the refrigerant vapor is drawn in as shown in Fig. 4.4.>. "he

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intake (lowpressure vapor) enters at one end of the compressor and isdischarged (compressed vapor) at the opposite end.

/ince there are four lobes on the male rotor and six on the female rotor3 themale rotor3 the male rotor will revolve more rapidly than the female rotor. "herotors are helixes so the pumping action will be a continuous action ratherthan pulsating as with a reciprocating compressor. In the absence of thisreciprocating motion there is very little vibration during

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B& Beinnin of *o+(ession&

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C& F)ll *o+(ession of t!((ed 2!(o&

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%9&. CENTRIFU5AL COMPRESSORS

+entrifugal compressors are used successfully in large refrigerating systems.

In this type compressor3 vapor3 as it is moved rapidly in a circular path3 movesoutward. "his action is called centrifugal force.

"he vapor is fed into housing3 near the center of the compressor. A disk withradial blades (impellers) spins rapidly in this housing forcing vapor against theouter diameter.

"he pressure gained is small so that several of these compressor wheels orimpellers are put in series to create greater pressure difference and to pump asufficient volume of vapor.

"his type of compressor looks like a steam turbine or an axial flow aircompressor for a gas turbine engine.

"he centrifugal compressor has the advantage of simplicity. "here are novalves or pistons and cylinders.

"he only wearing parts are the main bearings. umping efficiency increases

with speed so compressors operate at high speeds.

Fig. 4.4.! is a crosssection through a twostage centrifugal type compressor."he driving motor is mounted between stages.

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"he stator or casing of a centrifugal compressor is usually made of cast iron.It has a changing radius inside to adapt itself to the vapor pickup by theimpellers.

"he casing (cylinder) also holds the main bearings3 the oil pressure producingpump as well as the refrigerant vapor intake and exhaust ports. It also holdsthe shaft seal where the shaft extends or sticks out from the casing for thepower drive3 when an external motor is used. 7oth the first stage and secondstage have ad;ustable inlet vanes to control the capacity of the pump.

%9&8 ROTOR CONSTRUCTION

"he rotor or impeller in a centrifugal compressor is keyed to the compressorshaft. It is made of cast iron or steel and is specially designed to move thevapors without going above gas velocity limits and without having vaportrapping pockets. A typical rotor is shown in Fig. 4.4..

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