AIR CON 03 OVER

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LESSON

3LECTURE

MAINTAIN COMPRESSOR UNLOADERS

SUB - OBJECTIVE

At the end of this lesson the trinee !ill "e "le to#lerns the need for $o%&ressor 'nlodin() *e identifies h+dr'li$, &ress're ndsolenoid-o&erted "+&ss nd s'$tion $'toff 'nloders, nd identifies $orre$t'nloder o&ertion) *e d'sts &ress're-o&erted 'nloders) *e tro'"leshootsnd re&irs solenoid-o&erted 'nloders)

.)/ INTRODUCTION

When an air conditioning system is designed, a compressor capacity is selected to

handle the peak (maximum) load. In most systems the cooling load is constantlychanging, so the typical operating load for a compressor might be only 50% of the peakload. If a compressor designed to handle a peak load operates at full capacity hen theload is only 50%, it ill compress more refrigerant !apor than can be used by thee!aporator. "his causes suction pressure to fall. #o suction pressure can cause oilmigration, free$e the e!aporator, andor cause erratic operating conditions hich causecompressor failure.

0)/ MATC*IN1 COMPRESSOR CAPACIT2 AND LOAD

&ne ay to match compressor capacity and system load is to cycle a compressor on

and off. 'ut this is not the best solution. It in!ol!es freuent compressor starts and highpoer consumption. It also causes ear and tear on the compressor and its electricalcomponents.nother ay to ad*ust compressor capacity to load is to use se!eral smallercompressors in parallel instead of one large one. s the load rises and drops,additional compressors are sitched on or off to meet it. "he disad!antages of thisapproach are high initial cost, a greater use of floor space, and increased maintenance

3)/ CAPACIT2 CONTROL DEVICES

"he most efficient and least costly ay to match compressor capacity and load is to use

capacity control de!ices. capacity control de!ice regulates the amount of refrigerantgoing through the compressor. It can either be built into the compressor (compressorinternal) or separate from it (compressor external).ll compressor+internal methods of capacity control decrease a compressors capacityhile keeping the compressor running.

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3). COMPRESSOR UNLOADERS

"he most common internal capacity control de!ice is the unloader. n unloader reducesa compressors capacity by stopping the flo of refrigerant !apor through some of thecompressors cylinders. "he unloader does this by one of the folloing methods=


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suction cutoffs cylinder head is flat. It has a co!er plate and four bolts directly oppositeits capacity control !al!e.

I1) .). UNLOADER T2PES

3)4 BASIC UNLOADER OPERATION

6I7. shos the difference beteen the operation of a bypass and suction cutoffunloader. In a bypass unloader (6igure , 6I7. ) the discharge manifold is closed bya check !al!e. "he compressors piston dras refrigerant !apor from the suction lineinto the cylinder head and pushes it back into the suction line. "he refrigerant !apor isrecirculated beteen the discharge and suction side of the unloaded cylinder. In asuction cutoff unloader (6igure ', 6I7. ) the cylinder head is cut off from the suctionline by an unloader piston. 4uction gas is pre!ented from entering the cylinder bank.'ecause refrigerant gas is not dran into the suction cutoff unloaders cylinder, itspiston mo!es up and don ith little resistance. s a result, its compressor consumesless poer and runs more efficiently than a compressor ith a bypass unloader. "he

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operating temperature of a suction cutoff unloader is also reduced at part load, hilethe bypass unloaders operating temperature remains the same.

I1) .)0 BASIC OPERATION O B2PASS AND SUCTION CUTO UNLOADERS

3)5 IVE BASIC PARTS O UNLOADERS

'ypass and suction cutoff unloaders ha!e fi!e basic parts, as shon in 6I7.


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n unloaders operation is controlled by a capacity control !al!e. "he control !al!etriggers the loading and unloading of all cylinders in a particular bank. &n start+up thecontrolled cylinders are unloaded to reduce starting torue. capacity control !al!e is eitheror=. 9ressure+operated (acti!ated by a drop in suction pressure).'. 4olenoid+operated (acti!ated by electrical signals from a pressurestat, thermostat, orstep controller). "he pressure+operated and solenoid+operated capacity control !al!es

are used ith either bypass or suction cutoff unloaders. lthough the to !al!es lookdifferent mounted on the unloader cylinder head, as shon in 6I7.


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B2PASS CAPACIT2 CONTROL VALVESUCTION CUTO CAPACIT2 CONTROL8IT* PISTON9 VALVE : :PISTON REMOVED9I1) .)5 PRESSURE-OPERATED CAPACIT2 CONTROL VALVES

"he !al!e has to parts= () the bypass piston. "he bypasspiston is only used ith the bypass unloader. When the control !al!e is used ith thesuction cutoff unloader, the bypass piston is remo!ed and discarded. 6I7.


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"he solenoid+operated control !al!e is also called an electric capacity control !al!e. sshon in 6I7.


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I1) .)< SUCTION CUTO UNLOADER =S INTERNAL PISTON ASSEMBL2

3)6)3 UNLOADER *EAD 1AS>ET

s shon in 6I7.


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I1) .)./B2PASS AND SUCTION CUTO VALVE PLATES AND 1AS>ET

3)6)5 VALVE PLATE 1AS>ET

s shon in 6I7.


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3)< LOADIN1 T*E B2PASS VALVE


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3), 9art').

I1) .).3 UNLOADIN1 T*E LOADED B2PASS VALVE

;. With the poppet !al!e open (point ;, 9art ') discharge gas passes through thebypass passage into the suction manifold (point B, 9art ').

B. enting discharge gas reduces pressure behind the bypass piston. "his allos thepistons internal spring to force the piston to the left (point ?, 9art ').5. "he seating of the bypass piston against the control !al!es body opens the bypassport. "his pro!ides a path for the refrigerant gas to !ent to the suction side of thecylinder (point 5, 9art ').?. reduction in discharge pressure allos the check !al!es internal spring to mo!ethe check !al!e upard and seat it against the !al!e plate. "his isolates the cylinderfrom the discharge manifold (point @, 9art '). "he cylinder is no unloaded. s long asthe compressor operates, the cylinders ithout 2nloaders keep the discharge manifoldat discharge pressure. :oe!er, the discharge gas cannot mo!e the unloadedcylinders 'ypass piston to close the port. "his is because the open poppet al!e

bleeds the discharge gas to the suction side.

3)


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piston assembly is installed, remo!ed, and ser!iced through the co!er plate on theopposite side of the unloader head from the capacity control !al!e."his unloader entirely shuts off suction gas from the cylinder head hen it is closed(unloaded

I1) .).4 CUTA8A2 O SUCTION CUTO UNLOADER

3)


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I1) .).5 LOADIN1 T*E UNLOADED SUCTION CUTO VALVE

>. When the suction pressure rises to a point abo!e the control set point springtension, the pressure against the diaphragm forces it to the left (point < in 9art ' of6I7.


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pressure+operated unloader is controlled by suction pressure and actuated bydischarge pressure. "here are to ad*ustments to determine hich pressure ill loadand unload the cylinders. "he to ad*ustments are= ()pressure differential. "he location of these ad*ustment points is shon in 6I7.


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I1) .).? CONTROL SET POINT C*ART

>. 3efer to the ontrol 4et point hart in 6I7.


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I1) .)0/ BAC>STOPPIN1 T*E PRESSURE DIERENTIAL SCRE8

"o ad*ust the differential to the pressure at hich you ant the cylinder to unload, follothese steps. "he steps refer to 6I7. 0 and to the 'ypass -ifferential ur!e hart in6I7.


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I1) .)0. B2PASS PRESSURE DIERENTIAL C*ART

3).. C*EC>IN1 SET POINT AND DIERENTIAL ADJUSTMENTS

&nce you ha!e set the !al!es control set point and differential, you must check themfor proper operation=. ttach a compound pressure gauge to the suction port.;. ttach an ammeter to the compressor. &nce you ha!e attached the compoundpressure gauge and ammeter to the compressor, sloly close the suction ser!ice !al!eto loer the suction pressure. ontinue closing the !al!e until the cylinder unloads.When the compressor unloads, there is a change in the sound le!el and a drop in thecurrent dra.

3).0 TROUBLES*OOTIN1 UNLOADER OPERATION

If the unloader is not operating according to the control set point and pressuredifferential settings, there are se!eral steps to take to find and fix the problem.

3).0). PRESSURE - OPERATED B2PASS

.

>. heck the strainer in the bleed orifice, as shon in 6I7. >. If it is dirty, clean orreplace it.;. 8ake sure the check !al!e, hich pre!ents refrigerant from entering thedischarge manifold, is mo!ing freely, as shon in 6I7.;. If it is faulty, replace it.B. If items in steps > and ; are okay, the capacity control !al!e body is bad and shouldbe replaced.

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I1) .)00 STRAINER IN BLEED ORIICE

I1) .)03 TESTIN1 C*EC> VALVE OR REE MOVEMENT

3).0)0 PRESSURE - OPERATED SUCTION CUTO

abo!e.>. 3emo!e the co!er plate and check to see if the unloader piston is mo!ing freely. If itis not, replace it.;. If the piston assembly is mo!ing freely and the set point and differential cannot bead*usted, replace the control !al!e.

3).0)3 ELECTRIC - OPERATED B2PASS

. heck the electric solenoid. "his is done in one of to ays. With the solenoidenergi$ed, hold a scredri!er to the top of the solenoid coil to check for magnetism, asshon in 6I7. B. "he second method is to place a clamp+on ammeter around theelectric lead to the solenoid and check for current, as shon in 6I7. 5. If magnetismor current is not detected, replace the solenoid.;. heck the strainer in the bleed orifice.B. 8ake sure the check !al!e is mo!ing freely.

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5. If steps < through B are okay, replace the capacity control !al!e.

I1) .)04 C*EC> SOLENOID OR MA1NETISM C*EC> SOLENOID OR CURRENT

3).0)4 ELECTRIC - OPERATED SUCTION CUTO

for the electric+operated bypass !al!e.>. heck the strainer in the bleed orifice.;. 3emo!e the co!er plate and check the piston assembly for free mo!ement.B. If steps < through ; are okay, replace the capacity control !al!e.

3).0)5 *2DRAULIC UNLOADER

"he hydraulic unloader is acti!ated by the crankcase pressure, but poered by thecompressor lubrication system. It controls capacity by holding the suction !al!e disc off

its seat. "his ensures that no refrigerant gas is pumped hen the cylinder is unloaded."he ma*or components of a hydraulic unloader system are=. :ydraulic relay;. 2nloader poer elementB. al!e lifting mechanism.ie of 6I7. ?, shos the hydraulic unloader components simplified. ie 'shos the details and location of components. /ote that the surge chamber, shon in!ie ', is used in large capacity compressors only. It dampens rapid changes incrankcase pressure to reduce rapid cycling.

(6I7.


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I1) .)06 *2DRAULIC UDILOADBR CO>PODIENTS

3).0)6 *2DRAULIC UNLOADER OPERATION

t start+up, all controlled cylinders are unloaded. "hey remain unloaded until the oilreaches the unloader mechanism. :ydraulic 2nloader &peration from 4tart+up to#oading("he folloing simplified 6I7. make the explanation easier, but they do notsho the location of the actual components.)


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I1) .)0; CONTROL OIL DRAIN OPEN

;. "he increase in control oil pressure at the left side of the hydraulic relay piston, at @,

causes it to mo!e to the right.>. 4ee 6I7. C. s the needle !al!e mo!es toards the !al!e seat, the flo of controloil to the crankcase through the oil drain, at B, is reduced. "his causes the control oilpressure to increase as oil from the compressor oil pump circuit, at 4, flos through theorifice, at ?, to the hydraulic relay.

I1) .)0< CONTROL OIL DRAIN CLOSES

B. 4ee 6I7. D. 8o!ing the relay piston to the right allos the compressor oil pump,at C, to feed oil into the unloader poer element, at D. "he pump oil pressure in theunloader poer element forces the piston upards and pi!ots the lifting fork, at


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I1) .)0? SUCTION VALVE SEATED

3).0); Unloder O&ertion fro% Loded to Unloded

, to the right,compressing the control !al!e spring, at ;.;. "he needle !al!e mo!es off its seat, at B.B. "his allos the control oil to bleed from the hydraulic relay and the capacity control

!al!e through the oil drain, at 5, to the crankcase.

I1) .)3/ DROP IN CRAN>CASE PRESSURE

5. 4ee 6I7.


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@. 4ee 6I7. . s the oil pump pressure to the poer element drops, the unloaderpiston, at C, mo!es donard draining oil, at D, to the crankcase.C. "he lifting fork, at > refrigerant.

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"o ad*ust the control point=


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3).5 COMPONENT MALUNCTIONS AND T*EIR S2MPTOMS

3).5). SEMI- *ERMETIC RECIPROCATIN1 COMPRESSORS

C'se of Mlf'n$tion S+%&to%:s9.< :igh suction pressure, lodischarge pressure>.> /o change in system pressureith unit running>.; onsiderable noise in cylinderhead>.B -ifferent head temperature inmulti + cylinder compressors

;. #oose compressor ;.< /oisy compressor. hold+donbolts

3).5)0 UNLOADERS

.< /o response to load changesor ad*ustment.

;. -efecti!e control !al!e body(both types)

;.< ompressor ill remain loadedor unloaded.

B. -efecti!e control !al!e bodyandor check !al!eA andor piston

assembly (solenoid+type)

B.< 1lectrical parts check out okay,but compressor does not load after

starting.5. apacity control !al!e stuckopen or lifter pins stuck in the upposition or pluggedclosedpneumatic opening (hydraulic type).

5.< ompressor does not load.

?. 4tuck unloader element orlifter pins stuck in the don position(hyd type).

?.< ompressor does not unload.

@. 9artially plugged control oilstrainer (hyd type).

@.< 3apid unloader cycling.

3).5)3 OIL


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>.> Fnocking noise incompressor.

;. old oil and refrigerantmixture in the crankcase

;.< &il foaming in the crankcase.

B. Wrong oil type for the unit B.< Wax formation in theexpansion !al!e and tubing.

5. ontaminated oil 5.< orroded parts.

3).5)4 OIL SEPARATORS

.< :igh suction pressure,

normal discharge pressure.

3).5)5 OIL PUMPS


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TURNS IN ROM BAC>STOP CLOC>8ISE B2PASS PRESSURE DIERENTIAL CURVEI1) .)35

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SUCTION CUTO PRESSURE DIERENTIAL CURVEI1) .)36TURNS IN ROM BAC>STOP CLOC>8ISE

E@ERCISE B#

3).? TROUBLES*OOT AND REPAIRA SOLENOID OPERATED UNLOADER STEPS

. heck the iring for continuity beteen the control source and the solenoid. 3eplaceif defecti!e.;. heck the solenoid for proper operation. If there is poer at its terminals but it doesnot ork, check the solenoid coil. 3eplace the coil if it is open or shorted.B. 9erform the folloing steps if all the electrical parts are okay but the unloader stilldoes not ork.5. 3emo!e the capacity control !al!e.?. heck the strainer in the bleed orifice for blockage.@. 8ake sure the check !al!e is mo!ing freely (if you are orking on a bypass+typeunloader).C. 3emo!e the co!er plate from the unloader piston assembly (if you are orking on asuction cutoff+type unloader).

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D. 3eplace the piston assembly if it is not mo!ing freely.

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