capacity control of reciprocating compressors used in refrigerati

8/22/2019 Capacity Control of Reciprocating Compressors Used in Refrigerati

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International Compressor Engineering Conference School of Mechanical Engineering

1972

Capacity Control of Reciprocating CompressorsUsed in Refrigeration Systems

D. L. BoydChrysler Corporation

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Boyd, D. L., "Capacity Control of Reciprocating Compressors Used in Refrigeration Systems" (1972). International Compressor

Engineering Conference. Paper 6.hp://docs.lib.purdue.edu/icec/6

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pounding of the cylinder unloading mechanism by thevalve hammering on the l i f t e r rods. To avoid this

the lever is in fact a s t ra ight piece of spring

wire which deflects unti l enough movement of the

bellows causes t to overcome the detent mechanismand move the pis ton quickly to the next posi t ion.In pract ice a loose f i t t ing stamped stee l lever is

used along with the spring wire as a precaution

against binding.

The piston sleeve and body are c lose f i t t i ng to

minimize oi l leakage and the lat ter two parts

combine to provide precisely located ports andconnecting passages to the cover plate where dr i l l

ed holes are arranged to connect the valve to tub

ing in the crankcase.

To summarize the descript ion of this mechanism i tis mounted on the inside of a cover plate in such away that increasing suction pressure result ingfrom increased refrigerant flow a t the evaporator

compresses the bellows and spring causing motion

of the spring wire lever which tends to pull the

valve piston out of the valve. Sufficient bending

of the lever wil l snap the pis ton out one step.Oil under pressure wil l then be fed to the hydraulic

mechanism of a particular cylinder which moves to

drop the l i f t e r r ing and rods le t t ing the suction

valve open and close normally. Conversely areduction in refrigerant flow at the evaporator

will cause a drop in suction pressure unti l the

balancing spring moves the bellows inward enough to

move the pis ton one step back. This vents the port

of a particular unloader to the crankcase and the

l i f t e r spring or springs in the hydraulic mechanism

force out the oi l and the ring and l i f t e r rods open

the suction valve and firmly hold t wide open. Aselect ion of pressure balancing springs permits

control from suction pressures below one atmosphere

up to about 85 psig covering a temperature range

from -800F to SQOF with the commonly used halocarbon refrigerants . The mechanism is also adapt

able to external drives connected to the outer end

of the movable rod.

Another make of suction pressure sensor-control ler

ut i l izes the detenting spool pis ton and the

spring balanced bellows but in place of the leveris an interesting oi l valving hydraulic act iondesign. Briefly movement of the bellows opens

or closes a needle valve and the spool pis ton

having a similar detenting design to that described

above is moved in one direct ion as a hydraulic

pis ton and returned by the force of a compressed

spring. There is a small bleed in the oi l supply

ci rcui t to the piston such that the bellows operated needle yalve may balance the outflow of oi l to

hold the posi t ion of the pis ton unchanged over

come the bleed to move the pis ton against the

spring or close and l e t the spring return the

piston. The piston serving as a spool valve is

control l ing the unloader oi l supply as described

in the preceding paragraph. A design of this type

includes ingenious means for damping of bellows

and detenting pis ton movement and balancing of

small port valves and bleed or if ices .

Finally we wil l look a t the electr ical ly control

led systems which generally are of more recent

40

development. Basically these dif fer from the

suction pressure control led type in that only the

oi l valving part of the system is bui l t in to the

compressor as a cover a s s ~ m b l y The sensing partof the system may be elsewhere and connected

elect r ica l ly .

One such unloader control consists simply of

three-port solenoid valves which control the

supply of oi l to individual steps of unloading.Energized the solenoid valve supplies oi l under

pressure to the cylinder unloader mechanism

deenergized i t rel ieves the oi l to the crankcase

unloading the cylinder or cylinders.

Another unloader control ut i l izes much smaller less

expensive solenoid valves as pi lot valves which

supply oi l under pressure to dpring loaded spool

pistons. Movement of the l a t te r control oi l to the

unloader mechanisms. A bleed is used in conjunc

t ion with each solenoid valve so that oi l supplied

by the solenoid valve can be bled off when the

valve closes permitting the spool pis ton to return.The ori f ice is very small and is protected by asintered metal f i l t e r from small foreign particleswhich might obstruct the opening. The advantage of

this pi lot operated system is that larger portingfor oi l supply and drain is possible economically

and more rapid response by the cylinder unloading

mechanism is assured. Control of the solenoid

valves by staged pressure or temperature sensors is

straightforward with temperature control being the

more common Pneumatic control through staged

pressure electr ic switches is also possible.

Important to much of the internat ional market

where s tar -del ta type of electr ic motor s tar t ing

is commonly used is the avai labi l i ty of compressors

which can be completely unloaded so that the s tar t

ing motor is not overloaded. I f the compressor

has unloaders in a l l cylinders t is no problemwith electr ic unloader control to hold a l l cylinders

unloaded unti l af ter s tar ter t rans i t ion where the

motor has come up to speed. Then the power to the

capacity control system can be closed and normal

control assumed. Except when s tar t ing the com-pressor should never be operated completely unload

ed as f r ic t ion wil l eventually cause seriousinternal overheating and damageto the compressor

wil l resul t .

There are dis t inct advantages and disadvantages

to both the suction pressure sensing and the

temperature sensing type of unloader control . TI1e

former is very satisfactory in systems having an

ai r cooling evaporator coi l usually some distanceaway from the compressor. Normally the compressor

capacity follows the dictates of the thermal

expansion valve or valves i f there is more than

one ci rcui t and/or coil . Thermostats may control

solenoid valves in the l iquid refrigerant l ines to

the expansion valves so that flow is stopped

entirely when the thermostats are sat is f ied. I f

a l l flow is stopped the compressor operating a t

minimum capacity wil l p ~ t m p the boil ing refrigerantout of the evaporators reducting the suction

pressure unti l a pressure control is tripped andopens the compressor control ci rcui t stopping the

compressor. No matter what the circumstances when



~ ~

I Unloader spring

adjusting nur

2 U n load u spring3. U a loader ell ows

4. Uoloader oil tu be

5 Pi s to n ·

SLIDING YO KE ND R MP UNLOADER WIIH SUCTION PRESSURE SENSOR CONTROLLER

6 Plunger

7. Spring

8. Unloader lifter armass embly

9- Unloader lifter ring

ass embly

41

I 0. Suctioo valve plate

11. Su ct ion valve plate

spring

12 . Cylinder liner

13 Unloader ramp

14 . Linka ge

15 . Indexing piston

16 Indexing spr;. g s

17. Indexing ball

18 . Un loader body

19. Oil sup ply tube



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UNLO DER BELLOW S ND SPRING ASSEMBLY

42

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capacity control of reciprocating compressors used in refrigerati

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