Capacity Control of Reciprocating Compressors Used in Refrigerati
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Transcript of Capacity Control of Reciprocating Compressors Used in Refrigerati
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Purdue University
Purdue e-Pubs
International Compressor Engineering Conference School of Mechanical Engineering
1972
Capacity Control of Reciprocating CompressorsUsed in Refrigeration Systems
D. L. BoydChrysler Corporation
Follow this and additional works at: hp://docs.lib.purdue.edu/icec
Tis document has been made available through Purdue e-Pubs, a ser vice of the Purdue University Libraries. Please contact [email protected] for
additional information.
Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at hps://engineering.purdue.edu/
Herrick/Events/orderlit.html
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
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~ ~
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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LOCK NU r
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UNLO DER BELLOW S ND SPRING ASSEMBLY
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