Capacity Control of Reciprocating Compressors Used in Refrigerati.pdf

7/25/2019 Capacity Control of Reciprocating Compressors Used in Refrigerati.pdf

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I+#+a'+a C*#00 E+%'+##'+% C+$##+!# S!& $ M#!&a+'!a E+%'+##'+%

1972

Capacity Control of Reciprocating CompressorsUsed in Refrigeration Systems

D. L. BoydChrysler Corporation

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B5, D. L., "Caa!'5 C+ $ R#!'!a'+% C*#000 U0# '+ R#$'%#a'+ S50#*0" (1972). International Compressor

Engineering Conference. Pa# 6.&://!0.'.#.#/'!#!/6

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pounding of the cylinder

unloading mechanism by

the

valve 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 mechanism

and 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

and

connecting 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 t

is mounted on the inside of a cover plate

in

such

a

way

that

increasing suction

pressure

result ing

from 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 a

reduction 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.

A

select 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

halo

carbon 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 lever

is

an interesting

oi l

valving

hydraulic

act ion

design.

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

operat

ed

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 part

of 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 a

sintered metal f i l t e r from small foreign particles

which might obstruct

the

opening.

The

advantage of

this

pi lot operated system

is that

larger porting

for 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

problem

with

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

serious

internal 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

distance

away

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

refrigerant

out of

the evaporators

reducting

the suction

pressure

unti l

a pressure

control

is tripped and

opens the compressor

control ci rcui t stopping the

compressor. No

matter what the circumstances

when



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Capacity Control of Reciprocating Compressors Used in Refrigerati.pdf

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