U.S. patent number 3,795,117 [Application Number 05/285,695] was granted by the patent office on 1974-03-05 for injection cooling of screw compressors.
This patent grant is currently assigned to Dunham-Bush, Inc.. Invention is credited to Clark B. Hamilton, Harold W. Moody, Jr..
United States Patent |
3,795,117 |
Moody, Jr. , et al. |
March 5, 1974 |
**Please see images for:
( Certificate of Correction ) ** |
INJECTION COOLING OF SCREW COMPRESSORS
Abstract
A liquid injection expansion valve permits liquid refrigerant at
high pressure, bled from the condenser, to be injected into the
working chamber between the screws and intermediate of the suction
and discharge sides of a helical rotary screw compressor for
cooling the refrigerant working fluid and the captured oil. A
solenoid valve limits bleeding of liquid refrigerant from the
condenser at high compressor loads. A thermostat sensing the
temperature of the screw compressor discharge, modulates the liquid
injection expansion valve downstream of the liquid injection
solenoid valve. A compressor unloader slide valve may port oil and
the liquid refrigerant into the working chamber. The condenser may
be positioned at a height considerably above that of the screw
compressor to increase the head of the bled liquid refrigerant to a
pressure higher than the screw compressor discharge pressure.
System oil pressure may be supplied to a fluid pressure operated,
direct acting, on-off control valve upstream of the liquid
injection expansion valve and within the bleed line, under control
of a solenoid valve which is responsive to the temperature of the
oil leaving the oil pump, where such temperature is proportional to
compressor loading.
Inventors: |
Moody, Jr.; Harold W.
(Farmington, CT), Hamilton; Clark B. (Hartford, CT) |
Assignee: |
Dunham-Bush, Inc. (West
Hartford, CT)
|
Family
ID: |
23095336 |
Appl.
No.: |
05/285,695 |
Filed: |
September 1, 1972 |
Current U.S.
Class: |
62/197; 62/505;
62/228.1 |
Current CPC
Class: |
F25B
1/047 (20130101); F04C 29/042 (20130101); F25B
31/008 (20130101) |
Current International
Class: |
F25B
1/04 (20060101); F25B 1/047 (20060101); F25B
31/00 (20060101); F04C 29/04 (20060101); F25b
001/00 () |
Field of
Search: |
;62/196,197,505,468,469,470,471 ;418/201 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Perlin; Meyer
Attorney, Agent or Firm: Sughrue, Rothwell, Mion, Zinn &
Macpeak
Claims
What is claimed is:
1. In a closed loop refrigeration system including, in order: a
screw compressor, a condenser, and an evaporator, and having a
refrigerant circulating therebetween, the improvement
comprising:
means for bleeding high pressure refrigerant from said system and
injecting said liquid refrigerant into said screw compressor
working chamber intermediate of the suction and discharge sides of
the compressor for limiting the discharge temperature of the
refrigerant gas, and
means responsive to compressor load for controlling said bleeding
and injection means.
2. The refrigeration system as claimed in claim 1, wherein said
means for bleeding and injecting refrigerant comprises a bleed line
coupled at one end to said compressor working chamber and coupled
at the other end to said system intermediate of said condenser and
said evaporator and wherein a valve responsive to compressor load
is positioned within said line for controlling the flow of liquid
refrigerant therethrough.
3. The refrigeration system as claimed in claim 2, wherein said
valve comprises an on-off valve and wherein a thermostat responsive
to the compressor discharge temperature is operatively coupled to
said on-off valve for controlling operation of the same.
4. The refrigeration system as claimed in claim 3, further
comprising a variable flow valve positioned within said bleed line
and means further responsive to compressor discharge temperature
operatively coupled to said variable flow valve to modulate flow of
liquid refrigerant through said bleed line.
5. The refrigeration system as claimed in claim 2, further
comprising an oil separator at the discharge end of said screw
compressor to separate oil from the refrigerant working fluid, and
wherein; said means responsive to compressor load for controlling
the flow of liquid refrigerant through said bleed line comprises a
thermostat for sensing the temperature of said separated oil and
means operatively coupling said thermostat to said valve for
controlling operation of the same.
6. The refrigeration system as claimed in claim 5 further
comprising: a variable flow valve positioned within said bleed
line, and a thermostat responsive to compressor discharge
temperature operatively coupled to said variable flow valve to
modulate flow of liquid refrigerant through said bleed line.
7. The refrigeration system as claimed in claim 3, further
comprising an oil separator at the discharge end of said screw
compressor to separate oil from the refrigerant working fluid, and
wherein; said means responsive to compressor load for controlling
the flow of liquid refrigerant through said bleed line comprises a
thermostat for sensing the temperature of said separated oil and
means operatively coupling said thermostat to said valve for
controlling operation of the same.
8. The refrigeration system as claimed in claim 2, wherein said
condenser is located at a height, considerably above that of said
screw compressor, and said bleed line leading from said condenser
to said screw compressor is ported to said compressor working
chamber in the vicinity of the discharge side of the same, such
that the liquid refrigerant injected into the compressor working
chamber under control of said valve is at or above the compressor
discharge pressure.
9. The refrigeration system as claimed in claim 5, wherein said
condenser is located at a height, considerably above that of said
screw compressor, and said bleed line leading from said condenser
to said screw compressor is ported to said compressor working
chamber in the vicinity of the discharge side of the same, such
that the liquid refrigerant injected into the compressor working
chamber under control of said valve is at or above the compressor
discharge pressure.
10. The refrigeration system as claimed in claim 7, wherein said
condenser is located at a height, considerably above that of said
screw compressor, and said bleed line leading from said condenser
to said screw compressor is ported to said compressor working
chamber in the vicinity of the discharge side of the same, such
that the liquid refrigerant injected into the compressor working
chamber under control of said valve is at or above the compressor
discharge pressure.
11. The refrigeration system as claimed in claim 2, further
comprising a reciprocating slide valve for variably opening the
working chamber to the suction side of the screw compressor,
injection passage means carried by said slide valve, and wherein,
said means for coupling one end of said bleed line to said
compressor working chamber comprises means operatively coupling
said bleed line to said slide valve injection passage means.
12. The refrigeration system as claimed in claim 11, wherein said
valve comprises an on-off valve, and wherein a thermostat
responsive to compressor discharge temperature is operatively
coupled to said on-off valve for controlling the operation of the
same.
13. The refrigeration system as claimed in claim 11, further
comprising: a variable flow valve within said bleed line, and a
thermostat responsive to compressor discharge temperature
operatively coupled to said variable flow valve to modulate flow of
liquid refrigerant through said bleed line.
14. The refrigeration system as claimed in claim 5, further
comprising: a reciprocating slide valve for variably opening the
working chamber to the suction side of the screw compressor,
injection passage means carried by said slide valve, and wherein,
said means for coupling one end of said bleed line to said
compressor working chamber comprises means operatively coupling
said bleed line to said slide valve injection passage means.
15. The refrigeration system as claimed in claim 14, wherein said
valve comprises an on-off valve and wherein a thermostat responsive
to the compressor discharge temperature is operatively coupled to
said on-off valve for controlling operation of the same.
16. The refrigeration system as claimed in claim 14, further
comprising a variable flow valve positioned within said bleed line,
and a thermostat responsive to compressor discharge temperature
operatively coupled to said variable flow valve for modulating the
flow of liquid refrigerant through said bleed line.
17. The refrigeration system as claimed in claim 2, wherein said
valve comprises a variable position valve and wherein said means
responsive to compressor load comprises means responsive to the
temperature of the compressor discharge for modulating said
valve.
18. The refrigeration system as claimed in claim 17, further
comprising means responsive to the condensing temperature of the
refrigerant working fluid for additionally modulating said
valve.
19. The refrigeration system as claimed in claim 18, wherein said
valve comprises a movable valve member normally biased to valve
closed position, and means responsive to the temperature of the
compressor discharge tending to operate in opposition to said bias,
and further means responsive to the condensing temperature of the
refrigerant working fluid on said movable valve element tending to
maintain said valve element in valve closed position.
20. The refrigeration system as claimed in claim 19, wherein said
valve comprises a tubular valve housing, annular means defines a
fixed valve seat, a movable valve stem is concentrically positioned
within said valve seat and includes an enlarged portion closing off
said passage defined by the valve seat and said valve stem, a coil
spring concentrically carried by said valve stem biases said valve
to closed position, a diaphragm operatively contacts said valve
stem at one end and defines with said valve casing, a first closed
chamber and a second closed chamber on opposite sides thereof, a
bulb is positioned in heat conducting relationship to the
compressor outlet and is coupled to said first chamber by capillary
means, said chamber, said capillary means and said bulb carry a
thermo-expansive fluid, means fluid connects said second chamber
directly to said compressor at its discharge side, whereby, said
diaphragm is responsive to pressure differentials between said
chambers and acts directly on said valve stem to open the valve
against the bias of said concentric coil spring.
21. In a refrigeration system including a helical rotary screw
compressor, a condenser, and an evaporator, in that order, within a
closed loop, with refrigerant circulating therebetween, and wherein
said screw compressor includes a compressor housing defining with a
pair of intermeshed screws rotatably mounted therein, a compressor
working chamber and having a capacity control slide valve movable
within the rotor housing and away from a fixed valve stop
downstream of the compressor intake port for variably opening the
compressor working chamber to said intake port, the improvement
comprising: means carried by said slide valve for injecting liquid
refrigerant bled from the refrigeration system downstream of the
condenser into said screw compressor working chamber intermediate
of the suction and discharge sides of the same and downstream of
said fixed stop, for limiting the discharge temperature of the
refrigerant gas.
22. The refrigeration system as claimed in claim 21, wherein:
hydraulic motor means effects movement of said slide valve and
includes a slidable piston, rod means couples said piston at one
end and said slide valve at the other, said rod means includes
conduit means for feeding said liquid refrigerant, said slide valve
includes means coupled to said conduit means defining a liquid
refrigerant chamber, and at least one passage extends through a
wall of said slide valve and fluid connects said liquid refrigerant
chamber to said working chamber downstream of said fixed valve
stop.
23. The refrigeration system as claimed in claim 22, wherein; said
rod means comprises a plurality of concentric tubes, said slide
valve includes wall means defining a cavity concentrically
surrounding said rod means, said cavity is separated by wall means
intermediate of the axial ends of said cavity to form upstream and
downstream closed chambers extending transversely across said
cavity, and wherein first conduit means carried by said tubes means
is fluid connected to a source of pressurized oil and to said first
chamber, and has a passage opening up into the working chamber
downstream of said fixed stop, said tubes further include second
conduit means fluid coupled to said bled liquid refrigerant and to
said second chamber and fluid passage means fluid coupled to said
second chamber, and porting into said working chamber downstream of
said first passage, whereby liquid refrigerant is injected into
said working chamber downstream of the point of injection of said
lubricating oil regardless of the position of said capacity control
slide valve.
24. The refrigeration system as claimed in claim 22, wherein; said
hydraulic motor includes a piston mechanically coupled to said
slider valve by a piston rod extending therebetween, said rod
extends the full length of said slide valve and axially through the
center of the same, partition means define first and second axially
spaced fluid sealed chambers within said slide valve, fluid passage
means is carried by said shaft and extends the length of the same,
plug means carried by said shaft separate said passage means at
said partition means, and means fluid couple one of said fluid
passages to a course of lubricating oil, and the other of said
passages on the opposite side of said plug to said bled liquid
refrigerant and further passage means associated with each chamber
fluid couple respective portions of said shaft passage to said
chambers and said chambers to said screw compressor working chamber
downstream of said fixed valve stop.
25. In a refrigeration system including, in order, a screw
compressor, a condenser, and an evaporator in a closed loop with
refrigeration circulating therebetween, a port opening up into said
working chamber of the screw compressor intermediate of the suction
and discharge sides of said compressor, a bleed line fluid coupled
to said loop downstream of the condenser and to said port, a
compressor discharge temperature responsive liquid injection
expansion valve positioned within said bleed line for modulating
the flow of liquid refrigerant through said bleed line to said
port, and on-off valve means within said bleed line upstream of
said expansion valve, the improvement comprising: an external
source of fluid pressure, and wherein said on-off valve comprises a
direct acting fluid pressure operated, valve positioned within said
bleed line upstream of said liquid expansion valve, and said system
further includes means responsive to compressor load for coupling
said fluid pressure operated valve to said external source.
26. The refrigeration system as claimed in claim 25, wherein said
external source of said fluid pressure comprises the compressor oil
system, said pressure responsive on-off valve, is fluid coupled to
said system oil pressure by an oil line including a solenoid valve
therein, and said means responsive to compressor load for
controlling the flow of fluid pressure from said source to said
valve comprises a thermostat in heat receiving position with
respect to said compressor discharge and means operatively coupling
said thermostat to said solenoid valve.
27. The refrigeration system as claimed in claim 26, wherein said
thermostat comprises a thermal expansion bulb, fixed to said
conduit coupling the discharge side of the compressor to the intake
side of the condenser, and said means operatively coupling said
thermostat to said solenoid valve comprises an electrical circuit
including said solenoid valve and normally open thermostat
contacts, remote from said thermal expansive bulb and responsive to
expansion of temperature responsive material carried by said
bulb.
28. The refrigeration system as claimed in claim 26, further
comprising a by-pass line coupled to the oil pressure supply line
intermediate of the solenoid valve and the fluid pressure operated
valve and acting as an oil return, and fluid restriction means
carried with said by-pass line to limit flow therethrough and to
insure the continued operation of said on-off valve in response to
energization of said solenoid valve.
29. The refrigeration system as claimed in claim 27, further
comprising a by-pass line coupled to the oil pressure supply line
intermediate of the solenoid valve and the fluid pressure operated
valve and acting as an oil return, and fluid restriction means
carried with said by-pass line to limit flow therethrough and to
insure the continued operation of said on-off valve in response to
energization of said solenoid valve.
Description
BACKGROUND OF THE INVENTION
1. FIELD OF THE INVENTION
This invention relates to helical, rotary screw compressors, and
more particularly, to a simplified system of liquid injection to
control and limit the discharge temperature of the refrigerant
working fluid and the lubricating oil carried thereby.
2. DESCRIPTION OF THE PRIOR ART
In general, compressors are pumps that are used to raise gases or
refrigerant from one pressure level to a higher pressure level. In
the process, the vapor or gas is superheated by the work of
compression. Through thermodynamic relationships, operating
temperatures can be predicted by applying isotropic or polytropic
compression processes. With all types of compressors, the higher
the compression ratio
[compression ratio = (discharge pressure/suction pressure)]
The higher the discharge temperature that will be reached.
It is desirable to control and limit discharge temperatures so that
dangerous levels are not reached that may injure components and
lubricants and shorten their useful life. In the past, many methods
have been employed to inject fluids into gas streams for the
purpose of cooling or limiting compressor discharge temperature. In
air compressors, water mist has been sprayed into the compression
area, which vaporizes during compression to thereby limit
temperatures. In other compressors, oil injection has been used to
accomplish lower discharge temperature.
Attempts have also been made to inject liquid refrigerant into the
refrigerant vapor or working fluid as it is being compressed. This
has been accomplished by injecting liquid refigerant or refrigerant
rich oil into the suction side of the compressor where the
refrigerant evaporates and reduces the net inlet suction volume of
the compressor, decreasing the capacity of the compressor. Attempts
have further been made to add liquid to the gas discharge from the
compressor.
In the conventional systems employing axial screw compressors, the
need for oil cooling limits the discharge temperatures that the
system can tolerate. In systems with water cooled oil coolers, the
range of operation is usually established by water temperatures
available. In the case of minimizing the use of water or in air
cooled systems due to the ambient temperature of the air, there is
a problem in maintaining tolerable discharge temperatures. One way
of maintaining the discharge temperatures is through the use of
liquid injection along with some oil injection. In such a case,
locaion of the port for the liquid injection is critical in that,
if the liquid injection port is on the inlet or suction side of the
machine, the effect of the liquid being injected greatly affects
the volumetric efficiency of the compressor due to the fact that
the liquid will expand, flash off as it hits the low pressure
environment. There is also an effect on the horsepower requirements
of the machine, because the expanded liquid then is in gas form and
goes through a pressure range change and exits at the machine
discharge pressure.
If the point of liquid injection occurs at the discharge side of
the machine or after the gas has actually left the compressor
discharge area, not only is the pressure condition at the system
highest and thus there is an inherent requirement for an external
pump to pressurize the liquid to be injected above the maximum
compressor pressure of the system, but the added unit constitutes
an extra component which adds to the cost of the unit, and affects
the reliability of the system. Most importantly, where compressive
systems are designed hermetically, there is a rather confined
distance from the point of compressor gas discharge from the screw
compressor itself to where the same gas contacts and envelops the
motor winding of the hermetic electric motor and the space between
the two does not provide sufficient time or room for the liquid
injected at this point to properly expand and cool the discharge
gas prior to entering the motor compartment.
Conventionally, lubricating oil has been injected into the working
chamber, that is, the space occupied by the intermeshed screws in a
helical rotary screw compressor, for the dual purposes of
lubricating the intermeshed screws and to provide the necessary
seals between the rotating screws and the stationary housing.
Further, since the load on the compressor varies at times between
relatively large limits, the capacity of helical rotary screw
compressors has been modified by incorporating a capacity control
slide valve within the rotor housing and slidable parallel to the
axis of the screw. Axial movement of the valve is programmed by a
solid state, temperature initiated hydraulic actuated control
arrangement. The slide valve shifts longitudinally between limits
with the slide valve in closed position and against a valve stop
when the compressor is fully loaded, in which case all the gas
flows through the rotor housing from the intake to the discharge
side of the screw compressor. Unloading is achieved by moving the
valve away from the valve stop to create an opening within the
rotor housing through which the suction gas can return to the inlet
port area before compression of the same. Thus, in principle,
enlarging the opening in the rotor housing effectively reduces
compressor displacement. One mode of insuring that lubricating oil
is injected into the working chamber and between the intermeshed
screws has been to provide an axial passage in the mechanism
connecting the slide valve to a reciprocating fluid motor and
creating a closed chamber at the discharge side of the slide valve
with one or more radial ports opening up into the working chamber
downtream of the contact area between the end of the valve and the
stationary valve stop. In this case, as the slide valve opens to
reduce the capacity, oil injection occurs within the working
chamber closer to the discharge side of the compressor.
It is, therefore, an object of the present invention to eliminate
the necessity for a separate pump in liquid injection cooling of a
screw compressor and to effect liquid injection cooling of a screw
compressor without materially affecting the volumetric capacity of
the compressor or increasing the horsepower required.
SUMMARY OF THE INVENTION
In general, the objects of the present invention are met in
conjunction with a screw compressor operating as a component within
a refrigeration system wherein compressed gas is condensed to high
pressure liquid within a condenser and expanded in an evaporator or
chiller for cooling a refrigeration load, and then returned as a
low pressure gas to the inlet side of the screw compressor. The
invention involves bleeding high pressure refrigerant liquid from
the condenser and directing it through a liquid injection expansion
valve and into the screw compressor working chamber intermediate of
the suction and discharge sides of the same. A thermostat sensitive
to the compressor discharge temperature or to compressor load
either variably controls the volume of bled liquid refrigerant
which is injected into the screw compressor, or cycles the bleed
line on and off. A solenoid valve may be interposed in the bleed
line intermediate of the condenser and the liquid injection
expansion valve, with the solenoid valve being under control of a
thermostat bulb sensitive to the temperature of the oil, preferably
at the discharge side of the oil pump and downstream of the screw
compressor. In another embodiment, the condenser is located at a
height considerably above that of the screw compressor so that the
gravity head acts in conjunction with the normal high pressure of
the liquid refrigerant within the condenser to provide liquid
refrigerant within the bleed line at a pressure considerably above
the discharge gas pressure of the screw compressor so that the
refrigerant may be injected into the screw compressor at the
discharge side of the same or slightly upstream of the discharge
side.
A helical rotary screw compressor employs a capacity control slide
valve within the rotor housing movable away from a fixed valve stop
for reducing the capacity of the compressor. A chamber carried by
the valve may be supplied the bled liquid refrigerant which is
ported into the working chamber, downstream of the end of the slide
valve which contacts the stop when the compressor is fully loaded.
The liquid refrigerant chamber may be sealed from a second chamber
of the capacity control slide valve which ports lubricating oil
into the compressor working chamber; the refrigerant chamber
preferably being downstream of the oil chamber. Coaxial oil and
liquid refrigerant lines may be coupled to the respective sealed
chambers of the capacity control slide valve from opposite ends, or
by means of concentric passags direct flow to the chambers of the
slide valve or by means of parallel flow lines from the same
end.
In another embodiment, a thermal expansion valve within the bleed
line and responsive to the temperature of the compressor discharge
gas modulates the flow of liquid refrigerant through the bleed line
to the compressor injection port. An on-off valve is interposed in
the bleed line downstream from the thermal expansion valve and is
operatively coupled to an oil pressure line at the discharge side
of the oil pressure pump. The valve is direct acting in response to
system oil pressure. The oil pressure line, in turn, carries a
solenoid operated valve which is responsive to the temperature of
the oil at the discharge side of the pump which is representative
of compressor load. A by-pass oil line by-passes the oil pressure
operated valve and carries a restriction or orifice between the
valve and the sump.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a preferred embodiment of the
liquid refrigerant injection cooling system for a helical, rotary
screw compressor.
FIG. 2 is a schematic diagrammatic view of a refrigeration system
employing another embodiment of the present invention.
FIG. 3 is a schematic view of a refrigeration system incorporating
a third embodiment of the liquid refrigerant injection cooling
system of the present invention.
FIG. 4 is yet another schematic view of an alternate embodiment of
the liquid refrigerant injection cooling system of the present
invention, as applied to a refrigeration system.
FIG. 5 is an enlarged sectional view of the liquid injection
expansion valve employed in the liquid refrigerant injection
cooling system of FIG. 1.
FIG. 6 is a schematic, sectional view of a portion of a helical
rotary screw compressor of the type employed in the system of FIG.
1, employing a capacity control slide valve to port the liquid
refrigerant to the working chamber of the compressor.
FIG. 6a is an enlarged sectional view of the slide valve forming a
portion of the compressor of FIG. 6.
FIG. 7 is a sectional elevational view of a portion of a helical
rotary screw compressor similar to that of FIG. 6 but illustrating
an alternative mode of porting the liquid refrigerant to the screw
compressor working chamber.
FIG. 8 is a perspective view of the refrigeration system of the
rotary screw compressor type employing the improved liquid
refrigerant injection cooling system of the present invention.
FIG. 9 is a schematic representation of the liquid injection
cooling system of the present invention as applied to the
refrigeration system of FIG. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference to FIG. 1 illustrates in perspective view a refrigeration
system incorporating a helical, rotary screw compressor which
employs the liquid refrigerant injection oil cooling system of the
present invention, in one form. the principal components of the
closed loop refrigeration system are: the helical, rotary screw
compressor assembly 10; the condenser 12 which receives the
compressor discharge through conduit 14; the chiller or evaporator
16, which is connected to the discharge end of the condenser 12 via
conduit 18 and a filter drier 20 incorporated in conduit 18
intermediate of condenser 12 and chiller 16. Conduit 22 connects
the discharge end of the chiller to the intake or inlet side of the
axial screw compressor 10. Lubricating oil is circulated through
the compressor assembly 10 and in fact mixes to a limited extent
with the refrigerant working fluid, and is separated by oil
separator 25 located after hermetic electric drive motor 24, at the
downstream or discharge side of the screw compressor. The separated
oil enters oil sump 28 through pipe 27 and is pressurized by an oil
pump (not shown) at one end, prior to being returned to the
compressor assembly 10 after passage through an oil cleaner or
filter 30. The oil filter 30 receives oil from sump 28 through line
32 and discharges the same through a plurality of lines 34 at the
discharge end of the oil filter 30. These components of the
refrigeration system and the oil system are otherwise
conventional.
In general, the refrigerant working fluid which may comprise freon
or the like, enters the right hand end of compressor assembly at
10, through inlet 22 in gaseous or vapor form at relatively low
pressure and is compressed by the helical rotary screw compressor
26 for discharge over and across the hermetic motor windings (not
shown) of the electric motor 24 and exits from oil separator 25.
The high pressure vapor discharge from the compressor passes
through conduit 14 into the condenser 12 where the vapor is
condensed by contact with tubes containing coolant such as water or
air and is discharged therefrom through conduit 18 to filter drier
20. The high pressure liquid refrigerant then expands within
chiller 16 to cool the load consisting of water or other heat
exchange fluid within the chiller or evaporator 16. The expansion,
modulation and delivery of liquid refrigerant to the evaporator or
chiller 16 is suitably controlled by means 36 intermediate of the
filter drier 20 and the intake side of the evaporator or chiller
16. Further, for the purposes of the present invention, lubricating
oil at a relatively low temperature leaves the oil filter 30 via
lines 34 to various points along the compressor assembly 10 for
lubrication purposes or is employed as hydraulic motor fluid for
one or more hydraulic motors such as that associated with
compressor unloader 40, for instance, via line 42.
The present invention is directed to a liquid refrigerant injection
system for limiting the discharge temperature of the compressor and
thus the maximum temperature to which the system oil is subjected.
In this respect, high pressure liquid refrigerant is bled from
conduit 18 downstream of the filter drier 20 as at 44 by a small
diameter bleed tube or line 46 through shut off valve 48. The
liquid refrigerant passes through a solenoid valve 50 within bleed
line 46 and flows to a liquid injection expansion valve 52 which
modulates the flow of liquid refrigerant through the bleed line 46
and into the screw compressor 26, line 46 opening into the
compressor working chamber intermediate of the discharge and
suction sides of compressor 26. A second manually operable shut off
valve 54 is positioned within the bleed line 46 intermediate of the
screw compressor 26 and the liquid injection expansion valve 52 to
assist in isolating the injection system. The liquid injection
expansion valve 52 is of the modulating or variable flow type.
It is important to note that the solenoid valve 50 may be a pilot
operated, solenoid energized, two position valve such as that
produced by ALCO Controls Corporation of St. Louis, Missouri, a
division of Emerson Electric Company, under the trade designation
230R8.
In this case, a thermostat bulb 60 may be filled with a temperature
expansive material such as a liquid or gas, which operates a
thermostat associated with the panel (not shown) causing the
normally open contacts to close at a predetermined oil temperature
which, in turn, closes the electrical circuit to the solenoid which
is integral with valve assembly 50.
This is in contrast to the liquid injection expansion valve 52
which is nonelectrical in operation and which is coupled to a
temperature expansion valve bulb 56 by a capillary tube 57. The
bulb and capillary tube carry a temperature expansive material
which may be liquid, gas or part liquid and part gas and which
expands in response to increased temperature to variably shift a
movable valve element within valve 52 to modulate the flow of
liquid refrigerant to the injection port of the screw compressor
26. Essential to the operation of the expansion valve 52, is
equalizing line 53 which couples the expansion valve 52 to the
screw compressor 26 at the discharge side of the same.
Turning to FIG. 5, it is noted that the liquid injection expansion
valve 52 consists essentially of a dumb bell shaped valve housing
or casing 80 expanding at its upper ends and supporting a diaphragm
82 across the chamber 84 defined by that end of the valve housing,
the diaphragm 82 exerting pressure on a movable valve stem 86 which
is movable axially, being supported by a fixed guide member 88, the
stem carrying a disc 90 which is movable therewith, within housing
80 and against which one end of a compression spring 92 abuts, the
other end abutting against the fixed guide means 88. The end of the
valve stem 86 carries an enlarged diameter valve element 94, which
is normally biased against an annular valve seat 96 preventing the
liquid refrigerant in the bleed line 46 to pass through the
horizontal inlet and discharge through the vertical outlet at the
bottom of the casing. Chamber 84 is divided by the diaphragm into
an upper section 98 which is coupled by means of capillary tube 57
to the bulb 56. Further, the equalizing line 53 opens up into the
valve casing 80 and being in fluid communication with the lower
section 100 of working chamber 84 and acting on the bottom of the
diaphragm 82 in opposition to the pressure exerted by the
thermo-expansive material which additionally fills the upper
chambers section 98 as well as the capillary tube 57 and bulb
56.
The temperature responsive bulb 56 is operatively positioned with
respect to the compressor discharge, that is, it is mounted on or
adjacent to conduit 14 which couples the discharge side of the
compressor to the inlet side of the condenser 12. Alternatively, it
could sense oil temperature downstream of the oil separator 25,
which is proportional to compressor load. However, this is only one
of the parameters affecting the operating of the liquid injection
expansion valve 52, the other being the pressure within equalizing
line 53 which provides the valve 52 with a signal respresentative
of condensing temperature and which acts on the opposite side of
the diaphragm to the signal indicative of a compressor discharge
temperature. The compressive force acting on the valve stem 86
through the fixed disc 90 and thus on the diaphragm in the same
direction as the force emanating from the equalizing line 53, is
purposely set to assist in keeping the valve closed in conjunction
with the force representative of condensing temperature from line
53. The present invention is directed to a system in which it is
desired to keep a fixed temperature differential between the
condensing temperature and the compressor discharge temperature
regardless of operating conditions of the helical rotary screw
compressor. It is a characteristic of the oil separator 20 that it
requires approximately a 35.degree. temperature differential
between the condensing temperature of the refrigerant and the
discharge temperature of the refrigerant to operate efficiently.
Depending upon the pressure that the oil experiences in the sump or
at the oil separator and the temperature, the oil absorbs to a
greater degree or to a lesser degree, refrigerant. If the
temperature of the oil is less than 140.degree. F., it has a
tendency to pick up refrigerant. Valve 50 and bulb 60 insure that
the temperature of the oil stays above 140.degree. F. The function
of valve 52 is to maintain a 45.degree. superheat temperature
difference between the condensing temperature and the discharge
temperature. This is accomplished by selecting a spring having the
desired spring constant and providing a charge within the power
assembly, that is, the temperature responsive material within bulb
56 capillary 57 and the upper power assembly chamber section 98 and
acting on the diaphragm 82 in opposition to the spring 90. Only
under these two conditions will the valve open to variably supply
the liquid refrigerant to the working chamber of the compressor in
an attempt to maintain the desired conditions.
This may perhaps be best appreciated by reference to standard
refrigerant pressure enthalpy curves wherein a compressor inlet
condition is defined as an inlet pressure and an inlet temperature
with the inlet temperature being above the saturated vapor level,
for instance, which in that case, the refrigerant vapor has suction
superheat, but if it lies on the saturated vapor line, the suction
gas does not have superheat. During compressor operation, the
predicted discharge temperatures basically follow an isotropic line
on the pressure and enthalpy diagram. These lines are basically to
the right of the saturated vapor line and, depending on the inlet
conditions of the compressor, the compression ratio and the actual
discharge pressure, the condition of the gas exiting from the
compressor will be at different temperature levels, meaning that
the actual temperature as compared to the condensing condition at
which the compressor is operating will have various discharge
superheats. The effect of the valve 52 is to fix the discharge
superheat, that is, to maintain the same in terms of the minimum
discharge superheat for proper system operation and, in particular,
to insure proper operation of the oil separator.
The temperature responsive bulb 56 is operatively positioned with
respect to the compressor discharge, and in the illustrated
embodiment it is mounted on or adjacent to conduit 14 which couples
the discharge side of the compressor to the inlet side of the
condenser 12. Alternatively, it could sense oil temperatures
downstream of oil separator 25, which is proportional to compressor
load. Compressor discharge temperature, modulated by condensing
temperature therefor, controls the flow rate of liquid refrigerant
through the liquid injection expansion valve such that, in general,
as the temperature of the compressor discharge increases, more
liquid refrigerant is injected through the expansion valve which
opens to a greater extent to increase the flow of liquid
refrigerant to the screw compressor 26. Preferably, a sight glass
58 is placed in the bleed line 46 to visually ascertain the extent
of refrigerant flow through that line.
In contrast, the solenoid valve 50 is an electrically powered
on-off valve, which is responsive to oil temperature, to permit
initial flow of refrigerant to the axial screw compressor 26 to
limit the maximum temperature of the working fluid being compressed
and thus the temperature rise of the oil within the system. In this
respect, thermostatic bulb 60 senses the temperature of the oil
from sump 28 at the discharge end of the same as it passes through
conduit 32 prior to entering oil filter 30 and signals the solenoid
of the liquid injection solenoid valve 50 to open the valve in
response to rise in oil temperature to a predetermined minimum
value of say 140.degree. as previously described.
In operation of the embodiment of FIG. 1, liquid injection is not
required by the machine during its off cycle or when the unit is
running at low refrigeration loads, since at low loads, the
discharge temperature of the compressor working fluid is at a point
where additional cooling is not required. Since the temperature of
the oil is a function of the temperature of the discharge working
fluid from the compressor, the oil temperature at the oil pump
outlet is insufficient to turn the solenoid valve on. Assuming the
load on the chiller or evaporator 16 increases to the point where
the oil temperature reaches the predetermined minimum level
necessary to open the solenoid valve 50, liquid refrigerant flows
through the bleed line 46, at slightly less than the discharge
pressure from the compressor unit 10, to the liquid injection
expansion valve 52. The thermostat bulb 56 modulates the volume of
liquid injection expansion valve 52 and only the amount of liquid
refrigerant is injected into the compressor which is sufficient to
maintain the gas discharge temperature within a predetermined
range. Thus, as load increases, the discharge temperature of the
working fluid increases and bulb 56 senses the demand for more
liquid refrigerant to be injected into the screw compressor just
upstream from the discharge side of the screws. Thus, as discharge
gas temperature increases, the expansion valve 52 opens wider to
deliver more liquid refrigerant through bleed line 46 to the
compressor 26, and conversely as the load within the evaporator or
chiller falls off, the compressor discharge temperature decreases
and the expansion valve will throttle back to reduce the supply of
liquid refrigerant delivered to the compressor. In the illustrated
embodiment, it is not necessary to employ an external separate oil
cooler. Since oil seeps into and forms a small part of the working
fluid passing through the compressor, it must be separated
therefrom prior to passing the refrigerant through the condenser
and chiller or evaporator, since the presence of oil interferes
with the heat transfer function of the refrigerant. After the
working fluid passes through the separator which removes the oil
from the refrigerant, it passes through conduit 14 to the intake
side of condenser 12. Oil, in turn, accumulates within sump 28,
then is driven by a pump back to the compressor assembly 10. The
precentage of oil in the refrigerant which is a liquid and at
relatively high pressure in the condenser, is very small, from zero
to three per cent or less and does not materially interfere with
the liquid refrigerant which is injected through bleed line 46 just
upstream from the discharge side of the screws. Some minor flashing
or vaporization may occur intermediate of the liquid expansion
valve and the port (not shown) within the screw compressor 26, but
most of the expansion and vaporization takes place within the space
defined between the lobes of the screws which space captures the
working fluid gas which is compressed as it moves from the intake
side to the discharge side of the intermeshed screws. The purpose
of shutoff valves 48 and 54 is to isolate the bleed line components
from the main components of the refrigeration system for
maintenance purposes.
The thermostat 60 which is responsive to the temperature of the oil
at the oil pump discharge and which controls the on-off solenoid
valve 50, is positioned to be responsive to the temperature of the
oil, since the outside surface of the oil line heats up much
quicker than the outside surface of the refrigerant discharge
conduit 14 and therefore the change in the oil temperature conduit
surface anticipates the subsequent increase in the temperature of
refrigerant working fluid conduit exterior surface and effects
liquid refrigerant injection into the screw compressor at the time
that injection is needed rather than at some point subsequent
thereto.
From the above, it is seen that the present invention truly limits
the discharge temperatures at the heart of the compressor and by
selecting the point of injection of the liquid refrigerant, cooling
is achieved without materially affecting the volumetric capacity of
the compressor. Injection occurs when the suction stroke has been
completed and the compression stroke is well in process, and may in
fact occur just at the point of full compression prior to discharge
from the compressor. Further, the injection of liquid refrigerant
during the compression stroke permits the heat required to vaporize
the liquid refrigerant to remove a significant part of the heat
generated in the compression process and thereby materially
reducing the discharge temperature of the working fluid and, of
course, the maximum temperature to which the oil is subjected.
Further, since the pressure level of the injected refrigerant is
only slightly less than the discharge pressure of the working
fluid, the horsepower requirement to raise the vaporized
refrigerant to the full discharge pressure is minimum and the very
small horsepower increase necessary to achieve this end does not
seriously affect the overall efficiency of the compressor.
Turning to FIG. 2, there is illustrated schematically a
refrigeration system similar to that of FIG. 1 which incorporates
liquid refrigerant injection for oil cooling purposes and when the
liquid injection system components are modified to some extent. In
this case, the screw compressor unit or assembly 110 comprises a
screw compressor motor 124 for driving the screw compressor 126
which receives the gaseous or vaporous refrigerant working fluid
through intake line 122 and compresses the same for discharge over
the screw compressor motor 124, the high pressure vapors exiting
from the screw compressor through discharge conduit 114. The oil
separator 125 is illustrated schematically, in this case, as being
downstream of the screw compressor motor and upstream of a
conventional water cooled condenser and positioned within line 114
intermediate of a screw compressor assembly 110 and condenser 112.
Again, high pressure liquid refrigerant leaves condenser 112 via
conduit 118 and passes to the chiller or evaporator 116 via filter
drier 120. Schematically, a liquid refrigerant bleed line 146
permits some of the high pressure liquid refrigerant to be bled to
the screw compressor 126 for injection through an injection port
170 defined by the headed end of the arrow which lies intermediate
of the suction and discharge sides of the screw compressor 126.
Refrigerant vapor returns from the chiller or evaporator 116 to the
suction side of the screw compressor through line 122.
The simplified liquid refrigerant injection control system in this
embodiment, consists essentially of a thermostat in the form of a
temperature sensitive bulb 156 which carries along with capillary
157, a temperature responsive material which upon expansion
modulates the valve 152 to thereby variably increase the delivery
of liquid refrigerant to the injection port 170 of screw compressor
126. In like manner to the embodiment of FIG. 1 equalizing line 153
provides a second input to valve 152 responsive to condensing
temperature. While the system contains a temperature sensistive
expansion valve 152 and while the valve modulates the flow of
refrigerant in direct proportion to the temperature at the
discharge side of the screw compressor unit 110 and while this
variation simplifies the external components of the system, this
variation in the control scheme may be less desirable, since it is
necessary that the valve 152 maintain a good tight shut off
condition, that is, one in which at no load or when the machine is
in compressor off portion of the cycle, liquid refrigerant will not
be injected into the working chamber through injection port 170.
Alternatively, the bulb 156 could be operatively positioned at oil
separator 125 and responsive to oil temperature at the separator or
within the oil sump.
Turning to FIG. 3, a further embodiment of the present invention is
shown for a refrigeration system which is identical in most
respects to the refrigeration system of FIG. 2. In this case,
similar components are given similar numerical designations. The
only difference in this embodiment, is the fact that the bleed line
146 carries a solenoid operated on-off valve 150 similar to liquid
injection solenoid valve 50 of the first embodiment and, in which
case, the electric control line 157 provides electrical current to
the solenoid under control of thermostat operated switch contacts
at a control panel 171 intermediate a thermostat bulb 172
operatively associated with the discharge line 114 at the discharge
end of the screw compressor assembly 110. Bulb 172 could
alternatively be positioned so as to sense directly the temperature
of the separated oil at or downstream of oil separator 125. The
thermostat bulb 172 carries a temperature responsive material such
as a liquid or a gas along with capillary tube 173 which is coupled
to the thermostat (not shown) within panel 171. Switch contacts
close the curent through line 157 which includes the solenoid
associated with valve 150 and which is powered from a source (not
shown). The solenoid valve 150 merely cycles on and off and the
function of the thermostat is to either allow injection of liquid
refrigerant via port 170 to the screw compressor 126 or prevent
such injection of liquid refrigerant thereon. In this case, there
is no modulation or variation in the rate of flow of refrigerant to
the injection port 170 and, at high temperatures, liquid
refrigerant is injected into the screw compressor while at low
working fluid discharge temperatures, there is an absence of liquid
injection. The thermostat 172 merely functions to signal the valve
150 to open or close and thus continually cycles the valve during
operation of the system. Again, while the system is simplified, a
possible detrimental effect is the fact that the cycling of the
components may reduce the life of the same well below the design
life of the major components of the machine, such as the screw
compressor, etc.
Reference to FIG. 4 illustrates a fourth embodiment of the
invention, again as applied to a refrigeration system essentially
identical to that of FIGS. 2 and 3 and, in which case, the like
components are given like numerical designations. In the embodiment
of FIG. 4, however, it is desirable to increase the pressure of the
liquid refrigerant in bleed line 174 prior to injection of liquid
refrigerant into the compressor at injection port 176 which is very
close to the discharge side of the screw compressor as compared to
discharge port 170 of embodiments illustrated in FIGS. 2 and 3. In
this case, the condenser 112 rather than being positioned close to
and at the same height as the other components of the refrigeration
system, is positioned at a much greater height. For instance, the
refrigeration system may be one incorporated within a relatively
tall building as, for instance, where the condenser 112 is
positioned on the roof, perhaps some 10 or 15 stories above the
screw compressor assembly 110, and the other components of the
system which may well be in the basement of the same building. In
this case, within the line 118 leading from the condenser to the
drier filter 120 and at the condenser itself, the bleed line 174
creates a static pressure head relative to the injection port 176
which adds materially to the pressure of the liquid refrigerant
emanating from condenser 112. In fact, the combined pressure head
of the liquid refrigerant within belled line 174 may be in excess
of the discharge pressure of the screw compressor discharge working
fluid. Again, the system employs a temperature sensitive bulb 156
carrying a temperature expansible material as does capillary 157 to
modulate operation of valve 152 proportional to the temperature of
the discharge gas, further modulated by the condensing temperature
fo the working fluid via equalizing line 153 in similar fashion to
the embodiment of FIGS. 1 and 2, the bulb 156 being operatively
associated with the conduit 114 at the discharge side of the screw
compressor assembly 110. Thus, the valve 152 variably controls the
flow of liquid refrigerant at high pressure which enters the screw
compressor through the injection port 176, in this case very close
to the discharge side of the same. In each of the embodiments of
FIGS. 1, 2 and 3, the condenser may be a distinct height advantage
over the other components of the system.
Reference to FIG. 6, illustrates an alternate embodiment of the
present invention in which the liquid refrigerant is ported to the
working chamber of the helical rotary screw compressor by means of
a capacity control slide valve associated therewith. Like elements
to the embodiment of FIG. 1 are like numbered. The screw compressor
may be, in general, identical to the compressor illustrated in FIG.
1 as at 10 and including the hermetically sealed type and including
from left to right, the screw compressor itself, as at 26, the
hermetic drive motor 24 and terminating in an oil separator 25
which drains the oil to a sump (not shown) through an oil discharge
passage 27, the compressed refrigerant working fluid exiting
through an axial port 29 to gas discharge conduit 14. To the left
of the screw compressor, beyond an intake opening 200 within the
compressor housing 202, is a hydraulic motor 212 fixed to one end
of the rotor housing, and extending outwardly thereof. Essentially,
the unloader 40 consists of a reciprocating capacity control slide
valve 204 mounted within the rotor housing 202 and fixed at one end
to an outer tubular shaft or rod 206 which is concentrically fixed
to an inner shaft or rod 208, the slide valve 204 moving therewith
at one end of both shafts 204 and 206 in response to shifting of
piston 210 within cylinder 214 of the unloader hydraulic drive
motor 212. The inner shaft 208 sealably carries a fitting 216,
closing off one end of tubular shaft 204, while the opposite end of
the inner shaft is fixed to an end wall 218 of slide valve 204.
Concentric shafts or rods 206 and 208 are sealably and slidably
carried by the compressor housing. Valve 204 reciprocates between
limits defined by casing end wall 219 and a valve stop 220 and,
when moved to the position illustrated in FIG. 6, provides a
relatively large opening 222 within the bottom of the rotor
housing. The working chamber 224 carries the intermeshed male and
female screws or rotors 226 and 228. The capacity control slide
valve forms no part of the present invention as such, and, when in
the closed position, the flow of all of the gas emanating from
suction passage 200 passes through the rotor housing and is
compressed by the intermeshed screws for discharge at the right
hand side of the screw compressor filling the discharge chamber
230. Unloading is initiated by the slide valve 204 as it moves from
left to right and away from the fixed valve stop 220 to thus create
the variably sized opening 222 through which the suction gas can
return from the rotor housing to the inlet or suction port
associated with passage 200 before the working fluid has been
compressed. As there has been no sufficient amount of work done on
the return gas, reduced compressor capacity is obtained at no loss.
Shifting of the piston 210 occurs by selectively porting
pressurized lubricating oil or hydraulic fluid selectively to one
side or the other of a piston through passages 232 and 234.
The present invention in the embodiment of FIG. 6 provides the
means for supplying both oil to the working chamber 224 under high
pressure for lubrication and sealing of the rotary screws and also
for porting liquid refrigerant to the working chamber, the
expansion of which achieves cooling of the screws, the working
fluid and any oil in contact therewith. In this respect, a suitable
passage 240 is formed between the inner and outer shafts 206 and
208 through which oil under pressure is delivered from flexible
line 236 and the fitting or coupling means 238. Passage 240 extends
axially, FIG. 6a, and opens up into an oil chamber 242 defined by
an end wall 244 of the slide valve 204 and a partition member 246
as well as spaced walls 248 and 250 at the top and bottom of the
slide valve 204. Oil is ported directly to the working chamber 224
through one or more inclined passages 252, thus permitting the oil
to reach the intermeshed screws intermediate of the suction and
discharge sides of the compressor.
The present invention is directed to the employment in this
embodiment of a second axial conduit, preferably by making the
inner shaft 208 hollow, that is, providing it with a bore 254 and
supplying by means of a second flexible conduit 256, liquid
refrigerant via bleed line 46 and coupling means or fitting 216 to
a liquid refrigerant chamber 260 defined by the right hand end wall
218 of the slide valve 204, upper and lower walls 248 and 250 and
the partition 246, which spaces the same and sealably surrounds the
internal shaft 208. The bore 254 terminates near the right hand
end, and one or more radial passages 266 permit delivery of the
liquid refrigerant to chamber 260. Further, one or more oblique or
inclined passages 268 downstream of the oil injection passages 252
open up into the working chamber 224 to insure liquid injection of
the refrigerant at predetermined points in the compression cycle
when the valve is fully closed and the compressor is working at
maximum capacity. Of course, as the valve shifts from left to right
and moves to open position, the point of injection for the liquid
refrigerant changes and shifts mechanically, however, because the
capacity is reduced, the net effect is positive rather than
negative in terms of compressor efficiency, etc.
Rather than employ concentric tubes or shafts in which oil and
liquid refrigerant passages are formed between or within any given
tube or tubes, spaced parallel tubes may be employed, in which case
the liquid refrigerant and the oil travel in parallel paths from
the fluid motor to the slide valve, in this case the parallel flow
paths terminate at respective oil and liquid refrigerant chambers.
An alternative arrangement for delivering both oil to its injection
port within the capacity control slide valve and the refrigerant to
its injection port, is shown in FIG. 7. Again, the drawing shows a
portion of a hermetic compressor similar to that of FIG. 6, and
like elements are given like numerical designations. In this case,
the shaft 206 connects the piston 210 of the hydraulic motor 212 to
the slide valve 204, is provided with a first bore section 270
which acts as a central oil passage, this bore section being
provided with one or more radial openings 272 permitting the oil to
enter the oil chamber 242 again defined by end wall 244, partition
246, a top wall 248, and a bottom wall 250. Oil under pressure is
delivered to shaft 206 through line 236 via fitting 238. A plug 274
closes off the hollow shaft 206. Extending coaxially therewith but
in the opposite direction is a second bore section 276. Shaft 206
protrudes through the right hand end wall 218 which forms with top
and bottom walls 248 and 250 and the partition 246, a liquid
refrigerant chamber 260. A series of radial passages 282 permits
the liquid refrigerant which enters through a flexible conduit 284
coupled to the outer end of shaft 206 by coupling means 286 to fill
the chamber and to be injected through one or more inclined
passages 268 which are ported to the working chamber 224 in similar
manner to the embodiment of FIG. 6, downstream of oil passages 252.
In this case, the costruction is somewhat simplified in that the
necessity for multiple concentric tubes or shafts is eliminated and
the oil and refrigerant are essentially isolated from each other.
Further, the natural introduction from opposite sides permits the
injection passages of the oil to lie upstream of the refrigerant
injection passages.
It is to be noted that the liquid refrigerant injection system in
which refrigerant is injected through passages carried by the slide
valve, is controlled identically to the manner set forth with
respect to those systems illustrated in FIGS. 1-4 inclusive.
Further, with respect to the systems of the present invention and
with respect to the employment of a slide valve for porting the
refrigerant to the working chamber, it is the injection of the
liquid refrigerant which maintains lower operating temperatures for
the compressor. This is especially advantageous for refrigeration
systems in which water is unavailable as a coolant, especially
where water is normally employed as a liquid for an oil cooler
incorporated within the oil system. For a large ammonia
refrigeration system which is air cooled, this system is
particularly advantageous especially where the oil cooler may be
dispensed with, eliminating all the problems and foul ups in terms
of oil leakage and water requirements. Of course, this system has
application also to refrigeration systems in which an oil cooler is
required to partially maintain the oil temperature within defined
limits.
In the systems previously described, the methods involve bleeding
high pressure refrigerant liquid downstream of the condensor and
directing it through a temperature modulated expansion valve where
it enters the screw compressor working chamber through a port which
lies intermediate of the suction and discharge sides of the
compressor. A thermostat sensitive to the compressor discharge
temperature, for instance, variably controls the volume of bled
liquid refrigerant through the expansion valve which operates in
conjunction with a solenoid valve normally interposed in the bleed
line downstream of the liquid injection expansion valve. The
solenoid valve may be controlled by a second thermostat which is
sensitive to the temperature of the oil in the sump, and preferably
at the discharge side of the oil pump. The temperature varies with
compressor load and the solenoid valve acts to ensure that liquid
refrigerant flows through the bleed line to the liquid expansion
valve only during compressor operation and when the compressor is
running above the minimum load condition. The solenoid valve is
often subjected to chattering which is detrimental to the component
life, and is also undesirable due to the noise problem.
Since the liquid refrigerant passing through the bleed line is at a
pressure approximate to, or normally, slightly less than the
discharge pressure of the compressor, and since it is injected into
the compressor working chamber at a port location quite close to
the discharge side of the compressor, the pressure differential
across the bleed line is relatively small. Essentially the flow in
the liquid injection assembly constituted by the bleed line is
modulated by three mechanisms; (1) the thermal expansion valve; (2)
the port location which is the available differential between the
liquid line; and (3) the point of the compression cycle of the
screw compressor at which injection occurs. This differential in
turn is effected by the operating conditions relative to the built
in compression ratio of the screw compressor. In other words, for a
given built in compression ratio and port location, the liquid
injection is shut off and the operating differential (P.sub.D
-P.sub.S) drops to some point below the built in ratio because the
liquid pressure drops to, or below the port pressure. The fourth
mechanism comprises the injection port size; for instance if the
port size drilled into the compressor is too small, the injection
rate is restricted at high flow rates due to the .DELTA.P in the
port itself.
An investigation of valve chattering, further indicates that a
condition which is normally present when valve chattering occurs is
the existence of a solid column of liquid downstream of the thermal
expansion. The effect of the solid column of liquid refrigerant
feeding directly to the motor is to produce a "water hammer"
effect. This pulsation, combined with the operation of the pilot
operated solenoid valve in the marginal .DELTA.P range, is the
basic cause of the liquid injection solenoid valve chattering.
Attempts have been made to minimize the valve chattering in the
marginal .DELTA.P ranges by such steps as moving the injection port
towards the suction side of the compressor to increase the .DELTA.P
from discharge pressure to the injection port. While this minimizes
the chances, it does not completely eliminate the possibility of
operating with a solid column of liquid, however, the moving of the
port may have a highly adverse effect on performance.
Further, attempts have been made to improve the pilot operated
solenoid valve such that the pilot operates at approximately a zero
pressure differential, but, such valves are necessarily
expensive.
Reference to FIG. 8 illustrates in perspective view, a closed loop
refrigeration system incorporating a rotary screw compressor which
employs the improved liquid refrigerant injection cooling system of
the present invention in an alternative form. The principle
components of the refrigeration system comprise; the axial, rotary
screw compressor assembly 310 consisting of the axial screw
compressor 326, the electric drive motor 324 and oil separator 325
axially positioned in that order from the suction or intake side of
the assembly as defined by conduit 322. The refrigeration system
further comprises a condenser 312 which receives the compressor
discharge through conduit 314 and cools the same by water or other
heat exchange medium passing therethrough. The discharge of high
pressure liquid refrigerant from condenser 312 occurs through
conduit 318 which connects the condenser 312 to the evaporator or
chiller 316, with the refrigerant passing through a filter dryer
320 incorporated within conduit 318 intermediate of condenser 312
and chiller 316. The conduit 322 connects the discharge side of the
chiller or evaporator 316 to the intake or suction side of the
axial screw compressor 326. Lubricating oil is circulated through
the compressor assembly 310 and, in fact, mixes to a limited extent
with the refrigerant working fluid during compression, but the
lubricating oil is separated from the working fluid by oil
separator 325 associated with the electric drive motor 324 at the
downstream or discharge side of the screw compressor. The separated
oil accumulates with an oil sump 328 via pipe or passage 327 and is
pressurized by the oil pump (not shown) within sump 328 prior to
being returned to the compressor assembly 310 after passage through
an oil cleaner or filter 330. The oil filter 330 receives oil from
sump 328 through line 332 and discharges the same through a
plurality of lines 334 indicated generally at 334 at the discharge
side of the oil filter 330. The components of the refrigeration
system and the oil system as previously described, are otherwise
conventional and form no part of the present invention.
In general, the refrigerant working fluid which may comprise Freon
or the like, enters the compressor assembly 310 at the right hand
end through inlet or intake conduit 322 in gaseous or vapor form at
relatively low pressure and is copressed by the rotary screw
compressor 326 to a relatively high pressure for discharge over and
across the motor (not shown) of electric motor 324. The refrigerant
as a high pressure gas exits from the oil separate 325 and passed
to the condenser 312 where it condenses, by contacting with a
coolant such as water. The high pressure liquid refrigerant then
passes to the filter dryer 320 via conduit 318 and to the chiller
or evaporator 316. The high pressure liquid refrigerant expands
within the evaporator 316 to cool the load which may consist of
water or other heat exchange fluid within the evaporator 316, with
the expansion, modulation and delivery of liquid refrigerant to the
evaporator or chiller being suitably controlled by means 336 within
line 318 intermediate of the filter dryer 320 and the intake side
of the evaporator or chiller 316.
For the purposes of the present invention, and which forms no part
of the present invention, in brief, lubricating oil at relatively
low temperature leaves the oil filter 330 via lines 334 to various
points along the compressor assembly 310 for either lubrication
purposes or to be employed as a hydraulic motor fluid for hydraulic
motors such as that associated with compressor unloader 340; oil
passing to the unloader for instance via line 342.
The present invention is directed to an improvement in a liquid
refrigerant injection system which limits the discharge temperature
of the compressor working fluid and thus the maximum temperature to
which the system oil is subjected. In that respect, high pressure
liquid refrigerant is bled from conduit 318 downstream of the
filter dryer 320 at a bleed or tap point 344 by a small diameter,
bleed tube or bleed line 346 under control of a manually operated
shut-off valve 348. The liquid refrigerant is directed through
bleed line 346 to a liquid injection expansion valve 352 which
modulates the flow of liquid refrigerant to the screw compressor
326 and to an injection port (not shown) at the termination of line
346 which opens up into the screw compressor working chamber
intermediate of the discharge and suction sides of the compressor
326. A second manually operable shut-off valve 354 is positioned
within the bleed line 346 intermediate of the screw compressor
injection port and the liquid injection expansion valve 352 to
isolate the injection system when acting in conjunction with
shut-off valve 348.
The liquid injection expansion valve 352 is of the modulating or
variable flow type and preferably, is non-electrical in operation.
A thermostat bulb 356 is coupled, at one end of a capillary tube
357 and coupled at the other end directly to valve 352. The bulb
356, and the capillary tube 357, carry a temperature expansive
material which may be liquid, gas or part liquid and part gas but
which expands in response to increased temperature to variably
shift a movable valve element (not shown) within valve 352 and to
thereby modulate the flow of liquid refrigerant within bleed line
346 to the injection port of the screw compressor 326. An
equalizing line 353 couples the expansion valve 352 to the
compressor 326 at the discharge side of the compressor. The liquid
injection expansion valve 352 operates identically to that of the
previous embodiments.
The temperature responsive bulb 356 is operatively positioned with
respect to the compressor discharge, and in the illustrated
embodiment it is mounted on conduit 314 which couples the discharge
side of the compressor to the inlet side of condenser 312 and lies
adjacent to the compressor assembly 310. The compressor discharge
temperature therefore controls the flow rate of liquid refrigerant
through the liquid injection expansion valve 352 such that, as the
temperature of the compressor discharge increases, more liquid
refrigerant is injected through the injection port associated with
compressor 326 under control of the expansion valve 352 which opens
to a greater extent.
It is to this basic type of liquid refrigerant injection cooling
system, that the present embodiment is directed. Further reference
to FIGS. 8 and 9 indicate that the liquid refrigerant bleed line
346 has positioned, downstream from the manual control valve 348 a
fluid pressure operated direct acting, on-off valve 364 which is
connected to oil sump 328 through supply line 368 and is provided
with a return or by-pass line 370. The by-pass line 370 couples the
oil supply line 366 to the oil sump 328 through a fluid restriction
or orifice 372 such that, when the oil passes through the supply
line 366 as a result of energization of solenoid valve 372, the
valve 364 is maintained in open position. However, the orifice 372
permits relief of the system pressure after the solenoid valve
again closes to drain the line through restriction 372 back to the
sump. Energization of solenoid valve 374 occurs much in the same
manner as that of the previous embodiments. That is, a thermal
expansion bulb 360 or a thermostat bulb is filled with a
temperature expansive material such as liquid or gas and which is
coupled by capillary tube 362 to a panel not illustrated in FIG. 8
but illustrated in block form at 376 and FIG. 9, panel 376
including a thermostat (not shown) having normally open contacts
but closing in response to expansion of the material filling bulb
360 and capillary 362 so as to close an electrical circuit to the
solenoid associated with solenoid valve 374 through electrical line
378. The solenoid valve therefore comprises an on-off valve
permitting the selective application of high pressure oil to the
direct acting fluid pressure operated valve 364. Upon application
of the high pressure fluid, valve 374 moves from fully closed to
fully opened position permitting the expansion valve 352 to
modulate the flow of liquid refrigerant to the compressor injection
port.
From the above description, it is apparent that since the shut-off
valve 364 which is responsive to compressor load is not pilot
operated, and since it is not dependent upon a pressure drop across
the valve to maintain the valve open, chattering due to minor
changes in pressure differential is eliminated. As long as the
solenoid valve 374 which is responsive to the oil temperature and
indirectly the compressor load is open, the direct operating, fluid
pressure responsive valve 364 is maintained in open position and
high pressure refrigerant liquid is available to the compressor
working chamber via the injection port to the extent that the
modulation valve 352 varies the flow of same. Since the operation
of valve 364 is independent of the controlled medium, that is the
liquid refrigerant passing through the feed line, the operation
will be unaffected by modulation of the liquid flow through line
346 and the elimination of the modulation effects in terms of valve
364 virtually eliminates the chances of valve chattering.
To review the operation of the liquid injection system of this
embodiment, it is apparent that liquid injection is not required by
the machine during its off cycle or when the machine is running at
low refrigeration loads, since the discharge temperature of the
compressor working fluid at low loads is at a point where
additional cooling is not required. Since the temperature of the
oil is a function of the temperature of the discharge working fluid
from the compressor, the oil temperature at the oil pump outlet
side sump 328 is insufficient to heat thermal expansion or
thermostat bulb 360 to the point where solenoid valve 374 is
energized. With valve 374 closed, the high pressure oil leaving
sump 328 via line 368 is cut off from the oil pressure operated
valve 364 preventing flow of liquid refrigerant within bleed line
346. However, assuming that the load on the chiller or evaporator
316 reaches a predetermined minimum value necessary to cause the
oil temperature in turn to rise to the point where the thermostat
contacts within panel 376 close, the solenoid valve 374 is
energized, opening the oil pressure line 366 to valve 364. High
pressure oil flowing through line 366 acts on normally closed valve
364 to open the same with the valve remaining open but without any
modulating effect on refrigerant flow in line 346. The liquid
refrigerant passing through bleed line 346 is modulated by the
temperature of the compressor discharge, since the thermal
expansion or thermostat bulb 356 which operates on bulb 352 through
capillary 357 is in contact with the discharge line 314 from the
compressor which passes the high temperature compressed gas
directly to condenser 312. Further, equalizing line 353 senses
condensing temperature at the discharge side of the compressor and
the valve will variably inject liquid refrigerant into the
compressor working chamber. The thermostat bulb 356 modulates the
volume of liquid passed by the liquid injection expansion valve 352
and only the amount of liquid refrigerant is injected into the
compressor which is sufficient to maintain the gas discharge
temperature within a predetermined range. As load increases the
discharge temperature the working fluid increases and bulb 356
senses the demand for more liquid refrigerant to be injected into
the screw compressor just upstream from the discharge side of the
screws. Regardless of the modulation effects on the liquid passing
from the main liquid refrigerant flow line 318 connecting condenser
312 to evaporator 316 through dryer 320, valve 364 remains open as
long as the load on the compressor is above the minimum value.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that the foregoing and other changes in
form and details may be made therein without departing from the
spirit and scope of the invention.
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