U.S. patent application number 11/042949 was filed with the patent office on 2006-07-27 for screw compressor acoustic resonance reduction.
This patent application is currently assigned to YORK INTERNATIONAL CORPORATION. Invention is credited to William Joseph Fox, Robert Clarence JR. Sleighter.
Application Number | 20060165543 11/042949 |
Document ID | / |
Family ID | 36696934 |
Filed Date | 2006-07-27 |
United States Patent
Application |
20060165543 |
Kind Code |
A1 |
Fox; William Joseph ; et
al. |
July 27, 2006 |
Screw compressor acoustic resonance reduction
Abstract
A screw compressor for acoustic resonance reduction and
refrigeration systems including the screw compressor are disclosed.
The screw compressor has an acoustic barrier integral with the
compressor housing for use with a slide valve assembly that
includes a piston connected to a slide valve. A shaft connecting
the slide valve and the piston passes through an aperture in the
barrier with the piston and the slide valve disposed on opposite
sides of the barrier. The acoustic barrier protects the piston from
discharge pulses of compressed fluid exiting the rotors of the
screw compressor, thus attenuating acoustic resonance, reducing
compressor noise and premature wear of the slide valve assembly and
compressor housing.
Inventors: |
Fox; William Joseph; (Dover,
PA) ; Sleighter; Robert Clarence JR.; (Fayetteville,
PA) |
Correspondence
Address: |
MCNEES, WALLACE & NURICK LLC
100 PINE STREET
P.O. BOX 1166
HARRISBURG
PA
17108-1166
US
|
Assignee: |
YORK INTERNATIONAL
CORPORATION
York
PA
|
Family ID: |
36696934 |
Appl. No.: |
11/042949 |
Filed: |
January 24, 2005 |
Current U.S.
Class: |
418/201.1 |
Current CPC
Class: |
F04C 29/0035 20130101;
F04C 29/068 20130101; F04C 18/16 20130101; F04C 29/061 20130101;
F04C 28/12 20130101; F04C 18/023 20130101 |
Class at
Publication: |
418/201.1 |
International
Class: |
F04C 2/00 20060101
F04C002/00; F01C 1/16 20060101 F01C001/16; F04C 18/00 20060101
F04C018/00; F01C 1/24 20060101 F01C001/24 |
Claims
1. A screw compressor comprising: a compressor housing having a
passage for the flow of a fluid therethrough from a suction side of
the compressor to a discharge side of the compressor, the housing
including an integral acoustic barrier; a pair of intermeshed
rotors disposed in the passage, the rotors configured to increase
the pressure of the fluid flowing from a suction port on the
suction side of the compressor through the passage to a discharge
port on the discharge side of the compressor; and a slide valve
assembly movably disposed within the compressor housing to adjust
the capacity of the compressor, the slide valve assembly comprising
a slide valve connected to a piston by a shaft, wherein the piston
and the slide valve are disposed on opposite sides of the acoustic
barrier and wherein the acoustic barrier includes an aperture sized
to receive the shaft, the acoustic barrier attenuating acoustic
resonance and noise in the compressor and wherein the acoustic
barrier is positioned a preselected distance from the discharge
port.
2. The screw compressor of claim 1, wherein the piston is disposed
in a piston cylinder, at least a portion of piston cylinder defined
by the compressor housing, the piston cylinder sized to receive the
piston and in fluid communication with the suction side and the
discharge side of the housing by a conduit assembly.
3. The screw compressor of claim 2, wherein the compressor housing
comprises cast iron.
4. The screw compressor of claim 2, wherein the slide valve
assembly includes a single piston.
5. The screw compressor of claim 2, wherein the slide valve
assembly further includes a biasing means to urge the piston in a
first direction and wherein a refrigerant gas introduced into the
piston cylinder from the discharge side by the conduit assembly
urges the piston in a direction opposed to the first direction.
6. The screw compressor of claim 2, wherein the compressor housing
further includes a piston housing portion, wherein the piston
housing portion includes at least a portion of the piston cylinder
and at least a portion of the conduit assembly.
7. The screw compressor of claim 2, wherein the piston cylinder is
sized to receive the piston such that substantially no refrigerant
gas leaks across an interface between the compressor housing and
the piston.
8. The screw compressor of claim 2, wherein the acoustic barrier
comprises a portion of an end wall of the piston cylinder.
9. The screw compressor of claim 1, wherein the slide valve
assembly further comprises a spring disposed between the piston and
the acoustic barrier, the spring having a bias urging the piston
away from the acoustic barrier.
10. The screw compressor of claim 1, wherein the compressor housing
comprises an intake housing, a rotor housing, a discharge housing,
and a piston housing.
11. A method for reducing acoustic resonance in a screw compressor
comprising the steps of: passing a refrigerant gas through a
passage of a compressor housing of a screw compressor from a
suction side of the compressor to a discharge side of the
compressor through a discharge port, a portion of the passage
defined by a pair of intermeshed rotors disposed in the compressor
housing passage; providing an acoustic barrier integral with the
compressor housing, the acoustic barrier positioned a preselected
distance from the discharge port; providing a slide valve assembly
in the compressor housing, the slide valve assembly comprising a
slide valve connected by a shaft to a piston, wherein the shaft
passes through an aperture defined by a barrier integral with the
compressor housing, the slide valve and the piston on opposite
sides of the barrier; and modifying the flow of the refrigerant gas
through the passage by moving the slide valve from a first position
with respect to the intermeshed rotors to a second position with
respect to the intermeshed rotors.
12. The method of claim 11, wherein the step of passing a
refrigerant gas includes passing 134a refrigerant gas.
13. The method of claim 11, wherein the compressor housing for
passing a refrigerant gas through the compressor housing comprises
a cast material.
14. The method of claim 11, wherein the step of providing a slide
valve assembly in the compressor housing comprises: providing a
slide valve assembly in the compressor housing such that the piston
is movably disposed within a piston cylinder of the compressor
housing, the piston cylinder sized to receive the piston, wherein
moving the piston results in moving the slide valve, thereby
modifying the flow of gas and controlling the capacity of the
compressor.
15. The method of claim 14, wherein the moving of the slide valve
from a first position to a second position with respect to the
intermeshed rotors is accomplished by varying a pressure on one
side of the piston with respect to a pressure on an opposing side
of the piston.
16. The method of claim 14, wherein a portion of the compressor
housing defines at least a portion of an end wall of the piston
cylinder.
17. The method of claim 16, wherein the portion of the compressor
housing that defines at least a portion of the end wall of the
piston cylinder is the acoustic barrier.
18. The method of claim 14, wherein the step of providing a slide
valve assembly further comprises: providing a slide valve assembly
further comprising a biasing means to urge the piston in a
direction opposite the acoustic barrier.
19. The method of claim 14, wherein the moving of the slide valve
from a first position to a second position with respect to the
intermeshed rotors is accomplished by introducing a high pressure
gas into the piston cylinder from the discharge side to increase
pressure on one side of the piston, thereby urging the piston in a
direction opposite the first direction urged by the biasing
means.
20. A refrigeration system comprising: a condenser; an evaporator;
an expansion valve; and a screw compressor comprising a compressor
housing having a passage for the flow of a fluid therethrough from
a suction side of the compressor to a discharge side of the
compressor, the housing including an integral acoustic barrier, a
pair of intermeshed rotors disposed in the passage, the rotors
configured to increase the pressure of the fluid flowing from a
suction port on the suction side of the compressor through the
passage to a discharge port on the discharge side of the
compressor, and a slide valve assembly movably disposed within the
compressor housing to adjust the capacity of the compressor, the
slide valve assembly comprising a slide valve connected to a piston
by a shaft, wherein the piston and the slide valve are disposed on
opposite sides of the acoustic barrier and wherein the acoustic
barrier includes an aperture sized to receive the shaft, the
acoustic barrier attenuating acoustic resonance and noise in the
compressor and wherein the acoustic barrier is positioned a
preselected distance from the discharge port, wherein the
condenser, evaporator, expansion valve and screw compressor are
serially connected for the passage of a refrigerant
therethrough.
21. The refrigeration system of claim 20, wherein the slide valve
assembly of the screw compressor comprises a single piston.
22. The refrigeration system of claim 20, wherein the refrigerant
comprises 134a refrigerant.
Description
FIELD OF THE INVENTION
[0001] The present invention is directed to a screw compressor
having a slide valve used in gas compression, and more particularly
to a screw compressor for use in heating, ventilation, air
conditioning and refrigeration systems that reduces acoustic
resonance, slide valve and compressor housing wear, and compressor
noise levels.
BACKGROUND OF THE INVENTION
[0002] Screw compressors are widely used to compress many types of
gases, particularly those used in heating, ventilation, air
conditioning and refrigeration (HVAC&R) systems. The capacity
of the screw compressor is typically regulated by a mechanism
called a slide valve.
[0003] Rotary screw compressors of the type which are utilized for
compressing a refrigerant gas or other fluid conventionally include
a pair of rotors having intermeshing lands and grooves helically
disposed about the periphery thereof. A working fluid enters the
compressor from an inlet port and enters the grooves between the
rotors. The rotation of the rotors forms chevron-shaped compression
chambers in which the working fluid is received and which chambers
diminish in volume as the chambers move toward an outlet in the
compressor's outlet casings. The working fluid is discharged when
the crest of the rotor lands defining the leading edge of a
compression chamber pass an edge of the discharge ports in the
discharge casing of the compressor. Lubricant is provided to the
rotors to lubricate the contacting metal and to seal the
compression chamber.
[0004] To provide a controlled variation in volumetric capacity
simultaneously with controlling compression ratio, it has been
known to utilize a slide valve, which communicates along its full
length with the bores of the compressor in which the rotors are
mounted. The end of the slide valve is moveable relative to the
rotors and its position is used to control the amount of working
fluid being compressed and thus vary the overall capacity of the
compressor in a controlled manner. The slide valve configurations
of rotary screw compressors allow an infinitely variable capacity
to be maintained, which simultaneously results in very efficient
compressor loading.
[0005] The position of the slide valve is controlled by pressures
in a cylinder which act on a piston attached to the slide valve via
a shaft, which together make a slide valve assembly. The rotors of
the screw compressor rotate at high rates of speed, and multiple
sets of rotors and/or compressors may be configured to work
together to further increase the amount of fluid that can be
circulated in the system, thereby increasing the operating capacity
of a system. While the rotors provide a continuous pumping action,
each set of rotors and/or compressor produces pressure pulses as
the pressurized working fluid is discharged at the discharge port.
Typically, the entire slide valve assembly is exposed to the
discharge pulses as the working fluid leaves the rotors. These
discharge pulses are of very high energy and may result in
vibrations causing excessive wear to the slide valve assembly and
increased noise levels in the compressor. Notably, the pulses
reflect off of the piston and are transmitted back through the
shaft to the slide valve.
[0006] At certain slide valve positions, the distance from the
discharge port to the piston is equal to one half of an acoustic
wavelength of the discharge pulse. When operating at these
positions, a resonance is created that multiplies the effect of the
discharge pulse on the slide valve assembly. This in turn cause
even faster wear to the slide valve assembly, including the slide
valve, piston seal and mating surfaces of the slide valve assembly
with the compressor housing. These vibrations are also transmitted
to the compressor housing.
[0007] U.S. Pat. No. 5,979,168 discloses a slide valve for use in a
screw compressor that reduces the size of the volume available for
compression by opening a portion of the volume to suction pressure.
The slide valve is gas operated by, and is in fluid communication
with, gas from a single source, the refrigerant gas from the
discharge pressure side of the screw compressor. The apparatus of
the '168 Patent includes a slide valve assembly having a valve
portion, a piston and a connecting rod connecting the valve portion
to the connecting rod. Discharge gas from the compression chambers
is discharged into a discharge passage. A partition member is
positioned in the discharge passageway and includes an aperture
penetrated by the connecting rod. The partition member forms a
barrier for lubricant discharged with the gas flow stream, but gas
is allowed to pass through the aperture in the partition member. As
a result, the gas on either side of the aperture is at the same
pressure, but the gas passing through the aperture is essentially
free of lubricant. The partition member also includes a weepage
hole to drain any small amount of lubricant that may make its way
through the aperture to the opposite side of the partition
member.
[0008] The stated purpose of the '168 Patent is to reduce lubricant
in the gas admitted into the valve to operate the slide valve, but
it does not recognize the problem of vibration and acoustic
resonance.
[0009] U.S. Pat. No. 5,044,894 discloses a double piston system for
adjusting a slide valve and a slide stop of a screw compressor. The
pistons of the '894 Patent are located in a control housing forming
two separate piston chambers. Concentric annular rods extend from
the pistons, through the control housing, to the slide valve and
slide stop. Oil supplied from an oil separator is applied to the
piston chambers to cause movement of the pistons and a resulting
change in slide valve and slide stop positions. Seals on the rods
seal the piston chambers from one another and from the discharge
chamber. While the two piston system of the '894 Patent opines that
the use of the seals on the rods could isolate the pistons from
variations in the discharge manifold, it fails to recognize the
problems caused by the reflection of energy from discharge pulses
through the slide valve assembly, including acoustic resonance,
noise, and excessive wear. Furthermore, because oil is introduced
to the piston chambers, the '894 Patent presents a different
mechanical system having a different resistance and likely
different damping characteristics which may not produce a
resonance.
[0010] In view of the foregoing, there remains a need to provide
for a screw compressor that attenuates acoustic resonance and
reduces wear of the compressor housing and slide valve and reduces
compressor noise levels.
SUMMARY OF THE INVENTION
[0011] Accordingly, it may be desirable to provide a screw
compressor that overcomes these and other disadvantages in the art.
Particularly, it is desirable to provide a screw compressor that
provides for a slide valve assembly that separates the piston from
the discharge by introducing a damping device between the piston
and the slide valve. More particularly, it may be desirable to
provide a screw compressor that has a damping device integral with
the compressor housing, which damping device separates the piston
from the discharge port and dampens sound, vibration and noise in
the compressor.
[0012] The present invention is directed to screw compressors and
compression systems that include the screw compressors. The screw
compressor comprises a compressor housing having a passage for the
flow of a fluid therethrough from a suction side of the compressor
to a discharge side of the compressor, the housing including an
integral acoustic barrier, a pair of intermeshed rotors disposed in
the passage, the rotors configured to increase the pressure of the
fluid flowing from a suction port on the suction side of the
compressor through the passage to a discharge port on the discharge
side of the compressor, and a slide valve assembly movably disposed
within the compressor housing to adjust the capacity of the
compressor, the slide valve assembly comprising a slide valve
connected to a piston by a shaft, wherein the piston and the slide
valve are disposed on opposite sides of the acoustic barrier. The
acoustic barrier includes an aperture sized to receive the shaft,
the acoustic barrier attenuating acoustic resonance and noise in
the compressor and the acoustic barrier is positioned a preselected
distance from the discharge port.
[0013] The present invention is also directed to a method of
attenuating acoustic resonance in a screw compressor. The method
comprises passing a refrigerant gas through a passage of a
compressor housing of a screw compressor from a suction side of the
compressor to a discharge side of the compressor through a
discharge port, a portion of the passage defined by a pair of
intermeshed rotors disposed in the compressor housing passage,
providing an acoustic barrier integral with the compressor housing,
the acoustic barrier positioned a preselected distance from the
discharge port, providing a slide valve assembly in the compressor
housing, the slide valve assembly comprising a slide valve
connected by a shaft to a piston, wherein the shaft passes through
an aperture defined by a barrier integral with the compressor
housing, the slide valve and the piston on opposite sides of the
barrier, and modifying the flow of the refrigerant gas through the
passage by moving the slide valve from a first position with
respect to the intermeshed rotors to a second position with respect
to the intermeshed rotors.
[0014] An advantage of the present invention is that an integral
acoustic barrier placed between the piston of the slide valve
assembly and the discharge from the rotors greatly reduces the
possibility of creating an acoustic resonance resulting from
periodic discharge of compressed refrigerant gas from the
compression chambers as the integral barrier attenuates sound and
vibration.
[0015] Another advantage of the present invention is that an
integral acoustic barrier placed between the piston of the slide
valve assembly and the discharge from the rotors reduces wear to
the slide valve assembly.
[0016] Yet another advantage of the present invention is that an
integral acoustic barrier placed between the piston of the slide
valve assembly and the discharge from the rotors decreases
compressor noise levels.
[0017] Still another advantage of the present invention is that an
acoustic barrier formed integrally with the compressor housing can
be produced as part of the manufacturing process of the compressor
housing, decreasing the number of additional parts in the
compressor housing, reducing possible sources of leaks and
potential failures, while simplifying assembly.
[0018] Other features and advantages of the present invention will
be apparent from the following more detailed description of the
preferred embodiment, taken in conjunction with the accompanying
drawings which illustrate, by way of example, the principles of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a refrigeration system according to an exemplary
embodiment of the invention.
[0020] FIG. 2 is an enlarged partial cross-sectional view showing
the slide valve assembly of the screw compressor in the
refrigeration system of FIG. 1, showing the compressor in a fully
loaded position.
[0021] FIG. 3 is the enlarged partial cross-sectional view of FIG.
2 showing the compressor in a fully unloaded position.
[0022] FIG. 4 is a perspective view of a piston housing portion of
a compressor housing.
[0023] FIG. 5 is a graphical representation of results of screw
compressor vibration testing.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Introducing a barrier integral with a compressor housing of
a screw compressor to separate the piston and slide valve portions
of a slide valve assembly physically separates the piston from the
discharge gases. The discharge gases are high pressure gases
produced as the gases exit each of a number of compression chambers
formed between intermeshed screws or rotors. The gases are
discharged from the compression chambers in a periodic manner as
the rotor or rotors rotate, producing pulses. The frequency and
amplitude of the pulses are dependent on a number of factors,
including but not limited to rotational speed of the rotors, as
well as the number of compression chambers available for
compression. Consequently, due to the position of the integral
barrier, the pulses are attenuated and less energy is absorbed by
the piston and reflected back through the slide valve assembly
through the shaft, thereby reducing the amount of vibration in the
slide valve assembly and decreasing the risk of premature wear of
the assembly. Not only does the attenuation of vibrations or pulses
lower the acoustic resonance, but the distance from the discharge
port to the point of energy absorption is fixed. That is, the
energy is now absorbed by the barrier which remains a constant
distance from the discharge port, even while the distance of the
piston is varied from both the discharge port and the integral
barrier. By appropriately selecting the distance from the discharge
port to the integral barrier, the possibility of acoustic resonance
is further reduced as the distance selected should correspond to an
acoustic wavelength that does not result in resonance during normal
compressor operation.
[0025] Referring first to FIG. 1, a refrigeration system 10
comprises a screw compressor 12, a condenser 14, an expansion valve
(TVX) 16 and an evaporator 18 serially connected to permit the flow
of refrigerant therethrough. The compressor 12 includes a
compressor housing 20 that contains the working parts of the
compressor 12. As shown in FIG. 1, the compressor housing 20
comprises an intake housing 100, a rotor housing 200, a discharge
housing 300, and a piston housing 400.
[0026] Refrigerant gas at suction or low pressure is directed from
the evaporator 18 to a communicating suction area at an intake
passage on the suction side 105 of the compressor 12. A motor is
used to drive the rotors. The motor may include a motor stator 110
comprised of laminations and coil heads. The motor further includes
a motor drive 120 connected to a rotor 210 by way of a drive shaft
130, which rotor 210 is matingly engaged with a second rotor (not
shown) via intermeshing lands and grooves. The rotors revolve in an
accurately machined cylinder with the rotor housing 200. The two
rotors are not of the same shape. Rather, one is generally termed a
male and the other is generally termed a female. One rotor, usually
the male, typically rotates or revolves more rapidly than the other
rotor, usually the female.
[0027] Refrigerant at suction pressure flows into the rotor housing
200 at a suction port 107 and enters compression pockets defined
between the surfaces of the mating rotors. By the counter rotation
and meshing of the screw rotors, the compression pockets, also
referred to as lobes, are reduced in size and are axially displaced
to the high pressure discharge side 305 of the compressor 12, where
the then-compressed refrigerant gas is discharged into a discharge
passage 305 of the discharge housing 300. The compressed
refrigerant eventually exits the compressor 12 to the condenser 14
and continues through the refrigeration cycle. Typically, screw
compressors include oil separators (not shown) for separating
lubricant entrained in the compressed refrigerant from the
refrigerant. In addition, a structure such as a sump (not shown) is
typically included for storing the excess lubricant.
[0028] As better illustrated in FIGS. 2 and 3, showing the
compressor 12 in a fully loaded and fully unloaded state,
respectively, the compressor 12 includes a single piston slide
valve assembly 500 which controls the capacity of the compressor
12. The slide valve assembly 500 comprises a slide valve 510 and a
piston 520 rigidly connected to one another by, but oppositely
disposed on, a shaft 530. The slide valve 510 forms a portion of
the boundary of the rotor housing 200, providing the ability to
adjust the amount of the rotor threads exposed to the suction
inlet, and the loading of the compressor 12.
[0029] Compressed refrigerant gas exits the rotors into the
discharge passage on the discharge side 305 at a discharge port
310. The discharge port 310 is comprised of two portions, the first
being a radial portion 312 which is formed by a discharge end 512
of the slide valve 510 and the second being an axial portion 314
which is formed by the discharge housing 300. The geometry of the
compressor housing 20 is such that the size of the radial portion
312 is controlled by the position of the slide valve 510.
[0030] The slide valve assembly 500 is adjusted to control the
position of the slide valve 510 over the rotors by pressure applied
to the piston 520. The piston 520 is contained in a piston cylinder
405 of the piston housing 400 sized to receive the piston 520, the
piston 520 having one or more seals 522 such that the piston 520
effectively divides the piston cylinder 405, which is in fluid
communication with the suction side 105 and the discharge side 305
of the compressor, into two distinct chambers, one chamber on
either side of the piston 520. Despite the seals, sufficient
lubricity is provided so that by loading pressure to the piston 520
via either side of the piston 520, the piston 520 is displaced and
thus the rest of the slide valve assembly 500, including the slide
valve 510 which is attached to the opposite end of the slide valve
shaft 530, moves, increasing or decreasing compressor capacity.
[0031] As shown with respect to FIG. 3, when the slide valve 510 is
unloaded, biasing means, such as a spring 540, urges the piston 520
to the right of the cylinder 405. This in turn moves the slide
valve 510 to the right. In this position, the slide valve is said
to be unloaded and the slide valve 510 reveals a recirculation port
230 for refrigerant gas to return uncompressed to the suction side
105 of the compressor 12. To increase capacity, pressure is
introduced into the piston cylinder 405 from the discharge side 305
through a piston cylinder opening 420, counteracting the spring 540
and moving the piston 520 and hence the slide valve 510 to the
left. The fully loaded position is shown in FIG. 2. In this
position, the recirculation port 230 is fully closed. All suction
gas introduced into the rotors is now compressed. When the slide
valve 510 is fully unloaded, as shown in FIG. 3, the recirculation
port 230 is fully open, allowing a portion of the refrigerant gas
to by-pass the rotors, circulate around the motor and return to the
suction side inlet. In a partially loaded condition, the
recirculation port 230 is partially closed by the slide valve
510.
[0032] During operation, and particularly when the slide valve
assembly 500 is adjusted so that the compressor 12 is only
partially loaded, compressed gas is discharged from the rotors in
pulses, which pulses are typically of very high energy. As
previously discussed, in screw compressor designs, this can lead to
vibrations and produce an acoustic resonance that results in severe
wear to the slide valve assembly 500, including galling that can
lead to premature failure of the compressor 12.
[0033] In the present invention, an acoustic barrier 410 is
introduced into the compressor housing 20 to reduce the effect of
the pulses on the slide valve assembly 500 and overcome the
problems of excessive vibration and wear to the assembly 500,
undesirable noise levels within the compressor 12, as well as to
overcome the significant damage such as galling of the slide valve
assembly 500 that results when the distance from the piston 520 to
the discharge port 310 matches an acoustic wavelength of the
discharge pulse. In severe cases, the harmonic effects can damage
the compressor housing 20 itself. The barrier 410 is formed
integral with the compressor housing 20, typically during casting
of the compressor housing 20.
[0034] The barrier 410 includes an aperture 415 sized to snugly
receive the shaft 530 of the slide valve assembly 500. The piston
520 and the slide valve 510 are on opposite sides of the barrier
410, separating the piston 520 from the discharge pulses coming
from the discharge port 310. Also, as shown in FIG. 2, the barrier
410 may further define a portion of an end wall 407 of the piston
cylinder 405.
[0035] It will be appreciated that the acoustic wavelength is a
property of the refrigerant and may vary at different temperature
and pressures of operation. Furthermore, the frequencies generated
by the compressor 12 are based on rotor 210 design and operating
speed of the compressor 12. Thus, it will further be appreciated
that acoustic wavelengths and compressor 12 frequencies that result
in resonance can vary widely, but that the acoustic barrier 410 can
appropriately be positioned a preselected distance from the
discharge port 310 to allow the slide valve 510 to move over the
full length of a desired range while still protecting the piston
520 from pulses existing the discharge port 310.
[0036] Movement of the piston 520 is controlled by the introduction
or evacuation of refrigerant into or from the chamber of the piston
cylinder 405 opposite the biasing means, the piston cylinder 405
being in fluid communication with both the suction side 105 and the
discharge side 305 of the compressor 12. To load the compressor 12
and increase its capacity, high pressure gas is introduced through
a piston cylinder opening 420 which acts as an inlet carrying high
pressure gas from the discharge side 305 of the compressor 12
through a conduit assembly 425. Conversely, to unload the
compressor 12, gas may be removed through the piston cylinder
opening 420 which now acts as an outlet by venting the gas to the
suction side 105 of the compressor 12 through the conduit assembly
425. The function of the cylinder opening 420 as an inlet or outlet
can be controlled by a solenoid valve (not shown), which is
typically part of the conduit assembly. When it is necessary to
load the compressor, refrigerant can be introduced through the
opening 420 through the solenoid valve which places the opening 420
in fluid communication with the discharge side 305 of the
compressor 12. Likewise, when it is determined that the compressor
12 should be unloaded, refrigerant can be withdrawn through the
opening 420 through the solenoid valve which places the opening in
fluid communication with the suction side 105 of the compressor 12.
This describes well known slide valve operation.
[0037] Preferably, the slide valve assembly includes a spring 540
disposed between the piston 520 and the end wall 407 provided by
the barrier 410. The end wall 407 advantageously serves as a spring
stop. The spring 540 urges the piston 520 to remain in place in the
absence of high pressure gas introduced through the piston cylinder
opening 420, so that the slide valve 510 automatically is unloaded
in the absence of high pressure gas.
[0038] The aperture 415 defined by the barrier 410 approximates the
diameter of the shaft 530 but with sufficient tolerance to permit
the shaft 530 to slide back and forth when the piston 520 is
displaced without causing damage to the slide valve assembly 500 or
the compressor housing 12. The piston 520 is designed and
manufactured to allow the piston 520 to slide freely in the piston
cylinder 405 without the flow of gas around the piston 520.
Typically, a seal 522 is provided to prevent gas leakage around the
piston 520. Thus provided, the piston 520 may be protected from
discharge pulses without the need to provide any seals or other
non-integral pieces on the shaft 530 of the slide valve assembly
500 or attached to the compressor housing 20.
[0039] As previously discussed, the acoustic barrier 410 is an
integral part of the compressor housing 20. Preferably, the barrier
410 is cast as an integral part of the piston housing portion 400
of the compressor housing 20 although it is possible, but
considerably more expensive, to machine the compressor housing 20
from a forging. As shown in FIG. 4, the piston housing 400 is a
piece of unitary construction cast from iron with a mating surface
402 that attaches to the compressor housing 20. The piston housing
400 comprises two passages, the piston cylinder 405, and a second
passage 430 which forms part of the discharge passage on the
discharge side 305 of the compressor and which directs the
compressed refrigerant to the condenser. As best seen in FIGS. 2
and 3, a piston housing end plate 404 is attached to the piston
housing 400 after the slide valve assembly 500 has been assembled.
The piston housing end plate 404 provides an end wall 409 of the
piston cylinder 405 opposing the end wall 407 provided by the
barrier 410. By producing the barrier 410 integral with the
compressor housing 20, which is typically massive, particularly
when compared to working components such as the slide valve
assembly 500, energy from the discharge pulse is attenuated
throughout the compressor housing 20 reducing the risk of damage to
other working parts of the compressor 12. Certain materials, such
as gray cast iron, are particularly effective in attenuating
vibrations.
[0040] Screw compressors with an integral barrier according to
embodiments of the present invention may be used in any compression
system, but are particularly useful in refrigeration systems using
high density refrigerants such as 134a, which because of its high
density, is more likely to conduct the discharge pulses at higher
energy levels, thus resulting in more vibration and greater damage
in the absence of a barrier.
[0041] The effectiveness of embodiments of the present invention is
further illustrated as shown in the following non-limiting
example.
EXAMPLE
[0042] A screw compressor having a slide valve assembly according
to conventional methods, i.e., without a barrier separating the
piston and the slide valve was compared against a screw compressor
according to an exemplary embodiment of the present invention, but
the compressors were otherwise identical. Both compressors were
tested using 134a refrigerant as the refrigerant gas. Compressor
vibration at 355 Hz was then measured as a function of various
slide valve positions. During testing, each compressor was
maintained at each slide valve position long enough to achieve
steady state operation.
[0043] As shown graphically in FIG. 5, which displays peak forces
in G loading versus percent slide valve loading, the compressor
without any barrier between the piston and slide valve exhibited
significantly higher peak G's, particularly over slide valve
positions from about 10 to about 80 percent, where 100 percent
corresponded to a slide valve position for a fully loaded
compressor while 0 percent corresponded to a slide valve position
opened to allow maximum recirculation of refrigerant. FIG. 5
demonstrates that vibrations were reduced from in excess of 2 G's
in screw compressors with no barrier (designated as "original")
down to under about 0.2 G's in screw compressors with an integral
barrier (designated as "wall"), an entire order of magnitude
decrease.
[0044] While the invention has been described with reference to a
preferred embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the scope
of the invention. In addition, many modifications may be made to
adapt a particular situation or material to the teachings of the
invention without departing from the essential scope thereof.
Therefore, it is intended that the invention not be limited to the
particular embodiment disclosed as the best mode contemplated for
carrying out this invention, but that the invention will include
all embodiments falling within the scope of the appended
claims.
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