U.S. patent number RE36,274 [Application Number 09/158,475] was granted by the patent office on 1999-08-24 for method of manufacturing valve system for capacity control of a screw compressor.
This patent grant is currently assigned to Coltec Industries Inc. Invention is credited to Arthur R. Legault, John Q. Richardson, Jan A. Zuercher.
United States Patent |
RE36,274 |
Zuercher , et al. |
August 24, 1999 |
Method of manufacturing valve system for capacity control of a
screw compressor
Abstract
A lift valve communicating with a compression chamber of a
.[.variable capacity.]. screw compressor is set forth including a
valve housing, a piston reciprocally received within the valve
housing, a shaft having a first end connected to the piston and a
second end extending from the housing. The valve further includes a
valve element connected to the second end of the shaft having a
valve surface exposed to the compression chamber and a
reciprocation mechanism for reciprocating the piston within the
housing. The reciprocation mechanism includes a first pressure
passage communicating with the housing adjacent a side of the
piston, and a second pressure passage communicating with the
housing adjacent and opposed side of the piston, wherein the valve
surface is positively displaced toward and away from the
compression chamber of the .[.variable capacity.]. screw compressor
in response to the application of fluid pressure to at least one of
the first and second pressure passages .[.to vary the capacity of
the screw compressor.].. Further, the lift valve is manufactured
integral with the manufacturing of the compression chamber of the
.[.variable capacity.]. screw compressor. This manufacturing
process includes securing at least one lift valve to a housing of
the .[.variable capacity.]. screw compressor in an operating
position. Once secured to the housing, the shaft and consequently
the valve element is fully extended from the valve housing and
maintained in such position thus simultaneously machining an inner
surface of the compression chamber and the valve surface such that
the valve surface forms a continuation of the inner wall of the
compression chamber when the .[.variable capacity.]. screw
compressor is operating at full capacity.
Inventors: |
Zuercher; Jan A. (Spanish Fort,
AL), Richardson; John Q. (Daphne, AL), Legault; Arthur
R. (Suwanee, GA) |
Assignee: |
Coltec Industries Inc (New
York, NY)
|
Family
ID: |
23358563 |
Appl.
No.: |
09/158,475 |
Filed: |
September 22, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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346244 |
Nov 23, 1994 |
5556271 |
|
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Reissue of: |
706301 |
Aug 30, 1996 |
05694682 |
Dec 9, 1997 |
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Current U.S.
Class: |
29/888.023;
29/888.02; 418/201.2 |
Current CPC
Class: |
F16K
31/122 (20130101); F04C 28/125 (20130101); F16K
27/029 (20130101); F04C 28/16 (20130101); Y10T
29/49236 (20150115); Y10T 29/49242 (20150115) |
Current International
Class: |
B23P
15/00 (20060101); B23P 015/00 () |
Field of
Search: |
;29/888.023,888.02
;418/201.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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59-131791 |
|
Jan 1983 |
|
JP |
|
5-18374 |
|
Jan 1993 |
|
JP |
|
89 10489 |
|
Nov 1989 |
|
WO |
|
Primary Examiner: Cuda; Irene
Attorney, Agent or Firm: Cummings & Lockwood
Parent Case Text
This is a Divisional application of Ser. No. 08/346,244, filed Nov.
23, 1994 now U.S. Pat. No. 5,556,271.
Claims
What is claimed:
1. A method of manufacturing a lift valve for use in a .[.variable
capacity.]. screw compressor comprising the steps of:
securing at least one lift valve to a housing of the .[.variable
capacity.]. screw compressor in an operating position, said lift
valve including a valve housing, a shaft extending from and
reciprocally received within said valve housing and a valve face
.[.seethed.]. .Iadd.secured .Iaddend.to a remote end of said
shaft;
fully extending said shaft from said valve housing;
maintaining said shaft in said fully extended position, and
simultaneously machining an inner surface of said compressor
housing and said valve face;
wherein said valve face forms a continuation of said compressor
housing when said .[.variable capacity.]. screw compressor is
operating at full capacity.
2. The method as defined in claim 1, further comprising the step of
maintaining an angular orientation of said valve face with respect
to said valve housing during said machining step.
3. The method as defined in claim 1, wherein said step of
maintaining said shaft in said fully extended position includes
pressurizing at least one chamber within said valve housing.
4. The method as defined in claim 3, wherein said chamber is
pressurized with a substantially incompressible fluid.
5. The method as defined in claim 3, wherein said incompressible
fluid is oil.
6. The method as defined in claim 1, wherein a plurality of lift
valves are secured to said housing prior to said step of machining
said inner surface of said compressor housing.
Description
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a valve system for controlling the
capacity of a screw compressor. Particularly, the present invention
is directed to double acting lift valves for controlling the
capacity of a screw compressor as well as the manufacture of such
double acting lift valves.
BACKGROUND OF THE INVENTION
Rotary screw compressors of the type set forth herein comprise two
rotors mounted in a working space which is limited by two end walls
and a barrel wall extending therebetween. The barrel wall
necessarily takes the shape of two intersecting cylinders, each
housing one of the rotors. Each rotor is provided with helically
extending lobes and grooves which are intermeshed to form chevron
shaped compression chambers. In these chambers, a gaseous fluid is
displaced and compressed from an inlet channel to an outlet channel
by way of the screw compressor. Each compression chamber during a
filling phase communicates with the inlet, during a compression
phase undergoes a continued reduction in volume and during a
discharge phase commimicates with an outlet. A rotary screw
compressor of this type is disclosed in U.S. Pat. No.
4,435,139.
Rotary screw compressors of this kind are often provided with
valves for regulating the built-in volume ratio for the capacity of
the compressor. When continuous regulation is required, slide
valves are often used, however, as with other regulation needs, it
is sufficient to use lift valves. Such lift valves are mounted in
the barrel wall of the compressor or may be mounted in one of the
end walls and in this regard, normally in the high pressure end
wall.
Several solutions for controlling the capacity of screw compressors
operating at a constant number of rotations have been proposed. One
such solution is disclosed in U.S. Pat. No. 5,108,269 issued Apr.
28, 1992. This solution provides radially positioned valves in the
side wall of the barrel with the valves being opened so as to
communicate the particular compression chamber with either the
inlet or outlet manifold. However, as will be discussed in greater
detail hereinbelow, with such valves, compression losses due to
leakage clearance valve and between the valves and the rotors are
experienced to the extent that full capacity cannot be
realized.
Of the above noted solutions, the use of conventional slide type
valves which constitute a portion of the barrel of the compressor
has the advantage of providing a wide control range and the
possibility that at a constant working pressure ratio in the
compressor a relatively constant built in pressure ratio within the
greater part of the control range can be brought about by means of
a suitable dimensioning of the axial discharge port. The main
disadvantage of slide valves is that they are expensive to
manufacture in that close tolerances and accurate centering are
required. Further, the actuating system which is normally a
hydraulic system is also relatively expensive and complicated.
Another solution is to use a rotary type valve wherein the valves
are in communication with slots formed in the barrel through which
gas is recirculated to suction to create at partial loads. This
valve arrangement has the advantage of being less expensive to
manufacture than conventional slide valve types, however, the
capacity control is not as accurate as with slide valve
arrangements. Further, built-in pressure ratio drops with
decreasing loads are experienced. Moreover, leakage is obtained
across the slots along the rotor bores, particularly at higher
loads and at full loads. This shortcoming will be described in
greater detail hereinbelow with respect to FIG. 7b. Accordingly, it
has been determined that the use of lift valves achieves an
economic balance between the need for accurate capacity control as
well as the need for minimizing manufacturing costs and operating
costs. Lift valves of this type have been known and permit
successive compression nodes within the barrel to communicate with
one another, thus, effectively reducing the capacity of the
compressor. One such valve is disclosed in U.S. Pat. No. 4,453,900
issued Jun. 12, 1984. Further, such valves may communicate an
overlying compression node with a recirculation passage which
returns pressurized fluid to the suction side of the compressor.
However, it is noted that the opening of the lift valve is directly
dependent upon the compression spring as well as the internal
pressure of the compressor. However, the actuation of such valves
is unreliable due to friction, corrosion and other environmental
factors which often degradate the positioning of this type of lift
valve. Further, while the face of the valve element takes on the
approximate shape of the barrel, the valve element is separately
formed by casting or other process within predetermined tolerances.
In order to economically manufacture such valve elements, the
tolerances must be some what relaxed which may result in the
leakage of pressurized fluid between compression chambers thereby
degrading the efficiency of the compressor.
Clearly there is a need for an accurately controlled and
inexpensively manufactured valve system for controlling the
capacity of a oil flooded rotary screw type compressor. Such a
valve system to include a plurality of serially positioned lift
valves which may be readily manufactured within a zero tolerance,
with each when opened reducing the capacity of the compressor a
predetermined mount.
SUMMARY OF THE INVENTION
A primary object of the present invention is to overcome the
aforementioned shortcomings associated with known valve
systems.
Another object of the present invention is to provide a series of
lift valves for effectively controlling the capacity of a screw
compressor.
Yet another object of the present invention is to provide a series
of double acting lift valves for accurately controlling the
position of the lift valve and thus the capacity of a screw
compressor.
A further object of the present invention is to ensure reliable
operation of the double acting lift valves by providing a two way
shaft seal about an exposed end of the valve for preventing leakage
from the valve and oil leakage into such valve.
An even further object of the present invention is to provide a
series of lift valves wherein operating losses due to leakage about
the valve are minimized while assembly costs are reduced.
A further object of the present invention is to provide a series of
double acting lift valves for controlling the capacity of a screw
compressor wherein a surface of each valve which is exposed to a
compression chamber of the screw compressor forms an effective
continuation of a surface of the compression chamber of the screw
compressor.
Yet another object of the present invention is to machine the
surface of each valve simultaneously with the machining of the
surface of the compression chamber of the screw compressor in order
to reduce manufacturing cost as well as operating losses.
A further object of the present invention is to positively and
accurately axially position the surface of each valve during the
machining of the surface of the operating chamber of the screw
compressor.
An even further object of the present invention is to maintain the
radial positioning of the surface of each valve during the
machining of the surface of the compression chamber as well as
during the operation of the screw compressor.
Yet another object of the present invention is to provide a series
of lift valves wherein each lift valve housing is a single cast
unit thereby minimizing leakages associated with related valves and
reducing assembly costs.
These as well as additional objects of the present invention are
achieved by providing a series of lift valves communicating with a
compression chamber of a variable capacity screw compressor with
each valve including a valve housing, a piston reciprocally
received within the valve housing, a shaft having a first end
connected to the piston and a second end extending from the
housing. Each valve further includes a valve element connected to
the second end of the shaft having a valve surface exposed to the
compression chamber and a reciprocation mechanism for reciprocating
the piston within the housing. The reciprocation mechanism
including a first pressure passage communicating with the housing
adjacent a first side of the piston, and a second pressure passage
communicating with the housing adjacent an opposed side of the
piston, wherein the valve surface is positively displaced toward
and away from the compression chamber of the variable capacity
screw compressor in response to the application of fluid pressure
to at least one of the first and second pressure passages to vary
the capacity of the screw compressor.
Additionally, the lift valve is manufactured integral with the
manufacturing of the compression chamber of the variable capacity
screw compressor. This manufacturing process includes securing at
least one lift valve to a barrel portion of the variable capacity
screw compressor in an operating position. As mentioned above, the
lift valve includes a valve housing, a shaft extending from and
reciprocally received within the valve housing and a valve surface
of a valve element secured to a remote end of the shaft. Once
secured to the housing, the shaft and consequently the valve
element is fully extended from the valve housing. The process
further includes maintaining the shaft in the fully extended
position, and simultaneously machining an inner surface of the
compression chamber and the valve surface such that the valve
surface forms a continuation of the inner wall of the compression
chamber when the variable capacity screw compressor is operating at
full capacity. In this manner, zero tolerance is evidenced between
the valve structure and the surface of the compression chamber.
These as well as additional advantages of the present invention
will become apparent from the following detailed description of the
invention when read in light of the several figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view of the screw type compressor and
supporting controls to which the present invention may be readily
adapted;
FIG. 2 is a perspective view of a partially cut away screw
compressor incorporating valves in accordance with the present
invention;
FIG. 3 is a block schematic view of the overall operation of the
screw compressor in accordance with the present invention;
FIG. 4 is a perspective view of a screw compressor housing
incorporating the present invention;
FIG. 5 is an elevational view of the lift valve in accordance with
the present invention;
FIG. 6 is a cross-sectional view of a lift valve in accordance with
the present invention;
FIG. 7A is a cross-sectional view of the lift valve in accordance
with the present invention in operation in the screw compressor
housing;
FIG. 7B is a cross-sectional view of a prior art spiral or turn
valve in operation in the screw compressor housing, and
FIG. 7C is a cross-sectional view of a prior art lift valve in
operation in the screw compressor housing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention relates to improved lift valves and improved
methods for manufacturing such lift valves for rotary screw
compressors. FIG. 1 is a diagrammatic view showing the compressor
system 100 to which the present invention may be readily adapted.
Compressor system 100 preferably includes an improved oil-flooded
rotary screw compressor 102 and an electronic control system 104.
In the preferred embodiment of the invention, the compressor 102 as
well as the several capacity reduction lift valves 322 (only one
illustrated) are controlled in accordance with the electronic
control system described in a co-pending application entitled
"System And Methods For Controlling Rotary Screw Compressors,"
naming Steven D. Centers and Paul Burrell as inventors, filed Nov.
23, 1994 and assigned to the same assignee as this application.
This related co-pending application is hereby incorporated in the
present disclosure by reference, and constitutes the primary source
of detailed disclosure of the electronic control system. However,
those features of the control system that are most relevant to the
operation of the present invention will be described briefly in
enough detail to facilitate use of the inventive capacity reduction
lift valves. Referring again to FIG. 1, compressor 102 is powered
by an electric motor 214. Electronic control system 104 includes
control housing 236 (containing the main electronic control
components of the system), and relay housing 106 containing relays
and switchgear for the system. Air end 314 of compressor 102 is
connected to a air/lubricant reservoir 312, which provides air to
service air output 346.
As referred to hereinabove, compressor 102 is provided with four
capacity reduction lift valves. When actuated, each of these valves
acts to effectively bypass a part of the compressor screw, reducing
the capacity compressor 102 by approximately 12.5%. Thus, by
opening one valve, a 12.5% reduction in output capacity is
obtained, and by opening all four valves, capacity of the
compressor is reduced by 50%. Intermediate levels of capacity of
reduction, such as 25% and 37.5%, are similarly obtained by opening
from one to four of the capacity reduction valves. For clarity,
only one capacity reduction valve, valve 322, is shown in FIG. 1.
Each of the capacity reduction valves is a positive double acting
air operated valve, and each is controlled by a four way solenoid
valve in response to signals from the electronic control system
104. The four way solenoid valves for controlling the four capacity
reduction lift valves are designated in the drawing as SV1, SV2,
SV3, and SV4.
Compressor 102 has an inlet valve 336 controllable to vary the
amount of inlet air supplied to compressor 102. When inlet valve
336 is closed, no air is provided to compressor 102, so compressor
102 is "unloaded" and runs freely with minimal compression load.
When inlet valve 336 is fully open, the compressor is "loaded" or
provided with input air. Inlet valve 336 can also be controlled to
open partially in a "modulated" operating mode, so that compressor
102 is only partially loaded. The operation of inlet valve 336 is
controlled by solenoid valves SV5 and SV7 which respond to signals
from electronic control system 104. Valve SV5, when activated,
closes inlet valve 336 and unloads compressor 102. Valve SV7, when
activated, partially closes inlet valve 336 so that compressor 102
is only partially loaded. Valve SV7 is connected to a proportional
regulator. Thus, when activated, valve SV7 provides closing
pressure through the proportional regulator to inlet valve 336 that
varies with the pressure in reservoir 312. As system pressure is
increased, the amount of closure of inlet valve 336 upon activation
of valve SV7 is also increased. Electronic control system 106 is
also connected to blowdown valve SV6 which can be activated to
release pressure from the system when unloaded and at shutdown.
Referring now to FIG. 2, the compressor 102 will now be described
in greater detail. Specifically, the compressor 102 is a constant
velocity oil flooded rotary screw type compressor which is driven
by an electric drive motor 214 which drives the main shaft 6 which
is supported by bearing assembles 8 and 10 which are housed in
bearing housings 12 and 14 respectively. Positioned at the end of
the main shaft 6 is a positive displacement lubricant pump 16 for
providing efficient lubricant injection under all operating
conditions. Secured to the main shaft 6 is a first rotor 18 while
secured to a second rotary shaft (not shown) mounted parallel to
shaft 6 includes a second rotor 20. The second shaft is similarly
mounted in bearing housing 22. As discussed hereinabove, the screw
type compressor includes an inlet valve 336 which controllably
moves between a closed position as illustrated in FIG. 2 and a
fully opened position when the screw compressor is operating at
full capacity. Further, when the screw compressor is operating at
less than full capacity, the inlet valve 336 may be positioned
somewhere between a fully opened and fully closed position or
oscillated between such positions as described in the above-noted
copending application.
As discussed hereinabove, lift valves 322, 324, 326 and 328
communicate with the compression chamber 24 formed within the
barrel 26 of the compressor 102. As illustrated in FIG. 2, a bore
28 is provided in the barrel 26 which may selectively provide
communication between compression nodes and consequently reduces
the capacity of the compressor 102. Alternatively, bore 28 may
communicate with passage 29 in the barrel housing for returning
pressurized fluid to the suction side of the compressor. As
discussed hereinabove, when in the open condition, each of these
valves act to effectively by-pass a part of the compressor screw
and thus reduce the capacity of the compressor by approximately
12.5%. Accordingly, by opening all four valves, the capacity of the
compressor is reduced by 50%. It is the structure and process of
manufacturing the lift valves 322, 324, 326 and 328 which
constitute the essence of the present invention. Accordingly, these
valves will be discussed in greater detail hereinbelow.
FIG. 3 is a block schematic diagram of air control line connections
and air control equipment in accordance a preferred embodiment of
the invention. Again, this control system is discussed in detail in
the above-noted copending application and will be only briefly
discussed herein. The air control equipment includes a control
panel 302 having a pressure switch 304, an air filter indicator
switch 306, a line pressure transducer 308, and a reservoir
pressure transducer 310. Separator scavenges 311 of reservoir 312
are connected to air end low pressure point 314 of compressor 102
through line filter orifices 316, sight gauges 318, and line
filters 320.
The four way solenoid valves SV1 through SV4 are connected to
control lift valves 322, 324, 326, and 328 respectively. Valves SV1
through SV4 are preferably four-way positive action solenoid
valves. An air supply input for valves SV1 through SV4 is connected
to a pressurized air outlet of reservoir 312 by way of pressure
regulator 330 and automatic line filter 332. Pressure regulator 330
may be omitted if the compressor system 100 will not be operated
above 125 psi full load pressure. Valves SV1 through SV4 can also
be connected by two lines to low pressure point 333 below air
filter 334, on inlet valve 336 which is installed on the air intake
port of compressor 102. These two lines provide exhaust ports for
valves SV1 through SV4, for each direction of stroke of the
valves.
The provision of double action lift valves 322, 324, 326, and 328
rather than single action lift valves provides a significant
advantage in the context of compressor system 100. This feature
will be described in greater detail hereinbelow.
A reservoir air output 337 is connected to reservoir 312 to carry
the compressed air output of the compressor to the customer's
service air piping system, and thus to the equipment operating on
the compressed air generated by compressor system 100. Air output
337 is connected through an after cooler 339 to a minimum pressure
check valve 341, the output of which is connected to the customer's
service air piping system at service air output 346. Reservoir air
output 337 is also connected to a solenoid operated blowdown valve
SV6 which is connected to a muffler 343. When blowdown valve SV6 is
actuated, air pressure in reservoir 312 is released to the
environment through muffler 343.
The pressurized air outlet of reservoir 312 is connected by an air
line to reservoir pressure transducer 310, and a mechanical
pressure gauge 338 is connected to the same line next to reservoir
312. Similarly, a pressurized air output of reservoir 312 is
connected to an input of automatic line filter 340. The output of
automatic line filter 340 is connected to one air input side of
shuffle valve 342 and to the input of pressure regulator 344. The
output of pressure regulator 344 is connected to a non-common
connection of three-way solenoid valve SV7. The other air input
side of shuffle valve 342 is connected to the customer's service
air at service air output 346 of compressor system 100.
The output of shuffle valve 342 is connected to pressure switch 304
and to a non-common connection of three-way solenoid valve SV5. The
common connection of three-way solenoid valve SV5 is connected to
one air input side of shuffle valve 350. The other air input side
of shuffle valve 350 is connected to the common connection of
three-way solenoid valve SV7. The remaining non-common connection
of each of three-way solenoid valves SV5 and SV7 is open for
exhaust. The output of shuffle valve 350 is connected by an air
pipe to the input of gauge/pressure regulator 354. The output of
gauge/pressure regulator 354 is connected to the inlet valve 336
control side.
These particular air connection configurations and the use of
three-way valves SV5 and SV7 are significant because they allow
inlet valve 336 to receive operating air pressure more quickly
during startup, so that inlet valve 336 can be immediately closed
to provide an unloaded startup of compressor 102. At startup, there
is no pressure in reservoir 312. There may, however, be pressure in
the customer's service air line, due to stored pressure in an
external reservoir and/or because other compressors are running to
pressurize the service air line. It has been determined that when
service air pressure is available, it is advantageous to make use
of this pressure for startup control during the period before
reservoir 312 is pressurized.
At startup, the existence of pressure in the service air line and
the lack of pressure in reservoir 312 will bias shuttle valve 342
to connect the service air line to three-way solenoid valve SV5.
Three-way solenoid valve SV5 is then actuated to transmit the
service air pressure to shuttle valve 350, while three-way solenoid
valve SV7 is controlled to connect its common connection to the
exhaust end. The service air pressure biases shuttle valve 350 to
connect the service air pressure to control inlet valve 336. Valve
SV5 is then actuated, which will unload compressor 102 prior to
starting motor 214. In this way, compressor system 100 can be
started without any loading, minimizing startup power usage and
transient currents. When sufficient pressure is available in
reservoir 312, air from reservoir 312 is provided to bias shuttle
valve 342 toward three-way solenoid valve SV5, allowing
transmission of the reservoir air to the inlet valve 336 control
side.
Referring now to FIG. 4, the barrel portion 26 of the screw
compressor housing is illustrated in detail. The barrel portion 26
is formed by casting and subsequently machined to receive the
respective rotors. The barrel wall necessarily takes the shape of
two intersecting cylinders, each housing one of the rotors 18 and
20. As discussed hereinabove with respect to FIG. 2, lift valves
322, 324, 326 and 328 of which only lift valve 322 is illustrated
communicate with the compression chamber 24 within the barrel 26 by
way of bores 28. The double acting lift valve 322 includes a
mounting flange 323 which permits the double acting lift valve 322
to be secured to the barrel 26 by way of bolts 325 (one of which is
shown). In order to assure proper alignment of the lift valve with
the barrel 26, opposed bolt holes 321 in flange 323 as well as the
barrel 26 are staggered. By doing so, the lift valve can only be
mounted in one orientation. Also provided is a gasket 327 for
providing a seal between the barrel 26 and mounting flange 323. The
remaining lift valves 324, 326 and 328 are similarly mounted to the
barrel 26 in this manner.
In accordance with the present invention and in order to form a
more efficient screw compressor, each of the double action lift
valves are secured to the barrel 26 in a manner discussed with
respect to FIG. 4 and machined along with the machining of the
surface 25 of compression chamber 24 within the barrel 26.
Referring to each of FIGS. 5, 6 and 7a, it can be noted that the
surface 402 which is exposed within the compression chamber 24 of
the barrel 26 takes on a concave shape due to its machining along
with the machining of the compression chamber 24 of the barrel
26.
Referring to FIG. 5, the double action lift valve 322 includes a
housing 410 which accommodates a piston (not shown) and piston stem
412. Formed integral with the piston stem 412 is a valve element
414 which includes the concave surface 402. Additionally, a flange
416 is provided for positioning the valve against the barrel 26
when the valve is in the fully extended position as illustrated in
FIG. 7a. Again, the double action lift valve includes a mounting
flange 323 which is cast with the housing 410 for securing the
valve in place. In order to seal both pressurized air within the
housing 410 as well as sealing out any oil which may leak past the
flange 416, a two-way shaft seal 418 is secured to an end of the
housing 410. The inner details of the lift valve 322 will now be
discussed in greater detail with respect to FIG. 6.
As can be seen from FIG. 6, the piston stem 412 is integrally
formed with a piston member 411 which is reciprocally received
within the housing 410. The piston stem 412 and piston member 411
may also be separate units secured to one another in any known
manner. Further, it should be noted that the housing 410 is in the
form of a one-piece cylinder casting. With previous lift valves,
the valve casing or housing 410 is formed from multiple sections
which are secured to one another using sealing gaskets and the
bolts. However, it has been determined by casting a single piece
housing, not only are previous leakage points eliminated, the
assembly time for assembling the lift valve is also reduced.
Further, with the one-piece construction, the flange 323 as well as
bolt holes 321 can be so oriented that the lift valve 322 can only
be mounted on the barrel in a single orientation thereby
eliminating incorrect installation of the lift valves if such
valves are removed for shipping or service as referred to
hereinabove. Again, it is critical that the lift valves be
installed in the orientation in which they are initially
manufactured such that the concave surface for 402 is properly
oriented within the compression chamber 324. Additionally, in order
to assure that the piston 411 and piston stem 412 do not change
orientations with respect to the housing 410 after manufacturing, a
square pin 414 is received within a square hole 416 formed in the
piston 411. In doing so, the square pin 414 will prohibit any
rotation of the piston 411 with respect to the housing 410. While
the particular embodiment illustrated in FIG. 6 includes the square
pin 414 and square hole 416, any mechanism for maintaining the
orientation of the piston 411 with respect to the housing 410 may
be utilized. The primary concern is to assure the proper
orientation of the concave surface 402 within the compression
chamber 324. Such an orientation may be maintained by any
acceptable means.
When the lift valve 322 is assembled, two pressure chambers are
formed, one being pressure chamber 418 between the end of the
housing 410 and the piston 411 the other being a second pressure
chamber 426 formed between the piston member 411 and the two-way
shaft seal 418. Again, as described hereinabove, the two-way shaft
seal 418 is provided in order to seal in both directions, that is
the two-way shaft seal 418 seals in pressurized air within the
pressure chamber 426 and seals out any oil external to the
valve.
As discussed hereinabove, each of the lift valves 322, 324, 326 and
328 are actuated and de-actuated by way of four-way solenoid valves
SV1, SV2, SV3 and SV4 respectively. That is, in order to manipulate
the piston 411 within the housing 410, pressurized air may be
provided to either one of pressure chambers 424 or 426 while the
other of the pressure chambers are exhausted. That is, in order to
force the valve element 414 into the fully extended closed
position, pressurized air is provided to the pressure chamber 424
through passage 425 while the pressure chamber 426 is exhausted
through passage 427. It should be noted that both passages 425 and
427 are positioned in a lower portion of the valve housing 410.
This assures that condensation will be properly drained from the
chamber 424 and 426 respectively. Likewise, should it be desired to
operate the screw compressor at less than full capacity,
pressurized air is supplied to the pressure chamber 426 through
passage 427 of one or more of the lift valves while the pressure
chamber 424 is exhausted through passage 425 in order to
reciprocate the piston 411 and consequently the valve element 414
to an open position. As discussed hereinabove, four way solenoid
valves SV1, SV2, SV3 and SV4 are controlled to selectively
pressurize and exhaust pressure chambers 424 and 426 in response to
a demand placed on the compressor system. In order to isolate the
pressure chambers 424 and 426 from one another, piston 411 is
provided with seals 428 and 430. Also, seal 432 is provided in the
two-way shaft seal which is secured to an open end of the housing
410.
As discussed hereinabove, the surface of the valve element 414 is
machined integral with the machining of the surface 25 of the
compression chamber 24 of the barrel 26. That is, during the final
machining of the compressor chamber side walls 25, in order to form
the requisite tolerance between the rotors and such side wall, each
of the lift valves 322, 324, 326 and 328 are positioned in their
operating position secured to the barrel 26. In this regard, the
piston 411 and consequently the valve element 414 must be fully
extended and maintained in the fully extended position throughout
the machining process and particularly when the surface 402 itself
is being machined. In order to do so, the pressure chamber 424 is
filled with pressurized hydraulic fluid or oil which assures that
the valve element 414 will remain in its fully extended position
assuming such fluid to be incompressible. Accordingly, once the
lift valves 322, 324, 326 and 328 are secured to the barrel 426,
pressure chamber 424 is filled with an incompressible fluid at
which time the final machining of the wall 25 of the compression
chamber 24 is carried out. In doing so, the surface 402 of the
valve element 414 exactly matches and forms a continuation of the
wall 25 of the compression chamber 24 which minimizes any leakage
around the rotor as the rotor passes over the surface 402.
Referring now to FIGS. 7A through 7C, the distinct advantage of the
present invention over prior art valving systems will become
clearly apparent. The present invention is illustrated in FIG. 7a
wherein the valve element 414 is positioned in its fully extended
position. As can be seen from FIG. 7A, the surface 402 of the valve
element 414 forms a continuation of the surface 25 of the
compression chamber 24 of barrel 26. Accordingly, as rotor 18
rotates past the valve element 114, there is no leakage between the
surface 402 of the valve element 414 and the rotor 18. This is
achieved because the surface 402 is machined integral with the
surface 25 of the barrel 26. Further, the positioning of the valve
element 414 is assured due to the positive displacement of the
piston within the double acting lift valve. While the
aforementioned prior art devices illustrate lift valves having
concave surfaces, such lift valves are formed by way of a separate
manufacturing process and subsequently positioned within the
compressor housing. Accordingly, these lift valve surfaces are
manufactured to within predetermined tolerance, however, such
manufacturing process cannot practically duplicate the curvature of
the compression chamber surface 25 and thus leakage by the rotor
may still exist in such systems.
Referring to FIG. 7B, clearly when using a turn and spiral valve
variable capacity design, numerous ports 50 are provided near the
bottom center line of the barrel 26'. As discussed hereinabove,
these ports are as deep as the housing material is thick and
consequently air in the higher pressure compression pocket blows
around the tips of the rotors 18 and 20 as they pass these ports.
Clearly, the efficiency of the device is significantly reduced and
full capacity cannot be achieved.
The poppet type valve illustrated in FIG. 7C includes a planar
surface 52 on the valve element 54 which also allows blow by around
the rotor 18 resulting in a reduction in the efficiency of the
system. Further, such a poppet type valve relies on a single acting
piston to close the valves, thereby relying on the internal air
pressure and/or a spring force to move, the valve to the open
position. Often times, the opening pressure may be low and
consequently these valve designs may stick or operate erratically,
again failing to provide the user with the maximum savings under
part load conditions.
Clearly, it can be seen that by utilizing double acting lift valves
having a single valve cylinder casting with a valve element which
is machined in conjunction with the machining of the compression
chamber wall provides an advantageous capacity control system
wherein the compressor can realize 100% efficiency when the double
acting lift valves are in the closed position and which may
accrately control the capacity reduction as desired.
While the present invention has been described with reference to
referred embodiments, it will be appreciated by those skilled in
the art that the invention may be practiced otherwise than as
specifically described herein without departing from the spirit and
scope of the invention. Therefore, it will be understood that the
spirit and scope of the invention be limited only by the appended
claims.
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