Stepping type unloading system for helical screw rotary compressor

A stepping piston is projected into the path of a slide valve drive piston to limit piston movement and thus the slide valve towards maximum unload position determined by piston bottoming out against the cylinder wall. The slide valve main drive piston stroke is also correlated to desired slide valve positions along the intermeshed helical screw rotors of the helical screw rotary compressor to provide, for example, stepped unloading at compressor full load, two-thirds full load, and one-third full load.

BACKGROUND OF THE INVENTION 
This invention relates to helical screw rotary compressors, and more 
particularly, to an improved stepping type unloading system for 
controlling compressor capacity and discharge pressure of the machine by 
stepping of a screw compressor capacity control slide valve. 
DESCRIPTION OF THE PRIOR ART 
One form of positive displacement gas compressor is the helical screw 
rotary compressor in which a gaseous working fluid is trapped within the 
closed threads of intermeshed helical screw rotors defining a decreasing 
volume working chamber. The helical screw rotors are mounted for rotation 
within intersecting bores with coplanar axes defining the barrel portion 
of a screw compressor casing. Conventionally, to control the capacity of 
the compressor and to control the pressure ratio or pressure of the 
working fluid at compressor discharge, a slide valve is provided to the 
compressor and carried within a longitudinally extending recess within the 
barrel portions of the casing, in open communication with the bores, and 
partially overlying respective sides of the intermeshed screws. U.S. Pat. 
No. 3,088,659 to H. R. Nilsson et al and entitled "Means for Regulating 
Helical Rotary Piston Engine" is exemplary of the employment of such a 
slide valve on a helical screw rotary compressors. 
Further, the longitudinal or axial position of the slide valve itself is 
normally controlled by a hydraulic linear motor comprising a cylinder 
normally an extension of the compressor casing itself, which slidably and 
sealably bears a piston connected to the slide valve member by way of a 
piston rod which extends therebetween. Further, by modulating the flow of 
hydraulic fluid to a closed chamber on one side of the piston, and/or by 
relieving fluid pressure within the chamber on the opposite side of the 
piston, the piston is shifted. The piston slideably moves the slide valve 
member relative to intermeshed helical screw rotors to thus variably 
control the size of a bypass opening formed between the end of the slide 
valve member proximate to the suction port opening to the intermeshed 
screw rotors, and a fixed stop. As such, a portion of the suction gas 
entering the working chamber as defined by the intermeshed grooves and 
lands of the rotors, is returned to the suction or low pressure side of 
the machine without compression. When the slide valve is at the point 
where its end face contacts the fixed stop and closes off the bypass 
passage, the compressor operates at 100% capacity, that is, full load. In 
turn, by shifting the slide valve member to its full extent away from the 
fixed stop and to the point where there is no cut off between the suction 
and discharge sides of the intermeshed helical screw rotors, no 
compression of the gas takes place and the compressor is operating at full 
unload. 
Such modulating type capacity control arrangement is adequate and, in fact, 
highly desirable for larger helical screw rotary compressor systems and is 
advantageous in maximizing the efficiency of the gas compressor system. 
For smaller size compressors, requiring less sophisticated control 
arrangements, not only is there no need for such modulating capacity 
control, but the use of such modulating capacity control system renders 
the overall system unduly expensive. 
It is, therefore, an object of the present invention to provide a helical 
screw rotary compressor with an improved slide valve capacity control 
system which permits operation at multiple selected load conditions which 
is simple, highly effective, is relatively inexpensive and which will meet 
most system demands required of small size helical screw rotary 
compressors. 
SUMMARY OF THE INVENTION 
The present invention is directed to stepping type slide valve unloading 
system for a positive displacement helical screw rotary compressor. A 
compressor casing is provided with a barrel portion defined by 
intersecting bores with coplanar axes located between axially spaced end 
walls and having a low pressure suction port and a high pressure discharge 
port in communication with the bores at opposite ends of the barrel 
portion. Helical screw rotors having grooves and lands are mounted for 
rotation within the respective bores with the lands and grooves of 
respective rotors intermeshed. An axially extending recess is provided 
within the barrel portion of the casing in open communication with the 
bores. A slide valve member is longitudinally slidable in the recess with 
the innerface of the slide valve member being complementary to the 
envelope of that portion of the bores of the casing structure confronted 
by the opening of the recess communicating with the bore portion of the 
casing. The valve member is in sealing relation with the confronting 
rotors. At least a portion of the discharge port is located within the 
barrel portion of the casing with the slide valve member being movable 
between extreme positions, with the end of the slide valve member 
proximate to the suction port variably closing off a bypass passage in 
open communication with the suction port and functioning to bypass 
uncompressed gaseous working fluid. The slide valve member is normally of 
sufficient length to cover the entire remaining length of the confronting 
portion of the rotor structure throughout the range of movement of the 
slide valve member between its extreme positions. A linear drive motor for 
the slide valve member comprises a cylinder, a main drive piston sealably 
and slidably positioned within said cylinder and a piston rod connecting 
the piston to the slide valve member. The piston forms, with the cylinder, 
an inboard chamber on the side of the piston proximate to the slide valve 
member, and an outboard chamber on the opposite side thereof. Means are 
provided for supplying and relieving hydraulic fluid pressure to at least 
one of said chambers for shifting the slide valve member between the 
extreme positions. 
The improvement resides in a stepping piston carried by the linear motor 
and shiftable between retracted and projected positions with respect to 
one of said chambers to limit piston movement between the slide valve 
extreme positions to thereby define with the main drive piston of the 
linear motor, three distinct capacity control step positions for the slide 
valve member. 
The inboard chamber may open directly to the compressor discharge port such 
that, absent fluid pressure application to the outboard chamber, the 
piston is shifted to its extreme unload position as defined by the end of 
the cylinder forming the outboard chamber. The cylinder is preferably 
provided with a cylindrical casing extension portion at its outboard end, 
the casing extension portion defining a stepping cylinder. A stepping 
piston is sealably mounted within the stepping cylinder bore and has a 
portion projecting from the inboard face thereof which is projectable into 
the outboard chamber of the main drive linear motor and being of a length 
such that when the stepping piston is at its extreme inboard position with 
respect to the slide valve member, the projection extends fully into the 
outboard chamber of the main linear drive motor to provide a positive stop 
for the linear drive motor piston, some distance from the outboard end of 
the linear drive motor cylinder. The system further includes means for 
selectively supplying hydraulic fluid pressure to the stepping cylinder 
outboard chamber to drive the projection portion of the piston from 
retracted position to projected position within the main linear drive 
cylinder outboard chamber and/or to the outboard chamber of linear drive 
motor. 
The means for supplying to and relieving hydraulic fluid pressure from the 
outboard chambers of said main linear drive motor and said stepping 
cylinder may comprises a hydraulic pressure source and conduit means 
connecting said source of hydraulic pressure to the outboard chamber of 
both said main drive cylinder and said stepping cylinder and for returning 
hydraulic fluid from said outboard chambers to a system sump. Selectively 
operable valve means provided within said conduit means selectively 
connects each of said outboard chambers to said source of hydraulic 
pressure or to said sump to relatively cause said main drive piston to 
drive said slide valve member against said fixed stop and to maximum load 
condition for the compressor, or to drive said stepping cylinder piston to 
projected position to prevent compressor discharge shifting of said main 
drive piston to the end of the main drive motor outboard chamber for 
partially unloading the compressor or opening both the main drive cylinder 
and said stepping cylinder outboard chambers to the sump to permit the 
compressor discharge pressure to cause said main slide valve motor piston 
to nearly bottom out against the end of said slide valve drive cylinder, 
remote from the intensified screwrotors with the slide valve member of 
maximum unload position.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference to FIG. 1 shows the stepping type unloading system for a helical 
screw compressor forming one embodiment of the present invention. The 
control system has application to a helical screw rotary compressor, 
indicated generally at 10, comprised principally of a compressor section 
12 formed by intermeshed helical screw rotors 14 and 16 and a slide valve 
section indicated generally at 18. The rotary drive motor for the helical 
screw rotary compressor is purposely not shown, although such is needed 
for rotatably driving one of the rotors 14, 16. Additionally, the system 
comprises a high pressure hydraulic fluid pressure source indicated 
schematically by arrow 20 and a sump for return of the hydraulic fluid or 
indicated by the arrow 22. Conduit means indicated generally at 24 directs 
the hydraulic fluid under pressure to the slide valve section 18 and the 
return of the same to the sump. 
With respect to compressor 10, the compressor 10 comprises a casing 
indicated generally at 26 including a central barrel portion or section 
28, of modified cylindrical form, formed of cast metal and closed off at a 
suction or low side end by an end bell or end wall 30. The opposite 
highside or discharge side is closed off by end bell or end wall 32. While 
not shown, the casing sections are sealed to each other by means of 
O-rings and the like and are bolted or screwed to each other to permit 
disassembly. The casing central barrel portion or section 28, located 
between end walls 30, 32, is provided with a compression chamber or 
working space formed by two intersecting bores as at 34 which bear 
respectively the helical screw rotors 14, 16 whose axes are coplanar and 
which extend, in this case, horizontally through the barrel portion 28 of 
the casing. The helical screw rotary compressor 10, in this respect, is 
conventional, and both the male and female rotors have helical lands and 
intervening grooves which intermesh, with the rotors mounted to rotate in 
the bores by means of suitable bearings, being journaled by shafts as at 
36 bearing the rotors 14 and 16. Multiple anti-friction bearings 38 may be 
employed for mounting the shafts 36 and thus the intermeshed rotors for 
rotation about their axes. One shaft 36 may extend through end end wall 30 
and may be directly coupled to the rotor of an electrical drive motor or 
the like (not shown) which act to drive the intermeshed helical screw 
rotors. One of the rotors functions to drive the other. The compressor 
casing central barrel section 28 is provided with a low pressure suction 
port 40 at or adjacent one end wall 30 which opens to the intermeshed 
helical screw rotors at that end of the machine. 
The central barrel section 28 of the compressor is additionally provided 
with a longitudinally extending recess 42 which opens at one end to a high 
pressure discharge port 44 while its opposite end terminates at a bypass 
passage 46 which opens transversely to suction port 40. Slidably mounted 
within recess 42, is a longitudinally slidable slide valve member 50 
sealably configured to recess 42 and bearing a peripheral portion 50a 
which faces and makes sliding contact with peripheral portions of the 
intermeshed helical rotors 14 and 16 and which forms a part of the 
envelope for the compression process occurring within working chambers 
defined by the intermeshed helical screw rotors 14 and 16, the casing 
section 28 and the slide valve member 50. Conventionally, end face 50b of 
the slide valve, proximate to the suction port 40 and thus the low side of 
the machine, is flat, at right angles to the slide valve member axis and 
abuts, when in extreme left position in the figures, a fixed abutment or 
stop 52. The slide valve member 50 and stop 52 define a variably sized 
bypass opening 54 leading from the intermeshed helical screw rotors 14 and 
16 and bores 34 to the bypass passage 46. Passage 46 is connected to the 
suction side of the machine via casing cavity 48. 
While a portion of the opposite end face 50c of the slide valve member 50 
is vertical and at right angles to the axis and flat, there is a 
peripherally relieved portion 56 of face 50a of the slide valve member 50 
which forms with the casing, a common high pressure axial and radial 
discharge port 44 for the compressor, leading to compressor casing 
discharge port 44a. 
Conventionally, the slide valve member 50 is sealably carried within the 
casing section and is driven between two longitudinally displaced extreme 
positions. The present invention includes a modified hydraulic linear 
drive motor indicated generally at 60. In that respect, the end bell or 
end wall 32 is provided with a cylinder 62 having an internal cylindrical 
bore 64 coaxially aligned with the longitudinal axis of the slide valve 
50. The cylinder bore 64 sealably and slidably bears a main drive piston 
66 for the slide valve section 18, which piston is connected to the slide 
valve member 50 by way of a piston rod 68. The piston 66 is provided with 
a groove 70 within its periphery, bearing an O-ring or equivalent seal as 
at 72. The piston 66 defines with the cylinder a sealed inboard chamber 
74, proximate to the slide valve member 50, and on its opposite face, to 
the right of piston 66, a sealed outboard chamber 76. 
Unlike the prior art helical screw rotary compressors, the outboard chamber 
76 is not closed off simply by an end wall or plate which spans across the 
open end of the cylinder 62 housing the main drive piston for the slide 
valve member 50. In this case, there is provided a stepping piston 
assembly indicated generally at 78 including a stepping piston cylinder 80 
open at its left end and being closed off at its right end by spherical 
end wall 82. The cylinder 80 is partially closed off, at the left, by a 
vertical end wall 84 which extends radially beyond the periphery of the 
cylinder 80 to close off main drive motor outboard chamber 76, thus 
forming an enlarged radial flange. End wall 84 is provided with a circular 
opening 86 at its center which opens to the interior of the hollow 
cylinder 80. Cylinder 80 is formed with a circular bore 87, within which 
is slidably and sealably mounted a stepping piston indicated generally at 
88. Stepping piston 88 is of a diameter slightly less than the diameter of 
the bore 87 within which it is positioned. Piston 88 bears a groove 90 
within its periphery within which sits an O-ring seal 92. Piston 88 seals 
off outboard chamber 94 within the stepping piston cylinder 80. Integral 
with the stepping piston 88, is a reduced diameter cylindrical projection 
96 having a diameter on the order of the circular hole 86 within wall 84 
within which, the projection 96 rides. 
Thus, the piston 88 is T-shaped in cross-section with an enlarged headed 
end interiorly of the stepping cylinder casing 80. Further, the length of 
the projection 96 is such that with the main drive piston 66, driven to 
the right, such that its face remote from the slide valve member 50 nearly 
contacts end wall 84 of the stepping piston assembly 78 and the projection 
96 is retracted almost completely into casing 80 with its end face 96a 
nearly flush with the face of end wall 84. Wall 82 prevents full 
retraction of projection 96 from outboard chamber 76, although cylinder 80 
could be lengthened to achieve this end. 
In order to effect axial displacement of main drive piston 66 of the main 
linear drive motor 60 for the slide valve member 50, as well as 
independently, the projection 96 of the stepping piston 88 into the linear 
drive motor outboard chamber 76, the system employs means for effecting 
the controlled application of hydraulic pressure to chambers 76 and 94, 
respectively. 
In that regard, the system as indicated previously is provided with conduit 
means at 24 for directing the flow of hydraulic fluid under pressure from 
a source 20 to said chambers 76 and 94 and the relief of such hydraulic 
pressure by return of hydraulic fluid to the sump indicated by arrow 22. 
Specifically, supply conduit or pipe 98 divides at point 100 such that one 
supply conduit portion 98a connects to one side of solenoid valve 106 
while the other side 98b connects to one side of a second solenoid valve 
108. Supply and return line 101 connects the other side of solenoid valve 
106 to chamber 94 of stepping piston assembly 78, opening to that chamber 
via hole 102 within cylinder end wall 92 of that assembly. 
A supply and return line 103 directs hydraulic fluid under pressure to the 
outboard chamber 76 of the linear drive motor for the slide valve member 
50, being connected to a small diameter passage 104 within end wall 84 and 
opening, at port 104a, to the outboard chamber 76. 
Solenoid valves 106 and 108, are two position valves. That is, the valves 
are spring biased by way of springs 110 to normally, absent energization 
of solenoids as at 112, connect lines 101 and 103 to a common sump or 
fluid return line 114 leading to the sump as indicated by arrow 22. Line 
114 is connected via sump line 114b to valve 106, and via sump line 114a 
to valve 106. Movable valve members 111 within the solenoid valves permit 
selective communication, via passage 118, in each instance, of supply line 
98 to respective supply and return lines 101 and 103 respectively. 
Alternatively, by way of passages 120 within movable valve members 111, 
and sump or return lines 114a, 114b, connection of the supply and return 
lines 101 and 103 is effected to the common sump line 114. 
As may be further appreciated by reference to FIG. 3, when end face 50b of 
the slide valve member abuts the end face 52a of stop 52, the bypass port 
or gap 54 is closed off and the bypass passage 46 cannot return 
uncompressed working fluid back to the suction side of the machine as 
defined by casing cavity 48. This is one extreme capacity control or 
loading position for the compressor. It is the full load position in the 
illustrated and exemplary embodiment. The maximum volume of working gas is 
compressed with all of the gas taken in the suction side of the machine, 
via port 40, being compressed at a compression ratio defined by machine 
parameters and being discharged under high pressure at discharge port 44 
to the right of the intermeshed rotors 14, 16. Under the stepping control 
scheme, the slide valve member 50 steps partially to the right, FIG. 2, to 
the point where main drive piston 66 abuts end face 96a of the stepping 
piston projection 96 when it is maintained in projected position in that 
figure by application of fluid pressure to chamber 94. This position of 
the slide valve 50, FIG. 2, represents, in an exemplary fashion, 
two-thirds loading of the compressor. Further step unloading is permitted 
to the extent that the piston 66 nearly abuts end wall 84, that is, is 
fully displaced to the right with the stepping piston 88 near fully 
retracted as seen in FIG. 1. In this position bypass port or opening 54 
leading to bypass passage 46 is open to its maximum with very little of 
the working fluid being compressed by the intermeshed rotors most being 
returned to the suction side of the machine prior to compression. 
In normal operation, the sequence occurs from FIG. 1 to FIG. 3. Referring 
to FIG. 1, it is seen that the slide valve member 50 is to its extreme 
right position with piston 66 nearly abutting end wall 84 and displacing 
the projection 96 of the stepping piston 88 to the right with the stepping 
piston 88 adjacent end wall 82 of assembly 78. This is the position 
occurring at start up (or shortly after start up), where the pressure of 
the discharge gases filling the inboard chamber 74, displaces piston 66 to 
the right. The developed force acting on the main drive piston 66 is in 
excess of that acting on end face 50b of the slide valve member tending to 
shift the slide valve member 50 to the right within its recess 42. 
Further, with solenoid valves 106 and 108 de-energized, the biasing 
springs 110 tend to shift their movable spool members 111 to the right, 
thus connecting supply and return lines 101 and 103 to the common sump 
line 114 to drain outboard chambers 94 and 76, respectively. The 
compressor operates at its minimum capacity, that is, to its fullest 
unload capability. 
In the illustrated embodiment, the step unloading (or step loading, as the 
case may be) is from one-third loaded condition, as shown in FIG. 1, 
through a two-thirds loaded condition, FIG. 2, to compressor full load 
condition of FIG. 3. To sequentially achieve that end, by reference to 
FIG. 2, it may be seen that solenoid valve 108 remains de-energized such 
that the outboard chamber 76 is unpressurized. With solenoid valve 106 
energized however, the applied fluid pressure within outboard chamber 94 
of the stepping piston assembly 78 is high enough to overcome the 
discharge pressure differential acting between the inboard face of piston 
66 and the end face 50b of the slide valve member 50, such that the 
projection 96 of stepping piston 88 projects to its fullest extent into 
outboard chamber 76 of the main linear drive motor for the slide valve 
member 50. This effectively acts as a stop to prevent further movement of 
piston 66 towards end wall 84 under such conditions. 
With the solenoid operated valve 106 energized, hydraulic pressure is 
applied as at arrow 20 to common supply line 98 and passes by way of 
branch line 98a and passage 118 within the solenoid valve spool 111 to 
supply and return line 101. Thus hydraulic fluid under pressure applied to 
chamber 94 effects the displacement of stepping piston 88 and its 
projection 96, to the left, FIG. 2. Meanwhile, supply and return line 103 
leading to outboard chamber 76 remains connected to the common sump line 
114 by way of sump return branch line 114b and passage 120 of spool 111 of 
the solenoid valve 108, which solenoid valve remains de-energized. 
In order to step the slide valve member 50 to the left and to its extreme 
load position, and to close off bypass port 54, fluid pressure must be 
applied to the outboard chamber 76 of the linear drive motor to effect the 
displacement of piston 66 further to the left than that shown in FIG. 2 
and against the discharge pressure acting within inboard chamber 74 on the 
opposite face of piston 66. This is achieved, FIG. 3, by energization of 
the solenoid valve 108 to shift the fluid connections to supply and return 
valve line 103 from sump line 114 to the hydraulic pressure supply line 98 
via branch line 98b and passage 118 within the valve spool 111 for that 
solenoid valve. 
Stepping piston 88 has the purpose of automatically creating a step 
unloading procedure should a reversal in operation occur, that is, with 
the compressor operating, if the fluid pressure applied to the outboard 
chamber 76 of the main drive linear motor is terminated and that chamber 
is open to the sump as indicated by arrow 22, while solenoid valve 106 
remains energized, the compressor will simply step unload from the full 
load condition of FIG. 3 to a two-thirds load condition as seen in FIG. 2. 
Alternatively, if solenoid valves 108 and 106 are both de-energized or if 
valve 106 is de-energized initially with valve 108 energized, upon 
termination of energization of solenoid valve 108, the system will revert 
to the condition shown in FIG. 1 which is at maximum unload and with the 
piston 66 nearly abutting end wall 84 to terminate any further movement of 
the slide valve member 50 to the right. 
While the three steps in the loading/unloading procedure are illustrative 
of one set of equal capacity change steps of a typical loading or 
unloading sequence, the compressor may be manufactured such that the slide 
valve moves from full load to full unload position with a one-half 
unload/load intermediate stepped position for a three step sequence. 
Alternatively, other slide valve step positions may be effected as well as 
a greater number of stepped positions, determined by utilizing additional 
piston assemblies similar to that at 78. 
While the invention has been particularly shown and described with 
reference to a preferred embodiment thereof, it will be understood by 
those skilled in the art that the various changes in form and details may 
be made therein without departing from the spirit and scope of the 
invention.