US3869227A

US3869227A - Variable capacity rotary screw compressor having variable high pressure suction fluid inlets

Info

Publication number: US3869227A
Application number: US449263A
Authority: US
Inventors: Erich J Kocher; Whitney I Grant
Original assignee: Vilter Manufacturing LLC
Current assignee: Copeland Industrial LP
Priority date: 1974-03-08
Filing date: 1974-03-08
Publication date: 1975-03-04
Anticipated expiration: 1992-03-04
Also published as: SE7502274L; SE411243B

Abstract

A variable capacity multiple inlet rotary screw compressor having a pair of helical rotors mounted within a housing for compressing fluid drawn from a low pressure suction fluid chamber and a high pressure suction fluid chamber and discharging to a discharge chamber. A slide valve is provided in the housing to control the capacity of the compressor by changing the point of low pressure suction fluid cut off to the rotors. The point of high pressure suction fluid admission being changed in synchronism with the movement of the slide valve to connect the working chambers of the rotors to the high pressure fluid suction fluid chamber simultaneously with or just after low pressure suction fluid cut off.

Description

Kocher et a1.

1 1 Mar. 4, 1975 VARIABLE CAPACITY ROTARY SCREW COMPRESSOR HAVING VARIABLE HIGH PRESSURE SUCTION FLUID INLETS Inventors: Erich J. Kocher, Milwaukee;

Whitney 1. Grant, Muskego, both of Wis.

Vilter Manufacturing Corporation, Milwaukee, Wis.

Filed: Mar. 8, 1974 Appl. No.: 449,263

Assignee:

US. Cl 418/15, 418/159, 418/201 Int. Cl. F0lc 1/16 Field of Search 418/15, 159,201,202,

References Cited UNITED STATES PATENTS 5/1963 Nilsson et a1. 418/201 X 3/1969 Schibbye 418/201 X 12/1970 Linneken 418/201 X 9/1973 Persson et a1 418/159 X FOREIGN PATENTS OR APPLICATIONS 663.144 5/1963 Canada 418/201 Primary Examinew-C. J. I-Iusar Assistant E.\'aminer-Leonard Smith Attorney, Agent, or Firm-James E. Nilles [57] ABSTRACT A variable capacity multiple inlet rotary screw compressor having a pair of helical rotors mounted within a housing for compressing fluid drawn from a low pressure suction fluid chamber and a high pressure suction fluid chamber and discharging to a discharge chamber, A slide valve is provided in the housing to control the capacity of the compressor by changing the point of low pressure suction fluid cut off to the rotors. The point of high pressure suction fluid admission being changed in synchronism with the movement of the slide valve to connect the working chambers of the rotors to the high pressure fluid suction fluid chamber simultaneously with orjust after low pressure suction fluid cut off.

14 Claims, 14 Drawing Figures PATENTEI] MAR 4i975 SHEET 1 BF 5 mm mm NR. mm x S a mm mm mm mm A. 1. mm 2 6m Q @h 5 mm. NMJI 5 v n on w m? P QR PATENTEI] "AR 4 I975 SHEET 2 UF 5 FIGS 0 8/! fag aw m \J,

f 6 6 4 M 9 9 9 L 0 l x 9) 8 8 W PATENTEU 4 I975 SHEET 3 0F 5 VARIABLE CAPACITY ROTARY SCREW COMPRESSOR HAVING VARIABLE HIGH PRESSURE SUCTION FLUID INLETS BACKGROUND OF THE INVENTION Multiple suction variable capacity screw compressors utilize a slide valve to vary the capacity of the compressor between minimum and maximum capacity positions. The slide valve is used to control the low pressure suction fluid inlet producing a change in the pressure at the point of admission of the high pressure suction fluid. In a copending application Ser. No. 403,195, filed Oct. 3, 1973 and entitled Variable Capacity Rotory Screw Compressor", the point of injection of the secondary or high pressure suction fluid is maintained constant with respect to the low pressure suction fluid cut off in order to maintain the constant pressure at the point of admission of the high pressure suction fluid. This was accomplished by introducing the high pressure suction fluid at a fixed distance from the low pressure suction fluid cut off.

SUMMARY OF THE PRESENT INVENTION The multiple inlet variable capacity screw compressor of the present invention provides a change in the point of admission of the high pressure suction fluid vapor with a change in the position of the slide valve in order to admit high pressure suction fluid at the point of low pressure suction fluid cut off. In one embodiment of the invention, this is achieved by advancing or retracting the point of injection of the high pressure suction fluid vapor with respect to the position of the working chamber between the rotors. An electrohydraulic sensing system is used to sense the position of the slide valve and to open or close angularly offset plug valve assemblies. In the alternate embodiment of the invention, a continuing change in the point of admission of the high pressure suction fluid is provided with a continuing change in the position of the slide valve. Control valve assemblies are provided in each of the rotors of the compressor which are controlled by an electro-hydraulic circuit that maintains a synchronized relationship between the position of the slide valve and the control valve assemblies through an electrically balanced circuit.

DRAWINGS FIG. I is a plan view partially in section of the variable capacity compressor according to the present invention;

FIG. 2 is a section view taken on line 2--2 of FIG. 1 showing the inlet passages from the low pressure suction fluid inlet chamber to the working chambers of the COITIPI'CSSOI';

FIG. 3 is a view taken on line 3-3 of FIG. 1 showing the connection of the high pressure suction fluid chamber and the discharge chamber to the working chambers of the compressor;

FIG. 4 is a section view taken on line 44 of FIG. 3 showing the position of the slide valve and the plug valve assemblies in the minimum capacity position;

FIG. 5 is a plan view similar to FIG. 1 showing the hydraulic circuit for controlling the plug valve assemblies;

FIG. 6 is a plan view similar to FIG. 5 showing the slide valve in the maximum capacity position and the hydraulic control circuit in the maximum capacity position;

FIG. 7 is a plan view partially in section showing another embodiment of the variable capacity compressor of this invention;

FIG. 8 is a view taken on line 8-8 of FIG. 7 showing the connection of the low pressure suction fluid chamber to the working chambers of the rotors;

FIG. 9 is a view taken on line 9-9 of FIG. 7 showing the connection of the discharge chamber to the working chambers of the rotors;

FIG. 10 is a view taken on line 10-10 of FIG. 7 showing the connection of the high pressure suction fluid chamber to the rotors;

FIG. 11 is a perspective view of the control cylinder which is used to control high pressure suction fluid admission to the rotors;

FIG. 12 is a section view taken on line 12-12 of FIG. 9 showing the control valve assembly for controlling the admission of high pressure suction fluid to the grooves in one of the rotors; and

FIG. 13 is a view similar to FIG. 12 showing the control valve assembly in the maximum capacity position; and

FIG. 14 is a schematic diagram of an electrohydraulic circuit for controlling the movements of the control assemblies.

DESCRIPTION OF THE INVENTION The variable capacity multiple inlet rotary screw compressor 10 of this invention generally includes a pair of oppositely rotating constant mesh rotors l2, 14 which define working chambers 16 within a housing 18. The working chambers 16 are in fluid communication with a low pressure suction fluid inlet chamber 20 at one end of the housing 18 and to a high pressure suction inlet chamber 22 at the other end of the housing 18. Fluid drawn from the

chambers

20 and 22 is compressed in the working chambers 16 is discharged to a discharge chamber 24.

The capacity of the compressor 10 is adjusted by means of a slide valve 26 which controls the low pressure suction fluid cut off. The admission of the high pressure suction fluid is varied so that communication with the working chambers 16 occurs simultaneously with or just after low pressure cut off. In FIGS. 1 through 6, this is achieved by means of a number of plug valve assemblies 28. The timing of the operation of the plug valve assemblies 28 in relation to the position of the slide valve 26 is controlled by means of an electro-hydraulic circuit 30 which includes a cam operated switch assembly 32 that is connected to the slide valve 26.

In a variable capacity compressor of the type contemplated herein, as the slide valve 26 is moved between maximum and minimum capacity positions, the plug valve assembly 30 must be opened or closed so that high pressure suction fluid cannot bypass through the working chambers 16 between. the rotors l2 and 14 back to the low pressure suction chamber 20. More particularly and referring to FIGS. 5 and 6, it should be noted that as the

rotors

12 and 14 intermesh to form the working chambers 16, the working chambers will initially register with the low pressure suction chamber 20, then the high pressure suction chamber 22 and finally discharge through the discharge chamber 24. In order to obtain maximum efflciency, the high pressure suction fluid chamber 22 should be in communication with the working chambers 16 simultaneously with or just after low pressure cut off but prior to pressure build up in the working chambers 16. As the point of low pressure cut off is changed by moving slide valve 26 to reduce capacity, the point of admission of high pressure suction fluid to the working chambers 16 must be varied so that the high pressure suction fluid enters the working chambers 16 in accordance with the new low pressure suction cut-off.

THE VARIABLE CAPACITY COMPRESSOR OF FIGS. 1-6

More specifically, the housing 18 includes two intersecting

bores

34 and 36 closed at each end by

end walls

38 and 40. The low pressure suction chamber 20 is provided between the end wall 38 and outside wall 39. The high pressure suction chamber 22 is provided between the end wall 40 and outside wall 41. A low pressure suction inlet port 46 is connected to the chamber 20. A high pressure suction inlet port 48 and a discharge port 50 are connected to the high pressure suction chamber 22 and discharge chamber 24, respectively. Chamber 22 is separated from discharge chamber 24 by means of a partition 52.

The low pressure suction inlet chamber 20 is connected to the working chambers 16 within the housing 18 by means of a pair of arcuate slots 54 provided in the end wall 38. The high pressure suction inlet chamber 22 is connected to the working chambers 16 by

ports

56a and 56b in end wall 40. The discharge chamber 24 is connected to the working chambers 16 through arcuate slots 58 in end wall 40.

The

rotors

12 and 14 are mounted for rotation on shafts 55 which are supported in a parallel spaced relation by

end walls

38 and 40. Each rotor includes a number of helical lands 60 and grooves 62. The wrap angle of the land 60 on the male rotor 12 is approximately 270 and the wrap angle of the land 60 on its female rotor 14 is approximately 180. The rotors l2 and 14 are positioned within the working cylinders defined by the

end walls

38 and 40 and the parallel bores 34 and 36. The compressor can be driven in a conventional manner by an electric motor 64 connected to a shaft 57 on rotor 14.

COMPRESSOR CAPACITY CONTROL The capacity of the compressor 10 is varied by means of the slide valve 26 which is positioned in a bore 66 provided in the housing 18 in a parallel relation to the

bores

34 and 36. The slide valve 26 forms a movable wall for portions of the wall of the

bores

34 and 36 in the housing 18 and is axially movable in the bore 66 from the minimum capacity position shown in FIG. 5 'to a maximum capacity position shown in FIG. 6. The slide valve 26 can be moved to the maximum capacity position by means ofa hydraulic piston and cylinder assembly 70 connected to one end of the slide valve 26 by a shaft 67 and is normally biased to the minimum capacity position by means of a spring 68.

The point or timing of admission of low pressure suction fluid or vapor into the working chambers of the rotors l2 and 14 is determined by the position of the face or end 72 of the slide valve 26. In the minimum capacity position of the slide valve 26, the end 72 of the slide valve is spaced from end wall 38 connecting a portion of the

bores

34 and 36 for the

rotors

12 and 14 to the bore 66 for slide valve 26. The working chambers 16 of the rotors will not close until the point of contact of the helical land 60 of the rotor 12 seats in groove 62 of rotor 14 and passes the end 72 of the slide valve 26. The stroke of the compressor at the minimum capacity position will be equal to the distance of the end 72 of the slide valve 26 to the end wall 40 at the discharge end of the

rotors

12 and 14.

HIGH PRESSURE SUCTION FLUID ADMISSION The location of the point of admission or introduction of the high pressure suction fluid or vapor into the working chambers 16 between the rotors l2 and 14 is determined by opening or closing the

plug valve assemblies

28a and 28b provided in the high pressure suction chamber 22. In this regard, each of the plug valve assemblies 28 includes a valve member 74 which is positioned for movement into a closing position with respect to the ports 56 in end wall 40. The valve members 74 are actuated by means of double acting piston and

cylinder assemblies

76 and 78 provided on the end of the housing 18. In the minimum capacity position of the slide valve 26, (FIG. 5) the high pressure suction fluid must be admitted into the chambers 16 as they are closed by the end 72 of the slide valve 26. Since only a portion of the working chambers 16 formed between the

rotors

12 and 14 is used in the minimum capacity position of the slide valve 26, the outer ports 56a should be opened and the inner ports 56b closed. As the slide valve 26 is moved toward the maximum capacity position the full length of the working chambers 16 defined by the rotors l2 and 14 will be used and the ports 56a closed and the ports 56b opened to allow the high pressure suction fluid to enter the working chambers 16 at an earlier point in the rotory motion of the

rotors

12 and 14.

Means are provided for synchronizing the opening and closing of the

plug valve assemblies

28a and 28b with the position of the slide valve 26. Such means is in the form of an electro-hydraulic circuit 30 shown in FIGS. 5 and 6. In the minimum capacity position of the slide valve as seen in FIG. 5, the plug valve assemblies 280 should be in the open position and the plug valve assemblies 28b should be in the closed position.

In the maximum capacity position (FIG. 6) of the slide valve 26, the plug valve assemblies 28a should be closed and the plug valve assemblies 28b should be opened. This is accomplished by means of the solenoid actuated

valves

80 and 82. The solenoid valve 80 is connected to a source of hydraulic fluid 84 under pressure. The solenoid valve 82 is connected to a reservoir 86. In the minimum capacity position of the slide valve 26 shown in FIG. 5, fluid under pressure will flow through the solenoid valve 80 and

lines

88 and 90 to the cylinders 78 for the plug assemblies 280 to move the pistons 76 to the open position. Fluid under pressure will also flow through

line

92 and 94 to the cylinders 78 for the plug valve assemblies 28b to move the valve members 74 to the closed position. Fluid discharging from the cylinder 76 in the plug assemblies 28a will flow through the

lines

96 and 98 to the solenoid valve 82 for discharge to the reservoir 86. Fluid discharging from the cylinder 78 for the plug assemblies 28bwill discharge through

lines

100 and 98 to the solenoid valve 82 for discharge to the reservoir 86.

In the maximum capacity position of the slide valve 26 shown in FIG. 6, the

solenoid valves

80 and 82 are reversed to open plug valve assemblies 28b and close plug valve assemblies 28a. In this position, fluid under pressure will flow through solenoid valve 80 and lines 96 to cylinder 76 and

lines

98 and 100 to cylinders 78. Fluid discharging from cylinders 78 will flow through

lines

94 and 92 and from cylinders 76 through

lines

90 and 88 through solenoid valve 82 to reservoir 86.

The

solenoid valves

80 and 82 are controlled by means of the cam actuated switch assembly 32 which is connected to respond to the movement of the slide valve to open or close the electrical circuit to the

solenoid valves

80 and 82. The assembly 32 includes a switch 102 and a cam 104. In FIG. 5, the cam 104 is shown in the minimum capacity position with the switch 102 open. Rotation of the cam 104 to the maxi mum capacity position of the slide valve as seen in FIG. 6 will close switch 102 and energize the

solenoid valves

80 and 82.

CONTINUOUS HIGH PRESSURE SUCTION FLUID CONTROL In the alternate embodiment of the invention (FIGS. 7 through 14), a variable capacity compressor 106 is shown having a continuous high pressure suction fluid control. This is accomplished by utilizing a pair of control valve assemblies 108 mounted within

rotors

110 and 112 to control the admission of high pressure suction vapor or fluid to the working chambers 114 of the

rotors

110 and 112.

In this regard, the compressor 106 includes acompressor housing 116 having a pair of

cylindrical bores

118 and 120 closed at one end by means ofa low pressure suction fluid or vapor housing 122 and at the other end by means of a high pressure fluid or vapor discharge housing 124. A high pressure suction fluid or vapor housing 126 is mounted on the discharge housing 124.

Fluid or vapor is pumped from a low pressure suction chamber 130 in housing 122 and a high pressure suction fluid or vapor chamber 132 in housing 126 to a discharge chamber 134 in discharge housing 124 by the

rotors

110 and 112. The capacity of the compressor 106 is adjusted by means of a slide valve 136 which controls the low suction fluid pressure cut off as described above. High pressure suction fluid admission is controlled by means of the control valve assemblies 108. The timing of the operation of the control valve assemblies 108 is controlled by means of an electrohydraulic circuit 138 which synchronizes the position of the control valve assemblies 108 with the slide valve 136.

More particular, the low-pressure suction fluid housing 122 includes an inside wall 140 and an outside wall 142 spaced apart to define the chamber 130. The housing 122 is connected to the compressor housing 116 by a number of bolts 144. Fluid communication between the chamber 130 and the housing 116 is provided by means of a number of arcuate slots 146 provided in the end wall 140 as seen in FIG. 8. A pair of

rotor bearing openings

148 and 150 are provided in the inside end wall 140. Low pressure suction fluid is admitted to the chamber 130 through an inlet port 152.

The discharge housing 124 provides fluid communication between the discharge chamber 134 and the compressor housing 116 by means of a pair of arcuate recesses 154 provided in the end face 156 of the housing 124. Fluid is discharged from chamber 134 through port 164 in housing 116. The housing 124 is mounted on the compressor housing 116 by means of bolts 158.

A pair of

rotor bearing openings

160 and 162 are provided in the discharge housing 124 in axial alignment with the

rotor openings

148 and 150, respectively, in housing 122.

The high pressure suction fluid or vapor housing 126 includes a pair of threaded openings 166 which are axially aligned with

openings

160 and 162. An enlarged bore 170 is provided in each of the openings 160 on the inside surface of the housing 126. A pair of diametrically opposite slots 172 are provided in each of the bores 170. The bores 170 are intersected by the chamber 132. High pressure suction fluid or vapor is admitted to chamber 132 through an inlet port 175.

The rotors and 112 are mounted for rotation within the housing 116 and cooperate to provide working chambers 114 for compressing fluid admitted from the low pressure suction chamber and the high pressure suction chamber 132. Rotor 110 is journalled for rotation in bearings 174 provided in the opening 148 in the end wall and the opening in the discharge housing 124. The rotor 112 is journalled for rotation in bearings 176 provided in the opening 150 in the end wall 140 and the opening 162 in the discharge housing 124. The rotors 110 and 1112 can be driven by any suitable means such as a motor connected to a shaft 178 provided on the end of the rotor 112. Rotor 110 includes an axially extending opening 180 which is closed at one end by end wall 182 and is connected to the grooves in the rotor by radially extending ports 184. Rotor 112 is similar to rotor 110 and includes an axial bore 180 connected to the grooves of the rotor by ports 184. Axial movement of the

rotors

110 and 112 is prevented by twin ball thrust bearings 113. The

rotors

110 and 112 are sealed by spring biased seals 115.

LOW PRESSURE SUCTION FLUID CONTROL The point of admission of low pressure suction fluid or vapor into the working chambers 114 between the

rotors

110 and 112 is controlled by means of the slide valve 136 in the same manner as described in the embodiment shown in FIGS. 1 through 6. In this regard, the point of low pressure suction fluid cut off is determined by the position of the end 137 of the slide valve 136. In the minimum capacity position of the slide valve 136, the end 137 will be spaced from the end wall 140 connecting a portion of the

bores

118 and 120 to the bore 134 for the slide valve 136. The working chambers 114 will not close until the point of contact of the lands and grooves of the

rotors

110 and 112 pass the end 137 of the slide valve 136. The stroke of the compressor at the minimum capacity position will be equal to the distance of the end 137 of the slide valve 136 to the face 156 of the discharge housing 124. The slide valve 136 is moved in substantially the same manner as shown in FIGS. 1 and 2.

HIGH PRESSURE SUCTION FLUID CONTROL ASSEMBLIES High pressure suction fluid is admitted to the working chambers 114 through the blind bore 180 and ports 184 provided in the

rotors

110 and 112. The timing of admission of the high pressure suction fluid or vapor from the high pressure chamber 132 is controlled by means of the control assemblies 108 provided in the bores 180 in

rotors

110 and 112. In this regard, each of the control assemblies 108 includes a hollow cylinder 186 as seen in FIG. 11. The cylinder 186 is pro vided with a central bore 190, an axially extending arcuate alot 188 and an opening 192 in end wall 194. A pair of axially extending guide bars or rods 196 are provided at one end of the cylinder 186. Fluid communication is provided from the bore 190 through the wall of the cylinder 186 by means of the arcuate slot 188. The cylinder 186 is positioned for axial movement within the bore 180 of the rotor 110 with the slot 188 aligned with the radial ports 184 in the rotor 110. The cylinder 186 is prevented from rotating in the bore 180 by means of the rods 196 which are aligned with the slots 172 in bore 170.

The cylinder 186 is biased to a minimum capacity position by means of a spring 198 provided within the bore 190 of the cylinder 186 and bearing against the housing 126. In the maximum capacity position shown in FIG. 13, the end l88b of the slot 188 will be aligned in the radial plane of ports 184. In the minimum capacity position shown in FIG. 12, the end 1880 will be moved beyond the radial plane of the ports 184 to cut off the high pressure suction fluid.

Means are provided for moving the cylinder 186 axially in the bore 180 of the rotor 110. Such means is in the form of a hollow tube 200 which extends through the bore 190 and the opening 192 in the end wall 194 of the cylinder 186. The tube 200 is sealed in the opening 192 by means of an O-ring seal 202. The tube 200 is retained in a fixed position in the cylinder 186 by means of a pressure seal 204 provided in the opening 166 in the housing 126 and a lock nut or cap 206 which is threadedly received in the threaded opening 166. Hydraulic fluid is admitted to the tube 200 through an inlet port 199 and discharged through a port 201. I-Iydraulic fluid under pressure is admitted to the bore 180 through tube 200 into the space between end wall 194 and the bore 180. When the pressure of the fluid builds up sufficiently to overcome the force of the spring 198, the cylinder will move axially in the bore 180.

The cylinder 186 is lubricated for free movement in the bore 180 by means of a passage 208 provided in the side wall of the cylinder 186 and a number of ports 210. As hydraulic fluid is admitted into the bore 180 it will flow through the passage 208 and the ports 210 into the space between the cylinder 186 and the walls of the bore 180.

ELECTRO-HYDRAULIC CONTROL Means are provided for synchronizing the position of the cylinder 186 with the position of the slide valve 136. Referring to FIG. 14, a schematic electrohydraulic diagram is shown for synchronizing the movements of the cylinders 186 with the position of the slide valve 136. The electrohydraulic circuit includes an electrical circuit 212 which is used to control

solenoid valves

214 and 216 in the hydraulic circuit 138.

More particularly, the electric circuit 212 includes a first potentiometer 220 which is connected to the slide valve 136 by means of a rod 221 and is used to convert the mechanical movement of the slide valve 136 to an electrical signal. The electric signal is transferred through

electric lines

222, 224 and 226 to a pair of bridge balancing relays 228 and 230 through

lines

223, 225 and 227. The bridge balancing relays 228 and 230 are connected to a pair of

potentiometers

232 and 234, respectively. The

potentiometers

232 and 234 are connected to the cylinders 186 by rods 235 and convert the mechanical position of the cylinders 186 to an electrical signal which is transmitted to the respective

bridge balancing relays

228 and 230.

Each of the bridge balancing relays 232 and 234 is connected to an oil supply solenoid valve 214 and to an oil drain solenoid valve 216. Each of the oil supply solenoid valves 214 is connected to the inlet line 199 to the tubes 200. Each of the oil drain solenoid valves 216 is connected to the outlet port 201 to the tubes 200. The oil supply valves 214 are used to control the flow of hydraulic fluid from a source 236 of hydraulic fluid under pressure such as a lube oil pump. Each of the oil drain solenoid valves 216 is used to control the discharge of hydraulic fluid from the tube 200 to the suction chamber 130.

In operation, both of the

valves

214 and 216 are normally closed when the slide valve 136 is in the minimum capacity position. When the slide valve 136 is moved from the minimum capacity postion to an intermediate position or to the maximum capacity position, the potentiometer 220 will move a corresponding amount indicating the mechanical movement of the slide valve and providing an electrical signal to each of the bridge balancing relays 228 and 230. The initial change in the electrical signal will unbalance the

relays

228 and 230 opening the oil supply solenoid valves 214 and allowing hydraulic fluid to flow into the tubes 200 to move the cylinders 186 toward the maximum capacity position. The movement of the cylinders 186 will be sensed by the

potentiometers

232 and 234 providing a signal of the postion of the cylinder 186 to the bridge balancing relays 228 and 230. When the potentiometer 220 for the slide valve comes to a stop, the

potentiometers

232 and 234 will continue to move until the signal from the

potentiometers

232 and 234 is balanced with the signal from the potentiometer 220 in the

relays

228 and 230. When the signals are balanced, the solenoid valve 214 will close.

When the slide valve 136 is moved in the opposite direction, the movement of the potentiometer 220 will again produce a signal to the bridge balancing relays 232 and 234 which will cause the valves 216 to open allowing fluid to flow from the tube 200 to the low pressure chamber through lines 238 or to a reservoir. The cylinders 186 will move toward the minimum capacity position due to the bias of the spring 198 moving the

potentiometers

232 and 234 until the signal is balanced with the signal of the potentiometer 220.

We claim:

1. A variable capacity rotary screw compressor comprising: a housing having a pair of parallel cylindrical bores, a pair of oppositely rotating constant mesh rotors positioned in said bores and defining working chambers, a low pressure fluid chamber in fluid communication with said working chambers at one end of said rotors, a discharge chamber in fluid communication with said working chambers at the other end of said rotors, a blind bore in each of said rotors, a plurality of radial ports in said rotors, said ports lying in a common plane, a high pressure fluid chamber connected to each of said blind bores, cylinder means in each of said bores for selectively advancing or retracting the point of admission of high pressure fluid to said ports in said rotors, a slide valve mounted in said housing for controlling the low pressure fluid cut off between minimum and maximum capacity positions with respect to said rotors, and electrohydraulic means connected to respond to the position of the slide valve for controlling said high pressure fluid control means.

2. A variable capacity rotary screw compressor comprising:

a housing having a low pressure chamber and a high pressure chamber,

means for separating said high pressure chamber into a high pressure fluid inlet chamber and a high pressure discharge chamber,

a low pressure fluid inlet port connected to said low pressure chamber,

a high pressure fluid inlet port connected to said high pressure chamber and a fluid discharge port connected to said high pressure discharge chamber,

a pair of oppositely rotating constant mesh rotors having helical lands and intervening grooves defining working chambers, said rotors being mounted within said housing to provide pumping and compressing action and being connected at the inlet end to said low pressure chamber and at the outlet end to said discharge chamber,

means at the outlet end for connecting the high pressure chamber to the working chambers defined by said rotors,

a slide valve mounted within said housing for movement between minimum and maximum capacity positions with respect to said rotors,

and electrohydraulic means connected to respond to the position of said slide valve and being operatively connected to said high pressure fluid connecting means to change the angular point of admission of high pressure fluid to the working chambers in relation to the minimum and maximum capacity positions of said slide valve.

3. The compressor according to claim 2 wherein said connecting means includes a pair of angularly offset ports for each of said rotors connecting the high pressure chamber to said rotors and a valve member corresponding to each of said ports mounted for movement between open and closed positions with respect to the corresponding port and including means for connecting said valve members to said electrohydraulic means whereby one of said ports in each of said pairs of ports is opened when the slide valve is in the minimum capacity position and the other of each of said pairs of bores is open when the slide valve is in the maximum capacity position.

4. A variable capacity multiple inlet rotary screw compressor comprising:

a housing having a low pressure suction inlet port, a high pressure suction inlet port and a discharge port,

a pair of oppositely rotating constant mesh rotors defining working chambers within said housing, said rotors being positioned in said housing to provide pumping and compressing action and being connected to draw fluid from the low pressure inlet port and said high pressure inlet port and to discharge the compressed fluid to the discharge port,

means mounted within said housing for controlling the low pressure suction inlet fluid cut off for said rotors,

means for varying the point of admission of the high pressure inlet fluid from said high pressure inlet port,

and electro-hydraulic means connected to respond to the position of the cut off means for controlling the high pressure varying means whereby said high pressure suction fluid is admitted to the working chambers simultaneously with or just after low pressure suction fluid cut off.

5. The compressor according to claim 4 wherein said rotors have helical lands and intervening grooves having wrap angles of less than 360.

6. The compressor according to claim 4 wherein said high pressure varying means comprises a number of plug valve assemblies operatively connected to said electro-hydraulic means for connecting the high pressure suction inlet port to said working chambers.

7. The compressor according to claim 6 wherein said plug valve assemblies include a number of ports angularly displaced in a plane radially outwardly from the axis of rotation of said rotors and a corresponding num ber of valve members positioned for movement into engagement with said ports for selectively connecting said ports to the working chambers defined by said rotors.

8. The compressor according to claim 4 wherein each of said rotors includes a bore and a number of radially extending ports connecting said bore to said working chambers, and said varying means comprises a control valve assembly mounted in said bore in each of said rotors to control the point of communication of said bore with the working chambers.

9. The compressor according to claim 8 wherein each of said control valve assemblies includes a hollow cylinder positioned in said bore in each of said rotors, said cylinder including an arcuate slot positioned to intersect the radial plane of said ports and including means for moving said cylinder axially in the bore of said rotors to angularly displace the point of intersection of said ports with said arcuate slot.

10. A variable capacity rotary screw compressor comprising: a housing having a low pressure suction fluid chamber, a high pressure suction fluid chamber and a fluid discharge chamber, a low pressure fluid inlet port connected to said low pressure chamber, a high pressure fluid inlet port connected to said high pressure chamber and a discharge port connected to said discharge chamber, a pair of oppositely rotating constant mesh rotors having helical lands and interven ing grooves defining working chambers, said rotors being mounted within said housing to provide pumping and compressing action, said working chambers being connected at one end to draw fluid from the low pressure chamber and at the other end to discharge compressed fluid to the discharge chamber, means for selectively providing fluid communication between said high pressure fluid chamber and said working chambers, a slide valve movably mounted within said housing and including cut off means for controlling the ca pacity of said compressor, and means connected to said slide valve and to the high pressure fluid communicating means for controlling the point of admission of high pressure fluid from said high pressure chamber to said working chamber in relation to the position of said slide valve.

11. The compressor according to claim 10 wherein said controlling means comprises a control valve assembly mounted in each of said rotors.

12. The compressor according to claim 11 wherein said rotors each includes a bore and a number of radially extending ports connecting said bores to said intervening grooves and said control valve assembly includes a hollow cylinder positioned in said bore in each of said rotors, said cylinder including an arcuate slot positioned to intersect the radial plane of said radial ports in each of said rotors and further including means for moving said cylinder axially in said bore of said rotors to angularly displace the point of intersection of said ports with said arcuate slot.

13. The compressor according to claim wherein said high pressure fluid communication means comprises a number of ports in said rotor connecting said high pressure chamber to one end of said working chambers and said controlling means includes a number of valve members corresponding to said ports and being mounted for movement between open and closed positions with respect to said ports.

14. The compressor according to claim 13 wherein said controlling means includes a first variable resistor connected to respond to the movements of the slide valve, a pair of bridge balancing relays connected to said first variable resistor to sense the change in the position of said first variable resistor, a second and a third variable resistor connected to each of said bridge balancing ralays, means for connecting said second and third variable resistors to respond to movements of said selectively providing means and hydraulic circuit means connected to said bridge balancing relays to control said selectively providing means to vary the point of fluid communication between said high pressure fluid chamber and said working chambers.

Claims

2. A variable capacity rotary screw compressor comprising: a housing having a low pressure chamber and a high pressure chamber, means for separating said high pressure chamber into a high pressure fluid inlet chamber and a high pressure discharge chamber, a low pressure fluid inlet port connected to said low pressure chamber, a high pressure fluid inlet port connected to said high pressure chamber and a fluid discharge port connected to said high pressure discharge chamber, a pair of oppositely rotating constant mesh rotors having helical lands and intervening grooves defining working chambers, said rotors being mounted within said housing to provide pumping and compressing action and being connected at the inlet end to said low pressure chamber and at the outlet end to said discharge chamber, means at the outlet end for connecting the high pressure chamber to the working chambers defined by said rotors, a slide valve mounted within said housing for movement between minimum and maximum capacity positions with respect to said rotors, and electrohydraulic means connected to respond to the position of said slide valve and being operatively connected to said high pressure fluid connecting means to change the angular point of admission of high pressure fluid to the working chambers in relation to the minimum and maximum capacity positions of said slide valve.

4. A variable capacity multiple inlet rotary screw compressor comprising: a housing having a low pressure suction inlet port, a high pressure suction inlet port and a discharge port, a pair of oppositely rotating constant mesh rotors defining working chambers within said housing, said rotors being positioned in said housing to provide pumping and compressing action and being connected to draw fluid from the low pressure inlet port and said high pressure inlet port and to discharge the compressed fluid to the discharge port, means mounted within said housing for controlling the low pressure suction inlet fluid cut off for said rotors, means for varying the point of admission of the high pressure inlet fluid from said high pressure inlet port, and electro-hydraulic means connected to respond to the position of the cut off means for controlling the high pressure varying means whereby said high pressure suction fluid is admitted to the working chambers simultaneously with or just after low pressure suction fluid cut off.

5. The compressor according to claim 4 wherein said rotors have helical lands and intervening grooves having wrap angles of less than 360*.

7. The compressor according to claim 6 wherein said plug valve assemblies include a number of ports angularly displaced in a plane radially outwardly from the axis of rotation of said rotors and a corresponding number of valve members positioned for movement iNto engagement with said ports for selectively connecting said ports to the working chambers defined by said rotors.

10. A variable capacity rotary screw compressor comprising: a housing having a low pressure suction fluid chamber, a high pressure suction fluid chamber and a fluid discharge chamber, a low pressure fluid inlet port connected to said low pressure chamber, a high pressure fluid inlet port connected to said high pressure chamber and a discharge port connected to said discharge chamber, a pair of oppositely rotating constant mesh rotors having helical lands and intervening grooves defining working chambers, said rotors being mounted within said housing to provide pumping and compressing action, said working chambers being connected at one end to draw fluid from the low pressure chamber and at the other end to discharge compressed fluid to the discharge chamber, means for selectively providing fluid communication between said high pressure fluid chamber and said working chambers, a slide valve movably mounted within said housing and including cut off means for controlling the capacity of said compressor, and means connected to said slide valve and to the high pressure fluid communicating means for controlling the point of admission of high pressure fluid from said high pressure chamber to said working chamber in relation to the position of said slide valve.

13. The compressor according to claim 10 wherein said high pressure fluid communication means comprises a number of ports in said rotor connecting said high pressure chamber to one end of said working chambers and said controlling means includes a number of valve members corresponding to said ports and being mounted for movement between open and closed positions with respect to said ports.