Abstract:
A system and method are provided for the automatic regulation of a balancing pressure to be applied to a rotor of a rotary screw compressor. The system includes a microprocessor which computes a balancing pressure to be applied to the rotor in response to an input of various compressor operating parameters such as suction pressure, discharge pressure, and percent capcity.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to compressors and, more particularly, to a system and method for the automatic regulation of a balancing pressure to be applied to a rotor in a rotary screw compressor. 
     BACKGROUND OF THE INVENTION 
     Rotary screw compressors of the type wherein a gas is introduced through an intake port into compressor rotors, and discharged through a discharge port in a compressed state are well known in the art. In these compressors, an inherent pressure differential exists between the intake or suction side and the outlet or discharge side of the compressor which tends to force the rotors to move toward the intake side In order to balance the rotors, it is known to utilize an oil pump or discharge pressure to provide hydraulic pressure acting on pistons located on the intake or suction side of the rotors which bias the rotors against the pressure differential. A problem with such compressors is that the balancing pressure on the pistons is not responsive to the various operating parameters other than outlet pressure of the rotary screw compressor 
     There is a need for a rotary screw compressor wherein the balancing pressure applied to the pistons is responsive to the various operating parameters thereof. This invention is directed to satisfying this need 
     SUMMARY OF THE INVENTION 
     An object, therefore, of the invention is to provide a new and improved rotary screw compressor of the character described. 
     In the exemplary embodiment of the invention, a rotary screw compressor is provided with a casing having intersecting cylindrical cavities within which meshing male and female rotors are located on parallel axes. The compressor includes an inlet at a low pressure end and an outlet at a high pressure end. Further, the compressor includes means for sensing the pressure of a gas flowing through the inlet, means for sensing the pressure of the gas flowing through the outlet, and microprocessor control means for computing a balancing pressure, in response to inputs or sensed parameters of the pressure of the gas sensed at the inlet and the outlet, which is applied to the male rotor to adjust the relative longitudinal position thereof. 
     According to the invention, the male and female rotors include shafts extending axially into first and second cylindrical bores in an inlet housing at the low pressure end of the casing and in first and second cylindrical bores in an outlet housing at the high pressure end of the casing. A piston is located within a chamber defined by a portion of the first cylindrical bore and attached to an end of the male rotor shaft such that the balancing pressure counters the shaft forces The piston and shaft may move axially within the first cylindrical bore to apply the balancing pressure. 
     Further, the compressor includes an oil pump associated with the compressor including conduit means for supplying oil at balancing pressure to the chamber for moving the piston and shaft axially and adjusting the relative longitudinal position of the male rotor. 
     Still further, a modulating valve is provided between the oil pump and the chamber for regulating the flow of oil from the pump to the chamber. The valve is responsive to the balancing pressure signal computed by the microprocessor means. 
     Additionally, the compressor has means for controlling the capacity of the compressor. 
     Means are provided for sensing the position or flow of the capacity control means and feeding the same to the microprocessor control means for computing the balancing pressure to be applied to the male rotor. 
     Still further, the compressor is provided with a check valve, and means, flowing through the check valve, for applying pressure to the male rotor at outlet pressure rather than balancing pressure. 
     Other objects and features of the invention will be apparent from the following detailed description taken in connection with the accompanying drawings 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of this invention which are believed to be novel are set forth with particularlity in the appended claims. The invention, together with its objects and the advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying drawings, and which like reference numerals identify like element in the figures and which: 
     FIG. 1 is an exploded perspective view of the components of a typical rotary screw compressor; 
     FIG. 2 is a horizontal section of a typical rotary screw compressor; 
     FIG. 3 is a perspective view of the capacity control means, with a section through the casing; 
     FIGS. 4A-4D illustrate, in a schematic fashion, the compression process in a rotary screw compressor; 
     FIG. 5 is a schematic of a system according to the invention for the automatic regulation of a balancing pressure to be applied to a rotor in the compressor; and 
     FIGS. 6 and 7 illustrate, in a schematic fashion, the axial forces on a rotary screw compressor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings in greater detail, and first to FIG. 1, a typical rotary screw compressor, generally designated 10, is illustrated. The term &#34;compressor&#34; is used herein and in the claims hereof generically to include a prime mover that operates in a reverse mode such as a turbine, expander or the like. 
     As shown therein, rotary screw compressor 10 includes a casing 12 having intersecting cylindrical cavities 14 and 16 within which meshing male and female rotors 18 and 20, respectively, are mounted on parallel axes. The male rotor 18 and female rotor 20 include shafts 22 and 24, respectively. Shaft 22 of male rotor 18 is connected at one end 26 to a drive shaft (not shown) which drives shaft 22 and, therefore, male rotor 18 to rotate in the direction of arrow A, which in turn drives rotor 20 in the direction of arrow B. 
     The compressor 10 further includes an inlet housing 28 at a low pressure end of casing 12, and an outlet housing 30 at a high pressure end of casing 12. Casing 12 includes an opening 32 through which a gas enters and travels in the direction of arrows C into the rotors 18, 20. An outlet or discharge port 34 is located within outlet housing 30 for the flow of compressed gas in the direction of arrows D. 
     As shown in FIG. 2, shaft 22 of male rotor 18 is located within a cylindrical bore 36 in inlet housing 28. Further, a piston 38 is attached to the end of shaft 22 of male rotor 18, and is located within a cylindrical portion 40 of bore 36 which defines a chamber 42. In a like manner, shaft 24 of female rotor 20 is located within a cylindrical bore 44 of inlet housing 28 which includes a portion 46 defining a chamber 48. As shown therein, shaft 24 is connected at one end to a shaft 50 of an oil pump 52. 
     As shown in FIG. 3, the casing 12 of rotary screw compressor 10 further includes an axially extending recess 54 in communication with cylindrical cavities 14, 16. A slide valve 56 is mounted on a shaft 58 for axial movement within recess 54 in the direction of arrows E. As shown therein, slide valve 56 has interfaces 60 in sealing engagement with rotors 18, 20, a front face (not shown) adjacent to outlet or discharge port 34 and a rear face 61. 
     As further shown in FIG. 3, a slide stop 62 is mounted on a shaft 66 for axial movement within recess 54 in the direction of arrows F. Slide stop 62 has interfaces 64 in sealing engagement with rotors 18, 20, and a front face (not shown) adapted to engage rear face 61 of slide valve 56. Slide stop 62 is movable independently of slide valve 56 to provide an opening of selected size between rear face 61 of slide valve 56 and front face (not shown) of slide stop 62. As indicated by arrows G, the opening allows some of the gas to return to the suction side of cylindrical cavities 14, 16 before compression. 
     In a typical operation of the rotary screw compressor illustrated in FIGS. 1-3, a gas 63 at an inlet or suction pressure P s  is drawn axially into opening 32 of casing 12 (FIG. 1). Thereafter, as shown in FIG. 4A, gas 63 is drawn into rotors 18, 20. Male rotor 18 includes lobes 70 which function as pistons that roll in cylinders 72 of female rotor 20. Therefore, as rotors 18 and 20 turn, gas 63 is trapped within casing 12 and rotor cylindrical cavities 14, 16, as shown in FIG. 4B. Thereafter, as shown in FIG. 4C, lobes 70 of male rotor 18 reduce the volume in cylinders 72 of female rotor 20 to compress trapped gas 63. The process is completed as shown in FIG. 4D compressed gas 63, at an outlet or discharge pressure P D , is discharged through outlet 34 (FIG. 1). 
     Due to the inherent pressure differential (ΔP=P D  -P S ) which naturally exists between the inlet or suction side and the outlet or discharge side, rotors 18, 20 normally are forced axially toward the left in FIG. 2. In order to compensate for the leftward movement of the rotors 18, 20, oil pump 52 is used to supply oil at a balancing pressure P B  to chamber 42 via inlet 74. The oil at a balancing pressure P B  acts against a face 76 of piston 38 to force the piston 38 to move male rotor 18 to the right in FIG. 2. Although not shown in FIG. 2, it is understood that a piston could be located within chamber 48 to moVe female rotor 20 to the right in response to the force of oil at a balancing pressure within chamber 48. As shown schematically in FIG. 5, a system, generally designated 74, according to the present invention is provided for automatically regulating the balancing pressure of oil to be supplied to chamber 42 to move piston 38 and, correspondingly, male rotor 18 to the right in FIG. 5. 
     As shown therein, various operating parameters are sensed and fed to a microprocessor 76 which compares the operating parameters, through an algorithm to be described later, and modulates the balancing pressure P B  of oil to be supplied to chamber 42. The balancing pressure of oil flowing from pump 52 to chamber 42 via conduits 78, 80 is controlled by a modulating or variable throttle valve 82 which opens or closes in response to instructions from processor 76. In the situation wherein female rotor 20 includes a piston 84 within chamber 48, oil at balancing pressure P B  may be supplied to chamber 48 via conduit 86 connected to conduit 80. 
     As shown in FIG. 5, microprocessor 76 is provided with a parameter input of inlet or suction pressure P S  and outlet or discharge pressure P D  to generate a pressure differential (ΔP=P D  -P S ), and an output of balancing pressure P B . 
     Additionally, and only wherein the compressor includes a capacity control such as slide valve 56 and/or slide stop 62, microprocessor 76 may be provided with an operating input parameter of pump capacity ratio %C which is generated by sensing the axial positions of slide valve 56 and slide stop 62. 
     Of course, it is understood that the subject invention is not limited to the parameter inputs of P S , P D , and %C, but, rather, is applicable to any other operating input parameters which may have an affect on the balancing pressure of oil to be supplied to chamber 42. 
     System 74 further includes a check valve 88 and a safety oil circuit 90, 92 for supplying oil to chamber 42, and at outlet or discharge pressure P D  in the event that gear pump 52 fails. 
     A generalized control algorithm for system 74 now will be described with reference to FIG. 6. From the forces shown in FIG. 6, it follows that: 
     
         F.sub.B =F.sub.C +F.sub.G 
    
     Since F B  =P B  A B , it follows that: ##EQU1## Where: 
     F G  =Gas/oil pressure forces against end of piston 
     F B  =Force to balance rotor loads axially 
     F C  =Compression forces axially f (D 1   2 , P S , P D  -P S ), V i , %C, k) 
     P S  =Suction pressure 
     P D  =Discharge pressure 
     P B  =Balancing pressure 
     %C=Percent capacity or valve position 
     V i  =Volume ratio 
     k=Specific heat ratio of gas compressed 
     D 1  =Diameter of rotor 
     A more detailed control algorithm for system 74 now will be described with respect to FIG. 7. If the rotor shown therein is balanced axially, 
     
         F.sub.B =F.sub.2 -F.sub.3 +F.sub.C +F.sub.4 +F.sub.5 +F.sub.6 
    
     Since 
     F B  =P B  A B , 
     F 2  =P 2  A 2 , 
     F 3  =P 3  A 3 , 
     F 4  =P 4  A 4 , and 
     F 5  =P 5  A 5 , it follows that: 
     F 6  =F A  A 6   
     
         P.sub.B A.sub.B =P.sub.2 A.sub.2 -P.sub.3 A.sub.3 +F.sub.C +P.sub.4 A.sub.4 +P.sub.5 A.sub.5 +P.sub.A A.sub.6 
    
     Furthermore, ##EQU2## Therefore, ##EQU3## Where: 
     P B  =Oil pressure in balancing chamber 
     P S  =Suction pressure 
     P D  =Discharge pressure 
     P 2 , P 3 , P 4 , and P 5  are functions of P B , P S , and/or P D   
     F C  =Force of compression gases 
     Specifically, F C  =f [D 1   2 ,P s ,(P D  -P S ),V i , %C,k] 
     The present invention thus provides a system for the microprocessor control of the balancing pressure of oil to be applied to a rotor which is superior to the prior art. The present invention also provides for an operating parameter input of pump capacity ratio %C in a system for controlling the balancing pressure. 
     It will be understood that the invention may be embodied in other specific forms without departing from the spirit or central characteristics thereof. The present examples and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the invention is not to be limited to the details given herein.