Variable capacity screw compressor
1451 Apr. 15, 1975 [54] VARIABLE CAPACITY SCREW COMPRESSOR Anders Lundberg, Norrkoping, Sweden [75] Inventor:  
 [73] Assignee: Stal-Refrigeration AB, Norrkoping,  
 Sweden [22] Filed: Aug. 15, 1973 [21] Appl. No.: 388,600  
 [30] Foreign Application Priority Data Aug. 28, 1972 Sweden 11092/72 [52] US. Cl 417/440; 418/201 [51] Int. Cl F01c 1/16; F040 17/12; F04b 23/00 [58] Field of Search 418/159, 197, 201-203;  
 [56] References Cited UNITED STATES PATENTS 3,088,658 5/1963 Wagenius 418/201 3,088,659 5/1963 Nilsson et a1. 418/201 FOREIGN PATENTS OR APPLICATIONS 218,309 11/1961 Austria 418/159 Primary Examiner-John J. Vrablik Attorney, Agent, or Firm-Eric Y. Munson [57] ABSTRACT A variable capacity screw compressor comprising a casing, housing two intersecting bores defining a working chamber in which a pair of intermeshing screws comprising helical lands and intervening grooves are mounted to rotate and to compress a working fluid as it is being passed axially through the working chamber from a low pressure inlet wall to a high pressure outlet wall, the inlet wall having an inlet passage in substantially axial alignment with the axes of the rotary screws for filling the ends of the grooves facing the axial inlet passage with working fluid. The capacity of the compressor to operate within a predetermined range of load variations from full load to no load conditions is regulated by a slide valve located on the high pressure side of the compressor and being axially displaceable in response to varying load conditions to expose a by-pass opening for venting partially compressed working fluid directly into the grooves of the compressor screws at a location between the bypass opening and the axially aligned inlet passage to thereby interrupt admission of cold working fluid into the grooves from the inlet passage.  
 4 Claims, 6 Drawing Figures VARIABLE CAPACITY SCREW COMPRESSOR BACKGROUND OF THE INVENTION In modern screw compressors of known design, capacity control is achieved by tapping off or bypassing a certain amount of the partially compressed gaseous working medium from the rotor operating chamber of the compressor and returning the unloaded or bypassed gas to the inlet end for admixture with the intake gas, which is introduced through the inlet gate located axially at the inlet plane of the rotors. To accomplish this control, an axially movable slide valve, which forms part of the casing surface of the rotor housing, is located below and between the rotors, serving as adjustable exit for the tapped-off gas, which is led outside the rotor chamber casing, through channels provided in the compressor housing, and back to the inlet end piece.  
  In some compressor designs, the return channels also serve as draining passages for oil from the bearing on the outlet side and from any shaft seals, carrying the oil to the inlet gate, where it is drained into the operating chamber of the compressor, pumped out on the high pressure side, and separated from the gaseous working medium. It may well happen in conventional compressors of this type that, during unloading, the tapped-off gas, which is heated by the oil sprayed into the working chamber and by the heat of compression work which may be required to overcome the pressure drop in the gas being tapped off, squirts out and up into the inlet chamber together with possibly hot draining oil and thus heats up the intake or low pressure gas. Especially in low temperature installations, this occurrence works against the low temperature requirement of the intake gas and results in considerable heat losses with a consequent loss in efficiency.  
 SUMMARY OF THE INVENTION The present invention overcomes the disadvantages of the prior art described above by, reducing the efficiency loss resulting from the heat loss when the compressor is under partial load while permitting a simplified construction of the rotor housing and inlet end piece.  
  The invention is based on the concept that a portion of partially compressed working medium is by-passed to the inlet side which implies that the slide valve must be arranged on the high pressure side. Furthermore, the by-passed medium must not be returned to the inlet chamber but to an opening in the cylindrical casing surrounding the working chamber in order to produce the intended effect and the inlet passage must be located in the end wall of the casing so as to extend substantially in axial alignment with the rotors.  
  More specifically, the present invention provides at least one opening in the rotor chamber casing surface of the compressor housing, through which tapped-off gas is reintroduced into the rotor chamber, instead of being brought back to the inlet gate, as in the prior art. This opening is located approximately along a helical line on the casing surface which corresponds to the path of the helical ribs of one of the rotors and which passes through the rear edge of the inlet gate. Therefore, the opening will be in communication with the particular helically shaped rotor space which at that specific moment is being closed off by the rear edge of the inlet gate after filling from the inlet gate has been completed.  
  Thus, the tapped-off gas from the unloading valve will be supplied through the casing opening to the various helical rotor spaces during the final stage of their intake or suction process, so that the mixing of cold intake gas with hot exhaust gas in the inlet end piece is avoided, and the resultant disadvantages eliminated.  
  This invention will be described in full detail with reference to the accompanying drawings, which illustrate a preferred embodiment of the novel concepts contained herein.  
 DRAWINGS FIG. 1 is a partial longitudinal sectional view of a compressor constructed according to this invention;  
  FIG. 2 is a transverse sectional view, showing the inlet end piece of the compressor as seen from the inside;  
  FIG. 3 is a transverse section view, showing the outlet end piece of the compressor as seen from the inside;  
  FIG. 4 is a cross-sectional view through the compressor of this invention; and  
  FIG. 5 is a developed plan view of the inside surface of the female rotor chamber casing of the compressor.  
  FIG. 6 is a developed plan view similar to FIG. 5 showing both rotor chamber casings developed from 7a to 7b of FIG. 5.  
 DESCRIPTION FIGS. 1-4 show the screw compressor of this invention with inlet end wall 1, rotor housing 2, outlet housing wall 3, and helical rotors 4a and 4b mounted within rotor casing portion 4 of rotor housing 2. Gaseous working medium unloading means is here illustrated as axially movable slide valve 5. The gas is drawn into the compressor and the helical grooves therein through intake connection 6 by means of inlet gate 7 which is located in the inlet end plane of rotors 4a and 4b and in axial alignment with the axes thereof whereby the gas will pass into the grooves in an axial direction from the end facing the inlet gate 7. At full capacity, slide valve 5 rests against stop 8; during partial load conditions, slide valve 5 is displaced axially from stop 8 and a bypass opening 9 is created to permit exhaust gas to flow out into rotor housing 2. The tapped-off gas proceeds through conventional closed channels in housing 2 to opening 10 located in the wall of casing 4 of the rotor chamber and is drawn into the thread volume of rotors 4a and 4b.  
  The shape of inlet gate 7 may be clearly seen in FIG. 2, which shows the inlet end wall 1 as seen from inside rotor housing 2. Similarly, FIG. 3 shows outlet gate 12 in outlet end wall 3, as seen from inside rotor housing 2. Outlet gate 12 is provided in its lower portion with a recess complementary to and adapted to receive slide valve 5. The positions of male helical rotor 4a, female helical rotor 4b and slide valve 5 relative to each other can readily be observed in FIG. 4, as well as outwardly extending ribs or lands 13 of female rotor 4b, which define intervening spaces or grooves 15.  
  The location of exhaust opening 10 is governed by the principle illustrated in FIG. 5, which is a developed plan view of the inside surface of rotor chamber 4 of housing 2. Unbroken double lines 13 indicate the location of the ribs or lands of female rotor 4b as they extend across the surface of casing 4 in the direction of the arrow. Broken line 14 indicates the line on the surface of casing 4 where the rotors diverge (see FIG. 4), i.e., the upper middle edge of rotor housing 4 and where spaces or grooves 15 between rotor lands 13 of rotor 4b are blocked off by the projecting lands of male rotor 4a. As rotor 4b is turned in the direction of the arrow, the end of grooves 15 are exposed and extended sequentially so that gas is drawn in from inlet gate 7 and passed into the ends of the grooves in an axial direction. This intake continues until the particular land 13 defining the rear flank of a specific groove 15 reaches the rear edge 7a of inlet gate 7 whereupon the groove 15 in question is closed off. It is clear that the length of the rotors and the pitch of their helical configuration are chosen so that the closing position for any land 13, indicated by broken line 11 (see also FIG. 1) extends from rear edge 7a of inlet gate 7 to the outlet end of line 14.  
  It is also clear that exhaust or recirculation opening 10 should be located along line 11 so that opening 10 is closed off from any given groove 15 no later than simultaneously with the closing off of this same groove 15 by rear edge 7a from inlet gate 7. In this way the tapped-off exhaust gas will be supplied to spaces 15 during the final stage of the intake process for each space 15. Therefore, depending on the degree to which slide valve is opened in response to varying load conditions, more or less of the tapped-off exhaust gas will be drawn directly into grooves 15 through casing opening block off gas from intake gate 7, thus controlling the capacity of the compressor.  
  With further reference to FIG. 5, it will be apparent that at A the groove is so short that its crosssectional area will be substantially reduced. However, at B the groove has acquired its maximum crosssectional area as its length increases progressively from B,C,D and E. By reason of the axial inlet gate 7, the  
 grooves are filled with working medium from the end facing the inlet and passed axially through the working space until one of the corresponding lands a, b, c, d or e is about to pass the upper edge of the exhaust port 10. At the moment of passage of the land, the corresponding groove becomes aligned with the by-pass opening 9 exposed by the axial displacement of slide valve 5, allowing partially compressed working fluid to enter the inlet end of the groove and thus preventing further working fluid from being drawn into the groove from the axial inlet gate. In other words, the partially compressed working fluid entering the groove acts as a stopper or plug. Obviously some mixing will take place between the partially compressed by-passed working fluid and the working fluid ahead of the by-pass, as well as with the non-compressed medium entering through the gate 7. However, the effect of such mixing may be disregarded as the consequence thereof is insignificant in comparison with the effect that would result from by-passed fluid being blown back through gate 7 and inlet 6. It should be understood that if the by-passed working fluid should be blown back through the port 10, it would have very little time to expand in the groove and escape therefrom before the groove is again closed when the land at d passesthe rear edge 7a.  
  FIG. 6 shows both intersecting bores of the casing developed from to 7b and which include a second opening or port 10. It will be clear that the double lines in the lower half of FIG. 6 represent the lands 13 between the grooves 15 in the female rotor 4b, while the double lines in the upper half represent the narrow grooves 16 between the lands 17 of the male rotor shown in FIG. 4.  
 I claim: 1. A variable capacity screw compressorcomprising: a. a casing, housing two intersecting bores defining a working chamber in which a pair of intermeshing screws having helical lands and intervening grooves are mounted to rotate and compress a working fluid as it is being passed axially through said working chamber from a low pressure inlet wall to a high pressure outlet wall, said walls being located at opposite axial ends of said casing; b. an inlet passage in said inlet wall located in substantially axial alignment with the axes of said screws for admitting working fluid into the ends of valve to vent partially compressed air from the&#34; working chamber; 7  
 e. means for passing said vented partially compressed 1 working fluid directly into the grooves of said 1 screws at a location between said by-pass opening and said axially. aligned inlet passagewhereby to interrupt admission of low pressure working fluid.  
  2. A variable capacity screw compressor according to claim 1, in which the means for passing the vented working fluid comprises a port in the working chamber casing, said port being so located that the leading flank of the lands of said screws will commence to pass across the rear edge of said axial inlet passage as the 7 following grooves become aligned with said port.  
  3. A variable capacity screw compressor according to claim 2, in which a port is provided in said casing for each of the two bores.  
  4. A variable capacity screw compressor according to claim 2, in which the width of the port in the casing is of the same order of magnitude as the distance between the leading flanks of two successive lands. 7