Patent Publication Number: US-6217304-B1

Title: Multi-rotor helical-screw compressor

Description:
This is a continuation-in-part of copending U.S. patent application Ser. No. 09/106,620 filed on Jun. 29, 1998, now abandonded, which is a continuation-in-part of copending U.S. patent application Ser. No. 08/808,470 entitled MULTI-ROTOR HELICAL-SCREW COMPRESSOR filed Mar. 3, 1997 filed by David N. Shaw, now U.S. Pat. No. 5,807,091 which is a continuation of U.S. patent application Ser. No. 08/550,253 entitled MULTI-ROTOR HELICAL-SCREW COMPRESSOR filed Oct. 30, 1995 filed by David N. Shaw, now U.S. Pat. No. 5,642,992. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to helical screw type compressors. More specifically, the present invention relates to a multi-screw compressor having, e.g., a male rotor and at least two female rotors. 
     Helical type compressors are well known in the art. One such helical compressor employs one male rotor axially aligned with and in communication with one female rotor. The pitch diameter of the female rotor is greater than the pitch diameter of the male rotor. Typically, the male rotor is the drive rotor, however compressors have been built with the female rotor being the drive rotor. The combination of one male rotor and one female rotor in a compressor is commonly referred to as a twin screw or rotor, such is well know in the art and has been in commercial use for decades. An example of one such twin rotor commonly employed with compressors in the HVAC (heating, ventilation and air conditioning) industry is shown in FIG. 1 herein, labeled prior art. Referring to FIG. 1 herein, a cross sectional view of a male rotor  10  which drives an axially aligned female rotor  12  is shown. Male rotor  10  is driven by a motor, not shown, as is well known. Male rotor  10  has four lobes  14 - 17  with a 300° wrap and female rotor  12  has six flutes  18 - 23  with a 200° wrap. Accordingly, the compression-discharge phase of the axial sweep with respect to male rotor  10  occupies about 300° of rotation. The resulting gap between the male and female rotors requires oil to be introduced into the compression area for sealing, however, the oil also provides cooling and lubricating, as is well known. However, the introduction of this oil requires the use of an oil separation device, to separate the oil from the refrigerant being compressed in HVAC compressors. The primary benefit of the twin rotor configuration is the low interface velocity between the male and female rotors during operation. However, the twin rotor configuration is not balanced and therefore incurs large radial bearing loads and thrust loads. The obvious solution to alleviating the bearing load problem would be to install sufficiently sized bearings. This is not a feasible solution, since the relative diameters of the rotors in practice result in the rotors being too close together to allow installation of sufficiently sized bearings. 
     The prior art has addressed this problem, with the introduction of compressors employing ‘so-called’ single screw technology. Referring to FIGS. 2 and 3 herein, labeled prior art, a drive rotor  24  with two opposing axially perpendicular gate rotors  26  and  28  is shown. Rotor  24  is driven by a motor, not shown, as is well known. Rotor  24  has six grooves  30  and each gate rotor  26 ,  28  has eleven teeth  32 ,  34 , respectively, which intermesh with grooves  30 . The gate rotors  26  and  28  are generally comprised of a composite material which allows positioning of the gate rotor at a small clearance from the drive rotor. This clearance is small enough that the liquid refrigerant itself provides sufficient sealing, the liquid refrigerant also provides cooling and lubrication. The rearward positioning of gate rotors  26  and  28  and the positioning on opposing sides of drive rotor  24 , (1) allows equalizing suction of pressure at both ends of rotor  24  thereby virtually eliminating the thrust loads encountered with the above described twin screw system and (2) balances the radial loading on rotor  24  thereby minimizing radial bearing loads. However, the interface velocity between the gate rotors and the drive rotor are very high. Accordingly, a common problem with this system is the extensive damage suffered by the rotors when lubrication is lost, due to the high interface velocities of the rotors. 
     SUMMARY OF THE INVENTION 
     The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the multi-rotor compressor of the present invention. In accordance with the present invention, the compressor includes a male rotor which is axially aligned with and in communication with at least two female rotors. The male rotor is driven by a motor, in other words the male rotor is the drive rotor. The male rotor has a plurality of lobes which intermesh with a plurality of flutes on each of the female rotors. The pitch diameters of the female rotors are now less than the pitch diameter of the male rotor. 
     The male rotor comprises an inner cylindrical metal shaft with an outer composite material ring mounted thereon. The ring includes the lobes of the male rotor integrally depending therefrom. The lobes of the male rotor being comprised of a composite material allows positioning of the female rotors at a small clearance from the male drive rotor. This clearance is small enough that the liquid refrigerant itself provides sufficient sealing, however, the liquid refrigerant also provides cooling and lubrication. 
     The positioning of the female rotors on opposing sides of the male rotor balances the radial loading on the male rotor thereby minimizing radial bearing loads. Further, due to a larger diameter male drive rotor as compared to the male drive rotor in the prior art twin screw compressors, and therefore, additional distance between the rotors, any female radial bearing loads can be easily accommodated with sufficiently sized bearings. It will also be appreciated, that interface velocity between the male and female rotors during operation is very low, whereby the extensive damage suffered by the prior art single screw compressors when lubrication is lost, due to the high interface velocities of the rotors, is reduced. 
     The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
     FIG. 1 is a diagrammatic cross sectional view of a twin screw or rotor configuration in accordance with the prior art; 
     FIG. 2 is a diagrammatic top view of a single screw configuration in accordance with the prior art; 
     FIG. 3 is a diagrammatic end view of the single screw configuration of FIG. 2; 
     FIG. 4 is a diagrammatic cross sectional view of a tri-rotor configuration in accordance with the present invention; 
     FIG. 5A is a diagrammatic unwrapped pitch line study of the prior art twin screw or rotor configuration of FIG. 1; 
     FIGURE 5B is a diagrammatic unwrapped pitch line study of the tri-rotor configuration of FIG. 4; 
     FIG. 6 is a diagrammatic side cross sectional view of a compressor employing the multi-rotor configuration of FIG. 4; 
     FIG. 7 is a view taken along the line  7 — 7  of FIG. 6 with the discharge plate removed for clarity; and 
     FIG. 8 is a diagrammatic cross sectional view of a multi-rotor configuration in accordance with an alternate embodiment of the present invention; 
     FIG. 9 is an induction end view of the compressor of FIG. 6; 
     FIG. 10 is a view taken along the line  10 — 10  of FIG. 6; 
     FIG. 11 is a view taken along the line  11 — 11  of FIG. 6; 
     FIG. 12 is a discharge end view of the compressor of FIG. 6; FIG. 12A is a view taken along the line  12 A— 12 A of FIG. 12; and 
     FIGS. 13A-13P are views similar to FIG. 4 showing the following configurations of male rotors/lobes and female rotors/flutes combinations: 
     
       
         
           
               
               
               
             
               
                   
               
               
                   
                 No. Male 
                 No. of Female 
               
               
                 Figure 
                 Rotors/Lobes 
                 Rotors/Flutes 
               
               
                   
               
             
            
               
                 13A 
                 1/8  
                 2/5 
               
               
                 13B 
                 1/8  
                 2/6 
               
               
                 13C 
                 1/8  
                 2/7 
               
               
                 13D 
                 1/9  
                 2/5 
               
               
                 13E 
                 1/10 
                 2/5 
               
               
                 13F 
                 1/10 
                 2/6 
               
               
                 13G 
                 1/10 
                 2/7 
               
               
                 13H 
                 1/12 
                 2/7 
               
               
                 13I 
                 1/11 
                 3/5 
               
               
                 13J 
                 1/12 
                 3/5 
               
               
                 13K 
                 1/12 
                 3/6 
               
               
                 13L 
                 1/12 
                 3/7 
               
               
                 13M 
                 1/15 
                 3/5 
               
               
                 13N 
                 1/15 
                 3/7 
               
               
                 13O 
                 1/16 
                 4/6 
               
               
                 13P 
                 1/20 
                 4/7 
               
               
                   
               
            
           
         
       
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 4, a cross sectional view of a rotor configuration for use in compressors in accordance with the present invention is generally show at  40 . A male rotor  42  is axially aligned with and in communication with female rotors  44  and  46 . Male rotor  42  is driven by a motor, described hereinafter, and the male rotor drives the female rotors. The multi-rotor compressor of the present invention includes a plurality of commercially viable combinations between the number of female rotors  44 ,  46 , the number of flutes  56 - 61  on the female rotors, and the number of lobes  48 - 50  on the male rotor as will be described in more detail herein below. In this example, male rotor  42  has eight lobes  48 - 55  with a 150° wrap, female rotor  44  has six flutes  56 - 61  with a 200° wrap, and female rotor  46  has six flutes  62 - 67  with a 200° wrap. The pitch diameters  68 ,  70  of the female rotors  44 ,  46  are less than the pitch diameter  72  of the male rotor  42 . Accordingly, the compression/discharge phase of the axial sweep with respect to male rotor  42  occupies about 150° of rotation. Duplicate processes are occurring simultaneously on the top and bottom of the male rotor. 
     Male rotor  42  comprises an inner cylindrical metal shaft  82  with an outer composite material ring  84  mounted thereon. Shaft  82  is preferably comprised of steel, ductile iron or other material of comparable strength for supporting the rotor. Ring  84  includes lobes  48 - 55  integrally depending therefrom. Ring  84  is preferably comprised of a thermoplastic or other suitable composite material for use in compressors, i.e., suitable for high pressure application. The larger diameter male drive rotor as compared to the male drive rotor in the prior art twin screw compressors allows for the above described two piece construction. The smaller diameter male drive rotor in the prior art twin screw compressors could not be constructed as described above since a small diameter inner shaft would not be strong enough to properly support the rotor. The male drive rotor in the prior art moderate high pressure twin screw compressors is comprised of solid unitary metal piece. It will be appreciated that certain operating parameters of male rotor  42  such as inertial resistance, stiffness, vibration resistance and critical frequency are controlled by the relationship between the sizes of metal shaft  82  and composite ring  84 . The thickness of composite ring  84  is the distance between metal shaft  84  and pitch diameter  72  represented by the arrow labeled L1 and the distance between the pitch diameter and the tip of each lobe  48 - 55  represented by the arrow labeled L2. The thickness of a typical composite ring  84  ranges from one where L1 is at least half of L2 to one where L1 is the same distance as L2. A preferred male rotor  42  is comprised of a metal shaft having a radius represented by the arrow labeled R where R ranges from 1.67(L1+L2) in an embodiment with the thinnest preferable composite ring  42  to one where R is equal to L1+L2. 
     The significance of the lobes  48 - 55  being comprised of a composite material, is that it allows positioning of the female rotors  44  and  46  at a small clearance from the male drive rotor  42 . This clearance is small enough that droplets of the liquid refrigerant itself entrained in the gaseous phase of the refrigerant to be compressed provide sufficient sealing, cooling, and lubrication of the rotors. My copending U.S. patent application Ser. No.: 09/245,516, filed Feb. 5, 1998, and which is incorporated herein by reference, disclosed an example of apparatus and method for controlling the amount of liquid droplets entrained in the gaseous refrigerant. Accordingly, the need to introduce oil into the compression area, such as in the prior art twin screw compressors for sealing, cooling and lubricating is eliminated because the composite material can be adequately lubricated with liquid refrigerant. Further, the positioning of female rotors  44  and  46  on opposing sides of male rotor  42  balances the radial loading on male rotor  42  thereby minimizing radial bearing loads. Also, due to larger diameter male drive rotor as compared to the male drive rotor in the prior art twin screw compressors and therefore the additional distance between the rotors, any female radial bearing loads can be easily accommodated with sufficiently sized bearings. It will also be appreciated, that interface velocity between the male and female rotors during operation is very low, whereby the extensive damage suffered by the prior art single screw compressors when lubrication is lost, due to the high interface velocities of the rotors, is reduced. The low interface velocity results in minimal sliding action at the pitch band interface of the rotors. 
     Referring to FIGS. 5A and B diagrammatic unwrapped pitch line studies are provided. FIG. 5A is an unwrapped pitch line study of the prior art twin rotor of FIG.  1 . FIG. 5B is an unwrapped pitch line study of the rotor configuration  40  of FIG.  4 . 
     Referring to FIGS. 6 and 7, a compressor employing the rotor configuration  40  of the present invention is shown generally at  90 . Compressor  90  includes a hermetically sealed motor  92  having a drive shaft  94  which is integral with shaft  82  of male rotor  42  for driving the same. As described above, a bearing  96  is mounted at shaft  82  in between motor  92  and rotor  42  and a bearing  98  is mounted at one end of the shaft  82  to absorb any remaining radial bearing loads. Bearing  96  is shown as a cylindrical roller bearing. Bearing  98  is shown as a double row angular contact ball type. 
     Compressor  90  further comprises a housing having an inlet or induction housing portion  100  for induction into the compressor of the gaseous refrigerant with the entrained droplets of liquid refrigerant (from the evaporator of a cooling or refrigeration system), a main housing portion  102  and a discharge housing portion  104 . Alternatively, liquid droplets  103  of refrigerant could be introduced by atomization of the liquid droplets into the gaseous refrigerant at or near the inlet end of the compressor, such as by spray nozzles  101  located at or near the inlet to the compressor, as shown in FIG.  6 . 
     An induction side plate  106  and a discharge side plate  108  are mounted on male rotor  42  by a plurality of dowels  110  and bolts. Induction at housing portion  100  is shown in FIG.  9  and the induction side plate  106  is shown in FIG.  10 . The center line of the dowels lies at the intersection of ring  84  and shaft  82 , whereby cooperating semi-circular, longitudinal grooves  11 ,  10  are formed at the outer surface of shaft  82  and the inner surface of ring  84  for receiving the dowels. The outside diameter of plate  106  is equal to the root diameter of the male rotor  42 . The outside diameter of plate  108  is equal to the crest diameter of the male rotor  42 . Plates  106  and  108  serve two purposes: to secure ring  84  on shaft  82  and to equalize suction pressure at both ends of the male rotor  42  thereby virtually eliminating the thrust loads encountered with the prior art twin screw compressors. 
     Discharge porting is defined in housing  104  wherein trap pocket relief is provided. The problem of a trapped pocket is well known in the art of compressors. More specifically, the trap pocket is generated as a lobe reduces the area between the two flutes, a small void between the lobe and one of the flutes traps a pocket of compressed refrigerant. This trapped pocket of refrigerant must be relieved, otherwise the resistance generated by the trapped pocket may damage the compressor. 
     Housing  104  includes an inner circumferential surface  111  for receiving plate  108 . A clearance is defined between the outer circumference of plate  108  and the inner circumferential surface  111  of housing  104 . An inwardly countersunk surface  112  depends from surface  111 , which allows the clearance between plate  108  and surface  111  to be sealed by the liquid refrigerant, thereby minimizing leakage back to the low side of the compressor. Moreover, the discharge side of the male rotor  82  being sealed off from the high side by plate  108  causes the pressure on both ends of male rotor  82  to be equalized. Further, as is readily apparent to one of ordinary skill in the art, the high pressure at the interface of the discharge side of the male rotor  82  and the plate  108  acts on plate  108  in the direction to the right in FIG.  6  and acts on the lobes of the male rotor  82  in an equal and opposite direction (i.e., to the left in FIG.  6 ). These equal and opposite forces result in the elimination of the thrust loads on the male rotor. Countersunk surface  1   12  terminates at an opening or hole  114  with the shaft of the male rotor  82  disposed therein. Openings or holes  116  and  118  are also provided for receiving the shafts of the female rotors  44  and  46 , respectively, as best shown in FIG.  7 . Compression and discharge side  74  (i.e., the corresponding radial discharge area of male rotor  42  and the axial discharge port area of female rotor  44 ) communicates with discharge porting  120  and compression and discharge side  76  (i.e., the corresponding radial discharge area of male rotor  42  and the axial discharge port area of female rotor  46 ) communicates with discharge porting  122 . Discharge at discharge plate  108  is shown in FIG.  11  and at housing portion  104  is shown in FIGS. 12 and 12A. Since discharge porting  120  operates the same as discharge porting  122 , only discharge porting  120  is described in detail below. 
     Discharge porting  120  comprises a first stepped down portion  124  defined by a line  126  which represents the circumferential distance encompassed when surface  124  intersects inner circumferential surface  111 , an edge  128  which follows the root diameter of female rotor  44  and a curved edge  130  which communicates with the periphery of the remaining radial and axial port areas, such areas being well known and defined in the art. This first stepped down portion  124  provides relief on the female rotor side of the aforementioned trapped pocket, since such will be aligned with this portion. A second further stepped down portion  132  depends from stepped down portion  124  and generally aligns with the axial port area of female rotor  44 . Both portions  124  and  132  lead into a discharge opening  134  which generally aligns with the radial flow area. The discharge opening from discharge porting  120  and  122  are combined and form a single discharge output for the compressor. The operation of the compressor will now be more fully described. FIG. 4 shows the directions of rotation, the typical profiles of the rotors, and their typical pitch circles. It also shows compression and discharge side  74  occurring between the center male rotor  42  and left female rotor  44  and an identical and simultaneous process occurring on the bottom between the center male rotor  42  and the cooperating right female rotor  46  at compression and discharge side  76 . Note, too, induction ports  78  and  80  shown in FIG.  4 . 
     Each female rotor  44  and  46  cooperates with male rotor  42  in a manner well known to those familiar with the traditional twin screw rotary compressor, represented in FIGS. 1 and 5A. Referring again to FIG. 4, at each induction port  78 ,  80 , the lobes  48 - 55  and flutes  56 - 67  separate and are radially exposed to the inlet as seen in FIG. 10, which, compared with FIG. 4, is an opposite view of rotors  42 ,  44 ,  46  from the inlet side of the compressor. FIG. 5B schematically shows the cooperating pitch lines of the rotors (unwrapped so as to allow a two dimensional representation) as they would be seen when looking up at the bottom of FIG.  4 . Therefore, since the left and right female rotors  44 ,  46  are rotating clockwise in FIG. 4, the pitch lines in FIG. 5B of the left and right rotors  44 ,  46  will appear to move to the left, while the pitch lines of male rotor  42  will appear to move to the right, and so the pitch lines of male rotor  42  and female rotor  46  come together along the interface between them as seen in FIG.  5 B. At the same time, the pitch lines of female rotor  44  and male rotor  42  are separating along the interface between them as seen in FIG.  5 B. 
     Each space between the pitch lines in FIG. 5B represents a chamber, as would be readily apparent to a person of ordinary skill in the art. In the male rotor  42 , the chambers are formed between adjacent lobes; in the female rotors  44  and  46 , the chambers are formed within each flute. Looking at the center male rotor  42 , the chambers closest to the upper left corner are in communication with the low pressure side of the compressor. As male rotor  42  rotates to the right, the chamber reaches a point that extends from the lower left to the upper right comer of the profile of male rotor  42  shown in FIG.  5 B. At this position, it is closed off from the inlet and outlet sides of the compressor by an end plate as seen in FIG.  10 . As male rotor  42  continues to rotate to the right, the chamber interfaces with chambers carried by the right female rotor  46 . As these chambers turn into each other, they are reduced in size until they reach a point near the lower right corner of the profile of male rotor  42  and lower left comer of the profile of right female rotor  46 , at which point the chambers reach an opening in an end-plate  104  as seen in FIG. 11, and the compressed fluid is exhausted to the downstream or high pressure side of the compressor. 
     Thus, it can be seen that male rotor  42  interfaces and interacts with female rotors  44  and  46  to form closed rotating working chambers to compress a fluid, the working chambers reducing in volume as the rotors rotate to compress the working fluid. 
     While the above described embodiment has been described with a male rotor having eight lobes, whereby eight discharge pulses per revolution of the male rotor are generated for each of the female rotor for a total of sixteen pulses per revolution, it may be preferred that a male rotor having nine lobes (i.e., an odd number) be employed. The sixteen pulses per revolution actually only generate eight pulses per revolution, since two pulses occur at the same time, i.e., one for each of the female rotors. With a male rotor having nine lobes, eighteen pulses per revolution are generated, i.e., nine pulses per revolution for each of the two female rotors. However, none of these eighteen pulses occur during another one of the pulses, thereby generating a more even or smoother discharge flow, i.e., less noise. 
     Further, while the above described embodiment has been described with only two female rotors, it is within the scope of the present invention that two or more female rotors may be employed with a single drive male rotor. Referring to FIG. 8, a cross sectional view of a male rotor  140  is axially aligned with and in communication with three equally spaced female rotors  142 ,  144  and  146 . Male rotor  140  is driven by a motor, as described above. In this example, male rotor  140  has between ten and twenty lobes (e.g., twelve lobes would have a 100° wrap), female rotor  142  has between four and seven flutes (e.g., six flutes would have 200° wrap), female rotor  144  has between four and seven flutes (e.g., six flutes would have 200° wrap), and female rotor  146  has between four and seven flutes (e.g., six flutes would have 200° wrap). The male lobe wrap angle S° can easily be determined from the female flute wrap angle P°, the number of female flutes Z, and the number of male lobes X by the following formula: 
     
       
         S°=P°Z/X 
       
     
     Similarly, for any given male lobe wrap angle S°, the female flute wrap angle P° can be determined from the formula: 
     
       
         P°=S°X/Z 
       
     
     Again, where the pitch diameters of the female rotors  142 ,  144 ,  146  are less than the pitch diameter of the male rotor  140 . In all cases, the wrap angle of the male lobes is preferably less than 360°, and the wrap angle of the female flutes is always less than 360°. Referring to Table 1 the relationship between wrap angles is illustrated for various commercial combinations as discussed herein above. The comparison made in Table 1 is between a twin rotor of the prior art having a male wrap angle of 300° and various multi-rotor configurations in accordance with the present invention. An important aspect of the present invention is that the male wrap angle S° of the multi-rotor compressor is equal to that of a twin rotor compressor divided by the number of female rotors Y. For example, in Table 1 male wrap angle S° for two female rotors is 150° and S° for three female rotors is 100°. The wrap angle of the female rotor for many combinations of rotor attributes are shown in Tables 1 and can be determined in accordance with the formula given above. This relationship holds for any male rotor wrap angles. The relationships for male wrap angle S° equal to 360° divided by the number of female rotors is shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Degree of Wrap for Female Rotors for male rotors having 
               
               
                 300°/Y Degrees of wrap for various Lobe-flute Combinations 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 -Y- 
                 -Z- 
                   
               
               
                 No. of Female 
                 No of Female 
                 -X- Number of Male Lobes 
               
            
           
           
               
               
               
               
               
            
               
                 Rotors 
                 Flutes 
                 4 
                 5 
                 6 
               
               
                   
               
               
                 1 
                 5 
                 240 
                 300 
                 360 
               
               
                   
                 6 
                 200 
                 250 
                 300 
               
               
                   
                 7 
                 171 
                 214 
                 257 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 -X- 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 -Y- 
                 -Z- 
                 8 
                 9 
                 10 
                 11 
                 12 
               
               
                   
               
               
                 2 
                 5 
                 240 
                 270 
                 300 
                 330 
                 360 
               
               
                   
                 6 
                 200 
                 225 
                 250 
                 275 
                 300 
               
               
                   
                 7 
                 171 
                 193 
                 214 
                 236 
                 257 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 -X- 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 -Y- 
                 -Z- 
                 12 
                 13 
                 14 
                 15 
                 16 
                 17 
                 18 
               
               
                   
               
               
                 3 
                 5 
                 240 
                 260 
                 280 
                 300 
                 320 
                 340 
                 360 
               
               
                   
                 6 
                 200 
                 217 
                 233 
                 250 
                 267 
                 283 
                 300 
               
               
                   
                 7 
                 171 
                 186 
                 200 
                 214 
                 229 
                 243 
                 257 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Degree of Wrap of Female Rotors for male rotors having 
               
               
                 360°/Y Degrees of Wrap for various Lobe-flute Combinations 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 -Y- 
                 -Z- 
                   
               
               
                 of Female 
                 No of Female 
                 -X- Number of Male Lobes 
               
            
           
           
               
               
               
               
               
            
               
                 Rotors 
                 Flutes 
                 4 
                 5 
                 6 
               
               
                   
               
               
                 1 
                 5 
                 288 
                 360 
                 * 
               
               
                   
                 6 
                 240 
                 300 
                 360 
               
               
                   
                 7 
                 206 
                 257 
                 309 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 -X- 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 -Y- 
                 -Z- 
                 8 
                 9 
                 10 
                 11 
                 12 
               
               
                   
               
               
                 2 
                 5 
                 288 
                 324 
                 360 
                 * 
                 * 
               
               
                   
                 6 
                 240 
                 270 
                 300 
                 330 
                 360 
               
               
                   
                 7 
                 206 
                 231 
                 257 
                 283 
                 309 
               
               
                   
               
            
           
           
               
               
            
               
                   
                 -X- 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 -Y- 
                 -Z- 
                 12 
                 13 
                 14 
                 15 
                 16 
                 17 
                 18 
               
               
                   
               
               
                 3 
                 5 
                 288 
                 312 
                 336 
                 360 
                 * 
                 * 
                 * 
               
               
                   
                 6 
                 240 
                 260 
                 280 
                 300 
                 320 
                 340 
                 360 
               
               
                   
                 7 
                 206 
                 223 
                 240 
                 257 
                 274 
                 291 
                 309 
               
               
                   
               
               
                 *This combination yields a wrap angle greater than 360°.  
               
            
           
         
       
     
     The present invention further comprises a myriad of combinations between the number of female rotors, the number of flutes on the female rotors, and the number of lobes on the male rotor with each combination yielding different operating parameters such as noise, pumping capacity, rotational speed, discharge flow, etc. Table 1 illustrates many of the commercially viable combinations of lobes on the male rotor X, number of female rotors Y, and number of flutes Z on the female rotor with comparison to the prior art twin rotor configuration where Y=1. The first example given above is found in Table 1 for a male rotor having eight lobes, X=8, two female rotors, Y=2, and each female rotor having 6 flutes, Z=6. The second example is shown in the next column for a male rotor having nine lobes. As illustrated by Table 1 a relationship exists between a twin rotor configuration Y=1, a tri-rotor configuration Y=2, and a four rotor configuration Y=3. It can be seen from the examples given above and shown in Table 1 that the various combinations of rotor attributes with regard to an increase in the number of female rotors from Y 1  to Y 2  is governed by the following formula: 
     
       
         X 2 =(Y 2 /Y 1 )X 1 ±N/Y 1   
       
     
     where the number of lobes on the male rotor increases from X 1  to X 2  and where N is an integer ranging from 0 to (Y 2 −1). 
     It will be appreciated that certain geometrical and operational relationships are established in accordance with the present invention. For instance, the diameter of the male rotor increases with an increase in the number of female rotors as described herein above. The increase in diameter of the male rotor increases the stiffness of the rotor and increases the rotational speed of the female rotors thereby increasing the output capacity of the compressor. With reference back to FIG. 4, pitch diameter (PD)  72  of male rotor  42  increases in relation to the increase in the number of female rotors. If the pitch diameter  68 ,  70  of the female rotors  44 ,  46  remains constant the increase in the male rotor pitch diameter is expressed in accordance with the following formula: 
     
       
         PD 2 =PD 1 (Y 2 /Y 1 ) 
       
     
     where the number of female rotors increases from Y 1  to Y 2 . 
     It will be appreciated that the output capacity of a compressor, having a constant rotational speed of the male rotor and essentially identical female rotors, increases as the number of female rotors increases. In accordance with the present invention the output capacity C of compressor increases as the number of female rotors increases from Y 1  to Y2 in conformity with the following formula: 
     
       
         C=Y 2   2   
       
     
     Also, while the above example has been directed to a compressor for HVAC use, the multi-rotor configuration of the present invention is equally applicable in other helical type compressors, e.g., compressors with working fluids such as helium, air and ammonia. Moreover, the multi-rotor compressor of the present invention may be extremely well suited for oil less air compression. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.