US3658442A

US3658442A - Compressor

Info

Publication number: US3658442A
Application number: US44131A
Authority: US
Inventors: Arnold M Heitmann; Robert O Chambers
Original assignee: Northern Research and Engineering Corp
Current assignee: Ingersoll Rand Energy Systems Corp
Priority date: 1970-06-08
Filing date: 1970-06-08
Publication date: 1972-04-25
Anticipated expiration: 1989-04-25

Abstract

A centrifugal compressor system has a compressor housing partitioned into a plurality of chambers and a gear chamber that has a common wall with the compressor housing. A compressor module support structure is mounted in each chamber on the common wall and receives a compressor module that has a pinion gear that extends past the common wall for engagement with a bull gear mounted for rotation in the gear chamber. Each compressor module has an inlet shroud that is supported on an intermediate partition of the compressor housing and a large plenum chamber is defined in front of each compressor module in part by that intermediate partition.

Description

United States Patent Heitmann et al.

[54] COMPRESSOR [72] inventors: Arnold M. l'leltmann, Swampscott; Robert 0. Chambers, Medford, both of Mass.

Northern Research and Engineerlng Corporation, Cambridge, Mass.

(22] Filed: June8, 1970 [21] Appl.No.: 44,131

[73] Assignee:

[56] References Cited UNITED STATES PATENTS 3,355,097 11/1967 l-lornschuch ..415/l79 Perrott ..417/243 Ford ..4l7/243 Primary Examiner-C. .I. l-lusar Attorney-Willis M. Ertman ABSTRACT A centrifugal compressor system has a compressor housing partitioned into a plurality of chambers and a gear chamber that has a common wall with the compressor housing. A compressor module support structure is mounted in each chamber on the common wall and receives a compressor module that has a pinion gear that extends past the'common wall for engagement with a bull gear mounted for rotation in the gear chamber. Each compressor module has an inlet shroud that is supported on an intermediate partition of the compressor housing and a large plenum chamber is defined in front of each compressor module in part by that intermediate partition.

16 Claims, 13 Drawing Figures PATENTED APR 2 5 I972 SHEET 18F 6 PATENTEUAFR 2 5 I972 SHEET 5 OF 6 QON @ON NON OON @Q VQ SHEET 8 OF 6 T ov RATENTEB APR 2 5 m2 COMPRESSOR SUMMARY OF INVENTION improved multi-stage centrifugal compressor arrangement employing a modular arrangement of impeller shafts having graduated rotational speeds so as to achieve advantageous compression conditions per compressor stage for the several impellers while providing a convenient and economical drive arrangement.

A further object of the invention is to provide novel and improved multi-stage centrifugal compressor arrangement which has a compact flow path arrangement through the impeller and cooler stages, requires a small amount of space and presents an attractive appearance.

A further object of theinvention is to provide a novel and improved multi-stage centrifugal compressor arrangement having conveniently arranged cooling and lubrication arrangements.

Still another object of the invention is to provide novel and improved arrangements for driving a multi-stage centrifugal compressor assembly.

A further object of the invention is to provide a gaseous medium compressor arrangement having improved efficiency.

In accordance with one feature of the invention there is provided a centrifugal gaseous medium compressor system having a compressor housing, with a plurality of compressor module support structures in the housing which support a corresponding plurality of compressor modules each of which includes an impeller and bearing assembly. Partition means in the housing divides the housing into a plurality of chambers with an inlet plenum chamber and a discharge chamber associated with each compressor module. The partition means further defines a flow path between the discharge chamber of one compressor module and the inlet plenum chamber of the next compressor module, and a cooler module including a series of heat exchanger surface elements is disposed in that flow path. The housing has an outer wall with individual ports in it through which the compressor and cooler modules may be withdrawn as individual units.

In a preferred embodiment each compressor module includes a centrifugal compressor impeller, a diffuser, and the necessary seals, bearings and pinion gear in a modular assembly, and each cooler module is a self-contained unit having heat exchanger surface elements and water manifolds and the partition means. The system further includes a gear casing that has a common wall with the compressor housing. A bull gear is mounted in that casing and the pinion gears of the compressor modules extend through the common wall into the gear casing and are engaged by the bull gear. A drive motor is disposed adjacent the gear casing and has a motor shaft that extends into the gear casing and the bull gear is mounted directly on that motor shaft.

In accordance with another features of the invention there is provided centrifugal gaseous medium compressor apparatus having a housing and a compressor assembly mounted within the housing. The compressor assembly includes an impeller assembly and an inlet shroud defining an inlet passage. A plenum chamber through which the gaseous medium to be compressed passes is immediately adjacent the compressor assembly inlet shroud; the plenum chamber being within a distance equal to the inlet diameter of the inlet shroud from the forward blade edge of the impeller and having a cross-sectional area at least four times the cross-sectional inlet area of the compressor assembly as defined by its inlet shroud. In preferred embodiments the cross-sectional area of the plenum chamber is'at least ten times the cross-sectional inlet area of the compressor assembly. This close coupled plenum chamber-compressor assembly arrangement provides a significant increase in compressor efficiency. Such arrangement also permits the close positioning of an inlet throttle valve without adversely affecting the performance of the compressor.

In a particular embodiment, the compressor housing is a rectangular housing in which three compressor modules and three cooler modules are mounted. This housing has dividing partitions which define the air passages and close coupled plenum chambers that route the air in the required sequence through the compressor and cooler modules. The compressor and cooler modules are assembled into the housing through access ports in the front of the housing and are easily inspectable from these ports and easily removed without disconnecting drive structure or piping. Water connections to the cooler modules are made on the back side of the compressor housing through appropriately sealed ferrule assemblies that facilitate insertion and removal.

The drive motor is of the face mounting configuration and an adapter plate bolted to the face of the motor serves as the rear wall of the gear casing. The hull gear is mounted directly on the drive motor shaft, thus eliminating the need for bull gear bearings and the problems of couplings and coupling misalignment. A common base for the compressor housing and the drive motor contains a lubrication oil module and control cabinet. The lubrication module is an assembly of the necessary lubrication oil conditioning equipment and typically includes a lubrication oil cooler, a pump, pressure regulating valves and temperature sensors.

Compressor controls include throttling means for throttling the compressor inlet to maintain constant discharge pressure and also to prevent surge. Other controls include sensing, drive motor overload protection and lube oil temperature pressure sensing, cooling water pressure and vibration sensors. Any one of the key elements of the assembly can be removed without disturbing other elements, including any of the rotor assemblies, any of the coolers, the lubrication oil module, or the control module.

Other objects, features and advantages of the invention will be seen as the following description of a particular embodiment progresses, in conjunction with the drawings, in which:

FIG. 1 is a perspective view of a three-stage centrifugal air compressor system constructed in accordance with the invention, showing a compressor module and a cooler module spaced from the compressor assembly housing;

FIG. 2 is a top plan view with parts broken away of the compressor system shown in FIG. 1;

FIG. 3 is side elevational view with parts broken away of the compressor system shown in FIG. 1;

FIG. 4a is a diagrammatic view of components of the compressor system;

FIG. 4b is an exploded diagrammatic view corresponding to FIG. 4a, showing the air flow path through components of the compressor system;

FIG. 5 is a sectional view showing details of a compressor module employed in the system shown in FIG. 1;

FIG. 6 is a perspective view with parts broken away showing details of a cooler module employed in the system shown in FIG. 1;

FIGS. 7-10 are sectional views taken along the lines 7-7,

8-8, 9-9, and 10-10, respectively, of FIG. 2; and

FIGS. 11 and 12 are sectional views taken along the lines 11-11 and 12-12, respectively, of FIG. 3.

DESCRIPTION OF PARTICULAR EMBODIMENT With reference to FIGS. 1-3, the apparatus includes a base 10 on which a compressor assembly housing 12 is mounted at its left end and a prime mover 14 in the form of an electric motor is mounted at its right end. A control panel 16 is mounted on one side of motor 14 and a system for supplying lubricating oil to the compressor assembly is mounted on the opposite side of motor 14. Chamber disposed between motor 14 and compressor housing 12 houses a bull gear 22 that is mounted directly on the shaft 24 of motor 14.

The compressor assembly in housing 12 includes three

compressor Stages

30, 32 and 34 and three

cooler structures

36, 38 and 40 disposed in rectangular housing 12. Each compressor stage and cooler structure is removable as a unit through an access port in the left end wall 42 of housing 12,

compressor stages

30, 32 and 34 being removable through

ports

44, 46 and 48 covered by

plates

50, 52 and 54, respectively; and

cooler stages

36, 38 and 40 being removable through

ports

56, 58, 60 and having

integral cover plates

62, 64 and 66, respectively. Inlet conduit 68 is attached to plate 50 and may include a throttling valve; and outlet conduit 70 extends upwardly from the top wall 72 of housing 12.

A diagrammatic view of the compressor assembly is shown in FIG. 4a and a corresponding exploded view is shown in FIG. 4b. Air is introduced into the compressor assembly through inlet conduit 68 to plenum chamber 74 which communicates with the inlet of first compressor stage 30. That compressor stage discharges air into discharge chamber 76 which translates the compressed air through cooler 36 to plenum chamber 78 at the inlet to compressor stage 32. The compressed air from that stage is discharged into chamber 80 for transfer to cooler 38 to plenum chamber 82 which communicates with the inlet to compressor stage 34. Compressor stage 34 is mounted to discharge air into discharge chamber 84 and transfer that air through aftercooler 40 and chamber 86 to outlet conduit 70.

A sectional view of compressor module stage 30 and plenum chamber 74 which is illustrative of the construction of module stages 32 and 34 (although those stages are of graduated smaller size) and their associated plenum chambers, is shown in FIG. 5.

The compressor module is received directly in a mounting boss 90 that projects forwardly from the front wall 92 of the gear case 20 and receives its primary support from this mounting structure. Additional support is provided by the interconnection of the inlet shroud 94 of the module with partition 96 in the compressor case between

chambers

74 and 76. The compressor impeller 98 and shaft 100 are an integral assembly, the impeller 98 being cast from a corrosion resistant steel and welded to shaft 100. The pinion gear 102 is press fitted on shaft 100. Shaft housing 104 includes hydrodynamic

radial bearings

106, 108, the impeller end bearing 106 of the three lobed type and the pinion end bearing 108 is of the simple journal type. Thrust bearing 110 is of the simple stepped type. The main thrust is towards the impeller and a thrust washer I12 captured by the pinion 102 provides the main positioning surface. A position control thrust bearing 114 located at the end of the bearing housing 104 uses the back face of the pinion as a runner surface. The shaft seal 116 is a two-stage seal, the primary seal or oil side being a mechanical or nose type seal and the second stage a simple labyrinth. The oil to the radial and thrust bearings is supplied from the lubrication supply line 118 into the compressor cartridge via an annular passage 120 formed by the stepped diameter of the bearing housing 104 and the receiving bore in the support boss 90. Lubrication flow control is established by the sizing of the drilled holes required to port the oil to various bearing supply areas. Passage 122 provides a vent. Drainage from the bearings is ported away from the bull gear and flows to an annular recess 124 covered by plate 126 in the forward face of the bull gear chamber 20.

The compressor diffuser 128 is formed by a flat plate having a series of vanes 130 cast thereon. This plate is bolted to the housing 104 and the shroud 94 is secured in accurately centered relation to the diffuser 128 and bearing cartridge 104 by dowel pins and bolts. The entire compressor module may be removed as a unit by unbolting the inlet bellmouth shroud structure 94 from partition 96 and sliding the unit out to initially disengage pinion 102 from bull gear 22 and then to remove the unit through the plenum chamber 74 and port 44.

Plenum chamber 74 is close coupled to compressor module 30, wall 96 being a partition separating plenum chamber 74 from compressor discharge chamber 76. The plenum chamber side of wall 96 is located at a distance from the front end 132 of impeller blades 134 of about one-half the compressor inlet diameter indicated by dimension 136 and chamber 74 has a cross-sectional area about 25 times the cross-sectional area of the compressor inlet and a length of about twice the inlet diameter 136.

A diagrammatic view of a cooler module is shown in FIG. 6. In a typical module, the intercooler includes a multipass, cross-counterflow arrangement having tube-fin heat exchange surfaces. The air side is formed of aluminum plates 140 spaced approximately 0.050 inch apart and the water side formed by inch copper tubing 142. Each intercooler module has two headers 144, 146, both headers having baffling 148 to provide the appropriate number of water passes. The inner header 146 containing inlet and discharge ports I50, 152. The flow is from header section 146-1 through tubes 142 to header section 144-1 and return through a second set of tubes 142 to header section 1462 and then return again to header section 144-2 for final pass back through still another set of tubes 142 to header section 146-3 and discharge through connection 152. These ports match up with appropriate water supply and drain

port structures

154, 156 in wall 92 of the compressor case and the interface junction is sealed by O-ring seal ferrules 158. Moisture condensed out during the intercooling process is collected by the fins 140 and drained to the bottom of the intercooler chamber chamber 160 and to a sump 162 (FIG. 8) which is piped through usual water traps to an overboard drain. Flexible seal structures 164 are mounted on either side of the cooler module and engage two

opposed walls

166, 168 of the chamber 160 in which the module is received. At the front end of the cooler module is a flange structure which is bolted with appropriate seals to the wall 42 of the compressor assembly housing 12, securing the module in position in the flow path in the compressor system. The entire cooler module may be removed as a unit by unbolting the plate structure and sliding the unit out to remove the unit through the port.

Further details of the assembly are indicated in FIGS. 7-12. An end view of the compressor assembly in FIG. 7 shows the access plates over the access ports in wall 42. In FIG. 8 there are indicated the cooling water inlet line 200 and cooling water outlet line 202 that communicate with each of the three cooler modules, lubrication supply line 204 which communicates with supply manifold 122 and lubrication drain line 206 which communicates with annular drain manifold 124. This manifold, as indicated in FIGS. 5 and 8, may be machined directly in the wall 92. With reference to FIGS. 9-12, plenum chamber 74 is defined by partitions 210,212, 214, top wall 72, transverse wall 96 and front wall 42. Discharge chamber 76 is defined by rearward extension of wall 210, side wall 218, top wall 216, lower wall 220,

partition walls

96, 212 and 214, and wall 92 between the compressor compartment and the bull gear compartment. Thus discharge chamber 76 has common walls with plenum chamber 74. Intercooler module 36 is disposed in a chamber whose upper wall is an extension of partition 214 and lower wall is a portion of the drain floor wall 220, the inner side wall is partition 222; and front and rear walls are

structures

42 and 92, respectively.

The plenum chamber 78 has as a top wall a portion of partition 214, a bottom wall part of partition 220, a side wall as partition 224, front wall 42 and rear wall 96. The discharge chamber 80 for compressor module 32 is defined by

partitions

214, 220, 222, 224, front wall 42 and rear wall 92. intercooler 38 is disposed between the discharge chamber 80 and inlet plenum chamber 82 to the third compressor module. Chamber 82 is bounded by side wall 226,

partitions

96 and 214, top wall 216, and front wall 42. The discharge chamber 84 for the third stage is formed between partition 230 and top wall 216, partition 210 and side wall 226, and partition 96 and rear wall 92. Aftercooler 40 is disposed in this chamber and the air after flowing through that aftercooler is discharged to conduit 70.

As an illustrative example, in this compressor assembly with an 800 horsepower motor, compressor stage 30 has a shaft speed of 61,800 rpm, compressor stage 32 has a shaft speed of 42, 400 rpm, and compressor stage 34 has a shaft speed of 29,100 rpm. A compressor unit of this frame size has an overall width of 4 feet and an overall height of 5 feet and is 6 feet, 9 inches long.

While a particular embodiment of the invention has been shown and described, various modifications thereof will be apparent to those skilled in the art and therefore it is not intended that the invention be limited to the disclosed embodiment or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

What is claimed is:

1. A centrifugal gaseous medium compressor system comprising a compressor housing, means partitioning said housing into a plurality of chambers, a gear casing having a common wall with said compressor housing, a bull gear mounted for rotation in said gear casing, a plurality of compressor module support structures mounted directly on said common wall, a housing outer wall structure opposite to and spaced from said common wall, said outer wall structure having a plurality of ports therein aligned with corresponding ones of said compressor module support structures, and a corresponding plurality of compressor modules supported in said module support structures, each said compressor module including a shaft and bearing assembly, an impeller, a pinion gear, a diffuser structure and an inlet shroud supported on said shaft and bearing assembly, each said inlet shroud being supported on an intermediate partition of said housing and each said pinion gear being disposed in said gear casing for driving by said bull gear, each said compressor module being removable as a unit from its support structure through the corresponding port in said outer wall structure of said housing.

2. A centrifugal gaseous medium compressor system comprising a compressor housing, means partitioning said housing into a plurality of chambers, a gear casing having a common wall with said compressor housing, a bull gear mounted for rotation in said gear casing, a plurality of compressor module support structures mounted directly on said common wall, a corresponding plurality of compressor modules supported in said module support structures, each said compressor module including a shaft and bearing assembly, an impeller, a pinion gear, a diffuser structure and an inlet shroud supported on said shaft and bearing assembly, each said inlet shroud being supported on an intermediate partition of said housing and each said pinion gear being disposed in said gear casing for driving by said bull gear, and an outer wall of said housing opposite said common wall having corresponding ports through which said compressor modules may be removed from said housing as units, and a cooler module disposed in said housing in a flow path between the discharge chamber of one compressor module and the plenum chamber of another compressor module, said cooler module including a series of heat exchanger surface elements, and said outer wall further including a port through which said cooler module may be withdrawn from said housing as a unit.

3. A centrifugal gaseous medium compressor system comprising a compressor housing, means partitioning said housing into a plurality of chambers, a gear casing having a common wall with said compressor housing, a bull gear mounted for rotation in said gear casing, a plurality of compressor module support structures mounted directly on said common wall, a corresponding plurality of compressor modules supported in said module support structures, each said compressor module including a shaft and bearing assembly, an impeller, a pinion gear, a diffuser structure and an inlet shroud supported on said shaft and bearing assembly, each said inlet shroud being supported on an intermediate partition of said housing and each said pinion gear being disposed in said gear casing for driving by said bull gear and an outer wall of said housing opposite said common wall having corresponding ports through which said compressor modules may be removed from said housing as units, said partitioning means cooperating with each said inlet shroud to define a plurality of plenum chambers between said inlet shroud and said outer wall, and each said plenum chamber being associated with a corresponding compressor module and having a cross-sectional area at least four times the cross-sectional area of the compressor module inlet passage as defined by said inlet shroud.

4. A centrifugal gaseous medium compressor system comprising a compressor housing, means partitioning said housing into a plurality of chambers, a gear casing having a common wall with said compressor hosuing, a bull gear mounted for rotation in said gear casing, a plurality of compressor module support structures mounted directly on said common wall, a corresponding plurality of compressor modules supported in said module support structures, each said compressor module including a shaft and bearing assembly, an impeller, a pinion gear, a diffuser structure and an inlet shroud supported on said shaft and bearing assembly, each said inlet shroud being supported on an intermediate partition of said housing and each said pinion gear being disposed in said gear casing for driving by said bull gear and an outer wall of said housing opposite said common wall having corresponding ports through which said compressor modules may be removed from said housing as units and an annular drain channel formed in said common wall and communicating via said module support structures with said shaft and bearing assemblies.

5. A centrifugal gaseous medium compressor system comprising a compressor housing, means partitioning said housing into a plurality of chambers, a gear casing having a common wall with said compressor housing, a bull gear mounted for rotation in said gear casing, a plurality of compressor module support structures mounted directly on said common wall, a

corresponding plurality of compressor modules supported in said module support structures, each said compressor module including a shaft and bearing assembly, an impeller, a pinion gear, a diffuser structure and an inlet shroud supported on said shaft and bearing assembly, each said inlet shroud being supported on an intermediate partition of said housing and each said pinion gear being disposed in said gear casing for driving by said bull gear and an outer wall of said housing opposite said common wall having corresponding ports through which said compressor modules may be removed from said housing as units and a drive motor disposed adjacent said gear casing, said drive motor having a motor shaft extending into said gear casing and said bull gear being mounted directly on said motor shaft.

6. The system as claimed in claim 5 wherein each said plenum chamber has a cross-sectional area at least four times the cross-sectional area of the compressor module inlet passage as defined by said inlet shroud.

7. The system as claimed in claim 6 wherein said partitioning means cooperates with each said inlet shroud to define a plenum chamber between said inlet shroud and said outer wall.

8. The system as claimed in claim 7 and further including a cooler module disposed in said housing in a flow path between the discharge chamber of one compressor module and the plenum chamber of another compressor module, said cooler module including a series of heat exchanger surface elements, and said outer wall further including a port through which said cooler module may be withdrawn from said housing as a unit.

9. The system as claimed in claim 8 and further including an annular drain channel formed in said common wall and communicating via said module support structures with said shaft and bearing assemblies.

10. A centrifugal gaseous medium compressor system comprising a compressor housing, a plurality of compressor module support structures in said housing, a corresponding plurality of compressor modules supported in said module support structures, partition means in said housing dividing said housing into a plurality of chambers with an inlet plenum chamber and a discharge chamber associated with each compressor module, said partition means further defining a flow path between the discharge chamber of one compressor module and the inlet plenum chamber of another compressor module, a cooler module support structure disposed in said flow path, a cooler module including a series of heat exchanger surface elements releasably secured to said cooler module support structure, said housing having an outer wall with individual spaced ports therein and aligned with corresponding ones of said compressor module support structures and said cooler module support structure through which said compressor and coller modules may be withdrawn as individual units and means for sealing said ports.

11. A centrifugal gaseous medium compressor system comprising a compressor housing, means partitioning said housing into two chambers, an aperture in said partitioning means, a gear casing having a common wall with said compressor housing, a bull gear mounted for rotation in said gear casing, a compressor module support structure on said common wall and extending into one of said chambers, a housing outer wall on the opposite side of said chambers from said common wall, said outer wall having a port therein aligned with said compressor module support structure, a compressor module supported in said module support structure, said compressor module including shaft and bearing assembly, an impeller, a pinion gear, a diffuser structure and an inlet shroud supported on said shaft and bearing assembly, said inlet shroud being secured in sealing relation in said aperture of said partitioning means, the second of said chambers being a plenum chamber through which the medium to be compressed is supplied and having a cross-sectional area at least four times the cross-sectional area of the compressor module inlet passage as defined by said inlet shroud, and said shaft and bearing assembly extending through said common wall so that said pinion gear is disposed in said gear casing for driving by said bull gear, said compressor module being removable as a unit from its support structure through said port in said outer wall of said housing.

12. Centrifugal gaseous medium compressor apparatus comprising a housing, a compressor assembly mounted within the housing, said compressor assembly including an impeller assembly and an inlet shroud that defines an inlet passage to said impeller assembly, structure defining a plenum chamber through which the gaseous medium to be compressed passes immediately adjacent the compressor assembly inlet shroud, said plenum chamber being within a distance equal to the inlet diameter of said inlet shroud from the forward blade edge of said impeller assembly and having a cross-sectional area at least four times the cross-sectional inlet area of said compressor assembly as defined by said inlet shroud.

13. The compressor assembly as claimed in claim 12 wherein the cross-sectional area of said plenum chamber is at least ten times said cross-sectional inlet area of said compressor assembly.

14. The apparatus as claimed in claim 13 wherein said apparatus includes a plurality of compressor assembly modules and associated close coupled plenum chambers and further including a cooler module disposed in said housing in a flow path between the discharge chamber of one compressor module and the plenum chamber of another compressor module, said cooler module including a series of heat exchanger surface elements, and said housing further including a port through which said cooler module may be withdrawn from said housing as a unit.

15. The apparatus as claimed in claim 14 and further including partitioning means cooperating with each said inlet shroud to define a plenum chamber between said inlet shroud and the outer wall of said housing.

16. The apparatus as claimed in claim 15 and further including a drive motor having a motor shaft, and a bull gear mounted directly on said motor shaft for engaging drive pinions of said compressor assembly nlodules.

Claims

5. A centrifugal gaseous medium compressor system comprising a compressor housing, means partitioning said housing into a plurality of chambers, a gear casing having a common wall with said compressor housing, a bull gear mounted for rotation in said gear casing, a plurality of compressor module support structures mounted directly on said common wall, a corresponding plurality of compressor modules supported in said module suppOrt structures, each said compressor module including a shaft and bearing assembly, an impeller, a pinion gear, a diffuser structure and an inlet shroud supported on said shaft and bearing assembly, each said inlet shroud being supported on an intermediate partition of said housing and each said pinion gear being disposed in said gear casing for driving by said bull gear and an outer wall of said housing opposite said common wall having corresponding ports through which said compressor modules may be removed from said housing as units and a drive motor disposed adjacent said gear casing, said drive motor having a motor shaft extending into said gear casing and said bull gear being mounted directly on said motor shaft.

16. The apparatus as claimed in claim 15 and further including a drive motor having a motor shaft, and a bull gear mounted directly on said motor shaft for engaging drive pinions of said compressor assembly modules.