EP0012895A1

EP0012895A1 - Centrifugal vapor compressor and a diffuser control therefor

Info

Publication number: EP0012895A1
Application number: EP79104971A
Authority: EP
Inventors: Phiroze Bandukwalla; Howard W. Kirtland; Gordon L. Mount
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 1978-12-26
Filing date: 1979-12-06
Publication date: 1980-07-09
Also published as: EP0012895B1; JPS5921100U; JPS6218719Y2; DE2966146D1; CA1119140A

Abstract

A diffuser control (26) for controlling vapor flow through a diffuser passage (16) comprising an annular recess (28) defined by a housing (12) of the diffuser passage (16) and in communication therewith, and an annular control ring (30) supported for movement within the recess (28) and the diffuser passage (16) between a throttling position, wherein the control ring (30) throttles vapor flow through the diffuser passage (16), and an open position for permitting a substantially free flow of vapor through the diffuser passage (16). The diffuser control (26) further comprises a plurality of springs (32) supported by the housing (12) for urging the control ring (30) toward the throttling position, a stop (34) for limiting movement of the control ring (30) at the throttling position, a low pressure conduit (38) for connecting the annular recess (28) to a low pressure source, and a seal (42, 44) for retarding vapor flow from a higher pressure side of the control ring (30) to a lower pressure side thereof when the control ring (30) is in the open position. A valve (40) is provided for regulating vapor flow through the low pressure conduit (38) and includes a first position for maintaining a low pressure in the annular recess (28) and a pressure difference across the control ring (30) for maintaining the control ring (30) in the open position, and a second position for maintaining a high pressure in the annular recess (28) wherein the springs (32) maintain the control ring (30) in the throttling position.

Description

This invention relates to centrifugal vapor compressors, and more particularly to a diffuser control for controlling vapor flow through a diffuser passage of a centrifugal vapor compressor
One of the major problems arising in the use of centrifugal vapor compressors for applications where the compressor load varies over a wide range is flow stabilization through the compressor. The compressor inlet, impeller and diffuser passages must be sized to provide for the maximum volumetric flow rate desired. When there is a low volumetric flow rate through such a compressor, the flow becomes unstable. As the vilumetric flow rate is decreased from a stable range, a range of slightly unstable flow is entered. In this range, there appears to be a partial reversal of flow in the diffuser passage, creating noises and lowering the compressor efficiency. Below this range, the compressor enters what is known as surge, wherein there are periodic complete flow reversals in the diffuser passage, destroying the efficiency of the machine and endangering the integrity of the machine elements. Since a wide rage of volumetric flow rates is desirable in many compressor applications, numerous modifications have been suggested to improve flow stability at low volumetric flow rates.
One of the most accepted and successful modifications has been the addition of guide vanes in the inlet of the compressor to vary the flow direction and quantity of entering vapor. Another widely known modification has been to vary diffuser configuration in response to the load on the compressor. Commonly, this is done by means of a diffuser coatrol ring which moved laterally across the diffuser passage to throttle vapor flow therethrough. Prior art variable diffuser control rings have been generally controlled by a mechanism arranged to locate and hold the control ring at any position between a full opened and a full closed position. Such control mechanisms are typically relatively expensive, often involving fairly complex mechanical and/or pneumatic components. Further,' because of the complex components, the manufacture and installation of diffuser ring control mechanisms are often difficult and time consuming tasks requiring expensive skilled manual labor. While continuously variable diffuser control rings often provide excellent results, it has been learned that very satisfactory results can be achieved with a diffuser control ring which has a limited number of discrete, spaced throttling positions. While obtaining these very satisfactory results, a discretely variable diffuser control, in accordance with the present invention, is, at the same time, much simpler than prior art diffuser controls. This simplicity facilitates and reduces the cost of construction, installation, and maintenance of the diffuser control and improves the reliability thereof.
More specifically, these and other advantages are attained with a diffuser control for controlling vapor flow through a diffuser passage comprising an annular recess defined by a housing of the diffuser passage and in communication therewith, and diffuser restriction means supported for movement within the recess and the diffuser passage between a throttling position, wherein the restriction means throttles vapor flow through the diffuser passage, and an open position for permitting a substantially free flow of vapor through the diffuser passage. The diffuser control further comprises urging means supported by the housing for urging the restriction means toward the throttling position, first stop means for limiting movement of the restriction means at the throttling position, a low pressure conduit for connecting the annular recess to a low pressure source, and sealing means for retarding vapor flow from a higher pressure side of the diffuser restriction means to a lower pressure side thereof when the restriction means is in the open position. Valve means is provided for regulating vapor flow through the low pressure conduit and includes a first position for maintaining a low pressure in the annular recess and a pressure difference across the diffuser restriction means for maintaining the restriction means in the open position, and a second position for maintaining a high pressure in the annular recess wherein the urging means maintains the restriction means in the throttling position.
This invention will now be described by way of example, with reference to the accompanying drawings in which:

Figure 1 is an elevational view, partly in section of a portica of a centrifugal compressor having a two position diffuser control in accordance with one embodiment of the present invention;
Figure 2 is similar to Figure 1 but with the compressor he is three position diffuser control in accordance with a second embodiment of the present iivention, the diftuser control thereof being shown in an open position; and
Figures 3 and 4 show portions of the compressor shown in Fig. with Figure 3 showing the diffuser control ring in an intermediate throttling position and Figure 4 showing the diffuser control in a full throttling position.

Referring to Figure 1, there is shown portions of vapor compressor. 10 constructed according to a first embodiment of the present invention. Generally, compressor 10 is of the well-known centrifugal type wherein vapor to be compressed is induced to flow in an axial direction into a vaned impeller connected to a suitable driver for imparting rotary motion thereto. As the vapor is compressed during its travel through the compressor, it is directed radially from the impeller to a diffuser passage communicating with the tip of the impeller. More particularly, compressor 10 includes housing 12 with inlet passage 14 and diffuser passage 16 formed therein. Only portions of housing 12 are shown, it being understood that such a construction is conventional in equipment of the kind under consideration. Impeller 18 affixed to shaft 20 by nut 22 is provided in housing 12 between inlet passage 14 and diffuser passage 16. Inlet guide vanes 24 journaled for rotation in housing 12 are positioned about inlet passage 14 to control the direction and quantity of vapor flow therethrough. Diffuser control, referenced generally as 26, is provided for controlling vapor flow through diffuser passage 16.
Diffuser control 26 comprises, generally, annular recess 28 defined by housing 12 and in communication with diffuser passage 16, and diffuser restriction means such as annular control ring 30. Control ring 30 is supported for movement within annular recess 28 and diffuser passage 16 between an open position, shown in full lines in Figure 1, and a throttling position, shown in broken lines in Figure 1. In the throttling position, control ring 30 throttles vapor flow through diffuser passage 16; and, preferably, in the open position, the control ring allows an unrestricted flow of vapor through the diffuser passage. Urging means 32 is provided for urging control ring 30 toward the throttling position. Preferably, the urging means includes resilient means such as a plurality of springs 32 positioned within recess 28. Springs 32 are preferably equally spaced about the circumference of control ring 30, forming a ring of springs with a radius equal to the radius of the control ring. Stop means 34 limits movement of the control ring at the throttling position for preventing the control ring from completely restricting vapor flow through diffuser passage 16. Preferably stop means 34 includes a surface of housing 12. More specifically, as control ring 30 moves forward, from left to right as viewed in Figure 1, and reaches the throttling position, flange 36 of the control ring abuttingly engages surface 34 and this abutting contact prevents further forward movement of the control ring.
Diffuser control 26 further comprises conduit means 37 and valve means 39. Conduit means 37 connects annular recess 28 to a pressure source having a pressure less than the pressure in diffuser passage 16, and valve means 39 regulates vapor flow through the conduit means. In the embodiment illustrated in Figure 1, conduit means 37 includes low pressure conduit 38 which connects annular recess 28 to, for example, an inlet line of compressor 10, and valve means 39 includes valve 40 located in the low pressure conduit. Moreover valve 40 includes a first position for maintaining control ring 30 in the open position and a second position for maintaining the control ring in the throttling position. Preferably, when valve 40 is in the first position, conduit 38 is fully open, maintaining a low pressure in annular recess 28 and a pressure difference across control ring 30 for maintaining the control ring in the open position. That is, the low pressure source is chosen so that the forces on control ring 30 resulting from the pressure differential thereacross, explained in greater detail below, when conduit 38 is open are greater than the forces on the control ring due to springs 32, wherein the vapor pressure forces maintain the control ring in the open position. Also, when valve 40 is in the second position, preferably low pressure conduit 38 is fully closed thereby, and vapor passes into annular recess 28 from diffuser passage 16 for maintaining a high pressure in the annular recess wherein springs 32 maintain control ring 30 in the throttling position.
Valve 40 is operated by positioning means 41, which may be of any appropriate type such as electric, pneumatic, or hydraulic positioners. Positioning means 41 is responsive to an operating condition of compressor 10 or its associated equipment which is indicative of the load on the compressor. Thus, if compressor 10 were to be used in a refrigeration machine, then the positioning means could be responsive to the temperature of the chilled water leaving the machine, this temperature being related to the quantity of refrigerant being lifted from the low side to the high side of the machine by the compressor. If compressor 10 were being used to compress air, then the positioning means could be responsive to ambient temperature, since this is an indication of the air density and therefore the quantity of air being compressed. Preferably, though, the position of valve 40 is determined by the position of guide vanes 24, sensed by sensing means 43. Sensing means 43 may include, for example, a limit switch (not shown) which is actuated by a guide vane or a control linkage thereof in response to movement of the guide vane to a predetermined position, indicative of a restricted flow of vapor through compressor 10.
Compressor 10, with diffuser control 26 described above, functions as follows. With valve 40 in the first position wherein conduit 38 is open, annular recess 28 is in communication, via conduit 38, with the low pressure source, and the pressure in the recess is approximately equal to that of the low pressure source. Vapor pressure in annular recess 28 is less than vapor pressure in diffuser passage 16 and a pressure difference exists across control ring 30, urging the control ring rearward, to the left as viewed in Figure 1. As mentioned above, the low pressure source is chosen so that the forces on control ring 30 resulting from the pressure differential thereacross when conduit 38 is open are greater than the forces on the control ring due to springs 32. Thus, control ring 30 is moved to and maintained in the open position, allowing maximum vapor flow through diffuser passage 16.
Second stop means 42 may be provided to limit rearward movement of control ring 30, and preferably the second stop means includes a surface of housing 12. More specifically, as control ring 30 moves rearward and reaches the open position, flange 44 of the control ring abuttingly engages surface 42 and this abutting engagement prevents further rearward movement of the control ring. With this arrangement, flange 44 and surface 42, when abuttingly engaged as described above, also function as sealing means for retarding vapor flow from the higher pressure, right side of control ring 30 to the lower pressure, left side thereof.
To move control ring 30 to the throttling position, valve 40 is moved to the second position, preferably wherein conduit 38 is fully closed by the valve. This may be done, as mentioned above, in response to any one of a number of conditions indicative of a restricted flow rate through compressor 10 such as the position of guide vanes 24. With conduit 38 closed, annular recess 28 is isolated from the low pressure source. Vapor passes into annular recess 28 from diffuser passage 16 through the interface between surface 42 and flange 44, equalizing the pressure in the annular recess and the diffuser passage. The vapor pressure forces on control ring 30 equalize. Forces from springs 32 dominate and push control ring 30 into diffuser passage 16 to the throttling position, wherein abutting contact between flange 36 and surface 34 prevents further forward movement of the control ring. In the throttling position, control ring 30 throttles refrigerant passing into diffuser passage 16, maintaining stable vapor flow therethrough at the relatively lower flow rate.
Referring now to Figures 2 through 4, there is illustrated a second embodiment of the present invention. As will be appreciated, the embodiments shown in Figures 1 and 2 through 4 have many corresponding parts, and generally corresponding parts are given like reference numerals. Diffuser control ring 30 of the embodiment shown in Figures 2, 3 and 4 includes three stable positions; an open position shown in Figure 2, an intermediate throttling position shown in Figure 3, and a full throttling position shown in Figure 4. In order to obtain this increased flexibility, the embodiment shown in Figures 2 through 4 includes, in addition to those elements discussed above with reference to Figure 1, second urging means 50 for urging control ring 30 toward the intermediate throttling position. Also, with the second embodiment of the present invention, conduit means 37 preferably includes low pressure conduit 38 and intermediate pressure conduit 58; and valve means 39 includes valves 40 and 62, with the former valve located in conduit 38 and the latter valve located in conduit 58.
Preferably, the second urging means includes second resilient means such as a plurality of second springs 50 positioned within recess 28. Second springs 50 are preferably equally spaced about the circumference of control ring 30, forming a second ring of springs with a radius also equal to that of the control ring. In the embodiment depicted in Figures 2 through 4, first and second springs 32 and 50 have a generally cylindrical shape, with each second spring 50 concentrically encircling a first spring 32. Force limiting means, referenced generally as 52, limits the forco exerted by second springs 50 on control ring 30 at the intermediate throttling position. In the illustrated embodiment, force limiting means 52 includes movable rings 54 and stop means such as stationary rings 56 which are secured to housing 12 and extend into annular recess 28. Rings 54 are slidably supported within recess 28 between second springs 50 and fingers 57 of control ring 30. Second springs 50 abuttingly engage rings 54, and this abutting contact maintains second springs 50 to the left of the rings. First springs 32, however, are slidable within ₃ rings 42, and rings 42 do not interfere with movement of first springs 32.
As control ring 30 moves between the open and intermediate throttling positions, rings 42 abuttingly engage and transmit force between second springs 50 and the control ring. When control ring 30 is in the intermediate throttling position, as shown in Figure 3, movable rings 54 abuttingly contact stationary rings 56. This contact prevents further movement of movable rings 54 to the right as viewed in the drawings. Control ring 30, though, is free to move further to the right and is urged so by first springs 32. As control ring 30 moves forward between the intermedite throttling and full throttling positions, movable rings 54 and second springs 50 become spaced from the control ring. When this occurs, force is not transmitted between second springs 50 and control ring 30.
Intermediate pressure conduit 58 connects annular recess 28 with an intermediate pressure source. For example, in case compressor 10 is a multi-stage compressor, the intermediate pressure source may be an intermediate stage thereof. In addition to the foregoing, in the embodiment depicted in Figures 2 through 4, valves 40 and 62 cooperatively define a first position for maintaining a low pressure in annular recess 28, a second position for maintaining an intermediate pressure therein, and a third position for maintaining a high pressure in the annular recess. The low pressure is sufficiently lower than the pressure in diffuser passage 16 so that, when the low pressure is maintained in annular recess 28, the pressure force on control ring 30 due to the pressure difference between the annular recess and the diffuser passage is greater than the forces on the control ring due to springs 32 and 50, and this first pressure force maintains the control ring in the open position. When the above-mentioned intermediate pressure is maintained in annular recess 28, the pressure force on control ring 30 is less than the combined forces on the control ring due to 32 and 50 but more than the forces on the control ring due solely to springs 32. This second pressure force maintains control ring 30 in the intermediate position.
Further, when valves 40 and 62 are in the third position and the high pressure is maintained in annular recess 28, the force on control ring 30 due to the pressure difference, if any, between the annular recess and diffuser passage 16 is less than the force on the control ring due to first springs 32, wherein springs 32 maintain the control ring in the full throttling position. Preferably, when valves 40 and 62 are in the first position, low pressure conduit 38 is open and intermediate pressure conduit 58 is closed by valve 62. When valves 40 and 62 are in the second position, conduit 38 is closed by valve 40 but conduit 58 is open, and when valves 40 and 62 are in the third position, the valves close conduits 38 and 58 respectively. Further, second valve 62 may be operated by second positioning means 66 which, like first positioning means 41, may be of any appropriate type such as electric, pneumatic, or hydraulic positioners. Also like first positioning means 41, second positioning means 66 is responsive to an operating condition of compressor 10 or its associated equipment which is indicative of the load on the compressor. Preferably, positioning means 66 is responsive to the position of guide vanes 24 as sensed by sensor 43.
The embodiment of compressor 10 illustrated in Figures 2 through 4 functions as follows. With valves 40 and 62 in the first position wherein low pressure conduit 38 is open and intermediate pressure conduit 58 is closed by valve 62, annular recess 28 is in communication with the low pressure source via conduit 38, and the pressure in the recess is approximately equal to that of the low pressure source. Vapor pressure in annular recess 28 is less than vapor pressure in diffuser passage 16 and a pressure difference exists across control ring 30, urging the control ring rearward, to the left as viewed in the drawings. The low pressure source is chosen so that the forces on control ring 30 resulting from the pressure differential thereacross when valve 40 is open and valve 62 is closed are greater than the forces on the control ring due to springs 32 and 50. Thus, the vapor pressure forces dominate and control ring 30 is moved to and maintained in the open position, allowing maximum vapor flow through diffuser passage 16.
Second stop means 64 may be provided to limit rearward movement of control ring 30, and preferably the second stop means includes a surface of stationary ring 56. More specifically, as control ring 30 moves rearward and reaches the open position, flange 70 of the control ring abuttingly engages surface 64 and this abutting engagement prevents further rearward movement of the control ring. With this arrangement, flange 70 and surface 64, when abuttingly engaged as described above, also function as sealing means for retarding vapor flow from the higher pressure, right side of control ring 30 to the lower pressure, left side thereof.
To move control ring 30 to the intermediate throttling position shown in Figure 3, low pressure conduit 38 is closed by valve 4C and intermediate pressure conduit 58 is opened. This may be done. for example, in response to guide vanes 24 moving to a predetermined position wherein vapor flow through compressor 10 is restricted. Annular recess 28 is in communication with the intermediate pressure chamber via conduit 58. The pressure in recess 28 is increased to approximately that of the intermediate pressure source, and the pressure difference across control cing 30 is decreased. The intermediate pressure source is chosen so that the vapor pressure forces on control ring 30 when valve 40 is closed and valve 62 is open are less than the combined forces on the control ring due to springs 32 and 50. Thus, control ring 30 moves from the open position toward the intermediate throttling position.
When control ring 30 reaches the intermediate throttling position shown in Figure 3, movable ring 54 abuttingly engages stationary ring 56, preventing further forward movement of the movable: ring and second spring 50. Thus, second springs 50 do not urge control ring 30 further to the right. First springs 32 continue to urge control ring 30 further to the right. However, the intermediate pressure source is chosen so that the vapor pressure forces on control ring 30 resulting from the pressure differential thereacross when valve 40 is closed and valve 62 is open are greater than the forces on the control ring due solely to second springs 50. That is, the force due to first springs 32 is insufficient to move control ring 30 to the right, second springs 50 are prevented by rings 54 and 56 from urging the control ring to the right, and the pressure differential across the control ring is insufficient to move the control ring to the left against the combined forces of springs 32 and 50. As a consequence, control ring 30 is maintained in the intermediate throttling position. In this position, control ring 30 throttles the vapor flowing through diffuser passage 16, providing a stable vapor flow therethrough at the reduced flow rate. Moreover, when control ring 30 is in the intermediate throttling position, movable ring 54 abuttingly engages both stationary ring 56 and fingers 57 of the control ring, and rings 54 and 56 and fingers 57 function as sealing means for retarding vapor flow from the higher pressure, right side of the control ring to the lower pressure, left side thereof.
To move control ring 30 to the full throttling position, shown in Figure 4, low and intermediate pressure conduits 38 and 58 are closed by valves 40 and 62 respectively. This may be done, as an example, in response to guide vanes 24 moving to a second predetermined position further restricting vapor flow through compressor 10. Vapor passes into annular recess 28 from diffuser passage 16 via the interface between fingers 57 and movable rings 54, and vapor pressures in the annular recess and the diffuser passage equalize, equalizing the vapor pressure forces on control ring 30. Forces from springs 32 dominate and push control ring 30 into diffuser passage.16 to the full throttling position, wherein abutting contact between flange 36 and surface 34 prevents further forward movement of the control ring. Control ring 30 further throttles vapor passing through diffuser passage 16, maintaining stable vapor flow therethrough even at the further restricted rate of flow through compressor 10.
The above-described preferred embodiments of the present invention, particularly the two positions of control ring 30 and three positions of control ring 48 provide stable vapor flow through diffuser passage 16 over a wide range of compressor loads. At the same time, the illustrated diffuser controls 26, since they do not require the complex mechanical or pneumatic components of prior art diffuser ring controls, are relatively simple to construct and install. This substantially reduces the cost and improves the reliability of the diffuser controls, and facilitates manufacture and installation thereof. Furthermore, the simplicity of diffuser controls 26, particularly the absence of any complicated mechanical linking arrangement connecting the control with, for example, guide vanes 24, makes the diffuser controls well suited for use on a retrofit basis.
While it is apparent that the invention herein disclosed is well calculated to fulfill the objects above stated, it will be appreciated that numerous modifications and embodiments may be devised by those skilled in the art, and it is intended that the appended claims cover all such modifications and embodiments as fall within the true spirit and scope of the present invention.

Claims

1. A diffuser control (26) for controlling vapor flow through a diffuser passage (16) comprising an annular recess (28) defined by a housing (12) of the diffuser passage (16) and in communication therewith; diffuser restriction means (30) supported for movement within the recess (28) and the diffuser passage (16) between a throttling position, wherein the restriction means throttles vapor flow through the diffuser passage, and an open position for permitting a substantially free flow of vapor through the diffuser passage; characterized by first urging means (32) supported by the housing (12) for urging the restriction means (30) toward the throttling position; first stop means (34) for limiting movement of the restriction means (30) at the throttling position; conduit means (37) for connecting the annular recess (28) to a low pressure source; first sealing means (42, 44) for retarding vapor flow from a higher pressure side of the diffuser restriction means (30) to a lower pressure side thereof when the restriction means is in the open position; and valve means (39) for regulating vapor flow through the conduit means (37) and including a first position for maintaining a low pressure in the annular recess (28) and a first pressure difference across the diffuser restriction means (30) for maintaining the restriction means in the open position, and a second position for maintaining a high pressure in the annular recess wherein the first urging means (32) maintains the restriction means in the throttling position.

2. A centrifugal vapor compressor (10) comprising a housing (12) defining an inlet passage (14) and a diffuser passage (16); an impeller (18) rotatably mounted in the housing (12) between the inlet (14) and diffuser (16) thereof; an annular recess (28) defined by the housing (12) and in communication with the diffuser (16) thereof; diffuser restriction means (30) supported for movement within the annular recess (28) and the diffuser passage (16) between a throttling position, wherein the restriction means throttles vapor flow through the diffuser passage, and an open position for permitting a substantially free flow of vapor through the diffuser passage; first urging means (32) supported by the housing (12) for urging the restriction means (30) toward the throttling position; first stop means (34) for limiting movement of the restriction means (30) at the throttling position; conduit means (37) for connecting the annular recess (28) to a low pressure source (14); first sealing means (42, 44) for retarding vapor flow from a higher pressure side of the diffuser restriction means (30) to a lower pressure side thereof when the restriction means is in the open position; and valve means (39) for regulating the vapor flow through the conduit means (37) including a first position for maintaining a low pressure in the annular recess (28) and a pressure difference across the diffuser restriction means (30) for maintaining the restriction means in the open position, and a second position for maintaining a high pressure in the annular recess wherein the first urging means (32) maintains the restriction means in the throttling position.

3. The invention as defined by claims I or 2 further including second stop means (42) for limiting movement of the restriction means (30) at the open position.

4. The invention as defined by claim 3 wherein when the valve means (39) is in the first position, the conduit means (37) is open; and when the valve means (39) is in the second position, th- conduit means (37) is closed thereby.

5. The invention as defined by claim 4 wherein the first sealing means (42, 44) includes the second stop means (42) and a surface (44) of the diffuser restriction means (30).

6. The invention as defined by claim 5 wherein the first and second stop means include, respectively, first (34) and second (42) surfaces of the housing.

7. The invention as defined by claim 6 wherein the first urging means includes resilient means (32) positioned within the annular recess (28).

5. The invention as defined by claim 7 wherein the resilient means includes a plurality of springs (32)

3. A diffuser control (26) as defined by claim 1 further comprising second urging means (50) supported by the housing (12) for urging the diffuser restriction means (30) toward an intermediate throttling position located between the open and full throttling postions, force limiting means (52) for limiting the force exerted by the second urging means (50) on the diffuser restriction means (30) at the intermediate throtalign position; and second sealing means (54, 56) for retarding the vapor flow from the higher pressure side of the diffuser restriction means (30) to the lower pressure side thereof when the restriction means is in the intermediate throttling position; and wherein the valve means (39) further includes a third position for maintaining an intermediate pressure in the annular recess (28) and a second pressure difference across the diffuser restriction means (30) for maintaining the restriction means in the intermediate throttling position.

10. A centrifugal vapor compressor (10) as defined by claim 2 further comprising second urging means (50) supported by the housing (12) for urging the diffuser restriction means (30) toward an intermediate throttling position located between the open and full throttling positions; force limiting means (52) for limiting the force exerted by the second urging means (50) on the diffuser restriction means (30) at the intermediate throttling position; and second sealing means (54, 56) for retarding vapor flow from the higher pressure side of the diffuser restriction means (30) to the lower pressure side thereof when the restriction means is in the intermediate throttling position; and wherein the valve means (39) further includes a third position for maintaining an intermediate pressure in the annular recess (28) and a second pressure difference across the diffuser restriction means (30) for maintaining the restriction means in the intermediate throttling position.

11. The invention as defined by claims 9 or 10 wherein the conduit means includes a low pressure conduit (38) for connecting the annular recess (28) to the low pressure source; and an intermediate pressure conduit (58) for connecting the annular recess to an intermediate pressure source.

12. The invention as defined by claim 11 further including second stop means (64) for limiting movement of the restriction means (48) at the open position.

13. The invention as defined by claim 12 wherein when the valve means (39) is in the first position, the intermediate pressure conduit (58) is closed thereby and the low pressure conduit (38) is open; when the valve means (39) is in the second position, the low (38) and intermediate pressure (58) conduits are closed thereby; and when the valve means (39) is in the third position, the low pressure conduit (38) is closed thereby and the intermediate pressure conduit (58) is open.

14. The invention as defined by claim 13 wherein the force limiting means (52) includes a movable communicating member (54) for transmitting force between the second urging means (50) and the diffuser restriction means (30) as the diffuser restriction means moves between the open and intermediate throttling positions; and third stop means (56) for limiting movement of the communicating member (54) when the diffuser restriction means (30 i.s at the intermediate throttling position wherein, as the diffuser restriction means moves between the intermediate throttling and full throttling positions, the communicating member (54) is spaced from the diffuser restriction means for preventing forie from being transmitted between the second urging means (52) and the diffuser restriction means.

15. The invention as defined by claim 14 wherein the first sealing means includes the second stop means (64) and a surface (70) nf the diffuser restriction means (30).

16. The invention as defined by claim 15 wherein the first urging means includes first resilient means (32) positioned within the annular recess (28); and the second urging means includes second resilient means (50) positioned within the annular recess (28).

17. The invention as defined by claim 16 wherein the communicating member includes a ring (54) slidably supported within the annular recess (28) between the second resilient means (50) and the diffuser restriction means (48); and the third stop means includes a ring (56) secured to the housing (12) and extending into the annular recess (28).

18. The invention as defined by claim 17 wherein the first resilient means includes a plurality of first springs (32); and the second resilient means includes a plurality of second springs (50) adjacent to and interposed between first springs (32).

19. The invention as defined by claims 2 or 10 wherein the low pressure source is an inlet line to the compressor (10).