COMPRESSORS

Abstract: A compressor comprising a rotatable shaft (10), drive means (1) for rotating the shaft and one or more impeller rotor stages (15, 16) mounted on the shaft, wherein the rotatable shaft (10) and rotor stages (15, 16) are hollow and means (22, 23, 12) are provided to allow a coolant fluid to flow axially through them to cool the rotor and compressor in operation if needed. A hollow tie bolt (12) is mounted through the shaft and rotors and the coolant (e.g. water) is flowed through the tie bolt (12).

Description: COMPRESSORS This invention relates to compressors. In particular, it relates to a compressor which provides a supply of compressed air or other working gas which is free of oil or other bearing lubricating material. This type of compressor is desirable when a source of compressed air is used to process food, pharmaceutical or other sensitive material which must not be contaminated by any substance borne with the gas. Attempts to design oil free compressors in the past have met with varying degrees of success but a problem still remains in achieving good cooling of the compressor mechanism, particularly with high speed rotary type compressors. The present invention arose in an attempt to provide an improved oil-free air or gas compressor of the rotary type. According to the present invention there is provided a compressor comprising a rotatable shaft, drive means for rotating the shaft and one or more impeller rotor stages mounted on the shaft, wherein the rotatable shaft and rotor stages are hollow and means are provided to allow a coolant fluid to flow axially through them to cool the motor and compressor if needed in operation. According to the present invention there is further provided a method of cooling the rotor of compressor having a rotatable shaft, drive means for rotating the shaft and one or more impeller rotor stages mounted on the shaft comprises the shaft and impeller stages being hollow, the method comprising causing a coolant fluid to flow axially through the shaft and impeller stages to cool rotor and the impeller in operation if required. The coolant fluid may be water or any other suitable liquid or fluid material. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawing which shows schematically a compressor. The compressor shown in the figure is a two- stage turbo-compressor but may of course have more stages than this. It is usual to have two or more stages in a compressor of this type. The compressor is driven directly by a high speed DC motor, shown as stator 1 and rotor 10, which may be arranged to drive at a speed of, say, 50,000-100,000 rpm, although this figure may vary depending on the use required of the compressor. The motor is controlled by an invertor (not shown) to run directly at the speed required by the compressor and thereby avoid the necessity for a speed- increasing gear box and its associated lubrication system. The motor is used to rotate the rotor 10 in conventional manner. The shaft carries permanent magnets 10a which are acted upon by the stator windings. This is connected to two turbo-compressor stages 2 and 3, the motor being positioned between the two compressor stages. Journal bearings 4, 5 are provided which support the shaft radially. The journal bearings will typically be hydrostatic, gas pressurised, ones with the process air or gas as the supporting medium. Since any compressor takes a finite time to start up the bearings are initially supplied with air or gas from a small accumulator 8 but once the compressor is up to speed then the supply is taken direct from the compressor. Many types of bearing may be used successfully with this arrangement. Although the design of the compressor stages 2, 3 is preferably such that any axial thrust is minimised, it will generally be found necessary to include a thrust bearing in the system. In the embodiment shown, this is in the form of a spiral groove thrust bearing 6, 7 which is used to carry any residual axial load. This spiral groove bearing also preferably operates using the process air or gas of the system. The shaft assembly itself comprises four main hollow components. A central rotor portion 10 lies wholly within the motor stator 1 and carries the permanent magnets 10a as described above. At each end of this central portion is a respective one of two end pieces 9, 11 which provide the bearing journals 4, 5 and also the thrust collar 6a for the thrust bearings 6, 7. The bearings are mounted outboard of the motor and inboard of the impellers, the thrust bearing being on the portion between the motor and the second impeller stage 3. Each of the outer portions is spigotted to the centre portion and respective impellers 15 and 16 are located by spigots onto the outer portions 9 and 11. Thus, an in-line system results in which central portion 10, outer portions 9, 11 and impellers 15 and 16 all rotate together under the action of motor 1. The assembly is clamped together by means of a hollow tie bolt 12 running axially through their common hollow centres. The tie bolt is hollow to allow for cooling water to be passed through it, and thereby through the centre of the shaft, impeller assembly during operation. Heat is inevitably generated in the rotor due to eddy current losses in the magnet, particularly at high speed motor operation and the use of coolant water through the central axis of the shaft-impeller assembly provides an effective way of cooling the system. The water is fed in at a non-rotating input 22 which feeds directly into the (rotating) tie bolt 12. To avoid leakage, two respective chambers 13, 14 are provided, one at the eye of each impeller 15, 16. Each has is own labyrinth seal. Water then exits the system at a non-rotating outlet port 23. During initial start up of the compressor, no water is passed through the shaft which is of course initially cool. At an intermediate speed during acceleration the chambers are pressurised slightly from air/gas compressed by the process. When this pressurisation has been achieved then the water is allowed to flow to begin cooling of the rotor 10 and of the impeller assembly if required. In the embodiment described, two high efficiency centrifugal compressor stages are employed. More than this may be employed in which case more impeller stages will of course be necessary. Each of the stages in the described embodiment comprises a backward sloping impeller 15, 16 followed by a vaned or vaneless diffuser or a pipe diffuser 17, 18 and this is sized to give maximum static pressure recovery before delivery the air into a low loss scroll 19, 20. The design of impellers, diffusers and scrolls is of course well known. The air or gas to be processed is arranged to enter the first compressor stage at a direction perpendicular to the machine's axis through a row of variable inlet guide vanes 21. This feature of the gas entering from a direction other than in-line and with variable guide vanes offers a considerable degree of flow control which is not possible with positive displacement type machines. The air or gas is then acted upon by the impeller 15 and leaves this stage at portion 2a in the figure to enter an intercooler (not shown) . Intercoolers themselves are well known and the intercooler has not been shown for clarity. The air or gas then passes through the intercooler before entering the second impeller stage at 3a. The process ensures a low power consumption and reduces the degree of after cooling which may be required in some applications. The air or gas is enacted upon by the second impeller 16 and allowed the exit the system in its processed form. Preferably the rotor mechanism (rotors, shaft, etc) is entirely or partially of ceramic. This helps to achieve thermal stability and lightness in the machine.

Claims: CLAIMS 1. A compressor comprising a rotatable shaft, drive means for rotating the shaft and one or more impeller rotor stages mounted on the shaft, wherein the rotatable shaft and rotor stages are hollow and means are provided to allow a coolant fluid to flow axially through them. 2. A compressor as claimed in Claim 1, wherein the shaft comprises a plurality of hollow sections. 3. A compressor as claimed in Claim 2, wherein a hollow tie bolt is mounted through the shaft and impellers, the coolant fluid being flowed through the hollow tie bolt. 4. A compressor as claimed in Claim 3, wherein non- rotating coupling means are provided to couple a source of coolant into and out of respective .opposite ends of the tie bolt. 5. A compressor as claimed in any of the preceding claims, wherein the rotors are of ceramic material. 6. A compressor as claimed in any of the preceding claims, wherein journal and/or thrust bearings are provided which are at least partially operated by the process gas or air. 7. A method of cooling a rotor of a compressor having a rotatable shaft, drive means for rotating the shaft and one or more impeller rotor stages mounted on the shaft, the shaft and impeller stages being hollow, the method comprising causing a coolant fluid to flow axially through the shaft and impeller stages to cool the impeller in operation. 8. A method as claimed in Claim 7, wherein a hollow tie bolt is mounted through the shaft and impellers and coolant fluid is caused to flow through the tie bolt. 9. A method as claimed in Claim 7 or Claim 8, wherein coolant fluid is only caused to begin to flow at an intermediate speed during acceleration of the compressor from rest. 10. A compressor substantially as hereinbefore described, with reference to, and as illustrated by, the accompanying drawings. 11. A method of cooling a compressor substantially as hereinbefore described, with reference to the accompanying drawings.

Inventors: GOZDAWA RICHARD JULIUS, GOZDAWA, RICHARD, JULIUS
Applicants: COMPACT RADIAL COMPRESSORS LTD, GOZDAWA RICHARD JULIUS, COMPACT RADIAL COMPRESSORS LTD., GOZDAWA, RICHARD, JULIUS
Application year: 1997
Main classification: F04D29
IPC codes: F04D25, F04D29