EP0883749B1

EP0883749B1 - Compressor

Info

Publication number: EP0883749B1
Application number: EP97902446A
Authority: EP
Inventors: Richard Julius Gozdawa
Original assignee: Corac Group PLC
Current assignee: Corac Group PLC
Priority date: 1996-02-02
Filing date: 1997-01-31
Publication date: 2001-12-05
Anticipated expiration: 2017-01-31
Also published as: DE69708850D1; AU1609197A; GB9602126D0; ATE210246T1; EP0883749A1; DE69708850T2; WO1997028372A1

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

This invention relates to compressors.

There is a demand for compressors which provide 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.

WO-A-95/24563 discloses a compressor in accordance with the preamble of claim 1.

According to a first aspect of the present invention there is provided a compressor comprising a rotatable shaft, drive means for rotating the shaft, at least one impeller rotor stage mounted on the shaft, and a tie bolt mounted through said at least one impeller rotor stage and through at least part of the shaft to hold said at least one impeller rotor stage to the shaft, wherein the rotatable shaft and said at least one impeller rotor stage are hollow, characterised in that the tie bolt has a hollow interior to enable a supply of liquid to be provided to flow axially through the rotatable shaft and said at least one impeller rotor stage, via the hollow interior of the tie bolt, as a coolant liquid.

According to a second aspect of the present invention there is provided a method of cooling a rotor of a compressor having a hollow rotatable shaft, drive means for rotating the shaft, at least one hollow impeller rotor stage mounted on the shaft, and a tie bolt mounted through at least part of the shaft and through said at least one impeller rotor stage to hold said at least one impeller rotor stage to the shaft, said tie bolt having a hollow interior, the method comprising providing a supply of coolant liquid and causing said coolant liquid to flow axially through the shaft and said at least one impeller rotor stage, via said hollow interior of said tie bolt, to cool the impeller.

The coolant fluid may be water or other suitable liquid.

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

A compressor comprising a rotatable shaft, drive means for rotating the shaft, at least one impeller rotor stage (15,16) mounted on the shaft, and a tie bolt (12) mounted through said at least one impeller rotor stage (15,16) and through at least part of the shaft to hold said at least one impeller rotor stage (15,16) to the shaft, wherein the rotatable shaft and said at least one impeller rotor stage (15,16) are hollow, characterised in that the tie bolt (12) has a hollow interior to enable a supply of liquid to be provided to flow axially through the rotatable shaft and said at least one impeller rotor stage (15,16), via the hollow interior of the tie bolt (12), as a coolant liquid.
A compressor as claimed in claim 1, wherein the shaft comprises a plurality of hollow shaft sections.
A compressor as claimed in claim 2, wherein the hollow tie bolt (12) is mounted through the plurality of hollow shaft sections.
A compressor as claimed in any one of the preceding claims, wherein non-rotating coupling means (13,14) are provided to couple a source of coolant liquid into and out of respective opposite ends of the tie bolt (12).
A compressor as claimed in any one of the preceding claims, wherein the rotors are of ceramic material.
A compressor as claimed in any one of the preceding claims, wherein journal and/or thrust bearings (4,5) are provided which are at least partially operated by the process gas or air.
A method of cooling a rotor of a compressor having a hollow rotatable shaft, drive means for rotating the shaft, at least one hollow impeller rotor stage (15,16) mounted on the shaft, and a tie bolt (12) mounted through at least part of the shaft and through said at least one impeller rotor stage to hold said at least one impeller rotor stage (15,16) to the shaft, said tie bolt (12) having a hollow interior, the method comprising providing a supply of coolant liquid and causing said coolant liquid to flow axially through the shaft and said at least one impeller rotor stage (15,16), via said hollow interior of said tie bolt (12), to cool the impeller.
A method as claimed in claim 7, wherein said cooling liquid is water.
A method as claimed in claim 8, wherein said coolant liquid is only caused to begin to flow at an intermediate speed during acceleration of the compressor from rest.