GB2422636A

GB2422636A - Vertical shaft gas compressor

Info

Publication number: GB2422636A
Application number: GB0608643A
Authority: GB
Inventors: Albert William Crook
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-03-28
Filing date: 2003-03-28
Publication date: 2006-08-02
Anticipated expiration: 2023-03-28
Also published as: GB2422636B; GB2399863A; GB0608643D0; GB2399863B; GB0307192D0

Abstract

A vertical axis gas compressor comprises a rotor arrangement which includes a vertical axis shaft 1 with an impeller 5 and bearing 3 mounted thereto, the compressor further comprising a stationary bearing arrangement 10 having a seat, the rotor arrangement being axially insertable into the compressor as an integral unit so as to seat the rotor bearing 3 onto the stationary bearing seat. The rotor may include an armature winding 2 of an electric motor mounted on the shaft 1. The rotor and stator bearings 3, 10 may have complimentary conical surfaces and may be separated by gas supplied by the compressor.

Description

* 2422636 Gas Compressor The present invention relates to a gas

compressor.

According to a first aspect, the present invention provides a gas compressor having a rotor shaft orientated in a substantially upright position.

Beneficially the rotor shaft is oriented substantially vertically. The rotor advantageously carries a compressor impeller on an upper portion of the shaft.

The compressor is preferably a radial flow compressor, beneficially an axial inflow compressor, the gas being directed to radial outflow via an impeller rotor stage carried by the rotor shaft.

It is preferred that a bearing arrangement is provided for supporting the rotor shaft, the bearing arrangement having a rotary bearing element mounted for rotation with the rotor shaft and a stationary bearing element cooperating with the rotary bearing element for supporting the shaft.

Preferably the rotary and stationary elements have correspondingly adjacent substantially frustroconical bearing surfaces. Preferably the bearing arrangement acts to take up both thrust (shaft axial) and journal (shaft radial) forces. The bearing arrangement is beneficiallyprovided in the region of an upper portion of the rotor shaft, preferably directly to the rear of the rotary impeller stage (or stages). The rotary bearing element preferably effectively sits in a seat or cradle defined by the stationary bearing element. The bearing surfaces of the arrangement act as the primary gas seal on the shaft and there is no requirement for a separate shaft seal.

The compressor therefore, in a refined form, comprises a vertical axis compressor with, in line from top to bottom, one or more compressor impeller stages carried by the rotor, and typically an armature of a high frequency electric motor to drive the rotor shaft. The rotor is supported at its upper end by the seated (conical) bearing arrangement acting as primary thrust bearing, journal bearing and seal for the compressor (limiting the leakage of compressed gas). A lower end journal bearing is typically provided at the lowermost portion of the compressor. This journal bearing is required to take very minimal bearing load.

It is preferred that the bearing arrangement is provided with a gas supply to the bearing surfaces to effect separation of the bearing surfaces for operation. As is well known in the art a compressor delivers gas at an elevated temperature. Typically the gas is passed through an after cooler and then to a receiver from which cooled gas at elevated pressure is drawn for whatever process it is to serve. Beneficially the gas supply to the bearing arrangement comprises a take-off downstream of the after cooler that may be from the receiver. In the event that the compressor delivers gas at elevated pressure to another compressor, the after cooler' then becomes and inter cooler'. Beneficially, the gas supply to the bearing arrangement comprises a take-off from the compressor compressed gas outlet. Preferably the bearing surface of the stationary bearing element is provided with gas supply ports or orifices to supply separating gas at pressure to the interface between the bearing surfaces of the bearing. It is preferred that three or more gas supply ports or orifices are provided, equally spaced about the periphery of the bearing. Advantageously, the compressor is provided with an operation control system ensuring that compressor start up is only effected when sufficient gas separation exists at the bearing surfaces interface. Oil lubrication for the compressor bearing arrangement is therefore not required making the compressor of the invention oil-free and therefore suitable for use in situations where oil-free running is important (food hygiene situations, clean-room environments and the like).

It is preferred that one or both of the bearing surfaces of the bearing arrangement comprise a friction andlor wear reducing material such as graphite. Beneficially, although there is separating gas supply to the bearing surface interface and the surfaces are generally smooth (beneficially frustroconical substantially smooth surfaces), the surfaces may include local formations arranged to have an effect of making the bearing self-generating when the rotor shaft rotates on its axis. Undulations machined in the surface of one or both bearing surfaces can have this effect.

Beneficially, the compressor is closed by a removable end casing element, preferably permitting the rotor shaft and compressor impeller to be mounted or de-mounted as an integral unit into and from the operating position.

According to a second aspect, the invention provides a method of assembling a gas compressor comprising an upright orientated rotor shaft, a bearing element and a compressor impeller mounted to the shaft above the bearing element and proximate an upper end of the rotor shaft, the method comprising advancing in the axial direction, the rotor shaft with the compressor impeller and bearing element mounted to the rotor shaft, so as to seat the bearing element in a seat formed by a stationary bearing element of the compressor.

Preferably, the rotor shaft with the compressor impeller and bearing element mounted to the rotor shaft, is lowered into the compressor casing so as to seat the bearing element in a seat formed by the stationary bearing element of the compressor.

Compressors according to the invention maybe used in combination (for example in series gas connection) with alternative compressors or compressors according to the invention. The pressurised gas output from a first compressor is directed to comprise the gas intake of a second connected compressor. Intercooling of gas may be provided between the connected compressors.

Preferred features and advantages provided by the compressor arrangement according to the invention will become apparent. The invention will now be further described, by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic sectional view of a compressor according to the invention having its rotor shaft axis orientated vertically; Figure 2 is a graphical representation of separation of the bearing elements against thrust force acting on the rotor shaft; and Figure 3 is a schematic flow diagram of high pressure and low pressure compressors according to the invention connected gas flow-wise in series.

Referring to the drawings, and initially to Figure 1, there is shown a vertical axis, oil-free gas compressor having a rotor shaft (1) with the permanent magnets of a motor armature (2) mounted to the shaft (1). The rotor shaft (1) is hung from a conical bearing (3) and is stabilised by a tail bearing (4). A single stage centrifugal impeller (5) is mounted at an upper end of the rotor shaft (1).

A stator (6) of the motor with its end windings is carried in the outer case (7) of the compressor that in turn carries a lower component part (8) of the diffuser and volute (9). A lower casing part (8) carries the stationary element (10) of the conical bearing that is fixed in its recess by the internal flange (11) of the casing (7). The upper part of the volute and the casing of the impeller (12) is mounted to lower casing part (8). Upper casing (12) carries the intake (13) with nose (14) that acts also as an emergency restraint upon the impeller, preventing the impeller from rising out of its seat in the conical bearing arrangement (10). The absence or removal of the upper casing (12) with or without intake (13) and nose (14) attached, permits the rotor assembly to be dropped into position, cradled in the frustroconical bearing surface of stationery bearing element (10), or to be lifted out, complete with the impeller (5) of the compressor mounted to the rotor shaft (1).

The conical bearing arrangement (3, 10) is effectively a gas bearing fed with high pressure gas from the receiver of the compressor via the inlet duct (15), gallery (16) and the three or more equally spaced orifices, one ofwhich is shown (17). Each orifice may feed in to its individual distribution channel (not shown) that spreads its supply of gas of enhanced pressure in an arcuate manner around the bearing. To provide for the individuality of the channels, in the instance of three orifices, each distribution channel may by way of example be limited to 1000 arcuate extent. An annular gap (18) exists between the perimeter of the impeller (5) and casing (8), ensuring that the upper end of the bearing is subject to the static pressure at the exhaust of the impeller (5). This static pressure is approximately one half of the stagnation pressure at the exhaust, and consequently, after diffusion of the flow, about one half of the static pressure in volute and receiver. The flow of high-pressure gas from the orifices separates the surfaces of the conical bearing whether the rotor is stationary or rotating. The flow escapes partly upwards to the annular gap (18), and partly downwards to the ambient pressure within the casing. In contrast with a gas bearing that depends upon motion for its effect - a self- generating gas bearing, the conical bearing as described above is an aero- static bearing. As will be described later, the surfaces of the conical aero-static bearing (3, 10) require a minimum pressure of supply for its bearing surfaces to be separated. The starting and stopping of the compressor is safeguarded in that respect when the receiver has a sufficient pressure, but initially an auxiliary supply of high-pressure gas is required. To make the conical bearing tolerant of accidental solid- solid contacts the stationary part of the bearing may be machined from a block of graphite. The block may also be machined with a slight undulation of the generator of its conical surface to produce three or more peaks and troughs to produce a self- generating action.

The tail bearing (4) might be an aero-static bearing, or because its duty is so light, a self- generating bearing of a kind known in the art such as a foil bearing.

Some results from a study of a particular design will now be stated. The design comprises a low-pressure (LP) compressor, and a second highpressure (HP) compressor. The arrangements is shown schematically in Figure 3. This illustrates the flows of air in g/s for the combination of LP and HP compressors. The LP compressor compresses air at atmospheric pressure to 3.7 bara. The HP compressor compresses the output from the LP compressor to 9.4 bara. The mass flow is 0.3kg/s at a speed of 72500 rpm.

With reference to Figure 1, the downward force acting upon the rotor shaft (1) of the vertical axis compressor are: the force arising from the static pressure over the upper face of the impeller (5) the force produced by the rate in the change in momentum of the flow as it changes in direction through impeller (5) from axial at inlet to radial at exhaust the weight of the rotor assembly the static pressure, slightly sub-atmospheric acting over the area of the root circle of the blades of the impeller (5) at entry.

The vertical thrust of the conical bearing arrangement (3, 10), together with a minor contribution from ambient pressure acting over the area of the minor circle ofthe bearing have to balance the sum of the downward forces. In the instance of the LP compressor the total downward force is 1600 N. The balancing of the forces is illustrated in Figure 2 in which the vertically acting force produced by a conical bearing fed with air via three orifices of 2.4mm throat diameter, is plotted against the thickness of the air film that separates the opposing conical surfaces. The balance point gives a film thickness of 541.tm.

The film thickness of the HP compressor is 44gm from a similar consideration.

Figure 2 demonstrates that a separation of the opposed surfaces of the conical bearing arrangement (3, 10 of Figure 1) of the LP compressor obtains that does not impose too arduous a requirement upon the accuracy of machining of the cones, nor upon the surface finish of the surfaces. The same considerations are true also for the conical bearing of the HP compressor.

The mass flows of Figure 3 show that the separations in the conical bearings of both the LP and the HP compressor are obtained at reasonable expenditures of compressed air. For instance, the conical bearing of the LP compressor draws 8.83g/s of air from upstream of the inter-cooler but a part of that flow is returned (but without the velocity head of the main flow) to the exhaust of the impeller.

The slightly greater mass flow shown between volute and inter-cooler than the mass flow at intake arises from the re-circulation noted above.

Figure 3 shows that the loss in mass flow from the conical bearings is approximately 5 to 6% of the mass flow at intake. Therefore the conical bearings do impose an affordable drain of the compressed air. The sizing of the orifices of the conical bearings is a compromise. The greater their size the greater will be the film thickness, but also the greater the consumption of compressed air.

The small mass flows, without velocity head, fed back into the volutes will reduce the final pressure in the receiver insignificantly.

in another embodiment of the invention an axial flow compressor stage may precede the radial flow impeller. I0

The invention in a preferred embodiment provides a vertical axis, oilfree gas compressor with a running line that from top to bottom comprises one or more compressor stages carried by the rotor and armature of a highfrequency electric motor. The rotor is supported at its upper end by a conical bearing that acts both as a thrust bearing, ajoumal bearing and as a seal that limits the leakage of compressed gas, and at its lower end by a journal bearing. The advantages of the invention in comparison with gas compressors that have horizontal axes are: a) A critical speed in the first mode that is very much higher and further removed from running speed. This comes about because the span between bearings of the vertical axis compressor is less than the span between bearings of a horizontal axis compressor. Also because the impeller of a vertical axis compressor is carried more rigidly than the impeller of a horizontal axis machine is carried by its relatively slender overhang. With rotor diameter taken as the basis of comparison, a span between bearings ofa vertical axis compressor is some four diameters combined with an overhang of the impellers of great rigidity. In comparison, the span between bearings of a horizontal compressor is some five diameters and with the impeller overhung from its proximate bearing by about a rotor diameter and carried by a shaft reduced in diameter. (it is known in the art that these differing spans and overhangs will lead to a very large difference between the critical speeds, in the first bending mode, of the two builds of compressor.) b) The conical bearing of the vertical axis compressor acts as its thrust bearing, and eliminates the need of a thrust collar with its thrust and surge pads. The reduction in the bearing span that has been noted above in a) is a result of eliminating the thrust assembly.

c) A seal of relatively small diameter has to be provided behind the impeller of a compressor with a horizontal axis. The conical bearing of a vertical axis compressor is also its seal, and the length of shaft otherwise needed to pass through a seal is eliminated. The elimination of a shaft seal also eliminates the relatively slender shaft of the overhang of the impeller of a horizontal axis compressor, and thereby raises the critical speed of the first bending mode.

d) In an end-assembled compressor, as opposed to a machine with a split casing, to complete the assembly of a compressor it is necessary to separate the impeller from its rotor after balancing. The design of the vertical axis compressor permits the rotor/impeller to be end assembled in the frame of the compressor as a unity after balancing. It is then certain that the state of balance established in the balancing machine cannot be disturbed by the separation of the impeller from its rotor and it replacement.

e) There is no journal loading of the tail bearing. In consequence the tail bearing may be reduced in diameter, and be simple in design.

The combination in the conical bearing of the functions of thrust and seal, and the elimination ofjournal loading of the end bearing, leads to a lesser number of components, and a greater ease of assembly of the compressor of vertical axis in comparison with a horizontal axis compressor.

Claims

Claims: 1. A gas compressor having: a rotor shaft orientated in a

substantially upright position, the compressor having a rotor shaft assembly comprising: i) the rotor shaft; ii) a shaft bearing mounted to the rotor shaft; iii) an impeller mounted to the rotor shaft upwardly of the shaft bearing element; iv) an armature mounted to the rotor shaft downwardly of the shaft bearing element; and a stationary bearing co-acting with the shaft bearing; wherein the stationary bearing and the rotor shaft assembly are configured to permit the rotor shaft assembly to be mounted to seat the shaft bearing in co-acting position with the stationary bearing, such that the rotor shaft assembly can be mounted to andlor de- mounted from the compressor as an integral assembly.
2. A gas compressor according to claim 1, wherein the stationary bearing is provided with a central aperture dimensioned to permit rotor shaft and assembly to be dropped into position.
3. A gas compressor according to any preceding claim, wherein the stationary bearing is configured to permit the rotor shaft mounted armature to pass through.
4. A gas compressor according to any preceding claim, wherein the compressor is a radial flow compressor.
5. A gas compressor according to any preceding claim, wherein the compressor is an axial inflow compressor, the gas being directed to radial outflow via a compressor impeller rotor stage carried by the rotor shaft.
6. A gas compressor according to any preceding claim, wherein a bearing arrangement is provided for supporting the rotor shaft, the bearing arrangement having a rotary bearing element mounted for rotation with the rotor shaft and a stationary bearing element cooperating with the rotary bearing element for supporting the shaft.
7. A gas compressor according to claim 6, wherein the rotary and stationary elements have correspondingly adjacent substantially frustroconical bearing surfaces.
8. A gas compressor according to claim 6 or claim 7, wherein the bearing arrangement acts to take up both thrust (shaft axial) and journal (shaft radial) forces.
9. A gas compressor according to any of claims 6 to 8, wherein the bearing arrangement is provided proximate an upper portion of the rotor shaft.
10. A gas compressor according to any of claims 6 to 9, wherein the rotor shaft carries a rotary impeller stage, the bearing arrangement being provided behind the rotary impeller stage.
11. A gas compressor according to any of claims 6 to 10, wherein the rotary bearing element sits in a seat defined by the stationary bearing element.
12. A gas compressor according to any of claims 6 to 11, wherein the bearing surfaces of the bearing arrangement act as a primary seal for the rotor shaft.
13. A gas compressor according to any of claims 6 to 12, wherein the bearing arrangement is provided with a gas supply to the bearing surfaces to effect separation of the bearing surfaces for operation.
14. A gas compressor according to claim 13, wherein the bearing surface of the stationary bearing element is provided with at least three gas supply ports or orifices to supply separating gas at pressure to the interface between the bearing surfaces of the bearing.
15. A gas compressor according to claim 13 or claim 14, wherein the compressor is provided with an operation control system ensuring that compressor start up is only effected when sufficient gas separation exists at the bearing surfaces interface.
16. A gas compressor according to any preceding claim, wherein the lower portion of the rotor shaft of the compressor is provided with a bearing arrangement for taking up minimal journal load.
17. A gas compressor according to claim 16, wherein a primary bearing arrangement is provided at an upper portion of the rotor shaft, the journal (rotating) load borne by the journal bearing arrangement proximate the lower portion of the rotor shaft being significantly less than the journal bearing load borne by the primary bearing arrangement.
18. A gas compressor according to any preceding claim, wherein the rotor shaft carries a compressor impeller on an upper end of the shaft and a bearing element mounted to the shaft behind the compressor impeller, the rotor shaft bearing element being seated in a seat formed by the bearing surface of a stationary bearing element.
19. A gas compressor according to claim 18, wherein the compressor is closed by a removable end casing element, permitting the rotor shaft and compressor impeller to be mounted or de-mounted as an integral unit into and from the operating position.
20. A gas compressor according to any preceding claim, wherein the rotor shaft is electrically driven.
21. A gas compressor according to any preceding claim, including an electric motor arrangement to drive the rotor shaft, the electric motor arrangement comprising an armature mounted on the rotor shaft and a stator carried by the compressor housing surrounding the armature.
22. A gas compressor system comprising a first gas compressor according to any preceding claim in combination with a second gas compressor, the pressurised gas output from one of the compressors being directed to be the gas intake for the second compressor.
23. A gas compressor system according to claim 22, wherein the first and second gas compressors comprise compressors according to any of claims 1 to 22.
24. A gas compressor system according to claim 22 or claim 23, wherein a gas intercooler is provided between the first compressor gas out-take and the second compressor gas intake.
25. A gas compressor according to any preceding claim, wherein a bearing arrangement is provided for supporting the rotor shaft, the bearing arrangement having a rotary bearing element mounted for rotation with the shaft and a stationary bearing element for supporting the shaft, the bearing arrangement being provided with a gas supply to the bearing surfaces to effect separation of the bearing surfaces for operation, the gas supply to the bearing arrangement comprising a take-off from the compressor gas outlet.
26. A method of assembling a gas compressor comprising an upright orientated rotor shaft, a bearing element and a compressor impeller mounted to the shaft uppermost of the bearing element and proximate an upper end of the rotor shaft, the method comprising advancing in the axial direction, the rotor shaft with the compressor impeller and bearing element mounted to the rotor shaft, so as to seat the bearing element in a seat formed by a stationary bearing element of the compressor.
27. A method according to claim 26, wherein the rotor shaft with the compressor impeller and bearing element mounted to the rotor shaft, is lowered into the compressor casing so as to seat the bearing element in a seat formed by the stationary bearing element of the compressor.