GB2088960A - Rotary Positive-displacement Gas Compressors - Google Patents

Abstract

A gas compressor for e.g. air, has rotary positive-displacement gas-compressing means 12, which may be of the sliding-vane type, and a cooling unit 17 comprising a fan 19 coupled to the compressing means so as to be driven therewith by e.g. an electric motor 14. The fan draws ambient air through heat-exchanging cores 20,21,22, the core 20 being used to remove the heat of compression from the compressed gas and the other cores being connected in series with each other and used to cool lubricating oil for the compressing means. <IMAGE>

Description

SPECIFICATION Compressor with Cooling Unit This invention relates to a gas compressor having a cooling unit and is particularly, but not exclusively, concerned with an air compressor.

Air compressors can conveniently be divided into two groups.

The first group comprises various types of piston compressor which, although being generally highly efficient, have a very high exit air temperature.

The second group comprises the rotary positive displacement compressors which can be split into three main types, the screw compressors, the meshing gear compressors, and the sliding vane eccentric rotor compressors. Compressors of this latter type are recognised as being the quietest of all compressors and comprise a stator supported within a casing, and a rotor mounted eccentrically within the stator and carrying a series of vanes which coact with the stator driving the rotor causes air to be drawn through an inlet in the stator, and the air is then compressed before it is discharged through an outlet in the system into a reservoir for the compressed air.With many types of rotary positive displacement compressors, it is common practice to use the compressor lubricatingoil as a coolant, by feeding the oil into the airstream so that a mixture of the oil and compressed air will be discharged through the outlet in the stator. As the oil has a much greater thermal capacity than the air, it absorbs a substantial proportion of the heat generated during compression of the. air and correspondingly restricts the temperature gained by the air. After being discharged through the outlet in the stator, it is necessary to separate the oil from the air and this is achieved by oil precipitation and filtration systems which are well-known in the art. It is known for other coolants to be used, for instance water, particularly in meshing gear compressors.

When the coolant is to be recirculated, it is essential to remove the heat gained during compression to preserve its effectiveness as a coolant and, when the coolant is the compressor lubricating oil also to avoid thermal degradation.

For these reasons, it is common practice for the coolant to be passed through a heat exchanger before it is passed again through the compressor.

It is also advantageous to cool the compressed air as this reduces its capacity to hold water.

Before the teaching of our published UK Patent Application 2017 216A, it was common practice for the coolant to be passed through a heat exchanger secured to one end of the motor/compressor unit, the heat exchange being with ambient air pushed through the heat exchanger by an axial-flow fan. US Patent 2 641 405 shows a typical motor/compressor unit of this type mounted on a wheeled chassis. Because of the position of the heat exchanger, it is impossible to service the compressor without first removing either the heat exchanger or the motor, and severe overheating can occur if the motor/compressor unit is operated with the heat exchanger end near to a wall or other obstruction.

Such motor/compressor units had no provision for cooling the compressed air, although separate compressed air cooling systems were available for insertion into the compressed air line from the compressor reservoir.

UK Patent 1 318 884 teaches that an annular oil cooler can be mounted between the compressor and its motor, but the construction of the oil cooler precludes any simple mounting connection between the compressor and the motor which consequently must be independently supported. Furthermore, the efficiency of the oil cooler will be adversely affected if either side of the motor/compressor unit is positioned close to a wall or any other obstruction.

In our aforesaid UK Patent Application, we have taught that both the coolant and the compressed air can be cooled in a single cooling unit having a housing interconnecting the compressor casing and the motor casing, the housing also serving to support the coolant and the compressed air heat exchangers, and further serving to define passages for the ambient cooling air which is sucked into the housing by a fan, mounted on a shaft interconnecting the motor and the compressor rotor, and is then blown through the heat exchangers. This arrangement has many advantages over the previously proposed motor/compressor units, in addition to the realisation of a particularly compact arrangement with integral cooling for both the compressor coolant and the compressed air. Its advantages include: A.The provision of a simple mounting connection between the compressor and the motor which enables, for the first time, the positioning of both the coolant and compressed air heat exchangers between the compressor and the motor.

B. The use of a single fan for passing ambient air through both heat exchangers.

C. Ready access for servicing the compressor, the motor and the side mounted heat exchangers.

D. Simplified mounting of the compressor and the motor.

In addition to the waste heat dissipated by the coolant and the compressed air heat exchangers, waste heat is also dissipated by the casing of the compressor and its compressed air reservoir, and the rate of dissipation is a function of the thermal gradient between these casings and the ambient air. With all hitherto proposed compressors having heat exchangers for the coolant and/or the compressed air the heated air issuing from the heat exchangers is discharged, sometimes quite indiscriminately, around the compressor and results in an increase of air temperature over at least part of the casings of the compressor and its compressed air reservoir. Such increased air temperature reduces the rate of heat dissipation by these casings and mean that the heat exchangers, and their associated fan or fans, must be of greater capacity.Furthermore the compressor is liable, at least under some conditions, to draw in air heated by the heat exchangers, with consequent loss of efficiency in the compression cycle.

With all hitherto proposed compressors having heat exchangers for the coolant and for the compressed air, the cooling air is blown by a fan through the heat exchangers, with the result that air-borne dirt will accumulate on the side of the heat exchanger which is hidden from the compressor operator. Any accumulation of such dirt will reduce the effectiveness of the heat exchangers and it is prudent to design both the heat exchangers and their associated fan or fans to be of sufficient capacity to provide effective cooling, even when the heat exchangers are partially clogged with such dirt.

Thus, with all hitherto proposed compressors, it is necessary to provide heat exchangers and an associated fan or fans, which have sufficient excess capacity to cope with increased ambient temperature around the compressor, and partial clogging of the air passages in the heat exchangers. In addition to the capital cost of providing larger heat exchangers and a larger fan, there is the additional operational cost of driving the larger fan An object of the invention is to provide a gas compressor having a more efficient cooling unit. A further optional object of the invention is to provide a gas compressor unit which will conserve energy.

According to the invention, a rotary positive displacement compressor has a compressor mechanism for compressing a mixture of gas and a coolant liquid and a cooling unit for the coolant liquid which includes a heat exchanger for the coolant liquid and fan mounted within the cooling unit and connected to be driven with the compressor mechanism, the cooling unit defining a heated air outlet, and the fan being arranged and positioned relative to the cooling unit so that it will suck ambient air through the coolant liquid heat exchanger and discharge it through the heated air outlet. In this manner, the waste heat can be discharged away from the casing of the compressor and its compressed air reservoir, thereby avoiding increasing the temperature of the air drawn into the compressor and the air surrounding the compressor casing.Furthermore, by arranging the fan to suck ambient air through the coolant liquid exchanger, it is possible to achieve a much more even air flow through the heat exchanger core, and it is possible to position the heat exchanger so that any airborne dirt adhering to it can be readily seen by an operator.

The cooling unit may also include a heat exchanger for the compressed gas, the fan being arranged and positioned so that it will also suck ambient air through the compressed gas heat exchanger and discharge it through the heated air outlet In this manner, it is possible to cool both the coolant liquid and the compressed gas within a single cooling unit. The coolant liquid heat exchanger preferably comprises a primary portion and a secondary portion, the primary portion being arranged between the compressed gas heat exchanger and the fan, and the primary and secondary portions being connected, so that the coolant liquid will pass through the primary portion before flowing through the secondary portion.In this manner, it is possible to achieve a two-stage cooling of the coolant liquid, which has a much greater thermal capacity than the compressed air, whilst minimising the overall cross-sectional area through which air must be sucked through the two heat exchangers.

The heated air outlet is preferably connected to a duct for conveying the heated air to a position remote from the compressor. In this manner, it is possible to convey the waste heat to a point remote from the compressor. This feature is particularly useful where the compressor is to be used in a warm climate, as the waste heat can be dumped outside a building containing the compressor, thereby avoiding an unwanted increase of ambient temperature in the building.

Conversely, where the compressor is to be used in a temperate or cold climate, the waste heat can be ducted into a space-heating system so that the otherwise wasted energy is usefully employed.

The waste air may also, or additionally, be ducted to a point where it can be used for process heating.

The ambient air may be constrained to flow through an air cleaning filter before it reaches the or either heat exchanger. In this manner, the cooling air can be cleaned, thereby preventing any clogging of the, or either, heat exchanger and any build-up of dirt on the fan. This feature is particularly useful where the compressor is to be used in dusty conditions.

The cooling unit preferably includes a casing secured to a casing housing and compressor mechanism, and the cooling unit casing supports the, or both, heat exchangers. In this manner, the cooling unit can be rigidly supported by the compressor casing to form an integral assembly similar to that proposed in our aforesaid UK Patent Application. A driving motor is preferably mounted on the cooling unit casing, on the side remote from the compressor casing, and the shaft upon which the fan is mounted is a drive shaft interconnecting the motor and the compressor mechanism. By this means, the compressor, cooling unit and motor form a single intergral assembly, the cooling unit casing providing a simple mounting connection between the compressor and the motor, and permitting ready access for servicing the compressor, the motor and the heat exchangers. The fan is preferablya centrifugal fan, the cooling unit casing defines a chamber which surrounds the outer periphery of the fan and leads to the heated air outlet, and the cooling unit casing also defines an air suction passage communicating with one end of the fan.

In this manner, the cooling unit casing is additionally used to define the stator and suction passage for the centrifugal fan. The cooling unit casing may also define an air suction passage communicating with the opposite end of the fan.

This enables both ends of the centifugal fan to suck air through the, or both, heat exchangers as the case may be. The chamber, formed within the cooling unit casing and comprising the stator of the centrifugal fan, is preferably of involute, or volute, form and the, or each, heat exchanger is positioned remote from the widest portion of the chamber. In this manner, the chamber is shaped to promote efficient operation of the centifugal fan, and the, or each, heat exchanger, as the case may be, is located in a position which minimises the overall dimensions of the cooling unit casing.

In the case where the cooling unit includes heat exchangers for both the coolant liquid and the compressed air, both heat exchangers are preferably mounted on the same side of the cooling unit casing. This feature enables the compressor to be positioned with any, or all, of its other sides close to walls or other obstructions.

The heat exchangers are preferably mounted sideby-side, and the heated air outlet is preferably upwardly directed.

The invention is now described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic plan view of a motor/compressor unit incorporating a cooling unit which is shown in section; Figure 2 is a perspective view of the cooling unit taken in the direction of arrow 2 in Figure 1; Figure 3 is a vertical section of the cooling unit taken along the line 3-3 in Figure 1; Figure 4 is an end elevation of the cooling unit, which is drawn to a larger. scale and shows more details, the elevation being taken in the direction of arrow 4 in Figure 2; Figure 5 is a side elevation of the cooling unit, also drawn to the same larger scale and in more detail, taken in the direction of arrow 5 in Figure 2, but having the heat exchangers removed;; Figure 6 is a plan view of the cooling unit, again drawn to the same larger scale and in more detail, taken in the direction of arrow 6 in Figure 2; Figure 7 is a transverse vertical section taken along 7-7 in Figure 6, and Figure 8 is a horizontal section taken along 88 in Figure 4.

With reference to Figures 1 to 3, a rotary positive displacement air compressor is indicated diagrammatically by arrow 10 and may, for instance, be a sliding vane eccentric rotor compressor such as that described in our published UK Patent Application No 2017 216A.

The compressor has a casing means 11 which houses the compressor mechanism 12 and also defines the usual lubrication oil sump located in a lower part of the casing means, as indicated by arrow 1 3, and a reservoir for the compressed air.

The casing means 11 will usually comprise a plurality of housings and associated covers. The compressor lubrication oil serves to lubricate the compressor mechanism and as a coolant liquid for the compressed air. The compressor mechanism is driven by an electric motor 14 by means of a drive shaft 1 5 which will usually include at least one disengageable coupling 16, for instance those described in our aforesaid published Patent Application. A cooling unit is indicated generally by arrow 1 7 and comprises a housing 1 8 which contains a centifugal fan 19, drivably mounted in any convenient manner, to be driven with the compressor mechanism 1 2 by the shaft 13, and three heat exchange cores 20, 21 and 22, provided with upper and lower header tanks of conventional form.The heat exchange core 20 is connected to receive compressed air from the compressed air reservoir in the compressor casing 11 and serves as a compressed air heat exchanger. The heat exchange cores 21 and 22 are connected in series to receive heated lubricating oil from the sump 13 and serve jointly as a coolant liquid heat exchanger, the cooled lubricant being returned to the compressor for reuse.

Whilst the construction of the fan 1 9 and its relationship with the interior of the housing 1 8 will be described later in some detail, a brief description is now given of their general operation. Rotation of the fan 1 9 sucks ambient cooling air through the heat exchange cores 20 and 21, in the direction indicated by arrow 23 in Figures 1 and 2, and also sucks ambient air through the heat exchange core 22 in the direction indicated by arrow 24. In passing through the heat exchange cores, the cooling air becomes heated and is discharged by the fan 1 9 through a vertically upwardly directed heated air outlet 25 in the direction indicated by arrow 26 in Figure 2.

The coolant liquid heat exchanger 21, 22 serves to transmit the waste heat, generated during the compression of the air, from the compressor lubricating oil to the cooling air, and the fan 1 9 serves to discharge heated cooling air through the heated air outlet and away from the casing 11 of the compressor 10. It has been found that the centifugal fan 1 9 will blow the heated cooling air several metres from the compressor casing 11 and that the transmission of the heated cooling air away from the compressor is assisted by directing the heated air outlet 25 upwardly to promote convection.In this manner, it is possible to prevent the waste heat from being transmitted to the air surrounding the compressor casing, and fresh cool air can be drawn through the heat exchange cores 20, 21 and 22 rather than recirculating heated air, as can readily occur with hitherto proposed designs in which cooling air is simply blown through the heat exchangers and is discharged indiscriminately around the compressor. As a direct consequence, the thermal gradient between the compressor casing and the adjacent air is maintained at an optimum value, thereby minimising the capacity required by both the compressed air heat exchanger 20 and the lubricant heat exchanger 21, 22. Also the waste heat can be prevented from mixing with the air drawn into the compressor thereby preventing a reduction in the thermal efficiency of the compression cycle.

The heat exchange cores 21 and 22 respectively comprise primary and secondary portion of the lubricant heat exchanger 21 , 22 which has to dissipate much more heat than the compressed air heat exchanger 20 due to the much greater thermal capacity of the lubricating oil. Thus, the lubricant is cooled in two stages, the first stage being in the heat exchange core 21, which receives the cooling air after it has been partially heated by flowing through the compressed air heat exchanger 20, and the second stage being in the heat exchange core 22, which receives fresh cooling air.

From Figures 1 to 3 it will be noted that the heated air outlet 25 is close to a side wall 27 of the housing 18, which is opposite to the heat exchangers, and that the housing 1 8 has end walls 28 and 29, a bottom wall 30 and top wall 31. The end wall 28 is provided with a cylindrical recess, as shown in Figure 2, for locating a spigot defined by the casing of the motor 14, so that its drive shaft 1 5 will be located coaxially within the housing 1 8. The motor 14 is secured axially to the housing 18 in any convenient manner, for instance by four unshown bolts which pass through a flange of the motor casing and engage threaded holes 32 in the end wall 28. The compressor 10 is secured in a similar manner to the opposite end wall 29.By mounting the heat exchange cores side-by-side on the same side of the housing 18, a particularly compact cooling unit is achieved in which the heat exchangers are readily accessible. The compressor can also be positioned with the side 27 of the housing close to a wall or other obstruction.

The housing 1 8 is preferably in the form of a one-piece casting and forms a rigid mounting connection between the compressor 10 and the motor 14, and a mounting for the heat exchangers. In addition, the housing 18 is formed with an integral web 33 defining an involute, or if desired a volute, chamber which constitutes the stator of the fan surrounding the outer cylindrical periphery of the centrifugal fan 19 and leads to the heated air outlet 25. The web 33 is formed with integral end flanges 34, 35 which are positioned at opposite ends of the centrifugal fan 1 9 and define two air suction passages leading respectively from the heat exchange cores 20 and 21 to the compressor end of the fan 19, and from the heat exchange core 22 to the motor end of the fan 19.Thus the web 33 and end flanges 34, 35 provide suction paths from the heat exchange cores into both axial ends of the centrifugal fan 19, and a single delivery chamber which conveys the heated cooling air to the heated air outlet 25.

By arranging for the fan 19 to suck cooling air through the various heat exchange cores, instead of blowing as taught by the prior art, we have found several significant advantages are achieved including the following: A. The cooling air flows through the entire available surface of each heat exchanger, with only minor variations in velocity between the various cooling air passages in the heat exchange core. As a direct result, the cooling air stream flows substantially evenly through the heat exchange cores, with a consequent improvement in cooling efficiency over the prior art in which the velocity of the blown cooling air varies considerably.In a typical prior art arrangement in which an axial fan is used to blow air through a heat exchange core, the approach velocity of the cooling air is much higher through the centre of the heat exchange core than through its corners.

B. Any air-borne dirt will be deposited on an outside surface of the heat exchange core where it can readily be seen by the compressor operator and can be removed, when necessary, without disconnecting the heat exchange core from the cooling unit.

C. The output of the centrifugal fan is not obstructed by the heat exchangers, as in the prior art, and the fan is able to produce sufficient pressure to blow the heated cooling air to a position remote from the compressor.

As shown in Figure 3, an air cleaning filter 36 can be positioned in front of the heat exchange cores 20 and 22 to filter the cooling air sucked through them. This means that, when the compressor is operated in dusty conditions, the cooling air will be filtered prior to entering the heat exchangers, and that clogging of the heat exchangers or the fan can easily be prevented by changing or cleaning the filter. Because the cooling air is sucked through the heat exchangers, the filter 36 can be placed on the outside of the heat exchangers, as shown, and is therefore readily accessible whereas, if the cooling air was blown in accordance with the prior art, the filter would have to be positioned between the fan and the heat exchangers and would accordingly be relatively inaccessible.

The inter-relationship between the heat exchangers, the fan and the housing 1 8 will be understood further with reference to the more detailed larger scale Figures 4 to 8 in which the same reference numerals have been used to identify equivalent features. From these Figures it will be noted that the housing 1 8 is formed with integral legs 37, 38 which serve to support the whole compressor/cooling unit/motor assembly, thereby facilitating acces for servicing the compressor, the motor and the heat exchange cores. From Figure 4, it will be noted that the motor mounting aperture in the end wall 28 is formed with four recesses to accommodate corresponding protrusions on the motor casing whereby the torque reaction on the motor casing, will be transmitted to the housing 18.

From Figures 5 and 8, it will be seen that the opposed end flanges 34, 35 of the web 33 are provided with respective annular lips 39, 40 which curve inwardly towards the opposite axial ends of the fan 19, there being a small axial working clearance between these lips and the corresponding end of the fan. It will particularly be noted from Figures 1 and 8 that the annular lip 39 has a greater internal diameter than the annular lip 40, thereby enabling the fan 18 to draw a somewhat greater air flow through its compressor end.

The design of the housing 18 applies a substantially constant sub-atmospheric pressure to the inner surface of the heat exchange cores 21 and 22 so that all the cooling air passages will be used substantially equally for heat exchange purposes. In order to achieve this effect, the relative diameters of the annular lips 39 and 40, and the spacing of the web 33 from the heat exchange cores, can be altered.

In Figures 5 and 7, the heat exchange cores are omitted so that the interior of the housing 18 can be viewed through an aperture 41 in which the heat exchange cores are normally fitted.

From Figure 7, it will be seen that the chamber formed between the fan 1 9 and the web 33 is of involute cross-section, and that a particularly compact cooling unit has been achieved by positioning the aperture 41 remote from the widest portion of the involute chamber, so that the heat exchange cores can be placed close to the axially of the fan 19.

As the heated cooling air is blown out of the single aperture 25 by the action of the centifugal fan 19, it has been confined, for the first time, into one narrow region and can accordingly be readily controlled and dissipated. In particular, it is possible to connect a duct to the aperture 25, so that the waste heat can be conveyed to a point remote from the compressor. This feature positively ensures that the waste heat does not increase the temperature of the air surrounding the compressor casing, or of the air drawn into the compressor, or of the air sucked into the heat exchangers. Moreover, this feature provides a substantial saving of energy when there is a demand for heated air, for instance for space heating or for process heating. To facilitate attachment of the duct, a flange 42 is formed around the outlet 25.

From Figure 8, it will be seen that the centifugal fan 1 9 is formed from two halves, each having an integral radial flange 43 and an integral mounting boss 44, the fan blades being formed as axially-directed depressions in the cylindrical periphery of each half. The mounting bosses 44 are a closing sliding fit on the drive shaft which interconnects the motor 14 and the compressor 10, and rivets 45 serve to drive the halves and to locate them axially. The flanges 43 are arranged back-toback and are secured together by a series of spaced rivets 46.

It has been found that use of the apparatus illustrated gives rise to a small temperature increase in the lubricating oil temperature when the filter 36 is used, as opposed to the unfiltered arrangement. However, the temperature rise is only small and is insignificant in terms of the overall operating improvement in dusty atmospheres when compared to prior art devices.

Claims (14)

Claims

1. A rotary positive displacement gas compressor having a compressor mechanism for compressing a mixture of gas and a coolant liquid, and a cooling unit for the coolant liquid which includes a heat exchanger for the coolant liquid and a fan mounted within the cooling unit and connected to be driven with the compressor mechanism, the cooling unit defining a heated air outlet, and the fan being arranged and positioned relative to the cooling unit so that it will suck ambient air through the coolant liquid heat exchanger and discharge it through the heated air outlet.

2. A compressor, according to Claim 1, in which the cooling unit also includes a heat exchanger for the compressed gas, and the fan is arranged and positioned so that it will also suck ambient air through the compressed gas heat exchanger and discharge it through the heated air outlet.

3. A compressor, according to Claim 2, in which the coolant liquid heat exchanger comprises a primary portion and a secondary portion, the primary portion being arranged between the compressed gas heat exchanger and the fan, and the primary and secondary portions being connected so that the coolant liquid will pass through the primary portion before flowing through the secondary portion.

4. A compressor, according to any preceding Claim, in which the heated air outlet is connected to a duct for conveying the heated air to a position remote from the compressor.

5. A compressor, according to any preceding Claim, in which the ambient air is constrained to flow through an air cleaning filter before it reaches the, or either, heat exchanger.

6. A compressor, according to any preceding Claim, in which the cooling unit includes a casing secured to a casing housing the compressor mechanism, and the cooling unit casing supports the, or both, heat exchangers.

7. A compressor, according to Claim 6, having a driving motor mounted on the cooling unit casing, on the side remote from the compressor casing, and the shaft upon which the fan is mounted is a drive shaft interconnecting the motor and the compressor mechanism.

8. A compressor, according to Claim 6 or 7, in which the fan is a centrifugal fan, the cooling unit casing defines a chamber which surrounds the outer periphery of the fan and leads to the heated air outlet, and the cooling unit casing also defines an air suction passage communicating with one end of the fan.

9. A compressor, according to Claim 8, in which the cooling unit casing also defines an air suction passage communicating with the opposite end of the fan.

10. A compressor, according to Claim 8 or 9, in which the chamber is of involute, or volute, form and the, or each, heat exchanger is positioned remote from the widest portion of the chamber.

11. A compressor, according to any of Claims 6 to 10 and in the case where the cooling unit includes.heat exchangers for both the coolant liquid and the compressed air, in which both heat exchangers are mounted on the same side of the cooling unit casing.

12. A compressor, according to Claim 11, in which the heat exchangers are mounted side-byside.

13. A compressor, according to any preceding Claim, in which the heated air outlet is upwardly directed.

14. A rotary positive displacement gas compressor, substantially as described herein, with reference to Figures 1 to 3 of the accompanying drawings.

1 5. A rotary positive displacement gas compressor having a compressor mechanism for compressing a mixture of gas and a coolant liquid and having a cooling unit substantially as described herein with reference to Figures 4 to 8 of the accompanying drawings.