SAFETY ARRANGEMENT FOR A VACUUM PUMP

Abstract: A vacuum pump or exhauster has a rotor (10) eccentric­ ally journalled within a cylindrical chamber (20). In order to prevent overheating, the exhauster housing (12) contains a valve arrangement (28) having a closure member (36) which is coupled to a thermostat (40) such that, above a predeter­ mined temperature, the closure member is lifted from its associated valve seat (34) to thereby admit atmospheric air to the suction side of the exhauster via a housing cavity (46).

Description: DESCRIPTION IMPROVEMENTS IN VACUUM PUMPS The present invention is concerned with vacuum pumps or exhausters. A problem with existing vacuum pumps (described hereinafter as exhausters) is that of overheating in certain circumstances which can lead to early rotor bearing failure or; if not prevented, to complete seizure and failure of the rotor bearings. The problem arises, for example, when the exhauster is pulling too high a vacuum for the cooling capabilities provided. The power consumption of an exhauster at constant speed remains substantially constant over the whole vacuum range. However, at peak vacuum no air enters or leaves the exhauster and substantially all the power consumed in rotating the exhauster rotor is converted into heat. The particular vacuum level at which overheating occurs depends on a number of factors such as (a) the rotor speed - cooling problems tend to be greater with higher speed machines; (b) the size of the exhauster smaller machines run cooler than larger ones; (c) the ambient temperature - if this rises the problem can be aggravated; (d) the presence or otherwise of large cooling fans, large ribs and correct design of internal porting can affect the ability of a given machine to tolerate internally generated abnormal temperatures; (e) the presence of worn exhauster blades; dirt - e.g. road dirt, building up to reduce cooling efficiency. One way of countering this problem would be to provide a sophisticated forced-air cooling system but this would considerably increase the overall dost of the machine. Large two-stage exhausters are often watercooled but are not suitable for many applications, such as on road tankers, being rather large, heavy and expensive. It is known to incorporate into vacuum pumps a simple spring-loaded valve which is adapted to limit at all times the vacuum pulled by the pump to a level well below the maximum of which the pump is capable. The disadvantage of such an arrangement is that full vacuum can never be pulled so that operating times are increased. This arrangement is therefore unpopular with users. It is an object of the present invention to provide an exhauster in which the problem of overheating at high vacuum levels is obviated. In accordance with the present invention, there is provided an exhauster having a temperature responsive valve which is arranged to admit atmospheric air to the suction side of the exhauster in the event that the operation temperature of the exhauster exceeds a predetermined value. Advantageously this takes the form of a snifter valve controlled by a thermostat located in a chamber in the exhauster housing. The invention is described further hereinafter by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a longitudinal section through an exhauster in accordance with the present invention; and Fig. 2 is a partial transverse section to an enlarged scale showing the snifter valve in the exhauster of Fig. 1. The exhauster illustrated in the drawings is basically of conventional design and will not be described in detail herein. It includes a rotor 10 which is mounted for rotation within a stationary housing 12 by means of a central shaft 14 journalled in bearings 16,18. The rotor is eccentrically mounted relative to a cylindrical chamber 20 defined within the housing 12 whereby radial vanes 22, radially displaceable in respective slots 24, draw in air from a suction region of the chamber and deliver this air through an outlet port. The shaft is driven by means of a moving belt at 26. Thus far, the exhauster is conventional. In order to reduce the vacuum at the suction side of the exhauster in the event that overheating is occurring, the exhauster includes a valve arrangement indicated generally at 28. As best seen in Fig. 2, the valve 28 includes an annular body member 30 which is screwed into a tapped bore 32 in the exhauster housing. The outer end of the body member 30 defines a frusto-conical valve seat 34 which is adapted to co-operate with a frustoconical closure member 36 carried by one end of a pin 38. The other end of the pin 38 engages a thermostat unit 40 contained in a chamber 42 of the exhauster housing whereby, when the temperature at the thermostat unit 40 exceeds a pre-determined temperature, the pin is displaced to the left as viewed in Fig. 2 to lift the closure member 36 from the valve seat 34 against the action of a helical. coil spring 44 which-acts in a sense to hold the valve closed. Opening of the valve admits atmospheric air to enter a housing cavity 46 which communicates directly with the housing chamber 20 in the suction region whereby to reduce the vacuum pulled by the exhauster. This has the effect of reducing the temperature in the section region so that the valve eventually closes again. The valve thus provides a means of limiting the temperature variations for the exhauster without losing the capability of pulling full rated vacuum. It should be noted that the invention is not limited to the particular valve configuration and position illustrated and any valve may be used which admits air to the suction side in dependence upon a predetermined temperature in the exhauster being exceeded.

Claims: CLAIMS 1. An exhauster have a rotor (10) eccentrically mounted within a chamber (20) in a stationary housing (12) to draw in air from a suction region and deliver this air through an outlet port, characterised by a temperature responsive valve (28) which is arranged to admit atmospheric air to the suction side of the exhauster in the event that the operating temperature of the exhauster exceeds a predetermined value. 2. An exhauster according to claim 1, wherein the valve (28) is disposed within the housing (12) of the exhauster. 3. An exhauster according to claim 1 or 2, wherein the valve (28) comprises a snifter valve controlled by a thermostat (40) located in a chamber (42) in the housing (12) of the exhauster. 4. An exhauster according to claim 3, in which the valve (28) has an annular body member (30) received in a tapped bore (32) in the exhauster housing, the outer end of the body member (30) defining a frusto-conical valve seat (34) which is adapted to co-operate with a frustoconical closure member (36) carried by one end of a pin (38), the other end of the pin (38) engaging the thermostat (40) contained in said chamber (42) whereby when the temperature at the thermostat exceeds said predetermined value the pin (38) is displaced longitudinally so as to lift the closure member (36) from the valve seat (34) against the action of a coil spring (44) which acts in a sense to hold the valve (28) closed.

Inventors: SCHMITZ, LOTHAR PETER
Applicants: THE DRUM ENGINEERING COMPANY LIMITED
Application year: 1982
Main classification: F04C29
IPC codes: F04C28, F04B49