Rotary kinetic fluid motors or pumps – Including heat insulation or exchange means – Cooling fluid contacts shaft – seal or bearing
Reexamination Certificate
1998-08-03
2001-10-02
Ryznic, John E. (Department: 3745)
Rotary kinetic fluid motors or pumps
Including heat insulation or exchange means
Cooling fluid contacts shaft, seal or bearing
C416S20400A
Reexamination Certificate
active
06296441
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to compressors of air or other gases and in particular, but not exclusively, relates to compressors for the pharmaceutical and food industries in which compressed gases free from oil are required.
Such a compressor generally comprises a centrifugal impeller overhung at one end or an impeller overhung at each end of the shaft of the armature of a high frequency electric motor. The armature of the motor carries permanent magnets which become ineffective at temperatures above some characteristic value. A major problem in the design of compressors of that type is to arrange cooling so that the magnets are not degraded by a too severe rise in temperature. The problem of cooling is compounded because of the absence of lubricating oil which otherwise would convect away heat from a compressor to its oil cooler.
Two mechanisms of generation of heat have to be regarded. Although in principle no eddy currents are induced in the armature of the motor, in fact some eddy currents exist because of inevitable departures from perfection in the practical application of the electromagnetic principles of the motor. The first mechanism is the generation of heat by these trace eddy currents. The second mechanism is the generation of frictional heat at the journal bearings and at the thrust bearing.
It may seem appropriate to carry away heat from both of these sources by flows of air or of gas derived from the compressor itself. Although that is a feasible method for carrying heat away from the bearings it is found from heat transfer calculations to be inherently inadequate by itself for the cooling of the armature. At the armature the essential requirement is that the temperature of the permanent magnets should not exceed some characteristic value which in turn requires the heat generated by the trace eddy currents to flow from armature to coolant under the limited temperature difference dictated by the limiting temperature of the magnets.
SUMMARY OF THE INVENTION
According to the present invention, a way of satisfying the criterion is by a flow of a suitable cooling liquid, preferably water, through a central bore in the armature.
According to the present invention, there is provided an oil free high speed gas compressor driven by a high frequently electric motor with a soft magnetic armature which holds permanent magnets arranged peripherally and a centrifugal impeller overhung at one or at each end of the shaft of the armature of the motor, wherein the temperature of the armature is held within the characteristic temperature of its permanent magnets by a flow of cooling liquid through a central bore in the armature or by a flow of cooling liquid through a central drilling through a tie-bolt in thermal contact with a bore in the armature.
The liquid should be in direct contact with the surface of the bore or if it passes through a central drilling through a tie-bolt, then it is required that, by shrink fitting or otherwise, there should be intimate thermal contact between the tie-bolt and the bore of the armature. Although it is possible that other liquids may be found to serve the purpose the heat transfer coefficients from solid to liquid possible with water, together with its other heat transfer properties make water the preferred cooling fluid. Water also permits the design of a low cost cooling system.
At the high speeds at which the compressor is designed to run, conventional bearing surfaces can be destroyed in the absence of a liquid lubricant. Journals and bearing pads of ceramic materials are therefore preferably used. The bearings are lubricated by air or by gas and now cooling by air or gas is feasible because the heat is generated by shear in the air or gas itself and in the absence of any heat transfer surface is convected away directly with the flow of lubricating air or gas. To provide adequate cooling of journal bearings and the thrust bearing under all conditions from start to shut down the bearings and thrust are arranged as aerostatic bearings fed from a receiver pressurised by the compressor. Some aerostatic supply of air or gas is maintained as a coolant at speed although the bearings become self generating.
The two sources of generation of heat are effectively isolated because of the low thermal conductivity of the ceramic bearings.
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Christie Parker & Hale LLP
Corac Group PLC
Ryznic John E.
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