Rotary expansible chamber devices – Non-metallic working member – cylinder or partition
Patent
1997-10-16
1999-04-06
Vrablik, John J.
Rotary expansible chamber devices
Non-metallic working member, cylinder or partition
418195, F04C 1852
Patent
active
058908846
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a positive displacement rotary compressor of the screw and gaterotor type.
It is known for instance from French Patent No. 1,331,998 (U.S. Pat. No. 3,180,565) to build compressors comprising a screw with grooves rotating in a bore of a casing and cooperating with at least one gaterotor having teeth meshingly protruding in said grooves to define with the casing variable volume chambers.
It is also known by U.S. Pat. No. 5,080,568 to mount gaterotors rotating around a shaft set inside the gaterotor and to locate the gaterotor shaft axially by securing the shaft to the casing on the side of the gaterotor exposed to the pressure.
This invention relates to a compressor to compress a gas between a low and a high pressure, comprising a screw provided with at least one groove, rotatably mounted in a casing and cooperating with at least one gaterotor carrying teeth which meshingly protrude into the groove to define with the casing a volume for increase of pressure of the fluid, one side of the teeth being exposed to the high pressure fluid, whereas the other side is exposed to the low pressure fluid, this gaterotor comprising a plastic sheet carrying the teeth in sealing engagement with the groove and located on the high pressure side of the gaterotor, supported by a metal support on the low pressure side, the high pressure side of the plastic sheet being in sealing proximity of a lip of the casing, thrust means to create a load pressing the gaterotor shaft against stopping means, the thrust load created by said means being higher than the maximum load carried by the gaterotor during normal operation of the compressor.
This construction has the remarkable effect to substantially reduce the damages created to the compressor by what is commonly called a liquid lock without a deterioration of the plastic teeth profile or of the compressor performances.
It is indeed known in the refrigeration and air conditioning industry that under certain conditions, particularly at start-up, the compressor may happen for a very short period, generally less than a second, to have to compress liquid instead of gas and this creates what is generally called a liquid lock.
Indeed, as the volume of the groove reduces to insure gas compression and as liquid is not compressible, when gas is replaced by liquid, enormous pressures are generated in the groove until something breaks or clearances are created through which the liquid can escape. The pressures created are many times the maximum pressures for which the compressor is designed, commonly 10 or 20 times higher.
According to the invention if such liquid lock happens, the pressure in the groove will generate an axial thrust on the gaterotor largely exceeding the thrust load generated by the thrust means and the gaterotor will move axially, thereby opening a gap between the high pressure side of the teeth and the lip of the casing, allowing the excess liquid in the groove to escape through this gap and thereby allowing to maintain the pressure in the groove within acceptable limits.
Nevertheless, when such axial displacement of the gaterotor happens, the flanks of the teeth of the gaterotors are no longer at their proper operating position. It is known that such displacement has the same effect as if the width of the groove was suddenly narrowed. The larger the displacement, the more the teeth have to distort so as to accommodate what looks like narrower grooves. If the gaterotor teeth were completely in metal, something would break, either the teeth or the threads, or be severely damaged; plastic is more flexible and does not break but should it stay for some time at the new position created by the displacement, it would wear so that the teeth adjust their shape to the seemingly narrower groove; when moved back to the original position, this wear would translate into increased leakage losses and substantially diminished efficiency performances.
For instance, in air-conditioning compressors delivering around 100 tons of refrigeration, with a drop of gaterotor of
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