Screw compressor having sealed low and high pressure bearing...

Rotary expansible chamber devices – Methods

Reexamination Certificate

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Details

C418S087000, C418S097000, C418S104000, C418S201100, C418SDIG001

Reexamination Certificate

active

06612820

ABSTRACT:

FIELD OF INVENTION
This invention relates generally to rotary compressors, and more particularly to rotary compressors of the positive displacement type including two or more rotors or screws disposed within a housing, supported by bearings, and formed with inter-engaging helical lobes and grooves.
BACKGROUND
It is disclosed in prior art of rotary compressors, that one rotor is driven and it in turn drives the other rotor through a gear system or directly without gears. The rotors do not contact each other or the housing, but have small clearances between tips on the lobes, mating surfaces on the rotors, and the inner surface of the housing. The housing is provided with an entrance port at one end and a discharge port at the opposed end, the discharge port proportioned to cause the pressure of the gas being compressed to be raised within the compressor before the gas is discharged. The compressor has a working chamber where a process gas is compressed and in some cases a liquid, such as oil, is injected into the chamber to lubricate the intermeshing rotors, seal the clearances between the rotors and casing, and to cool the gas being compressed. In the case where one rotor directly drives the other, the injected liquid transmits the driving force from one rotor to the other. Downstream of the compressor, this oil may be recovered by passing through a separator that allows the oil to be separated from the gaseous fluid. Such a compressor that utilizes a lubricating system for the rotors for sealing and cooling, and in most cases, force transmittal, is called a flooded screw compressor. It can achieve higher compression ratios than so-called dry compressors that omit a sealing lubricant in the working chamber and rely on a precision mating of rotors and precision gears to maintain a very close fit between moving parts for sealing (controlled leakage). It is desirable to provide systems where the gears and bearings supporting the rotors are also lubricated with a separate oil supply to a plurality of bearing and gear chambers that are separated from the working chamber by seals.
The following disclosure may be relevant to various aspects of the present invention and may be briefly summarized as follows:
U.S. Pat. No. 3,073,513 to Bailey teaches a flooded screw compressor that utilizes a separate pressurized oil supply tank and pump to provide oil for the working chamber. A certain viscosity oil is required to achieve the desired sealing with given clearances, volumetric ratios of oil and gas, and speeds of operation. The outlet from the compressor includes a separator where the oil is separated and recirculated to the pressurized tank. The bearings and gears are lubricated by a separate oil supply that comprises a ventilated tank and a pump that supplies oil to the bearings from which it drains back to the ventilated tank. It is suggested that labyrinth seals can be used at both ends of the rotors between the two oil systems.
However, a problem exists in that the seals separating the two oil systems, or separating one oil system from a process fluid, are either expensive to manufacture and maintain to provide leak proof seals or they are inexpensive and simple to maintain, but allow leakage between the working chamber and the gears and bearings. In the latter case, where leakage occurs, there is a problem when the process fluid in the working chamber is corrosive or forms a corrosive mixture when contacting the oil. Leakage of the oil, if present, and process fluids from the working chamber into the bearings and gears causes accelerated corrosion and premature failure of the bearings and gears. The labyrinth seals suggested by Bailey typically operate with some clearance and thus, some degree of leakage is to be expected. In this case, and especially at the high pressure end, some leakage of process fluids and working chamber lubricant would be expected to leak into the bearings and would be recirculated to all bearings and gears. When the process gas is highly corrosive, even a small amount of such leakage can be detrimental to the bearings and will considerably shorten the life of the bearings. A problem occurs because the process must be shut down and the bearings replaced before the wear of the bearings causes excessive variation in the clearance between rotors that may result in severe damage to the compressor. Frequent process shutdowns are expensive and decrease productivity.
SUMMARY OF THE INVENTION
Briefly stated, and in accordance with one aspect of the present invention, there is provided method for lubricating and sealing bearings and gears associated with a plurality of rotors of a screw compressor and isolating a process fluid to be compressed from a lubricant for the bearings and gears, the screw compressor having the process fluid and the rotors in a working chamber, the rotors having shafts supported by the bearings, the bearings contained in a plurality of bearing chambers, the shafts passing from the working chamber to the bearings in the bearing chambers, the working chamber having a low pressure inlet end and a high pressure outlet end for the compressible fluid, comprising: providing a low bearing chamber pressure to a first bearing chamber adjacent the low pressure inlet end of the working chamber, the low bearing chamber pressure at least equal to about 90% of the pressure at the low pressure inlet end of the working chamber; providing a high bearing chamber pressure to a second bearing chamber adjacent the high pressure outlet end of the working chamber, the high bearing chamber pressure at least equal to about 90% of the average pressure at the high pressure outlet end of the working chamber;
pumping oil to the bearings in the plurality of bearing chambers under pressure;
sealing the first and second bearing chambers from the working chamber by seals having a bore around each rotor shaft, the seals comprising a body having a first end adjacent the working chamber and a second end adjacent a bearing chamber and an inner groove in the bore intermediate to the ends, the inner groove of each seal connected to a source of buffer gas; providing a buffer gas to the seals adjacent the first bearing chamber, the buffer gas having a low pressure adjacent the groove greater than the low bearing chamber pressure, a portion of the low pressure buffer gas entering the first bearing chamber; providing a buffer gas to the seals adjacent the second bearing chamber, the buffer gas having a high pressure adjacent the groove greater than the high bearing chamber pressure, a portion of the high pressure buffer gas entering the second bearing chamber;
releasing the oil in the first bearing chamber and the portion of the low pressure buffer gas from the first bearing chamber to maintain the low bearing chamber pressure; and releasing the oil in the second bearing chamber and the portion of the high pressure buffer gas from the second bearing chamber to maintain the high bearing chamber pressure.
Pursuant to another aspect of the present invention there is provided a method for lubricating and sealing the bearings and gears associated with a plurality of rotors of a screw compressor and isolating a process fluid to be compressed from a lubricant for the bearings and gears, the compressor having a process fluid and the rotors in a working chamber, the rotors having shafts supported by the bearings, the bearings contained in a plurality of bearing chambers, the shafts passing from the working chamber to the bearings in the bearing chambers, the working chamber having a low pressure inlet end and a high pressure outlet end for the compressible fluid, comprising: providing a first bearing chamber adjacent the low pressure inlet end of the working chamber; providing a second bearing chamber adjacent the high pressure outlet end of the working chamber; pumping oil to the bearings in the plurality of bearing chambers under pressure; sealing the first and second bearing chambers from the working chamber by seals having a bore around each rotor shaft, the seals comprising a body having

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