Rotary expansible chamber devices – Methods
Patent
1996-08-22
1998-01-13
Vrablik, John J.
Rotary expansible chamber devices
Methods
418203, F04C 1816, F04C 2910
Patent
active
057072230
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a rotary screw compressor having a balancing piston.
In compressors of this type the thrust balancing device has the function to apply a force on the rotor that counterbalances the axial gas force in order to reduce the thrust load on the bearings. Such devices are generally known in the prior art. A problem, however, arises when the outlet pressure varies and in particular when also the inlet pressure varies. In such applications the gas force will vary with the result that the rotor might be under- or overbalanced at certain working conditions. This means that the load on the bearings might fall outside the range within which a sufficient bearing running life is attained. The gas forces also in general are lower during the starting period of the compressor than during normal working conditions. There is thus a need for the possibility to vary the thrust balancing force to appropriately balance the varying axial gas force.
This problem has been recognized, and is addressed in U.S. Pat. No. 3,932,073, U.S. Pat. No. 4,964,790, U.S. Pat. No.5,207,568,WO 91/12432 and PCT/SE 94/00947(published as WO 95/10708).
U.S. Pat. No. 3,932,073 discloses a device with an expansion valve, which connects the high pressure side of the balancing piston with a closed working chamber in the compressor. The valve should be automatically opened or closed, and when open it creates a pressure drop over a throttling device between an oil separator and the balancing piston in a way not further described U.S. Pat. No. 4,964,790 discloses automatic regulation of balancing pressure using a microprocessor which computes a balancing pressure to be applied to the rotor in response to parameters such as suction pressure, discharge pressure and percent capacity. U.S. Pat. No. 5,207,568 discloses a pneumatical balancing piston, which is affected by a pressure connected to a closed working chamber of the compressor. The pressure in the working chamber varies according to suction pressure to cause the piston to apply a variable counterbalancing force. WO 91/12432 discloses a balancing piston having an active pressure surface that by means of a valve can be exposed either to outlet pressure, to unthrottled inlet pressure or to throttled inlet pressure and a rear pressure surface that is exposed to unthrottled inlet pressure, which normally is about atmospheric pressure. The balancing force attained therethrough can be at either of three levels and also alter direction, so that the flexibility to adapt to different running conditions is increased. PCT/SE 94/00947 discloses means for continuously varying the pressure acting on the balancing piston. These means include first and second throttles in the return pipe from the oil separator to an oil injection port. Between the throttles there is a connection to a branch pipe which ends in a cylinder which houses the balancing piston. The balancing pressure acting on the piston will thereby vary as suction and delivery pressures vary in a way determined by the relation between the degree of throttling in the two throttles.
The known devices suffer from the drawbacks of either requiring circumstantial devices for varying the balancing force or presuppose devices that normally only are present in certain applications. There is thus a need for further improvements in this field.
SUMMARY OF THE INVENTION
The object of the present invention thus is to attain a thrust balancing device of a rotary screw compressor which is simple and reliable and which can be used in applications where the known devices not are sufficiently appropriate.
The balancing device according to the invention utilizes a high pressure for the active balancing force and either of two lower pressures of different levels for the force in the opposite direction, which thus reduces the net balancing force to different extents. This allows a lower balancing force during some working conditions, when the axial gas forces are relatively low such as during starting up of the comp
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patent: 4964790 (1990-10-01), Scott
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patent: 5207568 (1993-05-01), Szymaszek
patent: 5281115 (1994-01-01), Timuska
Englund Arnold
Timuska Karlis
Svenska Rotor Maskiner AB
Vrablik John J.
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