Identification and Quantification of Start Up Process in Oil Flooded Screw Compressors

Abstract

Oil injected screw compressors, used for air compression, refrigeration and air conditioning, are usually installed in simple packages which often do not include an oil pump to supply oil to the bearings and the working chamber, since the oil is pressurised, together with the gas being compressed, in the working chamber. Consequently, normal lubrication will only start when the back pressure reaches the chamber pressure,. Due to the lack of lubrication in this start up period, the rotors will be in direct contact with insufficient or no oil film between them, while the pressure in the compression chamber will increase causing the temperature to rise. The period of un-lubricated operation then depends on the size of the oil system and the length of the discharge piping. Also, during this time, due to the lack of oil in the working chamber, the leakage flow will be higher than normal. This will increase the overall temperature in the compression chamber. It is expected that some surface damage may occur on the rotors. When the compressor operates intermittently, with frequent starts and –stops, mode, this may result in rapid wear and decrease in the compressor performance. This paper addresses such issues through experimental investigation and simulation and defines the scope of work required to understand the oil flooded compressor process in transient operation mode. This will allow prediction of wear in such compressors and give some insight to the modifications required to increase the compressor reliability.