REFRIGERATION
OIL COOLING FOR SCREW COMPRESSORS D
uring the vapour compression process, the refrigerant heats up. The amount of temperature and enthalpy increase depends on the refrigerant type, the compression efficiency, the pressure and temperature entering the compressor, along with the discharge pressure. Refrigerants heat up in varying amounts according to their thermodynamic properties. For example, even though R404A and R507 are very similar refrigerants, the resulting discharge temperatures will be different after the compression on process. The return gas conditions also affect the final temperature of the refrigerant leaving the compressor. So, a higher suction superheat will result in a higher discharge temperature. The efficiency of the compressor also has an overall impact on the final discharge temperature. The higher the efficiency of the compression process, the lower the discharge temperature will be.
As the compression ratio increases, the heat of compression increases. It is very important that the discharge temperature of the compressor does not exceed the compressor manufacturer’s published limits. Ideally, it is recommended to stay well within the limit and not just hover at the limit. With reciprocating compressors, head cooling fans and liquid injection are the two common auxiliary cooling methods used. With screw compressors, oil cooling or liquid refrigerant injection are used. In twin-rotor screw compressor designs, the rotating helical rotors are coated with a film of oil. This oil film occupies the volume between the two rotors, as well as the volume between the rotors and the rotor housing. This oil maintains the dynamic seal required for compression.
Phil Boudreau 38
M e c h a n i c a l
Due to the large volume of oil entering the rotor area, approximately seven to 10 GPM for the compressor used in Table 1, there is the same amount of oil leaving the rotors via the discharge port. Also, this oil will be at the same temperature as the refrigerant leaving the compressor. The oil is then separated from the refrigerant, cooled if necessary, and then returned to the compressor’s oil injection port. The oil separator holds a generous quantity of oil to ensure there is always a good supply of oil available for compressor lubrication and also to maintain good high to low side sealing of the refrigerant compression process. With screw compressors, leakage will slowly increase with compression ratio. This also drives the discharge temperature upwards. Unloading a compressor has the effect of decreasing the effective length of the rotors. With a shorter effective rotor length, the discharge temperature tends to climb due to the additional leakage that takes place at higher compression ratios. This is the reason why the amount of unloading is usually limited at higher compression ratios.
Phil is the Ontario sales manager for Bitzer Canada Inc. and provides training and technical support for Bitzer’s clientele. He can be contacted at: pboudreau@bitzer.ca.
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