REFRIGERATION
OIL COOLING FOR SCREW COMPRESSORS − PART II I
n Part I in the November/December 2020 issue (available online at www.mechanicalbusiness.com), the need for oil cooling in screw compressors at higher compression ratios was discussed. In this article, we review the various methods of oil cooling in more detail; the function of thermostatic mixing valves in oil cooling applications; and points that cover the sequence of operation. As mentioned, liquid refrigerant injection oil cooling is very limited. The amount of injection cooling must not exceed a certain percentage of the total cooling capacity. When cooling the compression process using this method, the liquid injection may be fed into the economizer port or dedicated liquid injection port as recommended by the compressor manufacturer. Note the liquid is not fed into the suction port of the compressor as it can result in lubrication problems for the compressor. Water-cooled, refrigerant (DX) and air-cooled are the main methods used for oil cooling. The DX cooler simply uses a DX system to extract the heat while the water-cooled method uses an oil-to-water or aqueous solution type of heat exchanger with one or more valves to control the fluid flow. In situations where the oil cooler temperature drops below 20°C (68°F) during the off cycle and/or the volume of oil in the oil cooler and associated piping exceeds 25 litres or 6.6 U.S. gallons, the circuit must be designed so that
the oil leaving the oil separator bypasses the oil cooler. This can be accomplished by using a mixing valve which, in addition to regulating flow through the oil cooler, allows the oil to warm up quickly. In situations where the oil cooler is subject to cold ambient temperatures, it may be also necessary to add heat and insulation to the oil injection piping. In this type of application, it is also important to ensure that the oil separator is kept sufficiently warm during the off cycle and the oil separator is heated and insulated to minimize heat loss. The mixing valve is typically installed so that the hot oil from the oil separator is connected to port B and the cooled oil from oil cooler is connected to port C on the valve. These ports are both used as inlets in this case. Mixing valve port A is then connected to the oil injection line feeding the compressor.
FIGURE 1 WATER-COOLED OIL COOLER
An alternate way to connect this valve is in a diverting type of arrangement, where the hot oil from the oil separator is connected to valve port A, the oil cooler inlet is connected to valve port C, and the compressor’s oil injection line is connected to port B. In this case, the valve is used in such as way that results
Phil Boudreau
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M e c h a n i c a l
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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