Positive displacement compressors perform work on the gas in a repetitive noncontinuous process. The types…
The Role Of Reciprocating Compressors
In the early 1970s the builder of a large refrigeration facility would install a row of reciprocating compressors in the machine room to provide the compression requirements. Today, the same owner would choose one or two screw compressors to achieve the same capacity. At one time, reciprocating compressors were the standard choice for industrial refrigeration plants—sometimes combined with rotary-vane compressors. The high-volume capacity of the rotary-vane machine suited it for the low stage in a two-stage system. The screw compressor has made inroads into the reciprocating compressor market to such an extent that it is appropriate to evaluate the role of the reciprocating compressor for industrial refrigeration.
Here are some observations regarding the current role and future prospects of the reciprocating compressor in industrial refrigeration. During the several decades of its shrinking market, some manufacturers of reciprocating compressors disbanded their production. The result is that only the strong producers of reciprocating compressor remain today. Sales of reciprocating compressors seem to have stabilized at this lower level in North America, while in Europe and Asia the reciprocating compressors has maintained a strong position. The service and repair requirements remain less sophisticated for the reciprocating compressor.
Customers often prefer the screw compressor because of lower maintenance costs. The screw compressor might be run for 30,000 to 60,000 hours before overhaul, while plant operators have traditionally serviced reciprocating compressors annually. However, many manufacturers of reciprocating compressors are recommending that while the cylinder heads should be removed annually for a routine inspection that the machines can be operated between 10,000 and to 20,000 hours before overhaul. What has happened to the requirement of annual overhaul? To a considerable extent it had been a tradition, and in some cases was encouraged by the manufacturer to further the profitable sale of repair parts. When the operating conditions are not abusive, annual overhauls of compressors may not be necessary due to the improved quality of materials and manufacturing processes available today.
The efficiencies of the reciprocating and screw compressors are of the same order of magnitude at full load. In refrigeration capacities less than about 350 kW (100 tons of refrigeration) the reciprocating compressor is slightly more efficient. At part load, the reciprocating compressor with cylinder unloaders almost always enjoys a higher efficiency than the screw compressor. The first cost of a single reciprocating compressor is normally less than that of a screw compressor of the same pumping capacity. The first cost comparison switches in favor of the screw compressor in large sizes when one screw compressor can provide the capacity of multiple reciprocating compressors. The screw compressor can work against higher compression ratios than can the reciprocating compressor. Thus, a screw compressor is capable of operating single stage from a low-temperature evaporator in a two-stage system during periods of low capacity demands.
The maximum refrigeration capacity of the largest reciprocating compressor available on the market in a typical high-stage application is approximately 900 kW (250 tons of refrigeration), while one large screw compressor can deliver over 4000 kW (1200 tons). The owner of a large-capacity plant will achieve lower capital costs by choosing screw compressors, and accommodate the machines in much less space. If the refrigeration demand on the plant is greater than that capable of being provided by one reciprocating compressor, two units would be needed, which is more costly than one screw compressor, but the reciprocating compressor installation has the advantage of having one unit as a backup if the other is out of service.
It must be accepted that the reciprocating compressor will provide only a portion of the compressor capacity for the industry of the future. Its place is likely to be in plants of less than 1000 kW (300 tons), and many such plants exist. In many packaged refrigeration applications, such as ice makers, reciprocating compressors are usually favored. The treatment of reciprocating compressors in this book reflects the evaluation that reciprocating compressors, although providing much less capacity for the industry than the screw compressor, are still extremely important. Furthermore, some of the analyses of the performance of reciprocating compressors translate directly to screw compressors as well.
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