Reciprocating compressor compression ratio

Reciprocating piston compressors. Compression ratios are 1.2 to 6 select to keep outlet temperature <150°C. Efficiencies for reciprocating compressors are 65% for compression ratio 1.5 75% compression ratio 2, and 80% to 85% for compression ratios 3 to 6. Power 70 to 1200 kW/mVs. 

Non-lubricated screw compressors have very close clearances and thus they are designed for limited ranges of discharge temperature, temperature rise, compression ratio, etc., all of which can cause changes in thesc clearances. Lubricated compressors have a somewhat broader tolerance to changes in operating conditions, but they are still more limited than reciprocating compressors. 

So far, we have limited our discussion to adiabatic compression efficiency. This sort of inefficiency downgrades work to heat. For a given compression ratio, the temperature rise of the gas as it flows through the compressor may be excessive, thus indicating a low adiabatic compression efficiency. Both centrifugal and reciprocating compressors suffer from this common problem, which is the subject of Chap. 30. 

Let us assume that a reciprocating compressor has two cylinders working in parallel. Each cylinder has both a crank-end section and a head-end suction, where gas is compressed. In effect, we have four small compressors working in parallel. The inlet and outlet pressures, and hence the compression ratio, for each of these four minicompressors, is the same. The relative efficiency for each minicompressor is then... 

Figure 7.27. Efficiencies of centrifugal and reciprocating compressors, (a) Polytropic efficiencies of centrifugal compressors as a function of suction volume and compression ratio (Clark Brothers Co.), (b) Relation between isentropic and polytropic efficiencies, Eqs. (7.22) (7.23). (c) Isentropic efficiencies of reciprocating compressors (De Laval Handbook, McGraw-Hill, New York, 1970). Multiply by 0.95 for motor drive. Gas engines require 7000-8000 Btu/HP.

Reciprocating compressors No Compression ratio Flow rate Power input Piston displacement >100 1 >100 1 >100 1 10... 

Reciprocating compressor No Compression ratio Input rate and power Piston displacement >100 1 10 1 10... 

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