COMPRESSOR EFFICIENCY

The amount of gas pumped by the compressor will always be less than the physical displacement of the pistons in the cylinders. The re-expansion loss has already been mentioned and the other losses are illustrated in Figure 4.15 . Volumetric efficiency (VE) accounts for all the losses affecting the flow rate of the compressor. The reference point is the condition of the gas, pressure P1 and temperature T1 at the inlet or suction. Heat pick up due to motor losses is included in the case of enclosed types. In practice VE is close to linear with pressure ratio as illustrated in Figure 4.16. The major element of the volumet­ric loss in a piston machine is due to re-expansion. Its effect is not too serious because work is recovered in the re-expansion process.

	J r->rt
I I •U

Motor heat pick up

Pressure drops

Re-expansion Loss ‘ Leakage loss

Heat transfer

P2T2

P1T1

Volumetric efficiency (VE)

Actual volume flow at suction Compressor displacement

Figure 4.15 Volumetric losses Pressure ratio (PR) Figure 4.16 Typical VE characteristics for various compressor types

The energy efficiency of compression is defined with reference to the ideal adiabatic compression process. The isentropic power input is the minimum amount of power required to compress the gas, mass flow rate, m, from P1, T1, to P2. The actual power will always exceed the isentropic power because of the losses shown in Figure 4.17 . The magnitude of the losses will depend

Motor losses

P1T1

Over compression, pressure drops, leakage Friction and oil pump

Isentropic efficiency (IE)

Isentropic power input Actual power input

®P2T2

IE U

Figure 4.17 Power losses

On the compressor type, and an approximate order of magnitude is 10% motor loss (motor efficiency 90%), 10% friction losses and 10% flow and heat trans­fer losses. In practice values of isentropic efficiency (IE) above 70% are very good compressor efficiencies. It is difficult for designers to get much more out of the compressor. General IE trends are illustrated in Figure 4.18 . These do not relate to specific products, and manufacturer’s data must be consulted to establish specific values. Published data quote performance in terms of refrigeration capacity, power input and COP, and these values reflect the under­lying efficiency characteristics.

Pressure ratio (PR) Figure 4.18 Typical IE characteristics for various compressor types

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