Compressors
A compressor adds shaft work to a gas to increase its pressure and usually its temperature. For a steady-flow compressor with negligible heat transfer and changes in kinetic and potential energy, the specific work input is w_c = h_out − h_in. An isentropic compression endpoint represents the minimum ideal work; friction, turbulence, leakage, and heat transfer make actual work higher. Compressors are broadly classified as positive-displacement machines, which trap and reduce gas volume, or dynamic machines, which add velocity and convert it to pressure in a diffuser. A high pressure ratio can produce excessive discharge temperature and work, so multistage compression with intercooling is often used. Reciprocating-compressor capacity is affected by clearance re-expansion and volumetric efficiency, while centrifugal compressors must avoid surge at low flow.
Knowledge Check
1.What does a compressor primarily do to the gas passing through it?
2.For an adiabatic compressor with negligible kinetic and potential energy changes, what is the input specific work?
3.A compressor raises refrigerant enthalpy from 300 kJ/kg to 450 kJ/kg at a mass flow rate of 2 kg/s. What is the compressor power?
4.A compressor requires 250 kJ/kg of actual work for a process with an isentropic work requirement of 200 kJ/kg. What is its isentropic efficiency?
5.Air enters a compressor at 300 K with a pressure ratio of 8. For ideal isentropic compression with k = 1.4, what is the approximate outlet temperature?
6.Why is multistage compression with intercooling often used for a high overall pressure ratio?
7.Which statement describes positive-displacement compressors such as reciprocating or scroll compressors?
8.How does a centrifugal compressor primarily raise gas pressure?
9.What is compressor surge in a centrifugal compressor?
10.Why is an aftercooler often installed downstream of a compressor?