Compressor Calculator — Shaft Power, Isentropic Efficiency & Discharge Temperature (ideal gas; PTC 10 / ISO 1217 terminology)

The ideal (isentropic) specific work is derived from the steady-flow energy equation for a reversible adiabatic process. It depends on the inlet temperature, the pressure ratio and the ratio of specific heats γ = Cp/Cv of the gas. The actual specific work is the isentropic work divided by the isentropic efficiency (not a polytropic efficiency — convert one first), which accounts for internal irreversibilities (heat transfer through the casing, gas leakage, friction). ASME PTC 10 (centrifugal and axial machines) and ISO 1217 (displacement machines) are performance-TEST codes that define how these quantities are measured; the calculator uses their terminology but is not a test-code evaluation.

The actual discharge temperature follows from the isentropic temperature rise scaled by the efficiency. For a multi-stage machine the equal stage pressure ratio is the classical split for minimum total work. Between stages the gas leaves each intercooler at the suction temperature plus an approach (enter your intercooler outlet temperature; 10 K is assumed when you do not, and a limit exceedance caused only by that assumption is flagged for review rather than failed), so stages 2…N start slightly hotter than stage 1 — perfect intercooling back to the suction temperature is the 0 K case.

A reciprocating compressor is analysed via its polytropic P-V indicator diagram — the closed loop of compression (1→2), discharge (2→3), re-expansion of clearance gas (3→4) and suction (4→1). The clearance volume reduces the effective swept volume: gas trapped in the clearance expands back on the suction stroke before fresh charge is admitted, reducing the volumetric efficiency. The indicated work per cycle equals the enclosed area of the P-V loop, computed on the net induced volume (BDC − end of re-expansion), which correctly excludes the shuttle work of the clearance gas.