Calculate reciprocating compressor discharge temperature from suction, ratio, and k, per API 618.
| Method | Formula |
|---|---|
| Adiabatic | T2 = T1 × r(k-1)/k |
| Polytropic | T2 = T1 × [1 + (r(k-1)/k - 1) / η] |
Where: r = compression ratio, k = Cp/Cv, η = efficiency
Understand compression thermodynamics and temperature limits
Isentropic discharge temperature is T2 = T1 × (P2/P1)(k−1)/k, where T1 is suction temperature in Rankine and k = Cp/Cv. Actual T2 is higher due to inefficiency: T2,actual = T1 + (T2,isen − T1) / η, with η typically 0.80–0.92 for reciprocating cylinders.
Suction temperature, compression ratio (P2/P1), gas specific heat ratio (k = Cp/Cv), and isentropic efficiency. Higher k and higher ratio drive T2 up sharply; for natural gas (k ≈ 1.27), every additional unit of ratio adds roughly 40–50°F at typical suction temperatures.
Adiabatic (isentropic) assumes ideal compression with no losses. The polytropic option in this calculator divides the isentropic temperature rise by the adiabatic efficiency to estimate the real discharge temperature, which is always higher than the isentropic value.
API 618 sets 300°F (150°C) as the design discharge-temperature limit; 350°F (180°C) is the typical high-temperature alarm/trip ceiling and the limit for TFE wear parts. Sour gas (H2S > 100 ppm) drops the limit to 275°F per NACE MR0175. Mineral oil lubricants degrade above 350°F; synthetic PAO oils extend the limit to 400°F.