The suction pressure of a reciprocating compressor is 65 PSIG and the discharge pressure is 120 PSIG. The atmosphere is 14.7 PSIG. The compression ratio is closest to:

• A. 1.35
• B. 1.69
• C. 2.10
• D. 0.90

Answer: (B)

The main idea is that the compression ratio is the discharge absolute pressure divided by the suction absolute pressure. Since the pressures given are gauge pressures, you must convert them to absolute pressures by adding the atmospheric pressure (14.7 psi).

Suction absolute pressure = 65 + 14.7 = 79.7 psi

Discharge absolute pressure = 120 + 14.7 = 134.7 psi

Compression ratio = 134.7 / 79.7 ≈ 1.69

So the answer around 1.69 is the best fit. Using gauge pressures directly would misrepresent the ratio because atmospheric pressure affects both sides and must be included in absolute terms.