If a centrifugal compressor has suction pressure of 3 psi and discharge pressure of 10 psi, with atmospheric pressure 14.7 psi, the ratio of absolute discharge pressure to absolute suction pressure is closest to:

• A. 1.39
• B. 0.68
• C. 2.05
• D. 1.10

Answer: (D)

The ratio uses absolute pressures, so convert the given gauge pressures to psia by adding atmospheric pressure. Absolute suction pressure = 3 + 14.7 = 17.7 psia. Absolute discharge pressure = 10 + 14.7 = 24.7 psia. Divide: 24.7 / 17.7 ≈ 1.395, which is about 1.40 and closest to 1.39. Using gauge values alone would give a misleading result (for example, 10/3 ≈ 3.33 or other missteps), but the correct approach is adding atmospheric pressure to both pressures before forming the ratio.