Among the following, which ratio correctly matches a rotary vane compressor?

• A. 12:1
• B. 4:1
• C. 20:1
• D. 6:1

Answer: (B)

The key idea is the amount of pressure rise a single-stage rotary vane compressor delivers, expressed as a compression ratio. A rotary vane uses a fixed displacement per rotation, trapping and compressing gas to raise its pressure by a moderate amount in one pass. In typical ammonia refrigeration setups, a single-stage rotary vane provides a moderate compression ratio, around four to one. That means the discharge pressure is about four times the suction pressure, which matches common low- and high-side pressures for ammonia systems. Ratios like twelve to one or twenty to one would require a much larger lift per stage than a single-stage vane is designed to deliver, and would usually indicate multi-stage compression or intercooling. A ratio near six to one is less typical for a standard single-stage rotary vane in RETA-style questions. So the four-to-one ratio best fits the characteristic pressure rise of a rotary vane compressor.