The volume of vapor drawn into a compressor is expressed as?

• A. Liters per second
• B. Cubic feet per minute (CFM)
• C. Bar
• D. Gallons per minute

Answer: (B)

The idea being tested is that the flow into the compressor is a rate—the amount of vapor moving past the suction in a given time, i.e., a volumetric flow rate. In ammonia refrigeration practice, this suction gas flow is typically expressed in cubic feet per minute, which tells you how much vapor is entering the compressor each minute.

While liters per second could express the same idea in metric units, the standard convention used in RETA-style questions is cubic feet per minute. Bar measures pressure, not how much gas is flowing, so it doesn’t describe volume flow. Gallons per minute is a valid way to express flow, but the common, accepted unit in this context is CFM, which aligns with the practical measurements used for compressor suction.

Remember that the actual volume flow depends on the gas density at suction conditions, so the same mass flow could correspond to different volumetric flow rates if density changes.