Which type of compressor is typically controlled by a hot gas bypass valve for capacity control?

• A. Reciprocating hermetic compressor
• B. Centrifugal compressor
• C. Scroll compressor
• D. Screw compressor

Answer: (A)

Capacity control in refrigeration systems can be achieved by bypassing some of the hot, high-pressure discharge gas back to the evaporator. This method works well with a reciprocating hermetic compressor because its displacement is fixed, so you can modulate the system’s cooling output without changing the compressor’s speed or geometry. By sending hot discharge gas into the evaporator, the evaporating temperature and suction pressure rise, which reduces the net cooling effect and lowers capacity as needed. Other compressor types use different ways to adjust capacity—such as varying flow with vanes in centrifugal units or using unloading, variable-speed drives, or other control mechanisms in scrolls and screws—so hot gas bypass isn’t the typical method for them.