Which device is commonly used to modulate refrigerant flow by varying the amount of liquid refrigerant admitted?

• A. Thermostatic expansion valve
• B. Capillary tube
• C. Solenoid valve
• D. Check valve

Answer: (B)

Controlling how much liquid refrigerant gets into the evaporator requires a valve that can adjust its opening as conditions change. A thermostatic expansion valve does exactly that: it uses a sensing bulb on the evaporator (or suction line) to detect temperature and, based on that, modulates the valve opening to admit more or less liquid refrigerant. This continuous metering keeps the evaporator at the desired superheat, matching the load and preventing flood or starvation.

A capillary tube, on the other hand, is a fixed metering path with a tiny bore. It creates a predictable pressure drop but cannot actively adjust to varying conditions, so the flow isn’t modulated in response to load changes. A solenoid valve can open or close, offering on/off control rather than steady, variable metering, and a check valve prevents reverse flow rather than regulating the amount admitted.