The thermal expansion device and the ________ tubes are two popular refrigerant capacity controls.

• A. Capillary
• B. Piston
• C. Receiver
• D. Compressor

Answer: (A)

Two popular refrigerant capacity controls are the thermal expansion device and the capillary tube. A capillary tube is a fixed metering device: its small, constant diameter creates a controlled pressure drop as liquid refrigerant flows into the evaporator, metering the amount of refrigerant without any moving parts. This simple, inexpensive method is often used in smaller or economical systems. It contrasts with components like a piston, receiver, or compressor, which do not serve as fixed metering tubes for capacity control. The thermal expansion device (TXV or EEV) can adjust to load by modulating flow based on evaporator conditions, whereas the capillary tube provides a fixed restriction. Together they represent two common approaches to controlling system capacity.