CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
A client's small window air conditioner has no adjustable expansion valve. A technician explains that the unit uses a capillary tube as its metering device. Which statement best describes this device?
- AA fixed-length, fixed-diameter tube that meters refrigerant flow by pressure drop, with no moving parts and no ability to adjust for changing load conditions
- BA spring-loaded valve that mechanically adjusts opening based on measured superheat
- CA device that only meters refrigerant flow during the defrost cycle
- DAn electronic device that modulates flow using a stepper motor and sensor feedback
Show answer & explanationAnswer & explanation
Correct answer: A. A fixed-length, fixed-diameter tube that meters refrigerant flow by pressure drop, with no moving parts and no ability to adjust for changing load conditions
A capillary tube is a fixed-length, small-diameter tube that restricts refrigerant flow through pressure drop alone. Unlike a TXV, it has no moving parts and cannot compensate for changing load or ambient conditions, making systems using them sensitive to charge accuracy.
Why the other options are wrong
- B. This describes a thermostatic expansion valve (TXV), not a capillary tube.
- C. Capillary tubes meter refrigerant continuously during normal cooling operation, not just defrost.
- D. This describes an electronic expansion valve (EEV), which has active control components.
Capillary Tube Metering Device
A fixed-length, small-diameter tube used to restrict and meter refrigerant flow in smaller or simpler systems, relying purely on pressure drop with no moving parts.
- Common in window units and small refrigeration equipment
- Cannot adjust for varying load — charge must be precise
- Failure mode is typically restriction from debris or moisture
Memory trick: Capillary: thin and fixed, never adjusts, never mixed.