Capillary tube metering devices are described as which type of restriction?

• A. Fixed restriction devices
• B. Variable restriction devices
• C. Electrically actuated devices
• D. Pressure-regulating valves

Answer: (A)

Capillary tube metering devices are fixed restriction devices. The capillary tube is a short, narrow passage with a constant internal geometry (no moving parts or automatic adjustments). The flow into the evaporator is limited purely by the tube’s fixed diameter and length, so the amount of refrigerant that can pass through is determined by the pressure difference across the tube and the refrigerant’s properties at the operating temperature. Because there’s no mechanism to vary the opening in response to load or temperature, the restriction stays the same, unlike devices that dynamically adjust the flow.

Thermostatic expansion valves or other variable-restriction devices change the orifice size in response to superheat or temperature/pressure conditions, which is why they’re not described as fixed restrictions. Pressure-regulating valves control system pressure rather than metering flow, and electrically actuated devices describe actuation methods, not the fundamental restriction nature.