Which statement is not true about a capillary tube metering device?

Which statement is not true about a capillary tube metering device?

• A. they have no moving parts
• B. they respond to changes in pressure drop across the tube
• C. they maintain constant evaporator temperature
• D. they provide a fixed orifice for refrigerant flow

Answer: (C)

Capillary tubes meter refrigerant by using the pressure difference between the high-pressure liquid side and the low-pressure evaporator side. They are just a narrow tube with no moving parts, so they act as a fixed restriction. Because the flow is driven by the available pressure drop, any change in that pressure difference changes the amount of refrigerant that can pass through the tube, which in turn affects the evaporator conditions indirectly. What this device does not do is actively regulate the evaporator temperature. The evaporator temperature depends on the heat load and the overall system pressures; a metering device that actively maintains a set evaporator temperature would need to adjust flow in real time (like a thermostatic or electronic expansion valve). Since a capillary tube cannot modulate flow, it cannot maintain a constant evaporator temperature. The fixed, no-moving-parts nature, the dependence on pressure drop for flow, and the fixed restriction all describe capillary tubes correctly, while the idea of maintaining a constant evaporator temperature does not.

Capillary tubes meter refrigerant by using the pressure difference between the high-pressure liquid side and the low-pressure evaporator side. They are just a narrow tube with no moving parts, so they act as a fixed restriction. Because the flow is driven by the available pressure drop, any change in that pressure difference changes the amount of refrigerant that can pass through the tube, which in turn affects the evaporator conditions indirectly.

What this device does not do is actively regulate the evaporator temperature. The evaporator temperature depends on the heat load and the overall system pressures; a metering device that actively maintains a set evaporator temperature would need to adjust flow in real time (like a thermostatic or electronic expansion valve). Since a capillary tube cannot modulate flow, it cannot maintain a constant evaporator temperature.

The fixed, no-moving-parts nature, the dependence on pressure drop for flow, and the fixed restriction all describe capillary tubes correctly, while the idea of maintaining a constant evaporator temperature does not.