What effect does replacing a capillary tube with the same length but a smaller diameter have on evaporator pressure and temperature?

Replacing a capillary tube with the same length but a smaller diameter will result in lower pressure and temperature in the evaporator. This is primarily due to the increased flow resistance that the smaller diameter creates within the capillary tube.

In a capillary tube system, the refrigerant requires a certain amount of pressure drop to allow it to expand and change phase. When the diameter of the tube is reduced, the cross-sectional area decreases, which restricts the flow of the refrigerant. This restriction causes a higher pressure drop across the tube, leading to lower pressure at the outlet entering the evaporator. As a direct consequence of this reduction in pressure, the refrigerant will also have a lower boiling point, which means the temperature in the evaporator will decrease.

Consequently, as the refrigerant flows through the smaller diameter capillary tube, it experiences greater resistance, resulting in less refrigerant reaching the evaporator at the required pressure for proper cooling operation. This relationship between pressure, flow rate, and temperature is crucial in understanding the efficiency and performance of refrigeration systems, making it clear why lower pressure and temperature are observed when a smaller diameter capillary tube is employed.