The throttling of refrigerant from the high-pressure side (high side) to the low-pressure side (low side) of an air conditioner/heat pump may be accomplished by a variety of devices, the most common being thermostatic expansion valves, capillary tubes, and plug orifices. This study focuses on the adiabatic capillary tube, and is directed toward detailed data that would lead to means of selecting capillary tubes given the liquid line pressure, flow rate, and degree of inlet subcooling and evaporator pressure. Several capillary length and diameter combinations were tested using a test loop that provided control of the subcooling, and condenser and evaporator pressures. Tests were also performed to measure the distribution pressure drops and to determine the effects of coiling on the restriction characteristics of the capillary tube. Coiling was noted to increase the pressure drops by about 5%, while the loss factor for the distributors was found to be slightly higher than that for re-entrant flows.

KEYWORDS: capillary tubes, tubes, R22, refrigerants, testing, fluid flow, throttling, air conditioning, heat pumps, subcooling, pressure drop.