Five short-tube refrigerant expansion devices, with L/D ratios from 7.5 to 11.9, were tested on a three-ton split-system heat pump. The test results show that first-stage choking has occurred at the liquid subcooling temperature of 22.2 C° (40.0 F°). However, there is no indication that second-stage choking has ever occurred.

Based on the test results, two correlations were formed. One is an orifice model with the orifice constant as a function of the pressure differential and the level of liquid subcooling. This model is for liquid subcooling less than 22.2 C° (40.0 F°). The other is a first-stage choking model, which is for a level of liquid subcooling above 22.2 C° (40.0 F°).