Refrigerant Flow Through Flexible Short-Tube Orifices

Abstract

Flexible short-tube orifices were designed to decrease their diameter as the pressure differential across them increases under high outdoor temperatures. A series of tests for an R-22/lubricant mixture (mass fraction of oil 1.2%) were performed with two flexible short-tube orifices to develop flow data over a range of typical air conditioner operating conditions. One short tube had a modulus of elasticity of 7063 kPa and the other a value of 9860 kPa. Both short tubes had identical lengths (14.5 mm), entrance diameters (2.06 mm), and exit diameters (2.46 mm). The tests included both single- and two-phase flow conditions at the inlet of the flexible short tube. Upstream pressures were varied from 1179 kPa to 2144 kPa, which corresponded to saturated condensing temperatures of 29.4 to 54.4°C. Experimental results were presented as a function of pressure, subcooling/quality, evaporating pressure, and modulus of elasticity. Mass flow rates were compared with those of a rigid short tube. The flow rate through the flexible short-tube orifices was strongly dependent on the condensing pressure, subcooling/quality, and modulus of elasticity (which, in effect, changed diameter at different upstream pressures) of the short-tube material. However, the flow rate showed little dependence on the evaporating pressure. An empirical flow model was developed using the experimental data. This flow model was then combined with an air-conditioning system simulation model. Results from the model indicated that the flexible short tube provided approximately 2 to 3% higher capacity than the fixed expansion devices at temperatures lower than 25°C and higher than 45°C. However, the flow control with the flexible short-tube orifice was not as good as with a thermal expansion valve.