THEORETICAL INVESTIGATION OF AN ADIABATIC CAPILLARY TUBE WORKING WITH PROPANE / n-BUTANE / iso-BUTANE BLENDS

In this paper, a theoretical model is developed to predict refrigerant flow characteristics in adiabatic capillary tubes using propane/n-butane/iso-butane mixtures as working fluids in a domestic refigerator. This model is based on mass, energy and momentum conservation equations for a homogeneous refrigerant flow under different inlet conditions such as subcooled, saturated and two-phase flow. Effects of inlet pressure (8-16bar), inlet vapor quality (0.001-15%), inlet subcooling degree (1-15°C), mass flow rate (1-5 kg/h), propane mass fraction (0.5-0.7), capillary tube inner diameter (0.6-1.0mm) and tube surface roughness on the capillary tube length are predicted. Results showed that the present model predicts data that are very close to the available experimental data in literature with an average error of 2.65%. Pressure of hydrocabon mixture (HCM) decreases while its vapor quality, specific volume and Mach number increase along the capillary tube. Also, the results indicated that capillary tube length is largely dependent on capillary tube diameter. Other parameters such as mass flow rate, inlet pressure, sub-cooling degree (or quality) and relative roughness influence the capillary tube length in that order. Capillay tube length as function of the significant parameters is presented in equation form. Also, capillary tube selection charts either to predict mass flow rates of propane/n-butane/iso-butane mixtures through adiabatic capillary tubes or to select the capillry tube size according to the required applications are developed. The comparison betweenn R12, R134a and the hydrocarbon mixture (HCM) of propane/n-butane/iso-butane indicated that for a given mass flow rate, the pressure drop per unit length is about 4.13, 5.0 and 12.0 bar/m for R12, R134a and HCM, respectively. The ratios of average mass flow rate of HCM with propane mass fraction of 0.6 to those of R12 and R134a are about 0.62 and 0.67, respectively. Average capillary tube length for HCM with propane mass fraction of 0.6 is longer than those of R134a and R12 by about 30% and 48%, respectively.