A Generalized Performance Prediction Method for Adiabatic Capillary Tubes

Abstract

Generalized dimensionless correlations were developed to predict refrigerant mass flow rate in an adiabatic capillary tube, for both subcooled and quality inlet conditions. Dimensionless parameters were created using the Buckingham Pi Theorem. These parameters were based on the effects of tube geometry, inlet conditions, and fluid properties. The correlations were based on experimental performance data with refrigerants R-134a, R-22, and R-410A that encompass an extensive range of operating conditions, including: condensing temperatures from 79 °F to 127°F (26°C to 53°C), capillary tube inner diameters from 0.026 to 0.090 in (0.66 to 2.29 mm), capillary tube lengths from 20 to 200 in (508 to 5080 mm), and inlet conditions from 30°F (16.7°C) sub-cooled to 35% quality. Corresponding to this range of refrigerants, capillary tube geometry, and operating conditions, the experimental mass flow rate range was between 3 and 375 lbm/h (1.4 to 170 kg/h). Independent assessments of both correlations were made by comparing predictions to measured performance with R-152a. These data fell within the ±6% and ±13% prediction intervals for the subcooled inlet and quality inlet correlations, respectively. The accuracy of the subcooled inlet correlation was further assessed by comparing predicted performance to measured data reported by other investigators with refrigerants R-12, R-134a, and R-22. It was shown that the subcooled inlet correlation predicted the bulk of these flow data to within ±10%. However, when compared to reported data with R-600a (isobutane), however, the subcooled inlet correlation over-predicted mass flow rate by 0 to 20%.