Correlations for sizing adiabatic capillary tubes

This paper presents new correlations for the practical sizing of adiabatic capillary tubes used as an expansion device in small refrigerating and air-conditioning systems. The governing equation based on conservation of mass, energy and momentum is modelled. The developed model is used as an effective tool for studying the effects of relevant parameters on capillary tube length and developing the correlation. In this model, Colebrook's equation is used to determine the friction factor. The two-phase viscosity models are varied depending on the type of refrigerant and are based on the recommendations from past research. By varying the model input parameters, it is possible to show that for all refrigerants, the length decreases as the mass flow rate increases, increases as subcooling increases, increases as tube diameter increases, decreases as tube roughness increases and increases as condensing temperature increases. After the developed model is validated by comparing with existing experimental data, correlations for sizing capillary tubes, which contains the relevant parameters, namely condensing temperature, degree of subcooling, refrigerant mass flow rate, capillary tube inner diameter and roughness, are presented. Different from previous studies, correlations are presented for an extensive number of refrigerants and a wide range of operations. The developed correlations are validated with previous studies and found to agree well with the experimental data. The correlations can be used to integrate with system models working with alternative refrigerants for practical design and optimization. Copyright (C) 2003 John Wiley Sons, Ltd.