Evaporation of refrigerants in a smooth horizontal tube: prediction of R22 and R507 heat transfer coefficients and pressure drop

This paper presents experimental heat transfer coefficients and pressure drop results obtained during the evaporation of pure R22 and the azeotropic mixture R507 (R125–R143a 50%/50% in weight). The test section was a smooth, horizontal, stainless steel tube (6 mm ID, 6 m length) uniformly heated by Joule effect. The effects of heat flux, mass flux and evaporation pressure on the heat transfer coefficients have been investigated. Each working parameter was bound within the range: evaporating pressure 3–12 bar, refrigerant mass-flux 250–286 kg/m2 s, heat flux 10.6–17.0 kW/m2, respectively. Additionally the experimental results have been compared with existing correlations which characterize the evaporative heat transfer coefficient to assess the validity of these models for refrigerant mixtures.