Condensation heat transfer and pressure drop of low-pressure hydrocarbons and synthetic refrigerants

Synthetic refrigerant R-245fa is an alternative to R-134a in HVAC systems and is being considered for use in Organic Rankine Cycles (ORCs) for power generation from low-grade heat sources. Pentane, in combination with R245fa, is being considered for use in geothermal heat generation. An experimental investigation of condensation heat transfer and frictional pressure drop for pure R-245fa and pentane flowing through smooth horizontal tubes with internal diameters of 7.75 mm and 14.45 mm was conducted. These measurements were made over mass fluxes ranging from 100 to 600 kg m(-2) s(-1), operating pressures from 122 to 610 kPa, and nominal vapor qualities ranging from 0.05 to 0.95. Results from this experimental study are compared with predictions from correlations in the literature, and agreement and differences are interpreted and discussed. The measured frictional pressure drops ranged from 0.25 to 100 kPa m(-1), and heat transfer coefficients from 1.5 to 15 kW m(-2) K-1 were observed. Heat transfer and pressure drop models that apply specifically to these low pressure fluids, tube diameters, and operating conditions are developed. These models provide significant statistical improvements in the predictions of the heat transfer coefficient and pressure drop over existing correlations. (C) 2020 Elsevier Ltd. All rights reserved.