HFO, condensation, heat transfer coefficient, pressure drop, GWP, BPHE
All commonly used Hydro-Fluoro-Carbon (HFC) refrigerants have a high Global Warming Potential (GWP), higher than 1000, and some countries have already enacted legislative measures towards a limitation in the use or a gradual phase-out of HFCs. HFO1234ze(Z) was identified as a new low GWP refrigerant, which has the potential to be a global sustainable solution particularly for heat pump application. HFO1234ze(Z) is a pure compound which exhibits low pressure and is classified by ANSI / ASHRAE Standard 34 (2010) as class A2L refrigerants (lower flammability and lower toxicity). Therefore, it can be used in direct expansion systems without the need for a secondary loop as alternative for HFC236fa and HC600a. This paper presents the experimental heat transfer coefficients and pressure drop measured during HFO1234ze(Z) saturated vapour condensation inside a small commercial BPHE: the effects of refrigerant mass flux and saturation temperature were investigated. The experimental facility consists of a refrigerant loop, a water-glycol loop and two water loops. In the first loop the refrigerant is pumped from the sub-cooler into the evaporator where it is evaporated to achieve the set condition at the condenser inlet. The refrigerant goes through the condenser where it is condensed and then it comes back to the post-condenser and the sub-cooler. A variable speed volumetric pump varies the refrigerant flow rate and a bladder accumulator, connected to a nitrogen bottle and a pressure regulator, controls the operating pressure in the refrigerant loop. The second loop is able to supply a water-glycol flow at a constant temperature used to feed the sub-cooler and the post-condenser. A set of 42 saturated vapour condensation data points with refrigerant down-flow and water up-flow was carried out at four different saturation temperatures: 25, 30, 35 and 40°C. The experimental results were presented in terms of heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show weak sensitivity to saturation temperature and great sensitivity to refrigerant mass flux. At low refrigerant mass flux ( < 20 kg/m2s) the heat transfer coefficients are independent of mass flux and condensation is controlled by gravity. For higher refrigerant mass flux ( > 20 kg/m2s) the heat transfer coefficients depend on mass flux and forced convection condensation occurs. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on the refrigerant mass flux. HFO1234ze(Z) shows heat transfer coefficients higher both than HC600a and HFC236fa and frictional pressure drop higher than HFC236fa and lower than HC600a. The experimental results were compared against theoretical models for condensation heat transfer coefficients (Nusselt 1916 and Akers et al. 1959) and a new linear correlation for two-phase frictional pressure drop was presented.