Predicting and optimizing the wet outer surface of water-sprayed finned-tube air-cooled condensers based on purely geometric arguments for inclined sprays: Application on heat transfer enhancement for the cooling of the refrigerant R134a

Abstract

During spray cooling, the determination of the heat energy gains due to water collection requires prior knowledge of the heater wetted surface. However, it is impossible to directly measure this surface, especially for complex geometries like finned-tube heat exchangers. In this paper, a model based on purely geometric arguments is proposed for the prediction of the wet outer surface of a water-sprayed finned-tube air-cooled condenser for an inclined spray. It is based on determining the number of wetted fins and tubes in the spray impact area and deduce therefrom the heater total wetted surface. Then, to understand the mechanism of spray cooling by examining the effects of spray inclination on heat transfers, the model is applied to the cooling of the refrigerant R134a. The results showed that the optimal horizontal spray inclination angle is 45° and increasing this angle from 0 to 45° allowed the wet surface to grow from 0.043 to 0.091 m². At β = 45°, the wet surface equalizes the spray impact area while the water collection rate tends to unity, which helped to intensify heat transfers from 1.718 to 1.744. Consequently, the refrigerant condensing temperature drops from 25.5 to 23.5 °C when β is varied from 0 to 45° and from 40 to 23.5 °C when mist is applied with 45° of inclination compared to the heater without mist. It is concluded that spray inclination allows to reduce the condenser vacuum rate and increase the water collection rate, which results in an enhancement in the condenser heat dissipation.