Investigating heat transfer and frosting performance of air source heat pumps with the impact of particulate fouling

Due to superior energy efficiency and emission-reducing potential, air source heat pumps (ASHPs) have been widely used in space heating and domestic water heating. However, the heating seasons usually coincides with the serious air pollution, hence particulate fouling appears on ASHPs' outdoor coils repeatedly, despite being de-fouled periodically. In this study, four levels of fouling, namely none fouling (0 g/4 m²), mild fouling (15 g/4 m²), moderate fouling (40 g/4 m²) and severe fouling (70 g/4 m²), were concretized on a finned tube heat exchanger successively, followed by experimentally investigating the negative effects of fouling on the heat transfer performance of an air source heat pump water heater (ASHPWH). Results demonstrated that the effects of fouling on the performance of ASHPs in heating mode would be exacerbated with decreased air temperature. In addition, the frosting performances at different levels of fouling were evaluated quantitatively, by depicting the frosting evaluation plots. The frosting evaluating plots suggested that fouling would expand the air temperature and relative humidity (RH) ranges where frosting appears, and would hasten frosting process. As verification, signal factor experiments were carried at different levels of fouling. The experimental results confirmed that the increase of fouling level would result in faster frosting process, but smaller final frost accumulation mass.