Frosting in heat pump evaporators part A: Experimental investigation

Abstract

Frost growth on an evaporator of a 5 kW air-to-water heat pump assembly was analysed using several experimental approaches. For comparative testing, the evaporators fin and tube surfaces were treated with a hydrophobic nano-coating and the difference in frosting was investigated. Besides thermodynamic measurement data, transient frost mass and extracted frost layer thickness was available through the use of a scale and image capturing techniques, respectively. Images using a thermographic-, a macro- and overview-cameras were analysed for several climatic conditions established in a climate chamber. Time intervals between defrosting at +4.5 ∘C ambient temperature increased significantly by around 20% for the coated evaporator compared to the uncoated evaporator. This difference, however, got reduced when lowering the air inlet temperature below the freezing point, where this behaviour got reversed and the coated evaporator needed shorter intervals between defrosting compared to the uncoated one. This trend is also supported by the extracted frost thicknesses from the macro-camera image stream and by analysing the transient brightness signal of full evaporator image streams characterized in this work. Frost mass growth rate is reduced between 16% and 26% for −2.0 ∘C to +5.5 ∘C ambient temperature, respectively for the coated evaporator compared to the mass growth rate of the uncoated one. Parametric studies of evaporator frosting of this 5 kW air-source heat pump at multiple air inlet stream conditions build up the base for comparison to the numerical work presented in Part B of this publication.