Airside heat transfer, airside friction, correlations, heat pump, optimization
The use of small diameter tubes in air-to-refrigerant heat exchangers has significant advantages, which include increase in heat transfer coefficient, reduction in size, reduction in material or weight and reduction in refrigerant charge. However, there are no air-side correlations for small diameter tubes below 2mm in the literature. Furthermore, conventional empirical correlation development relies on testing of samples, which is inherently time consuming, expensive and has a limited range of applicability. This paper presents equations for airside friction and heat transfer characteristics for bare tube air-to-refrigerant Heat eXchangers (HX) with tube diameters ranging from 0.5mm to 2mm, and are valid for 2 to 40 rows of tubes in both staggered and inline arrangements. The correlations presented in this article are developed based on comprehensive CFD simulations for a large design space and include experimental validation. More than 80% of source data can be predicted within 10% error and more than 90% within 20% error. In this paper we use these correlations to optimize the condenser and evaporator of a 3 ton heat pump unit using R410A as the working fluid. The HX optimization framework uses a Multi-Objective Genetic Algorithm (MOGA) and an in-house HX design tool based on a segmented Îµ-NTU method. The optimum designs exhibit more than 50% reduction in size, and up to 50% reduction in both air and refrigerant pressure drops, compared to the baseline tube-fin HXâ€™s using tube diameters larger than 7mm. In a system context the optimum HXâ€™s demonstrated the ability to reduce 50% of the refrigerant charge within the HXâ€™s and shifting majority of the system refrigerant mass to the connecting pipes. The COP is improved by 5%-7% for the same capacity.