Accuracy of simplified heating coil models based on manufacturer catalogue data

Abstract

The overall thermal performance of a heating coil, typical of those used in air conditioning systems, is investigated by two models based on the classical ε–NTU method. The first model, here taken as the reference model, is supported by detailed information about the coil geometry and published correlations for the air and liquid Nusselt numbers. The second model, much simpler, does not require a detailed knowledge of the coil geometry. It is based on mechanistic correlations with unspecified parameters that are calibrated for a particular coil using catalogue information. The purpose of this paper is to compare the prediction ability of both models for a typical HVAC heating coil.The reference model is first validated by comparing its results with those of a certified software provided by a coil manufacturer. The agreement is good in general, but errors are significant when the coil operates at low flow rates. Once validated, the reference model is used to generate two separate sets of coil performance data by systematically varying the mass flow rate and inlet temperatures of both fluids. The first dataset is representative of the conditions typically reported in manufacturer catalogues, while the second dataset includes a wider range of operating conditions that the coil can experience in practice (for example, lower than normal flow rates). The parameters in the non-geometrical model are calibrated using the first dataset. This fitted model is then applied to the second dataset for checking the prediction error. A good agreement is observed between the results, except in some cases where the water flow is laminar or in the transition regime.A practical case study of application of the simplified model for predicting the energy performance of a heating coil is finally conducted. The coil is simulated as part of a dedicated outdoor air system (DOAS) which conditions the ventilation air of a building. The DOAS does not incorporate energy recovery devices. Simulations are performed for the heating season by taking into account the local weather data. The water flow rate varies according to the strategy adopted in controlling the temperature of supply fresh air. The accuracy of different versions of the non-geometrical model is quantified.