Heat Transfer between Airflow in Ductworks and the Environment in Large HVAC Systems

摘要

The waste of thermal energy in air conditioning systems has been extensively studied during the last 20 years. In central Heating, Ventilation and Air Conditioning (HVAC) units used for large buildings, the air is treated in a central air conditioning station before being distributed. Treated air is then blown through ductworks that may have considerable lengths (hundreds of meters). Due to architectural limitations, the duct lines may pass through conditioned as well as unconditioned areas. Generally a temperature difference exist between the supply air and the ambient surrounding the duct, resulting in a non-negligible amount of heat exchange. This heat transfer is always considered as an undesirable effect for HVAC systems. Such a situation will provoke changes in the temperature of the supplied air that do not satisfy design criteria, creating as well noticeable discomfort due to inappropriate balance of the air temperature. Some efforts have been carried out to predict temperature changes across supply ducts. The influence of wall friction on the temperature of the airflow has been studied theoretically in. Thermodynamic equations for reversible process were applied for both adiabatic and polytropic processes to show that the effect of friction on the temperature can be easily neglected. It should be noted that the assumption of a reversible processes is in contradiction with the physical nature of frictional losses. Moreover, the way that the authors applied the equations is mathematically incorrect. Air temperature changes in ductwork due to friction losses, has also been studied. It was shown that under adiabatic flow conditions the change in enthalpy along two distant points in a duct corresponds to the change in the flow kinetic energy between these two points. The effect of frictional losses was implicitly take into account. Experiments were carried out to obtain heat transfer data between different type of ducts and the environment. The data was then used to calculate the steady state overall heat transfer coefficient, U, based on the internal heat transfer surface area. The variation of U as a function of the average air velocity was presented. Based on this work, a heat loss calculation procedure was recently proposed in. The main objective of the present work is to model airflow through ducts for estimating heat losses (or gains) to the environment due to different heat transfer mechanisms. In addition, the effects due to the emissivity of the outer surface, inside hydraulic diameter (ID), insulation thickness and temperature difference across the duct are presented. The results are compared with experimental data and correction factors are proposed in order to better estimate the airflow temperature in ducts.