AbstractsEngineering

An increased use of heat pumps is one of the measures that can be taken to reduce energy consumption on a large scale, particularly in areas where buildings generally are heated by electrical radiators. For a wider acceptance, and a major heat pump market expansion, it is crucial to develop heat pumps that cause minimal disturbance, especially in densely populated areas. Results from field measurements made by SP Technical Research Institute of Sweden indicate that the use of so called indirect heat pumps has potential to significantly reduce the noise level of ambient air heat pumps. The noise level caused by such heat pump has been shown to be highly influenced by the design of the air-to-fluid heat exchanger in its outdoor unit. It has been identified that it is mainly the air flow delivered by the fan and the resulting pressure drop in the air flow across the heat exchanger that together influence the level of noise. Hence, in order to achieve an acceptable noise level, the heat exchanger needs to be of such design that the necessary air flow and resulting pressure drop can be limited to a certain level. The overall purpose of this study is to propose a design of an air-to-fluid heat exchanger for an indirect ambient air heat pump system that allows for a well performing system as well as a low level of noise and cost. Two different types of heat exchangers, with flat tubes and round tubes, are designed and compared for suitability. Using relevant data and literature on heat transfer and heat exchangers, the necessary size (height, width and depth) and air flow of the different heat exchangers is calculated using Matlab, including the resulting noise level. According to the results of the study, a heat exchanger with flat tubes and plain fins is the most suitable out of the studied designs. It is shown that such a unit needs to be 700 mm high, 700 mm wide and 80 mm deep, therefore displacing a volume of around 0.04 m3An alternative design with flat tubes that instead has wavy fins is practically as suitable. Two round tube heat exchangers were also evaluated and both showed to be significantly less suitable than any of the flat tube heat exchangers, displacing more than twice the volume. The reason the flat tube heat exchangers turned out more suitable is shown to be that the heat transfer resistance on their tube side is significantly lower, while the resistance on the outside still is comparable to that of the round tube heat exchangers.