Dynamic Characteristics of a Free-Flow-Channel Heat Exchanger

The performance of sewage heat exchangers plays an important role in the sewage source heat pump systems when the sewage does not enter the heat pump unit directly, especially when untreated sewage is used as heat or cold source. In this article, a free-flow channel heat exchanger is proposed to be used in an untreated sewage source heat pump system. First the article designs the sizes of heat exchanger according to the real system requirements, and then it models and analyzes dynamic characteristics on the simulink platform. The transfer functions that are suitable for the heat exchanger are deduced. The analysis shows that the wall temperature is 9.1°C when the untreated sewage and water temperatures at the inlet are 15 and 7°C, respectively. The result is obtained when considering the temperature at the outlet is affected by the temperature at the inlet. The variable quantity of water temperature at the outlet is affected greatly by the variable quantity of water temperature at the inlet and the variable quantity of sewage temperature at the outlet by the variable quantity of sewage temperature at the inlet. Besides, the measured values of water temperatures at the inlet and outlet are also observed from the real system where the free-flow channel heat exchanger is adopted. The variable quantities of water temperature at the outlet are calculated according to the measured values, and are compared with the model results deduced by the transfer functions. The comparative result shows that the differences between the measured and model results of water temperature variable quantities at the outlet range from ?1 to 1°C, thus getting the water temperatures at the outlet according to the model results. The differences of the water temperature at the outlet between the measured values and the deduced results are from ?0.5 to 0.5°C. It illustrates the temperature at the outlet can be estimated by modeling the temperature variable quantity at the outlet.