The property diagram in heat transfer and its applications

Inspired by the property diagrams in thermodynamics, which distinctly reflect the performance and characteristics of thermodynamic cycles, we establish a state equation for heat motion and introduce a two-dimension property diagram, T-q diagram, in heat transfer to analyze and optimize the performance of heat exchangers, where heat flow is a state parameter for heat motion. According to the property diagram, it is convenient to obtain the influences of heat exchanger area, heat capacity rate and flow arrangement on the heat transfer performance during the analysis of heat exchangers and their networks. For instance, when analyzing the heat exchanger network in a district heating system, it is obvious to find that: if both the heat demand and the indoor air temperature in each branch of the network are the same, the total area of heat exchangers, the flow rate of water and the return water temperature in each branch are all the same; if the indoor air temperatures in different branches are different, the temperatures of the waters after flowing through different branches are different, which means that the mixing process of return waters with the same temperature is not an essential requirement to realize the best performance of district heating systems.