Design of floor structure in radiant floor system based on heat transfer effectiveness-number of transfer units method

Abstract: With the development of radiant floor system engineering applications in civil and agricultural buildings, an engineering problem has been gradually becoming urgent, that is how to match the terminal of a radiant floor system for changeable and flexible application. This problem is closely related to thermal performance of the floor. Therefore, it is very important and necessary to study the heat transfer process of the floor, and obtain the regulation and characteristics of heat transfer to solve it. As the core component of heat transfer in radiant floor system, the floor can be regarded as one kind of special heat exchanger. Based on the basic theory and parameters of heat exchanger, thermal performance can be analyzed and evaluated. Based on the application of thermal resistance in series, the heat transfer of multi-layer floor structure is equivalent to that of homogeneous structure, whose analytical solution and heat resistance of floor are obtained. Assuming that the heat conduction in pipes wall is one-dimensional problem, the thermal resistance of the wall is amended after the model based on the equivalent thermal resistance. Furtherly, combined with the basic theory of the thermal design of the heat exchanger, the heat transfer effectiveness ε is raised when the radiant floor is regarded as some kind of heat exchanger. According to the heat transfer effectiveness, the number of transfer units can be obtained, and then the ε-NTU (number of transfer units) method for thermal design of floor structure is creatively proposed, which is compared with the design graph and design tables in ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) Handbook. Finally, the ε-NTU method for thermal design of floor structure was put into practice for the designs of radiant floor system in the buildings located in Beijing. According to the supply temperature provided and the heating load calculated, the total thermal resistance of floor structure was obtained, and then the parameters of floor layers and pipe spacing were determined. The experimental data indicate that the temperatures of the rooms meet the design requirements very well. It shows that the ε-NTU design method can be more convenient for the design of floor heat transfer structure with flexible application. The corresponding floor structure can be determined according to the supply temperature, indoor temperature and heating load, and the supply temperature can be determined according to the different floor structures. The floor surface temperature under the supply temperature and the floor structure can be checked. A new way of thinking for the floor thermal design is provided.