基于效能-传热单元数法的地板传热结构设计

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

针对当前常用的线算图表在辐射地板传热结构工程设计中存在的问题,通过分析地板传热过程,将多层地板结构当量为单层均匀地板结构,求得了地板各传热环节的热阻及地板结构总热阻,并提出了地板传热效能值,进而从换热器的角度提出了用于地板传热结构设计的效能-传热单元数方法,并和ASHRAE手册设计方法进行比较。最后,在北京相应建筑地板辐射系统设计的工程案例中,通过该文提出的效能-传热单元数设计方法进行了地板热工设计,根据房间的热负荷和系统能提供的供水温度,计算所需的地板结构总热阻,进而确定了地板的埋层结构和对应的管间距,经试验测试,房间的温度较好地满足了设计要求。最终结果表明效能-传热单元数设计方法可较好适应多变工程设计条件,既可以根据供水温度确定对应的地板结构,也可以根据地板结构确定所需的供水温度,还可以校核一定供水温度和地板结构下的地板表面温度,可方便进行地板传热结构的设计,为地板辐射系统工程设计提供了一个新的思路。

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.