What Is a Heat Transfer Coefficient?

The heat transfer coefficient was previously called the total heat transfer coefficient. The current national standard is uniformly named the heat transfer coefficient. The value of heat transfer coefficient K refers to the amount of heat transferred through a unit area per unit time in units of watts per square meter in degrees under the condition of stable heat transfer. (W / · K, where K can be replaced by ), which reflects the strength of the heat transfer process

The heat transfer coefficient K of the wall is characterized by the wall (including all structural levels) under stable heat transfer conditions. When the air temperature difference between the two sides is 1K (1 ° C), it is transmitted through the wall area per square meter per unit time Heat in W / (M2.K). That is, the heat transfer coefficient K includes all structural layers of the wall and the air boundary layers on both sides. It characterizes the thermal performance of the wall insulation system, and studies have shown that a reduction in the heat transfer coefficient of the external wall will significantly reduce building energy consumption. ^[1]

For heat exchangers often used in air conditioning projects, if other additional thermal resistances are not considered, the heat transfer coefficient K value of the single-layer envelope structure can be calculated as follows:

K = 1 / (1 / h ₁ + / + 1 / h ₂ ) W / ( · ° C) ^[2]

At present, there are the following methods for on-site testing of the wall thermal resistance and heat transfer coefficient: heat flow meter method and power (called the hot box method in the industry, in order to maintain uniform and convenient communication, hereinafter referred to as the hot box method), unstable State methods are explained separately below.

The heat transfer coefficient is a process variable, and its size depends on factors such as the physical properties of the fluid on both sides of the wall, the flow velocity, the shape of the solid surface, and the thermal conductivity of the material. In the calculation of building heat loss, it is a parameter characterizing the total heat transfer performance of the envelope structure, and its value depends on the materials, structures and environmental factors on both sides of the envelope structure. The larger the heat transfer coefficient, the worse the thermal insulation effect of the envelope structure. For example, the heat transfer coefficient of a single-layer 3mm thick glass metal window is 6.4W / (mK), and the heat transfer coefficient of a 370mm thick plastered brick wall is 1.59W / (mK). ^[5]

The larger the K value, the more intense the heat transfer process. The heat transfer coefficient mainly depends not only on the physical properties of the hot and cold fluids and their respective average flow rates, but also on many factors such as the thickness of the solid wall surface and the thermal conductivity of the material. Generally, it is determined by means of specific experiments and calculated by the heat transfer equation. , Or by calculating the total thermal resistance R _t per unit area of the heat transfer process. The following table lists the approximate value ranges of several common heat transfer processes K: ^[6]