Heat transfer coefficient

The heat transfer coefficient was previously referred to as the total heat transfer coefficient. The current national standard specification is uniformly named as heat transfer coefficient. The heat transfer coefficient K value refers to the amount of heat transferred through an area of 1 square meter within 1 second under stable heat transfer conditions, with a temperature difference of 1 degree (K, ℃) between the air on both sides of the enclosure structure. The unit is W/(㎡· K), where K can be replaced by ℃. The heat transfer coefficient is not only related to the material, but also to the specific process. Air conditioning calculation
For heat exchangers commonly used in air conditioning engineering, if other additional thermal resistances are not considered, the heat transfer coefficient K value can be calculated as follows:
K=1/(1/Aw+ δ/λ+ 1/An) W/(㎡· ° C)
Among them, An, Aw - heat exchange coefficient of inner and outer surfaces, W/(㎡· ° C)
δ—— Pipe wall thickness, m
λ—— Thermal conductivity coefficient of pipe wall, W/(m ·° C)
Calculation formula
1. Calculation of thermal resistance of enclosure structures
Single layer structure thermal resistance
R= δ/λ (m2. K/w)
In the formula: δ— Material layer thickness (m)
λ— Material thermal conductivity [W/(m.k)]
Multi-layer structure thermal resistance
R=R1+R2+-- Rn= δ 1/ λ 1+ δ 2/ λ 2+----+ δ N/ λ N
In the formula: R1, R2, -- Rn - thermal resistance of each layer of material (m2. k/w)
δ 1 δ 2--- δ N - Thickness of each layer of material (m)
λ 1 λ 2--- λ N - Thermal conductivity coefficient of each layer of material [W/(m.k)]
2. Thermal resistance of enclosure structure
R0=Ri+R+Re
In the formula: Ri - Inner surface heat transfer resistance (m2. k/w) (generally taken as 0.11)
Re - External surface heat transfer resistance (m2. k/w) (usually taken as 0.04)
R - Thermal resistance of enclosure structure (m2. k/w)
3. Calculation of heat transfer coefficient of enclosure structure
K=1/R0 (w/(m2. k))
In the formula: R0- thermal resistance of the enclosure structure
Calculation of the average heat transfer coefficient of external walls under the influence of surrounding thermal bridges
Km=(KpFp+Kb1Fb1+Kb2Fb2+Kb3Fb3)/(Fp+Fb1+Fb2+Fb3)
In the formula:
Km - average heat transfer coefficient of the exterior wall [W/(m2. k)]
Kp - heat transfer coefficient of the main body of the exterior wall [W/(m2. k)]
Kb1, Kb2, Kb3- Heat transfer coefficient of the thermal bridge area around the exterior wall [W/(m2. k)]
Fp - Area of the main body of the exterior wall
Fb1, Fb2, Fb3- The area of the thermal bridge around the exterior wall
4. Calculation of heat transfer coefficient for aluminum alloy doors and windows
Uw=(Af * Uf+Ag * Ug+Lg)* Ψ g) /(Af+Ag)
In the formula:
Uw - heat transfer coefficient of the entire window W/m2 · K
Ug - Heat transfer coefficient of glass W/m2 · K
Ag - Area of glass m2
Uf - heat transfer coefficient of profile W/m2 · K
Af - Area of profile m2
Lg - perimeter of glass m
Ψ G - Linear heat transfer coefficient around the glass W/m2 · K