Views:0 Author:Site Editor Publish Time: 2020-05-12 Origin:Site
The increase of the heat exchange area of the plate heat exchanger is conducive to the increase of the heat exchange load, but when the heat exchange area increases to a certain degree, the Reynolds number becomes smaller, which may change from turbulent to laminar flow, which will reduce the heat transfer coefficient On the contrary, the heat load is reduced. This effect is sometimes even worse than the effect of using the original area for heat exchange. It is a matter of fact that the heat transfer area reaches a balanced and suitable value.
(1) Average temperature difference method
According to the basic equation of heat transfer, the required heat exchange area can be obtained as
F = Q / K. Δtm
Tip: Q—heat flow (W), △ tm—logarithmic average temperature difference (℃), F—heat transfer area (m2)
(2) Number of heat transfer units
The number of heat transfer units is a parameter that reflects the difficulty of the heat transfer process between cold and hot fluids, and also a parameter that measures the heat transfer capacity of the heat exchanger. The definition of the number of heat transfer units NTU can be more broadly expressed as
(NTU₁) = KA / C₁ or (NTU₂) = KA / C₂ = γ₁ (NTU₁)
In the formula, C₁and C₂—respectively the heat capacity of hot and cold fluids.
Obviously, as long as the NTU, C and total heat transfer coefficient K values are known, the heat exchange area can be obtained.
The size of the number of heat transfer units is related to the temperature efficiency ε and the ratio of the heat capacity of the Liang Huan thermal fluid γ. Temperature efficiency ε refers to the ratio of the temperature change of any fluid participating in heat exchange to the difference between the inlet temperature of the cold and hot fluid, that is
ε₁ = (t₁′－t₁ ″) / (t₁′－t₂ ′)
Or ε₂ = (t ₂ ″ －t ₂ ′) / (t₁′－t₂ ′) = γ₁ε₁
Where the heat capacity ratio γ is
γ₁ = C₁ / C₂ or γ₂ = C₂ / C₁ = 1 / γ₁
By establishing an energy balance method, the relationship between temperature efficiency and the number of heat transfer units and heat capacity can be obtained.
(3) Calculation of heat exchange area after the process combination is determined
Regardless of whether the average temperature difference method or the NTU method is used, a process combination must be set for the calculation of the heat exchange area of the board. The calculated heat exchange area and the heat exchange area of the combination of the process are equal or slightly smaller to meet the requirements of the working conditions. Otherwise, a new process combination shall be set and calculated again until the working conditions are met.