Calculation method of heat exchange area of plate heat exchanger

Many people have questions, does a Plate Heat Exchanger have a large heat exchange area and good heat exchange effect? This view is very one-sided. The heat transfer effect of plate exchange is determined by the heat transfer coefficient. In principle, reasonable heat exchange time will not be considered when designing plate heat exchangers. The load of heat exchange is mainly related to the average temperature difference, heat exchange area and heat transfer coefficient. The increase in the heat exchange area is conducive to the increase in the heat exchange load, but when the heat exchange area increases to a certain level, the Reynolds number becomes smaller and may change from turbulent flow to laminar flow. This will cause the heat transfer coefficient to decrease, and in turn reduce the heat transfer coefficient. Load, this effect can sometimes even be worse than the heat exchange effect using the original area. It is a principle that the extremes of things must be reversed. Therefore, it is most suitable for the plate heat exchanger's heat exchange area to reach a balanced and suitable value.

There are three main methods for calculating the heat transfer area of plate heat exchangers:

(1) Average temperature difference method

According to the basic equation of heat transfer, the required heat transfer 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) Heat transfer unit number method

The number of heat transfer units is a parameter that reflects the difficulty of the heat exchange process between hot and cold fluids, and is 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, C₁, C₂—are the heat capacities of hot and cold fluids respectively.

Obviously, as long as NTU, C and the total heat transfer coefficient K value are known, the heat exchange area can be obtained from.

The number of heat transfer units is related to the temperature efficiency ε and the heat capacity ratio γ of the thermal fluid. Temperature efficiency ε refers to the ratio of the temperature change of any fluid participating in heat exchange to the inlet temperature difference of cold and hot fluids, that is

ε₁ = (t₁′-t₁″)/(t₁′-t₂′)

Or ε₂ = (t ₂″-t ₂′)/(t₁′-t₂′) = γ₁ε₁

Among them, 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, the number of heat transfer units, and the ratio of heat capacity can be obtained.

(3) Calculation of heat exchange area after process combination is determined

Regardless of whether the average temperature difference method or the NTU method is used, a process combination must be set first to calculate the plate heat exchange area. The calculated heat exchange area is equal to or slightly smaller than the heat exchange area of the process combination to meet the requirements of the working conditions. , otherwise a process combination should be reset and calculated again until the working conditions are met.