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Due to the complexity of condensation heat transfer, changes in many factors will affect condensation heat transfer. The main influencing factors of condensation heat transfer include non condensable gas, gas flow rate, gas flow direction, liquid film Reynolds number and number of tube rows.

1. Non condensable gas

The presence of non condensable gas will have a very adverse impact on condensation heat transfer, for example, 1% (mass fraction) air in steam will reduce the condensation Heat transfer coefficient by 60%. Because as the vapor condenses, the concentration of non condensable gas at the interface will continue to increase. Before reaching the surface of the liquid film for condensation, the vapor must diffuse through the non condensable gas layer that accumulates near the interface, increasing heat transfer resistance; The decrease in vapor partial pressure causes the corresponding saturation temperature to decrease, thereby reducing the driving force of condensation temperature difference. Therefore, in the design of condensers, it is very important to avoid the accumulation of non condensable gases.

2. Gas flow rate

The effect of gas flow rate on condensation heat transfer can be ignored at low flow rates, but at high flow rates, the gas flow will generate significant viscous stress on the surface of the liquid film. In the flow rate range of industrial operation, the influence of gas flow rate on condensation Heat transfer coefficient is minimal.

3. Air flow direction

If the airflow direction is consistent with the flow direction of the liquid film, it will thin the liquid film, which is beneficial for condensation heat transfer; On the contrary, it will thicken the liquid film and reduce the condensation heat transfer efficiency. For example, a liquid film flows from top to bottom when the gas flow direction is consistent with it. Because the gas film has a stirring effect, it is beneficial for heat transfer; On the contrary, if the gas flows from bottom to top, it will reduce the downward flow speed of the liquid film, or even bring some of the lower condensate to the top, thus increasing the thickness of the upper liquid film, which will reduce the average heat transfer coefficient value. Therefore, it is better for the gas to flow from top to bottom in a vertical condenser.

In addition, in the condenser, because of the baffle plate, the gas flow direction is vertical to the pipe. In this way, the condensate is blown off due to the shear effect of the gas on the condensate, so that the actual average heat transfer coefficient value of saturated gas condensation is slightly larger than the calculated value, so the gas flow should be vertical to the pipe as far as possible.

4. Liquid film Reynolds number

In industry, the condensation process is mostly in the laminar flow area, where the relationship between the average heat transfer coefficient value and Reynolds number (Re) is different from the convective heat transfer without phase change.

When Re<2100, that is, in the laminar flow area, the average liquid film thickness increases with the increase of condensation, which makes the average heat transfer coefficient decrease with the increase of condensation, that is, the average heat transfer coefficient decreases with the increase of gas flow rate. The above situation is exactly the opposite of a heat exchanger without phase change, so this relationship should be considered when designing a condenser. It should be emphasized here that the influence of increasing condensation on the average Heat transfer coefficient and the influence of increasing gas flow rate are two different concepts, and should not be confused.

5. Number of pipe rows

When gas condenses outside the heat exchange tube bundle, the condensate from the upper row of pipes will drip onto the lower row of pipes, reducing the condensation heat transfer efficiency of the lower row of pipes. However, when the condensate drops, it will produce splashing and impact disturbance on the liquid film, which in turn strengthens the condensation heat transfer of the lower row of pipes. Therefore, comprehensive consideration should be taken into design.

6. The impact of condenser arrangement

There are two types of shell and tube condensers: vertical and horizontal. When there are a large number of condensers, there are two types of arrangements: series and parallel. Therefore, when selecting a model, one can directly choose one large or several small ones.