# Vacuum pump for negative pressure evaporation

Is non condensable gas pumped by vacuum pump during vacuum evaporation?

In the process of negative pressure evaporation, vacuum pump is often equipped to keep the equipment running under a certain pressure. When selecting the type of vacuum pump, it is necessary to consider the air volume. What kind of gas is the extracted steam from the vacuum pump? When selecting the type, the non condensable gas dissolved in the raw materials and the air leaked from the equipment are mostly considered. But in the actual operation, due to the limitation of cold source, there are often non condensing steam.

The following is a simple example to let you understand the distribution law of water in the vapor phase and liquid phase due to the existence of non condensable gas. (only rough calculation, assuming that the mole fraction of non condensable gas in water vapor is 1%, the non condensable gas is represented by air, and the pressure of the whole process is fixed at 10KPA). It is divided into three stages: the first stage is cooled to the saturation temperature of pure water 45 ℃, the second stage is cooled to 40 ℃ (the circulating water temperature is assumed to be 30 ℃, which is more common in industry), and the third stage is cooled to 35 ℃. It can be seen from the calculation results that when it is cooled to 35 ℃, about the same volume of water vapor as the non condensable gas exists in the vapor phase and is pumped away by the water ring vacuum pump.

It can be seen from the simple calculation that:

1) In addition to non condensable gas, the water ring vacuum pump also includes non liquefied water vapor. The amount of water vapor is affected by the cooling temperature. The lower the temperature is, the less the pumping volume is.

2) It can be predicted that the more non condensable gas is, the more water vapor will be pumped out under the same vacuum degree. Therefore, it is very important for vacuum system to reduce the content of non condensable gas as much as possible. If the content of non condensable gas in raw material is large, flash evaporation and other methods are recommended to remove.

You can continue to think about the influence of the content of non condensable gas on steam condensation under a certain cooling temperature and pressure (the ratio of water in the vapor phase to the liquid phase, that is, how much water is condensed). If the temperature and pressure of condensate water are known, can the content of non condensable gas be calculated? Can we judge the effect of non condensable gas emission.

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