Questions regarding vacuum pumps: “The vacuum level is too high, making condensation difficult”
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There is a principle in chemical engineering that states: \"For the condensable gases to be removed from vacuum equipment, as long as the vacuum level is not too high and allowing for easy condensation (usually achieved through water cooling), these gases should be cooled as much as possible to turn into a liquid form before being removed.\"； If condensation is difficult due to too high a vacuum level, and it is not economical to use another refrigeration system, then a steam jet pump is usually used first to raise the pressure in the mixture to a level at which steam can condense, thereby removing the condensable gases; after that, a gas vacuum pump is used to further remove the non-condensable gases. What does it mean that the vacuum level is too high and condensation is difficult? Some people don’t understand this; please help to explain it
Water vapor is the most common; let’s take water vapor as an example. At 15 degrees, the saturated vapor pressure of water is 17 mbar ； That is, when the pumping temperature is 15 degrees, no liquid water will form through condensation as long as the water vapor pressure is below 17 mbar. Therefore, at a water vapor partial pressure of 17 mbar, if the gas temperature is above 15 degrees, the gas is unsaturated vapor, even if it contains a large amount of water vapor. If you use a vacuum pump to remove it directly, the volume will be relatively large ； If you want to remove water vapor from the gas, you need to use water at around 7 degrees Celsius, not normal room temperature water at 32 degrees Celsius.
For the condensable gases that need to be removed from vacuum equipment, as long as the vacuum level is not too high and allowing for easy condensation (usually achieved through water cooling), they should be cooled as much as possible to form a liquid phase before being removed； If condensation is difficult due to too high a vacuum level, and it is not economical to use another refrigeration system, then it is often necessary to first use a steam jet pump to raise the pressure in the mixture to a level at which steam can condense, thereby removing the condensable gases. After that, a gas vacuum pump is used to further remove the non-condensable gases. In other words, if there are condensable gases in the vacuum system, it is preferable to first condense those gases and remove them as liquid, rather than using a vacuum pump to directly extract the gases. Because the volume increases by several hundred times when a liquid vaporizes. Pumping with a vacuum pump, especially a mechanical vacuum pump, is very slow. The second sentence means: if the vacuum level is too high, then the lower temperature is required for the liquid to condense. For example, the minimum temperature of the circulating water might be 30 degrees; you can’t just install a refrigeration unit to cool it down. In such cases, a steam jet pump is preferred, as it has high capacity but also high energy consumption; it raises the pressure of the condensable gases in the vacuum, then condenses them before removing them. For the subsequent stage, mechanical vacuum pumping can be used as appropriate to remove non-condensable gases.
Why 7 degrees of cold water? In principle, any condensate water with a temperature below 15 degrees should be able to condense, right?
1) The saturated vapor pressure of water at 15 degrees is 17 mbar. 2) It can be condensed only when the partial pressure of water vapor in the process gas is higher than 17 mbar ； 3) If the chilled water inlet temperature is 15 degrees, condensation generally does not occur, at a total process gas pressure of 17 mbar ； 4) It’s only barely possible if the outlet chilled water temperature is 15 degrees!
In other words, the saturated vapor pressure of a liquid is determined by the temperature of the liquid; to condense the vapor, it is necessary to make the vapor pressure greater than the saturated vapor pressure. Two approaches can be taken: one is to increase the pressure of the gas as a whole, thereby raising the vapor pressure above the saturated vapor pressure for condensation ； One is to lower the temperature of the liquid, thereby reducing the saturated vapor pressure; this allows the vapor partial pressure to exceed the saturated vapor pressure and lead to condensation. The book states: “When condensation is difficult due to too high a vacuum level, and it is not economical to use another cooling system, a steam jet pump is often used first to raise the pressure in the mixture to a level at which steam can condense, thereby removing the condensable gases; afterwards, a gas vacuum pump is used to further remove the non-condensable gases.” A high vacuum level means that the overall gas pressure is low, resulting in a vapor pressure that is lower than the saturated vapor pressure. If condensation is desired, the second method can be used: a refrigeration system is employed to reduce the saturated vapor pressure of the liquid, thereby enabling condensation. However, since the refrigerant is not cost-effective, the first method is employed to increase the pressure of the gas as a whole, so that the vapor pressure exceeds the saturated vapor pressure and condensation occurs. Is this analysis correct?
Yes, that’s the case. Either the temperature of the cooling medium must be reduced, or a pressure of process gas must be applied in order for condensate to form.
Well, thank you so much; it’s been a great help
:victory: You’re welcome!