The issue of ultimate vacuum in liquid ring vacuum pumps
Thread Content
We all know that the liquid ring vacuum pump is a type of vacuum generation method that operates by changing its volume. The ultimate vacuum level it can achieve is closely related to the type and temperature of the working fluid; in other words, the saturated vapor pressure of the working fluid at its operating temperature determines the ultimate vacuum level that the liquid ring vacuum pump can attain. For example, the saturated vapor pressure of water at 15°C is 1.7 kPa; therefore, a water ring vacuum pump operating at 15°C can achieve a vacuum level of up to 1.7 kPa (i.e., -99.6 kPa). So the question is: If a working fluid with a very low saturated vapor pressure at the operating temperature is used, can the vacuum pump achieve a higher vacuum level? For example, transformer oil or certain esters with high boiling points generally have a vapor pressure of only 0.1 kPa at 50°C; is it then possible to achieve such a high level of vacuum? I asked Foshan Water Pump Factory, and they said it’s impossible. It seems that there is a pressure difference ratio between the exhaust side and the suction side of the vacuum pump, and this ratio determines that no matter what working fluid is used, it’s not possible to achieve a stable vacuum level of less than -99 kPa Does anyone have any experience using this in such contexts? If that’s the case, then what is the principle?
For power generation turbines, there is a limit vacuum; beyond this limit, surge occurs in the last stages of the turbine blades, causing a significant deviation from the designed operating conditions. In addition to non-condensable gases, water vapor is also drawn out by the water ring vacuum pump. Water vapor is insoluble in transformer oil; therefore, transformer oil cannot be used as the working medium for such vacuum pumps. Water is inexpensive and easily available, and since it can meet the requirements when used as a medium, why use other more expensive alternatives? Moreover, water is relatively easy to cool, as it has the highest specific heat. It should be noted that water ring vacuum pumps also require a steam-water separator and heat exchanger to cool the working medium.