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Water saturated vapor pressure at different temperatures
1. Lowering the temperature of the working fluid in a liquid ring vacuum pump during operation is essential, as the discharge of gas carries away some of the working fluid. To maintain proper function, a continuous supply of working fluid to the pump chamber is necessary. The lower the temperature of the working fluid supplied, the lower its saturated vapor pressure. For example, referring to Table 2-25, which shows the saturated vapor pressure of water at different temperatures:
- At 15°C, it's 17.04 mbar
- At 20°C, it's 23.37 mbar
- At 25°C, it's 31.66 mbar
- At 30°C, it's 42.41 mbar
- At 35°C, it's 56.22 mbar
- At 40°C, it's 73.75 mbar
Assuming the pump operates at an absolute pressure of 80 mbar, the higher the water temperature, the smaller the difference between the operating pressure and the saturated vapor pressure. This increases the likelihood of cavitation. Therefore, by reducing the working fluid temperature, the operating point can be maintained in a safe range, effectively preventing cavitation.
2. Another important factor is the temperature of the suction gas. If the gas being drawn into the pump is too hot, the heat from compression and the gas itself will transfer to the liquid ring, increasing its temperature. This, in turn, raises the saturated vapor pressure of the liquid inside the pump chamber. By lowering the temperature of the suction gas, the liquid ring temperature decreases, leading to a reduction in the saturated vapor pressure, thus improving pump performance and reducing the risk of cavitation.
3. It’s crucial to operate the liquid ring vacuum pump within a safe area. When selecting the pump, the relationship between the operating point and the working fluid temperature must be carefully considered. The pump should run in the cavitation safety zone. For instance, when using water as the working fluid, Figure 2-93 illustrates how water temperature affects the exhaust volume curve. A boundary line separates two zones: the cavitation zone on the left and the safe operating zone on the right. The horizontal axis represents the operating pressure (in mbar), while the vertical axis shows the vacuum pumping performance coefficient. If the intersection point of the operating point and the corrected water temperature curve falls in the cavitation zone, the pump may experience cavitation. For example, if the operating point is at 50 mbar, the curve intersects the safe zone at 18°C but enters the cavitation zone at 26°C. This indicates that operating the pump at 50 mbar with water at 18°C is safe, but at 26°C, cavitation damage could occur.