Shortly after the vacuum pump started to work, the minimum pressure P'g appeared. The vacuum pump continues to operate. Because of the increase of vacuum pump temperature, the saturated vapor pressure and viscosity of vacuum pump oil rise and viscosity drop, and the inlet pressure slightly rises to PG. PG is the stable minimum pressure obtained by the vacuum pump, and also the limit pressure of the vacuum pump.
The limit pressure of the rotary vane vacuum pump is related to the type of vacuum gauge used in measurement. When the compression vacuum meter is used, the limit pressure value is about one order of magnitude lower than that measured by the heat conduction vacuum meter (thermocouple or piezometer, etc.). This is because the former can only measure the partial pressure of permanent gas, the latter is the total pressure of the compressed gas. The current regulations of our country use compression vacuum gauge to measure the limit pressure of oil seal mechanical vacuum pump.
The circulating oil quantity of vacuum pump also affects the limit pressure of vacuum pump. With the increase of oil quantity, the better sealing effect is, the relationship between the limit pressure and oil quantity of vacuum pump changes, and the saturation vapor pressure Po of oil is achieved. In fact, there is a lot of air and water in the vacuum pump oil. After the vacuum pump oil enters the vacuum pump cavity, the air and water absorbed in the vacuum pump oil are released. The relationship between the limit pressure and the oil quantity of the vacuum pump develops upward.
Generally, the vacuum pump oil should be kept on the specified oil level, so that the circulating oil quantity of the vacuum pump should be kept to a certain extent. For double stage vacuum pump, vacuum pump oil can enter high vacuum stage only after degassing at low vacuum stage, so that the amount of air discharge of vacuum pump oil in high vacuum stage can be reduced, so as to reduce the limit pressure of vacuum pump.
The limit pressure of vacuum pump is related to the working temperature of vacuum pump oil. The higher the oil temperature, the higher the saturated vapor pressure of the oil, the higher the limit pressure of the vacuum pump.