Characteristics of Roots vacuum pump

The positive exhaust blower can be used in series with the vane pump to obtain a higher pumping speed and lower ultimate pressure under medium vacuum than when the vane pump is used alone. Roots pumps, or roots blowers, consist of a pair of cocoon shaped rotors mounted on two parallel shafts. The clearance between the rotor itself and between the rotor and the pump housing is typically about 0.2mm. The two rotors rotate synchronously in reverse from 3000rpm to 3500rpm. Since the clearance between the rotor and the pump casing is not sealed by oil, such a speed is feasible.

The compression ratio of roots pump, or the pressure ratio of the inlet and outlet of the pump, is related to the pressure, and its maximum value is usually close to 100Pa. At a higher pressure, the compression ratio of the pump is smaller, which is due to the increase of the conductance in the clearance with the increase of the pressure. At low pressure, theoretically, the compression ratio should be kept constant, but actually it is decreased. The roughness of the outlet gas and the rotor surface makes the compression ratio loss at low pressure. When the rotor faces the high-pressure side, the gas will be adsorbed on the rotor surface, and when the rotor faces the low-pressure side, part of the adsorbed gas will be released. The maximum compression ratio is 44. The compression ratio of large J pump is higher than that of small pump, because the clearance of large pump is smaller than that of pump volume. The compression ratio of a light gas like nitrogen is 15-20% smaller than that of air. Compression ratio is a static quantity, which is obtained under the condition of zero flow. During the measurement, the air inlet of the pump is sealed and connected with a pressure gauge, and the air outlet is connected with the one-in-one coarse pumping pump, and the whole system is aspirated. Put the gas into the front pipe connecting the roots pump and the vane pump, measure the pressure P (front pressure) at the exhaust port of the roots pump, and measure the pressure P1 at the air inlet. The compression ratio is derived from P / P1.

When using roots pump to exhaust under high pressure, considerable heat will be generated, which will cause the rotor to expand and may damage the pump. In order to avoid overheating, the maximum pressure difference between the air inlet and the exhaust port of roots pump is specified. The maximum pressure difference is generally 1000Pa, but exceeding this value in a short time will not cause damage to the pump. In order to avoid heating, roots pump is connected by means of compression pump or transfer pump.

In the more common way of compressed exhaust, roots pump and vane pump are used in series. The rated pumping speed of vane pump is 5 to 10 times lower than that of roots pump. When pumping from atmospheric pressure, the bypass pipe bypassing the roots pump shall be opened or the roots pump shall be idle. At this time, the mechanical pump is responsible for all air extraction until the pressure of the previous stage is lower than the pressure difference recommended by the manufacturer, then start the roots pump and close the bypass valve. Some roots pumps have this bypass feature on the pump body.

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