The positive exhaust blower can be used in series with the vane pump to obtain a higher pumping speed and lower limit pressure under medium vacuum than when the vane pump is used alone. Roots vacuum pump, or roots blower, consists 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 at 3000rpm to 3500rpm. Since the clearance between the rotor and the pump housing is not sealed with oil, this speed is feasible.
The compression ratio of Roots vacuum pump, or the pressure ratio between inlet and outlet of pump, is related to pressure, and its maximum value is close to 100Pa. At higher pressure, the compression ratio of the pump is smaller, which is because the conductance in the sub clearance increases with the increase of pressure. In theory, the compression ratio should remain unchanged at a lower pressure, but it is actually decreased. The outgassing and the roughness of the rotor surface make the compression ratio lose at low pressure. When the rotor faces the high pressure side, the surface of the rotor will absorb gas, and when the rotor faces the high pressure side, a 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 space occupied by clearance of large pump is smaller than that of pump volume. A light gas like nitrogen has a compression ratio of 15-20% less than that of air. The compression ratio is a static quantity, which is obtained under the condition of zero flow rate. During the measurement, the inlet of the pump is sealed and connected with a pressure gauge, and the exhaust port is connected with a closed coarse pump, and the whole system is pumped. Put the gas into the front pipe of Roots vacuum pump and vane pump, measure the pressure P (front stage pressure) at the exhaust port of Roots vacuum pump, and measure the pressure P1 at the air inlet. The compression ratio is obtained from P / P1.
Under high pressure, considerable heat will be produced when the roots vacuum pump is used to exhaust gas, which will cause expansion of the rotor and may damage the pump. In order to avoid overheating, the maximum pressure difference between the inlet and outlet of Roots vacuum 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 vacuum pump is connected by compression pump or transfer pump.
In the common way of compressed exhaust, roots vacuum pump and rotary vane pump are used in series, and the rated pumping speed of rotary vane pump is 5 to 10 times lower than that of Roots vacuum pump. When pumping from atmospheric pressure, the bypass pipe around roots vacuum pump should be opened or the roots vacuum pump should be idle. At this time, the mechanical pump is responsible for all the pumping function, until the pressure of the front stage is lower than the pressure difference recommended by the manufacturer, then start the roots vacuum pump and close the bypass valve.