The blower with forward exhaust can be used in series with the rotary vane pump to obtain higher pumping speed and lower limit pressure under medium vacuum than when the rotary vane pump is used alone. Roots pump, or Roots blower, consists of a pair of cocoon-shaped rotors mounted on two parallel rotating axles. The clearance between the rotor itself and between the rotor and the pump housing is typically about 0.2 mm. The two rotors rotate in reverse synchronization from 3000 rpm to 3500 rpm. Because the clearance between the rotor and the pump housing does not need oil seal, such speed is feasible.
The compression ratio of Roots pump, or the ratio of intake pressure to outlet pressure, is related to the pressure. Usually its maximum value is close to 100Pa. At higher pressure, the compression ratio of the pump is small, which is due to the increase of the flow conductivity in the sub-clearance with the increase of pressure. In theory, the compression ratio should remain unchanged at a lower pressure, but actually it decreases. The compression ratio at low pressure is lost due to the outgassing and the roughness of the rotor surface. Whenever the rotor faces the high pressure side, the surface of the rotor will absorb the 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 the large J pump is higher than that of the small pump, because the clearance of the large pump occupies less space than the volume of the pump. The compression ratio of light gases like nitrogen is 15-20% less than that of air. Compression ratio is a static quantity, which is obtained under the condition of zero flow rate. When measuring, the intake port of the pump is sealed and connected with a pressure gauge, the exhaust port is connected with a rough pump, and the whole system is pumped. Put the gas into the front pipe connecting the Roots pump and the rotary vane pump, measure the pressure P (front pressure) on the side of the outlet of the Roots pump, and measure the pressure P1 at the inlet. The compression ratio is derived from P/P1.
When the Roots pump is used to exhaust at high pressure, considerable heat will be generated, which will cause the expansion of the rotor and possibly damage the pump. In order to avoid overheating, the maximum pressure difference between the intake and exhaust ports of Roots pump is stipulated. The maximum pressure difference is generally 1000Pa, but if it exceeds this value in a short time, the pump will not be damaged. To avoid fever, Roots pump is connected by pressing compression pump or transmission pump.
In the common way of compressed exhaust, Roots pump and rotary vane pump are used in series. The rated pumping speed of rotary vane pump is 5 to 10 times lower than that of Roots pump. When pumping from atmospheric pressure, the bypass pipeline bypassing the Roots pump should be opened or the Roots pump should be idled. At this time, all the pumping action is carried out by mechanical pumps until the pressure of the front stage is lower than the recommended differential pressure of the manufacturer, then start the Roots pump and close the bypass valve. Some Roots pumps have this bypass characteristic on the pump body.
