Positive exhaust blowers can be used in tandem with rotary vane pumps to obtain higher pumping speeds and lower ultimate pressures at moderate vacuum than when rotary vane pumps are used alone. Roots pumps, or Roots blowers, consist of a pair of cocoon-shaped rotors mounted on two parallel rotating shafts. The clearance between the rotor itself and between the rotor and the pump casing is typically about 0.2 mm. The two rotors rotate synchronously in opposite directions at a rotational speed of 3000 rpm to 3500 rpm. This speed is feasible because the clearance between the rotor and the pump casing is not sealed with oil.
The compression ratio of the Roots pump
, or the ratio of the inlet and outlet pressures of the pump, is related to the pressure, and its maximum value is usually close to 100Pa. At higher pressures, the compression ratio of the pump is less because the conductance in the subclearance increases with pressure. At lower pressures, in theory, the compression ratio should remain the same, but in practice it decreases, and the roughness of the outlet air and rotor surface causes a loss of compression ratio at lower pressures. Whenever the rotor faces the high-pressure side, the rotor surface adsorbs gas, and when the rotor faces the low-pressure side, a portion of the adsorbed gas is released. Its maximum compression ratio is 44. The compression ratio of the large J pump is higher than that of the small pump, because the space occupied by the clearance of the large pump is smaller than the pump volume. The compression ratio of a light gas like nitrogen is 15-20% less than that of air. The compression ratio is a static quantity and is obtained under the condition of zero flow. During measurement, the air inlet of the pump is sealed and connected with a pressure gauge, and the exhaust port is connected to a rough pump, and the whole system is pumped. Put the gas into the fore-stage pipeline connecting the Roots pump and the rotary vane pump, measure the pressure P (pre-stage 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.
Exhausting with a Roots pump at high pressure generates considerable heat, which causes rotor expansion and can damage the pump. To avoid overheating, a maximum differential pressure between the intake and exhaust ports of the 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. To avoid heat generation, the Roots pump is connected as a compression pump or a transfer pump.
In the more common way of compressing and exhausting, the Roots pump is used in series with the rotary vane pump, and the rated pumping speed of the rotary vane pump is 5 to 10 times lower than the pumping speed of the Roots pump. When pumping from atmospheric pressure, the bypass line bypassing the Roots pump should be opened or the Roots pump should be idling. At this time, all the pumping action is undertaken by the mechanical pump, until the pressure of the fore-stage is lower than the pressure difference recommended by the manufacturer, then start the Roots pump and close the bypass valve. This bypass feature is provided on the pump body of some Roots pumps.