Compression ratio Roots pumps

The booster (blower or Roots) pumps are high-throughput pumps with a low compression ratio. A booster pump is schematically shown in Fig. 1.8. The gap between the two counter-rotating rotors is of the order of 0.1 mm. The rotation (about 50rps) of the rotors,... 

The lower limit of the water vapor partial pressure is determined through the compression ratio of the Roots pump at the backing pressure, w/hich is determined by the saturation vapor pressure of the condensed water. Also, in this region the intermediate condenser must be able to reduce the vapor partial pressure to at least 60 mbar. The stated arrangement is suitable -when cooling the condenser with water at 15 °C - tor water vapor pressures between about 4 and 40 mbar. 

Permissible cut-in pressure will depend on the reduction ratios of the roots pump as against the roughing pump and is calculated by dividing the permissible pressure differential Ap, g by the compression ratio, reduced by the value of 1 ... 

The ultimate pressure of a Roots blower/backing pump combination is approximately that of the backing pump divided by the compression ratio (k0) for the Roots pump at this pressure. In practice, however, the calculated value should be multiplied by a factor of 4 to 5 to take into account performance deficiencies of the backing pump. 

Relatively high compression ratios can be achieved with Roots pumps at high vacuum pressures but there is a maximum pressure difference (Apmax) at which they can be satisfactorily operated. This is determined predominantly by the temperature rise of the gas at the exhaust of the pump. Pump manufacturers specify Apmax in their data for Roots pumps and it refers to continuous operation of a pump with no additional control features such as by-pass valves and similar devices. 

Figure 5.14 Special version of a roots pump with exhaust gas cooler to overcome a higher compression ratio (up to some lOOmbar).

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