Determination of a suitable backing pump

The required pumping speed of the backing pump is calculated from  

At an intake pressure of p = 1 -10 mbar the pumping speed for the high vacuum pump as stated in the catalog is about 100 l/s, subsequently 50 % of this is 50 l/s. Therefore the pumping speed of the backing pump must amount to at least 

At an intake pressure of p = 1 10 mbar the pump has already reached its nominal pumping speed of 400 l/s the effective pumping speed is now Sg(f = 200 l/s thus the required pumping speed for the backing pump amounts to 

If the high vacuum pump is only to be used at intake pressures below 10 mbar, a smaller backing pump will do in the case of the example given this will be a pump offering a pumping speed of 6 m /h. If the continuous intake pressures are even lower, below 10 mbar, for example, the required pumping speed for the backing pump can be calculated from equation 2.41a as  

The size of the ballast volume depends on the total quantity of gas to be pumped per unit of time. If this rate is very low, the rule of thumb indicates that 0.51 of ballast volume allovi/s 1 min of pumping time with the backing pump isolated. 

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