Roots pumps efficiency

As shown in Figure 2.25, it is more efficient to use a backing pump combined with one or two roots blowers (Fig. 2.27). The backing pump requires only 40 m3/h capacity, combined with a roots pump of 200 m3/h. This system evacuates the 1000 L also in 8 min down to 0.01 mbar, but below 0.1 mbar the system has a capacity of 200 m3/h or 2.2 10 3 g/s at 0.05 mbar. Such a pumping system is preferable for freeze drying compared with a large two-stage pump alone. 

For the efficiency of the Roots pump, the generally valid equation applies k... 

Equation (2.16) implies that the compression k attainable with a Roots pump must always be less than the grading k, between Roots pump and backing pump since volumetric efficiency is always < 1. When combining equations (2.13) and (2.16) one obtains for the efficiency the well known expression... 

The volumetric efficiency of the Roots pump is thus determined, leading to Seff. As ... 

Roots efficiencies of about 70% or more can be achieved if k0 > 20 and Sth/Sback < io. Although such values for the staging ratio are satisfactory in the medium vacuum range, lower values are desirable if there are large variations in operating pressures or if the Roots pump is working where k0 is small. 

The system has a high efficiency and will work within the limits of the Roots pump. 

For the efficiency of a Roots pump, generally the following relationship applies ... 

Wliere N = the speed of the pump/motor in revolutions per minute Q = the square root of the flow in gallons per minute at the Best Efficiency Point BEP. For double suction pumps, use A BEP Flow. NPSHr = the net positive suction head required by the pump at the BFiP. 

Fig. 25 shows the action of a Root s pump. Gas inlet and outlet are separated by only a narrow gap which enables a back flow of gas from the exhaust region to the inlet region. Although the efficiency of compression (maximum compression... 

The volumetric efficiency / of a Roots vacuum pump is calculated as... 

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