Vacuum Catalogue - Edwards

Effect of the Pipe Between the Secondary Pumpand the Rotary PumpAs we discussed above, the conductance of the pipe between therotary pump and the secondary pump can reduce the effectivepumping speed. This may mean that the critical backing pressure ofthe secondary pump is exceeded, even though the unrestrictedspeed of the rotary pump is adequate.If you know the bore and length of the pipe between the rotary pumpand the secondary pump, you can use equation (3) and the data inFigure 2 to calculate the pumping speed at the end of the pipe. Then,match this corrected speed to the maximum throughput of thesecondary pump.Alternatively, in Table 2, we give a quick guide to the maximum lengthof pipe you can use between the rotary and secondary pumps, if youwant the reduction in pump speed at 0.5 mbar to be no more than20%. In our calculations, we assumed that the pipes and fittings usedcorrespond to the size of the inlet port. That is, for example, NW25pipes and fittings are used for the RV pumps. If you use smaller pipesand fittings, you must use shorter pipes.Pump Model Pump Inlet Inlet Ø(mm)Maximum PipeLength (m)50 Hz 60 HzSpeedivac 2 ¼ inch BSP 12 0.8 0.8E2M0.7 NW10 10 0.7 0.7E2M1, E2M1.5 NW10 10 0.4 0.4RV3 NW25 25 7 6RV5 NW25 25 4 3.5RV8 NW25 25 2.5 2RV12 NW25 25 1.5 1.5E1M18, E2M18 NW25 25 1 1E2M28 NW25 25 0.8 0.7E2M40 ISO40 40 3.5 3.5E2M80 ISO40 40 2 1.5E2M175 ISO63 70 7 7E2M275 ISO63 70 4.5 4.5Table 2 – Maximum pipe lengths, for a maximum decrease in pumping speed of 20% at0.5 mbar13Page465Shop online at www.edwardsvacuum.com