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In order to save power and costs, plant designers often choose the lowest possible rated power for drive motors. But the kilowatt specification on the nameplate can be deceptive when it comes to real energy consumption. The motor’s rated power is only one of several factors influencing overall efficiency. The ‘service factor’ (SF), to be found on some nameplates, adds to the confusion by disguising the real maximum rated power. A systemic view is needed to achieve the optimal energy efficiency in a vacuum system.
Ultimate pressure and pumping speed are the essential variables for the selection of a vacuum pumps. Actual ‘vacuum performance’ is determined by these factors, ie, the vacuum level achieved in a certain period and available in the application. vacuum pump systems with completely different technologies can reach the same given performance level. Motor speeds, for instance, can differ dramatically. The rotary vane vacuum pump is currently the most widely used vacuum technology. At 1000 rpm, it achieves a similar vacuum performance as an oil-lubricated screw industrial vacuum pump does with up to 7000 rpm.
This difference in motor speed — but not in in performance — can also be reflected on the nameplate: on the screw vacuum pump, it possibly indicates a lower electrical rated power than on the rotary vane vacuum pump. But selecting a device only by looking at this figure would be a mistake. In the process, power consumption regularly and largely departs from the rated power. The motor with the smaller number in front of the kW specification does not necessarily use less power than the ‘larger’ drive. In fact, very often the exact opposite is the case, especially when the actual rated power is also disguised by the service factor on an American nameplate.
In order to compare the actual energy efficiency of different vacuum pump system realistically, power consumption and performance have to be measured in practice. German vacuum pump manufacturer Busch ran such parallel tests with two vacuum pumps:
A speed-controlled and oil-lubricated screw industrial vacuum pumps from another manufacturer with a specified rated power of 15 kW plus a service factor of SF 25. A rotary vane vacuum pump from its own product range with a rated power of 18.5 kW on the nameplate.
The test showed that in the range of the main load the power consumption of the smaller motor according to nameplate-rated power was nearly twice as high as that of the reference device.