The real energy consumption of vacuum pumps
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 pump. Actual ‘vacuum performance’ is determined by these factors, ie, the vacuum level achieved in a certain period and available in the application. Vacuum pumps 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 vacuum pump does with up to 7000 rpm.
Rated power vs real consumption
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 electric motors there is no linear relation between power consumption and the provided performance (shaft power). Optimal performance is usually achieved somewhere between 50 and 100% of the motor’s rated power. It is quite safe to assume that it delivers ideal performance at highest efficiency in a more or less wide range around 75% of the rated power. Below this range, the motor needs more power in relation to the actual performance, thus increasing the relative power consumption.
Confounding service factor
Of course, this is also the case when the optimal range is exceeded. When this happens, it can even go above 100% of the rated power — making use of the so-called service factor. The US American National Electrical Manufacturers Association has defined the service factor as a standard in the NEMA MG1-2011 handbook. It is indicated on the nameplate (Figure 1), specifying the degree to which a motor can be loaded beyond the rated power. The rated power is multiplied by the SF value to calculate the allowed degree of overloading: eg, with a SF of 1.25, the real maximum rated power is 25% higher than indicated. Combined with a rated power of 15.0 kW, the maximum permissible, thus real rated power is 15 x 1.25 = 18.75 kW.
The SF range should only be used temporarily, as NEMA also points out. However, in everyday practice, it is often already implied in process calculations, as vacuum generation rarely stays on a continuous level. Instantly starting the vacuum pump from standby mode or short peak loads, even if they are regularly targeted in short cycles, can be recorded as ‘temporary’ overloading. While the relatively low rated power, not calculating the service factor, suggests low power consumption, the actual pumping performance used is clearly above the nominal figure. Periodically running in the top gear of the SF range, the motor also works significantly outside of its efficiency optimum for much of the time. This kind of regular overloading can shorten its life cycle, too.
Testing actual power consumption
In order to compare the actual energy efficiency of different vacuum pumps 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 vacuum pump 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. This latter rotary vane vacuum pump, running at significantly lower speed, worked significantly more efficient despite its larger motor (Figure 2).
Comparing nominal energy consumption is definitely not enough to judge the overall efficiency of a vacuum supply system. A realistic assessment requires a complex and systemic approach. Besides ultimate pressure and pumping speed of the vacuum pump or the vacuum system, operating principle and lubrication have to be considered; their compatibility with the process has to be ensured. Further factors like the place of installation, the control or the connection between process and vacuum generation, the placement of the machines, process cycles and the option of a vacuum buffer can also have major influence on the energy and economic efficiency of the vacuum supply. A qualified vacuum specialist can be needed to assess all aspects and to select the optimal solution. To calculate the actual energy consumption, realistic process parameters have to be take into account.
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