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Putting vacuum pumps to the test

06 May 2019

An independent testing organisation has carried out a comparison of different vacuum pump designs – an oil-sealed screw vacuum pump and an oil-lubricated rotary vane vacuum pump. 

Figure 1: Realistic test setup with vacuum vessel, vacuum booster, and tested vacuum pump as backing pump.
Figure 1: Realistic test setup with vacuum vessel, vacuum booster, and tested vacuum pump as backing pump.

The TÜV Süd comparison test entailed the simulation of the working cycle of a vacuum packaging machine, a standard industrial process required for the packaging of many fresh foodstuffs (see Figure 1).

As is often the case with such applications, both vacuum pumps were additionally supported by an identical vacuum booster. In addition, the test setup and procedures were checked by a well-known manufacturer of vacuum packaging machines and the test was confirmed as being a realistic simulation.

As an application example, a packaging machine with a large chamber volume was chosen, such as is used in the packaging of meat or cheese products. Typically, such a machine with automatic product supply handles several cycles per minute. 

The machine was simulated using a 300l chamber and an 11.5m long pipe system between the chamber, vacuum booster, and vacuum pump. The chamber was evacuated cyclically to a vacuum level of 5 mbar.

 

The time for evacuation depended on the performance of the vacuum pumps. The time between evacuation cycles was set at 14 seconds – a typical time span for this size of packaging machines. The required pump-down time of the vacuum pumps and their energy consumption were recorded. 

The results of the various test runs were consistently unambiguous – the rotary vane vacuum pump (RVVP) evacuates faster (Figure 2) and consumes less energy than the screw vacuum pump (SVP). Depending on the set speed of the RVVP, this results in further shortened pump-down times or increased energy savings. For example, the RVVP is 11% faster in 40 Hertz mode and saves 42% in power consumption by comparison. 

Figure 2: Packaged units depending on speed or design of vacuum pump.
Figure 2: Packaged units depending on speed or design of vacuum pump.

In addition to the pump-down time and the energy consumption, the pumping speed and energy consumption were also measured during the test as a function of the inlet pressure. The specific energy consumption (SEC) at different vacuum levels was calculated from these measured values. This gives precise information about how many watts are needed to extract one cubic meter of air per hour to reach a certain vacuum level. Here, too, the RVVP is superior to the SVP in all vacuum levels. The energy savings are between 13 and 73%. At the vacuum level of 10 mbar (typical in practice), the RVVP consumes 38% less energy than the SVP.

A question of principle
Although both vacuum generation and compression are about gas extraction, the different objectives require different technical solutions. 

For compressors the compression ratio is usually 1:10; for vacuum pumps it is 1:100 to 1:1000 – thus much higher. Technically speaking, this means that, in a screw compressor, the two screws and the housing can all be manufactured with higher tolerances. This means that production is more cost-effective and the targeted compression ratio of 1:100 is achieved despite the increasing number of internal leakage. However, this is only because it is compensated by much higher rotation of about 7,000rpm at full load. The RVVP, on the other hand, is a pure vacuum pump ultimately enables a much higher compression ratio. It therefore provides constant performance from the beginning to the end of the evacuation with low energy consumption and only runs at a maximum speed of 1,000rpm. The lower speed reduces the mechanical load and thus the maintenance requirement. This also makes it possible to achieve significantly longer downtimes and lower machine life cycle costs. 

The SVP requires separate pressure control by means of an inlet control valve to prevent overloading the vacuum pump in the range between 1000 and 300 millibar. Between atmospheric pressure and rough vacuum, it works with significantly reduced performance. This, and the design adopted from compressor construction, contribute to extending pump-down times. It is these differences that ultimately had an impact on the results of the comparison test.

Conclusion
The R 5 RA 0630 C oil-lubricated rotary vane vacuum pump (RVVP) from Busch performed better than the screw vacuum pump (SVP), both in terms of pump-down time and energy consumption. The test results confirm the superiority of the best-selling vacuum pump in this performance class. 


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