Factors to consider when choosing vacuum pumps August 26, 2020
5 MIN READ
When the time comes to select a new vacuum pump or pumpset, be it for a new system or to upgrade an existing one it's important to take several key factors into account. These will determine the pump type(s) and the required pumping capacity.
Various factors affect a vacuum pump’s overall efficiency and capabilities. Each pump requires its own process conditions, has its own operating range, and offers its own sets of benefits and limitations. These characteristics depend on four factors:
- Vacuum level and pumping speed
- Process or application impact on pump
- Pump impact on process or application
- Cost and maintenance
Vacuum level and pumping speed
Vacuum level and pumping speed are critical considerations.
It affects each of the other factors. Typical classifications and ranges include:
- Rough vacuum: from 10 3 mbar to 1 mbar
- Medium vacuum: 1 mbar to 10 -3 mbar
- High vacuum: 10 -3 mbar to 10 -7 mbar
- Ultra-high vacuum: 10 -7 mbar to 10 -12 mbar
- Extreme high vacuum: less than 10 -12 mbar
Pumping speeds
Forevacuum pumps pumping speed is measured in m3h-1, and for high vacuum pumps l/s is used: 1m3h-1 = 3.6 l/s
There are two considerations when choosing the correct pumping speed:
- Pumpdown time before a process can start – this should be as short as possible to minimize production costs
- Process flows – here the pumping speed must be able to maintain an appropriate pressure when process gases are employed.
Depending on the target vacuum level and pumpdown time, a variety of different vacuum pump technologies might be necessary.
Forevacuum pumps — those operating in the rough and medium vacuum ranges. These types of pump operate by compressing gases and then exhausting to atmosphere. Applications include food packaging, heat treatment and freeze drying.
High and ultra-high vacuum pumps — such as turbopumps and diffusion pumps — operate only in conjunction with a forevacuum pump and operate by the molecular transfer method. Applications include coating, metallurgy, and analytical applications.
Ion, non-evaporable getter pumps, and cryopumps work by trapping gases. In the case of a cryopump, this is achieved by freezing the gases. Evacuation and occasional forevacuum is needed at certain stages. Applications include R&D, high energy beam lines and space simulation.
Pump Impact on Processes or Applications
Evaluating the impact of the vacuum pump on the application or process is just as important as pumping speed. Several factors can influence vacuum pump choice, including, but not limited to:
- Oil or hydrocarbon emissions or back streaming
- Vibrations produced/induced by the pump
- Noise
- Magnetic fields produced by the pump (rare)
- Heat emission
- Energy consumption
Forevacuum pumps
There are several types of forevacuum pumps
- Oil sealed rotary vane pumps
- Scroll pumps
- Diaphragm pumps
- Screw pumps
Rotary vane technology is a popular choice for medium vacuum applications, but will exhibit oil back streaming. This can be an issue if a process is sensitive to contamination, for example precision coating. If the application demands an oil free environment, diaphragm and scroll pumps in particular would be suitable. However, both have lower pumping speeds than a rotary vane pump and the scroll can generate particles from its tip seals and is best suited to benign applications. Moreover, diaphragm pumps have a poor base pressure.
For high pumping speeds with good base pressure, the addition of roots pumps in combination with a rotary vane pump will increase pumping speed, but oil back streaming will still occur. Screw drypumps are an oil free option with high pumping speeds even without an added roots pump and can also handle difficult process applications.
There are several choices available for an additional pump for high vacuum processes:
- Diffusion pumps
- Turbomolecular pumps
- Entrapment pumps including cryopumps
Both diffusion pumps and turbo pumps offer similar base pressures. As with RV pumps, diffusion pumps are a source of oil contamination, but offer high pumping speeds. Turbomolecular pumps are an oil free option. However, the maximum pumping speed of the largest turbo is only around 5% that of a large diffusion pump.
For processes requiring large throughput in the high vacuum range, similar to a diffusion pump but without the issue of oil contamination, a cryopump where gases are trapped using low temperature might be appropriate. These pumps do not require a backing pump whilst operating, but will need a forevacuum pump for regenerating - the frequency of which would be process dependent.
Cost and maintenance
Beyond initial capital costs, there is cost associated with operation and maintenance or service.
Diffusion pump operation is likely to be more expensive when compared with a turbomolecular pump in terms of energy and cooling water. Diffusion pumps can deal with dirtier, industrial processes, whereas turbo pumps are mainly used in clean processes.
Cryopumps offer a similar performance to diffusion pumps, are oil free and have lower energy costs, but are not suitable for dirty industrial processes.
Final thoughts
Making the right pump choice for effective vacuum generation requires an understanding of how pumps impact processes — and how processes impact pumps. Vacuum level needs, pumping speeds, running costs and maintenance also impact vacuum pump choice.
Choosing the wrong pump can be expensive and potentially damaging to your operation if the pump does not perform as required. No amount of reading can make up for a quick meeting with a team who can answer all the questions even the most fastidious research raises. Reach out and let's chat.
Ensure you make the right choice, with the help of our team of leading vacuum technology experts.
