When it comes time 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 into account several key factors. These will determine the type pump or pumps and their 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
Forevacuum pumps pumping speed is measured in m3h-1, and for high vacuum pumps l/s is used: 1m3h-1 = 3.6 l/s
In choosing the correct pumping speed there are two considerations:
- Pumpdown time before a process can start – this should be short as possible to minimise 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 mix 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 would include food packaging and 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 molecular transfer method. Applications would include coating, metallurgy, and analytical
Ion, non-evaporable getter pumps, and cryopumps work by trapping gases. In the case of a cryopump this achieved by freezing the gases. Evacuation and occasional forevacuum and is needed at certain stages. Application would include R&D, high energy beam lines and space simulation.
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Process or Application Impacts on Pump
The way processes may impact pumps is a key role in selection. The type of application requires the selected pump technology needs to be evaluated carefully. Such considerations are:
- Dust or debris from the process
- Corrosive gases like chlorine or acidic mixtures in the presence of water vapor
- High gas throughput
- Frequent venting
- Mechanical movements and shocks
- Heat radiation on the pump
- Particle or X-ray radiation
- Magnetic and electric fields
Pump Impact on Processes or Applications
Evaluating the impact of the vacuum pump on the application or process is just as important. Several variables can influence vacuum pump choice, including — but not limited to:
- Oil or hydrocarbon emissions or back streaming
- Vibrations produced/induced by the pump
- Magnetic fields produced by the pump (rare)
- Heat emission
- Energy consumption
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 particularly scroll pumps would be suitable. However both have lower pumping speeds than a rotary vane pump whilst 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 the use of an added roots pump and are also able to handle difficult process applications
If the process is high vacuum there are several choices available for an additional pump
- Diffusion pumps
- Turbomolecular pumps
- Entrapment pumps including cryopumps.
Both diffusion pumps and turbo pumps offer similar base pressures. As was with RV pumps diffusion pumps are a source of oil contamination but offer high pumping speeds. Turbomolecular pumps offers an oil free choice. 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.
Related: Explore our range of vacuum pumps on the Leybold product page.
Cost and Maintenance
Beyond initial capital costs, there is cost associated with operation and maintenance (service).
Diffusion pump operation is likely to be more expensive when compared with an turbomolecular pump in terms of energy and cooling water. Diffusion pumps can deal with dirtier, industrial processes, whereas turbo pumps are used mainly in clean processes.
Cryopumps offer a similar performance to diffusion pumps are oil free and have lower energy costs but are also not suitable for a dirty industrial process.
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 and running cost, 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. And no amount of reading can make up for a quick meeting with a team who can answer all those questions even the most fastidious research throws up. Reach out and let's chat. Click the button below and ensure you make the right choice, with the help of our team of leading vacuum technology experts.