What do you need to consider when choosing vacuum pumps?
Anyone without a deep understanding or knowledge of pumps might think that vacuum generation is simply a question of “plugging in a pump”, starting it up and waiting for the vacuum to drop to the required level.
But the reality is that there is far more to the process.
Vacuum pumps are used to remove air or gas molecules from a sealed volume thus creating a vacuum. The vacuum level can be controlled, for example, with a process gas at a specific pressure.
Finding the right vacuum pump not only requires a good understanding of the necessary vacuum level and application – it also requires an understanding of process conditions, the operating range and the benefits and limitations of each specific vacuum pump type.
In this blog, we’ll briefly outline the four factors to consider when choosing between different types of vacuum pumps.
1. Vacuum level
The pump selection heavily depends on the level of vacuum that needs to be obtained. Typically, the different pressure ranges in vacuum technology are defined as follows:
- 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)
In rough- and medium vacuum most gas molecules are within the volume of the vacuum chamber, whereas in ultra-high vacuum (UHV) and extreme high vacuum (XHV) most of the remaining molecules will be on or in the chamber walls respectively. Thus, different pump technologies will be required for the different vacuum pressure ranges.
It is also important to consider whether it is mainly about pumping down to the required pressure level or – for example – holding a specific pressure level while certain gas loads are introduced to the vacuum system (e.g. for process reasons). While some vacuum pumps are optimized for pump-down processes (but might struggle with high process gas loads), others are more capable of handling high gas loads.
Depending on the target vacuum level, a mix of different vacuum pump technologies might be necessary. Primary vacuum pumps, i.e. those operating in the rough and medium vacuum ranges, exhaust to the atmosphere and can operate in isolation. High and ultra-high vacuum pumps, such as turbopumps and diffusion pumps, need to exhaust to or operate with a primary pump to create a level of vacuum they can work from. Ion, non-evaporable getter (NEG) and cryopumps need initial evacuation and then occasional primary pump support (e.g. during reactivation or regeneration process steps).
2. Impact of the process on the pump
Dependent upon the application area and medium to be pumped, the choice of pump(s) will vary. For example, rotary vane (RV) pumps are suited for a wide range of low and medium vacuum applications, including research and development, analytical instruments, industrial and coating activities, freeze drying, process engineering and many more.
The use of oil as a sealant and coolant allows for very good pumping performance and suitability for many applications, including where dirt, dust or condensates might be present.
Scroll pumps, on the other hand, provide hydrocarbon-free vacuum by compressing gases using two tip-sealed spirals rotating eccentrically against each other. This results in low operation and maintenance costs. Compared to RV pumps, scroll pumps applications are mainly limited to processes, without dust or dirt which could damage the tip seals in a short period of time.
Taking this into account, the impact of the application on the selected pump technology needs to be evaluated carefully, as well as the potential impact of:
- Dust or debris from process
- Corrosive gases or mixtures in presence of water vapour like Chlorine
- High gas throughputs
- Frequent ventings
- Mechanical movements / shocks
- Vibrations
- Heat radiation load to the pump (during baking or by evaporators)
- Radiation (such as X-rays)
- Magnetic fields
3. Pump impact on application
Evaluating the impact of the vacuum pump on the application or process is just as important. There are several variables that can influence the choice between different types of vacuum pumps, including – but not limited to:
- Oil or hydrocarbon emission
- Vibration produced by pump
- Noise emission
- EMC emission
- Magnetic field produced by pump
- Particle emission
- Heat emission
- Energy consumption
Returning to the example products above, RV pumps are at a disadvantage because they cannot generate hydrocarbon-free vacuum due to oil emissions. Scroll technology, on the other hand, while capable of producing hydrocarbon-free vacuum, carries the risk of particle emission due to wearing down of the tip seal.
4. Investment and maintenance
As well as considering what needs to be achieved, initial capital costs, operating costs and maintenance needs should be assessed.
Taking two high vacuum pump technologies, turbomolecular pumps (TMP) and oil diffusion pumps, as examples, it is fair to state that the initial costs for TMP usually will be significantly higher compared to an oil diffusion pump. However, considering the cost of ownership over a five-year period, oil diffusion pumps might cost more due to higher energy and maintenance costs. For some products, economic advantages might apply from a certain pump size/performance class.
In this video, Dr Andrew Chew highlights the key factors that could be influencing the performance of your vacuum pump.
5. What are the different types of vacuum pumps?
There are two classifications for vacuum pumps. Primary pumps exhaust directly to atmospheric pressure (such as rotary vane, scroll, diaphragm, screwand multi-stage roots pumps) and secondary pumps which require the use of a primary pump to continuously support their operation (turbomolecular pumps and diffusion pumps) or to evacuate to a pressure at which they can begin to be operated ion getter, titanium sublimation, non evaporable getter and cryogenic pumps). Roots booster pumps are often combined with primary pumps to be a ‘primary’ pump pair but are strictly to be classed as secondary pumps.
Effective vacuum generation requires an understanding of needs and the different types of vacuum pumps available. Choosing the wrong pump can be a costly mistake and potentially damaging to your operation should it not perform as required.
To find out more about choosing the right vacuum pump for your operation, click the link below to download our free eBook, “Vacuum Pump Technologies Explained”:
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