Factors to Consider When Choosing Vacuum Pumps
When it comes time to select a new vacuum pump, it's important to take a thoughtful approach. At first glance, it might seem like a simple task, but, there are several factors to consider.
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
- Process or application impact on pump
- Pump impact on process or application
- Cost and maintenance
Vacuum level is perhaps the most critical consideration.
It affects each of the other factors.Typical classifications and ranges include:
- Rough vacuum: from 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
Different pump technologies are required to achieve different vacuum pressure ranges. Depending on the target vacuum level, a mix of different vacuum pump technologies might be necessary.
Forevacuum pumps — 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 forevacuum pump.
Ion, non-evaporable getter pumps, and cryopumps need initial evacuation and occasional forevacuum and high vacuum pump support at certain stages.
Process or Application Impacts on Pump
The way processes can impact pumps plays a significant role in selection. 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 the process
- Corrosive gases like chlorine or acidic mixtures in the presence of water vapor
- High gas throughputs
- Frequent ventings
- Mechanical movements and shocks
- Heat radiation on the pump
- Particle or X-ray radiation
- Magnetic and electric fields
As an example, rotary vane pumps are well-suited for many low and medium vacuum applications, such as:
- Research and development
- Analytical instruments
- Industrial and coating activities
- Freeze drying
- Process engineering
On the other hand, scroll pumps provide hydrocarbon-free vacuums 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 pump applications are mainly limited to processes without dust or dirt that could damage the tip seals.
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 backstreaming
- Vibrations produced/induced by the pump
- Magnetic fields produced by the pump (rare)
- Heat emission
- Energy consumption
Rotary Vane pumps are at a disadvantage because they cannot generate a hydrocarbon-free vacuum due to oil vapor emission.
On the other hand, while capable of producing a hydrocarbon-free vacuum, scroll technology carries the risk of particle emission due to tip seal wear.
Cost and Maintenance
Beyond initial capital costs, there is cost associated with operation and maintenance (service).
Diffusion pump operation is likely much more expensive when compared with an turbomolecular pump. Also, the cost of ownership over five years also includes factors like higher energy and maintenance costs.
Diffusion pumps can deal with dirtier, industrial processes, whereas turbo pumps are used mainly in clean processes.
Vacuum Pump Options
There are two overarching categories of vacuum pumps: forevacuum pumps and high vacuum pumps.
Forevacuum pumps exhaust directly to atmospheric pressure. They include:
- Rotary vane pumps
- Scroll pumps
- Diaphragm pumps
- Screw pumps
Secondary pumps rely on primary pumps to continuously support their operation as they cannot exhaust atmosphere, and require a low pressure at their exhaust, known as "backing."
Continuously supported (backed) high vacuum pumps include turbomolecular and diffusion pumps.
Ultra-High vacuum pumps include ion getter, titanium sublimation, non-evaporable getter, and cryogenic pumps.
Roots blower pumps are often combined with forevacuum pumps to act as a pump pair, but they are strictly classified as secondary pumps, as they are not designed to be used without a forevacuum pump.
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, cost, and maintenance also impact vacuum pump choice.
Choosing the wrong pump can be expensive and potentially damaging to your operation if the pump doesn't perform as required.