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Vacuum Pumps: The Complete Buyer's Guide
How do I choose vacuum pumps?
Choosing the right pump is not an easy task. There are many important factors to consider, and we cover them all here.
When calculating the value of these products in your laboratory, it’s important to weigh the upfront costs versus the costs of ongoing maintenance. As a little research will show, the initial price tag tells only a part of the story.
With so many different vacuum pumps, choosing the right one for your application can be a challenge.
There are various factors to consider when choosing the right model, including: ultimate depth of vacuum, free-air-displacement (flow rate), wet or dry operation (oil or oil-less) and chemical resistance.
These are the basic starting points for making an educated decision on buying the one that’s right for you.
There are various categories of vacuum pumps currently on the market, these include: direct drive, rotary vane, diaphragm, scroll, and more.
When picking your pump, it is important to take into consideration which chemicals you will be using and in what concentrations. This will ensure that you don’t pick the wrong product for the task at hand.
Each type may be used for various applications based on the requirements of the application; This is why choosing the correct model and size for your application is crucial for equipment longevity and system operation.
The first step in choosing a great pump is to determine the operational parameters of your desired application.
Depth of vacuum
Understanding the depth of the vacuum required for the desired application is paramount when choosing the correct pump.
- Low vacuum is used mostly for moving liquids around via filtration on basic glassware such as filter flasks.
- Medium vacuum is used for applications such as solvent recovery, where you would be evaporating solvents or reconditioning solvents.
- Medium to high vacuum can have a wider variety of use such as botanical extractions where it’s important to lower the boiling points of certain compounds so that they will become gaseous.
- High vacuum applications are best suited for the semiconductor industry, clean rooms, optics and more; all of which require a deeper vacuum.
- Ultra-High Vacuum (UHV) is used more in theoretical applications when studying space and certain nuclear reactions.
Selecting the right vacuum depth and correct flow rate will have a significant impact on your system.
A system with small volume will not need to have a large flow rate pump, because the system itself is small. You won’t need as much flow rate to clear small systems of atmosphere.
This is, however, the opposite for larger systems; the larger the system, the more beneficial it is to have a larger flow rate, otherwise you could be waiting significantly longer for a system to pull vacuum.
For example, if a user has a vacuum oven used for purging low-boiling point volatiles out of a product, high-vacuum depths (sub-Torr) are required to fully purge some solvents in addition to heat.
If the oven is small, a small direct-drive pump can be used to quickly evacuate the system, while for a larger volume oven, a larger (higher free-air-displacement) pump may be utilized.
The second step is determining whether the pump will be exposed to corrosive compounds.
Corrosion resistant? Oil or no oil?
Depending on what items are being degassed/purged in the oven, a corrosion-resistant pump may be a better fit for longevity.
Here is where utilizing a dry or oil-less pump may be necessary:
Understanding the driving forces behind a specific application is key to selecting a pump. For example, take benchtop fractional distillation. In benchtop-scale fractional distillation, the vacuum is an environmental operational parameter, while heat is the driving force.
A vacuum lowers the boiling points of compounds, while heat/agitation allows compounds to evaporate into the vapor phase, and these vapors can then be turned into a condensate.
Free air displacement (measured at atmospheric conditions) relates to the flow rate of air through a given pump, and it describes how fast a given pump will reach its ultimate vacuum.
For this reason, high-flow rate pumps are not necessary to perform this operation, once the system achieves high-vacuum, the pumping speed dramatically tapers off (as there are fewer and fewer molecules for the pump to move).
Additionally, utilizing too high of flow rates for distillation can cause molecules to condense in undesirable areas due to the pump pulling vapors past where they are intended to be collected.
A handy guide to the most common varieties
Once your parameters have been determined, the next step is to determine what pumps are applicable for the desired application, and choose the best option for budget, brand, integrity, and/or availability.
There are a variety of pumps that may be applicable to a specific operation, which is why it is important to understand the differences between the types.
Direct drive units are driven by an electric or gas motor. These pumps can achieve a deep vacuum and are economically priced.
The motor spins at a constant speed, based on the type of motor used. The pump of these units is directly connected to the drive shaft of the motor, so the pump mirrors the motor with regard to RPMs.
The simplistic design of this style helps contribute to their cheap cost. However, when the pump spins at higher RPMs, the motor will suffer from wear and tear faster. This will reduce the lifespan of the machine.
The vibrations from the motor get directly transferred to the pump, which is the main contributing factor of wear.
Other versions of direct drive pumps try to negate this problem by adding a gearbox, which allows the pump to run at a lower RPM than the motor.
Common applications for direct drive pumps are vacuum ovens, freeze dryers, and centrifugal concentrators.
Direct Drive pumps are not meant for aspiration, filtration or other applications requiring operation above 30 torr.
These are high-performance and often lower in cost; however, they use oil, which has to be replaced and can be costly.
The life expectancy of rotary vane pumps relies heavily on maintenance. “RV” pumps use oil to make a tight seal and remove heat from rotors by lubricating the parts. These pumps can reach deep ultimate vacuum levels and have variable displacement capacities ranging from low to high. These pumps work well for samples that contain solvents, aqueous vapors or high boiling points.
Maintenance on these types of pumps is recommended every 3,000 hours.
These pumps are “dry”, meaning they use no oil.
The elimination of oil in this application is also environmentally positive, as it reduces the hydrocarbon output from the device.
Diaphragm pumps use a series of valves in a pulsing open-and-closing motion to move air, which allows this pump to be run without oil.
The valves in the system are typically made of polytetrafluoroethylene (PTFE), which help prevent corrosion.
The main application for these pumps is for low-to-medium vacuum levels, but some diaphragm pumps have been designed for medium-to-high vacuum applications.
These vacuums usually have a higher upfront cost, but save your time and money on maintenance.
Hybrid (or combination)
Hybrid (or combination pumps) are both a diaphragm and a rotary vane pump in one.
Inside the pump, the purpose of the diaphragm portion is to keep a negative pressure on the oil in the rotary part of the pump, as this does not allow solvent or contaminates to condense in the oil.
This process allows the oil in these pumps to last up to ten times longer than regular rotary vane pumps.
For this reason, these pumps are better at handling acids and solvents than rotary vane pumps alone.
Applications recommended for hybrid pumps include freeze drying volatile or caustic solutions, such as acidic samples or harsh chemicals like acetonitrile, nitric acid, and others.
These are “dry” (oil-free) pumps which incorporate two spiral scrolls to compress air and vapors. These are then directed toward the exhaust.
Dry scroll pumps can be expensive, but they can also aid in savings down the road.
Because they are oil-less, they require much less maintenance and downtime than some of the previous pumps we’ve talked about. This equates to more uptime and cost savings over the life of the pump.
The elimination of oil in these units is again more ecologically sound, as their hydrocarbon output is lower.
Scroll pumps are known to be quiet, and they are better at handling water vapor than most pumps.
The scrolls inside the unit are typically made of metal, so even on the chemical/corrosion-resistant models, only samples lower than 20% of acids are recommended.
Compared to a diaphragm pump, the scroll pump can reach a deeper ultimate vacuum and has higher displacement capacities. These pumps are also used in degassing, distillation, and/or concentration applications.
Depending on the pump, scrolls are recommended to be changed approximately every 40,000 hours of use.
The various pumps discussed above are the most widely used in chemistry, production, and the pharmaceutical industry.
While other pumps (like diffusion and turbomolecular) are relevant, they were designed for more specific applications.
Diffusion pumps were the first type of high-vacuum pumps, and they are referred to as “gas-jet” pumps.
Gas-jet pumps work off of diffusion rather than conventional fluid dynamics. The gas cannot diffuse against the vapor stream and will be carried toward the exhaust, which in turn creates a vacuum.
These pumps make use of heated oil, which boils and captures gaseous molecules pushing other particles with it, creating a vacuum.
Diffusion pumps use silicone oil or polyphenyl ethers as the working fluid; Mercury can be used as well when dealing with instruments which require a clean vacuum.
Refilling these work fluids can be costly, but the reliability of these pumps is second to none.
There are no moving parts in these pumps!
The main use for diffusion pumps would be a mass spectrometer or other applications which require an extremely high vacuum.
Turbomolecular pumps are much less frequently used in production circumstances, due to the nature of the pump and prohibitively high cost.
These pumps work from the principle that gas particles can be pushed in a particular direction if they come into contact with a constantly moving surface.
This pump makes use of multiple fan blades which spin and hit the gas molecules, pushing them in a particular direction.
Turbomolecular pumps require a backing pump that pulls into a high-vacuum in order to be turned on.
If a turbomolecular pump is not started under high-vacuum, it can quickly burn up, as it is not intended to have large quantities of molecules/particles present around the fan blades.
Why do I need a cold trap or vacuum trap?
While most vacuum pumps employ one or more of these technologies, they are still susceptible to caustic chemicals. Sadly, these technologies do little to protect your pump from damage.
How traps work:
Vapors are passed from the system towards the pump, passing through the trap, where they condense due to the surface area/cold surface of the trap.
Condensing vapors externally from the pump will increase pump longevity, reduce the frequency of oil changes on your oil pumps, and prevent many undesirable circumstances from occurring.
We highly recommend that you use a trap when dealing with any sort of vapor to prevent pump damage or vapor concentration inside the pump/pump oil.
Additionally, when contaminated with condensed vapor, the oil inside oil pumps can change viscosity; thus reducing pumping performance.
All of these factors should be considered when selecting the right vacuum pumps for your laboratory setup.
These machines can be an expensive piece of labware; therefore, care should be taken when selecting the style and capacity that’s right for you.
We hope you’ve found this guide to be helpful, happy shopping!
Lab Society is not responsible for any incorrect uses/applications of these products, wrong models purchased, or any other incorrect use.