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Aligning Vacuum Choice with Application Demands

Factors to consider when determining the right vacuum pump for your lab

by
Carl Watkinson

Carl Watkinson is the business development manager, vacuum products division at Agilent Technologies.

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Although easily overlooked, vacuum systems play a vital role in the performance of laboratory processes ranging from early-stage research and analysis to product development and evaluation to quality assurance testing. In some cases, the vacuum simply removes gases or vapors from a system to eliminate their interference in subsequent analysis, such as mass spectrometry or electron microscopy. In other applications, the vacuum serves to lower the atmospheric pressure within the system, facilitating filtration or solvent evaporation at temperatures more conducive to product stability and shelf-life.

To address the specific demands of these applications, a variety of vacuum systems have been developed, facilitating work requiring anywhere from minimal vacuum to ultrahigh levels.

Note: The ultimate vacuum levels described below are approximations and true values vary among pumps within a category.

Water aspirator (ultimate vacuum: 10 Torr)

Mechanics: Attached to laboratory water supply, gases and vapours are drawn into the aspirator by the rapid flow of water past an aperture.

Applications: liquid aspiration, sample filtration, solution degassing

Simple to set up and operate, water aspirators are a low-cost option for labs performing low-vacuum processes. The vacuum levels generated by water aspirators can be quite variable, as they are determined by in-house water pressure and temperature. Aspirators waste and potentially contaminate large amounts of water, increasing resource costs and environmental impacts.

Diaphragm pumps (ultimate vacuum: 102 – 10-1 Torr)

Mechanics: In a pulsing motion, pistons flex membranes to pull and push vapors and gases through one-way valves that alternately open and close, much like an aquarium filtration pump.

Applications:

  • Low vacuum: sample filtration, solution degassing, liquid aspiration
  • Medium vacuum: rotary evaporation, vacuum drying, solvent recovery, backing small turbomolecular pumps

Oil-free operation reduces the risks of oil contamination in samples and lessens the need for vapor cold traps to protect the system. Although basic units rely on a single diaphragm head to create the vacuum, the use of multiple heads in series can produce higher vacuum levels and a more constant flow. Corrosion-resistant components are also available to protect the system from acid vapors and corrosive solvents. Diaphragm pumps can struggle to achieve the vacuum levels of oil-sealed pumps and tend to generate more noise and vibration.

Piston pump (ultimate vacuum: 10-2 Torr)

Mechanics: Piston pumps come in a variety of piston-cylinder configurations, but ultimately move a piston through a repeated cycle of drawing gases and vapors into a chamber through a one-way valve and then exhausting them via a second valve much like the pistons in a car.

Applications: distillation, filtration, freeze drying, glove box, backing turbomolecular pumps

Although basic units rely on a single piston to create the vacuum, the use of multiple pistons in series can produce higher vacuum levels and a more constant flow. Many piston pumps are oil-sealed, increasing the risks of sample and oil contamination. Thus, it may be necessary to also use a cold trap, which increases costs. Oil-free, O-ring-sealed systems are available. Piston pumps tend to be noisy and require noise reduction enclosures when used in a laboratory environment. 

Scroll pump (ultimate vacuum: 10-2 Torr)

Mechanics: There are two interleaved scrolls, with one fixed and the other moving eccentrically against it to create a pocket of space that draws in gases and vapors which are then moved toward the center of the scrolls where they are exhausted.

Applications: distillation, filtration, freeze drying, glove box, backing turbomolecular pumps

The mechanics of the scroll pump provide higher vacuum levels than many competitive technologies, as well as lower noise and reduced vibration during operation. The design also simplifies maintenance as the tip seal on the scrolls can be replaced rapidly. Models are also available that can handle corrosive solvents and acids. Although scroll pumps are typically more expensive than oil-sealed pumps, this cost can be somewhat offset by lower operating and maintenance costs of oil-free operation, as well as a lower environmental impact.

Rotary vane pump (ultimate vacuum: 10-2 – 10-3 Torr)

Mechanics: Rotary vane pumps use sliding vanes mounted in an off-center rotor to create pockets of space that draw in gases and vapors that are then compressed and exhausted as the rotor rotates. The combination of centrifugal force, spring balancing and oil creates an optimum seal on the sliding vanes that produces higher vacuum levels. 

Applications: distillation, freeze drying, Schlenk lines, thin-film coating, backing high vacuum pumps   

Connecting the intake of a second rotor to the exhaust of the first unit can further increase vacuum levels and increase flow rates. The need for oil increases the risks of sample contamination and the oil itself can become contaminated by the gases and vapors being pumped, necessitating the use of a cold trap. Rotary vane pumps may also generate high loads of oil mist if used for high-flow applications like filtration or aspiration, limiting their use in clean rooms or closed environments.

Turbomolecular pump (ultimate vacuum: 10-3 – 10-11 Torr)

Mechanics: Rapidly spinning blades or vanes collide with gas molecules, directing them into channels that compress and move them toward the exhaust port.

Applications: electron microscopy, mass spectrometry

Because air is too dense at atmospheric pressure for the turbomolecular pump to function alone, the system requires the use of a second, backing pump (e.g., diaphragm, scroll, or rotary vane pump) to bring the chamber to operating pressures. Sophisticated suspension systems and rotor balancing elements help to improve performance while also reducing noise and vibration.

The type or combination of vacuum pumps your lab requires depends not only on your current catalog of applications, but also on potential applications you might introduce in the future, as well as the nature of the solvents and other chemicals you use. Physical space available for the pump is also an issue to consider when deciding on a particular model, as you may want to factor in noise and vibration mitigation or portability for pumps used in high-traffic areas, or pump exhaust management for oil-sealed versus oil-free models. Fortunately, the choice is nearly limitless and continues to expand.