Vacuum pumps are used in dozens of laboratory applications, including filtration, evaporation, degassing, drying/ freeze drying, metals/materials processing, coating, and distillation. Vacuum pumps have perhaps the widest dynamic range of any lab device, operating from just under atmospheric pressure, 760 Torr, to 10-17 Torr—a difference of almost 10-20.
Most lab vacuum applications operate at pressures of between 1.5 Torr and 150 Torr, with the 75- to 150-Torr range typically served by house vacuum. Low-pressure applications run between 1.5 Torr and 10-3 Torr, while instrument-dedicated pumps can go as low as 10-8 Torr.
The two most common laboratory vacuum technologies are diaphragm pumps and rotary vane pumps. Diaphragm pumps are oil-free and suitable for applications above about 1 Torr, which covers the vast majority of filtrations and aspirations.
Rotary vane pumps use oil for sealing and lubrication and require substantially more maintenance than oil-free pumps. Higher pumping speeds and ultimate vacuums make them suitable for evacuating glove boxes and for drying/freeze drying.
A third technology, the scroll pump, is also an oil-less design. With ultimate vacuums of around 0.1 Torr, scroll pumps are suitable for applications requiring vacuums intermediate between diaphragm and rotary vane capabilities.
Welcome to OEM world
Historically, says Jack Abrams of Agilent (Santa Clara, CA), the lab vacuum marketplace is an “OEM world,” where most users outside of physics or semiconductors (both requiring extremely high vacuum) have little familiarity with manufacturers. “It’s not a user community that’s deeply knowledgeable about products and options. Users might not even know, for example, that there’s a turbo pump in your GC/MS. Lab workers care little about how a vacuum pump works,” Mr. Abrams adds, “but they sure care when it stops working.”
Matching pumps with applications
With vacuum, more is not necessarily better. “It’s always best to match vacuum with the application,” says Peter Coffey, VP of sales at Vacuubrand (Essex, CT).
For example, filtration requires modest vacuum (75–150 Torr). A pump pulling 10-3 Torr would evaporate the filter and cause channeling or rupture of the filter medium. More powerful pumps also contribute to noise pollution.
Because vacuum pumps last a long time, most laboratory workers have never purchased one. The most common practice is to replace an old pump with one just like it. According to Mr. Coffey, this practice leads to acquiring pumps that are inappropriate to applications; do not reflect current technology; and are larger, noisier, and more difficult to maintain than necessary. VACUUBRAND, like many pump manufacturers, offers extensive guidance to steer purchasers to the right pump.
Mr. Abrams cites one R&D customer whose freeze-drying chamber took several hours to achieve optimal vacuum. A more suitable pump could have reached the desired pressure in a few minutes.
Once the application is defined, the three essential characteristics of pumps are vacuum pressure capacity (also called ultimate vacuum), pumping speed, and electronic control. Control is important when the vacuum needs to be adjusted up or down, applied gradually, or maintained constant during a changing process (e.g., distillation).
Mr. Coffey rates control as the most significant recent development in pumps. “Productivity is a real issue in the labs with staffing levels as tight as they are, so a pump designed to manage an evaporation frees a lab worker from having to direct the process manually.”
The argument for oil-free pumps
Oil-free pumps are quieter, cleaner, and require much less maintenance than oil-sealed rotary vane pumps, and offer vacuum pressures that are appropriate for most lab applications.
The demand for oil-free vacuum pumps has been growing despite higher acquisition costs (about double for oil-free). It was initially believed that the cost of oil and oil disposal for twice-yearly oil changes was a significant driver, but that was not so. “It’s labor costs related to maintenance and related logistics,” says Abrams. An oil change takes only 20 to 30 minutes, but downtime for critical processes while the pump is being serviced is just as significant.
Pressed to provide one pearl of wisdom on new pump purchases, Mr. Coffey does not hesitate: “Never use an oil-sealed pump when an oil-free model will do the job.”
Dan McDougall, senior manager for laboratory products at KNF Neuberger (Trenton, NJ), concurs. “Oil pumps are used when an application requires a deeper vacuum than can be provided by the diaphragm pump. But for the most common lab applications, such as rotary evaporation, gel drying, filtration, and vacuum ovens, oil-based pumps are overkill. An oilfree vacuum pump supplies adequate, reliable vacuum without the hassle and expense of pump oil.”
What to look for
Mr. McDougall provides a thumbnail guide to pump selection:
- Match the pump to its intended use, considering materials of construction, desired flow rate, and ultimate vacuum.
- Make note of the solvents that the pump will encounter. Pumps using premium PTFE-wetted parts are recommended for pumping corrosive vapors, but other materials are suitable for less aggressive applications.
- Strive for low cost of ownership.
- Diaphragm pumps are oil-free; replacing a diaphragm is accomplished in minutes using simple tools. Oilbased pumps require oil changes and disposal, and repairs are complicated, expensive, and performed off-site.
- Consider the environment. Oilfree pumps eliminate the need to dispose contaminated pump oil and excessive water usage from an aspirator.
Angelo DePalma holds a Ph.D. in organic chemistry and has worked in the pharmaceutical industry. You can reach him at email@example.com.
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