Oil-free diaphragm pumps have earned a reputation for environmental- friendliness for low- to medium- pressure applications. At the low-pressure end, pumps do not continuously send water down the drain as do aspirators. “And there is no contaminated oil to deal with,” notes Dan McDougall, senior manager for laboratory products at KNF Neuberger (Trenton, NJ). The pumps have a long operational life with minimal maintenance and DC voltage models are said to be more energy efficient.
KNF’s flagship “green” DC-powered vacuum systems feature a Bluetooth- enabled controller and several environmentallyfriendly features, including superior solvent recovery thanks to an inlet separator and outlet condenser. Advanced remote control features and ease of installation in tight spots (cabinets, shelves, inside fume hoods) provide greater safety and flexibility.
“The remote control allows the user to operate the vacuum system without once opening the hood,” Mr. Mc- Dougall says. KNF has performed several studies documenting the energy savings related to keeping the fume hood sash shut.
At the very high-pressure end, Agilent Technologies (Santa Clara, CA) has broken through with three products based on turbomolecular TwisTorr “molecular drag” technology. These oil-free pumps increase pumping efficiency and performance in compact, energy-saving designs. Turbomolecular high vacuums are used in advanced analytical instruments and for thin film deposition, space simulation, fusion experiments, and particle accelerators.
High foreline pressure tolerance permits the use of a smaller, low-cost backing pump. “Turbomolecular pumps cannot discharge directly to the atmospheric pressure, only up to 10 mbar absolute pressure at its outlet, hence the need for a backing pump,” explains Mauro Nebiolo, director of global marketing for vacuum products at Agilent.
The turbomolecular pumps are suitable for achieving high vacuum and ultra-high vacuum, with operating pressure ranging from 10-1 to 10-11 mbar inlet pressure.
High vacuum may also be achieved through a diffusion pump, which is not as clean as the turbomolecular design, or cryogenic pumps. “The latter are far less flexible, however, due to the required regeneration cycles,” Mr. Nebiolo tells Lab Manager Magazine.
Ultra-high vacuums may otherwise be reached using sputter ion pumps, titanium sublimation pumps, and getter pumps. The latter rely on a “non-evaporable getter,” an aluminum- zirconium alloy that soaks up gases by forming non-evaporable or non-sublimable compounds with them.
Selecting a vacuum pump
Vacuum pumps are commonly specified both by ultimate vacuum and by flow rate. The flow rate is also known as pumping speed (the “volume flow rate” at the inlet, measured in volume per unit time) or free air displacement (the volume of air that flows into the pump, per unit time, when inlet and outlet are at atmospheric pressure). Ultimate vacuum is the deepest vacuum the pump can reach, the point at which the effective pumping speed is zero.
“The problem is that the pump can’t move any vapor at ultimate vacuum, and at a specified pumping speed, the pump is working as a fan, not a vacuum pump. No one uses a vacuum pump to work at either of these conditions,” says Peter Coffey, vice president of sales and marketing at Vacuubrand (Essex, CT).
Pumps with identical specifications for flow rate and ultimate vacuum can perform very differently between these two end points. “So if you only use these terms to select a pump, you don’t know what performance you will get out of the pump under actual lab conditions,” he says.
In other words, users need to know how the pump behaves between these two end points—when the pump is both creating the vacuum and moving vapor.
There are two ways to determine pump performance at the vacuum pressure you need. One is to ask your vacuum pump vendor. Your vendor is used to thinking in these terms and can advise you. The other is to rely on vacuum pump performance curves that all reputable vendors will provide on request.
The curves map a pump’s performance between the free air displacement (flow rate) specification and the ultimate vacuum. Depending on the pump engineering, the actual pumping speed falls off at a greater or lesser rate as the pressure approaches the ultimate vacuum. The better the pump, the more the specified pumping speed is preserved closer to the ultimate vacuum. This is why pumps that have identical specifications for ultimate vacuum and flow rate perform very differently—and are priced very differently as well.
Pump performance curves for two pumps with identical specifications (flow rate of 2 cfm; ultimate vacuum of 10 mbar) may appear identical at first glance. At working pressure, however, the less-capable pump will be capable of moving only a fraction of the vapor compared with the higher-performance pump.
For example, at 25 mbar—approximately the vacuum needed to evaporate water at room temperature—a more-capable pump may evacuate vapors twice as fast as a less-capable pump. Again, these pumps may have identical specifications, but an evaporative process can take twice as long for one pump as for the other.
At the lower pressures required for organic solvents, the differences between the two pumps may be magnified. For example, at 15 mbar, the available pumping speeds could differ by a factor of four. “This begins to have very real implications for the efficient completion of your lab work,” Coffey observes. “An evaporative application may take several times as long to complete with the pump with lower performance characteristics.”
Bottom line: By studying these curves, a company could save significant money by buying a smaller pump with a more favorable performance curve and still meet process objectives.