Thursday, July 12, 2012
Please check out our Laboratory Vacuum Pumps section for more information or to find manufacturers that sell these products.
A laboratory vacuum pump is an adaptable tool that can aid a wide diversity of research scientists and engineers. Laboratory vacuum pumps are used routinely in labs:
- To provide suction to drive the aspiration or filtration of liquid or suspended samples
- To induce or control solvent evaporation by reducing vapor pressure, as in ovens, rotary evaporators, gel dryers, and concentrators
- To improve instrument-detection sensitivity by evaluating air molecules that might obscure or contaminate samples, as in a mass spectrometer
- To collect gas samples from test chambers or the atmosphere
- To provide a negative pressure (that is, less than atmospheric pressure) environment to prevent escape of potentially hazardous sample materials.
The first two of these applications represent the vast majority of the uses for which lab vacuum pumps are purchased in chemistry and life science labs, so they will be the focus of this article. Vacuum pumps that support instrumentation sensitivity are typically integrated with the instruments they support, and are rarely purchased independently for labs, though we will touch briefly on high-vacuum applications used in instrumentation and physics work. Vacuum to produce a negative pressure environment would typically be provided through building engineering efforts. It is worth stating at the outset that lab vacuum pumps are designed to move air or vapors, and not to directly pump liquids or suspensions.
Many laboratories—especially in older lab buildings—are equipped with central vacuum (sometimes called “house vacuum”) systems. These systems can be a convenience, especially in teaching labs and in labs in which the main applications are filtration and aspiration. Many new lab buildings are being built without such systems for a variety of reasons:
- Competing uses on central vacuum systems often lead to conflicts between users who need stable vacuum, and those who connect and disconnect vacuum apparatus because they need brief, intermittent vacuum support. This vacuum instability can be especially problematic in modern, multidisciplinary lab buildings.
- The vacuum levels they can reach are modest—about 75 Torr in a new building—so the vacuum they provide is insufficient for many evaporative chemistry applications, which end up needing dedicated pumps anyway.
- Since central systems are installed during original construction, they are unable to adapt as needs change over the life of a lab building without being overbuilt to meet all possible future needs. This fact, plus the energy intensity of such systems for the life of the building, means they are frequently not considered a very sustainable option.
- House vacuum systems suck chemical vapors and biological aerosols away from applications and into vacuum tubing behind the walls. There they can lead to cross-contamination between vacuum applications in different labs, or simply condense in the tubing and deposit there as an unknown mix of chemicals over the life of the building.
- Finally, because house vacuum makes things “disappear” into the wall, users may be less careful to avoid aspirating fluids or particulates into the system than they might be in using a pump on their lab bench.
Another vacuum source that is familiar in many labs is the “water jet pump” or “vacuum aspirator.” These devices are attached to a lab sink faucet; the rapid flow of water through the device creates a vacuum in a side-arm that is connected to a vacuum application. Water aspirators were historically popular because of low acquisition costs and the relatively deep vacuum (e.g., 10–15 Torr). The vacuum levels vary with water pressure and water temperature, however, and even in modest use, they can waste 50,000 gallons of water per aspirator over the course of a year. Multiplied across all of the labs in a science building, the water waste and cost—and the treatment costs as the water is contaminated with entrained lab vapors—are substantial, even in areas of the country in which water is not a scarce resource. Some states have even banned water aspirators in labs for these reasons.
Figure 1 - VACUUBRAND MZ 2C NT diaphragm pump (courtesy of VACUUBRAND, INC., Essex, CT).
The inflexibility and limited capabilities of central vacuum systems, along with the environmental and operating cost issues affecting water aspirators, have faced more scientists with the need to choose individual vacuum pumps for their laboratories. Choosing the right vacuum pump will enhance convenience and productivity in your lab; choosing the wrong pump may interfere with your scientific objectives, and lead to substantial service demands and an unpleasant lab environment.
Choosing a lab vacuum pump
Historically, most vacuum pumps used in laboratories were oil-sealed, rotary vane pumps. Familiar forms include belt-drive and direct-drive models. The oil in these pumps is used to lubricate and seal the pumps. Because of the oil seal, these pumps can typically reach vacuum pressures of 10–3 Torr—substantially deeper vacuum than most dry lab vacuum pumps, which typically do not reach below 10–1 Torr. Vacuum levels of 10–3 are needed for freeze dryers, molecular distillation applications, and Schlenk lines.
A first rule in selecting a lab vacuum pump is to choose for the needs of the application. Suction applications, like filtration and aspiration, can occur effectively at a few hundred Torr, and many solvents in labs can be evaporated at room temperature at pressures of 1 Torr or greater. All of these applications are within the range of dry pumps.
Labs over the last two decades have turned increasingly to oil-free or “dry” pumps for certain applications in which the deeper vacuum of the oil-sealed pumps is not essential. Since these pumps do not use oil, there is no contact between oil and process vapors and no need for oil changes (or costly disposal of contaminated waste oil). This can substantially extend service intervals, and reduce the total lifetime costs of operation of dry pumps compared with oil-sealed pumps.
Common oil-free vacuum pumps used in laboratories are piston and diaphragm pumps (see Figure 1) and, more recently, scroll pumps. Each has advantages. Diaphragm pumps operate with a pulsing motion, like a heart, and so need no oil as a seal. Valves open and close alternately to move vapors in the right direction to create vacuum. Diaphragm pumps have a physical vacuum limit of about 0.45 Torr, and so models are available that can handle all suction applications, as well as evaporative applications for virtually all solvents at room temperature, except DMSO, which needs gentle heating to evaporate at these vacuum levels.
By selecting the right materials of construction for the flow path (the portion of the pump exposed to process vapors), diaphragm pumps can be made quite resistant to acid vapors and corrosive solvents. For corrosive applications, choose a diaphragm pump with a full fluoropolymer flow path. Since vapors from virtually all applications pass harmlessly through a corrosion-resistant dry pump, some manufacturers indicate that cold traps are not even needed to protect the pump except for extreme conditions. Vapors can be captured easily at atmospheric pressure upon release from the pump, or ducted to fumehoods for exhaust from the lab building, avoiding the expense and inconvenience of cold traps.
Piston pumps (see Figure 2) are an economical choice for laboratory vacuum applications that do not generate chemically aggressive vapors. With a range of adjustable vacuum levels down to 5 Torr, piston pumps are typically used for life science aqueous fume operations such as filtration, aspiration, and vacuum oven drying.
Figure 2 - Welch® model 2522 WOB-L® piston pump (courtesy of Welch-Ilmvac, Niles, IL).
Hybrid vacuum pumps that combine attributes of rotary vane and diaphragm pumps are available. These pumps are especially well suited for applications that need the “fine vacuum” (10–3 Torr) range of a rotary vane pump, but which have high vapor loads that would create a need for very frequent oil changes. Freeze dryers are a typical application. With a hybrid pump, the diaphragm pump keeps the oil in the rotary vane pump under vacuum, reducing the condensation of vapors in the pump oil, and substantially reducing the need for oil changes.
Scroll pumps are the other dry pump technology often seen in labs. They operate with two interleaved spiral scrolls that move eccentrically against one another, compressing air and vapors and moving them toward the exhaust. Scroll pumps reach deeper vacuum levels than diaphragm pumps—some newer models can reach 10–3 Torr—and higher pumping speeds, so they are attractive for applications, such as gloveboxes, needing high flow rates. Each of the two scrolls includes a tip seal that keeps vapors in the proper channel. The tip seal is a wear part, and requires periodic replacement. As the tip seal wears, it sheds small particles, which can be problematic for some applications. Scroll pumps do not have quite the corrosion resistance of diaphragm pumps; some manufacturers specifically recommend against use in corrosive applications, though the corrosion-resistance of some models has improved over the years. For applications in which these conditions are not a problem, however, the scroll pump offers a compact alternative to oil-sealed and diaphragm pumps with higher pumping speeds.
An important vacuum technology that is used widely in labs but much less frequently purchased for individual applications is the turbomolecular pump. These pumps can reach vacuum levels of 10–10 Torr and are often used in instrumentation, such as inside a mass spectrometer. The operating principle is molecular momentum; rapidly spinning vanes collide with air or vapor molecules and impart momentum that moves these molecules in the direction of exhaust. Air at atmospheric pressure is too dense for these principles to work, so turbomolecular pumps need a second pump (variously called a “backing pump,” a “forepump,” or a “roughing pump”) to bring operating pressures down to the range at which the turbomolecular pump can function. A rotary vane, diaphragm, or scroll pump is typically used as the roughing pump; by creating a vacuum of 1 Torr, for example, 99.9% of the air molecules have been removed (1 Torr out of 760 Torr), thereby enabling the turbomolecular pumps to continue pumping down to the desired levels needed for detection, say, 10–8 to 10–10 Torr. Since the manufacturers of turbomolecular pumps typically match the turbo pump with the roughing pump and supply both as an integrated package for specialized applications, selecting such equipment can only be effectively addressed in a separate article.
Ultimate vacuum and pumping speed
Laboratory vacuum pumps are specified with two different units: the “ultimate vacuum” and the “pumping speed.” Some understanding of how to use these data is important when selecting the right pump for your lab.
Ultimate vacuum is the lowest pressure level (deepest vacuum) that the pump can provide. By definition, it is the point at which the flow rate falls to zero, that is, at which the pump can no longer move vapors. Since, for evaporative applications, an essential aspect of vacuum pump performance is to move vapors away from an application, you should allow some cushion between the actual vacuum you need to achieve and the specification of the pump you choose. For example, at room temperature, the vapor pressure of water is about 20 Torr. If your objective is to use vacuum to evaporate water at room temperature, you would want a pump with an “ultimate vacuum” of somewhat less than 20 Torr, or the pump will be barely able to move vapor from the application.
The other key specification is the pumping speed, also referred to as “flow rate” or “free air displacement.” This unit is defined as the maximum rate of vapor movement of which the pump is capable; this occurs under conditions at which no vacuum is being produced. In other words, this unit describes the performance of the pump under conditions in which the pump is acting as a fan. While this gives a general indication of the relative pumping capacity, no one uses a vacuum pump as a fan; to serve its purpose it must pump against the resistance of vacuum.
With two units that describe how the pump will perform in general terms, but not under real operating conditions, how do you choose the right pump, and compare models from various manufacturers? The best approach is to study the pump performance curves (see Figure 3). These curves show the performance in the region between the ultimate vacuum and the pumping speed specifications. These curves are important because two pumps with identical specifications according to these two units may, in fact, perform very differently. For example, as the vacuum increases, and the pumping speed falls toward zero at ultimate vacuum, the rate at which the pumping speed declines will influence how big a pump you need to buy. One pump may have lost only 10% of its pumping speed at the vacuum at which another pump—with the same specifications—has lost 50% of its specified pumping speed. To compensate, you would need to buy a much larger pump for the same application.
Figure 3 - Vacuum pump performance graph: Two pumps with identical specs (10 mbar, 1.8 cfm) differ markedly in performance at 20 mbar.
If you don’t have performance curves available, ask your dealer rep or vacuum pump rep a simple question: At “x” Torr (the level at which you need to work), what pumping speed can I expect? Ask for evidence, and then compare pumps and pump suppliers on that basis for price, delivery, service needs, noise levels, and similar characteristics. With this approach, you will be comparing pumps of similar capability relative to your specific needs, and will be sure that you get a pump that will get the job done for you.
After you have settled on a pump that suits your needs for vacuum depth and pumping speed, there are still a few last matters to consider. Different choices among the following may be influenced by your budget, by the need to protect critical samples, the inclination to enhance the productivity of researchers and the reliability of results, or an organizational commitment to sustainability objectives:
- Am I working with corrosive solvents (including bleach in life science applications)? If so, look for pumps with corrosion-resistant flow paths.
- Do I need specific vacuum levels to achieve my scientific objectives? If so, look into the range of control options that use electronics to manage processes, both to free you for other tasks and to ensure that your work is reproducible and samples are protected.
- Am I concerned about emissions from vacuum applications ? Accessories are available for many pump models that will capture vapors either before or after the vacuum pump. Besides protecting oil-sealed pumps, vapor capture can both reduce building emissions and permit recycling or proper disposal of waste solvents.
- Is energy consumption a concern? If so, you may want to look into options that permit pumps to “multitask,” that is, operate more than one vacuum application at once. Another option is pumps with variable speed drives that adapt automatically to pumping requirements, conserving energy by pumping only as much as needed at a given moment to create or maintain the desired vacuum levels.
Purchasing a laboratory vacuum pump
Most manufacturers of laboratory vacuum pumps produce a variety of pump types that fit into more than one of the technology categories discussed above. Manufacturers of laboratory vacuum pumps (with their U.S. headquarters location and url) include: Agilent Technologies (Santa Clara, CA, www.agilent.com), Edwards (Sanborn, NY, www.edwardsvacuum.com), Gast Manufacturing, Inc. (Benton Harbor, MI, www.gastmfg.com), KNF Neuberger, Inc. (Trenton, NJ, www.knf.com), Labconco (Kansas City, MO, www.labconco.com), Oerlikon/Leybold Vacuum USA (Export, PA, www.oerlikon.com/leyboldvacuum/us/), Pfeiffer Vacuum (Nashua, NH, www.pfeiffer-vacuum.com), ULVAC Technologies, Inc. (Methuen, MA, www.ulvac.com),VACUUBRAND, INC. (Essex, CT, www. vacuubrand.com), and Welch-llmvac (Niles, IL, www.welchvacuum.com).
BrandTech Scientific, Inc. (Essex, CT), a distributor of VACUUBRAND pumps for North America, offers an interactive on-line guide to assist customers in finding the right vacuum pump for their specific laboratory needs. The Vacuum Pump Selection Guide can be found as a link on the BrandTech home page at www.brandtech.com.
Emily S. Tozzi is a freelance writer with a Master’s degree in Soils and Biogeochemistry from the University of California, Davis; e-mail: firstname.lastname@example.org. Armen I. Malazian is with the Department of Land, Air and Water Resources at the University of California, Davis; e-mail: email@example.com. American Laboratory/Labcompare would like to thank Peter G. Coffey, Vice President—Sales & Marketing, VACUUBRAND, INC. (Essex, CT), for his contributions to this article.
To view videos on laboratory vacuum pump systems, see: