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Working Flow Rate: How to Compare Vacuum Pumps D
by Peter Coffey
iaphragm vacuum pumps with flow paths constructed of fluo- ropolymers like PTFE (e.g., Teflon®, DuPont, Wilmington, DE) offer tremendous advantages for laboratories. Compared with the use of traditional oil-sealed rotary vane pumps for moving corrosive vapors, oil-free pumps offer much longer maintenance intervals (some in excess of 10,000 hours), much greater corrosion resistance, the elimina- tion of oil-mist, and freedom from oil changes and contaminated-oil disposal. Since the materials of construction are highly resistant to corro- sion, an appropriately sized pump eliminates the need for a cold trap to protect the pump in many applications.
When selecting a dry vacuum pump to support an evaporative lab ap- plication such as rotary evaporation, centrifugal concentration, or vacuum drying, it is critical to match the pump’s vacuum capacity and pumping speed to the application. With too little vacuum capacity, the evapora- tive process proceeds slowly, if at all. With vacuum much deeper than the application requirements, there is a probability of evaporating filtrate, or foaming and bumping that can lead to sample loss. As a result, most buy- ers pay close attention to the “ultimate vacuum”—the lowest pressure the pump can reach—of the pumps they plan to purchase.
Often overlooked in selecting a vacuum pump, however, is the importance of flow rate (also called pumping speed or air displacement) in matching a vacuum pump to an application. A pump’s flow rate is simply the volume of gas or vapor it can move per unit of time, with units variously reported as liters per minute (lpm), cubic feet per minute (cfm), or cubic meters per hour (m3
/hr). The flow rate required for a particular application is determined
by a number of factors, including the application’s vapor volume, process temperatures, time requirements, and system leakage. If a pump cannot generate enough flow at the desired vacuum level, it may take many times longer than expected to complete the application.
The difference between vacuum pump specifications
and actual vacuum performance Even buyers who try to consider flow rates in pump selection, however, may be misled by the standard industry practice for reporting this speci- fication in manufacturers’ and dealers’ catalogs. That is because every pump has a point (usually very near atmospheric pressure) at which it can move the most vapor. This maximum pumping speed is reported as the flow rate or free air displacement in the pump’s specifications. As the pump generates a vacuum, however, the actual pumping speed steadily decreases as the developing vacuum resists the pump’s efforts to move vapor. When the pump reaches its specified “ultimate vacuum,” the pump has an actual flow rate of zero, by definition. At this point, the pump can no longer perform any work, that is, it can no longer move the vapors generated by the evaporative application.
Buyers understandably rely on the pump specifications to compare two or more vacuum pumps. The problem is that pumps with the same ultimate vacuum and flow rate specifications often differ dramatically in their avail- able pumping speed at working vacuum levels, that is, the vacuum at which you need to operate your application. In the author’s view, this makes the conventional flow rate specification virtually useless for understanding the relative capabilities of two similarly specified pumps. For this reason, it is strongly suggested that vacuum pump buyers investigate the working flow rate of any pump under consideration—the available pumping speed at working vacuum—to ensure that the selected pump meets their vacuum needs efficiently and effectively.
Charting pump performance Every pump has a measurable working flow rate at every point in between
atmospheric pressure (the specified flow rate) and the pump’s ultimate vacuum (zero flow). To present working flow rates concisely across several orders of magnitude of vacuum pressures, those flow rates are often chart- ed as a performance curve on a log–log scale. Armed with a performance curve, you can determine the effective pumping speed at the vacuum level needed by your application. A well-designed pump will retain much of its pumping speed throughout its working range, with speed dropping off sharply close to the ultimate vacuum. Less capable pumps with the same specifications lose their pumping speed more quickly as vacuum develops in the application.
By comparing the pumping speeds of two different pumps at your working vacuum level, you can be sure you are selecting a pump with the optimum performance for your application. This is important because the differ- ences in pumps with the same endpoint specifications (ultimate vacuum and free air displacement) often differ markedly in their effective pumping speeds at working vacuum levels. For example, some commercial oil-free vacuum pumps lose more than 90% of their specified pumping speed before achieving vacuum levels typically used in a lab evaporative applica- tion. With such a pump, your evaporative application proceeds much more slowly, and your vacuum application becomes a bottleneck in your work.
In Figure 1, the pumping speeds of two hypothetical pumps are plotted on performance curves. Both pumps depicted have identical flow rates at at- mospheric pressure (~1.8 cfm) and ultimate vacuum (~10 mbar). Thus, their specifications are identical and, at first glance, both curves look similar. However, the two pumps differ substantially in the amount of pumping they can do under typical conditions of an evaporative vacuum applica- tion—their “working flow rates.” At 20 mbar, for example, the pump on the red curve can still move 0.73 cfm of vapor, while the pump depicted on the black curve can pump only about 40% as fast.
What does this mean to you, the scientist? If you were using vacuum to evaporate a flask of water at 20 °C (vapor pressure 23 mbar), the pump in
AMERICAN LABORATORY • 36 • JUNE/JULY 2013
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