Figure 1 – Pumping speeds of two hypothetical pumps plotted on performance curves.


red would evaporate the water more than twice as fast as the black-curve pump. Furthermore, as the water cools because of evaporative heat losses, its vapor pressure would go down. As that occurs, the red-curve pump in- creases its advantage over the black-curve pump. At 15 mbar, the red-curve pump has a 3× working flow rate advantage over the black-curve pump, and that advantage builds as both pumps approach their ultimate vacuum. The user of the pump represented by the red curve will routinely benefit from faster process times and higher process productivity.


Choosing your next vacuum pump In practice, when selecting a pump, you need to know where on the pump’s


flow curve your most demanding application will fall. By doing so, you ensure that your pump will have a sufficient pumping speed at working vacuum (working flow rate) to accomplish your operation in a reasonable time. When investigating vacuum pumps for purchase, ask your vendor to provide the flow curves for your examination so that you can determine the pump’s working flow rate at your application’s required vacuum depth. Alternatively, tell your vendor the vacuum level at which you need to oper- ate your vacuum application, and ask for the pump’s working flow rate at that vacuum level.


While differences in effective pumping speeds provide a partial explanation for the range in prices among vacuum pumps with similar specifications, you should not assume that pump prices always reflect differences in working flow rates. Even premium-priced pumps can differ considerably in working flow rates. Figure 2 shows the actual performance curves of two VACUUBRAND VARIO® vacuum pumps (Essex, CT) and the curve of a com- petitive pump (Pump X) with similar specifications. All three pumps have reported ultimate vacuum specifications of 2 mbar. The PC 3001 VARIOpro


—


the smallest of the three pumps—has a pumping speed specification of 2 m3


/hr. Pump X has a 3 m3 maximum pumping speed of 4.6 m3


/hr specification, and the PC 3004 VARIO reports a /hr.


Examining the performance curves reveals an interesting pattern not cap- tured by the pumps’ specifications. Pump X loses almost two-thirds of its specified pumping speed before it reaches a vacuum level of 100 mbar—the vacuum level achieved by an effective house vacuum system, and a suitable


Figure 2 – Performance curves of two VARIO vacuum pumps and the curve of Pump X with similar specifications.


level for vacuum filtration precisely because it minimizes evaporation. The result is that at the working vacuum level of 23 mbar, as in the hypothetical example above, Pump X has a pumping speed of only about 0.9 m3 PC 3001 VARIOpro


/hr. The , with a specified flow rate that is one-third less than Pump


X, pumps 60% faster at a working vacuum level of 23 mbar. The PC 3004 VARIO pump, with a flow rate specification 50% higher than Pump X, has a pumping speed at the working vacuum level that is three times that of Pump X.


This understanding of comparative work rates can offer interesting purchase options to the vacuum pump buyer. You could choose to pay considerably less for a smaller pump, and get comparable (or even bet- ter) performance, or you could choose a pump with a comparable price, and get pumping performance fully three times as great at working vacuum levels.


Summary The two specifications that are generally available when selecting a vacuum


pump are the ultimate vacuum and flow rate. Unfortunately, these data points are often inadequate for determining whether a vacuum pump is appropriate for a particular application or for comparing the relative per- formance of two vacuum pumps. This is because ultimate vacuum and flow rate are merely endpoints on a curve, and reflect a pump’s capabilities under conditions that are not representative of the pump in actual use.


Scientists selecting a vacuum pump for a lab evaporative application should investigate the working flow rate of the pumps under consideration, either by reviewing flow curves for the pump or by asking the pump vendors for the effective flow rate of their recommended pump at the most demand- ing operating conditions expected for the intended application. Doing so helps ensure that labs obtain the productivity advantages of the pump with better pumping speed at working vacuum, and in some cases save money in the purchasing process.


Peter Coffey is Vice President–Marketing, VACUUBRAND, INC., 11 Bokum Rd., Essex, CT 06426-1506, U.S.A.; tel.: 860-767-5341; e-mail: pcoffey@ vacuubrand.net; www.vacuubrand.com.


AMERICAN LABORATORY • 37 • JUNE/JULY 2013


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