Productivity in a Vacuum

Electronic vacuum control pays for itself in cost savings and lab productivity gains.

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With lab staffing squeezed and no relief in project deadlines, improved lab productivity is your only hope. You’re already working as hard as you can. Where can you find these elusive productivity gains? One often overlooked source is electronic control of vacuum applications. In fact, effectively using modern vacuum technology in a lab can make a big difference in your lab productivity, stretching critical staff resources by freeing hardworking PhDs and technicians to do real science instead of control equipment.

A hot analogy

Suppose I gave you two options for keeping water at 60°C for two hours.

Option 1: Put a thermometer in a flask with the water. Then light a Bunsen burner under the flask, and heat until it reaches 60°C. Turn off the Bunsen burner, but stay close; you’ll need to light the Bunsen burner again as the temperature starts to drift lower. Watch the thermometer for two hours, lighting and extinguishing the Bunsen burner, all the while thinking about the education you are wasting.

Option 2: Put the water on a hot plate, set it for 60°C, and go do something creative for the next two hours.
Now suppose you need to maintain an evaporative application at 20 mbar for two hours.

You have two choices:

Option 1: Attach your vacuum application to an electronically controlled vacuum pump. Set the desired vacuum at 20 mbar, and go do something creative for the next two hours.

Option 2: Attach your uncontrolled vacuum pump or central vacuum system (CVS) port to the vacuum application. Turn on the pump or open the vacuum port of the CVS. Pump until you reach the desired vacuum conditions. Turn off the pump or close the vacuum valve. When evaporation stops, turn the pump back on. Keep this up for two hours, several times a week, all the while thinking about the education and intelligence you are wasting on equipment control instead of discovery.

Why, in the first example, would you think it crazy to control temperature manually, yet you would treat electronic vacuum control as a lab luxury?

Why vacuum control?

Besides being used for fluid movement (aspiration and filtration), vacuum is most frequently used in labs to provide control over evaporation, typically to initiate or accelerate evaporation at temperatures below a solvent’s normal boiling point. To do so most effectively and efficiently, conditions should be kept as close as possible to the vapor pressure at operating temperatures; exceed it, and you will get “bumping” or boilover. As the temperature and vacuum conditions approach the solvent’s vapor pressure, the flow of vapor changes rapidly. In complex mixtures, this happens at several different points in the evaporation. Under such conditions, the relationship between the vapor flow from the application and the pump’s pumping speed leads to sudden changes in vacuum level. Control—whether manual or electronic—is needed to compensate in an attempt to maintain optimum conditions.

Not every lab vacuum application needs electronic vacuum control. In simple fluid movement applications, just enough vacuum control is needed to manage the process. Many labs still use rotary vane pumps—operating well below 1 mbar/Torr—for filtration applications. This is much more vacuum than needed and causes a series of problems. The oil-sealed rotary vane pump has to be “dumbed down,” often using an air bleeder valve—effectively introducing a controlled leak to prevent boiling in the collection vessel. All that air not only makes the vacuum application noisy, but can create a lab hazard by continuously mixing air with what may be flammable process vapors, much like a car’s carburetor. The pump operates very inefficiently in this vacuum range, and all that excess air creates oil mist, which contaminates the lab atmosphere unless a mist filter is added. This is the lowest form of vacuum control: compensating for the use of the wrong pump for the application.

Oil-free (dry) diaphragm pumps, with a typical vacuum range down to about 1 mbar, operate efficiently in the vacuum range needed for most evaporative and fluid movement applications. Even so, control is needed to optimize vacuum conditions. The simplest approach controls the vacuum manually with either an air-bleeder valve or a flow restrictor. Using an air-bleeder valve on a dry pump has the same noise and hazard risks as those associated with similar technology on a rotary vane pump. A flow restrictor works by increasing or decreasing the rate at which the vacuum pump removes vapors from the application. Both approaches give rough vacuum control but require human oversight; as vacuum conditions depart from the optimum, direct intervention is needed to avoid either boil-over or excessively long process times.

Two-point control

For more sensitive or critical evaporations, adjustments need to be more frequent and, therefore, will be more time consuming if done manually. For these applications, electronic control keeps the process within prescribed parameters without continuous staff oversight. The most common electronic control is “two-point” control, which works like hot plate temperature regulation. A vacuum controller is programmed with a target pressure level, the application is pumped down to that level, and then the vacuum supply is interrupted. As the pressure rises because of vapor flow from the application or system leakage, pumping resumes and the pressure is brought back within parameters (the “two points” in two-point control). The gap between the two points is referred to as hysteresis, and the control is represented in Figure 1.

Two-point control can be provided in one of two ways: by cycling the pump on and off, or by using a solenoid valve to intermittently isolate the application from the pump. The former is typically somewhat less expensive up front, since the controller simply operates a switch instead of an electromagnetic valve. The solenoid valve adds some cost but protects the pump from the life-shortening wear of repeated on-off cycles. With a solenoid valve, the pump operates continuously, keeping the pump warm enough to minimize the internal condensation of vapor that can reduce pump performance. In either case, however, the lab has taken a giant step toward efficiency and productivity by using electronics, rather than precious staff time, to operate the system. Two-point vacuum control is the approach commonly built into rotary evaporators.

Notice that with electronic vacuum control at one setting, the system does not respond as conditions change. Changing concentrations of component solvents and increasing solute concentration lead to changes in boiling points for which the controller cannot compensate. More sophisticated two-point controllers can program “ramps” that periodically adjust the pump’s operation to expected conditions in the evaporator. Following timed development runs, during which conditions are controlled and recorded manually, the electronic controller is programmed with parameters that keep the application close to optimum for subsequent runs, provided all starting conditions are replicated. While time consuming initially, programmed operation of later runs is another big productivity booster.

Continuous control

As advantageous as two-point control is over uncontrolled vacuum, it still falls short of optimal productivity in three ways: (1) development runs are needed to determine optimum vacuum conditions before programming the controller; (2) the benefits accrue only from repetitive runs of the same application conditions; and (3) the programmed control is still an approximation of optimum conditions, since conditions change continuously (and often nonlinearly). All these drawbacks can be addressed with pumping systems that use motor-speed control to respond to changing vacuum conditions, although some pumps use motor-speed control merely as an alternative to turning the pump on and off, and still require development runs and programming.

When equipped with self-adjusting feedback control, motor-speed-controlled pumps offer the ultimate in automated vacuum management and productivity. Such systems (1) detect vacuum conditions in the evaporative applications several times a second; (2) instantaneously adjust pumping speed and vacuum in response to changing conditions; and (3) maintain optimum conditions automatically. A two-hour vacuum run can proceed unattended and without development runs or programming, virtually eliminating over-boiling (bumping) and protecting samples (Figure 2). This continuous optimization even corrects for hysteresis in other parts of the system, such as water bath temperature control. And because conditions are continuously optimized, evaporations are typically completed 30 percent faster than those managed with two-point control.

A small investment in productivity

Given all the benefits, the only real objection to vacuum control has to be cost. There’s no question that adding electronic control to a pump also adds to its cost, but the productivity gains more than offset the cost increases. One point of comparison is between an oil pump—often seen as an economical choice—and a corrosion-resistant, oil-free pumping system with two-point control. As Figure 3 illustrates, the operating cost savings of using a controlled oil-free pump more than pay for the convenience and productivity benefits in just a few years. Consider the benefit of time savings. A scientist who is paid $50,000 per year costs about $25 per hour, excluding benefit costs. Saving just two hours a week in avoided pump-oversight time, 50 weeks a year, amounts to annual savings of $2,500 in scientist time. Much more important to lab productivity are the avoided costs of manual pump control, namely the critical thought and creative work that don’t get done while the scientist serves as a human pump controller. Extending this analysis to self-regulating pumps, the lab further avoids all the time lost to trial-and-error process development needed before programming two-pointcontrolled evaporation, and each run is completed up to 30 percent faster

The savings are real and significant and reward the insightful lab manager with productivity gains for years to come. So next time you see a hot plate controlling temperature electronically, ask yourself: Why don’t I use electronic controls on my vacuum pumps for the same benefits? Your lab may end up so productive that you’ll qualify for extra staff!

Categories: Laboratory Technology

Published In

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Survival

Published: January 1, 2009

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