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Vacuum Control – Why & How?

The simplest lab vacuum applications require little or no control; vacuum filtration and aspiration require only the simple application of modest suction.

by Peter Coffey,VACUUBRAND

The simplest lab vacuum applications require little or no control; vacuum filtration and aspiration require only the simple application of modest suction. Evaporative applications – concentration, distillation and roapplication tary evaporation, for example – benefit from electronically controlled vacuum to both to ensure reproducibility and avoid time-wasting manual control.

There are two main electronic vacuum-control options: two-point control, and pumping speed control.

Two-Point Control—Vacuum is controlled by disconnecting the pump from the application or turning off the pump when the desired vacuum level is reached; reapplying vacuum when the pressure rises above the desired level. Vacuum pressures oscillate above the target, as shown in the figure. The magnitude of variation is called the hysteresis. (The most familiar analogous device is a home thermostat, which keeps room temperature within preset tolerances by turning a furnace on and off.) The disadvantage of two-point vacuum control is that the optimal level must be the trough of the “wave,” with everything above the trough sub-optimal. To set the vacuum level lower would induce boil-over or “bumping.”

The preferred way to implement two-point vacuum control is by isolating the application from the pump with an electromagnetic valve operated by a controller, while the pump operates continuously. The valve responds quickly and continuous operation keeps the pump warm, reducing performance-robbing condensation in the pump.

The alternative approach is to repeatedly stop and start the pump to maintain vacuum levels. While this approach avoids the cost of the valve, in the longer term it can be more expensive, due to the aforementioned condensation and resultant damage to components. Some pumps also have difficulty restarting under vacuum.

To use two-point control effectively, knowledge of the solvent vapor pressure is required. Additionally, solvent mixtures require a multi-step program or manual intervention.

Speed Control—An alternative to two-point control of a pump is to use a speed controlled motor to pump only as much as is needed to achieve and maintain the desired vacuum. The pump operates at a variable speed; if it is a pumping down a vessel to reach the target pressure, it will pump faster; once the desired pressure is achieved, pumping speed is reduced. This pump speed reduction extends diaphragm life, conserves energy and reduces noise.

The scientific advantage of pump-speed control is that vacuum levels closer to the target pressure can be achieved without hysteresis. Evaporation is significantly faster than with two-point control or manual control, since optimal conditions are provided 100% of the time.

A further advantage of speed-controlled pumps is that they can be combined with an “adaptive” controller to create an integrated vacuum unit, such as VACUUBRAND’s VARIO® systems, that can determine multiple solvent boiling points in a mixture and automatically optimize vacuum to continuously changing solvent conditions without programming. These pumping units can virtually eliminate “bumping” without manual intervention or test runs to determine program parameters. They can also manage pumping speed to eliminate hysteresis altogether. By maintaining exacting vacuum conditions electronically, the scientist benefits from process reproducibility while saving precious time.