In our last blog entry, we reviewed the challenges presented by using uncontrolled vacuum in the lab, and took a look at manual options for vacuum control. In this note, we’ll examine options for electronic vacuum control.
Electronic vacuum control requires both detection and a control technique. The detection is provided by an integral vacuum gauge that tells the how near or far the application is from the desired vacuum conditions. The control techniques used to achieve the target vacuum level are what differ.
Eliminating the need for continuous oversight, two-point control systems detect pressure conditions and then control vacuum levels between two points: a high and low vacuum setting. There are two common techniques for managing the vacuum in the target range: turning the pump on and off, and using an electromagnetic (solenoid) valve to intermittently isolate the application from the pump. While two-point systems are relatively stable, this type of operation still leaves room for a bit of variability in the actual vacuum level.
Turning the pump on and off is a concept very much like the traditional approach to home heating systems; the pump is turned on when more vacuum is needed, and off when the vacuum moves deeper than the target value. With this approach, the vacuum level oscillates around the desired setting, a process called “hysteresis.” Turning the pump on and off adds to wear and tear on the pump, and risks causing condensation in the pump when the pump cools between operating cycles. It is also not quite as responsive as control with a solenoid valve.
Using a solenoid valve to isolate the application from the vacuum pump is the alternative. The pump continues to operate; a well-made pump will operate for prolonged periods without ill effects. The solenoid valve offers very fast response and long service life. The plunger-type actuator can be made of chemical resistant materials for corrosive applications. Any wear and tear over time occurs to a sacrificial component rather than the pump itself. While solenoid-based control is usually a bit more expensive than simple on-off control, the approach protects the pump and ensures fast control response.
“Variable speed drives offer the most accurate and reliable vacuum control.”
Variable speed drives
Variable speed drives offer the most accurate and reliable control over vacuum. By equipping the pump itself with a gauge and motor-speed control, the pump can respond to changes in vacuum pressure with appropriate adjustments in pumping speed. For example, when the target pressure is reached, the motor speed slows to only the level required to maintain the target pressure. Pumping speed is used to achieve exceptionally fine responses to changes in evaporation rates or application leakage that could affect vacuum levels.
These pumps can be operated to either hold a specific vacuum level with no hysteresis, or to follow a boiling point curve of a complex mix of solvents. Further, since it takes much more pumping speed (and energy) to establish vacuum than to maintain it, variable speed pumps can operate well below maximum pumping speed most of the time, resulting in very long service intervals and energy savings of 70 to 90 percent compared with fixed speed pumping systems. Returning to the home heating analogy, many modern furnaces are equipped with variable speed fans to achieve similar control and energy-saving advantages.