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A Short Course in Lab Vacuum, Part 2: Vacuum Control
Some applications, such as rotary evaporation, and to a lesser extent centrifugal concentration, are sensitive to bumping, foaming, and other problems that can arise due to uncontrolled evaporation of liquids. Control is required to keep these processes in check. VACUUBRAND® pumps can be equipped with a variety of controls to provide applications with the precise vacuum control they require.
Types of control: Gas Ballast Control: One type of vacuum control used in laboratories is gas ballast control. In this method, a controlled leak is introduced between the application and the vacuum pump. This inexpensive method provides a stable vacuum level for applications, provided the application itself is relatively stable, but it has several serious drawbacks. The first, and potentially dangerous, problem is if flammable solvents are used. This mixing of flammable solvent vapors and atmospheric air can create a potentially explosive condition. This method is also extremely noisy and reduces the efficiency of cold traps and other solvent recovery systems. When used with a rotary vane vacuum pump, it also greatly increases the amount of oil mist generated by the pump. For all of these reasons, VACUUBRAND® does not offer this control method on their pumps, and BrandTech does not recommend its use. Flow Control: Another relatively inexpensive method for approximate vacuum control restricts the flow between the pump and application. This method can be achieved in a less exacting manner by the use of a screw-type hose clamp on a vacuum line. A more precise method, as used on the VACUUBRAND® PC101 NT and PC201 NT is the use of an in-line PTFE diaphragm valve. This method tends to be a bit less stable than the gas ballast method, but is considerably safer and quieter than the gas ballast method. Electronic Two-Point Control: This method uses an electronic gauge and two switch points to control vacuum in a similar method to that of a home thermostat. When the vacuum reaches the desired level, the vacuum pump is disconnected from the application, either by switching the pump off, or by closing a solenoid valve between the pump and application. When the pressure rises slightly above that setpoint, or hysteresis, the pump is restarted, or reconnected to the application. The use of a solenoid valve, while slightly more expensive, is the preferred method in most applications, as it allows the pump to continue to run, reducing internal condensation of vapors. More sophisticated two point control systems allow the programming of 'ramps' by the addition of multiple pressure settings over time. Some pumps use variable motor speed to achieve preset ramped vacuum levels, however this should not be confused with the VARIO™ Automatic Control listed below, which is an interactive process.
VARIO™ Vacuum Control: This is a form of logic-based control for diaphragm pumps that requires the use of a electronic gauge and a variable speed motor . It can operate in three different modes, depending on the application. The simplest mode is Single Point Control. In this method, the pump simply speeds or slows up to maintain a specific preselected vacuum setpoint, rather than operating a valve. Because the pump can change its speed, an accurate and precise vacuum level can be maintained, provided the volume of the application is of sufficient size. The second mode is Automatic Control, used in evaporative applications.. In this mode, the pump is able to determine the volume of the application, by monitoring the change in pressure over time against the pumping speed of the pump. As the pump approaches a boiling point, the apparent volume of the application increases as solvent vapor begins to fill the dead air space. The controller then compensates to slowly approach the boiling point, virtually eliminating overboiling, or “bumping.” Because the pressure over time is continually monitored, the vacuum level compensates for changes in application temperature and solvent composition. This allows for automatic, continuously optimized vacuum, with evaporation up to 30% faster than two-point method. The third mode is Remote Control, and allows the interface between the vacuum pump and a Windows-based computer via a bi-directional RS232 port. This allows the control of all aspects of the pump, including starting, stopping, and operation of an optional solenoid vent valve. It allows the execution of complex pumping programs. Additionally, the use of the interface in the other modes can be used for process validation. Additional advantages of the VARIO™ system of control is reduced noise and electrical consumption, as well as increased service intervals, due to reduced wear on the diaphragms.
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