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Proportion-Airs QB3 electronic controller (top) piped to vacuum booster to increase flow. Combination can control pressure range from –14.5 to 250 psig with closed-loop accuracy. |
As manufacturers and machine builders discover new applications for vacuum, designers are finding that it is not always sufficient just to apply and remove vacuum in a process. To maximize productivity and ensure consistent quality in many cases, vacuum needs to be ramped up and down, and the level of vacuum must be accurately controlled. Unregulated vacuum can be just as harmful to a process as over-pressure.
Proportional electronic regulators for positive pressure have been around for some time. These devices control outlet pressure based on an electronic input or command signal. The same concept can be applied to the control of vacuum.
For vacuum applications requiring high accuracy, closed-loop regulators with onboard transducers should be considered. Such regulators typically have an accuracy of 0.2% of the scaled vacuum range. Their design incorporates two solenoid valves, an internal pressure transducer, and an electronic control circuit. The output working vacuum is proportional to an electrical input signal.
The control circuit activates the solenoid valves, which apply vacuum to the outlet port. One valve functions as inlet control, the other as exhaust. The vacuum source is connected to the exhaust port of the regulator. The inlet port of the regulator can be left open to atmosphere or plumbed to the appropriate pressure if the application calls for a range from vacuum through positive pressure.
The internally mounted transducer monitors vacuum (or pressure) in the work port and provides a feedback signal to the electronic control circuit. This signal is compared to the input command signal. If differences exist between the two, the control opens one of the solenoid valves to make the appropriate adjustment. Closing the control loop around the transducer ensures accuracy and repeatability.
Electrical command inputs can be 0- to 10-V DC, 4 to 20 mA, or 4- or 8-bit digital. Units may also provide an electrical monitor signal to send to a panel meter or controller for data acquisition or quality assurance needs. The transducer establishes the monitor signal — typically 0- to 10-V DC or 4 to 20 mA.
The majority of vacuum applications require flow rates higher than the flow capacity of typical electronic pressure regulators. In these cases, the regulator is piped to a volume booster. (Many relieving, air-piloted volume boosters can easily be adapted to handle vacuum.) When the vacuum on the dome of the booster equals the vacuum on the output side, the unit is satisfied — and that level of vacuum is maintained.
Because volume boosters are mechanical devices — with seals, springs, and sliding parts — they are less accurate than electronic regulators. To improve accuracy, some pressure regulator manufacturers build dual closed-loop models. These units can take downstream feedback from a second transducer. When they are combined-with boosters, the assemblies pass higher flow rates with the same accuracy as stand-alone single closed-loop models.
Two other vacuum-control methods may be of interest: controlling vacuum via a vacuum breaker and controlling positive pressure to a venturi vacuum generator.
In the vacuum breaker method, vacuum is taken from the source — usually a vacuum pump — and plumbed directly to the work area. An electronic pressure regulator fitted to a special booster — known as a vacuum breaker — is in the same circuit. The breaker is electrically commanded to maintain a certain level of vacuum. This command is compared to the actual vacuum in the system, which is sensed by an external transducer. The breaker remains closed until the vacuum reaches the commanded level. If the vacuum level goes beyond the command signal, the breaker opens to allow atmospheric pressure into the system to reduce it. An advantage of this arrangement is that the vacuum pump runs more efficiently.
The final method uses a dualloop regulator to control positive pressure to a venturi vacuum generator. This is a very economical way to regulate vacuum in a process because venturi generators are relatively inexpensive, and compressed air is readily available in most plants. The higher the pressure and flow of air through a venturi generator, the higher the vacuum level produced. The relationship between pressure and vacuum is seldom linear, but because the dual-loop regulator responds to a vacuum transducer in the process, vacuum levels are both controllable and repeatable.
If you are using vacuum in your system, you should consider the benefits of precisely controlling that vacuum using a closed-loop vacuum regulator.