ProSonix heaters typically use a pneumatic (air) modulating actuator to control the steam flow to maintain fluid temperature. Our ProJet heaters use an external steam control valve to modulate steam pressure, and the others (I, J or C series) have an actuator mounted directly on the heater to control steam mass flow.
Most modern control systems use an electronic temperature controller to maintain the desired temperature in the process. That means there must be some way to convert the electronic signal from the controller, to a pneumatic signal to the actuator.
The simplest method is to use an I/P (current to pressure) transducer. This device converts the electronic control signal (typically 4-20mA) into a proportional air pressure. The pressure then controls the actuator position. For example, if the actuator uses 10-30 psig to stroke 0-100%, the transducer will convert the air pressure as shown in the table below.
|Air Signal to Actuator
In many cases, the I/P transducer works very well. However, the transducer lacks any feedback to insure that the actuator goes to the desired position. For example, if there is excessive friction from the stem packing, the controller may send out a 50% position air signal, but the actuator may only go to 40% travel. If absolute position is important, it is helpful to add a device to the actuator to ensure the actuator agrees with the controller output signal. These devices are known as Positioners.
Electro-Pneumatic (EP) Positioner
An electronic positioner combines an I/P transducer with position feedback to ensure that the actuator goes to the position determined by the controller. The positioner is mounted directly on the actuator and reads the actual stem position. Is the stem position disagrees with the control signal, the positioner will adjust the air pressure to fix the error.
The advantages of a positioner include:
- Faster actuator operation
- Additional closing force at 0% travel
- Full stroke capability within air pressure limits