Figure 1. For heat/cool process using control-valves, the controller may have two 4 to 20 mA outputs that command the valves to open progressively from closed to any position.

In part 1 of this brief series (April 2008), I looked at the most common application for split-range controllers -- the heating and cooling of the barrel zones of plastics extruders. This month, I will continue the discussion and look at managing heat/cool processes using control valves.

You see the control arrangement in figure 1 in the process industries (e.g., bulk chemical, oil-refining, food processing). It differs from the control system in shown last month, where valves and contactors snap open and snap closed.

In this setup, the controller may have two 4 to 20 mA outputs that command the valves to open progressively from closed to any position.
  • Output 1 would increase and open one valve to deliver heat to the process in response to temperature falling through the setpoint.
  • Output 2 would increase and open the coolant valve in response to a temperature increasing through the setpoint.

More Considerations

Given the maximum heating demand of the process, you can work out the flow needed (gas, oil, steam etc.), then choose the nearest valve size and flow characteristic.

Check the delivery pressure and the temperature upstream of the valve and make sure that they are reasonably stable.

Given the maximum cooling capacity demanded, you can choose a valve to suit, and again make sure that the upstream temperature and pressure are reasonably stable.

For accurate tuning and consistent control performance, you need both valves to be linear in respect of heat and cool delivery and to obey the milliamp (mA) signals from the controller. You can compensate for excessive non-linearity in heat or cool delivery by bending the mA signal into a compensating curve. If custom curves are not built into the temperature controller, you can insert a signal conditioner between the controller output and the heat or cool device.

To achieve a smooth transition between heating and cooling, you should eliminate overlap and dead zone around the point where the heat and cool outputs should both be zero. You can either:
  • Mechanically trim the zero position of one or both valves.
  • Trim the bottom end of the mA signals.
If you allow overlap, you will invite control loop instability because of doubled gain (which equals halved proportional band) in the overlapped zone.

If you allow a dead zone between heat and cool, you will invite aimless wandering of temperature inside that zone.

Choice of Control Valves

There is no end to the variety of industrial control valves on offer and you can spend big on them, so keep an eye on cost/benefit. Here is a small sample of actuations available:
  • Pneumatic: diaphragm with force balance between milliamp and diaphragm air pressure.
  • Pneumatic: diaphragm or cylinder with position feedback.
  • Motorized valve with position feedback: Note that delivery of heat or cool does not necessarily track valve position.
  • Motorized or pneumatic, with flow feedback (of fuel or heating or cooling fluid).
  • Digital interface combined with any of the above feedback signals.

Some valves are rich in features and lay claim to having intelligence. Look at the specifications and consider the cost and benefits. Read the many articles on how to select a control valve.

Finally, prearrange the controller’s response in these events -- open circuit sensor; short circuit sensor; temperature too high or too low; air failure or power failure -- both with the controller working and with the controller malfunctioning. Consider a duplicate controller or a safety limit instrument.