Temperature Control Loop

DCS
1

Temperature Control

Components

  • Temperature Indicator
  • Control Valve
  • Temperature Controller

Strategy

The temperature of the fluid downstream of the valve is regulated using feedback control.

  • Measured variable - Temperature of stream
  • Manipulated variable - Flowrate of steam
  • Disturbances
    • Flowrate of water
    • Temperature of water
    • Temperature of steam

Operation

If the temperature of the stream increases then the valve is closed. If it decreases the valve is opened.

This control system will work regardless of the cause of a change in temperature, which can be brought about by a change water or steam temperature or of water flowrate.

Unlike the previous example it is of course necessary to measure the temperature downstream as this is what is to be regulated!

With any proposed control system simply ask yourself: “Does moving the valve change the measured variable?” If the answer is “No” then the control system cannot work.

2

Temperature Control Systems

In this section the control of temperature is to be discussed. Again only simple feedback loops are considered.

To change the temperature of something it is necessary to add or take away energy. This can be achieved in one of two ways.

  • Transfer energy indirectly, using a second stream, through coils, tubes, jackets etc. The second stream could be, for example, steam, cooling water, another process stream or even a source of power as in an electric element.
  • Mix in a second stream directly. This stream will have a different energy content from the original.

There are advantages and disadvantages for both methods. With the first there is the problem of transferring heat through the walls of the ‘coil’. In the second the energy is absorbed directly but with the additional problem of increased flowrate/volume.

Diagrams of these alternative schemes can be found below.