The type of temperature control employed in controlling the dryer
chamber temperature is termed "Cascade Control" and is a form of "Feed
Forward Control". By this means we are able to accurately control the rate
of rise of chamber temperature without fear of excess hot air input from the
burner. This can difficult with stand alone instrumentation but our
software engineering department has used the latest PLC technology to achieve
spectacular temperature profiles. The PLC monitors and controls
both the chamber and burner temperatures.
The control of chamber temperature has always been difficult in large
chamber dryers containing several tonnes of wet clay. The large mass of ware
means the chamber temperature reacts much more slowly than the burner duct
temperature, which can rise and fall rapidly. The principle of cascade control
is illustrated below.
Two thermocouples are used, one in the burner duct and one in the chamber itself. The chamber thermocouple is fed into the first three term control loop (PID 1) along with the chamber set-point (rsp). The output from this control loop (outp) is used as the set-point for the second control loop (PID 2) whose measured value (pv) is the burner temperature. It is the output from this loop that controls the burner gas valve motor. This ensures a safe limit between the two temperatures whilst maintaining the desired rate of rise of the chamber temperature without the risk of high temperature input which could damage the ware.
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