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Topic Title: Temperature Control Process Lag
Topic Summary: Problem with tuning
Created On: 23 November 2009 09:20 AM
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23 November 2009 09:20 AM
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tomtom01

Posts: 1
Joined: 25 July 2008

Trying to loop tune the temperature outlet of a furnace, the temperature element is some 100 metres away from the actual outlet, process lag is approx. 4 minutes, any suggestions on PIDS. I'm cascading the temperature controller onto the furnace fuel pressure control. Any help would be most appreciated. Struggling !!!
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24 November 2009 10:22 AM
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oneye

Posts: 124
Joined: 25 February 2008

Without wishing to state the obvious, is there any way of shortening the 4min. process lag. On the assumption this to be impractical, and that the furnace fuel pressure control response is a great faster than 4 minutes, the only option is slow (or slug) the 3 term controller action to compensate.

How this is achieved depends on what type of controllers you have.
I'm more used to embedded PLC 3 term control and have in the past, written algorithms to compensate for process shortcomings.

In your case I would want to set up variables to clamp the furnace fuel rate of change providing the temperature was close to setpoint.
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26 November 2009 11:52 AM
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oneye

Posts: 124
Joined: 25 February 2008

I was not only indicating proportional control but 3 term PID. If the furnace fuel control was binary (that is either on or off) it would be more difficult to control, but proportional control could still be used where the
output is pulsed with a variable mark/space ratio suitably interfaced to the fuel controller.
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11 December 2009 12:10 AM
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cmatheson

Posts: 207
Joined: 16 January 2003

Are you looking at flue gas temperature or the heat transfer medium? Can you 'bump' the output of the temperature controller (by a small percentage) and get a record of temperature vs time? Is there a significant loss of heat in 100 metres or is it just 'dead time'? and are you interested in the temperature in the furnace or at the point where it is being measured , or somewhere else?

-------------------------
 Chris Matheson MInstMC
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