I have a doubt going on in my mind, blowers in furnace are running at full speed. If we want to control the rpm of blower, then what parameters should be taken into consideration for setting up a PID control loop? flow of air/ temperature readings or some other parameters? What will be appropriate?

 

If you are talking about the combustion blower that supplies combustion air, maybe a CO2 monitor would work. Or just open loop it based on the current fuel demand. i am not real sure there is all that much to be gained from slowing the blower down in such a case though.

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Bob

 

The purpose of the furnace is important. If this is simply a heater, than the blower RPM should be controlled as a function of the controlled temperature. If this is a combustion furnace, than air flow must be proportioned to fuel flow. In either case, the output of the PID should be sent to the speed setpoint of a Variable Frequency Drive (VFD.) In a fan, airflow is directly proportional to fan rotational speed (RPM.) Many VFDs have a 4-20ma input port which may be defined as the speed setpoint, or it may be defined as the input to an internal PID controller process variable.

Dick Caro

 

Thanks for replying..

I'd like to explain the situation with more clarity.

These are cooling air blowers which are running in full rpm. Whether these should be controlled wrt air flow in the duct or temperature reading of some locations. Though installing temp measurement devices might be difficult but still which will be more accurate?

 

Your explanation isn't any clearer...

- Why don't you want to run the fans at full speed?

- What are you trying to control?

- You say these are "cooling fans". What exactly are they cooling? Is the furnace too hot? too cold? cooling too quickly?

- What will happen to the furnace if you slow the fans down?

If you can answer these questions then you may not even need our help. If you want to control something, you have to measure it. Decide what you want to control, and install the appropriate sensor.

 

The circulation blower should be controlled to maintain the temperature rise across the heat exchanger. Too little rise and you get condensation, too much and you waste energy and stress the heat exchanger.

Unless you have a variable burner, that limits you to the speed that
gives you the right heat rise. Which explains why nobody I've heard of does this. The small variation possible within the constraints isn't
worth the effort.

Regards
cww

 

I am going to assume that these are blowers that come on to cool the furnace after the heat source is turned off. I know that the natural cooling of a furnace can take days sometimes. I would use temperature as the feedback to your controller. You should already have some kind of temperature sensor for the heater control. You might be able to piggy-back that sensor, or retransmit it. What controller is being used for the heater? Many temperature controllers have heat and cool capability.