Damper Dilemma...

[Bluenoser337]

I have 2 wood boiler dampers that, based on 4-20mA signal from a pressure controller, operate in unison to control the air moving through the boiler firebox. The dampers are positioned as the 4-20 signal dictates, and their position is fedback from separate 100 ohm "slidewire" resistance devices. Each damper has a separate controller strictly for positioning. I'm convinced there is no device in the world that will replace "only the damper controller" so I have to come up with a plan "B". I have a new controller with slidewire feedback that is using the steam pressure signal. It will control one damper nicely, while monitoring its slidewire feedback...but if I add another controller in the steam pressure loop to control the second damper, they will not operate in unison. I could connect all this up to a PLC and add a couple of displays and switches for position control...but that's expensive. Wondering if anyone has any suggestions on how to make these units operate in unison. Assume there are 2 new separate PID controllers on the steam pressure signal, with a slidewire input from each damper to its respective controller. Thanks!

 

[hacksaw]

try a master/slave setup that will eliminate the cyclic behavior of two masters

 

[Bluenoser337]

hacksaw...don't see any easy way to do that in this case. Can you elaborate? Thanks!

 

[cbarn24050]

Hi, I am assuming you have the control inputs in series. You may have an earth loop problem, or maybe you dont have enough loop voltage. You need to find out why your dampers won't run together before you can find the solution.

 

[Bluenoser337]

The control inputs are in series. I'm thinking there's no way to be sure that the dampers will run in unison because each one would be on a separate PID controller if I replace the old units...even though the same pressure input is being used by both.

 

[jsummerfield]

In cascade mode, the steam pressure controller output is the setpoint for the damper position controllers. Consider the output units from the pressure controller to be the required damper position. Each damper controller has the desired damper position for the setpoint - and the actual damper position feedback. This should work whether using individual controllers or a PLC.

Cascade controller tuning guidelines need to be applied.

John

 

[Bluenoser337]

JSUMMERFIELD...that's the way the old units work...one pressure controller operating 2 damper controllers. The old damper controllers had no setpoints. They would just operate the dampers inversely proportional to the 4-20 signal from the pressure controller. The new damper controllers have a setpoint for reacting to the input signal...but no damper position setpoint. That's what's got me in this "bind". Thanks for the post!