PID controller

chuck172 said:

I wonder, can these pid controllers actually modulate the speed of the combustion fan?

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Any of the available economical controllers I've studied all have relay outputs and/or 8-12VDC SSR control outputs, either of which is modulated by percentage-of-time-active for a fixed-period duty-cycle. IIRC there may be some that mention models with 0-10VDC output in their datasheets, but I've yet to find one for sale. So getting a continuous signal suitable for controlling a variable speed small motor controller is bit of a problem.

Converting from a two-second on-off cycle to a continuous voltage could be done easily enough, but there's another problem, which is that a variable speed motor control can only go about so slow. I had my exhaust fan set up on a manual-set variable speed motor control and switched back and forth with a relay from full speed to as-slow-as-possible using a PID controller. I was trying to limit stack temperature to 350 degF, but once the unit got going good it would go way up past setpoint with the fan running only on slow speed. I could adjust stock inlet damper to work well when the fire was going good, but then it was damped a bit much for starting a fire. I tried on-off fan control with a 80 second cycle but was getting smoke on fan activity transitions, which was a no-go for me.

Since I wanted to be able to shut down the draft inlet port completely when it was time to shut the fire down, I ended up with a field expedient control that controls flue temperature by modulating the inlet damper with a Honeywell zone valve heat motor, and the inlet damper closes when the burn cycle is done. This cost less than $100 and a Saturday afternoon and has been working quite well.

got any pictures? sounds interesting. do think this could be done on the exhaust side with an inline damper?

2.beans said:

got any pictures? sounds interesting. do think this could be done on the exhaust side with an inline damper?

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Not many pictures, but it has been discussed a fair amount:

https://www.hearth.com/econtent/index.php/forums/viewthread/69395/P45/

https://www.hearth.com/econtent/index.php/forums/viewthread/51909/

https://www.hearth.com/econtent/index.php/forums/viewthread/35892/#381738

Most all are on the draft side. Because of temperature, ash fouling, maintaining negative draft on boilers that require it, seems like there would be some disadvantages of trying to restrict exhaust flow. Would there be any advantages of damping the exhaust instead of the draft?

Nonetheless, it should be possible to isolate the actuator from the stack heat and use a Belimo, Honeywell, Erie, or somesuch damper control with 4-10mA input with a PID controller with 4-20mA output to control a stack damper.

--ewd

I'd be mighty uncomfortable throttling the exhaust- if something goes wrong, you've got a recipe for asphyxiation or fire...

exhusto makes a fan damper for just such purposes, controlled by firebox pressure with a integrated pid. I think the controller is a ecb 10 and exhausto is now enervex. For our application of a pulsing firebox pressure you would also need a input signal filter with a relatively long filter time to even out the noisy process value.

pybyr said:

I'd be mighty uncomfortable throttling the exhaust- if something goes wrong, you've got a recipe for asphyxiation or fire...

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my boiler is naturally aspirated. cut the draft down and the fire slows down.

chuck172 said:

So not to hi-jack a previous thread:
I understand that some are using pid controllers (cheap from ebay) wired to thier combustion fans.
My Tarm has an aquastat to shut down the fan at a set temperature, when the stack temps go down and the fire burns out.
I wonder, can these pid controllers actually modulate the speed of the combustion fan? I think that would really be good upgrade.

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In looking at this myself for the addition of a forced-draft fan to my big wood furnace, I tried a blower, pulled from a scrap bin (of course) that works well for lots of draft. However, to modulate it, the only reasonable way turned out to be a swinging damper that covers the intakes to the fan. The fan is a squirrel-cage type with one intake and a cast fan shroud with the 120Vac motor bolted to the outside. I would think the PI controller could look at the draft pressure either at the outlet of the fan, or at the flue breaching. I still have not worked out a way to make the fan damper close 100% sealed to shut off the air, (secondary air at back remains open) but it is in the works. I went this way as modulating the fan was more difficult than getting a cheap 0-10VDC modulated damper actuator. Many are around, and I just happen to have a couple of Belimos that were removed 'noisy'. Have you moved more on this project?