Controlling Stack Temperature With PID

[rwh442]

I have a Thermo Control boiler that has a tendancy to overfire and the route I took to control this issue was a thermocouple in the stack, PID temperature controller and damper actuator on the air intake. My wood has not been the best this first season and I did not want to take the barometric damper route due to what I have read about those possibly feeding chimney fires with air. True/false who knows - I did not want to chance it when I'm off at work. My "system" runs in series with the factory setup (which is near non existent in my opinion).

Do any of the gurus on this site have any experience with this? I would like any comments on the pros/cons of doing this. Do the Eko's, Econoburn's etc do this already?

The thing thas has me baffled the most is creosote buildup. At a constant stack temperature of 450 degrees how does this happen? Reduced stack velocity? I can run at 450 degrees for 7 hours straight but dependent upon wood condition I can have creosote buildup. If I run at 550 degrees I have minimal creosote buildup but my burn times or shorter. The 450 degree setting maintains the boiler water temperature longer for the afternoon.

I have never had problems with this boiler meeting the heat demands of the house. I have had problems with creosote buildup. I should be much better next year with better wood (seasoned) but again any inputs on this would be appreciated.

 

[steam man]

The problem with using flue temp as a a control sensing point is that it doesn't tell the whole story in that the burn cycle is not consistent. Primarily O2 should be measured for an efficient burn along with flue temp. It also seems to me that draft control is a crude way of inferring 02 levels in the stack gas. I realize your wood is not the best but I would consider raising the stack temp to get a cleaner burn. At least while using wet wood. Your stack temp sounds low to me with a non-gasifying boiler anyway. You may be asking too much of your unit. I would give serious thought to raising the temp as you said burns cleaner thus more efficient. Drier wood will would tell a better story I would think. As another poster on the forum has said he had better results with a barometric damper by giving cleaner burns. Sounded logical to me. Barring 02 control I would consider using a damper if Thermo Control recommends and code has no problems. I would suck it up and raise the temp for now while using wet wood. Getting the cleanest burn you can will keep stack fires down. Obviously 450 deg stack temp (aka, reduced draft) is not giving the combustion temps you need for a clean burn.

Mike

 

[Eric Johnson]

High stack temps don't necessarily translate into lower creosote formation. Don't ask me why, but that's been my experience in the past with conventional wood-fired boilers. And dry wood helps--but not a great deal.

I had a Marathon Logwood in the basement of my previous house, hooked up to a 7" insulated stainless steel chimney that ran right up through the middle of the house. It was maybe 25 feet from the stove outlet to the top of the chimney. Straight shot. Because I could easily clean the chimney from the basement, I did it once a week. Why? Because creosote formation was unpredictable; some weeks I'd get a lot and some weeks almost none at all. By doing it every week, I was always confident there wasn't enough to light off and burn. I don't know what my stack temps were, but they said to run that boiler with water temps in excess of 200 degrees to encourage secondary burn. I did run it that hot, but doubt that I ever got much secondary burn.

So good luck. From my experience, if creosote is a problem, frequent cleaning is the only good solution. Let us know if you come up with a better method.

 

[sparke]

Just to clarify. Is the creosote build up in the chimney or the boiler water jacket? If it is inside the boiler you need to take a look at return water temp.

 

[mpilihp]

Hi Rob H I was wondering if you would mind sharing what you are using for a PID controller and actuator? I have a conventional wood boiler with a barametric damper controller (for the boiler damper door) and I have issues with maintaining a consistant stack temp. It works well for cycling the water jacket temp, IE as pump runs and water jacket temp cools it opens and gets the temp up to 180 when the pump turns on again, problem is sometimes the flue stack temp races out of control, so much that the magnetic gauge on the stack pipe falls off. I would like to try a mechanical control that would activate the damper in the stack. But I dont know what to get or use.

Could you tell me what you bought and where?

Thanks

 

[leaddog]

just one word of caution. If you use any sort of auto-draft control on the stack don't make it so it can shut off tight. If you have a fire going and you have a malfuction and it closes you could have a back draft and pump smoke and bad stuff back.
leaddog

 

[rwh442]

Leaddog - There is nothing in my stack except for the thermocouple - the draft control is on the air intake on the door. Would this still cause the back puff if it was shut closed? I do have the temperature controller shutting down the "stock" air intake damper when it reaches an alarm limit - meaning that the stack temperature was taking off way too fast and too high - which I have seen before - and then opens up again after the blast off has subsided.

Sparke - The creosote buildup in the boiler itself is not terrible - I am not an expert but I would think the buildup is normal. My highest delta T is around 24 degrees so I do not think the return temperature is an issue. I agree with what Nofossil said though - I can run weeks with no creosote in the chimney and then after one burn I have creosote. It has to be the quality of the wood. Can't wait until next year.

Mphilihp - I bought all my draft control equipment from Ebay. The thermocouple is a type K sheathed unit - you can find these on Ebay or from companies like Omega or Marlin Manufacturing for around $25 - make sure that you use type K thermocouple wire back to the controller. The temperature controller I am using is a Yokogawa UT320, again Ebay for $40, that has PID control with both relay, 4-20 mA and pulse outputs. Caution - most temperature controllers only have one control output so make sure you know what you are buying. Search Ebay for those temperature controllers - you will find a lot of them. Make sure it has PID temperature control. As far as the damper actuator goes mine ended up being an obsolete Honeywell unit that accepts a 4-20 mA control signal and rotates 95 degrees - $85 for that one. Belimo and Erie make damper actuators also. Patriot Supply had a Honeywell with 4-20 mA for I think $85 also - they make so many variations of those so watch what you buy. A bi-directional damper control using the relay control may work just as good also.

I guess all the input brings me to the big question - when does creosote ignite? Can I run the stack at 800 degrees? 900 degrees? I'm sure this is not cut and dry either!

 

[leaddog]

Leaddog - There is nothing in my stack except for the thermocouple - the draft control is on the air intake on the door. Would this still cause the back puff if it was shut closed? I do have the temperature controller shutting down the "stock" air intake damper when it reaches an alarm limit - meaning that the stack temperature was taking off way too fast and too high - which I have seen before - and then opens up again after the blast off has subsided.

Sorry I thought you were putting it in your stackand slowing down the fire that way. slowing down by controling the intake shouldn't be a problem.
I use a PID controller to modulate my water temp output from my boiler using a honeywell 3-way valve and a honeywell damper-valve motor. Like you say watch what you buy as there are LOTS of configureations. relay, 135ohm, 4-20ma, 0-5v, 0-10v. I found probes from china that run less than $10 including shipping on ebay and most pid's but not all will take k, j, and many other types.
http://cgi.ebay.com/Thermocouple-K-...ryZ50971QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I used one of these for measuring my stack temp connected to a PID controller just using it for a thermometer. If you watch you can pick up used and sometimes new PID's for under $20 and any will work to read temp and you don't care what output they have. If you are using them with the output make sure they are the right type like stated above.
leaddog

 

[mpilihp]

Hi Rob H thanks for the info ill start looking into it.

Leaddog I initially was thinking of using it to control the stack damper but the more I think of it I could run the chain from the bi-metal damper control to the draft air intake door through a pulley and have it moved side to side by this actuator. IE have in normal state run through a pulley and be PULLED away causing the chain to be pulled to the side. As long as the stack temp never goes over say 700 deg it doesnt move, and the bi-metal control can open and close the draft door to maintain boiler jacket water temp at 180 deg. If stack temp jumps up over 700 the actuator would move the pulley over causing the chain to travel more vertical and closing the door preventing overtemp. Sounds possible.

~ Phil