Help sought for temperature control of coal boiler.

[nixy]

I think you misread TCMTech's post.

He said



Which would mean that just burning wouldn't cause tar formation. It would require short cycling of the fire to produce the tar. So if you had not been short cycling for the fire the past 20 years, then no noticeable tar deposits would have been formed in the past 20 years.

Sort of like how burning alone doesn't cause carbon monoxide to be produced. It requires burning in an oxygen defficient environment.

Also, you didn't seem to respond to the mention of explosive gasses or carbon monoxide in TCM's response.

The system HAS been short cycling for the past 20 years. It it has been running at low settings for extended periods. I think there is confusion over the exhaust gas temperature, which will be far higher than the that recorded on the flue (stack). Nonetheless, the surface mounted temperature WILL be a function of EGT. Perhaps I should have made that clearer?? Didn't think I needed to.

 

[tcmtech]

What I suspect is that when your boiler is running it is managing to get just over the bottom thermal and time limits needed for the exhaust gasses to burn off the coal tars and related materials so up to now you have been lucky and have not had any problems. You drop below that point and you will start getting exactly what I described.

I honestly cannot see how you can come to the conclusions that your concept is a good idea since the very operators manual you posted clearly states that pretty much everything you are doing is wrong, dangerous and just a bad idea in general.

No boiler system should ever controlled by the stack temperature because stack temperatures have no exact correlation to water temperatures and heating times on a properly functioning system.
The stack temp and water temperature are not directly proportional and the burn rate VS temp change relations can be drastically different with very small external changes in working parameters.

What exactly are you thinking this will do to your advantage?
I guarantee it wont save fuel but it will create more problems than you already have at some point.
I have strong suspicions that your heating system is possibly badly designed, improperly sized, or missing its true and correct control systems and critical components as per whats suggested in the manual.

Plus I doubt it is presently up to code or meets insurance requirements and regulations if your even insured at all. If it is up to code and meets insurance requirements this design change will void any of that.

Without more explanation as to how the system is set up and how its being used I can not come up with a valid reason to think that this is any form of safe heating system to be using. Its just a disaster waiting to happen.

 

[nixy]

What I suspect is that when your boiler is running it is managing to get just over the bottom thermal and time limits needed for the exhaust gasses to burn off the coal tars and related materials so up to now you have been lucky and have not had any problems. You drop below that point and you will start getting exactly what I described.

I honestly cannot see how you can come to the conclusions that your concept is a good idea since the very operators manual you posted clearly states that pretty much everything you are doing is wrong, dangerous and just a bad idea in general.

No boiler system should ever controlled by the stack temperature because stack temperatures have no exact correlation to water temperatures and heating times on a properly functioning system.
The stack temp and water temperature are not directly proportional and the burn rate VS temp change relations can be drastically different with very small external changes in working parameters.

What exactly are you thinking this will do to your advantage?
I guarantee it wont save fuel but it will create more problems than you already have at some point.
I have strong suspicions that your heating system is possibly badly designed, improperly sized, or missing its true and correct control systems and critical components as per whats suggested in the manual.

Plus I doubt it is presently up to code or meets insurance requirements and regulations if your even insured at all. If it is up to code and meets insurance requirements this design change will void any of that.

Without more explanation as to how the system is set up and how its being used I can not come up with a valid reason to think that this is any form of safe heating system to be using. Its just a disaster waiting to happen.

What exactly are you thinking this will do to your advantage?

It will prevent this..........

............... boiler is running it is managing to get just over the bottom thermal and time limits needed for the exhaust gasses to burn off the coal tars and related materials

This system WILL prevent the exhaust gas temperature from dropping too low while the system is at idle. At present, the temperature can fall unchecked. And as you've pointed out, coals that aren't hot enough risk gasification without combustion.

Saving fuel is NOT a requirement here.

 

[tcmtech]

This system WILL prevent the exhaust gas temperature from dropping too low while the system is at idle. At present, the temperature can fall unchecked. And as you've pointed out, coals that aren't hot enough risk gasification without combustion.

Saving fuel is NOT a requirement here.

If the fire is going out it is not getting enough air because of the intake is shut down to far or insufficient draft is being formed on the stack side of the system. If there was a change in the quality of the coal it may very well just need far more idle air to stay burning.
Page 2 of the operators manual explains how to set the idle air draft for correct idle burn characteristics.

If the system is not pulling sufficient draft for the idle air to work on its own the system is at times running to low of water temp or there are problems with the outgoing stack system being improperly designed, or restricted some place or the draft stabilizer is letting in too much air and preventing the boiler from breathing properly when idle.
Draft stabilizers are good but too much air flow can cause problems as well. See page 6.

If the system is cooling down too much and losing the draft can be caused by a bad thermostat letting the water temp drop to far. If so it can suggest that the stack could be getting tarred up at the coolest points furthest from the boiler which could be an additional source of the idle draft problem or there is too much draft stabilizer air flow which is preventing the actual boiler from breathing properly when idle. See pages 12 and 13.

Over all this still sounds like a problem with improper maintenance and tune up procedures or problems with the system in other ways.
Bad or lower grade fuel, bad thermostats, bad draft control system, stack restrictions, and or wrongly adjusted controls.