What Causes Overheating of Grates in Stokers?

coalisorganic · Post by **coalisorganic** » Wed. Jul. 30, 2008 10:34 am

What sort of operating conditions cause overheating of the grates or the stoker mechanism? I have heard that having too much draft and no barometric damper can cause this. I have also heard that letting the underside of the grate fill up with fines or ash can "insulate" the grate and make it overheat.

Is any of this true? Can grates/stokers be damaged by having too much air blowing thru the coal bed? Can you tell there is too much air/grate too hot by looking at the burning coal, or do you have to instrument the grate for temperature measurements? Does anybody know what the max recommended temperature for a cast iron burning grate should be?

Thanks for any info you can provide.

LsFarm · Post by **LsFarm** » Wed. Jul. 30, 2008 11:06 am

Both of your examples are correct.. if you have fines restricting the air flow under the grate, the grate won't be cooled by the combustion air. if it is completely blocked by fines, the coal won't burn completely or at all.

If you have excessive draft, the flames will be very high, and you might even have the fire burn back into the base of the hopper if it can get air from there..The heat output from the stove will be low because the heat is drawn up the chimney,, and the chimney temps will be very high.

All stoves should have a barometric damper installed so that the draft is regulated, steady and the adjustements to the stoker or air settings are consistant.

Greg L

Post by **coaledsweat** » Wed. Jul. 30, 2008 3:08 pm

I would think grate overheating is more of a problem with hand fired units than stokers. Would you agree with that Greg?

coalisorganic · Post by **coalisorganic** » Wed. Jul. 30, 2008 3:14 pm

Thanks Greg. I had not thought of the chimney temp, I will have to put in a sensor for that.
IF I had a way to measure the height of the flame above the grate, and if I could use that measurement to change the amount of air blowing into the grate, do you think it would be a valid way of controlling the heat output of the stove? In other words, is the height of the flame the only information needed to define the heat output of the burner or the temperature of the grates ASSUMING that the grate is not blocked by fines/ash? Is there some condition possible where a fire of low height is creating too much heat in the grates or the stove, again assuming there is no grate blockage?

Flame is electrically conductive apparently so that would be "easy" to measure.

LsFarm · Post by **LsFarm** » Thu. Jul. 31, 2008 5:10 am

I've seen quite a few flat bed stokers with burned notches in the sides of the grate, I'm sure this is from the lower portion of the grate not having any open holes and the fire burning really hot in the middle of the grate, burning a notch in the sides.. Maybe Matthaus can post some photos of some of his burnt up grates out of his old stoves..

Hand fired grates are for sure more likely to get overheated because of the fire just sitting on the grates, and if ash is allowed to build up in the ash pan, this insulates the grate from the cool incoming air, and you soon will have sagging grates from being run too hot..

But it can happen in stokers as well.. there was a thread last spring about a Harman Mag Stoker owner who had way too much draft and burn up a set of grates every season.. really was overfiring the stove to make up for all the heat going up the chimney.. a properly set barometric damper fixed the problem if I remember correctly..

Greg L

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coalisorganic · Post by **coalisorganic** » Thu. Jul. 31, 2008 8:49 am

Thanks!
This is a case of the more I learn the more I find out how little I know.

How often (how many pounds or tons?) should the grate holes be cleaned out? Is the stuff in there weak, or do I need a drill to clean it out?

WNY · Post by **WNY** » Thu. Jul. 31, 2008 12:51 pm

Depending on how much you burn, I normally can make it thru a season (3-5 ton) without having any problems with an over amount of fines below the grates. If you get a warm day and you shut it down during the season, doens't hurt to clean it out. Depends on the coal and how you monitor your system. Like Greg and others, about overfiring, drafting, etc...Good to keep an eye on your draft, it will Definitely burn hotter when it gets really cold outside.

I am going on 4 years with the keystoker and my Hyfire is probably about the same vintage, the plates are both nice and flat.

LsFarm · Post by **LsFarm** » Thu. Jul. 31, 2008 12:59 pm

Don't worry about measuring the flame,, if you have a barometric damper in your flue, set to the correct draft, the flame height will be limited by the draft.. You sound like you will be taking care of your stove/stoker, so don't worry about the grate, it will last a very long time, decades with normal use.. and cleaning.

Greg L

coalisorganic · Post by **coalisorganic** » Thu. Jul. 31, 2008 2:09 pm

So I am I right in thinking that there are TWO separate things to watch out for with grates: Fines build up in the space under the grate, AND fines clog some of the holes IN the grate?

I will be taking care of the stoker/boiler alright, since I am going to be building it

I have seen some stokers burning so I have some idea of what the flame is supposed to look like, and my weatherizing efforts will be finished in the whole house this year so I think I will need only a small fire.. I am planning on reducing the airflow so I get burning over the whole length of the grate, while at the same time not dumping unburned coal.. maybe I can measure ash temperature as it drops off the grate, use that info as a warning or as a control mechanism..
Thanks everyone for all the information.