Coalgun- Puff backs & Explosions

Bob wrote:I think the combustion tube is a round vertical tube where there is a column of coal with ash at the bottom, then a layer of burning coal, and then a layer of coal that is not yet burning. I believe the combustion tube is different than the sight tube, which is not vertical but rather is angled and opens to the side of the boiler.

Another rookie question: How can you tell where the fire and ash levels are at in the combustion tube?

lsayre wrote:OK, here is a newbie/rookie question for you: Is what Axeman calls the combustion tube the same thing that AHS refers to as the sight tube? As far as I can tell, my fire level has al\ways been at or below the sight tube, and I've never seen ash through the sight tube.

I believe they are referenced the same between manufacturer's. the sight tube is where the flapper seals against when the blower runs. The ash will always be below the sight tube but the level of ash below the sight tube is what has to be dialed in.

The sight tube looks into the side of the combustion tube.

In the Axeman brochure diagram the bottom of the sight tube appears to be about 1/3 of the way down the combustion tube.

What I think I have found is that if I can see hot glowing coals when the unit is idling--at least several minutes after the last fan operation--then the fire is "high" enough to avoid puff backs.

Bob wrote:What I think I have found is that if I can see hot glowing coals when the unit is idling--at least several minutes after the last fan operation--then the fire is "high" enough to avoid puff backs.

Exactly. The combustion tube is about a foot tall from the lower rim to the observation port. Normally that area is filled about half way with ash. If the ash gets thicker, it's a thin fire and if the ash gets thinner it's a thick fire. Thin and thick ash levels will lower or raise where the actual fire is in the combustion tube because it can only support a given quantity of coal at one time at idle. The real problem stems from the amount of unburned coal over the fire. A thick fire will be deeper in the bed and have a lot more fresh coal on top of a massive fire that cooks the volatiles out of the fresh coal and you get a big boom. Get the fire away from the grate and the booms will go away.

coaledsweat wrote:Exactly. The combustion tube is about a foot tall from the lower rim to the observation port. Normally that area is filled about half way with ash. If the ash gets thicker, it's a thin fire and if the ash gets thinner it's a thick fire. Thin and thick ash levels will lower or raise where the actual fire is in the combustion tube because it can only support a given quantity of coal at one time at idle. The real problem stems from the amount of unburned coal over the fire. A thick fire will be deeper in the bed and have a lot more fresh coal on top of a massive fire that cooks the volatiles out of the fresh coal and you get a big boom. Get the fire away from the grate and the booms will go away.

coaledsweat, how do you get the fire to move further away from the grate. Do you lower the thermal ash grate temperature setting to accomplish this?

Besides curing/preventing the big bang...your efficiency may improve...less unburnt coal in the bucket too.

McGiever wrote:Besides curing/preventing the big bang...your efficiency may improve...less unburnt coal in the bucket too.

How is this benefit achieved? I'm at 125 degrees on my thermal ash grate monitor setpoint, and I'm still seeing lots of unburnt coal in my ashes. I thought this sign of inefficiency might be due to the boiler idling too much due to the warmer weather.

lsayre wrote:coaledsweat, how do you get the fire to move further away from the grate. Do you lower the thermal ash grate temperature setting to accomplish this?

Yes, lowering the ashing set point and the beast will build a thicker ash-bed and push the fire higher. I believe the start up is pretty critical for a happy fire with no problems. When I start mine, I make sure the combustion tube is about half full of ash. Then I wad up paper, pile in some cedar shingles and then some match light briquettes. A few coffee cans of coal and take off the belt that feeds and ashes and then fire it up. Once the fire is going good, I'll add some more coal and let that get going well and then shut it down, replace the belt and let her eat. If your fire doesn't go all the way around the combustion tube evenly, it will ash deeper to get to the set point. It may be that a fast start up lends itself to a problematic fire due to its shape.

I just moved my ash grate control setpoint down to 120 degrees. That is the lowest setting recommended in the AHS manual, and it is suggested to use it only when the weather is generally on the warmer side. I don't want my fire moving so high up that it approaches the hopper. Is there any real fear that this could happen?

lsayre wrote:I just moved my ash grate control setpoint down to 120 degrees. That is the lowest setting recommended in the AHS manual, and it is suggested to use it only when the weather is generally on the warmer side. I don't want my fire moving so high up that it approaches the hopper. Is there any real fear that this could happen?

It's pretty warm right now. What was the previous set point? If you turned it down 10*, it may only move the fire an inch or so. Keep in mind, You are trying to control a fire that can get to a few thousand degrees with a device reading in the low 100s several inches away. It isn't the most accurate method, but it is reliable.

ran all summer on 109, now running 120. never went out. during irene no power 5 days stayed on. this last storm it went out after 7 days. running at 130 i get to much unburned at current temps.

i also run a field controls fan in a can highly recommend it.

lsayre wrote:I just moved my ash grate control setpoint down to 120 degrees. That is the lowest setting recommended in the AHS manual, and it is suggested to use it only when the weather is generally on the warmer side. I don't want my fire moving so high up that it approaches the hopper. Is there any real fear that this could happen?

I don't think that is a risk so long as you have the hopper lid on and the seal is good. I think the risk with going too low is that the fire will go out. {Note added 11/11/11: the post I was responding to was asking about adjustment of the grate control setpoint and the risk of the fire moving into the hopper --presumably with a setting of 120 degrees. As noted in subsequent posts if the ashing mechanism is not working at all the fire can move up into the hopper and recent model AHS boilers have a temperature sensor to shut down the fan if this happens. But it continues to be my belief that the fire moving into the hopper is not a risk with "reasonable" settings of the grate control--and I include settings up to 10 degrees below 120 in the realm of reasonable in some circumstances where both the ambient air temperatures around the boiler are low and the boiler set temperature is low.}

I have been operating at a set point of 120 so far this year and I am getting the most complete combustion I have ever experienced. I have been operating since mid October and have had no puff backs with the setting at 120.

But I don't think 120 is a magic setting--my current theory is that the appropriate setting in a particular location is affected by the temperature setting of the boiler (I am currently operating at 170) where a lower temperature setting goes with a lower grate control set point. I also think the appropriate setting is affected by the ambient air temperature where the boiler is located--with lower ambient air temps suggesting a lower grate set point.

I have to jump in here and say a few things. First, i run steam (aquastat at 190-230) so i am set at 140 for the ash year round and don't touch it. I do get unburnt all summer long. Fire is about 1 layer of coal down, you can always see it glowing under the fresh coal. My sight door, with no port, swings on a 1/4 20 stud, which i have threaded a 1/4 20 nut, then the door, then another nut. It swings as free as it wants and is never closed unless it is running. No CO in the basement. Also, my 86 year old chimney goes straight up through the center of my house and has draft no matter what. Probably drafts on the high side and i think that also helps to never have puffbacks. My opinion is that the coal fire likes to burn hot and by cranking it down to 120 you are making the problems worse. Try burning buck if you are getting overshoots on your temps for hot water baseboard.

lsayre wrote:

McGiever wrote:Besides curing/preventing the big bang...your efficiency may improve...less unburnt coal in the bucket too.

How is this benefit achieved? I'm at 125 degrees on my thermal ash grate monitor setpoint, and I'm still seeing lots of unburnt coal in my ashes. I thought this sign of inefficiency might be due to the boiler idling too much due to the warmer weather.

From what you describe I believe you have a thin fire...and I also agree that if you turn the ashing any lower it will just get thinner (worse).

In my mind, I would switch off the grate switch and run the hot water coil straight down a drain and get comfortable nearby and watch the fire climb and get REALLY hot. When it climbs high enough to be even w/ the sight tube...turn the water off and switch back on the grate switch and set ashing controller to 130*.

And now your ashing controller will behave in a whole new way...give it some time to stabilize before making any adjustments.

This will be the quickest and most painless way to get it set correctly.

My AHS S130 does not have the thermo-ash controller. It has a 10 minute percentage cycle timer that is powered whenever the combustion fan is running. My year round setting is 2-3 minutes. This means 20-30% of the time the time the fan is running the grate is also moving. I do not see any glowing coals when I look into my inspection port door. When the combustion blower is running a quick peak will show burning gases being sucked into the boiler water tubes. I do not know the ash height in the stack of ash - burning coal - unburnt coal.

My puff backs ended when I had my boiler factory modified to the tombstone style inspection port cover and flapper with rotatable opening. My theory is idle draft improved when the air heated by the front of the boiler rises and enters the open flapper. This improved draft reduces the concentration of out gassing coal gases.

The boiler also came from the factory with a switch to turn off the grate motor. I assume it's for turning it off while you remove the ash bucket. If you forget to turn it back on you will get a hopper fire. How quickly depends on how often the combustion blower comes on.