Prill Combustion Air

I've had my Prill running for about 3 weeks. Using 50 lbs per day in this mild weather (30s at nite, 60 daytime). I added a barometric damper yesterday. It seems to be keeping the draft steady at 0.04" or 0.05". It would climb to 0.07" before the baro was installed, even in mild weather. Seems like the damper is mostly closed when the stoker is in operation, and opens slightly after the stoker stops. Once the flue cools some, it closes again.

I'm having trouble deciding how to set the amount of combustion air. The combustion fan is driven by the same motor as the coal auger. You control the air with a disk on the fan intake. As set, I'm getting about 1/4" of soot on the firebox tubes over the 3 weeks I've been burning it. When I bought it, the tubes were clean (I doubt the previous owner cleaned them, they looked untouched).

I know the bit. coal I've been burning has lots of volatiles. The previous owner burnt bit. coal that was about 8,000 btus per pound. Mine is 12,000 btus. More volatiles, maybe more moisture too. Wondering if I need to provide more combustion air. Any tips on how the set the air? And how does that work with my baro? Does one affect the other?

Also, 3 weeks of burning has produced a significant buildup of ash in the elbow at the back of the stove, and the thimble. As you can see from the pic below, I installed tee's when I put my baro in, to aid in cleanout (baro is in the top tee, bottom one is for dropping ash out). If I peak into the stovepipe from my baro, I can see ash and quite a few sparks flying when the combustion fan is running. That makes me think I have to much air. But the soot, coupled with the several minutes it takes to really get the fire burning once it starts stoking makes me think I don't have enough air. No sure what to do.

Another item to muddy the water - while setting the baro, I had the stove stoking for about 15 mintues (so far this season, it's mostly run on the "keep fire" timer, about 3.5 minutes per cycle). Stoking for 15 minutes produced lots of heat, seemed like I was overfiring it. The thin sheet metal the firebox is built of was warping a little. I had to turn it off it was getting so hot. I'd be afraid to run it that long again very often. The flames were over 12" long above the firepot. Granted, that was before I got the baro set. Maybe I should stay with 8,000 BTU coal. Makes me think it would be nice to be able to set my feed rate - maybe I don't want an entire grate-ful of coal burning? I don't have that option though.

I'm full of questions. Any tips would be great. (how to set combustion air, mostly). Thanks.

Steinke

You should be looking for a bright, yellow flame when the unit is running and up to "speed" after 10 or 15 minutes. A dark, lazy, smoky flame means you need more air, and a hard, white flame with a lot of sparks means you need less air.

Is there adjustments for coal feed - multi-step pulleys? Sounds like you have more BTUs than you need if it mostly runs on the idle timer (or the idle timer is running the stoker too much or too long). But, that will also contribute to sooting up the tubes because the fire really never gets hot enough to burn completely. It would be better to run the coal feed at the lowest setting if you can and allow the unit to actually run on the thermostat call more often.

But, figure with 12,000 BTU coal, and a feed rate of maybe 9 or 10 lbs. an hour, that's a LOT of heat output.

Chris

OT: This is off topic but I saw the bow and it reminded me of when I use to have a PSE Laser Magnum. I was pretty deadly with that sucker. We use to shoot at the range and did a lot of hunting.

I had read something in a combustioneer manual about a white flame meaning too much air. No sure I have a white flame yet, but then again, I'm usually only seeing the flame for less than 5 minutes. I definitely have sparks, and tall flames on the heat exchange tubes.

I had not thought about the beginning of the burn being inefficient. If that's all I have right now with the mild weather, maybe some soot is just a fact of life for now, until colder temps get here that require longer runtimes.

I cannot adjust the feed rate. The stoker/auger just turns at a constant rate. No step pulleys. Some could certainly be added. But to lower the feed rate, I'd need a smaller pulley on the motor, or a larger pulley on the gearbox, I think. I'll have to give that some thought. These stoves were made in Sheridan, WY, which has local Powder River Basin coal, more in the 8,000 BTU range. I'm using coal on the other side of the BigHorn mountains. It is way different. Maybe I need to do some tweaking, or just try a few weeks of the PRB coal.

The furnace came set at the highest timer settings. About 1 hour idle time, and 3.5 minutes of stoking. It keeps a fire just fine sitting idle for an hour. I don't think I want to reduce either setting.

If I'm using 50 lbs of 12,000 btu/lb coal per day, and getting just 50% efficiency, that is 300,000 btus per day. That sounds like alot of heat. The house has actually been too warm most days.

Thanks for your thoughts. I'll chew on this for awhile yet.

On the topic of archery - I often think that keeping a bow near the furnace is not the best storage solution, but it's just an old youth Pearson bow that gets used for a little target practice. I've been chasing elk for 5 years now in WY with other bows. Finally nailed a bull this year at 30 yards. It was worth the work. Tastes better than beef.

Steinke

you'll have to do the same thing I just got done fiddling around with. my previous pennsylvania kittaning coal was about 13,500btu/lb but a very light coal by volume, whereas the eastern ohio coal i'm burning now is about the same btu, but much more dense, which is unusual in coals that are otherwise fairly similar. so I had to reduce the size of the pulley on the stoker motor to reduce the feed rate. made all the difference in the world; sure it'l take longer to begin the firing cycle, but it won't overfill the furnace, it won't warp the heat exchanger, it soots much less, and just barely reaches the limit switch after 1/2hour of stoker on time. adjusting just the air will drive you mad, start by getting an adjustable pulley and reducing the feedrate (who cares if the belt gets a little slack in it while reducing the drive pulley size). However, with denser coals especially during mild weather, you will get sooting, and some smoke is normal even during a long firing cycle, ideally you will have a barely visable light grey coming from the stack, otherwise you will be using too much air and fill up your pipe with flyash.

OK - I think I'm going to fiddle with other pulley sizes. Not sure why they wouldn't have built the stove so it's more adjustable. Maybe they just dialed the design in at a happy medium for the PRB coal and left it at that.

In tinkering with building my own stoker design, I found an old bodine gearmotor that has lots of torque, and about 5 rpms. Only problem is that it is a DC motor. Turns out that may be a good thing, since you can then adjust the speed with a frequency controller. Now I just need to find a cheap controller/rectifier, and I'll be on my way to designing my own stoker. Sounds like feed rate control is a good thing to have.

Thanks for the tips Berlin.

Steinke

I've done quite a bit of fiddling with some of the same things. I have a small pulley now to decrease the coal feed as much as possible. The original pulley had three different steps on it, all too much feed. The new one is a bit bigger than a silver dollar. Haven't had problems getting the belt adjusted as there's lots of room to slide that motor out. As for the plate that controls the air flow, am told it should never be opened more than 1/4 inch. I've stopped mine down to about 1/8. Idle time is set at 2 minutes every hour. Burning about 9600 BTU coal. I have never figured out how to stop the build-up of ash in the elbow. I did add another section of pipe this year to see what kind of difference that would make. I clean my flues out about once a month. I'll know next week if there are improvements over last year.

I closed the disk on the combustion air intake to about 1/8". Seems to reduce the fly ash alot. Firebox seems to have more soot with the lower combustion air rate. Seems like a catch-22 - if I slow the combustion air down to reduce ash buildup, I don't have enough air to burn the coal. Still need to find a smaller motor pulley to reduce the feed rate to see if that helps. I wonder if lower velocity blower would help.

Kudos to Charlie for sending me what little documenation Prill has for this stoker. I scanned it into a PDF file (attached) for those interested.

Steinke

If it were me, I'd take the fly ash over soot any day of the week. Fly ash can be vacuumed up vs. soot which just makes a bigger black mess the more you screw with it.

You only have two variables to play with - air and fuel. More fuel per unit of time (minute, hour, etc.) = more BTUs into the home. You need X amount of air to burn Y amount of fuel in the proper "mixture". So, as you add more "throttle" and feed more fuel, you need to open the "carb" up and add more air to keep the same mixture.

Set the fuel feed in lbs. per hour to get the required heat output to heat your house with the stove running a good % of the time, and then adjust air for a clean burn. Colder weather means you maybe need to feed more fuel and adjust air accordingly.

Chris

Hi Steinke,,, does your 'new' Prill have an over the fire hot air tube?? Something like a pipe that is aimed down at the firebed?? If so, try increasing the air through this over-the-fire vent, it will add oxygen to the volitiles being burnt off and hopefully reduce the soot build up..

If your stove/furnace doesn't have any over-the-fire air vent,, then I'll go with Chris' comments, I'd much rather have extra fly ash than extra soot.. Soot is a mess to ckean up,, fly ash not too bad..

Take care,, Greg L

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Greg;

If you have secondary air it has to be hot to burn the volatiles off thats where most coal stoves fall

short and cannot get a clean burn with bit coal .It I usually hard to get this preheated air to the fire so the

burning process can be completed.If you bring this preheated air up hot enough you have a coal

gasification process going and a complete burn with no volatiles going up the flue and very good heat

output.The air needs to be hot because coal burns much more controlled than wood which can use

ambient air and it will combust, the high heat of the wood fire heats the air as it mixes with the gases

in the firebox and stays above the combustion temp of the gases so it burns freely,thats why the flue of

wood burning stove runs so hot,coal runs at about 1/2the temperature,so preheated air is needed.

BigBarney

HI BigBarney, yep, i'm aware that the over-the-fire air for burning off Bituminous volitiles has to be hot air.. I don't think I said otherwise. Many made-for-bituminous stokers have a preheated air vent that dumps air over the fire. I was asking Steinke if his Prill has this air vent for burning off the volitiles.

In my Big Bertha boiler I made a preheated hot air vent for burning off the volitiles from a fresh load of coal . It was very successful at burning off the volitiles..but the vent didn't last very long due to the very hot/corrosive environment in the firebox.. When conditions were right, the volitiles were burnt off with a blow-torch effect, very impressive.

Some of the retrofit bituminous stokers like my Iron Fireman use the underfeed stoker to bring the coal into a hot environment so the volitiles can boil out of the coal and rise through the hot coal bed to be burnt off with the coal.

Greg L

The Prill has no hot air tubes for good secondary combustion. It has just a few small holes in the access door, but they do not do anything for secondary burn. Not sure how the Big Bertha or Axeman/EFM pots work, but mine has an area in the middle where coal is rising that is black - the coal does not begin to burn until it moves radially outwards over the area where air is flowing. So it seems to me that the coal is not really getting preheated much on this setup.

Also, I can see your point on the fly-ash vs. soot. However, based on the flames I was seeing in my stovepipe (see pic), I really think I had too much air. The baro was doing it's job, but the air was pushing the flame up the smokepipe. Still going to find a smaller pulley just to test.

Steinkebunch:

It seems as though you have too much air flow if the flame is in the flue 4 feet above the stove,does the stove have baffles inside

to redirect the flame front to get more heat extracted ? Try less air in the primary burner.

The flame looks good, blue flames with orange tips,good secondary burn but in the wrong place outside of the heating body of

the stove.Can you get the extra air in above the pot where it would be a lot more effective? Some older designs in stoves used

a tube above the fire blowing down onto the glowing bed of coals and creating a excellent secondary burn.

BigBarney

There is a baffle on the rear interior of the firebox that prevents the flame/heat from exiting directly into the flue (you can just barely see it near the center of the pic below). So I'm not sure if the flame is actually exiting the stove, or if it's just igniting in the smokepipe. I agree that it's a nice flame, but needs to be inside the firebox, and not in the chimney. I got noticeably mor heat from the stove when the flame was in the smokepipe.

I reduced the primary air, and it solved the chimney flame problem, but now I'm not getting the secondary burn, but I'm getting lots more soot. I think I'm wasting alot of unburnt volatiles out the chimney

I'm willing (actually excited) to build some preheated secondary tubes, but not sure how to go about it. I'm guessing I'd need to tap a tube into the combustion blower discharge duct, run the tube thru the flames above the pot, and daylight the tube downward toward the center of the burn ring. Or maybe spread the preheated air out more, right beneath my heat exchange tubes (see pic below). Stainless steel likely best, but maybe try steel pipe or tubing for now. If not buried in coals, it may last OK.

Anybody got any photos of this kind of secondary setup on an underfed stoker?

Photo below was taken after burning only a few hundred pounds of coal. There's quite a bit more soot a month later.

Steinke