How Much Coal/Hour?

For the engineering types out there, please check my numbers.

Just doing a calculation on the power it takes to heat 265 cfm (the advertised flow rate of a Keystoker 90K) from 60F to 160F is roughly 20 KBTU/hr. Air's heat capacity (assuming 0.17 BTU/lb/F) and density (0.074 lb/ft^3) vary with temp, but not by much for the purpose of calculating the heat output actually reaching the air.

Put another way, if a Keystoker really put 90 KBTU/hr into an air steam of 265 cubic feet per minute, the stove would heat that air from 60F to 480F, roughly.

If my numbers are right, or close, the 90K should be renamed the 20K.

Thoughts?

I can’t answer your question about BTU vs CFMs, but you guys found an interesting problem. No one wants to waste coal with pushing it off while it red hot. I can tell you with my hand fired stove there probably isn't much life in my coals at the bottom of my fire. Even if I shake red hot coals out, they aren’t very black. Maybe they assume at 100% feed rate (90,000 btu) that almost all of the energy is burnt out. Maybe 90% burnt up. Someone that engineered a stoker would really have to chime in here.

Two burner pots would be great. Running them both so they wouldn’t push hot coals over the sides would still give you more than 90k btu. I think the dual Alaska I seen was 180,000 btu.

Personally I would hate to see it pushing hot coals over the sides. Most everyone is fine and dandy until we hit around 0 degrees... if you have to push hot coals over the sides for do maybe 2-3 weeks a year maybe it isn’t that bad. In our area, historically the nightly low average temperature is already on the rise. That isn’t holding true this season, but we are almost half though February.

A couple of years ago I added an hourmeter to track my Van Wert Anthratherm coal stoker burner motor, measured how much coal (buck) in pounds that fit into my feed barrel (225# from the top lip to auger), and, averaging over five full barrel samples, found it uses 11.4 pounds per hour.

Haven't looked at it for a while, but last April started to track usage again.

Here are the results in tabular form, and a bar chart for those who may be interested. Note: Bar chart data are extrapolated to 'fill in' partial monthly results for April 2006, and February 2007 - this is why the run time percentages don't add up between the table and chart.

I have a Yellow Flame stoker rated at 150K BTU.

I play with the feed adjustment depending on the outside temp. When it's 0-10°, I've got the fuel feed maxed out and usually see a full tray of hot coals. Quite a sight.

At that rate, I consume about 150#/day. So 6-7#/hr, at the max burn rate I've run so far.

These posts have me a little worried, I haven't yet decided on a coal stove but it is between a Keystoker Koker 160,000 BTU unit and a Keystoker 250k btu furnace. I would rather save myself a few thousand bucks and get the smaller unit which can be direct vented but if it won't put out at least 150k BTU's it isn't going to work. The propane furnace I have now is rated at 144k btu and 90% efficiency and while it will maintain the minimum temp I want when it is -10 I would like to have some reserve.

e.alleg
You won't need the maxmium BTUs until it really get cold. You can always burn less in a bigger furance / stoker, but you can't get more out of your smaller 160k coal furnace. If you don't want to waste coal, it sounds like you need a bigger stoker. I know I wouldn't like running at full feed rate, just to watch hot coals being pushed off. You might never need full feed of that big 250k furnace.

I have a Yellow Flame stoker rated at 150K BTU.

I play with the feed adjustment depending on the outside temp. When it's 0-10°, I've got the fuel feed maxed out and usually see a full tray of hot coals. Quite a sight.

At that rate, I consume about 150#/day. So 6-7#/hr, at the max burn rate I've run so far.

With a 150k btu stoker maxed out shouldn't you be using around 10 pounds an hour? With 7 pounds an hour you are only producing around 100k btu input. I am new to this coal burning so I may be missing something. If so maybe an old pro can jump in and set me straight.

gambler wrote:

I have a Yellow Flame stoker rated at 150K BTU.

I play with the feed adjustment depending on the outside temp. When it's 0-10°, I've got the fuel feed maxed out and usually see a full tray of hot coals. Quite a sight.

At that rate, I consume about 150#/day. So 6-7#/hr, at the max burn rate I've run so far.

With a 150k btu stoker maxed out shouldn't you be using around 10 pounds an hour? With 7 pounds an hour you are only producing around 100k btu input. I am new to this coal burning so I may be missing something. If so maybe an old pro can jump in and set me straight.

I'm new at this too. I have no way of knowing whether I've reached the rated output of the rig. Only that I've had the fuel feed at max reach and seen a nearly full bed of hot coal burning. And that I've consumed 150-165# on the coldest days.

What I don't know is whether the furnace has been stoking the entire 24-hr period. Or the instantaneous consumption rate when it is.

We also shouldn't forget that a range of efficiencies has been suggested for these machines. Some have been posted in the "annual economy" thread.

With the cold snap we've had, I think many folks are seeing their stoves run at max output. That raises a question among many of us: "What's max output and how is it defined?"

From my engineering days, heat output is calculated by the amount of energy consumed by a machine multiplied by its efficiency. Thus, a machine that burns 100 KBTU/hr of fuel with an efficiency of 90% is rated at 90 KBTU/hr. That's 90 KBTU/hr that does some good by heating the house.

It seems, though, that the stove builders have come up with some new math. There's just no way that a Keystoker 90K puts anything near 90 KBTU/hr into the air it blows. That much power going into a 265 cfm airstream would raise the temp of that air by over 400 degrees. And, my stove can't consume nearly enough coal per hour to make that kind of heat.

With my stove tuned to burn at max tilt, the exit air is at 170F. Doing the math, that's about 20 KBTU/hr. Also, it can only burn about 70 #s per day....again that's about 20 KBTU/hr.

The problem is that when one buys a new furnace or stove, one must size the stove according to the heat load it must maintain. If the stove makes less heat than a house loses, the house gets cold.

Bottom line - buy a high powered stove because the factory ratings are wildly overstated.

Hello brent, your basic math and physics is right, but you are neglecting a major factor.

These stoves give off huge amounts of radiant heat. I would say that at least half the heat output from a Keystoker is from radiant heat off the front, top and sides of the stove body. I'm not sure how you would calculate this heat output, probably have to create a test cell to run the stove in with instrumentation.

As for the Keystoker or any other model from MOST, but not all stove makers, they are advertising the input BTU's from a fully burning stoker, with hot coals falling off the end of the stoker bed.

You have to multiply the BTU input by the effeciency of the stove. If you are only going to count the heat from the 265cfm blower, and not count the radiant heat from the stove body, then I'm gonna guess the effeciency is going to be less than 40-50%.

We have had numerous threads about lack of heat output from stoves placed in basements, where all the radiant is wasted trying to warm up the walls of a masonry basement, and the warm air distribution fans are pulling 55* air off the concrete floor and trying to warm it effectively and send that up either the stairway or warm the floor above the stove. This just doesn't work.

These coal burning stoves are NOT furnaces. They are heating appliances meant to be put in the living space where all the heat, radiant and convective is given off to the living space.

If they were furnaces, they would have cold air return boxes, filters, hot air outlet pipes, and most important INSULATED sides and top, so the radiant heat is not lost to the basement, but is added to the hot air output.


So the average stove is rated with BTU input, and where and how the owner places the stove will for a great extent determine the heat output from the appliance.

Greg L

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LsFarm wrote:As the the Keystoker or any other model from MOST, but not all stove makers, they are advertising the input BTU's from a fully burning stoker, with hot coals falling off the end of the stoker bed. ...Snip...These coal burning stoves are NOT furnaces.

As Greg points out stoves are room appliances not central heating appliances. The marketing department gets to define the data sheet specs. Kind of like the peak HP rating of electric motors vs the delivered work HP. Not a lie but not the most useful info. In the case of coal boilers the BTU rating IS delivered BTU. For example the Axeman-Andersen and the semi clone AHS S-130 do delivery 130,000 BTU. The A-A model specs were verified in a 1953 Bureau of Mines two year evaluation of the product. I would expect coal furnaces (NOT stoves) to be similarly rated on BTU output. Do not select a STOVE based on the BTU rating of your existing non-coal fueled furnace or boiler nameplate data and expect it to heat your house. And don't expect home brew fans, ductwork and return air paths to turn a stove into a furnace. A furnace has a built-in heat exchanger optimized for heat transfer to the circulating air. A stove radiates heat like asphalt on a hot sunny day. Two different heat transfer principles.

Yanche

It is true that we have both apples and oranges here, in that coal can be burned in both stoves and furnaces.

But mine's a furnace:

Yep, stockingfull's furnace is an example of what I was trying to describe: a return cold air box with air filter, hot air outlets to main hot air trunk lines and most importantly insulated sides, top and back to keep the heat in the furnace to tranfer to the hot air, not to radiate out to the surrounding basement walls and floor.

The effeciency of the Yellow Flame furnace speaks for itself.

Nice furnace stockingfull, any chance of some photos of the stoker unit??

Greg L

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Here's the stoker:

Lower level, from right to left: motor, centrif. blower, reduction gear, cam.

Upper level, left to right: drive arm, drive collar and clutch, feed tray arm.

(My fuel feed is by gravity onto a reciprocating burn grate, past a check plate. The whole grate moves back and forth. I can adjust the length of that throw.)

I sent an email to Keystoker about not being able to achieve max feed rate to get the theoretical 90K btu input to the stove. See reply below

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It could be a couple of problems. Is the gear motor working properly? Is the cam on the gear motor wore or is the hole in the cam more towards the center of the cam. It should be at the very end of the nylon cam. Are the nylons in the pusher bar properly adjusted? There should be no side to side play. But the pusher shouldn’t be tight in the chute. How big is your fire? To get max feed the fire should be all the way to the end of the grate. However if you max out the feed it might cause hot coals to be pushed off the grate. Your combustion air might have to be increased if your chimney can handle it. The draft should be a -2 using a draft gauge.



We use 13500 btu/lb this is what our local coal companies said

to use.
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I will check take apart and check after heating season is over. At 4.5 lbs per hour I have hot coals going to end of grate. 6.66lbs per hour is only going to put more hot coals over the grate. This does not agree with what they have printed in thier manual where they say you should have about 2" of ash at end of grate when burning for extended period of time.