On Friday Rob R. suggested that coal on the side opposite the auger packs a bit tighter.
RICHARD2 wrote: ↑Sat. Jan. 21, 2023 1:55 am
Rob R. wrote: ↑
I think the coal packs into the pot a bit tighter on the side opposite the auger. With clean coal the air is able to come up through the coal as intended, but when you start getting fines the air takes the path of least resistance.
That idea fits the observed facts; I will try to confirm it.
To be able to look at the coal distribution pattern in the pot, I shut the boiler off Saturday so the fire would die out undisturbed. On Sunday I was able to see how the coal in the pot was distributed. Using an artist’s small pallet trowel, I excavated the fire pot in layers about ½” thick, trying not to disturb or mix in the underlying coal. As mentioned above, the demarcation between the uneven height and burning was actually at an angle relative to the door from about 10 o’clock to 4 o’clock; but for simplicity I will refer to it as simply the left and right side.
I removed the ash from the ring, and the top ½” layer of mostly burnt coal. On the right side, which had been lower and poorly burning, I found a layer of very small pieces and fines about ½” thick while on the left side the coal was a mix of full size pieces. In the next lower layer, the right side had some rice and barley pieces mixed in with lots of fines, while the left was again the same mix of full size pieces. At the very bottom on the left where the auger enters, the coal looked the same as it is in the bin, but to the right, between the last blade of the auger and the pot bushing, it was about 80% fines. I cleaned out the bottom of the pot.
I wondered if I could see the separation happening, so I hand cranked the auger, and took photos every few turns. In the sequence below, you can see the coal being ‘sorted’ as it rises in the fire pot. My auger enters from the left, and curiously, from the second photo on, the angled line of the separation is visible.
Later Sunday evening, I saw that earlier in the day [when I had my head the in boiler], Ken C and Stoker Don had posted comments making similar observations. More on their posts below.
I obtained from my silo a coffee can sample of the “unscreened” mish-mash of coal purported to be “buck”, and sorted it. First photo shows the three sizes isolated. To my eye the volume of the pile on the left of the largest pieces is about 1.5 times the amount of mid-size pieces in the center, which in turn is about twice the size of the right side pile of small pieces; and the smaller pile of fines about two-thirds the amount of small pieces.
I used US coins for scale in the close up photos. The diameter of a quarter is 15/16”; a nickel is 13/16”; a dime is 11/16”.
The largest pieces sit on a quarter; the mid-size pieces are on a nickel, with one piece sitting on copper tubing with a ½” i.d., and the smallest pieces are pictured on a dime.
Using round holes to sort coal:
Rice coal passes through 7/16” and is retained by 3/16” .
Buck passes through 9/16” and is retained by 5/16”.
Pea passes through 13/16” and is retained by 9/16”.
To be “Buck” size every piece should pass through a hole smaller than a dime. And “Pea” should pass through a nickel size hole.
So in round numbers this load of coal is about 25% buck and rice, 10% fines, and 65% pea. An EFM auger cannot successfully transport pea coal; even with buck many on here report ‘sounds’ coming from the auger tube.
The foundational theory of an automatic coal stoker is that proper, and most efficient burning, is achievable by supplying a measured amount of air to burn a measured amount of coal. We set the amount of air with the damper, and the amount of coal by the number of teeth. But to actually work that way, the coal itself must be uniform, both in size and quality. I know this, but until Sunday, when I saw the coal separating as it fed up into the pot, I had thought that the mixture of sizes would rise symmetrically, and the only negative result would be the loss of heat/efficiency resulting from some of the larger pieces being pushed out half-burnt.
Ken C. wrote: ↑Sun. Jan. 22, 2023 11:50 am
When you emptied the burner pot.. did you empty to the point of fully exposing the auger?
Fines will gather against the lower inside of the burner pot and sort of melt fast and form a crust partially blocking the pathway up to the grates. I have had an EFM-520 for 40yrs. I have cleaned the lower internal fire pot several times using a scraping tool such as a wide blade screwdriver. I clean out the ash and good coal down to the auger. Then use the hand crank and turn the auger backwards pushing the collected fines from the bushings on the auger then scrape off the accumulated coal gunk.
There was an accumulation of fines in the bottom of the pot but they had not yet hardened so I was able to remove them easily.
As you can glean from the quote I use in my profile, I respect your 40 years experience with EFM over 'a lot of theory', but I always thought that the reason the pot auger bushing was made with the groove spiral opposite to the direction of the auger flights, was that while rotating in the same direction, the auger shaft flights pushed coal toward the center and the bushing spiral pushed fines toward the center also.
StokerDon wrote: ↑Sun. Jan. 22, 2023 3:56 pm
Well, that is the problem. Underfed stokers are designed to feed and burn 1 very consistent size of coal. Once it starts pushing 2 or more sizes up the auger, they will separate while being pushed upward into the pot. At that point you have to parts of the pot that require different amounts of air to fully burn the coal.
Yes, you are absolutely correct. I just wish I had known about the sizes separating while being pushed up --- I could see the result in the fire, but did not understanding the
Why. You have answered the question I posted.
StokerDon wrote: ↑Sun. Jan. 22, 2023 3:56 pm
The simplest solution is to just turn the air up to try to burn the smaller sizes.
I wish there were a simple solution.The air is supplied to the whole air pot, and enters the coal bed through the holes in the grates. It is the density of the smaller pieces and fines on the right side that causes the air to seek the path of least resistance through the larger pieces on the left side. Increasing the volume of air will not change that dynamic -- the path of least resistance will just get more air. Increasing the volume of air also risks blowing gases out the auger pipe.
For the coal bed to burn evenly, it must have equal resistance to the flow of air. That is the point you made first-- "designed to feed and burn 1 very consistent size of coal." Evening the density within the coal pot is the only way to get the air to flow evenly and hence to burn evenly.