Coalgun- Puff backs & Explosions

As I understand it the puff-backs only occur immediately after the fan shuts off, which is when the boiler reaches its operating temperature and is also right at the time of the highest temperature glowing coal the boiler is going to see, regardless of if it has been idling for a long period or not.

lsayre wrote:As I understand it the puff-backs only occur immediately after the fan shuts off, which is when the boiler reaches its operating temperature and is also right at the time of the highest temperature glowing coal the boiler is going to see, regardless of if it has been idling for a long period or not.

Please don't be fooled and dismiss so quickly what I wrote.

You missed my point, I am saying the longer idle has set the stage for the puff-back.

The aquastat can be satisfied and the burn-pot can still be sluggish (think slow response of coal) in the combustion process.

Volatiles must be consumed at a timely rate or...boom!

franco b wrote:

McGiever wrote:Volatiles must be consumed at a timely rate or...boom!

Exactly. Those volatiles might very well be CO as well. When troubleshooting any mechanism it is often helpful to ask, how could I deliberately make this happen. There is a flaw in the basic theory of operation of this machine that is unable to cope with conditions existing at shut down. Supply proper air and heat at that point and there will be no boom.

Did I mention somewhere within this long thread that Jeff Gingerich of AHS (former owner of the company) told me that puff-backs are always due to poor anthracite coal (I'm assuming here that he means lots of volatiles and perhaps lots of dust, or poor grade as to size) and insufficient draft? I contend that if the sight tube door does not spring open properly and immediately and sufficiently when the fan shuts off there is a condition of poor draft across the top of the super hot coals at the immediate juncture where the unburned coal is riding atop them, regardless of how well the chimney is drafting.

When I asked Jeff about opening the small rotating window port on the sight tube cover he told me to close it all the way if my coal was good and my draft was also good. He said they only added it for bad draft and bad coal.

Perhaps also as the mines have over the years "highgraded" their mining, thereby mining the very best and easiest anthracite coal to get to first and therefore long ago, the remaining coal itself is slowly getting to the point where even the best of it remaining is quite often no longer as good as it was when this boiler design originated?

franco b wrote:I think you understand it perfectly. The coal bit is a cop out. When you shut down an oil burner the air and the fuel are shut down. You can't just shut down one half and not the other without trouble. If the shut down were staged it probably would be OK. Freddy did just that with a timed release of over fire air.

Why is the quality of the coal itself to be considered a "cop out" when considering this serious matter? What if Jeff Gingerich actually knows what he is talking about (as I am confident that he does).

Alternate thought, new perspective: I wonder how much draft is required to keep the sight tube cover drawn up against the sight tube, even after the fan shuts off (even if only for a brief time)? I.E., could too much draft for a brief time (a high wind gust perhaps) keep the door closed for just long enough to evolve unburned volatiles, and then when the wind gust stops and the door finally swings open this allows air to rush in and the volatiles to go poof?

Or could perhaps the paint on the sight tube cover slightly melt and get tacky and fuse to the likewise slightly melted and tacky paint on the sight tube rim, thereby not allowing for the cover to immediately spring open when the fan shuts off? I actually noticed a tendency to such stickiness when I was adjusting the spring tensioner on the sight tube cover.

I think there is agreement that puff backs occur at the end of forced draft firing when the flapper door pops open and air floods in over the fire mixing with explosive gases and find an ignition source.

I would hope that there would be agreement with the observation that puff backs don't happen most of the time--even in boilers prone to them.

We also know that AHS has a variable opening in the flapper door and tells the customer to use the opening if they have problems with puff-backs. I know from personal experience that even with that opening full upen puff backs can still ocur. So the question is why that can happen.

My theory is that during some boiler operations--either because it is a relatively short cycle, because the fire level is low (thin layer of ash under the burning coal, or a combination of both--there is effectively no ignition source (it is buried too deep under fresh coal) while the forced draft is operating to ignite the explosive gases above the coal bed but when the door pops opens and there is no longer a forced draft the burning coal ignites the gases and a puff pack occurs. My theory goes on to say that if the fire level is sufficiently high (think enough ash layer under the burning coal) then with the opening in the flapper door admitting oxygen there is an ignition source and the gases ignite during forced draft operation and there is not puff back at the end of forced draft operation. I think the level of the fire has two effects--first to bring the burning coal high enough to be an effective ignition source during forced draft firing and second by generating relatively less explosive gases when the fire level is high because the bed of coal that is being heated to give off gases is smaller.

Please note that what I have posted is a mix of what I suggest are "facts" combined with what I offer as theory. I welcome comments or alternative views on both.

franco b wrote:I think you understand it perfectly. The coal bit is a cop out. When you shut down an oil burner the air and the fuel are shut down. You can't just shut down one half and not the other without trouble. If the shut down were staged it probably would be OK. Freddy did just that with a timed release of over fire air.

Freddy had a fire that was too fat. Stop ashing and get the fire up where it belongs and the puffbacks will go away. Then adjust the ashing to keep it there. Half of the combustion tube should be filled with ash. If it isn't, then get used to puffbacks.

coaledsweat wrote:Half of the combustion tube should be filled with ash. If it isn't, then get used to puffbacks.

I have had puff backs with the over half of the tude filled with ash so that isn't the only solution. As stated repeatedly, the fan must be running when the grate is ashing. Wiring the grate motor so that it only gets voltage when the fan is running should help accomplish this goal. I'm going to try to do that this weekend and will provide updates.

Jeff has been using the poor coal quality and damper excuses for years and that will not prevent the booms.

I wonder if the slowly idled the blower to a stop if it would help.

rychw wrote:
I have had puff backs with the over half of the tude filled with ash so that isn't the only solution. As stated repeatedly, the fan must be running when the grate is ashing. Wiring the grate motor so that it only gets voltage when the fan is running should help accomplish this goal. I'm going to try to do that this weekend and will provide updates.

Jeff has been using the poor coal quality and damper excuses for years and that will not prevent the booms.

How do you know how much ash you have? The only observation point I am aware of is the port with the flapper valve and that looks into the upper part of the tube. While I can see differences in the amount of "fire" I see with different settings of SV I can't see how much ash there is.

Take digital cam snap pic with flash from underside while its not blowing. Turn off blower and ash and wear Gloves.

There is a relationship between sv temp and ash, the colder the sensor the ash the higher it is, no ash and the sensor will read 600+.

Adjust sv slowly down 5 at a time until you have less unburned, and burning coal layer is up higher so flame is starting level with sight.

Let me make a positive contribution to one of the problems a severe puff back causes, damaging the barometric damper. You can absolutely prevent any damage by installing a barometric damper using the concentric stovepipe method as suggested in the A-A manual for high draft chimneys.

See: AXEMAN 101 *Class Is In Session!*

For a detailed photo look here: 5" up to 6" flu pipe

Image coped here also



Note the smaller 5" stovepipe inside the larger 6" stovepipe as viewed through the 6" barometric damper (door removed). Now when a puff back occurs or even an explosion the 5" stovepipe directs the blast directly into the chimney breach. Yes, the barometric damper door will swing but it's not likely to become unhinged. In the above photo the boiler connection would be on the right. The chimney breach on the left.

ValterBorges wrote:I wonder if the slowly idled the blower to a stop if it would help.

I've given considerable thought how I would engineer a solution to the puff back problem by changing the combustion blower shutdown sequence. Here's what I theorize would eliminate any problems. Before the cutting power to the blower I would open the flapper door slowly, letting the blower run for a minute plus. Opening it would change the air path from through the fire to up through the tombstone cover. Doing it slowly and gradually would hopefully not supply sufficient oxygen for a puff back. The blower is still running and it's exhausting normally through the boiler breach. This should exhaust any combustionable gases. Then after some time I would cut power to the blower and allow the flapper door to open by spring force and hang open as now designed.

The slow opening of the flapper door could be achieved with a single cycle mechanical timer motor literally driving a cam. The cam would be on a vertical shaft located below the inspection port flapper door. The cam would touch an extended tab on the bottom of the flapper door. When the cam is at the low point the flapper door is fully closed, but as it rotates it slowly opens the door. At the end of it's cycle, the flapper door is partially open. Since the blower is still running it's trying to keep the flapper closed by suction but the cam is prying it open at the bottom. After some time the blower motor turns off and the spring fully opens the flapper door.

Your thoughts?