Effieciency of stokers

Re: Effieciency of stokers

PostBy: coaledsweat On: Fri Jan 30, 2009 1:51 pm

Yanche wrote:If you are looking to design a low operating cost system, size you coal boiler so it does NOT meet the heating requirement on the coldest design day of the year. Then meet the additional heat requirement with a second boiler, oil or gas fired. That way most of the time your coal boiler is operating at near peak efficiency. The coldest day of the year is a rare event and most of the time you will never need the second boiler. Having a second boiler is a wonderful backup for when the other one needs service.


You will need to pay attention when plumbing and wiring controls for this scenario, the average install does not have the capability to operate two boilers in tandem.

In general, your solid fuel appliance can have a lower BTU rating than the gas/oil unit you have in place due to the fact that it's output is continuous, the gas/oil units start and stop to meet load demand. My Burnham oil boiler has a gross input of 160,000BTUs and a net output of 139,000BTUs. The old hand fired Steel King is about 90,000 BTUs and heats the house better than the oil burner.

You should be thoroughly confused by now, does this help? :)
coaledsweat
 
Stoker Coal Boiler: Axeman Anderson 260M
Coal Size/Type: Pea

Re: Effieciency of stokers

PostBy: Yanche On: Fri Jan 30, 2009 5:01 pm

Agreed it's not an average install. The title of the thread may be efficiency but it likely really is about operating economy. Clearly there is a trade off between complexity, efficiency and capital investment. A primary secondary piping system would easily handle the two boilers.
Yanche
 
Stoker Coal Boiler: Alternate Heating Systems S-130
Coal Size/Type: Anthracite Pea

Re: Effieciency of stokers

PostBy: joeblack5 On: Sat Jan 31, 2009 10:38 am

Sorry for the confusion:
The thread was meant to figure out what can be done to increase the efficiency of the boiler.
I noticed from my hand fired energy king that a large fire and a high demand resulted in a very high stack temperature. This heat is lost and even with the low price of coal that is a shame.

My thought was to oversize the boiler with a smaller fire ( all in reason) so that it has a larger heat exchanger and can get more heat out of the gasses.
Similar to the ideas behind a smaller nozzle in an oil boiler, or small hot wood fires in a soap stone stove.

[quote][/quote]
AHS130 Stoker boiler 21 Dec 2008 Yanche 5 post

Additional savings oil savings could be achieved by reducing the oil nozzle size. This would make the oil boiler more efficient. But the down side is it also reduces it output BTU capacity.

End of quote.

So in contradiction to the above and my own thoughts why it would be good for thermal efficiency to run a boiler of stove to the max.
Thermal efficiency as BTU of coal in versus BTU in hot water out versus BTU in flue lost.

Later johan
joeblack5
 

Visit Lehigh Anthracite

Re: Effieciency of stokers

PostBy: coaledsweat On: Sat Jan 31, 2009 6:27 pm

joeblack5 wrote:Sorry for the confusion:
The thread was meant to figure out what can be done to increase the efficiency of the boiler.
I noticed from my hand fired energy king that a large fire and a high demand resulted in a very high stack temperature. This heat is lost and even with the low price of coal that is a shame.

My thought was to oversize the boiler with a smaller fire ( all in reason) so that it has a larger heat exchanger and can get more heat out of the gasses.
Similar to the ideas behind a smaller nozzle in an oil boiler, or small hot wood fires in a soap stone stove.
So in contradiction to the above and my own thoughts why it would be good for thermal efficiency to run a boiler of stove to the max.
Thermal efficiency as BTU of coal in versus BTU in hot water out versus BTU in flue lost.


The efficiency is in the design, there isn't a lot you can do to improve it. The design efficiency and operating efficiency are two different things. A tractor trailer is designed to haul 80,000 or so pounds. It would not make sense to ship 8 bags of peat moss in it. So you want to match the load to the design limit to stay at the sweet spot.

It gets pretty involved, but like I said, don't get caught up in it.

More here.
http://www.bbs-engr.com/images/BoilerEffeciencyArticle.pdf
This link is broken, either the page no longer exists or there is some other issue like a typo.
coaledsweat
 
Stoker Coal Boiler: Axeman Anderson 260M
Coal Size/Type: Pea

Re: Effieciency of stokers

PostBy: lsayre On: Sat Oct 13, 2012 8:32 pm

I've determined that during the winter months of the past mild heating season the overall real world efficiency (overall operating efficiency) of my AHS S130 in my particular home and DHW heating application was somewhere between 60% and 65%.

The U.S. Bureau of Mines Report #4936 on the similar Axeman-Anderson boiler stated that with pea coal it is up to 84% efficient when firing, and that it was only about 23.6% efficient during the summer months while being used only to heat DHW (I.E., when it is not firing very often). The bottom line is that the higher the percentage of the time that its firing the better chance you have of achieving its maximum potential efficiency.

Yanche is right in stating that the ideal is to size the boiler to struggle in meeting the homes heating needs on the anticipated coldest days of the year.

I don't know what the ideal BTU output requirement for my home is, but I do know from heating it with a 22.5 KWH electric boiler (76,770 input/output BTU's) it started to get cold in my house and it struggled to stay at or above 60 degrees inside when outside temperatures stayed below zero for a day or two. If AHS made an S85 or S90 it would be more ideal for my house from a strictly home heating perspective. The S130 is a bit over-sized for heating the house only, but it is also heating my DHW so that adds to the daily BTU demand. I am not likely achieving the S130's maximum overall operating efficiency potential in my application. But I imagine that if it was 60% to 65% efficient for the 4th mildest winter on record, it should do a bit better in more normal winters.

The bottom line though is that I'm saving money.
lsayre
 
Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Stockton Anthracite Pea
Other Heating: Resistance Boiler (13.5 KW)

Re: Effieciency of stokers

PostBy: Pacowy On: Sat Oct 13, 2012 9:49 pm

lsayre wrote: Yanche is right in stating that the ideal is to size the boiler to struggle in meeting the homes heating need on the anticipated coldest days of the year.


I don't follow how this can be stated as a general conclusion. Early in this thread stovepipemike pointed out how transfer efficiency is driven in (large) part by the heat exchange surface area. If you have, for example, an EFM 520 that you have to run hard, all else equal you could produce the same net output with less coal by switching to a 700, because a 700 has a much larger heat exchange area (43.3 vs. 28.3 sf) than does a 520. Sure a bigger boiler probably has more standby losses, but I'd want to see the study that proves the standby losses always are more significant than the efficiency gains.

I think the Dead Men had a bunch of good reasons for installing coal boilers that seem under current standards to be oversized. Someone may choose to put in a system that leaves no room for error - or is even planned to come up short some of the time - but I certainly wouldn't. To me, one of the advantages of cheap btu's is that you can use enough of them to keep your family comfortable, and still save big-time relative to other fuels. I wouldn't trade that for what may well be the false economy of an undersized boiler.

Mike
Pacowy
 
Stoker Coal Boiler: H.B. Smith 350 Mills boiler/EFM 85R stoker
Coal Size/Type: Buckwheat/anthracite

Re: Effieciency of stokers

PostBy: lsayre On: Sat Oct 13, 2012 9:54 pm

The 1945 study that lead to the Anthratube design indicated that efficiency went up as the fire box area went down.
lsayre
 
Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Stockton Anthracite Pea
Other Heating: Resistance Boiler (13.5 KW)

Re: Effieciency of stokers

PostBy: Rob R. On: Sat Oct 13, 2012 10:02 pm

lsayre wrote:The 1945 study that lead to the Anthratube design indicated that efficiency went up as the fire box area went down.


I read that the % of heat transferred from the firetube itself decreased as the diameter increased. They couldn't achieve a proper burn with a small diameter tube, so they increased it and added a secondary heat exchanger to improve the efficiency.

Mike is absolutey right regarding the EFM units, with an equal firing rate a 700 will scavenge more heat than a 520. If the 700 is a plate boiler with a wet baffle, the difference is even greater.
Rob R.
 
Stoker Coal Boiler: EFM DF520
Hand Fed Coal Stove: Hitzer 50-93
Coal Size/Type: Lehigh Rice
Other Heating: Dad's 1953 EFM Highboy

Re: Effieciency of stokers

PostBy: lsayre On: Sat Oct 13, 2012 10:13 pm

Rob, the pertinent info is to be found on page 58 of the 1945 report.
lsayre
 
Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Stockton Anthracite Pea
Other Heating: Resistance Boiler (13.5 KW)

Re: Effieciency of stokers

PostBy: Pacowy On: Sat Oct 13, 2012 10:25 pm

lsayre wrote:The 1945 study that lead to the Anthratube design indicated that efficiency went up as the fire box area went down.


Are you saying stovepipemike and a whole slew of boiler designers were wrong, and that we can all make do with KAA-2's and other small boilers? It seems like you're trying to take efficiency out of the context of the need to perform a given task.

Mike
Pacowy
 
Stoker Coal Boiler: H.B. Smith 350 Mills boiler/EFM 85R stoker
Coal Size/Type: Buckwheat/anthracite

Re: Effieciency of stokers

PostBy: Rob R. On: Sat Oct 13, 2012 10:28 pm

Yes, they are referring to the amount of heat produced in the tube vs. the amount transferred through the wall of the tube to the water...something different what I was describing regarding the surface area in a secondary heat exchanger.
Rob R.
 
Stoker Coal Boiler: EFM DF520
Hand Fed Coal Stove: Hitzer 50-93
Coal Size/Type: Lehigh Rice
Other Heating: Dad's 1953 EFM Highboy

Re: Effieciency of stokers

PostBy: lsayre On: Sat Oct 13, 2012 10:34 pm

Mike, I'm saying that by my best reckoning it requires a boiler capable of about 85,000 to perhaps 90,000 BTUH of reliable output to fully keep my particular house warm when it remains below zero degrees F. outside (since 77,000 BTU's is not quite up to the task when its that cold), but that level of BTU output would not permit me to simultaneously heat my homes DHW. I have no idea what the BTU requirements to heat and provide for DHW are for any other home.

I also intended to state (though I probably did not do it very well) that I believe that to improve upon 60% to 65% efficiency for the long haul my boiler would have to be smaller than it is now.
lsayre
 
Stoker Coal Boiler: AHS S130 Coal Gun
Coal Size/Type: Stockton Anthracite Pea
Other Heating: Resistance Boiler (13.5 KW)

Re: Effieciency of stokers

PostBy: McGiever On: Sat Oct 13, 2012 10:52 pm

I believe the talk in the beginning paragraphs, in regards to firepot diameters, were for NON FAN INDUCED (natural draft) boilers.

The later paragraphs then talk of adding Draft Inducing Fans where then the stack temps where so high it became necessary to also add secondary heat exchangers in order to have more manageable stack temps and up the efficiency.
McGiever
 
Stoker Coal Boiler: AXEMAN-ANDERSON 130 "1959"
Hot Air Coal Stoker Stove: HARMAN MAGNUM
Hand Fed Coal Stove: RADIANT HOME AIR BLAST
Baseburners & Antiques: OUR GLENWOOD 111 BASEBURNER "1908"
Coal Size/Type: PEA / ANTHRACITE, NUT-STOVE / ANTHRACITE
Other Heating: Ground Source Heat Pump
Stove/Furnace Make: Hydro Heat /Mega Tek

Visit Lehigh Anthracite