Effieciency of Stokers

Although I have never seen a AA in person it seems a marvel of technology.

Does any body have any thoughts what determines the thermal efficiency of a boiler?
a: Is it the coal burning process it self: hand fed / stoker mechanism / burning principle/ how complete does the coal burn and why.
b: the size of the exposed surface area to the hot gasses.
c: does low stack temperature always mean that the thermal efficiency is high,or can it also indicate a poor burning process with a lot of coal waste?

Does a carpet stoker versus block stoker has an effect on the efficiency just the way it transports the coal?
Is burning coal in a hand fed boiler more efficient then a stoker unit ?

Is the heat exchanger surface area in the AA larger then other boilers of similar BTU range or is the heat exchanger cleaning process that makes the difference?

I would think that the burning process would not make a whole lot of difference and that it is mostly the heat exchanger area.
Would like to hear of real life experiences.

Thanks Johan

Joe~ It goes somewhat like this: Combustion efficiency is the percentage of the available BTU's in the fuel that is converted to heat.Combustion efficiency is determined chemically by the measurement of things like carbon dioxide,excess oxygen,and net flue gas temperature.These readings are combined and charted to arrive at the combustion efficiency percentage. These indicators are items that can be field adjusted. Each different type of fuel will have its own ultimate performance criteria.These measurements are taken with specialized instruments. Transfer efficiency is the percentage of available BTU's that are transferred to the conditioned space.Transfer efficiency is largely established by the engineer that designs the equipment.Size of internal areas exposed to flue gas,how long the flue gas remains in the unit before exit to the chimney,density of boiler/furnace plate will affect transfer efficiency.These things are not adjustable. Transfer efficiency can however be slightly influenced by how well equipment is maintained. An example would be the deposits of any insulating material on the heat transferring surfaces of the equipment such as fly ash in the case of coal ,or soot/carbon in the case of oil,cresote in the case of wood etc. These deposits will slow the transfer of btu's.to the heating medium. Transfer efficiency is not usually measured in the field. Hope this helps , Mike

Thanks Mike,

From the first part, the combustion efficiency, how much is this number effected by the stoker design? Would for example a carpet stoker support better combustion efficiency then a burn pot and why?

Later Johan

The efficiency doesn't stop at combustion, that just gets you the most heat out of the fuel. You have to get the most heat from the combustion into your heat transfer medium, in a boilers case, that is the water. Designs like the Axeman and AHS have the steel combustion chamber with a water jacket completely surrounding the fire and that fire is in direct contact with the steel. Hand fired boilers for the most part and some stokers have two walls that are wet and two, typically the front and rear of the boiler, where the outside firebox wall is exposed to the room resulting in radiant heat losses that don't see the transfer medium. The aforementioned boilers also have a long convoluted path through the water jacket for the exhaust gasses, this allows the most heat to be captured prior to disposing of them. Less heat up the chimney, more in the water.
The design of the stoker itself has little to do with the overall efficiency of the appliance.

Wouldn't Transfer efficiency be the important one? Basically transfer as much heat as possible before it gets to the flue, right? I can't seem to find any efficiency ratings for my 90k New Yorker, wood/coal boiler. I wanted to know the difference between it and my Kaa-2 (when I get it). I think Keystoker gives net and gross BTUs. can you get transfer efficiency from these numbers?

Ed

efo141 wrote:Wouldn't Transfer efficiency be the important one? Basically transfer as much heat as possible before it gets to the flue, right? I can't seem to find any efficiency ratings for my 90k New Yorker, wood/coal boiler. I wanted to know the difference between it and my Kaa-2 (when I get it). I think Keystoker gives net and gross BTUs. can you get transfer efficiency from these numbers?

Yes, transfer is important. The New Yorker is a hand fired, right? Most hand fireds run in the 65-70% efficiency range. The stokers run from 75-85% efficiency.

coaledsweat wrote:

efo141 wrote:Wouldn't Transfer efficiency be the important one? Basically transfer as much heat as possible before it gets to the flue, right? I can't seem to find any efficiency ratings for my 90k New Yorker, wood/coal boiler. I wanted to know the difference between it and my Kaa-2 (when I get it). I think Keystoker gives net and gross BTUs. can you get transfer efficiency from these numbers?

Yes, transfer is important. The New Yorker is a hand fired, right? Most hand fireds run in the 65-70% efficiency range. The stokers run from 75-85% efficiency.

So I should burn 5%-20% less coal with the Kaa-2?

efo141 wrote:can you get transfer efficiency from these numbers?

Yes & no, kind of, approximately. If you know the rated BTU and know how many pounds per hour it uses, that kind of gives you overall efficiency. But, toss that out the window, as mentioned, hand feds run about 70%, stokers higher, boilers higher than hot air. The carpet boilers seems to be a bit less than the *huge fan* boilers like the AA & AHS. In any appliance the coal, the draft, seemingly even the phase of the moon can make things run differently. For sure my AA likes to run. It burns more efficiently when it's dead of winter than in the warmer days of Fall.

Just another thought ~ Heat [btu's] will transmit approximately 32 times faster from metal to water than it will transmit from metal to air.Once again ,it has all to do with the amount of internal surface area.That is where great effeciencies are achieved,on the drafting table. Mike

True, boilers have much higher efficiency and can store heat. Heating a home is a tricky choice, it is much easier to select something for an industrial application where the load is fixed. Each design will have a sweet spot, a point at which it is most efficient. The A-A seem to love a big load, others will be happier with a lighter demand. But don't focus too much on efficiency, it can be a trap. A very efficient boiler that is oversize will cost you big time in standby losses, one that is undersized may struggle to heat your home. The 5% that you seek may turn into an even bigger loss in installation and operating costs than the efficiency could ever make up. A big boiler with low draft will be a problem as is a small one with too much draft and the other way around. The installation will be a more important consideration than the boiler's design itself. An A-A or AHS 260 are very efficient boilers, in a 1600' house, they would be a mistake. Size and choose a boiler based on you and your home's needs, not by some arbitrary number that in all probability will never be known. The chimney height, the fuel you use and the settings you apply to the appliance will have a greater impact on costs than the units efficiency rating.

Absolutely agree with you coaled sweat,the sad part is that customers do not often associate proper application and sizing with overall comfort and agreeable fuel costs.Heat loss calculation is often overlooked or downplayed. A wrong application is like having an alligator in the basement,it simply keeps eating with little return on investment.

Would a boiler designed for 140000 BTU run more efficient at feed rate equivalent to 70000 or 10000 BTU because the hot gasses can stay longer in the boiler? How much? 10%?

Are there any stokers that can automatically adjust their feed rate in a proportionate way depending on the outside temperature , average demand and so run more efficient. similar to gas boilers that adjust the water temperature by a modulating burner in stead of the on / off style?

It would not be all that hard to convert stokers with a DC motor so that the feed rate can be adjusted with a voltage or keep the AC motor and put a frequency converter on it.

Later Johan

Any boiler will be most efficient, i.e. most economical to operate, when it's operating near its maximum capacity. As coaledsweat points out an oversized unit would be a mistake. 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.

Thats why I am going with the smallest stoker boiler Kaa-2 I can find. I like the idea of heating my hot water year round if oil shoots up again,and it will idle at 5k. I have heard for years your boiler should run steady on the coldest day of the year if your system is perfect. I don't think many heating pros will install that tight if there is no back up heat. I have my oil boiler in case the Keystoker can't keep up on the coldest days.

I think these reason are why people have been so pleased with the KAA-2 design. It probably operates near it's maximum efficient more then many of the boilers out there.