By: steamup On: Tue Jan 29, 2013 8:01 pm
Lots of information flying about here. I have been reading with great curiousity.
To be simply put, we are having reality of the unknown variables screw up what theory says should work.
Let's look at the combustion air fan side of things:
Using stoichiometry combustion calculations, one can calculate the amount of air needed to combust coal. However, in the real world, it is impossible to achieve complete combustion with the exact amount of air for the chemcial reaction. Excess air is required. The men who have passed before us learned that you need about 40% (rule of thumb) excess air for anthracite. It is different for hand fired vs. stoker and I am sure different designs require different excess air. Not to bore you with the calculation, but if you run it with 40% excess air, the 60 cfm blower used is the right size as far as capacity vs theory.
Too little excess air leave unburned coal and gas. Increases the potential for an explosion.
Too much excess air and a lot of heat is lost to heating the air (yes, it can be considerable)
Now here is the problem. Different coal qualities require different air quanities. I have seen excess air requirements for Semi-anthracite listed as high as 70%. Other variables are static pressure. Fans are pumps for air and have a curve that varies with resistance. Things that vary resistance are air hole sizes in the grate, ash buildup in the boiler, coal size, the amount of fines, air density, etc. Also, as I have found with my Keystoker, ash dust buildup on the fan blades reduce performance. Draft at the outlet of the boiler will affect the fan static resistance.
What ever the issue is, I applaud Leisure Line in responding as best they can.
Now let's look at the application side of things.
If I have a boiler that is advertised at 110 MBH input at 90% I could in threoy get 99 MBH output. But no way would I count on it.
Why not? - because it is based on the best conditions with good coal and a clean boiler that has has time to come up to full fire. We are dealing with solid fuel and it behaves very different than push button fuels that are instant on and instant off.
Any boiler sizing should take in account pickup. Regardless of fuel, it is industry practice to use a 15% de-rating factor for hot water systems. (steam is 35% by the way) This would mean don't hook up to a load greater than 84 MBH and expect to keep up.
Now with coal, I haven't seen a pickup factor listed. One has to realize that there is significate time lag from the call of heat to full fire. I would tend to think a greater pickup factor is required sizing a coal stoker boiler than a push button fuel boiler. I won't go into hand fired here as that leads to a more complicated discussion.
Now we haven't defined the load.
If I calculate a load for a building, I use design weather conditions. These conditions are a theoretical temperature based on statistics. Now realize, if an area has 0 deg. F. design temp, it is usually based on it not going below that more than 2-1/2% of the time of the year. You are going to exceed these temperatures at some point for some time. In my case, it was -5 to -17 below zero for 3 mornings in a row in my area that has a 2 deg. F. winter design temperature. In other words, you have to properly oversize beyond the pickup factor. Better add 10 to 15% to those numbers.
That means better not hookup to greater than a 75 -76 MBH calcualted load. That includes domestic hot water!!.
Now here is the problem with Domestic hot water and tankless coils. They are an instaneous heater. However, the boiler doesn't respond instantenously. Also, just cause they can flow 5 gpm, the don't guarentee you can get the temperature rise you desire. Let's see, 5 gpm x 8.33 lb/gal x 1 btu/lb x 50 deg water to 110 deg water x 60 min/hr = 149,940 BTUH. Don't expect any boiler with less than this output rating to handle a sustained hot water load. That doesn't include any heating load. The best one can do with a coal boiler is use the mass of the boiler to generate the hot water until it catches up. If a boiler has 12 gallons of 180 deg water, expect to get about 14 gallons of 110 deg domestic water and wait for the boiler to catch up. At 5 gpm, you got about 3 minutes of hot water. That shouldn't be a problem for you Navy guys.
Now bottom line to this rambling, is application, application, application. One must be very careful in sizing small boilers. Not much room for error.
