Next, the computer considers each square inch of the firebox walls and heat exchanger surface. It uses the calculated incoming radiation plus the heat calculated in the smoke right against it, the heat transfer coefficient of the wall, smoke turbulence effects, wall thickness, calculated air temperature and air velocity on the other side of the heat exchanger wall to determine the net heat transfer through that square inch. Then it does the same for every other square inch.
Now that it has calculated all the heat exchangers, it seems that it would be done, but it's not. Since every bit of heat removed from the smoke reduces the available smoke heat in later (upper) parts of the unit and since every bit of heat transferred to the warm air warms it up, much of the heat exchanger system figures to work slightly differently. Therefore, the computer takes all of its results and uses that as its own input to refine the results. Often, it does this many, many times. It is only satisfied when the results for EVERY square inch show a metal temperature within 1°F of its previous results.
Then it gives a printout of the total heat exchange, radiation and convection, and average metal and smoke temperatures for each portion of the units and for the whole unit. Several efficiency figures are also printed along with total output and percent contributions of the various portions. We have run many hundreds of these simulations with different fire sizes, draft settings, and with different construction features in our various products.
Using the computer we can see if a modification to a baffle or heat exchanger will improve or degrade performance before building prototypes. Some interesting results of these computer simulation; for moderate sized fires the stove walls should slope in a 17 degree angle; for large fires, the angle approaches 15 degrees and for small fires, 19 1/2 degrees.
The simulation which follows is a fairly representative one; it took 9 1/2 minutes to calculate on a high-speed computer, shows the design performance of a JUCA B-3B, giving an overall net efficiency of 77% while giving full house heating output with air warmed to 136°F, with JUCA's standard 465cfm blower.
|Model B-3B Configuration|
|Using 8 lb/hr wood|
|465 cfm blower|
|100% excess air|
|Flame temp is 2500°F|
|cfm used by fire = 28.8|
|effective area on fire 50.6412 sq in|
|Heat in smoke is 41,990.4 Btu/hr|
|Radiation is 15,349.3 Btu/hr|
|Radiation absorbed by other wood is 6139.73 Btu/hr|
|Radiation escaping fire is 9,209.6 Btu/hr|
|Average Smoke velocity is 0.586653 ft/sec|
|HEAT EXCHANGER QUANTITIES (Btu/hr)|
|Area||Conv In||Rad In||Conv Out||Rad Out||Metal Temp||Energy In Smoke||Air Temp||Smoke Temp|
|HEAT OUTPUT TO HOUSE (Btu/hr)|
|AREA||CONV OUT||RAD OUT||AV TEMP||% CONTRIB|
|Total Energy From Fire||51,200.0 Btu/Hr|
|Wasted Heat||11,680.8 Btu/Hr|
|Heat To The House||39,519.2 Btu/Hr|
|OUTPUT TO HOUSE (Btu/Hr)|
|35,092.40||WARM AIR||AT 136°F AIR TEMPERATURE|
|2,221.61||UPPER RADIATION||AT 136°F SHELL TEMP|
|39,516.04||TOTAL Usable Output|
Heat is given off 88% as warm air and 12% as radiation.
Flue Gas Temperature is 408°F
Various conditions of fire size and blower choice affect the final Overall Efficiency results. In general, they range from 76% to 81% for the Standard blower. The larger optional blowers slightly imporve overall efficiency to 79% to 84% for the various optional blowers.