Heat transfer in OWBs
OK boys and girls, this is not a sales pitch to sell you lots of crap that you do not really need on an outdoor wood boiler. As a matter of fact, you can save money by not buying a lot of stuff that they sell on outdoor wood boilers. Why? Becasue a lot of the features are not really needed. In two cases in particular, stainless steel water jackets and forced air draft fan.
Yes, CB does sell both the SS options and forced air drafts on their boilers. But you do not need either one. Strainless steel in particular is not great for several reasons. One is that it is really hard to weld SS. Welding fixes in the field are going to be really hard to do. Another reason is that SS is about $1,000 more expensive to buy. Another reason is that SS does not conduct heat as well as plain carbon steel does. And finally, in the case of CB units, their carbon steel is really thick stuff and it is not apt to rust through anyway. As long as you keep the right level of corrosion inhibitor in the water and tank (stays good for about 3 years before needing replaced) you will get no rust on that side of the boiler. Creosote is actually very mild on steel surfaces. I have the data and can post it if needed, but creosote will not cause a sttel tank to rust from the firebox side of the boiler. That leaves the ash area of the boiler. You can use Ashtrol in the ashes, which is basically agricultural lime to ballance the acid pH of ashes in your boiler pan to keep the ashes from rusting out the pan. I do not use it, and I do not have any rust in the firebox of my boiler. The place that the boiler is most apt to rust is if you are burning green or wet wood and it seeps into the ashes and gets trapped between the ashes and the steel. I scrape the ash pan down to the steel once a week with a plain garden hoe, and that loosens any chunks of ash and/or water that are next to the steel. Again, after using it for the better part of 2 heating seasons, there is no rust in our plain steel firebox.
Now, on to forced air draft. Again, forced air drafts are specific to the design of the firebox. I can only say that in the case of the Central Boiler design they are not really needed. In actuality they are not needed in many designs of boilers out there on the market today. Yes, CB sells them as an option, becasue people like the ones on this forum seem to have it in their heads that a fan is somehow better than natural convection for moving gasses around in their boilers. In the case of the CB, the damper is more than enough to get the fire going and keep the fire hot. As someone else here stated, it can take longer to get the fire going with a natural draft than with a fan. That may be so, from a cold start. If you let the fire go out all the time and let the water get stone cold between firing, then you may indeed need a fan to get it going fast enough to heat your house. But in the case that you fill the unit with enough wood on a regular basis, like 2x a day, that will not be a problem. I can let our fire go out and the water will still stay above 120 before I can get more wood on it the next morning. There are always enough coals in there to light the fire with more logs tossed into the boiler the next morning. And from 120 to 165 it usually only takes about an hour to get there. And during that time the house is still 70 degrees, and the hot water is still hot from the night before.
Now, as for the thermodynamics of a fan vs the magic of natural convection? As others here have pointed out, CB does sell fans on their units as an option. About 10% of their boilers are sold with them. That leaves 90% of the rest of us w/o forced air fans on our boilers. What makes our boilers work without fans? Simple. A principle of thermodynamics is that heated fluids and gasses rise. Why? Becasue warmer fluids and gasses are less dense than colder ones. Being less dense, they rise and float above the cooler mass. So, when the damper door opens, hot air escapes out the top of the smoke stack. That causes a vacuume in the boiler chamber, and that in turn causes the cold air outside the damper door to be sucked in to replace the hot air that escaped out the top of the boiler. That cold air then meets the flames of the burning wood inside the boiler, and is heated, that air rises becasue it is now less dense, and the process continues until the damper door closes.
That whole process is called convection, one of the processes of heat transfer. It is one of the three basic processes to heat transfer. Convection, as described for gasses above, condution, and radiation. The energy that comes from your firewood in your boiler is radiated out in the form of low level light waves. We see some of that energy in the form of flames, and we feel most of that radiation in the form of heat. When the fire in the boiler is going, there is both radiant heat and convection heat being delivered to the surface of the steel in the firebox. A good boiler will have a heat trap above the firebox for keeping in some of the convection heat to allow it time to transfer to the steel. The radiated heat is also absorbed by the steel. Now the third process of heat transfer ocurs in the steel itself. The heat is conducted to the rest of the steel through the molucules of the steel. They ones nearest the fire vibrate with more energy and that causes a chain reaction to move the energy (heat in this case) to the rest of the steel in the boiler.
Now, we have a hot fire, with convection fueling the fire to burn, and the energy from the fire is released and transfered to the steel by convection and radiation. Then the heat in the steel is transered to the rest of the steel and then to the water on the other side of the steel jacket. Now more convection happens as the steel releases its heat energy to the water. Convection works in gasses and fluids. As the water gets hot in the boiler, it rises. That causes what is called natural convection in the boiler. At the same time there is a pump that circulates water from the boiler to the house, and back to the boiler. That is called induced convection. If you add a fan to the damper of the boiler, you also induce convection.
Now, the difference between the fan and the natural convection (or draft) air supply boiler. Again, it is design specific. You have to have a damper at or near the bottom of the burning chamber to supply cold air at a low point so that natural convection will draw the hot air out the top and through the stack. Placed too high and the draft will not work right. You also need the right diameter stack and one that is tall enough to provide a good draft to allow gasses to escape the burn chamber and allow more cold air to enter through the damper. A stack that is too low will not work right either. Same as any masonry house fireplace. Now if that draft is not designed right, or you want a bigger air supply then you can add a fan to force the airflow through the system. This will indeed induce the wood to burn faster, and in many cases hotter. But... at the same time you are adding more air flow to the air entering the burn chamber. In many cases that I have seen, the forced air fans actually force a lot of the heat that you otherwise want to trap in the firebox right out of the stack before it can be transfered to the steel, water, and your house. You see, more air flow is not always better. A stong air flow can actually casue you to burn more wood. While the wood itself may be burning nice and hot and more efficiently.... you may also be losing a lot of that heat out the stack. So efficiency is a trickey word to use in the wood boiler industry. What you want is good heat transfer, and as much of the heat in the firewood to be transfered to your steel boiler and in turn transfered to your boiler water tank. Good heat efficiency, not good wood burning efficiency (although both are desirable). That heat in turn neds to get to your heat exchnagers, in turn to be delivered to your hot water heater and you in the shower, and to the forced air or hydronic heating system to heat your house.
The main problem with most starved-air OWB systems that are are talking about here (all but the likes of Greenwood and high-efficiency indoor wood boiler heaters) is that they lose a lot of energy in the off cycles. This is becasue when the damper closes of the fan is turned off, the wood is still being heated. It basically is being turned into charcoal, and the wood gasses are allowed to escape before they are burned. One reason that I try to use the minimum wood needed her to keep our fire going here is that I do not want to BBQ the firewood before it flame burns. Charcoal is made that way: wood is heated but not burned, which drives off the wood gasses leaving the charcoal. Those wood gasses have a lot of energy in them, and that is one reason that these types of boilers have less efficiency than the likes of the Greenwood systems. But the advantages of these systems is that they do not need to be filled as often, and they burn at lower temperatures, and they store energy in the wood that has yet to burn, and in the water tanks. Water has a good heat capacity, and that is why it is used in these types of systems.
Well, that should be enough to confuse everyone here and fan the flames...