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[peacmar]

Well this is going to be exactly what the title says. I've spent the last three years feverishly studying wood combustion and boiler design. I have countless notebooks full of information that I have collected and I wish to share it with all of you. I think that the home built section of this forum is the best place for all of this as it can be used when designing your own OWB. One thing I want to mention from the get go... I spend most of my internet time on an Android phone because of the portability and ability to jump online in my spare time as I usually don't have time to sit at a computer. So please bear with me when it comes to spelling and grammar. Auto correct is much a downfall as a blessing. I will add to this in bits and pieces as time permits. And as this begins to grow, hopefully admin or mods can reorganize it into something nice for everybody. As the information I have to share can be of use to all solid fuel heating devices we discuss here, of all brands. I will site sources when I can, but most all or what I have is from hand written notes. I can assure you though that it is all available online in a simple Google search. With that being said, here we go

[peacmar]

I'm going to start with combustion. I have some numbers to share that are of scientific origin that pertain to complete wood combustion. I'll list them for now then explain later when I have a moment.

BTU: amount of energy to raise one pound of water 1 degree Fahrenheit.

Pounds of water/gallon: ~8.3


Btu stored in 1 pound of 0% moisture cellulosic biomass(aka wood): 8600

Volume of air by weight to wood by weight for complete combustion: 6.364:1

Btu's lost to evaporate 1 pound of water retained within wood: 1200

Approximate cubic feet of air at "standard conditions" in 1 pound of air: ~13.07

           *standard conditions means between freezing point and 100 degrees Fahrenheit, with between %45-%70 humidity level. This means that each pound of air contains approximately %3 water by weight.

Minimum temperature for complete combustion of all released wood gases in an oxygen rich environment : 1200 degrees Fahrenheit

1 boiler horsepower: 33475 Btu/hour

Surface area required for 1 boiler horsepower: 5 square feet horizontal flue gas travel, 10 square feet vertical flue gas travel

btu required to turn one pound of 212 degree water to 212 degree steam at atmospheric pressure: 970

For air temperature between -100oC (-150oF) and 100oC (212oF) the specific heat capacity = 0.240 (Btu/lboF)


For most grades of mild steel (structural and boiler tube) the specific heat capacity = .122


To be continued....

[jackel440]

I look forward to this information
maybe I can sit down and figure out exactly what i have done with my stove and wood  I am using.
Oh and I completely agree about the android phone.I use my phone most of the time also.Yes that auto correct feature can get you in trouble sometimes
I do love my andriod phone though

[peacmar]

Well, as I get into this more and more, I see that I'm going to be jumping around a lot through my information and I need to eventually start somewhere and take this in an organized direction. My above post with the numbers will soon include some examples of calculations and other info and I will edit it to reflect upon its contents. Somewhat like a glossary for the following. I've put much thought into this and feel that it would be best to start with the end result and work backwards. In regards to heat, specifically. 

For the sake of this discussion, I am going to start with heat load necessary to achieve the desired results. You can come about this number by any means that seems realistic to you. Your existing heating system would be best of course. but however you do it keep in mind that this number, in btu value requirements, will be the determining factor of everything that follows. So for the sake of this discussion, let's start with a theoretical house needing 65000 btu per hour. Keep in mind that this value changes with the outside temperature and the insulation of the dwelling, but its a starting point. One can reach this value a number of ways with a few simple calculations which will be discussed later when I discuss transferring the heat to the house and sizing exchangers.

So 65000 btu an hour, and wood contains 8600 btu per pound, so the following equation should net the amount of wood we need to consume per hour to generate that amount of heat.

65000/8600=7.55 pounds of wood consumed per hour.

But not everybody cab afford to burn kiln dried 0% moisture content wood. Aren't we trying to lower our heat bill? So now we take into account the moisture content value of our wood. Good quality seasoned wood that has been properly harvested and stored should have a moisture content of >%20 before it is burned for best possible results and burn efficiency. A moisture meter is the most accurate way to check this. So now we have to adjust our btu values for the moisture content per pound of wood we will be burning.

1 pound of wood contains 8600 btu

It takes 1200 btu to evaporate a pound of water

Our pound of wood contains %20 moisture content by weight, so it is %20 water and %80 wood.

This leaves us with .8 pounds of wood and .2 pounds of water.

8600*.8=6880 btu

1200 btu to evaporate a pound of water

1200*.2=240 btu lost to evaporate the portion of water in the wood.

So this leaves us with 6880 btu available and 240 lost to water so we have

6880-240=6640 btu available per pound of wood we hold in our hand at %20 MC by weight.
So now we need to calculate our wood consumption using our corrected heat values.

65000/6640=9.78 pounds of fire wood consumed per hour to create our needed heat value. We will modify this later once we determine efficiency and thermal mass storage, but for now its the basis of every design aspect of our wood burning boiler that will heat our home.

[peacmar]

Now we're going to take a look at the combustion side of things. As every OWB owner knows, this is something that most manufacturers overlook in terms of efficiency. Do they work? Yes. Do they consume massive amounts of wood allowing it to smolder for hours and smoking out everything within a half mile radius? Most certainly. This is one area where most OWB owners envy the gasifier owners to a great degree. The fatal error of all water jacket wood burning boilers that causes such an inefficiency is breaking the number one rule in boiler design. Something that even the designers of the first steam boiler knew before starting and is still rule number one today.

Never allow the flame front to contact any metal containing water backed surface.

As most of you already know, the water draws the heat from the metal and the surface of the metal draws combustion heat away from the flame. A cold fire wastes lots of unburnt fuel in the form of smoke.
      So the first part of wood combustion is the obvious, HEAT. Make as much of it as you can and waste as little as possible. So back up to third grade and we have the fire triangle.  We have heat and fuel so throw in some air and we are heating our water. But the question is how much air? Tis' easier said than done. So we now have to figure out how much air is required to burn our "charge" of fuel. While there are very precise amounts of oxygen necessary to properly and completely burn wood, we don't have a very precise supply of it readily available. So we must take what we can get. With a scientific air composition analysis using percentages and the periodic table of elements, we can use molar analysis and molar weight to determine that approximately 6.364 parts of air will contain enough oxygen to completely burn one part of wood when measured by weight. We are measuring our wood by the pound so we will measure our air by the pound also for the moment. If we work backwards again we can say that 9.78 pounds of wood will require 62.06 pounds of air to completely burn.

9.78*6.346=62.06

Now there is a rule of thumb about combustion of any fuel:

Air is never consistent.

So to make sure we have enough oxygen present in our air we feed the fire a little more just to be sure that everything is consumed. Not to much because we don't want to waste our precious BTU's on heating excess air but just enough. Somewhere around %150 of what is needed by calculation. And if your heat exchange method is efficient enough you can grab that heat wasted on excess air back up before its gone. 

So this leads us to the adjusted air flow of:

62.06*1.5=93.09 pounds of air per pound of wood consumed.

At this time we have not yet considered time as a factor to any of our calculations except for the required heat load.

Now we have to convert our needed volume of air to a more readily available unit of measure when talking about our method of moving air. Most fans are rated by cubic feet a minute (cfm) not pounds per minute. So we must convert our pounds of air to cubic feet with the following bit of simple math:

93.09*13.07=1216.7 cubic feet of air to burn 9.78 pounds of %20 MC wood.

1216.7 cubic feet of air seems like a lot, but we must remember that we are using this air over the period of an hour and by dividing by 60 we will calculate our air flow in cubic feet per minute.

1216.7/60=20.7

If you know anything about fans you will realize that a simple computer fan is really all you need to supply the proper amount of oxygen to feed our fire and heat our home at this point. Of course that would mean that our fire will be burning endlessly and continuously for all the days we need heat. We will cover mass storage at a later time.

[BoilerHouse]

Very good info.  Thanks for taking the time to post it.  Hopefully you will have time to post more. I use a value of 6050 BTU/LB when doing my efficiency tests.  It is a number that I have seen others use and it is quoted in the Juca Stove Maufactures site in the technical section.  There appears to be some variability in the quoted value of BTU/LB for wood. Regardless, it is useful to measure the effectiveness of any changes made to our OWB by sticking with a single specific number.

[peacmar]

It's been a very busy week and I will continue to expand on all of the above. The heat value of 8500 btu per pound for 0% moisture content (kiln dried) wood was determined by the University of Missouri by scientifically determining The calorific heat value using basic chemistry and through use of conversion tables we can determine btu value. The energy measured in btu to evaporate water into steam which removes it from the wood is another science of its own, but the values are available anywhere online and are all constant. I use ~8.3 US pounds of weight measured per gallon of water for all the above calculations.

[peacmar]

Thermal mass storage

I will be covering the use if water as the heat storage medium here, there are many different options and phase change materials available, oils and wax, I have even seen grease and cooking oil used. But water is the safest and most affordable for most.

So we have the basic information as stated above.

1 btu to increase 1 pound of water 1 degree Fahrenheit.

Approximately 8.3 pounds of water in a gallon, this is the figure that will be used for our calculations.

So above, we determined that our theoretical home requires 65,000 btu/hour for heating needs. I am also going to add in domestic hot water heat based on a 40 gallon water heater. And a 50 degree F ground water temp. And also will use the rate of 4 gpm for our rate of water flow and an average use of potable water for about 20 min duration. We will be calculating for 12 hour burn cycles with heat storage, and at this time our system will have no idle time in the burn cycle. Just all out burn till the wood supply is depleted and the fire goes out.

So we have 40 gallons of water we want to heat from 50 degrees to 120 ( considered a safe usable temperature).

We take our final temperature, subtract our input (ground) temperature, and we have our temp differential.

120-50=70 degrees F

It takes 8.3 btu to heat one gallon of water 1degree F and we have 40 gallons to heat 70 degrees so we break it down like this:

8.3*40=332

Multiply by 70 degrees F

332*70=23,240 btu to heat our water heater from cold.

Now we will calculate our heat exchange rate for the 4 gpm and for our final figure we will use the higher of the two to ensure that we have sufficient heat storage necessary.


I am including this calculation for those who have a water to water heat exchangers plumber in line before the water heater for continuous and never ending hot water. I will address sizing the heat exchangers at a later time.


So we have 4 gallons of water flowing in a minute, that we want to raise 70 degrees and on average the water shouldn't flow for more than 20 minutes.

4*8.3=33.2

33.2*70 degree increase =2324 btu/minute per flow of incoming water.

2324*20 minutes water run time =46480 btus used in the 20 minutes the water runs.

Now that we have discovered that our running water uses more heat from storage than heating the entire tank from cold, we will use the amount of 46480 btu for our final calculation. And to be safe we will say that with a household of 3 inhabitants, showers, laundry and dishes included, we will multiply that figure 3 times to ensure proper heat capacity.

46480*3=139,440 btu worth of hot water per burn cycle

Now we calculate our homes heating needs. Based upon average heating needs through most if the winter of course because there will be some cold times when the burner must run more often and there will be warm days when it may not run at all.

We take our hourly requirement of 65000 btu per hour, multiply by 12 hours, and arive at the heating needs per 12 hour cycle.

65000*12=780000

Now add in the hot water needs:

780000+138440=919440 btu worth to total storage capacity needed.


We have now determined that we must be able to store 919,440 btu worth of heat to be able to slowly pump that into our home over a 12 your period and supply our heating and hit water needs. Now we will determine our water capacity in gallons needed to store up this heat for use over time.

I will be using a forced air system which can operate from 120 to 200 degrees. Our differential temperature will be 80 degrees F. These calculations can be adjusted to suit your systems needs be it baseboard or in floor heating by adjusting the usable temperature differential. This calculation will not include the initial heating of the thermal storage if this where to be a seasonally used system.

So we start at square one once again, with the mist basic units of measure.

1 btu to heat 1 pound of water 1 degree F

Our pound of water must absorb and store 80 btu to increase its temperature from 120 to 200 degrees F then slowly release it over a 12 hour period. Each gallon of water weighs 8.3 lbs so each gallon of water can retain 664 btu of usable heat.

80*8.3=664

We must have 919,440 btu of usable heat, so we divide our needed heat capacity by the amount each gallon hold to determine our water capacity in gallons.

919,440/664=1384.6 gallons of water needed to store our 919,440 btu needed over a 12 hour period.

Now if you can find a tank that holds exactly 1384.6 gallons of water your the most lucky person in the universe. But odds are you won't, so its always best to shoot for the next largest size available because there will be some heat lost through insulation, ground lines, ect. 

So there you have it, your mass storage is calculated. I'm guessing that 1500 gallons would probably be the easiest size to find. Multiple tanks can be used also. One thing to consider is stratification. Heat rises. Use your plumbing to your advantage. Add heat to the top always and take heat from the top always. Heat doesn't technically rise, it goes where it wants to. But hotter water is more buoyant than cold.

[peacmar]

 We have covered most of the basics of burning our fuel and now I'm going to discuss in a little more detail the actual design and layout of the burn chamber of our wood burner and how to extract as much of that heat as possible.

    From a basic stand point, there are two different ways to heat our water, a standard burn box with a flue and a gasifier with heat exchangers to extract the heat. I'm going to start with the burn box method, while offering some basic information and a explain what is theoretically a "good design" and then ill move into the gasifier and heat extraction methods. As the two are very different in concept.

Well most of you get the basic concept, have a fire burning inside a sealed and controlled box or cylinder and have that surrounded by water. While this is mostly a very simple concept there is a bit of a science, both literally and figuratively, about how we should burn our fuel to get the most out of it. For many years, even the most notable wood boilers have been of the poorest design. Trying to extract heat directly from the combustion chamber to heat the water. This is where we loose all efficiency and is the cause of the smoke problems that have given this heating method such a bad reputation. The single most detrimental aspect is the extraction of heat from the fire itself. So much energy has been put into making as large a fire box as possible for the longest burn times that the idea of efficiency has been forgotten. One of the most important ingredients in a fire is heat. If we rob the fire of heat we rob it of its energy and lose efficiency as a result. Ironically, this has been known for over a century by boiler makers abroad, yet somehow has been forgotten in the past few decades. The best possible designs are ones that promote complete and thorough combustion first, then extract the heat from the flue
 gas afterwards. That is what we will be discussing here. I will cover a few design concepts that have been used throughout history and have proven themselves over time. I will also include pictures and drawings to elaborate and explain. These will be added as time permits, so check back often. As this is growing and evolving as it comes along. Much like any project

At this point we have identified our single most important ingredient. Heat, its what we want and what we need. As long as we have a place to put it then the more we have the better. So let's take an analytical look at how the fire burns inside the fire box, and also some past used and proven designs that take advantage of some things mother nature provides for us.

Burning wood is a pretty basic phenomenon. stack some wood, add some heat, and it consumes oxygen from the air around it to make the fire that we see. An open camp fire is very efficient in using its fuel because air is plentiful. But once we confine that fire to I side a burn chamber of a boiler there is no longer an excessive of air and it can no longer function in its natural state. So one of our first goals must be to allow the fire itself to operate in as natural of a manner as possible. The shape of the burn box is one of the most influential factors in efficiency of a fire. The fire must have ample room for flames to form and a proper place for the coal bed to form underneath. So we will start from the bottom up and construct the perfect fire with near perfect combustion.

First we need a base. It must withstand many hundreds of degrees of heat, be able to allow oxygen rich air to pass through in a metered amount. It must support the coals and allow suffocating ashes to fall through. It must reflect heat from the coals back into the fire and not absorb very much. And a good base will provide a central air source and channel the fuel towards the center where the primary air source is. A V trough design with a small flat area in the center seems to be the best. Round works, but the sides may not be steep enough to funnel the fuel towards the air source. Think of 3 fire bricks, one laying flat, and the long way. A standard split is 9 inch long, so that will be the width of out base. Then have two more fire bricks, one on each side, sloped uphill at about 30 degrees. These will act as ramps to funnel the fuel towards the air source in the center. This would make a very good base for a burn chamber. Use enough fire bricks to make it to whatever length you choose.

Now we are going to start a fire on this base. Once its established and burning properly, a bed if coals will form, and these will drive our continuous process of combustion. The heat from the coals, about 50% of the energy in the fire, will pyrolise the unchared wood and cause it to release its wood gas and char in the process. The heated and buoyant wood gas rises away from the coal bed. Straight sides work best, or slightly tapered in to channel it together are good also. This wood gas contains the other 50% of the energy contained in our wood. In some cases, especially with soft woods, it can contain as much as 70% of the stored energy. It is very dirty at this point. Containing hydrogen, CO, Co2, complex carbon chains, and an array of inert components. Along with various amounts of water vapor and steam. It is this wood gas that usually gets wasted and is seen in the form of smoke. Our goal here is to add additional air, or secondary air, to the mix in hopes of burning off the remaining particles and releasing the remaining heat energy. Secondary air must be added high enough above the coals to ensure that it has no influence on them, yet low enough that there will be ample room present in the burn chamber for mixing, combustion, and expansion. Taller is better usually. Head room is always ok. This is also another area we want to retain as much heat as possible. Because this wood gas has components that can be very difficult to ignite and require very high temperatures. This is where most OWB fail terribly. There are many different ways to direct the heat. Separation of the combustion from the water jacket is the best and should be first on the list. Second is possible insulation. Fire bricks and refractory are very good measures to take. Third is adding heat. Specifically from the secondary air flow. If the secondary air where to be routed through pipes that lay in the coal bed before being directed upwards the air will be super heater by the coals and this heat will help ignite the wood gas. This is a method I have had great success with personally.

To quickly recap. We have a sturdy, funnel shaped base with air inlets in the bottom to feed the coal bed. We have sides that are tall and straight, possibly taper inwards and narrower at the top. And the top must also be insulated so that it reflects the heat back towards the fire. I have found with my experiments that the shape means nothing. Round, flat, any sort of polygon. It makes no difference. It is only important that this top portion does not steal away any heat from the flame front. Some manufactures use a heavy steel baffle, and this will work well. But cast iron would be better. And refractory of sorts would be best. A fire box completely lined with fire brick or castable is the most durable form of insulation. The fire brick surfaces will achieve high enough temps to ignite the wood gas and also prevent any creosote build up. If the refractory is cast directly or the bricks cemented to the firebox itself, then any heat that passes through can then be absorbed by the surrounding water jacket.

There are many ways to introduce air to the fire, and if the layout is well thought out, then the options are endless. Cross-draft works very well if nozzles are placed and adjusted properly. The Garn units make use of this concept. It works very well. Under fire air has been a timless method. However you go about it, keep in mind that the airflow must penetrate the coal bed thoroughly and completely. Often times a piece of pipe that lays horizontally in the coal bed with many small.holes drilled along both sides on the horizontal plane will make for very good air distribution. Place an ash cleanout slot underneath that falls into an ash pan and gravity will keep.everything clear. A muffler shop can bend up pieces very quickly and cheaply and replacement every couple years is a small price to pay. Secondary air pipes can be constructed in the same fashion. Bent into a u shape with one leg in the coal bed and the other up above the fire with a row of small holes pointed towards the fire, the air will be well heated and promote combustion very well.  A slip fit design with a heavy wall.pipe passing through the water jacket and into a fan plenum with inflow controls will allow very accurate control. And if well placed will allow for easy and quick adjustment of air flow as conditions change throughout the seasons.

Now that we have very complete and efficient combustion, and very few by products, we must collect as much heat as possible before our hot flue gasses leave the chimney. There are many ways to.do this. Multiple single flue passes such as in early steam boilers and now in the Garn. Fire tubes, water tubes, and the least efficient, large baffled heat collection chambers to absorb the heat through the walls. How every you go about it, always keep in kind that you must be able to easily clean the surfaces routinely. Especially if your stove isles often. This is something we try to avoid but sometimes we just can't. And this is when buildup forms in the fire box and heat exchanger and insulates the surfaces depleting efficiency. A thorough cleaning every so often will keep things running at best, and also prevent moisture from building up and attacking the surfaces if the metal. Creosote is very acidic when wet and can eat corrosion right off steel. Is becomes a repetitive process and the material slowly gets thinner and thinner.

[peacmar]

Due to space requirements, I'm going to start a new section dedicated specifically to the internal heat Exchange methods and how to design and calculate capacity.

I'm going to start out with a tried and true method of heating water and explain how we can determine our capacity from that information. The basis of most all water heating calculations is boiler horsepower. More specifically the amount of energy a boiler can convert into steam. We are only heating water not making steam. Hopefully... For everybody's sake.... I will not get into all the technical details at this very moment, but the heat transfer rate measured in btu is the same regardless of heating water or making steam.

1 boiler horsepower =33,475 btu per hour

We can use the specific heat capacity of steel, most grades of carbon steel are very very close in value, to determine the transfer rate of heat through the material. This rate is rather constant. Velocity doesn't have a very large effect, but turbulence does. Also, due to something called laminar air flow,  the temperature of the flue gas does not have as large of an influence as one would think. The delta T between two different heat carrying materials usually determines the rate of heat transfer. This is very true for water. But when hot air flows through a tube or pipe, there is a thin layer of molecules that sticks to the surface and does not flow with the rest. This layer has an insulating effect, as the heat from the moving air mass must transfer its heat to these molecules, then to the steel surface. Most gasses have a very poor heat transfer rate, air especially. Which is why it is the best insulation and what holds heat in our home. More on that later..... 

So, after my long winded explanation, we come to our next bit of information.

It takes 5 square feet of horizontal traveling surface area, or 10 square feet of vertical traveling surface area, to conduct one boiler horsepower worth of heat from flue gas to water.

There are many things we can do to increase this efficiency, I will discuss that momentarily, Just the basics for now. 

First we will look at horizontal traveling flue gas. The reason that horizontal flue passages conduct heat better is because of the natural buoyancy of hot gasses. First think of the hot flue gas flowing through in a very smooth and steady stream. This is not reality, but paints a nice mental picture. Picture each hot gas molecule following one another in a single file line, in perfect rows, much like a marching battalion. If this where to happen, the molecules on the outer edges would cool as they travel down the side of the pipe until they reach the same temperature as the pipe. But the ones in the middle would still be hot because they haven't touched the cooler pipe yet to give up their heat. We would waste massive amounts of heat. This is where physics come into play. The molecules on the outer edges cool and figuratively fall to the bottom of the pipe as they are cooler and more dense than the hot center molecules. Now that the cool molecules are moving out of the way, the hotter center ones rise to the top of the pipe where they give up their heat upon contact, and the process repeats itself about a million times over. The word is turbulence. Our best friend in this circumstance. When flue gasses travel horizontally they have convection currents that cause turbulence and allow more heat carrying molecules to get stired around and make contact with the tube or pipe surface. Hence the greater heat conducting. Vertical flue passages don't get to experience this phenomenon naturally as the hot gasses rise right up and out and the cooler parts almost cling to the surface and insulate it rather than transfer heat. To counter act this the use of turbulators can cause the needed turbulence to help promote efficiency. They can be spiral shaped, flat waves, I have even seen chains used. The turbulators are inserted in the tube or pipe that the flue gas travels in to help stir things up a bit. They can be used in both horizontal and vertical flue passages, but have a greater effect on vertical.


Now I'm going to show a few examples and some simple math to investigate some different scenarios and boiler tube designs based on our above home heating example.

Horizontal 5 square feet =720 square inches
Vertical 10 square feet = 1440 square inches
Both are capable of creating 33,475 btu/hr

I am going to use the horizontal method for our example. Simply double the area requirement for vertical, unless using turbulators as the efficiency will be near that of horizontal and those figures can be used. I am also going to round numbers to the next higher inch or foot. As once again more is better.


We are going to take our above calculated heat capacity of 919,440 btu and say we want to create that in a period of 6 hours, half our needed storage time.

So first we must divide 919,440 by 6 hours to determine our required btu/hr output.

919,440/6=153,240

Now divide that by 33,475 to determine boiler horsepower

153,240/33,475=4.57boiler horsepower.  We will round it to 5 boiler HP.

Multiply by 5 to calculate required surface area in square feet.

5*5=25 square feet

 But before we can play with different combinations of tube or pipe diameter and amounts of tubes we have one more very important calculation to make.

Now that we have determined our burn rate of approximately 153,000 btu/hr we need to make some airflow calculations to determine our minimum flue size, which will determine a number of other factors when it comes to the fire tubes in our boiler. Once we determine our minimum flue size we will calculate the surface area, and the sum of the surface area of our boiler tubes must be equal or greater than the sum of the flue area to prevent major air restrictions and quite possibly a very pressurized fire box that could turn explosive. Although this is generally only possible when a fire has been idle and smoldering for hours and the fire box is full of flammable wood gas.

To calculate our flue size we need to first determine airflow needed to feed the fire to make our required heat of 153,240 btu in one hour. We will use 20% MC wood again as this is well seasoned and best to use.

11/17/11

Ok so apparently I have misplaced my notes on thermal expansion calculations and thermal dynamics. So I will edit this info in later on when I come across it.

For now we can use some figures based on commercial built gasifier boilers.

6" flue= 140,000 btu/ hr max
8"=205,000
10"=275,000

As soon as I find the info ill put it up, and also show how a draft inducer can actually move more air through the same size pipe with negative pressure. But for now, and most builds actually, the above sizes will be reasonably close.

Slightly larger diameter piping is ok when using a fan to meter air flow conditions through a fire. Technically, there is no reason to worry about a draft so one could go as large as economically possible. But it is important to never undersize and choke a fire or wood gas could back up and the results could potentially become catastrophic when fresh oxygen is being forced in. 

So using the above numbers and calculations we see that we must use at least an 8" diameter flue pipe for our heat requirements. It is the surface area of this flue diameter that'll help us determine our heat exchanger tubes. We must never have a total combined cross sectional surface area smaller than the flue.

11/26/11
Now that we have our flue diameter we can calculate our cross sectional surface area required and then we will start running numbers on different pipe diameter to determine a good combination of size and quantity that will make 25 square feet and provide us with our heating needs.

Some basic geometry will give us the area of the flue:

Surface area of a circle
(pi)r^2

(3.14)4^2=50.24

Now if we where using a very high efficiency exchanger consisting of hundreds of very small diameter tubes the .24 inches would be significant, but most of us will use something larger and more easily available so we will round to 50 square inches for our value. Having the flue slightly larger than the exchanger is always better any way.

Now there is one little rule if circles that's very handy and can make this a very simple matter.

If you have a circle that is half the diameter of another, its area is 1/4 that of the larger.

So for an 8" flue, we would need four 4" tubes to retain the same surface area and for the internal pressure to remain constant.

Every time you cut the size in half, multiply the quantity by 4

For a 8" flue:
4@4"
16@2"
And so on...
 If you want to use 1.5" pipe you need to calculate the area of 1.5" pipe (1.76) and divide 50.24" by  that for the number of tubes needed.

50.24/1.76=28.54

28 tubes would suffice, as we are going to be operating at the lower end of the 8" flue capacity. I would make it 29 if it where at The upper end.

I'm going to use 16 2" inside diameter tubes for our example of how to calculate the minimum length needed for the surface area of our heat exchanger tubes.

So we know we need 25 square feet.144 square inches in a square foot tells us we need 3600 square inches of surface area minimum. If the design of your boiler allows more length always use it. More surface area is better than less or the minimum.

To determine the surface area of a tube we must calculate the circumference first, then multiply by the length. If you where to cut a slit down one side of a tube and un roll it you would have a rectangle, where area is length times width.

It is best to use th inside diameter measurement of the tube for surface area as this is the actual contact area. The actually measurements of our tube when ordering will be 2.25"x.125" wall thickness to give us 2" I.D.

Formula for circumference is Pi*diameter or PiD

3.14*2=6.28

Now we can combine all of our tubes as though they where one large flat sheet:

6.28*16=100.48"

That gives us the length if one side of our rectangle. Now using the formula for surface area:

L*W=A

We can work backwards with some simple math rules:

5 boiler horsepower equals 25 square feet or 3600 square inches.

100.48*W=3600
Or...
3600/100.48=35.82 inches long

Seems pretty short, but technically it will suffice. Of course if you make them longer, say the length of your boiler, you will ensure the efficiency if heat transfer. When designing your boiler, gasifier or burner, play with different sizes and combinations to see what works best with the materials you have at hand. Keep in mind that this example is based on mathematically calculated value, real world results always differ. So keep in mind that one thing they don't teach in engineering school,

just because it works on paper doesnt always mean its true.


There are also a number of other factors to include. thermal expansion, gas temperatures, Charles laws of thermodynamics. But I will touch upon these more in the gasifier section. As a multiple pass boiler would become too easily plugged by the creosote of a naturally burning wood burner.





***I just realized I made a few errors in my math, I will correct them next time I work on this. The surface area for the tubes calculated to 5 square feet and I should have calculated for 25 square feet. If you wish to calculate your own in the mean time don't forget this important multiplication as I have currently in the above example.****


7/13/12- corrected calculations for tube length.

I want to point out that round shapes are better in boiler design. They may net fewer inches of surface area, but are stronger. There is much physical moment within the metal as it heats and cools. Expansion and contraction. A Sharp corner or weld joint creates a stress riser, a point where stresses concentrate and cause physical material failure. A round tube is like an endless line, the stress is always perfectly and evenly distributed around the circumference. All metal fails eventually. It's called fatigue, if ee can minimize these dresses we can prolong the lifespan of our creation.

[peacmar]

Reserving this space for heat load and calculations, heat exchangers, water flow requirements, efficiency, and basic hook up lay out and requirements- based on information available from asme code and hvac standards.

[peacmar]

Grabbing another slot...

[peacmar]

And yet another... I'm starting to feel like maybe I should contact a publisher....


Not the first time its crossed my mind either.....

[willieG]

if i ever get around to building another OWB this thread is gonna be my bible!

[peacmar]

That's exactly what I was hoping for. I've searched long and hard for information and never ever found one place that has it all. It was always a matter of chasing dead ends, researching 50 things at once. Studying other areas of science and physics and chemistry,  applying my experience in the welding and fabrication, machining, and pipe fitting fields, using my natural ability to absorb information, and putting the puzzle together for myself. My goal is to now create what I could never find for those who may not have the time or resources to do what I was able to do. And for once, have all the information available for the individual that wakes up one day and decides to build something. I must say that the co-gen and gasifier fields and they're advancements have contributed a great deal to my gathering of information. Along the way of learning all this I also was able to learn how to run an engine in wood. Expand my knowledge of chemistry and physics (I was always a bit of a nerd, just one that also liked to build cars and weld stuff in shop class) and with all the tools I have at home I use for building dragsters-my seasonal mostly winter hobby- I was able to make it all come together at some point and continue to learn from there. Come spring I will be starting my second wood burner, gasifier, high efficiency with 1200 gallons storage in its own dedicated building. And I  will document that here also. And hopefully it will inspire someone else to do the same.