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Chimney size, Chimney bends, and Tuyere size


Glenn

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From the archive:  A0027 Chimney Size
by Glenn Conner

There has been several discussions on the PROPER size chimney for a forge. Everyone is looking for one number or size to fit all situations and it just does not exist, as each situation is different. 

The diameter of the chimney is just one factor. Other factors include how many elbows or bends there are, or is it a straight stack, is it a hood or side draft or super sucker type transition to the chimney, the height of the chimney, does the chimney have a chimney cap, the location of the chimney to the nearest trees, buildings, etc, the wind currents in the area, the weather conditions such as heavy weather during a storm or a clear day, and etc, etc..

Let us just deal with the diameter of the chimney for a moment.

The following table shows the diameter in inches vs area in square inches

3 = 7.1
4 = 12.5
5 = 19.3

6 = 28.3
7 = 38.5
8 = 50.3

10 = 78.5
12 = 113.1

14 = 154
16 = 201.1
18 = 251.5

The sweet spot for most forges is suggested to be about 100 square inches of area. That means 10 inch or 12 inch diameter chimney should work, with the larger being suggested as better. Again you need to consider the type forge and size of the fire into the equation.

You can see from the chart that you would need 3 each 6 inch chimneys combined to have the 100 square square inches of area, or 2 each 8 inch chimneys combined. We are not going to go into the fluid dynamics of the air flow as we are just trying to show relationships. 

You could use a square chimney. A square 10 inches on a side would hit the 100 square inches. 

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Another post from the archive 

 

Sometimes the answer to a question is found in the old reference books. The answer is also related to the forges and work being done at that period of time (before 1945), by blacksmiths that WORKED 10 hours a day, and earned their living by what and how much they produced.
 

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12th ed Machinery's Handbook 1945

Air persssures and pipe sizes for forges

Blacksmith's forges require air pressures varying from 1-1/2 to 6 ounces per square inch. Small forges with the blower close to them are adequately supplied with 1-1/2 ounce pressure. If the blower is some distance away and a long discharge pipe with many bends leads to the forge, even through the latter be small, it may be necessary to carry 3 ounces pressure or more, to overcome the friction of the air ducts. Large forges usually require from 3 to 6 ounces pressure. The table "air pressures and pipe sizes for forges" gives the diameters of discharge mains for various tuyere sizes and numbers of forges.

For a one forge set up the numbers are diameter forge tuyere in inches vs diameter discharge main at the blower in inches.

3/4 forge tuyere......... 1-1/2 diameter discharge main at the blower in inches
1 forge tuyere............ 1-1/2 diameter discharge main at the blower in inches
1-1/4 forge tuyere...... 2 diameter discharge main at the blower in inches
1-1/2 forge tuyere...... 2 diameter discharge main at the blower in inches
1-3/4 forge tuyere...... 2-1/2 diameter discharge main at the blower in inches
2 forge tuyere............ 3 diameter discharge main at the blower in inches
2-1/4 forge tuyere ......3 diameter discharge main at the blower in inches
2-1/2 forge tuyere...... 3-1/2 diameter discharge main at the blower in inches
2-3/4 forge tuyere...... 4 diameter discharge main at the blower in inches
3 forge tuyere ........... 4 diameter discharge main at the blower in inches
3-1/2 forge tuyere..... 4-1/2 diameter discharge main at the blower in inches
4 forge tuyere ........... 6 diameter discharge main at the blower in inches

The book goes on to give forge tuyere sizes and diameter discharge main at the blower for up to 10 forges connected to the same air supply.


I saw nothing related to CFM of air required for the forge, only ounces of pressure.

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The discussion so far has NOT covered the size or configuration of the forge and how much coal is actually been dumped into the forge and onto the table.

I found the following in reference in some of my notes. The source was not listed.
 

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For machine blacksmithing forges should be from 36 to 42 inches in diameter and from 26 to 30 inches high, the top of the tuyere being from 4-1/2 to 7 inches lower than the top of the forge. As there are no standards or definite data for guideance in determining the size of the opening for tuyeres or the depth at which they should be placed below the level of the hearth. The table below gives what the writer considers to be proper dimensions for work varying from i inches to 10 inches in diameter, when the blast is delivered at a pressure of 8 ounces per square inch or over. Work over 10 inches in diameter can be more uniformly and economically heated in a furnace.

Table of sizes and arrangement of the tuyeres.

Size of the opening in tuyere..................................3/4 inch................1 inch
Distance between top of tuyere and top of forge ...4 inch .................5 inch
Size of the supply pipe...........................................1-3/4 inch.............2 inch
Size of the work to be done.....................................1/4 to 1 inch........1 to 2 inches


The forge is 3 to 3-1/2 FEET (a meter) in diameter. The top of the tuyere is lower than the top of the forge by 4-5 inches with additional coal on top of that. The air blast is 8 ounces per square inch or greater.

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And as long as your digging in the archive

Posted February 17, 2007 (edited)

In reading a magazine called Fuel Oil News, April 2006, I stumbled onto a article called Flue Pipe Design by George Lanthier, who thanks Tim Begoske and John Cotton of Field Controls for the chart. The following information relates to the fuel oil industry but may provide guide lines for forge chimneys.

Mr. Lanthier refers to a rule of thumb about “the chimney connector shall not be longer than 75% of the portion of the chimney above the chimney connector inlet.” If you have (as in his example) a 35 foot tall chimney and you deduct 5 feet for the height of where the flue pipe goes into the chimney, leaving an actual chimney height of 30 feet. Seventy five percent of that height leaves a working dimension for the flue pipe of no more than 22.5 feet.


Refer to the chart and follow along.

Using a Tee and a 90* elbow
If we come off the back of the heater with a short horizontal run of pipe (18”), a 90* elbow, a vertical rise (24”) into a Tee and a horizontal run (23”) into the chimney we have about 5.5 feet of pipe. Add to a 90* elbows with an equivalent of 11 feet, a Tee with an equivalent of 38 feet and we have a Total Equivalent Pipe Length in Feet of 54.5 feet. 54.5 feet is well beyond (almost 2.5 times) our target length of 22.5 feet

Using 90* elbows
If we come off the back of the heater with a short horizontal run of pipe (18”), a 90* elbow, a vertical rise (24”) into another 90* elbow and a horizontal run (23”) into the chimney we have about 5.5 feet of pipe. Add to that 2 of the 90* elbows with an equivalent of 11 feet each (22 feet for 2 elbows) and we have a Total Equivalent Pipe Length in Feet of 27.5 feet. This is still beyond out target length of 22.5 feet.

Using 45* elbows
If we come off the back of the heater with a short horizontal run of pipe (7”), a 45* elbow, a slant rise (48”) into another 45* elbow and a horizontal run (5”) into the chimney we have about 5 feet of pipe. Add to that 2 of the 45* elbows with an equivalent of 5 feet each (10 feet for 2 elbows) and we have a Total Equivalent Pipe Length in Feet of 15 feet.

I would like to better understand the whole process of chimneys and what makes them draw or not draw. If anyone has any additional material, please provide it with references if they are available so we can better study this matter.

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In regards to chimney size. Ive seen this equation for induced flow stack effect (chimney draft/draught) a few times in separate sources, but the easiest to find was from Wikipedia. Keep in mind this is assuming it's just a straight verticle chimney with wide open ends and no friction from bends or elbows.

1041808493_Screenshot_20190130-0248102.png.d14422f3873f1c568b7743d95659c638.png

The main forces that drive a naturally drafted chimney are:

Q- draft

A- cross sectional area of chimney(diameter)

h- height of chimney.

All the Ti/To mumbo-jumbo- temperature difference between the inside of the chimney and outside of chimney, measured at the exit of the chimney.

C and g don't have much bearing on this description. g is gravity and C is based on the shape of the entrance and exit of the chimney.

By looking at this equation, the area of the chimney has the biggest effect on draft. Height and temperature have less pronounced effects, but are still very important.

With a forge, it's a bit if a balancing act. With a bigger diameter chimney you have to heat up all that extra volume in the chimney to get the draft. Too large and the smaller fire of a forge just isn't up the the task. That's why going to larger diameters doesn't work as well.

Taller chimneys help pull more air, but they to have a limit. Too tall and the gasses cool down inside the chimney before they exit and have nowhere to go but down (cold air sinks). The ceiling for that is higher than most people will get to, so for simplicity's sake let's just say taller is better. On a side note. Notice I'm talking about height, not length. This equation is measured vertically from the top of the entrance to the bottom of the exit of the chimney. Any distance covered horizontally actually reduces draft, so a chimney that is 20ft long but goes at a diagonal 15ft up will draft less than a 20ft chimney that's straight up and down.

For the temperature part. A forges fire is hot but it's relatively small, so it can't heat up a huge amount of air. If you have the entrance of the chimney too far away from the heat source, it will pull in more surrounding air, cooling the air so the chimney can't draw properly. But if you have it closer to the fire it will pull in the heat before it dissipates and will draw better.

As Glenn said above there is no one chimney fits all solution for a forge, but the consensus seems to be a 10-12in chimney, as tall as you can get it, and have the entrance as close to the heat source as you can get it. You might be able to get away with a 8in if you have enough height to compensate for the smaller diameter.

Another side note. If there isn't any air coming in to your shop to replace the air going out the chimney it won't draft. Air going out= air coming in and visa-versa. No air coming in means no air going out.

I think that about covers my understanding of how a chimney works. I hope it doesn't confuse more than it explains.

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Glenn, i just unpacked and reread some notes from Francis Whitaker and his book " The Blacksmiths Cookbook".

Ive reviewed this thread and not found this. It appears we all, me included, are forgetting a prime detail. 

He states flue diameter should be 1/3 the width of your forge.  His example is that a 30" wide forge needs a 10" flue pipe, and that 12" is better.  So a riveters forge of 18" would work with a 6" flue and a 36" forge works with a 12".  I believe these are minimums.  

He also states flue height should be 12'-15'. I'm sure this is a straight pipe.  He uses the formula for volume of a pipe as already mentioned.  

The key here is forge size. I had designed a rumsford type fireplace once and do remember that fireplace size was a factor, and ive never associated that with a forge.  

And, without question all other parameters apply. 

Hope this helps 

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There are alot of variables to take into account, I havent installed a forge chimney, but Ive installed a fair few gas fires and solid fuel fires/stoves.

A warm chimney is what you are aiming for. IF you have a single skin 12" pipe, chances are a 8" twin wall will work just as well. No calculations on that atm.

So if you live in a cold climate with a 12" single stack, thats going to cool down pretty quickly.

I plan on using 6" inch twin wall flue pipe on my forge as a base size, this is adequate for approx 20-30kw solid fuel(68,000-102,000 btu). I havent calculated what my potential forge heat ouptut will be yet, still on planning side, unless anyone knows a rough figure. But a large multi fuel stove gives off approx 12kw

Chimneys/forges need adequate combustion air. So good ventilation is required to complete a good working chimney.

Extractor fans also play a huge part on the performance of chimneys. If you are running a large extract fan in your shop it will be having a negative effect of the efficiency of your chimney.

This is just a few thoughts, fly by post, it may help.

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On 1/30/2019 at 10:25 AM, anvil said:

He states flue diameter should be 1/3 the width of your forge.  His example is that a 30" wide forge needs a 10" flue pipe, and that 12" is better.  So a riveters forge of 18" would work with a 6" flue and a 36" forge works with a 12".  I believe these are minimums.

I haven't read this book yet, and I'm sure the author has much more practice in building and using forges and chimneys than me, but this doesn't make sense to me. My logic is telling me the chimney size should be based off the size of the fire pot, or more specifically the size of the fire instead of the size of the forge. The rest of the forge is essentially table space and shouldn't effect the way the chimney works. If you have a larger fire pot in a 18" forge, then a 6" flue wouldn't be up to the task. The 1/3 rule may be true for commercially available forges, but the only commercially available forge I've used was set up outside, so I can't say either way what chimney would work with it. I'll have to get that book and give it a read.

On 1/30/2019 at 11:54 AM, Casapa said:

A warm chimney is what you are aiming for. IF you have a single skin 12" pipe, chances are a 8" twin wall will work just as well.

You are correct about wanting to keep the chimney as warm as possible, but let's take a look at the difference of 8" vs 12". Using the the equation above, and assuming the temperature of the gas leaving the chimney is 400°f (semi-random number pulled from another post on flue temp) and keeping all other variables the same. 10ft of 8" pipe will move 232ft³/minute. 10ft of 12" will move 524ft³/minute. To get the same amount of air movement from the 8" you would need to extend the chimney to 50'. By using the insulated pipe, you reduce the temp loss. This will increase the temp at the chimney exit, where the measurement is taken, and you would be able to cut down on that length. If you plan on using the 6" it would take 160' to get the same draft as only 10' of 12" pipe. I'm not saying that 8" or 6" won't work, just that you will need more height to be as effective as a 10" or 12".

For those interested I put together a little "chart" illustrating the equivalent draft of different size flues.

10' of 12" pipe= 20' of 10"= 50' of 8"= 160' of 6"

10' of 10" pipe= 25' of 8"= 75' of 6"

10' of 8" pipe= 25' of 6"

Again this is assuming it's a perfect world and all factors besides height and diameter are the same. I did check with several other random temp values and heights and these numbers seems to hold true whether it's a 100° or 5000° difference from chimney to outside air and from 1ft to 100ft of height.

Also, I apologise if I seem direct, abrubt or even rude in any of my posts, its not my intention. I'm not like this in person, but I've been told by many people I need to be more personable when I write. I try, but it's tough. Just call me on it if it gets too bad.

Oh, and your welcome 8up, I'm glad I can help.

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A factor not covered in "normal" stack dynamics is one if forge stacks. The lack of waste heat compared to fire byproducts. The smoke just isn't very hot and regular calculations break down. There are two ways to prevent creosote or soot build up in a stack. The current commercial method is near total combustion and high stack temperature. A lack of byproducts and no cool surface to condense on means a clean stack.

Once I learned to operate of wood stove properly you can't get a coffee can of junk out of the stack a year.

The other method is velocity, keep the smoke  moving fast and byproducts don't have time to condense on the stack. The second method is what a coal forge is stuck with a coal fire just doesn't shed much heat. The larger the stack Dia. the slower a given volume of smoke can rise. Too large an opening allows too much ambient air to be drawn in so you have two things working against draw: it's cooler AND it's more volume. 

This is why you see side draft hoods drawing so hard, they can't draw a lot of Ambient air, it's just not available. 

I can see where you're intuiting fire pot size as a factor but it just doesn't follow in my head, numbers and all. I've been wrong plenty though. 

Frosty The Lucky.

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