Secondary Air Distribution System

Re: Secondary Air Distribution System

PostBy: wsherrick On: Tue Nov 26, 2013 1:46 pm

Air volume as well as velocity must be considered. The given draft at any particular time will determine how MUCH air is drawn into the firebox. The existing pipe has the capacity to deliver much more air than the draft will pull in or more than the fire needs. The placement of the pipes right over the top the fire box (as shown) is correct as the air needs to go exactly over the top of the coal. Not to place the air here will reduce efficiency by simply cooling off the combustion area above the fire.
The pipe within a pipe regular idea is good as is the spinner damper idea. If you use the spinner damper idea you can perfect the air distribution by increasing or decreasing resistance to the flow of air by having the larger holes at the far end of the pipe and smaller ones toward the beginning of the pipe. In this set up the air distribution is automatic and works the same every time. The smaller holes will increase the velocity of the air going through those holes where as the larger holes will have more air going through them at a slower rate. If the holes are all sized correctly then the volume of air through each one will be about the same, which is what you want.
The pipe within a pipe idea would do the same thing as long as the holes were properly sized.
With a strong draft, simply reducing a space for air flow may not restrict the total amount air going through it. A smaller space may serve to simply speed up the flow of air not necessarily reduce it. That is why a means to restrict the actual inflow of air at the source is a sure way of restricting the total amount of air admitted at any given time.
wsherrick
 
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Re: Secondary Air Distribution System

PostBy: Lightning On: Tue Nov 26, 2013 1:47 pm

franco b wrote:I like the idea of a another pipe fitted inside the 1 1/4 pipe but only 2 inches long. Inner pipe capped and having 1/2 inch holes drilled around the periphery. By sliding in or out the 1/2 holes are more or less exposed.

I like this plan!! I may end up with this for regulating the secondary air volume. Seems simple and effective!

franco b wrote:I was concerned that having equal sized holes in the distribution pipes that most of the air would be distributed nearest the supply in the front of the stove. I think I would have considered a series of holes smaller at the front and bigger at the back to equalize distribution.

Ya know, that crossed my mind too, and I'm sure there will be some variance of distribution as it flows down the pipe. Another thing I could have considered is spacing the holes a little more at the front, then closer together towards the back where there would be less pressure. This is why I chose a bigger pipe instead of a smaller one. I thought the bigger pipe would supply the smaller holes easier than a smaller pipe. Kinda the same concept as a gas grill orfice, which is huge compared to the size of the holes, which offers better distribution front to back on the burner.. And notice on a grill orfice, the holes are spaced evenly and are the same size. :D

From what I'm seeing so far, the distribution seems pretty even.. I'll see better as time progresses :)
Lightning
 
Hand Fed Coal Furnace: Clayton 1537G
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Re: Secondary Air Distribution System

PostBy: wsherrick On: Tue Nov 26, 2013 2:01 pm

The inner pipe idea might have issues with corrosion and ash getting in there. After a summer of non use, it could lock up on you.
wsherrick
 
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Re: Secondary Air Distribution System

PostBy: Lightning On: Tue Nov 26, 2013 2:18 pm

wsherrick wrote:The inner pipe idea might have issues with corrosion and ash getting in there. After a summer of non use, it could lock up on you.

You mean the inner pipe going down the full length of the tube right? With the alignment of each hole? Yeah.. I'm not convinced there won't be any warpage from heat either :lol: ... But the 2 inch inner tube acting as a valve seems good to me so far, Or a spinner type thing like you suggested.. :)
Lightning
 
Hand Fed Coal Furnace: Clayton 1537G
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Re: Secondary Air Distribution System

PostBy: wsherrick On: Tue Nov 26, 2013 2:22 pm

Franco's idea has great merit. The principal goal is to make something as reliable and simple as possible. All of the brains should be spent on making it work well all by itself with minimal fiddling needed.
wsherrick
 
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Re: Secondary Air Distribution System

PostBy: Sunny Boy On: Tue Nov 26, 2013 2:32 pm

wsherrick wrote:The inner pipe idea might have issues with corrosion and ash getting in there. After a summer of non use, it could lock up on you.

Ash can only get in through those feed holes if ash can somehow manage to "swim upstream" against the force of air coming out of the feed holes. As long as there's some feed air, I doubt ash getting into the tubes would be a problem.

Preventing corrosion of the tubes inside can be easily dealt with too. One I use a lot in my work is Bostic brand "Never-seez", or "Blue Moly". http://www.bostik-us.com/our-brands/never-seez

Bostic's is the only brand of the many brands of anti-seize that I've used/tested in 40 years of work and found that it's not only good at resisting high heat (comes in different temp ratings up to over 2400F.), They're also very good at anti-corrosion at high, or low temps in nasty, corrosive environments.

Coating the tubes during assembly should prevent then from sticking.

There are other high temp anti-corrosion products also.

Paul
Sunny Boy
 
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Re: Secondary Air Distribution System

PostBy: Lightning On: Tue Nov 26, 2013 2:48 pm

Thanks partner! That's great information! :D
Lightning
 
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Re: Secondary Air Distribution System

PostBy: Lightning On: Tue Nov 26, 2013 3:04 pm

One thing I should notice right away is a more even burn of the coal bed. Before it seemed the back burned faster since air used to just spill onto that part of the coal bed.

Here's a current peak in the fire box. I have the secondary air flow cut back to about 25%.
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Lightning
 
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Re: Secondary Air Distribution System

PostBy: Wanna Bee On: Tue Nov 26, 2013 3:32 pm

Sunny Boy wrote:If you can't get a conduit in a size that is a close fit inside the 1-1/4 you can cut it lengthwise with a Dremel, or air grinder cutoff tool, opposite where your going to drill holes. Using a wedge, such as the blade of a screw driver, or cold chiseled, tapped carefully with a hammer along the length of the cut, you can spread the inner pipe so that it will fit better inside the 1-1/4 pipe. The split will not be near the holes, so it will not have any affect on air control.

A piece of metal rod through the end of the inner pipe should make a simple handle. Make the handle holes the same in each pipe and you can use the handles as a way of seeing how the air feed holes adjustment is matching one side to the other for balancing of the air going in left and right.

Paul

Wouldn't even need to drill holes if you slit the inner tube. Just use the slot for opening and closing the holes.
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Re: Secondary Air Distribution System

PostBy: Sunny Boy On: Tue Nov 26, 2013 4:03 pm

Lee,

There's a reason the holes in the gas BBQ grills are the same size along the length of the burners, no mater how far from the inlet.

And yes, because gases are compressible, there will be differences in air flow through the large pipe and thus the feed holes as draft force changes. However, making the feed holes different sizes down the length of the pipe may seem like a better way to self regulate air flow at each, but gases don't flow like many folks think.

Size does mater here too ! :D

Hole size has a disproportional affect on gas flow. One thing that is happening is that, gases don't like to flow right at the surface of whatever they are being channeled over. There's no movement for quite a few thousandths of an inch above the surface. And, the square-cut edges of holes such as those made by a drill, makes it even more restrictive because of turbulence caused right at the very edge of the hole. It's called "edge affect".

Since that region of non-movement stays about the same, the smaller the hole the greater the proportion of the hole's cross sectional area gets reduced for allowing air flow. In other words, at the same air pressure, restriction of air flow through smaller holes is not proportional to how much their cross section is reduced.

By making the total of all the feed hole's cross sections about the same as the large pipe, they actually have less affective square area because of the edge affect reducing their actual cross section that will flow air compared to how well the large pipe will flow at the same pressure. To get them equal to flowing the same air flow at the same pressure and speed, you would have to use more small holes then their collective cross sectional square area is.

So, because the large pipe can feed more air than the total of all the feed holes in that pipe, that's the reason the flames seem the same along the length of the pipe. The small holes are self regulating by evening out the air flow at each hole along the length of the pipe.

If you were to use larger feed holes at the far end of the tubes their lower resistance to airflow very likely would allow then to flow more air than the smaller holes nearer the opening of the large pipe.

In other words, the even sized flames you see are because you made all the feed holes the same and with slightly more restriction than the large pipe has.

You smart thing you ! :D

Paul
Last edited by Sunny Boy on Tue Nov 26, 2013 5:02 pm, edited 2 times in total.
Sunny Boy
 
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Re: Secondary Air Distribution System

PostBy: Sunny Boy On: Tue Nov 26, 2013 4:04 pm

Wanna Bee wrote:
Sunny Boy wrote:If you can't get a conduit in a size that is a close fit inside the 1-1/4 you can cut it lengthwise with a Dremel, or air grinder cutoff tool, opposite where your going to drill holes. Using a wedge, such as the blade of a screw driver, or cold chiseled, tapped carefully with a hammer along the length of the cut, you can spread the inner pipe so that it will fit better inside the 1-1/4 pipe. The split will not be near the holes, so it will not have any affect on air control.

A piece of metal rod through the end of the inner pipe should make a simple handle. Make the handle holes the same in each pipe and you can use the handles as a way of seeing how the air feed holes adjustment is matching one side to the other for balancing of the air going in left and right.

Paul

Wouldn't even need to drill holes if you slit the inner tube. Just use the slot for opening and closing the holes.



Good point !

Paul
Sunny Boy
 
Hand Fed Coal Boiler: Anthracite Industrial, domestic hot water heater
Baseburners & Antiques: Glenwood range 208, # 6 base heater, Modern Oak 118.
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Re: Secondary Air Distribution System

PostBy: wsherrick On: Tue Nov 26, 2013 4:15 pm

Sunny Boy wrote:Lee,

There's a reason the holes in the gas BBQ grills are the same size along the length of the burners, no mater how far from the inlet.

And yes, because gases are compressible, there will be differences in air flow through the large pipe and thus the feed holes as draft force changes. However, making the feed holes different sizes down the length of the pipe may seem like a better way to self regulate air flow at each, but gases don't flow like many folks think.

Size does mater here too ! :D

Hole size has a disproportional affect on gas flow. One thing that is happening is that, gases don't like to flow right at the surface of whatever they are being channeled over. There's no movement for quite a few thousandths of an inch above the surface. And, the square-cut edges of holes such as those made by a drill, makes it even more restrictive because of turbulence caused right at the very edge of the hole. It's called "edge affect".

Since that region of non-movement stays about the same, the smaller the hole the greater the proportion of the hole's cross sectional area gets reduced for allowing air flow. In other words, at the same air pressure, restriction of air flow through smaller holes is not proportional to how much their cross section is reduced.

By making the total of all the feed hole's cross sections about the same as the large pipe, they actually have less affective square area because of the edge affect reducing their actual cross section that will flow air compared to how well the large pipe will flow at the same pressure. To get them equal to flowing the same air flow at the same pressure and speed, you would have to use more small holes then their collective cross sectional square area is.

So, because the large pipe can feed more air than the total of all the feed holes in that pipe, that's the reason the flames seem the same along the length of the pipe. The small holes are self regulating by evening out the air flow at each hole along the length of the pipe.

If you were to use larger feed holes at the far end of the tubes their lower resistance to airflow very likely would allow then to flow more air than the smaller holes nearer the opening of the large pipe.

In other words, the even sized flames you see are because you made the all feed holes the same and with slightly more restriction than the large pipe has.

You smart thing you ! :D

Paul


Very interesting and informative. You have given me something to think about. If you smoothed out the exit holes would that change anything. I would think that you would want some turbulence, I'm not sure on that point.
wsherrick
 
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Re: Secondary Air Distribution System

PostBy: Lightning On: Tue Nov 26, 2013 4:44 pm

Wow. Edge effect. I didn't realize there was a name for it. But somehow I wondered if there would be more friction from air leaving a bunch of little holes verses one big hole. Cool man!!! :D
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Re: Secondary Air Distribution System

PostBy: Sunny Boy On: Tue Nov 26, 2013 4:57 pm

wsherrick,

Not the exits, but, yes, by "bell-mouthing" the entrance of each feed hole, you will reduce the edge affect and increase airflow - without having to increase the hole size. How you'd do that to all those holes from inside that long tube, I'd be interested to see. ;)

With a bit of experiment, it might even be possible to come up with the correct size feed holes for that size fire bed. The result would be, not having to adjust anything. As draft pressure changes, the air flow volume through the feed holes will change because the air can change speed through the feed holes. If I remember correctly, up to a point. Then, I think resistance increases out of proportion to pressure difference and starts to limit flow. Much like motor speeds can often be limited by the size of the carburetor.

As long as the total flow area of feed holes in the large pipe does not exceed the flow area of the large pipe, the feed holes should pretty well do the self regulating. Size of the feed holes then becomes the question.

See, we're back to "size matters". :D

Paul
Sunny Boy
 
Hand Fed Coal Boiler: Anthracite Industrial, domestic hot water heater
Baseburners & Antiques: Glenwood range 208, # 6 base heater, Modern Oak 118.
Coal Size/Type: Nuts !
Other Heating: Oil &electric plenum furnace

Re: Secondary Air Distribution System

PostBy: Lightning On: Tue Nov 26, 2013 5:09 pm

Sunny Boy wrote:wsherrick,

Not the exits, but, yes, by "bell-mouthing" the entrance of each feed hole, you will reduce the edge affect and increase airflow - without having to increase the hole size. How you'd do that to all those holes from inside that long tube, I'd be interested to see. ;)

With a bit of experiment, it might even be possible to come up with the correct size feed holes for that size fire bed. The result would be, not having to adjust anything. As draft pressure changes, the air flow volume through the feed holes will change because the air can change speed through the feed holes. If I remember correctly, up to a point. Then, I think resistance increases out of proportion to pressure difference and starts to limit flow. Much like motor speeds can often be limited by the size of the carburetor.

As long as the total flow area of feed holes in the large pipe does not exceed the flow area of the large pipe, the feed holes should pretty well do the self regulating. Size of the feed holes then becomes the question.

See, we're back to "size matters". :D

Paul


Paul, this insight is revolutionary for this experiment!! I knew I wouldn't want the area of the little holes to exceed the area of the pipe and now I know why!! 8-)
Lightning
 
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