Secondary Combustion Designs: Tubular Manifold versus Solid Plate with holes

[Corie]

Just as the topic says, I know some stoves like Pacific Energy use a solid top baffle with holes drilled in it to supply secondary combustion air, whereas stoves like quadrafire and some jotul models just a stainless tubular manifold, usually fed by a series of rectangular tubes.

I know that representations of each are owned by several members on the board, any opinions? Any feelings on which is better? Someone with experience with both tops would be great I have my own feelings, but I want to know what the consensus is amongst those who actually know what they're talking about.

Thanks in advance

 

[MountainStoveGuy]

I would simply look at grams per hour. I havent researched which is better, burn tubes or baffle style. I would bet there comparable.

 

[Rob From Wisconsin]

Just swapped-out stoves in my basement.
The previous was "tubes", the new stove is
a plate drilled w/ holes. It seems the new stove
w/ plate holes works much better than the previous
w/ SS tubes. Whether this is the cause, or maybe it
is an all-around better stove for my application, I
cannot tell.....

Rob

 

[precaud]

I would expect the tubular designs to be more problematical with time. They just can't dissipate heat as well.

 

[MountainStoveGuy]

Jotul uses both designs in there stoves.

 

[precaud]

Are you a Jotul dealer? I noticed the new 118 is all tubular. So far it's been an underwhelming design, from user reports I've heard.

 

[wahoowad]

I would not think one design is better or worse than the other, instead it is a given implementation per stove model.

 

[precaud]

A tube will create more turbulence, which enhances mixing (a good thing). But a suspended tube cannot dissipate heat as well and is going to burn out more quickly than a plate (a bad thing). Given the tradeoffs, I'd go with a plate design.

 

[Corie]

I was leaning towards that as well precaud. It seems a solid plate design has more thermal mass and can thus hold the heat necessary for secondary combustion longer.

The problem right now though, is that all of my calculations have been based on a tubular manifold design. Now that I consider the plate design to be better, it's back to the drawing board :-/

 

[MountainStoveGuy]

Boy, this forum shure gets technical. And i learn something new every day here. But baffle design is one i have not heard debated. Some of the top manufactures use both designs. That would be reason enough for me to not make a buying decision based on that. I would not sacrifice style of stove if the secondary burn system is one type or another. Now if the stove you like happens to have the baffle system you prefer then thats great, otherwise i would just get what you want. In the seven years i have been doing this i havent had to replace tubes or baffles. Maybe thats because of the stoves i sell, or the fuel type that we burn here isnt hard on tubes.
This thread is intresting.

 

[Corie]

If I was buying a stove, I'd agree with you 100%. The type of secondary combustion chamber would be of no interest to me, as long as the stove worked. However, since I'm designing the stove from the ground up, and since literature and writeups concerning this type of thermal engineering/thermodynamics is pretty scarce, I've found it's better to talk to those with some experience on the topic.

For kicks later, i'll do a comparison chart of secondary combustion type versus firebox size, versus emissions

 

[MountainStoveGuy]

Your making your stove from scratch? i didnt realize that. I would be intrested to hear your findings. Good luck on the project Corie.

 

[Corie]

Trying to at least!

Right now I've only got some parts, some ideas and a LOT of calculations written on about 9.5 million pieces of paper.

 

[Mike Wilson]

We covered this on a previous thread. As far as Jotul is concerned, Their newer designed stoves all have plates, the older designs are tubes. That should be an indication of what's considered progress in the reburn world...

-- Mike

 

[Roospike]

Just some input: The Pacific Energy top baffle is all Stainless steel . They are covered 100% lifetime if you should have a problem . The only way you can get / order the top baffle is to be an owner of the stove and have one that needs replaced , they are not sold as itself . My thought at first before buying a stove was to build it for i own my own fabrication shop and can build just about anything steel i set my mind to but after all the thought and input about EPA , secondary burn chambers , EBT ofr extended burn time set up . I ended up buying a new stove . I could build my own family car but there again , why . For me it was just not worth the time unless your going to get into the business of building and selling and that would be another thing. I have built many , many very nice shop stoves over the years and would of been 100% quality for a home before EPA come into play but when it comes to secondary burn chambers , EBT , ect...ect ... you are getting into a different animal . If one was to have all the extra time on there hands to come up with a new house/home stove to build and was going to be a one-off for there own use it would be interisting . But there again all the extra time to do so for a one-off stove is the big issue . At least for me it was.

 

[Corie]

Not me! I'm just a mechanical/thermal engineering student with too much free time and parents that need a better stove.


I made a chart comparing firebox size to emissions to type of secondary, etc.

It is at best, worthless. The lowest emissions come from stoves that use tubular secondary manifolds, but so do the highest. It seems the level of engineering or the way the system is engineered has a lot more to do with the emissions than does the system used.

 

[Roospike]

I guess my next questions would be of once you have built your new stove how are you going to find out what the emissions will be ? I'm sure you could send it to a test lab or something but i'm sure thats not cheap . Can u get a hold of some kind of test equiment ? What would be great is if one could buy a already made secondary burn chamber and build a stove around it but i already know i'm not the first person to think that becuse the stoves co wont sell just the secondary burn chamber and such other wise i would of went down that road myself.

 

[Corie]

Building a stove around a combustion chamber would be too easy anyway.

THe reality is, this stove not only has to be able to burn wood and have a functioning secondary chamber, but it is also going to burn coal. In fact it's going to burn coal as it's primary fuel, so if all of my number are wrong and my secondary chamber doesn't work at all, I'll still have a pretty nice coal stove.

 

[Todd]

I'm all for "Do It Yourselfers" but what about insurance and inspections after the install? I bet it won't pass without a UL label.

What about Catalytic technology? I bet it would be easier to make a cat stove with a bypass damper?

 

[Corie]

We don't have inspections here so that's irrelevant, we've have three different stoves installed, two were done by installers and one by me. Inspection is not necessary.

I thought about catalytic technology as well. It's a more simple technology and it will more than likely work. It just isn't much of a challenge. We'll see though. I haven't decided on anything totally yet, other than the fact that the main body of the stove is designed, and the door and cast iron grates are here. I have some time before I start doing fab work on whatever secondary system I decide on, so we'll see.

As you said todd, I might just cop out and do a cat combustor- After all, I already have two six inch round models at the house.

 

[Roospike]

I haven't decided on anything totally yet, other than the fact that the main body of the stove is designed, and the door and cast iron grates are here. I have some time before I start doing fab work on whatever secondary system I decide on, so we'll see.

.

What are your designs of the stove you mentioned ? ie: how thick is the top plate? , how thick is the body? , ash pan ? , cf of the fire box ? ect ..... Are you thinking of making the secondary burn chamber of stainless steel ? i hope . walk us through this Corie . We are going to need to see some weld pic's too ya know . Were going to turn this into a fabrication forum . What welder ya running ? You going to have the steel sheet sheared , you cutting it to size your self ? Alright , I'll stop with the questions .......... well at least untell you can catch up with them , ha .

 

[heatxchanger]

Corie,

For what it may be worth, last year Vermont Castings/Majestic came out with a new non-catalytic burn chamber design which they call "EverBurn". They claim much longer burn times with this burn chamber design as compared to other brands of non-cat stoves. Anyway. if you are really interested in this stuff, you should certainly stick your head into a Vermont Castings stove with "EverBurn" to see the design. Let us know what you find - tubes? baffles? or ?

The reason I mention this is that it appears that Vermont Castings has winner with the "EverBurn" burn chamber as they are selling a lot of stoves with EverBurn.

 

[Corey]

Looks pretty close to call to me, just averaging all the tube stoves and all the plate stoves looks like 3.2gph to 3.5gph, respectively. Which would probably border on statistically insignificant given the small dataset.

Just on thought experiments, it would seem that the plate would have several advantages. In the plate, you would have a large volume of air moving relatively slowly, which means it would get very hot and help with the secondary combustion. With tubes, you have a smaller amount of air moving faster which means less heating time and the possibility of cooler air. Plus, the plate offers one continuous surface to re-direct heat into the firebox, while the tubes are intermittent sections of tube and firebrick. The one advantage of the tubes is their ability to make the air flow turbulent.

FWIW, now that I have my TIG welder up and running, one of my summer projects is going to be to retrofit a plate type combustor to my old stove and dispense with the tube style that I installed a couple of years ago. Not that the tube style doesn't work great...looks like a natural gas burner with 6" long flames shooting out when everything is up to temp. One design feature I hope to be able to incorporate into the plate is to dimple the surface (fire face) or maybe install little tabs as "spoilers". My thought is that if you ever watch the fire lick at any surfaces in the wood stove, you can see that it never really touches the surface. It encounters a layer of laminar flow which actually holds the flame anywhere from ~1/8" to 1/4" off the surface. By dimpling the surface, the laminar flow will be disrupted which will lead to more turbulent flow of the smoke and hopefully more thermal transfer of heat into the secondary air. This is one advantage of a tube system, is that all those tubes really mix up the smoke.

I also experimented this season with a "reverse cant" of the firebrick on the stove top. Most stoves seem to either have a flat layer of firebrick on the roof, or one that is lower in the back and higher toward the front. I raised mine higher in the back which directs a substantial portion of the smoke right back at the secondary air bar...where it promptly lights off in big long flames.

Corey

 

[roggeo]

Speaking of secondary burn, is anyone familiar with the old, presumably original VC Defiant, which has a bottom opening secondary chamber on one end (opposite the loading end)?? Or is that chamber for something else. It makes sense as a kind of downdraft exhaust, but its very large, at least 6 inches by the depth and hgth of the stove. I found this used one in perfect condition stamped 1975, but have never seen this end chamber detail and wondered if it was for a water jacket, or had contained a water jacket, but had been tampered with. I'd appreciate any first hand information on this model.
Thanks, Roggeo

 

[Corie]

One design feature I hope to be able to incorporate into the plate is to dimple the surface (fire face) or maybe install little tabs as “spoilers”.

Corey, you beat me to it!

I had also been pondering the idea of "turbulence generators" to help the air-smoke mixture burn completely. I've got a long ways to go, but hopefully it'll turn out ok.

Unfortunately, I just got an email from Woodmans Parts Plus that my shaker grates are on back order, which does not make me happy.