Amount of Heat Radiated

I'm not sure that the radiated heat from the stove pipes must be the same. If one stack produces a better draft than the other (once the pipes reach a stable temperature) then more heat will be leaving the stack and not transferring to the pipe. Will a 33% increase of stack diameter be enough for a critical difference, probably not. Then again????

why? do you just enjoy putting people to work solving theoretical problems for kicks?

Well, you finally figured me out...... I come out of that hot @$$ shop for my breaks, get on here and try to think of all kinds of silly $h!t just to waste people's time.... and thank you for your input on these silly questions over the years.

the question id confusing...it almost sounds as if he is using the vent pipe as an heat exchanger to heat a space? in any case engineers back in the 70s were finding ways to save energy everywhere including furnace manufacturers. the old vent stacks in our homes were huge wasters. out of that R&D came combustion blowers, improved exchangers, speed controlled fans etc. the dual walled pipe is now commonplace....now back to our regular scheduled program....

I thought you were testing out homework problems on us before assigning them to the students, the same way Napoleon would have his written orders to his generals read by a fool outside the command tent. If the fool was in any way confused by the orders, they were taken back inside to be rewitten.

Granted, that makes by wonder what you value our opinions for, however, I can accept the ambiguity.

*puts on floppy three-tailed hat and walks off, bells jingling*

Both, one or the other. All depends on what that flame level is. Though the same level in each. At a high flame level the the larger one may radiate more heat. The heated air velocity in the smaller one would have greater barrier level and allow less heat exchange to the surface of the tube. As you lower the flame level on both you would approach a point when they are the same. Lower yet and the smaller tube will radiate more heat.

That is if the exhaust end of those tubes are vented outside.

Exactly right!

Air is an insulator. It convects heat well, but it does not conduct heat well.

If the flame is small and the air is free to move (as 'stovepipe' implies) there will be a small column of hot air right around the flame carrying the heat upward. That column of heated air will be closer to the smaller pipe and thus there will be more heat lost to the small pipe. The larger pipe will barely heat up at all.

If the flame is large, then the heated column of air will reach the walls of the larger pipe and then it's larger surface area will allow it to absorb, and radiate, more heat than the smaller pipe.

I utilize stove pipe both straight and 90 degree elbows for a heater system...... I have been using 8" all these years, but someone suggested going smaller dia. for more heat and less propane.

The whole rigging runs about $450 in pipe and I want to try and not make a mistake on size.

use double wall.its safer with a lower surface temp

NO! The double wall reduces the heat transfer through the pipe. Unless I'm badly mistaken, Netmaker wants to maximize that heat transfer.

The double wall tubing is for use with water heaters and furnaces where much or most of the heat has been removed from the gasses before they enter the tube.

I am going to disagree with most of the answers on here so far.

I think everything else being equal, the velocity of exhaust gasses will be slower in the larger pipe and the surface area larger. Therefore more of the heat in the exhaust gasses will be lost.

If one were to measure the exhaust gas temperature in both cases I am pretty sure it would be cooler coming from the larger pipe.

Short answer.... the 8" pipe will radiate more heat.

That's what I was thinking. The temperature of the exhaust gases when they leave each pipe will be the key. If the fuel was a more complex fuel (home heating oil, kerosene, etc.) than propane there might be a tell tale creosote condensate if the flue stack temperature dropped too much.

I also think that the surface area is the key. How do you dissipate or, inversely, increase the heat flow - increase the surface area. Additionally, the increased mass of the larger pipe should retain and re-radiate heat longer after the system is off.

I tend to agree...in principle.

However, if the propane flame is centre to the pipe (assumed) then the heat transfer to the 8" pipe wall will probably be somewhat less than in the 6" pipe, lowering the exhaust gas velocity, and therefore the heat lost in the flue.

Therefore, and considering the larger surface area, the 8" pipe must radiate more heat.

I'm sure there is a law that I should have considered here....

Troy36 is right. The thermal resistance of the space between flame and walls depends on the surface too. For those who have used heat sinks to remove heat from devices, is a bit more obvious.

Well, we've managed to do it again.

Completely anal and overly technical confusion reins!

<unsubscribe>

I want to generate as much heat with as little propane using THIS set up.With that said, 6" or 8" pipe?????
I am NOT trying to make a truly comfortable room, just knock off the chill and get rid of some of the winter humidity.. Remember, in MY neighborhood, 60F is official winter ( and start of Gumbo season) 40F is dead of winter, ANYTHING in the 30's is Bitter winter and anything white is called sneaux!

classic cogeneration

What is your heat source... are you using something like a propane soldering torch?

If yes, this will be a disappointment. BTU's is BTU's.

This is a different configuration than your original sketch.

Consider that automobile radiators use a lot of thin pipes rather than 1 big pipe to get rid of engine heat. Take a look at how much room you have for the piping, then choose either 6 or 8 inch pipe - depending on which will give you the greatest amount of surface area for the room you have.

Check this out:

http://www.thediyworld.com/Passive-Wood-Stove-Heat-Exchanger.php

I have 8" single wall black pipe now. I was told that 6" would give me as much heat AND I could cut back on the flame itself.... just asking here for thoughts before I buy new pipe....the 8" is rusted and needs to be replaced anyway. Its a $450 expense to re-fit with 8" and slightly less with the 6"....its just a lot of work.

Sorry for any confusion on sketches. I have been in and out of the office and back and forth to the shop.

Why do you need a flue at all? There are self-contained propane heaters often used for temporary space heating, with a 33 kg gas bottle and a ceramic matrix to give radiant heat. The warm air that's left goes into the room. No doubt there is some minimum ventilation needed but it shouldn't be hard to find that out, and your application doesn't sound too critical. That uses all the heat and saves the cost of the flue.

I actually tried one but it was too little heat and it would not carry far enough. This system, though primitive, really warms up my old shop....I am just trying to see if I can cut back on that propane....and it never hurts to ask questions here anyway.

I use a 25 gallon bottle every 15 work days.....with a work day being between 6-14 hours x 6 days a week. The shop never gets hot, just slightly warm and this system seems to kill the humidity issue a little better than a space heater.

Plus, it gives me a place to warm up a pot of gumbo at lunch and my CDM coffee w/ chicory!

Sounds like draughts and a lack of insulation may be a big part of the problems.....they should be fixed so that the heat stays in!!

This will work???? It so simple.

Simple and easy. Fins give you way more surface area than a cylindrical zig-zag, and are probably cheaper and safer.

I think you'd be better off flattening the 8" pipe (once you're away from the burner). So you get more radiating surface (than the 6" pipe) with less cross sectional area (than the 8").

im thinking "fins" are better

...got to vent the exhaust (propane) or we get a nasty smell.....

If flatten it too much we might get leaks.....plus the pipe heats up alomg the ceiling and warms up the rest of shop[ a little.

Understand, we do not get really cold down here....but we are a bunch of candy @$$'$ when it comes to any weather under 40F. ha ha!

Keep this configuration. See Andy #36 about internal vanes. If your pipe was a straight chimney the 6" pipe might be the go ( google laminar flow ) to get the hotter gas closer to the wall. OR add internal vanes to 8". But in this case your bends disrupt the laminar flow and force contact with the walls. The long horizontal section also slows down draught increasing contact time and it is possible that 6" pipe would cause the system to have too little draught. This could be corrected by increasing the height of the final chimney; negating any savings in cost.

In short I think you have hit on the correct setup. If it is oversized you can just add a final restriction on the chimney as Lyn said.

Perhaps you could put a 8" to 6" step down joiner just outside your wall and add a 6" elbow and chimney BEFORE you change anything and see if you can maintain a flame ( that is to say the combusted gas doesn't blowback ) You may be able to economise a little by putting the horizontal section in with 6".

Jim

Understood. Thank you again for the explanation.

Heat source is a propane burner, set up on a stable platform much like a giant pilot light. Propane is outside the shop and fed into the burner via UL listed hose for burners. I do not know how much pressure is pushing the propane. I use the in-line thermometer to gauge. In the first vertical pipe which is now 8" dia. and 30" high, there is a thermometer placed through the pipe and bolted on with the "stem" of the thermometer inside the pipe. I simply regulate the amount of propane until the gauge hits 350F.

So, with that said: 6" or 8" which will be the most efficient????

There are probably better configurations but in this configuration the 8" will be more efficient. (more surface area and lower exhaust gas velocity)

Once you change your configuration from straight path to torturous path you totally change the outcome. What you are doing is creating baffles that slowdown the passage of the hot gases and forcing them to contact the pipe wall at each turn, vastly improving the overall heat transfer due to the elimination of the annular stagnant air space.

My guess is it won't matter which size pipe you use. What will matter is whether you will have cooled your hot gas stream too much. So much so that the natural draft that you are relying on to remove the (smelly) exhaust will become negative with severe negative consequences; i.e., the water vapor will condense and run out the bottom of the pipe, the pipe will corrode, and most importantly the heavier products of combustion will spill into the room (CO and CO2 in particular!

In this case your higher heat transfer efficiency works against you in a deadly fashion. If you choose to extract more heat in this fashion then you may have to investigate the use of an induced draft fan to suck the products of combustion out of the pipe and to ensure a negative pressure in your room. TANSTAAFL and the Law of Unintended Consequences rule in this type of endeavor.

Understood. The 8" pipe has always 'drafted' the CO2 and odor very safely away and outside. I was just curious about the 6".

So, to close this chapter, can I just expect to replace the same 8" pipe and forget about the 6"?

IMHO I like much better the idea of adding the radiating fins on the outside of either size stack pipe. This clever sheet metal work could even be added or removed from the bottom of the stack to draw more or less flue temperature to keep a draft to remove the dangerous part of the gasses from your room.

So, I'm back.

Some points to ponder.

There is truth to the fact that hot air is escaping out the pipe. The amount of heat that escapes depends on how high the flame is and how fast the air is flowing.

The more heat you put into the pipe the more air will go out the pipe, taking the heat with it. Right now, I don't think you are controlling this.

Any air that goes out the pipe from inside must be replaced by cold air from outside.

You want the air to flow out of the pipe to carry the bad gases out with it. But, you want the air to stay in the pipe until the maximum amount of heat has been transferred from the hot air into the pipe.

Air flow through the pipe should be modulated. For this, you need a damper located as near to the outlet of the pipe as practical. This will keep the hot air in the pipe longer.

But, you want enough air flow to keep the air flowing OUT of the room and fresh air coming in.

Buy a cheap cigar and use it to tune things so that the air never gets stagnant and always flows into the pipe/burner.

I think either size pipe will do and some of the finned do-dads will help.

Keep it simple.

You are right...my wood stove in the house has a damper and ...by regulating that gadget we get good heat and no smoke. I can make that happen out in the shop.

The fins seem like a good idea too....

But the damper on your wood stove has a different function. It allows you to regulate the amount of air going into the fire, thus letting it burn higher, depending on how much heat you want radiated from the stove.

In this case your heat is regulated by the flame and not the air and the damper is just to keep the air in the pipe longer to allow more heat to be radiated out.

So, air flow is important not so much to control flames but just to pull all the CO2 and other gasses out of the room.

I always enjoy these sessions, keep 'em coming.

Understood now

But I could still put one in and just make sure it was opened enough to stall the heat from escaping BUT not enough to cause a back up of the exhaust fumes???

That's what the cigar is for. Redneck flow.meter.

All the rednecks around here dip. They could spit in the flue I suppose. I wouldn't put it past 'em. They spit on everything else.

Totally guesswork (I haven't looked at anyone else's answer yet), I would guess that the wide tube would give off more of the heat.

My thoughts are that the wider the tube, the slower the heated air and the larger the contact area. Therefore the heated air would slow even more down as its temperature drops, even longer contact time....

Am I right?

Some homework questions are fun (even if I am actually wrong for some reason!!)

If you add at intervals what would look a vane, but fixed, you will cause the gases to spin and due to centrifugal force, they will tend to "brush" more against the inside walls and give more heat up.

They will also slow down the air movement, giving even more time for the gas to give more heat up.

It would work best in the larger tube.

Put the vanes where they are easy to get at for cleaning, just in case, though they should not get dirty quickly...

External fins would, as many have said, improve things more.

As a youngster, having motor bikes with chromed exhausts, we would wind thick copper wire around the first 18" (?) of each exhaust pipe, to keep the temperature down, to stop the chrome becoming "blued"....it helped.

It was a pig to wind and took two people to do it (sometimes 3!!). I "pre-wound" mine on a slightly smaller tube and removed the pipes (new gaskets each time) to do the work.....but "full throttle" buddies, would still burn the pipes somewhat in spite of the copper wire....

What looked worse,the blue pipe or the copper wire????

I guess that when reselling, the clean looking chrome might make it look like that you never rode with full throttle.....

It was a lot of trouble, I would not bother today (if I had a bike!!), too lazy!!

Here's an idea why not use the propane to heat water in a boiler and run it to a radiator with a fan to get heat in the room, just like you would for a house. That is what I do except I use wood for initial fire and when I am away for more than a day the gas burner takes over to heat the water / antifreeze to keep the concrete drives open, heat the house and garage. It's a good working system for me in the Midwest and boy can it get cold around here. Maybe not as cold as the Dakotas around Tcm's but IMHO it is still cold. Quit a few days in the minus degrees last year and from all the planning of the locals they are looking for it to be as bad this year. Duke

I hope you got a few interesting ideas by posing this question.

my professional background is commercial HVAC contracting. the entire premise is Mickey mouse but some here enjoy looking at almost any problem and start brainstorming.....thats the fun. of all the points made Ram's had the most value because he was eluding to "spill", this is when the byproducts of combustion that fail to rise out of the vent due to poor flow usually caused by restriction in the line or at the vent cap. since these in concentration are harmful they ARE important. so the entire venting topic has been beaten to death. I suggest you just buy a ceramic style heater and scrap the initial design and call it a day

Thank you for the information. I did try a ceramic heater but it did not put out enough heat . I am heating a 30x50 area....not very warm...just killing the chill so-to-speak.

One day, when I can afford ( if the gov. doesn't run our small business into the ground) , I going to have a professional come in here and insulate everywhere like it should be done and then I'll plop me a big @$$ Central AC/Heat unit right outside!

I am very mindful of the exhaust and I am confident that it is venting well. No odor, no dripping, no CO2 build up or my smoke alarm would go off.

Have you looked into something like this?

http://www.waldeneffect.org/blog/Stove_pipe_fans/

This seems overkill for the heat available.

I'm liking the radiating fin idea.

Apparently not...

I searched Wiki for "too much power", it has no idea what I'm asking about.

THAT IS INTERESTING..... Thank you for the idea.

OK.

I've got some ideas to try.

Sorry but the boiling water and radiator idea might be a bit much for my lazy @$$ to get into. Besides, if my wife senses boiling water anywhere, she will start dumping in seasoning and chicken parts for a gumbo.........work day over!

Thank everybody for their ideas. I've got my head wrapped around what I think is going to work now. Yes, I also read in the Farmer's Almanac that it was going to be another cold winter, even down here.

60F =Cajun winter begins, 40F=Dead of Winter, 30F Bitter Cold Winter, 20F(anything) we close down the state until it gets back to Bitter Cold Winter.* ANYTHING white that falls out the sky is SNEAUX.... and we close the state and the Interstate Highway , regardless of amount 1/8", 1/4".... ha ha!

Thanks to everybody again... stand by for the next Silly @$$ Question!

Quick suggestion- google infra red chicken barn heaters. These are basically induced draft versions of what you have.

An 8" pipe has more surface area than a 6" pipe of equivalent length, but the question you should be answering is - what are the temperature differentials between the incoming draft/ outgoing draft and the pipe surfaces at each end?

This will tell whether you have enough overall surface area- more important IMHO than the diameter for this issue. Also note from your drawing that you appear to have a combination of radiant heat (which fins don't really help for) and fan driven conduction (which they do). Reflectors behind the pipe can really help direct radiant heat where you want it.

What????????????????????????

You say, "Also note from your drawing that you appear to have a combination of radiant heat (which fins don't really help for" You can't be serious!

And what is, "fan driven conduction"? Fans move air. Transfer of heat via air is convection, not conduction.

Thermal conduction - Wikipedia, the free encyclopedia

Convective heat transfer - Wikipedia, the free encyclopedia

brain phart for #2- yes, I meant convection (well, technically the transfer of thermal energy from the metal to the air molecule is conduction, but no point in digging myself in further...).

But no, fins don't normally add much benefit to radiation of thermal energy, as they are usually oriented in a manner so that they act as both an emitter and receptor, largely canceling out any benefit derived. If you can orient them properly then yes they will help, but you obviously can't have near as many fins as a system designed for convective (yes, that word again!) heat dissipation.

But, surface area is surface area.

Put this around an 8" pipe and the radiation is not directly pointed at the adjacent fin.

Add fan and go.

2", 3", 4" ??? IN-BETWEEN FINS.

I don't think it matters too much. Heat radiates in all directions out from the surface it is leaving.

More surface, more radiation.

Just make it look pretty.

WOW! netmaker you really know how to get a conversation started! And the hot-water boiler I was talking about can be as simple as a small gas water heater with an old car-truck radiator with a fan behind it. With a small pump to move the water from tank to radiator. My uncle used this cheap system for thirty plus years never failed him and my cousin still uses it to this day. Put a Y or T in the line and you have multiple source's of heat. You just need to set the thermostat on the water heater for the temp you want to put in the air and fan speed to adjust the movement in the room and it can take care of itself from the water heater controls, so you don't create a rocket launch site in the shop. just turn it off when you leave and turn it back on when you get in the shop. It won't take near as long to heat up as your radiant system. Oh one other thing I almost forgot you will need a expansion tank near the water heater for expansion of the water when it is heated. Duke

Duke,

Sounds like a real Ol' Tyme Midwest gizmo.

Works like a champ, when they have wood for the wood stove they run the coils around the stove instead of through the water heater. Using the water heater tank as a storage tank. and when they run out of wood they use propane in the water heater. Works great, their shop is 80x120 the ceiling height is around 25ft. We work out there in our shirt sleeves most of the winter. but there again when I visit my friends in Fla in the winter I'm in short sleeves and no jacket and their in winter jackets, and tell me I am part Eskimo when it comes to cold. Duke

Dear Mr.netmaker,

Under identical conditions of heat added to the body, the higher body i.e., 8" body will have a lesser temperature since 8 inch dia. piece will weigh (8/6)^2 i.e., 77.7% more than the 6" body and the Temperature will be 56.25% to that of the Temp. of 6" body.In other words, if 6" body is 100 deg.C, the 8" body will be 56.25 Deg.C only.

The RADIATION is (T6/T8)^4, where the Temp. will be in Kelvin. where T6 = 100 +273 = 373 Deg.K, T8 = 56.25 + 273 = 312.25 Deg.K, therefore the Radiation will be (373/312.25)^4 = 2.42. Therefore the 6" piece will radiate heat 2.42 times more than the 8" piece.

DHAYANANDHAN.S

If the wall thickness of the 8" pipe is the same as the 6" pipe it will only weigh 33% more.

It is a hollow pipe not a solid body.

Dear Mr.Troy36,

You are right, it is a hollow pipe, which I missed and taken as solid pieces.

In that case, the Level of Temperature, for the same amount of heating will result in higher temperatures, but the radiation Philosophy i.e. (T8/T6)^4 WILL REMAIN SAME, and the radiation will be more.

I am of the view that the Thickness of 8" Tube will be higher than 6" Tube.

Thanks for pointing out my error.

DHAYANANDHAN.S

Screw the flue. Just put more spice in that Cajun food and you'll be all set.