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Copper tube Options
davisonh
#1 Posted : Saturday, February 09, 2008 9:12:49 PM
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It's not advisable to install a copper coil directly inside a woodburning firebox because the fire can and does pass the melting point of copper and yes it will crack and stress but the main reason not to is because water will end up boiling inside the coil too quickly,possibly causing an explosion.The average temperature inside a running firebox with the door closed is 400-1000 degrees and higher with the fire running hot.Its a much better idea to run the coils on the outside of the woodstove and to insulate the coil before running to the storage tank.You'll definitely need a T&P valve and a tempering valve,that water'll leave the coil @ around 190 degrees or so,which is space heating temperature.You'll want the water temperature between 120-140 degrees max,190 degrees will tear your skin off.A tempering valve mixes cold water in with the hot to make it the right temperature.You'll also need a small expansion tank and I would pipe it at 3/4 inch.You'll need some way to control the fire also because you run the risk of boiling the water keeping the woodstove running.

Jeff@Restoule
#2 Posted : Tuesday, February 12, 2008 9:21:48 PM
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Would a flat coil of copper tube on the top of the stove, with some sort of insulating material sheet covering it to prevent heat loss, be a functional and safe method ?

My goal is to reclaim the maximum heat from the burned wood.

 

This will be an open system circulating the water through the heating coil and back to a solar storage tank perhaps 1000-2000 gallons in size. As long as the water is not boiling, the temperature should not matter as it will mix with the cooler water in the tank until the tank reaches its max setpoint(?) temperature.

 

I imagine a thermostat in the tank and a circulating pump would be in order but I might be tempted to make a self-circulating system by using a check valve before and after the heating coil.

 

I have found some stainless steel boiler tube on the internet. Could this be expected to survive a wood firebox ?

 

Are there any good resources for homemade boiler systems ?

If so, could someone please point the way.

 

Thanks

Jeff

 

davisonh
#3 Posted : Wednesday, February 13, 2008 1:40:32 AM
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OK first off in order for any coil to be able to handle the stresses involved in a firebox it will have to be of substantial thickness, .10 ga or higher in order to handle the weight of the wood in the box being burned because it'll be piled up against the inside of the firebox and be able to handle that stress under extremely high heat conditions of 1000 degrees and over.Yes a woodfire gets *that* hot and even then some.A wood fire is as hot (and sometimes can be hotter than) an oil burner or gas flame.The coil has to also handle the stress of cold water expanding and contracting extremely quickly.This is why normally coils are not used inside fireboxes but are used in water jackets surrounding fireboxes and thats par for the course for any boiler,gas wood , oil or coal..High carbon steel 1/4" thick or thicker is recommended firebox material for it can handle heat stresses of over 3000 degrees and expansion and contraction also.Stainless tends to get very brittle over time because of the nickel-iron-chromium content of the steel.It will eventually crack from the heat and not from corrosion.Carbon steel will not.The boiler tube you mention normally is used inside a water jacket or steam chamber for heating domestic hot water or process hot water either in a hot water or steam boiler.Yes thermosiphoning can be used with 2 check valves but you will not get the rapid heat transfer that you'd get from using a circpump and and HTX.(heat exchanger)I would go with a Taco 007 or 009( about 1-3 gpm),dont worry about the expansion tank I mentioned earlier,the storage tank you mentioned going to atmospheric pressure will suffice but you will want some way to shut the fire 'off' when that temperature hits max setpoint and oh yes it will heat up 2000 gallons of water in pretty quick order,my boiler heats up 175 gallons of water from 68-70 degrees to 190 degrees in a couple of hours with an average fire going.This is why I say you'll need a T&P valve and maybe a homemade sidearm heat exchanger (shell and tube HTX is another word for it)to heat your domestic hot water.Yes you will get all that from a common box woodstove.The way the fire in my boiler is controlled is with a spring loaded damper connected mechanically to a solenoid coil so that when the setpoints reached the damper solenoids' deenergized and the spring pulls the damper shut.Everybody asks me if the fire goes out when that happens and no it does not.It will if the fires' running very poorly or I lose power during the day and the solenoid does'nt reenergize in time to reheat the coals.Because the boilers' insulated even when the boilers' off the fire is still extremely hot(I measured 400 degrees in there 2 years ago with the damper shut!)so that when air hitsthe coalsthey reignite quickly.This may be what you'll want to do with your project.To get a better idea on your project go to any wood boiler webpage,the one I use is www.centralboiler.com and that'll give you a good rough idea on how to design your woodstove.
Jeff@Restoule
#4 Posted : Thursday, February 14, 2008 5:21:00 PM
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Thanks for all the information.

I was considering using stainless steel boiler tube because it wouldn't rust and contaminate the system. The stove is rectangular so I would drill holes through the sides, just under the top, and run the straight tubes through the holes such that they enter one side and exit opposite.

The outer ends of the tubes would be threaded and connected with copper tube and brass fittings to allow some flexibility. This would put the tubes above the fire so there would be no mechanical loading except for supporting their own length when filled with water.

Would the tubes be safe from thermal shocking as long as water is running through them all the time that the fire is on (turn on the pump and let it run until the fire goes out). Could a klixon switch be used to open the pump electrical circuit once the fire box has cooled below a set temperature ?

If the temperature would still be too high could I add a diffuser plate below the tubes to prevent direct exposure to the flames.

I have attached a sketch of my plan. Please excuse, CAD skills do not seem to translate well to MS Paint.

Thanks again

Jeff.

davisonh
#5 Posted : Friday, February 15, 2008 2:23:46 AM
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Yes you are definitely going to want the diffuser plate below the coils,the direct exposure to flame will be too much for stainless to take for very long before it becomes brittle and cracks.The thickness of the tube walls is the issue even with water running through them.You'll have a leak the first time you fire it,the expansion issues of the stainless will be too great.What it is is the thickness of the metal and the rate of heat transfer to air,for example if you leave the damper on a barrel stove open too long the sides of the barrel stove crack and warp from the heat because the metal is too thin ,it does'nt have enough mass to hold up to the amount of heat energy involved in a wood fire.   There has to be a deflector between the coils and the direct flame.This way the coils will be heated by superheated air and not direct flame.The deflector should be made out of 1/4" carbon steel stock.Yea the klixon will work but the best way would be an aquastat mounted in the return line(the input line)either a strap on aquastat or a dry well.Dont do a wet well they dont take expansion and contraction very well over time.

The water temperature should be 190 degrees maximum,even with the water running thru the system all the time at 3 gpm you're still going to want some sort of emergency shutdown because 2000 gallons of water will heat up pretty fast respectively. 5/8 tube hooked to 3/4 copper lines'll work ok.You'll be more worried about shutting the fire off if the water gets too hot,definitely a temp controller in the storage tank with a t&p valve is a must.Good luck!

Jeff@Restoule
#6 Posted : Tuesday, February 19, 2008 6:02:06 PM
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Thanks again.

I will have to find some thickwall, lowcarbon steel pipe. I wonder if that black steel waterpipe that is sold at the building supply store would be sufficient ?

Would you recommend a water filter before the pump to remove any rust created by the steel pipe ?

 

Jeff.

 

davisonh
#7 Posted : Wednesday, February 20, 2008 12:14:54 AM
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Yup,that sounds like your best bet with the black pipe;as far as the filters' concerned the best thing you can do for system water is first test the acidity and the nitrite level of your system water.If the waters too acidic you'll have to neutralize the water down to 6.7 ph or so.The nitrite level should be around 2000 ppm.Then I would add a gallon per 150 gallons of of boiler corrosion inhibitor.No hot supply line should have a filter in it no matter what!Dangerous to do so because you could lose your flow.
davisonh
#8 Posted : Sunday, March 16, 2008 1:33:22 AM
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Again I must reiterate the safety issue when it comes to working with steam & boiling water systems,for we had an incident where I work last week where a few lives,mine included could have been lost.We have in our plant water heating load jigs we use to throw a heat load across the equipment we test.Well when they were designed the T&P valves were not installed because they have a tendency to leak when the pressure diaphragms wear out from use.Well,I got called away from the testroom to repair equipment down the production line as did my boss and my coworker to go work on other equipment.Well,the flow switch thats' supposed to sense when there's no water flow failed and pressure built up to,we're guessing over 2-3000 PSI at well over 900-1000 degrees(it melted the solder joints!)and the jig blew up.Everywhere,through the walls,ceiling,etc.Thankfully no one was in the testroom at the time and it was relatively simple to fix again(and yes we put in a T&P valve!!!)and a new flow switch installed,though I am sure 10-15 men and women had to change a few intimate garments after that jig let go(I heard it go 500 feet away from the testroom!),so please,please install the correct safety equipment when  working with high pressure equipment or high voltage equipment also..
practicalman45
#9 Posted : Friday, April 18, 2008 6:04:01 AM
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Here is a link to a company that sells stainless steel coils for adding to woodstoves.

http://www.therma-coil.com/order.htm

There is a plumbing installation page too at the same site.

http://www.therma-coil.com/plumbing.htm

 

I make my own stainless steel coils and install them into woodstoves I make. They are similar to the "W" coils this company sells except I weld the corners together.  I send folks to this site for safety installation info.

Jeff@Restoule
#10 Posted : Friday, April 18, 2008 4:53:24 PM
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What grade of stainless are you using to make your coils ?

 

Jeff.

 

 

practicalman45
#11 Posted : Monday, April 21, 2008 7:32:43 AM
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If I buy new material for the coils it is 304 series stainless.  It measures 1" o.d., and has .120" wall thickness. Looks similar to standard 3/4" pipe, but it is slightly larger. (this is spendy material...)

 

  For connection fittings I buy 3/4" sch. 40, 304 stainless pipe nipples, cut the threaded ends off, and weld them onto the ends of the coil. This type of pipe would also work well for the whole coil if you can get long nipples of it, especially if it is already threaded nipples (regular pipe threaders cannot cut threads into stainless steel).  You need the 3/4" pipe thread ends to allow screwing on the dielectric corrugated copper flex water heater hookup tubes.

 

When available, I've also used recycled boiler tubes. These are about 1.375" o.d. and .065 wall thickness. Not sure of the exact designation, but I'm guessing that its a 300 series also. (304, 308, or 316)

 

Tack-weld the tubes together, and then weld a mounting bar across the whole set of coils before welding to keep them from warping when welded.  Stainless warps more than steel. I'll use 1" x 1/2" mild steel bar to weld across them, and sometimes a #4 rebar between the ends. The bracing can be regular steel for economy. Drill a hole in the brace to allow bolting the assembly inside the stove.

Tig welding with stainless steel rods is best,  but a mig welder with regular steel wire (ER70S6) can do an acceptable job. Its a lot quicker and easier than tig, just get good tight fits on your miter joints and use a series of overlapping tack welds. Before using any welded coils pressure test them with air and soap suds.

Jeff@Restoule
#12 Posted : Friday, November 28, 2008 4:25:01 PM
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The mild steel or "black" pipe, as would be purchased from a hardware store, seems to have a welded seam.

For this purpose, would it be advisable to search out a seamless pipe ?

Also, what schedule would be most appropriate ? The hardware store variety appears to be a schedule 40.

Would a diffuser plate, between  the flames and the pipe, still be necessary if using mild steel pipe of sufficient schedule or wall thickness ?

 

Thanks again.

 

FatherSea
#13 Posted : Thursday, December 11, 2008 10:57:37 AM
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Superb idea!! great advice!
practicalman45
#14 Posted : Monday, December 22, 2008 1:25:00 PM
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Here's another company now making stainless coils mandrel bent from one continuous length of stainless tube. They can make custom designs for you if you contact them. Their kits look pretty good and complete. Wish that I had the equipment to make those bends like this. Pretty hard for me to compete with the quality and value that these guys offer.

http://www.hilkoil.com/product.htm

davisonh
#15 Posted : Tuesday, December 23, 2008 1:21:59 AM
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Uhhh boy,I should'nt mention this but umm,my co. has a coil rolling machine Pman..I'll try to describe it.It'll roll from 1/2 inch to 1 inch s/s.I am extremely familiar with this machine,lol.It's an old(and I mean OLD, nameplate is 1925 vintage)Buffalo Forge heavy angle iron bender,a serious monster of a machine.Was run orginally off a leather belt from a ceiling drive in the old factory building we used years ago.We sent it to a machine shop 40 years ago to be rebuilt as a coil rolling machine then set it up on the ceiling drive in the old factory building(The ceiling drive in this old factory building was *water* driven guys, we had a factory pond in the old building;had a turbine driven 250 kilowatt AC generator too.From what they tell me I gues sthey had to adjust the amount of water going thru the sluice gates to adjust the amount of horsepower needed)Well anyway this machine is incredibly accurate.Our coils we roll up to 190-250' length and 1.5-3' in diameter normally and is used constantly every day.If you see an old angle iron bender Pman consider redoing to do coils..might be worth it..
Jeff@Restoule
#16 Posted : Tuesday, December 23, 2008 1:21:59 AM
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Posts: 134,494

I am considering adding a coil of copper tubing inside the firebox of my woodstove to heat the water in my storage tank.

I am concerned about the durability of the copper when exposed to direct flame and heat.

I know that copper has a high melting point but I have heard that it will harden and crack from such exposure.

 

Could anyone please relate their experiences .

Also, if anyone has any information regarding appropriate tubing diameter, wall thickness, length of coil and flow rate of the water, it would be very helpful.

 

Thanks

Jeff.

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