Thermal conductivity of 2" exhaust pipe.

[glort]

I have been hunting round the net looking for a calculator or some calculation I could understand but not being too bright with these things, have come up without an answer.

What I want to know is what the heat transfer rate from hot gas ( say 500oC) is to water ( Under 20oC) through 2" steel exhaust pipe would be. I don't have the flow rates for the gas nor the water but something rough will be very helpful.

The practical situation is I want to know how many feet of 2" Exhaust tube I need roughly to transfer the heat from my 200Kw/ hr Waste oil burner into water to heat my pool and home.  I'd like to get something pretty efficient so If I have a ball park figure I can always add some length to make sure I have enough area.
I realise there are a lot of different parameters but this is backyard engineering rather than pure science so close enough will be good enough in this case.

If anyone could give me an answer in BTU/ Kwh per Ft/ meter for the pipe or point me to a simple calculator or take an educated calculation as to how much length of pipe I need to sink the 200 Kw of energy, I'd sure be mighty appreciative.

[Jens]

I can't help you out with what you request but I can suggest some other things to think about ...

A 2" tube will be highly inefficient at transferring heat like you suggest. The gases in immediate contact with the tube will in fact cool down nicely but what about the majority of gases in the center of the tube ? I suppose you could insert a wavy strip of metal into the tube to induce turbulence to increase the heat exchange rate but how will you clean that contraption. You will have to count on regular cleanings to remove deposits just like any commercial heat exchanger.
Of course this brings me to the question of 'why not use something made for the purpose and rated with the data you want'?   
If your water is 20 C you will have a lot of deposits and condensation happening - be sure to have proper drainage and make sure you build in features for easy 'rodding' of the heat exchanger tubing. This is usually done with a gun cleaning brush on a stick and is obviously quite messy. You will be much happier if the heat exchanger is located outside. On a diesel engine that produced less than a tenth of the power you are proposing, I cleaned my tubes every 100 hrs of operation for maximum heat transfer. That could probably be extended to 200 hrs but I didn't run anywhere near that cold. With a heat exchanger you would run cold water into the cold end of the heat exchanger and run it to the hot end. That gives you most efficient heat extraction.
How are you planning to store the heat in order to be able to use it for house heating ? You can run the heat straight into the pool but you are obviously limited to how hot you can make the pool. Because of that limitation, heating the house will be almost impossible unless your house is equipped with in floor radiant heating throughout that is set up for the small temperature difference (lots of tubing in the floor).
The way this is normally done is that you have a heat storage device (pretty substantial for the amount of heat you are generating ... think 1000 gallons as a starting guess). You run that water through your exhaust gas heat exchanger and heat it to just below boiling (assuming it is not pressurized) and you shut the burner off when this is achieved. You use that water to heat the house (I used water to air heat exchangers in my forced air heating system - the heat exchanger is in the air plenum by the regular heater and the regular heater is backup in case of failure of the hydronic system).
You have a water to water separate heat exchanger to heat the pool. Both pool and house heat exchanger systems are thermostatically controlled to maintain the right temperature and the waste oil burner is thermostatically controlled by the heat storage tank.
Any system that is exposed to freezing temperature needs to be protected with antifreeze. Since you don't want to mess with antifreeze in the large heat storage tank, you will want a small, separate hot water loop with antifreeze to go from the exhaust gas heat exchanger to the heat storage tank.
All loops will of course need circulation pumps.
Be aware that although a small exhaust gas heat exchanger can be used if you are dealing with a large temperature difference, you will have to size things totally different when your heat storage tank swings between 70 or 80 and 100 degrees C.

As you can see, things get quite complicated very quickly but you indicated 'efficiently' and a 2" pipe will definitely not cut the mustard. A proper heat exchanger will be able to efficiently deal with the burner energy and pass it on.

Note that I have not approached the subject of draft for the burner as I know nothing about that subject ... but it will need to be addressed, possibly by providing a much larger gas flow cross-section through the heat exchanger than the 2" pipe.

Good luck ! Let us know what the final outcome of the system will be as I am sure that there are lots of interested people .....

[Ronmar]

Jens is right on, the gas flow as well as the water flow will both try to build laminar flow, and the fluids in contact with the transfer surface will equalize, then act as an insulator to keep heat transfer from effecting the majority of the fluids.  Much like the thin layer of water against the skin in a wetsuit acts as insulation to keep a diver warm.  You need turbulence in both fluids to increase efficiency, but turbulators in the hot gas stream from the oil burner will make it hard to clean if not thought out carefully.  Also as mentioned, you want to try and keep the fluid flows in opposition for best transfer.  You also need as much surface area as you can get away with.

In answer to your question, have you tried Engineeringtoolbox.com ?  Lots of good heat transfer and heat exchanger info there, such as these tables.

http://www.engineeringtoolbox.com/heat-transfer-coefficients-exchangers-d_450.html
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html   That second table the second listing shows a gas at atmospheric pressure to a liquid which probably best describes your project.

Many of their tables list the conductivity in watts/sq meter per degreeC per hour.  Most of the tables show the conversion from this figure to BTU/sqft/F/HR.  They also have an online calculator to convert the numbers for you.
http://www.engineeringtoolbox.com/thermal-conductivity-calculator-d_857.html

If I were trying to build a water heater, I might try forming a coil of 1/2" soft copper tube into a say 5-6" continous coil.  then I would pass the hot gas up the center and shroud in back down along the outside of the coil, with a removable turbulator in the gaps.  But since you mentioned pool water with chlorine?, copper is definitaly out as the chlorine will kill it in short order.  Next would be a steel coil, like that used in the small oil fired steam cleaners.  But that is probably not going to be easilly formed into a coil in the garage...

Good luck.

[Ronmar]

Or another option might be to use something like 3/4" Corrigated Stainless Steel Tubing(CSST) gas line to form a continous coil.  If my math holds up, a coil of this formed around a 6" pipe would use about 1.83' per coil, so a 75' section would form 40 loops with a foot or so pigtail on either end.  40 loops X 3/4" per coil = a 30" X 6" ID tube to blow the hotgas from your burner into, and with an outer ductwork to return the hotgas back down the outside of the coil, you could probably put together a reasonably efficient homegrown collector.  Not having the exact ID and OD of the tube to crunch numbers, I can only take a guess that 75' of this stuff will have between 390 and 700 sq/in of transfer area  At around $170 for a 75' section, not a horribly expensive experiment. Fittings are redilly available and it is made from 300 series stainless so it should resist pool chemicals fairly well.  You can easilly form it by hand around a piece of pipe.  And it is available at a bunch of places(HD, Menards ect) although the 75' section will probably have to be ordered.  About all you will have to do is strip off the outer yellow PVC sheething with a razor knife.  It is only rated for a max of 25 PSI, but if you are just using a circ pump to push water thru it, you should never get anywhere near that pressure...  They also make longer continous lengths, so you could have inner and outer coils and direct the gas flow back and forth down the length of the coils to allow more time for heat transfer and drastically increase your transfer area.

I am looking at a similar project to collect heat from a waste oil burner or a chip burner into a large mass storage tank, so I have had a few ideas bouncing around in my head for this:)

Good luck

[Jens]

The ideas of a coiled tube is all nice and dandy but how will you clean it ? Remember that exhaust gases flow in this tube. The corrugated tube is even worse as all the corrugations will fill up nicely with crud. IMHO it has to be a straight smooth tube (single pass) so that it can be rodded clean on a regular schedule.

One thing that has bugged me since I first replied ..... 200 KW and only a 2" exhaust pipe   I can see 20 KW but even then 2" seems suspect if it has any length to it.

[Dualfuel]

Wow this has been a fun question. I thought it odd that such a large discharge of heat, 200000watts/hr through a 2" pipe could possibly be laminar... turns out that the transition from laminar to turbulent flow is between 1ft/s and 2ft/s. Using the discharge temp as my reference I come up with 12.7 ft/s second to discharge 68000BTUs through a 2" pipe.
Because you picked the exact range where exhaust gases change their heat carrying capacity and density very drastically, I didn't get the length of pipe. BUT....and this is the big one....Can your burner push the exhaust out at 13 ft/s? If not then this question changes drastically. We need to know if your fan will be able to force the gas into the 2" restriction. If not then, we have to use bigger pipe. You have to move 40ft^3/s to realize the 68k BTUs you want. If this was an enigine, it would be no problem because they are positive displacement pumps, but if its a squirrel cage, it might gag on the little pipe and force smoke out the door..hate to say this but, I need more details.

[LowGear]

This one gets my little gray cells bouncing off each other too. 

I think I'd convert to 3 inch copper pipe.  If I  were using the 3 inch (or 4 inch if I scored a deal)  I'd beat it up a little with a soft stroke sledge hammer.  Or two 2 inchers with a cross fitting in the middle to mix things up.

Are you going to pump the water past or rely on some sort of thermosyphon?

Casey

Much like the thin layer of water against the skin in a wetsuit acts as insulation to keep a diver warm.

Really?

[Dualfuel]

OMG, I flubbed a conversion...I am getting some kind of crazy gas velocity...1834 ft/s to move 40ft^3/s through 2" pipe... maybe salamanders do this, or a jet...

Anyhow, the 2" pipe is losing 49 BTUs per foot starting at 932F but gradually transfers less and less as the gas inside cools. By the time its just above 212F its transfering 11BTUs per foot. I would say at that point it would be time to pull the pin and dump the gas before it condenses the steam (and acids).
BPJ

[Dualfuel]

Ok, so I ran into the intergration problem between 49BTUs/ft and 11BTUs/ft..and I am afraid...seeing as I am afraid to intergrate...I would plug and chug with a spread sheet using smaller and smaller intervals... but before I do that, is there anyone who could possibly remember which intergal that one was? Is it like the efficiency of a cooling fin? I think I could answer the question for the guy, by assuming the velocity is transitional, and having the intergal solved...anybody? Buehler?
BPJ

[Ronmar]

Jens, the water would flow thru the corrigated pipe.  The hot gas would pass thru the area formed by winding the tube around a mandrel.  Visualize a large spring...  The area that needs to be cleaned would be rather large and accessible by a chimney brush initially.  The corrigations being perpendicular to the tubing length, when wound around a form and shaped/coiled into a larger tube, would line up parallel to the axis of the larger formed tube.  How it is shrouded would determine how difficult it is to clean.  If the shrouding can be completely removed, it might not be all that bad, perhaps even doable with a pressure washer...  I guess I won't know till I build one:)

Like this:

http://www.ebay.co.uk/itm/KARCHER-HDS-601c-ECO-STEAM-CLEANER-BOILER-HEATING-COIL-/270504320385

[glort]

The outlet of the Burner ( which I just went and measured to be sure) is 40mm.  You can see a vid of the thing Running here:  http://www.youtube.com/watch?v=v0CEk_f0N7E

In the vid the elbow sitting over the top of the thing is 4" to induce some secondary air.  I did another vid of it the other week running horizontally and flat out at about 240Kw/hr and the thing does indeed sound like a Jet.  Hopefully blowing the heat through a pipe right on the outlet and surrounded by a water jacket will quiet the thing Significantly as its far too loud to be running for practical purposes as it sounds in the open.

I am trying to build the HE as cheap as possible which is why I thought of using the exhaust pipe.  I have rung some muffler shops and they tell me they don't have benders that can wind the pipe into a coil so I was thinking a loop ( Curved ends, straight in the middle) or a zig zag would do. I want to try and enclose the fire tube in a 44 gallon drum and pump the water through that.

I thought the bends and direction changing might induce some swirl in the gas and  I was thinking to dent it a bit with a hammer or drill a hole every so often and weld in a bolt so the gas flow is disturbed and makes better contact of the hot gasses with the pipe.  Exhaust Pipe here comes in 6.5m ( 20 ft?)  lengths so I was wanting to get some idea of how many lengths I'd have to use to transfer the majority of the energy into the water which I want to jacket the pipe with.

I don't envisage cleaning being a big problem. The output of the burner is very clean and produces no smoke or carbon buildup due to the thing running an oversupply of air. I was also going to add a second air inlet near the top of the burner to make use of some of the excess air the blower is generating and ensure the cleanest burn possible. Even if I do get some buildup, It wouldn't be hard just to take off the burner and flush the whole thing with water and maybe a bit of caustic if need be.
As I thinking of building the thing, also not much problem just to run it dry and burn any buildup out as it will all be welded metal so nothing to melt or burn.

I have another burner of a different design which I tried running in a 44 gallon drum with a car radiator on the top and an fan sucking through it. I took out both ends of the drum and sat it on bricks so as to try and get some cold air mixing and lowering the gas temp. Even though the radiator was rated for nearly double the output I was creating, I couldn't get it the gas temps low enough and it started melting the plastic fans on the cold side of the radiator and seemed to damage the fins on the rad as well somewhat. I thought something that was impervious to overheating would be the way to go hence the pipe idea.

I don't like the idea of coils in the gas stream much as whatever you put the assembly in  is always going to have more surface area of the container where the heat can escape, insulated or not, than the area of the tubing you want to pick up the heat.  Plus in this case, when using something like 1/2 of 3/4 tubing, you'd need 100 miles of the stuff to absorb that much energy and copper pipe is BLOODY expensive here.

I'm very open to different ideas with this as long as it's relatively cheap and simple for me to build.
It seems I can build burners with no trouble at all, I have lots to learn about heat exchangers though which will allow me to put the heat to the work I want it to do!

[Jens]

Jens, the water would flow thru the corrigated pipe.  The hot gas would pass thru the area formed by winding the tube around a mandrel.  Visualize a large spring... 

Ahhhh, I see ..... yes, that would easily clean with a pressure washer but I am not sure how efficient it would be. I could see having a loose (removable) baffle type thing in the middle and a shroud on the outside and that could work very well and be extremely easily cleaned. Cool (hot) idea

[Jens]

The outlet of the Burner ( which I just went and measured to be sure) is 40mm.  You can see a vid of the thing Running here:  http://www.youtube.com/watch?v=v0CEk_f0N7E

In the vid the elbow sitting over the top of the thing is 4" to induce some secondary air.  I did another vid of it the other week running horizontally and flat out at about 240Kw/hr and the thing does indeed sound like a Jet.  Hopefully blowing the heat through a pipe right on the outlet and surrounded by a water jacket will quiet the thing Significantly as its far too loud to be running for practical purposes as it sounds in the open.

I am trying to build the HE as cheap as possible which is why I thought of using the exhaust pipe.  I have rung some muffler shops and they tell me they don't have benders that can wind the pipe into a coil so I was thinking a loop ( Curved ends, straight in the middle) or a zig zag would do. I want to try and enclose the fire tube in a 44 gallon drum and pump the water through that.

I thought the bends and direction changing might induce some swirl in the gas and  I was thinking to dent it a bit with a hammer or drill a hole every so often and weld in a bolt so the gas flow is disturbed and makes better contact of the hot gasses with the pipe.  Exhaust Pipe here comes in 6.5m ( 20 ft?)  lengths so I was wanting to get some idea of how many lengths I'd have to use to transfer the majority of the energy into the water which I want to jacket the pipe with.

I don't envisage cleaning being a big problem. The output of the burner is very clean and produces no smoke or carbon buildup due to the thing running an oversupply of air. I was also going to add a second air inlet near the top of the burner to make use of some of the excess air the blower is generating and ensure the cleanest burn possible. Even if I do get some buildup, It wouldn't be hard just to take off the burner and flush the whole thing with water and maybe a bit of caustic if need be.
As I thinking of building the thing, also not much problem just to run it dry and burn any buildup out as it will all be welded metal so nothing to melt or burn.

I have another burner of a different design which I tried running in a 44 gallon drum with a car radiator on the top and an fan sucking through it. I took out both ends of the drum and sat it on bricks so as to try and get some cold air mixing and lowering the gas temp. Even though the radiator was rated for nearly double the output I was creating, I couldn't get it the gas temps low enough and it started melting the plastic fans on the cold side of the radiator and seemed to damage the fins on the rad as well somewhat. I thought something that was impervious to overheating would be the way to go hence the pipe idea.

I don't like the idea of coils in the gas stream much as whatever you put the assembly in  is always going to have more surface area of the container where the heat can escape, insulated or not, than the area of the tubing you want to pick up the heat.  Plus in this case, when using something like 1/2 of 3/4 tubing, you'd need 100 miles of the stuff to absorb that much energy and copper pipe is BLOODY expensive here.

I'm very open to different ideas with this as long as it's relatively cheap and simple for me to build.
It seems I can build burners with no trouble at all, I have lots to learn about heat exchangers though which will allow me to put the heat to the work I want it to do!

Woaaa .... way out of left field for that one. A most impressive burner indeed, totally different than what I had in mind! ... and very cheap to build!
Of course what we are seeing here is probably less than 1% efficient and probably requires constant supervision but definitely interesting!
Running the exhaust through a bunch of elbows in a 45 gallon drum and at the speed of gas flow we are seeing would probably result in lots of turbulence and laminar flow would not be an issue.
I suspect (but don't know) that no matter how you slice this, efficiency will not be stellar but with input energy being free or very cheap, this should not be a big concern.
If I were in your shoes, I would first try and establish what happens when you put a significant amount of pipe on the exhaust. Will the burner still work as well ? What will the noise be like ? I would also contemplate what Ronmar posted - introduce the exhaust into the bottom of a 45 gallon drum and take a soft 3/4" copper pipe which should be easily bent in a big loop, maybe use a 30 gallon drum on the inside to plug the central hole but fill it with water to absorb additional energy.
I am not sure if I am describing it well and I don't know how to do diagrams ... let me try again ...,
Take a 45 gallon drum, forget the elbow on the output of your burner, set it in the middle underneath your drum, and blow the exhaust straight up into the 45 gallon drum. Set a 30 gallon drum into the center of the 45 gallon drum on some fire bricks, maybe reinforce (welded plate?) the bottom of the 30 gallon drum that is receiving direct flame. The exhaust gases will hit the bottom of the 30 gallon drum (that is filled with water) and will deflect to the sides and forced up through the gap between the 40 an 45 gallon drums. BTW, I do not know how the diameter of the two drums compare and if the gap is suitable, Anyway, to continue, you have one or more large coils of soft copper curled in the gap with water running through it/them and the exhaust of the burner running on the outside of the copper pipe in the gap. I hope that makes sense .....
The water in the 30 gal drum could be preheated water that then runs through the copper pipe and on to your storage facility and back into the 30 gallon drum. The 30 gallon drum should be closed to avoid evaporative losses. There would be very little if any concern with exhaust back pressure because the effective exhaust diameter would be huge. Additional efficiency could be achieved bu insulating the outside of the 45 gallon drum or somehow recovering heat from this surface as well but that could be added later on. If necessary and deemed worthwhile, the hight of this contraption could be doubled to two drums high bringing the noise and any exhaust smell higher up as well as increasing efficiency at the loss of ease of cleaning.
The more I think about this the more I like it enormous possibilities .....
One could add fins on the outside of the copper tubes in the colder areas (top) to absorb more heat. These are, I believe, commercially available preformed and slide over the pipe and ran be (hard/soft?) soldered to the pipe. Don't know how cleaning would be effected.

Wow, so many thoughts, so little time to write them all down ...

[Dualfuel]

Hi, thanks for the video...the easiest thing to do with your technology level is to find a used boiler, or gas fueled water heater and run the burner into the boiler's firebox. Failing that...use or make some sort of water tube boiler. Go look at the heating system in a steam pressure washer. The fire plays over coils, in which, the water circulates.
With what I see on the video, your problem is going to be welding all that pipe together and not having it leak. So look for something already built.
BPJ

[Ronmar]

Glort
   That is why I was suggesting the CSST.  It is used for gas installations all over the world and is fairly inexpensive.  A 75' piece is $170 US and will form a coil 6" in diameter and 30" long(40 coils). A 250' section is around $390 us, and would allow you to form a multi layered coil.  In answer to your outside surface area being a large heat loss, you simply wind a final coil as the outside then insulate around the outside of that.  A 250' section would get you a 24" long heatex(32 coils) with 3 layers of coils one inside the other. One with a 6" ID, the next with a 9.25" ID and the last with a 12 3/4" ID.  The hotgas would pass down the center of the innermost coil, then be turned back down between the first and second coil to pass alsong that passage.  Back at the starting end it would again be turned back to pass bewteen the second and third coiled tube, with the third coil forming the outer layer along with an insulated skin.  This would keep the hotgas surrounded by water.

I suggest the coil along the lines of the steam cleaner boilers.  There is a reason they do this as a continous tube.  Welded structures don't heat cycle well.  Shell and tube boilers are not welded, they are swaged to allow for expansion and contraction of the structure.  When you weld, the tubes will grow in length when heated and wear on that weld hardened union and cause cracks and leaks.  Using coiled continous tube, the tube coil free floats and can expand and contract at will without failure.  If my math is correct, 250' of CSST should be around 112 SQ/FT of transfer area...