Recuperator plans?

[Cecil McKenzie]

I hope to build a recuperator to use with a Therm Jet 400 burner. I have a free piece of unknown type of stainless steel pipe . Inside diameter just shy of 3 inches about 1/4 inch wall thickness. Will this be big enough for exhaust to pass through easily?

Any suggestions for outside pipe. I have seen some that were not stainless but is that a bad idea? It doesn"t seem like it would need to be that thick . I think the therm jet can use hot air up to about 700 F.

What temperatures have been reached with simple designs? I don't want to go to great lengths to build something where I will have to dilute the air after it comes from the recuperator. . I am willing to make it more complex if that is needed to get into the 700 degree F. range.

The design I am contemplating is a pipe inside a pipe ,closed off top and bottom, intake at top , hot air out the bottom. Any thoughts will be appreciated and thanks to Steve Stadleman for information on used stuff from Arden.

[Brian Gingras]

check your thermjet specs, I believe the upper limit for combustion air into it is around 600-700.

[Cecil McKenzie]

Brian thanks. On one site I saw that the therm jet would take air from 300 to 799 degrees F. I think they have ceramic, silicon carbide and stainless steel interiors. I assume the stainless steel will take the hottest preheated air but i'm not positive about that.

I spoke with someone today about high pressure water blasting through stainless plate. He said he could make a hole within thousandth of the diameter of the interior pipe. I thought that I may be able to get away without welding if they can really do that. I could make the outside pipe a few inches shorter then hold the end pieces together with threaded rod .


My main concerns are is a three inch pipe enough exhaust and if a five inch pipe would be big enough
for the outside or would a six inch pipe be better?

[Brian Gingras]

ok, rule of thumb 1sq inch per 10k BTU = 40sq inches

3" pipe = 14.8 sq inches.
5" pipe = 24.64 sq inches.

Someone jump in but it seems to small to me. then with one inside the other it's tiny.

[Hugh Jenkins]

The general rules of exhaust volume change with recuperation. What size furnace are you building? Simple metal tube in tube systems work up to a point, but 700 degrees is not a problem. I would suggest using a stainless wood stove stack of 6" ID as your outer tube. The extra volume means longer residence time and better heat exchange.

Give more info and I will be more specific.

[Kraig Richard]

Size of an exchangers outside pipe is directly ratioed to performance, and limited by what kind of blower pressure available.

The closer the walls are to each other the closer the air will be to the core pipe where the actual exchange takes place.

A dead leg hanging inside the core (perhaps a 1 inch pipe?) puts the flue gas next to the wall where the actual exchange is taking place.

A three inch core diameter is pretty small and would probably not benifit from a core plug. Unless this is a really small furnace.

The use of spirals in bot h the flue and air cavities is alleged to enhance peformance.

The outside pipe can be of common steel as it sees no exteremes. go with whats available cheap and weight considerations.

Longer tubes can further enhance exchange potential.

[Charlie Correll]

The rule of thumb of 1 square inch for every 10,000 BTU input for exhaust sizes is a number that is useful for natural draft furnaces. Most of us now use fairly powerful blowers, and the inside of our furnace is somewhat over atmospheric pressure, rather than the slightly negative pressure required for a natural draft system. This overpressure keeps the heat in the furnace a little longer, rather than having the natural draft suck it out before the maximum heat is transferred to the furnace. My outlet size from my 300-500 pound furnaces is ten square inches, more than enough. At the top of my recuperator stack, the exhaust outlet is about two square inches, because at that point, the exhaust has cooled down from contact with the recuperator, and thus has contracted by a factor of about 2.5 according to the Ideal Gas Laws. Holding at 2100, the exhaust temp at the flue hole is slightly above that, and by the time it get to the top of the my stack, it’s 400 degrees plus.

Surface area is the key. At no point in my stainless part of my system is the incoming air more than 1/4 inch away from the exchange surface. The surface area on my stainless part is 1560 square inches, with an additional 270 square inches of exchange area consisting of silicon carbide. This initial ceramic heat exchanger cools off the gasses sufficiently to protect the stainless recuperator.

More exchange surface means more heat in the incoming air. Depending on the furnace size, a three inch pipe, with a cross sectional area of about 7 square inches should be enough for high fire. At low fire, cover part of the top so heat doesn’t drift out unecessarily.

[Kraig Richard]

Originally posted by Charlie Correll
.

........ At low fire, cover part of the top so heat doesn’t drift out unecessarily.


Back pressure and dampers are both under-discussed and under appreciated.

[Hugh Jenkins]

Charlie, great to see you jump in! What was being asked was about fairly low efficiency exchanger. Going for maximum heat recovery requires a very different burner system than the thermjet.

If that burner is to be used, you may have to be careful not to overheat your air. One reason that I do not recommend a simple metal outer tube is that you have a very hot surface that is not only losing heat but is potentially dangerous. The insulated wood stove stacks will hold the heat and stay much cooler on the surface. They come in lots of different lengths. Air in and out adapters will have to be made.

On the theoretical side, air flow velocity and volume are major issues as well. If the volumes are too small, the velocity is high and residence time is short so the exchange is reduced.

[Charlie Correll]

Originally posted by Kraig Richard
Back pressure and dampers are both under-discussed and under appreciated.

YES! Back pressure is the main variable which allows auto control systems to work at all. And why leave all that space for all the heat to leave when you are on low fire?

While I'm at it, pressure (back pressure is the proper term) does not equal flow (volume). Say you have a blower sitting in the middle of the room, blowing out air. Put a pressure gauge in it, at a right angle to the flow and it will read close to zero, or maybe even minus. Max flow, no backpressure. Then put your hand over the outlet, blocking flow, and slide the pressure gauge between your fingers. It will read high pressure, so it's max pressure and no flow.

[Charlie Correll]

Hi, Hugh!

Yes, Velocity is important. High velocity is better heat exchange, per se, because of a higher temperature gradient over the thickness of the exchange surface, and a thnner dead air barrier against the exchange surface. but then you need a VERY long heat exchanger. For our purposes, maximizing time in the recuperator for both exhaust gasses and combustion air is more important, so we lower velocity with wider chambers, still trying to keep them small enough for efficiency, but not too small, so things don't clog up. It's a practical matter.

Low temp recuperation helps. Anything helps. Money out is less on the equipment, but savings, both in money and CO2 output, are much less than in high temp recuperation. And fuel prices are only going up.

[Cecil McKenzie]

I am trying to retrofit an extant furnace. I have been using a 130 lb. crucible. In plan the chamber is 28 inches in diameter with one quadrant squared off so burner and exhaust are on flat surfaces. The interior is just over 10. cubic feet. Interior wall is 3 inches of Mizzou castable surrouded by 8 inches of fiber. The floor schedule was one layer of red building brick, 3 inches of insblock 1900 F. , two layers of 2400 F. soft brick, three inches mizzou.

One thing I did to try and save fuel when I built this furnace was to make the crown about 6 inches above the glass line. Glass is gathered through a throat that opens to give access to glass. The door is like on a Denver furnace counter balanced and either open or closed. Despite being 20 years old it is still in good shape .

I have purchased a 400 T. BTU therm jet with an eclipse blower and control system . It was not used with a recuperator so I have to add a stainless interior so I can reclaim some heat and hopefully save some money. All of this has been done with the idea I may be grasping at straws. If the price of propane continues to rise the savings may still not be enough. Electricity seems like it really may be the way to go but I don't have the time or the cash to make that big of a change before I need glass.

Hugh I hope added detail will help give you a more adequate picture of what I'm doing. I think you have a pretty good grasp of where I'm coming from. It sounds like I should be able to get 700 F. with out too much trouble. I hear you on the fire hazard aspect. My furnace is outside so I can see leaves blowing up against the stack.


Charlie... you are completely correct that it would be better to recuperate at a higher temp but I didn't feel like stepping off into both recuperation and burner design at the same time and necessarily expect it to work. Thanks for the input on sizing .

[Charlie Correll]

Cecil,

I understand about trying to get some recuperation and fuel savings without going off the deep end. Any preheated air helps. Nozzle mix, like the thermjet or mine is the way to go to prevent backburning. I think the 400K BTU burner is oversized for the furnace you have, but it should be nice and quiet! I melt my 200 pounder on 186K BTU at highest fire with the recuperation. If the air gets very hot in the 400K burner, you might get some clinkers, that is, black carbon deposit from cracked gas (see other thread, Carbon Chunk). But you are going to stay reasonably cool. The pipe in a sleeve will work, especially with the insulated stove pipe that Hugh suggests. The bottom is gonna get hot. Perhaps you can make the bottom foot or two out of refractory to help cool the exhaust by the time it gets to the pipe.

This is a free standing pot in there?

[Hugh Jenkins]

Cecil, what size is the current burner hole in the furnace wall? There are ways to get both burner and exhaust side by side if there is enough room. I am getting to be fairly good at retrofitting, but there are always surprises in any home built equipment. The flat inner wall you mention does make it easier.

Another design issue is gas pressure. What is that for you?