Stirling Engine with wood fired oven exhaust.

[kjramkunwar]

Hi all,

I came across this community while I was searching for different ways to use residual heat from my food processing operations. I found the discussions here very useful and exciting.

Now coming to the problem to adress -

We run a food processing buisiness and have an oven that is built upon an insulated J tube. This means , we have temperature around 1000 degree Celsius in the burn chamber and around 800 degree Celsius in the dome ,where we manufacture smoked/ roasted veggies. After cooking, the residual heat that escapes through flue is around 400 degree Celsius. The exhaust is mostly carbon dioxide and water vapour. Now that's a lot of heat to let escape.

With that much of heat, what kind of useful work be generated through a Stirling engine? What type of stirling engine will be suitable ?

I am a machinist by training, so I shall be able to make a Stirling Engine right from scratch, without the constraints of using off the shelf / available scrap cylinders and pistons. What I need is a workable design or at least some design guideline to start with.
Can the community help me here?

[Tom Booth]

We run a food processing buisiness and have an oven that is built upon an insulated J tube. This means , we have temperature around 1000 degree Celsius in the burn chamber and around 800 degree Celsius in the dome ,where we manufacture smoked/ roasted veggies. After cooking, the residual heat that escapes through flue is around 400 degree Celsius. The exhaust is mostly carbon dioxide and water vapour. Now that's a lot of heat to let escape.

....

Are you sure about those temperatures? Even just 400° Celsius is hot enough to melt lead. 800° C would melt aluminum. 1000° C is near the melting point of copper, just to roast vegetables?

[Tom Booth]

Is this a brick/masonry J tube type rocket stove? If that is the case, I can understand generating those kind of temperatures. Cooking indirectly above the stove in some way.

If that is the case, and those temperatures are right, is it possible to retrofit the hot side or end of the Stirling directly into the burn chamber?

Generally, the higher the heat, the better for the engine, and I don't think this would result in much, if any, reduction in temperature at the cooking area. Not with a wide open draft.

Getting enough heat to run any kind of Stirling engine shouldn't be any problem.

I would, in some way, if possible, also utilize the cold ingoing draft air for cooling the cold side of the engine.

[kjramkunwar]

Yes, it's masonary refractory j tube rocket married to a dome oven .

The burn chamber cannot be disturbed as any obstruction to the air flow in the burn chamber leads to inefficient burning = more wood required for same amount of output. So retrofitting the hot end in either the burn chamber or the heat riser are ruled out.

Probably the hot end can be incorporated in the dome - little above the opening of raiser . This opening is towards the rear of the oven . So the retrofitting a Stirling hot head here can be considered.

Alpha type may be !! Hot cylinder built into the oven and cold cylinder outside the insulated rear wall of the oven.

[kjramkunwar]

....

Are you sure about those temperatures? Even just 400° Celsius is hot enough to melt lead. 800° C would melt aluminum. 1000° C is near the melting point of copper, just to roast vegetables?

Yes, these temperature are possible with a rocket oven design . We do not use the direct flame to roast vagies. We will charr them if we do in fact we did charr a few initially.

The high heat is absorbed in the insulated refractory dome and roasting happens with radiant and convection heat indirectly.

We save a lot on the wood fuel by investing in refractory construction.

[Tom Booth]

...
The burn chamber cannot be disturbed as any obstruction to the air flow in the burn chamber leads to inefficient burning = more wood required for same amount of output. So retrofitting the hot end in either the burn chamber or the heat riser are ruled out. ...
...
Alpha type may be !! Hot cylinder built into the oven and cold cylinder outside the insulated rear wall of the oven.

The engine need not necessarily block air flow. Depending on the design of the stove and engine.

What I had in mind was something like:

IMG_20220117_140623468_20220117140845477.jpg (314.87 KiB) Viewed 5309 times

In other words, the hot cap of the engine would just be part of the back, or whatever wall of the burn box where the highest heat, direct flame would hit.

The cooler temperature at or near the oven would likely work, but less heat = less power of course.

[Tom Booth]

As far as utilizing the cold draft, I had such an idea years ago as a modification to my wood stove. I have yet to give it a try, but the steel stove I thought I could rather easily modify for that project has recently been replaced by a coal stove, so, I may get a chance to do some experimenting with it soon as that stove is no longer in use.

This was a sketch of the idea:

stirling_stove.jpg (26.34 KiB) Viewed 5307 times

The idea is just to be able to close/block the feed door/shoot which re-directs the incoming cool air past the cold side of the engine before entering the burn box area.

For a masonry stove, that would probably involve a complete rebuild of the stove designed around the engine. Probably more than you wanted to get into, though, for some stoves, if it's not all mortared together, or something, might just involve re-stacking some bricks?

https://youtu.be/GKcf5cYhYXU


Though, lately, I'm leaning towards the theory that actively cooling a Stirling engine is not necessarily beneficial. As I was saying, less heat = less power.

[kjramkunwar]

Thank you so much Tom..

It's great to connect with like minded people.

1. The idea of hot cap in the burn chamber is fantastic. I will give it a try for sure.

2. Utilising cold draft ? I'll have to get back to the drawing board and see how much of this can be achieved with things as they stand now.

On the alpha design, i have decided that I'll go the trial and error route and start my project right away rather than wait for a perfect design.

This is a side project of an ongoing business activity afterall!! So anything achieved .. even if it's as simple as providing drive to a vegetables cutting machine , or a pulverizer will be an added bonus in reduced costs.

I'll keep you posted on the project as it goes ahead.

[Tom Booth]

The type of engine I thought I might try in my wood stove was one of these flat plate engines, just because, well, they are flat, so would take up very little room, essentially making up the back wall of the firebox.

https://youtu.be/av2aKSG1aRA

That's what I imagined sandwiching between the firebox and the cold air being drawn in from outside . That could be rather unobtrusive, but I'm not sure it would be suitable for high temperatures or power production. But on the other hand, why not? Just make it out of materials that can take the heat.

A more tried and true design, ruged enough to take a lot of heat might be something like a scaled up version of one of these toy engines. For really high heat, like in a jet stove, the hot end should probably be stainless steel.

https://www.officeofsteamforum.com/rest ... ng-engine/


I was thinking the nose could be made out of an old fire extinguisher or welding gas bottle.


I was looking for something that I could use directly over a campfire.

Similar toy model engines can be found. I thought this one looked easy to take apart to possibly scale up. So bought one that was for sale on Facebook marketplace locally.

I like engines I can unbolt and disassemble and put back together, probably better than a NASA type, hermetically sealed engine.

I should be getting this In the mail soon:

Screenshot_20220121-201126.jpg (88.39 KiB) Viewed 5272 times

I think this is pretty much the same engine.


https://youtu.be/kSSz7T2Nnco


I'll be trying to do more or less the same thing, possibly working with some Amish in the area, several of whome have machine shops and build their own machinery, living off grid.

I'd be happy with something that could charge a 12 volt battery that could then power an inverter.

Maybe put the whole thing in a 30 or 55 gallon drum and pressurize it for more power.

IMG_20220121_204251409.jpg (316.83 KiB) Viewed 5272 times

[Tom Booth]

That little red engine arrived.

I simulated the masonry chimney of a rocket stove from a left over piece of cement board, cutting a hole through it to poke the hot cap of the Stirling through.

I think it may actually run better with the hot nose of the engine isolated It seems to be the way these engines were intended to run: poking through a chimney, judging by the old antique toy models in the link above.

https://youtu.be/R_QB5amihko

[Tom Booth]

Same type of engine. A little bigger:

https://youtu.be/BjzAqwyBgy8

[Tom Booth]

What about an old stainless steel fire extinguisher as displacer chamber? I've been able to find a number of these locally and various places online.

Cur off the base, plug the top. Turn sideways and it looks a lot like the hot cap on one of the above small models, but a bit bigger.

Screenshot_20220128-121830.jpg (75.67 KiB) Viewed 5171 times

Screenshot_20220128-121830_20220128123424891.jpg (94.21 KiB) Viewed 5171 times

[Tom Booth]

For better or worse, I just sent for a couple, similar to the above, used stainless steel fire extinguishers from eBay.

The rest of the engine shouldn't be too difficult to put together, I don't think. Maybe a piston and cylinder from an old lawnmower engine? Some kind of pipe or conduit to run in between.

[kjramkunwar]

Thanks Tom,

You have given me a lot to ponder on and start building !!

I'm starting on it and I'll keep you and the community posted on the progress.

[Tom Booth]

Thanks Tom,

You have given me a lot to ponder on and start building !!

I'm starting on it and I'll keep you and the community posted on the progress.

Fantastic!

I had one other thing to mention that I came across recently. If I was going to put a Stirling engine, essentially INSIDE a "blast furnace" type environment, rocket stove, or back of a wood or coal stove, for the vertical "flat plate" type engine in particular, I was planning on making the displacer out of something like fire brick or refractory cement.

In those flat plate type engines the weight of the displacer is not so much of an issue because it is balanced on its edge or hanging vertically and rocks or just swings back and forth so does not need to be lifted.

I always anticipated having to make some thin flat fairly light weight material from scratch and that would take some experimenting and trial and error, but then I came across these kiln shelves:


https://youtu.be/Spf6lMQithc

Just thought I would mention these, though, probably I'd end up making my own from refractory cement and perlite or something anyway, only because the kiln shelves are expensive and probably still heavier than necessary... But, you probably get the idea.

These things survive firing inside a kiln that heats up to cherry red, stays hot for hours, apparently do not crack or break or warp, lasting "forever" in and out of this extremely hot kiln environment.

They might even work in a very large LTD "pancake" style engine on top of a rocket stove or above a fire of any kind.

I guess LTD (low temperature difference) is a misnomer for an engine that is heated up to glowing cherry red, but that is in reference to the typical style, large flat plate shaped engines are USUALLY LTD, but this would be the same style engine but certainly NOT low temperature.

I think that is more along the lines of what you had in mind originally. Something sitting above the chimney area of a rocket stove

These.thin light weight kiln shelves might work for that also

Inside the kiln:

Resize_20220219_080412_2698.jpg (112.99 KiB) Viewed 5027 times