Interesting Regenerative Firebox Design

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Interesting Regenerative Firebox Design

Post by TriangleTom » Sat Apr 11, 2020 12:43 am

I saw this design for a regenerative hearth design drawn up by a blacksmith on Reddit

Image

and I think it would have a lot of interesting possibilities as a firebox, especially under forced draft. The addition of the integrated heat exchanger for combustion air preheat seems like it'd be good for efficiency.
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Re: Interesting Regenerative Firebox Design

Post by Lopez Mike » Sat Apr 11, 2020 2:27 am

Sitting here looking at this beast, a couple of things occur to me.

A: Is it important that the combustion air entering the burner be heated? Air has a fairly low specific heat. Doesn't take much to heat it up. Doesn't seem worth the trouble.

B: This thing produces hot air. Is that any good for heating up water? The parts of my boiler that generate most of the steam have a direct "view" of the flame. Radiant heat rather than convective transfer. Not that I totally ignore convective transfer as witness those clanking turbulator strips in my tubes. But I'm pretty sure that not that much is going on in the tubes of my VFT compared to the firebox walls and tube sheet.

Perhaps I'm not getting it. Also, my grasp of thermodynamics and such is fairly ad hoc.

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Re: Interesting Regenerative Firebox Design

Post by TriangleTom » Sat Apr 11, 2020 4:44 am

I can't speak to everything, but:

A: The back of my envelope says that with a diesel burner, a 150 degree F temperature increase of the incoming air would result in a roughly 2% decrease in fuel consumption. With that said, that's assuming a perfectly stoichiometric mixture so it would probably be a somewhat better gain in efficiency under normal operating conditions where there is an abundance of air. Does that make it worth it? Probably not, but I could swear one of the Audel's books talks extensively about combustion air preheat for marine steam applications, so I'm not confident enough to say one way or another without doing the work to find out myself.

B: I think that perhaps the region where the flame is being shot in to the combustion space would be a good location for a two drum or a B&W boiler setup. That way you'd get the advantages of both convective and radiative heat transfer. That said, I have to imagine that convection plays a large role in moving heat to the water. My day job involves a lot of work with hot water coils for HVAC applications, and I am constantly surprised at how much heat a small coil with a modest delta-t can pump into an airstream, or vice versa.
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Re: Interesting Regenerative Firebox Design

Post by Lopez Mike » Sat Apr 11, 2020 12:06 pm

Hmm. So let's say that my power plant is simmering away at around 5% overall thermal efficiency (I think that's a very high number). So a 2% increase bumps me up to a roaring 5.1%. One less hunk of worm infested doug fir 2 x 4 every 15 minutes maybe?

Granted, my steaming is stuck somewhere on a river in Africa during WW1 with Katherine Hepburn steering but I think I'll try to improve my pipe lagging first.

I don't have any hard information about convective v.s. radiative heat transfer. I've read a fair bit about locomotive boilers and the designers there generally regarded the tubes as a way to get rid of the combustion products; most of the serious work being done in the firebox. The fireboxes got longer and longer over the years.

Understand that I'm looking at this from the view of a VFT operator. It might well be that the hot air coming out of this burner/regenerator would be of more use in a water tube unit.

I admit freely a personal prejudice. I dislike the noise of most oil burners. I chase down engine gnashings and squeaks with a vengeance and seldom operate at above half throttle (unless there is another steamer in sight!).
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Re: Interesting Regenerative Firebox Design

Post by barts » Sat Apr 11, 2020 7:49 pm

Preheating combustion air does the following:

* increases flame temperature; this may reduce smoke as well.
* increases boiler efficiency about 1% for every 40F increase in combustion air temperature if the heat is scavenged from the exhaust.

Combustion air preheating is used basically on all large boilers... one handy way of doing preheating for our small boilers is to bring combustion air past the back of the refractory surfaces in the firebox or down through a double wall stack; this helps cut down on case and exterior stack temperatures as well.

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Re: Interesting Regenerative Firebox Design

Post by Lopez Mike » Sat Apr 11, 2020 8:12 pm

In this design the heat isn't scavenged from the engine or boiler stack exhaust. I don't think so anyway.

My take on this design so far is that if you want hot air, it might do that. But the way it looks, the combustion byproducts are mixed with the air fed to the burner. Doesn't make sense to me. That's what heat exchangers are for. This looks like the gas streams are mingled.

I'm open to explanations of my stupidity and/or ignorance. Both in great supply. But so far I don't get it.
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Re: Interesting Regenerative Firebox Design

Post by Oilking » Sat Apr 11, 2020 9:25 pm

Mike,
After looking at the 3D rendition, I think that the combustion air and exhaust gases pass through a plate type heat exchanger.

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Re: Interesting Regenerative Firebox Design

Post by Lopez Mike » Sat Apr 11, 2020 11:26 pm

Ah. I hadn't interpreted it that way. That makes it a lot more likely that the flame might not blow out!

But I'm still looking at heated air as the output which has its uses but I'm not sure that generating steam would be one of them.

I'm often startled by the intensity and commotion of the fire in my little boiler. And how little I have to show for it. A day's steaming would heat my house in Winter for a week.

Let's see. Maybe five H.P. output max. That makes less than 4 k.w. I'll have to guess at the heating value of scrap softwood but I'd be surprised if I got 3% thermal efficiency from axe to wave making. But remember, it's the nineteenth century. Right? Or maybe the early twentieth.

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Re: Interesting Regenerative Firebox Design

Post by Mike Rometer » Sun Apr 12, 2020 7:46 am

That's all counter intuitive to me, as an I.C. man! We want the air as cold as possible to squeeze as many of those tiny little oxygen atoms in there as we can get. 8-)
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Re: Interesting Regenerative Firebox Design

Post by Lopez Mike » Sun Apr 12, 2020 11:37 am

There is something to be said for high compression in the combustion cycle. I vaguely remember seeing a hybrid cycle I.C. system where there was a reciprocating free piston I.C. engine sandwiched between two mechanically coupled centrifugal turbines. One to first compress the air and the one after the piston engine to extract energy and drive the compressor turbine. How they got it started I have no clue. Limited yield nuclear device most likely. With a priest and two german engineers in attendance.

I don't see this in my launch quietly running on wood.
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