[Stoves] Improving Thermal Efficiency (TARP-VE)

[Crispin Pemberton-Pigott]

Dear Ken

 

You must be just dying to hook a Stirling engine to one of those things.

>Subsequently the flue has been lined with 1" of refractory - a partial
solution - 

And a conventional one. You will need to have a lining with very high
thermal shock resistance. These little systems are far more challenging
(material-wise) than industrial ones where everything stay hot for months at
a time.

.but I like the idea of using an annular heat shield that provides
accelerated cooling and air pre-heat.

If you get enough heat pulled off the surface you can get a good lifetime
out of your SS without going to really expensive materials. Remember that
the outer shell will form an effective insulation around the hot shell so
the air passing through needs to be known pretty well, and needed. 

It is really worth modelling the heat flow in Excel. Pretty straight forward
stuff. You may have to keep the hot wall thin in order to get its inside
temperature down into the 600 C range just to be safe. Remember 3CR12 can
operate up to 700 but its conduction is lousy (0.24).  To do that you will
have to have an preheated air temperature max probably 100 C below that, at
least. I have seen 500 C air coming through such an arrangement so it is
possible. The annular gap will set the volume and together with the flow
rate the total heat you can pull off. See if you can balance that with the
air demand, or the secondary air demand. Hence the suggestion for a
spreadsheet. The Vesto is recycling maybe 15% of the combustion heat (max)
through the preheater.

You may want to taper the preheater larger to give a lower velocity at the
hot end to allow for a higher heat transfer efficiency as the ΔT drops,
putting the hottest air end at the coolest burner end. This is
counter-intuitive compared with a hot water heat exchanger where the
temperature of the internal surface does not enter into the equation. You
have a different priority.  You need to see what the internal temperature of
the burner is above all, not the total preheating efficiency, in order to
prevent condensation of the chrome in the alloy (pulling away from the
oxidation sites).

You sure have a nice looking burner there! What is the O2 level at the
exhaust?

Regards

Crispin

 

Ken 

 

Doing sustainable energy work in Northern California.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.bioenergylists.org/pipermail/stoves_lists.bioenergylists.org/attachments/20120603/0ef31996/attachment.html>