Air Preheat over rated...

["Tom Reed" <tombreed@attbi.com>]

Dear Ever Cheerful Piet and All:

Air preheat is a nice qualitative idea, warm and fuzzy, but when you get to
the quantitative use, watch out.

First, air passing over a hot surface has a low heat transfer coefficient,
so unless you measure its temperature just before combustion, you may only
gain a few percent of what you wish for.

Secondly, hot air is harder to move with natural or forced convection - it
is more viscous and lower density.

Third, hot air doesn't penetrate a mass of hot gas nearly as well as dense
cold air.

I don't pretend to know all the aspects, but I am very leary of glib
qualitative principles.

Caveat Emptor...

TOM REED                  BEF STOVEWORKS     BEF GASWORKS
----- Original Message -----
From: "Peter Verhaart" <pverhaart@optusnet.com.au>
To: "Paul S. Anderson" <psanders@ilstu.edu>
Cc: <stoves@crest.org>
Sent: Friday, May 24, 2002 5:58 AM
Subject: Re: Temperature dilution


> Did Prasad really say: "it dilutes the temperature"? It gets mixed with
> something else? Beyond me.
>
> How would you go about getting air at a very high temperature? Remember
> this is not rocket science. If it were we could pulverise the wood and
> inject it entrained in a flow of air (preheated if necesary). If that is
> not hot enough we take oxygen, automatic ignition, temperature and heat
> output rate control, none of this a problem.
>
> Alas, we stovers have to make do with what nature provides and we all know
> nature is a poor provider. The best we can come up with is natural draft
> and if we are really smart and inspired and lucky we might come up with
> something that works.
>
> I am sure, bye the way, that research on preheated air has been done for
> industrial combustion systems. The air is not only preheated, it is also
> given the right velocity to do its job, far beyond the capability of
> natural draft.
>
> Piet (ever cheerful) Verhaart
>
>
> At 08:50 23/05/02 -0500, you wrote:
> Please consider if the problem of "temperature dilution" (mentioned in the
> above paragraph) would be less of a problem if the air entering was
already
> at a high temperature.  And I mean at a VERY high temperature, as hot as
> you can get it.
>
> >Has research been done on the impact of pre-heated primary and/or
> >pre-heated secondary air?  Anyone have references?
>
>
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