Date:         Sun, 30 Jun 2002 09:23:59 -0700
Reply-To:     Nancy Hughes <keithahughes@QWEST.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Nancy Hughes <keithahughes@QWEST.NET>
Subject:      Re: Humid Air - More  Comments and a bit of theory
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Well...

> I thought that enthalpy would come into it somewhere - honest I did...

Kinda the basic underpinning, IME.

> ... but what we need are some graphs of the total heat transfer capacity of
> a radiator matrix, at a given International Standard Atmosphere, for a range
> of humidities to establish whether the curves increase or decrease. Let's
> assome a dry surface too for now, with no water droplets around, nor thin
> films. Any takers?

Unfortunately, this is impossible to do *unless* you calculate the
result for a *specific* heat exchanger configuration. You would
then have calculated what's known as the "Effectiveness" (similar
to what most folks think of as "efficiency") of that particular
exchanger, under the conditions of the test.

The effectiveness will be dependent on numerous factors, such as
both static and velocity pressure of the air stream, and airstream
turbulence, that make each installation of any specific heat
exchanger different. Take for example, our Vanagon configuration.
There's a great deal of backpressure on the radiator matrix due to
the exit flow path restrictions. This will increase static pressure
within the matrix, decrease velocity pressure, and *probably*
increase turbulence although I think it would be hard to
generalize.

There may, indeed, be some 'standard' heat exchanger calculations
for varying humidity profiles hidden amidst the ASHRAE stuff, but
I've not run across them. They have a great deal of esoteric stuff,
such as a "standard test day" (including total insolation [INcoming
SOLar radATION], with incident angle profile, standard system draw
rates, temp/wind/humidity specs., and sky temp specs) for testing
solar/thermal devices (yes, I used to do this back when the world
was young:-), so I suppose a standard fin/tube heat exchanger
profile isn't unlikely.

Keith Hughes
'86 Westy Tiico "Marvin"