Re: A/C recirculation/warm air from dash ponderings..

[johnc@together.net]

excuse the continued friendly debate...

Robert Houk - WorkGroup Server Firmware wrote:

> <snip my ranting..roberts ranting >
> My A4 *DOES* fog up the interior glass. It does this on automatic. It

yeah, i agree this happens sometimes, yeah i turn up the heat too

>    <more snipping>

>    except (my usual caveat: i don't know what i'm talking about) don't you have to cool
>    the air below it's dew point to condense the water vapor?  and that dew point won't be
>    changed by going through the heater, so you heat it up, only to cool it down again, and
>    then you get nice frosty air in the cabin, right?
>
> No more so that cooling it before heating it. You are pumping the same
> amount of heat energy either way. It's "commutative", so to speak. I.e.,
> add 40F then subtract 40F is the same as subtract 40F then add 40F.
> Within reasonable limits.

(still disclaiming my own ignorance...)
right about absolute temp change, of course.
i believe wrong about removing moisture.
heating up air doesn't 'dry' or take moisture out, just expands the volume, so that by volume
there is less vapor. whereas lowering it down to an absolute dewpoint makes the vapor
condense, which then removes it.
air with a certain vapor content has to get to an absolute temp before the water condenses
out.  so if the air is say, 50% RH, and 40F its going to need to hit 25F or so before the
vapor condenses.
with your example, you pump it up to 80F, then down to 40F again, but never get near the 40F
dewpoint that will drive the water out of the air, you basically get the same air back again.

i think...?

> By the by, all of the above is not to say the A4 isn't a fine not-so-little
> little ole car, just that it has its faults, and the automatic climate

agreed, 100%.

--

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John J Cunningham III           Project Leader / Technical Analyst
                                             Country Home Products
johnc@together.net                            (802) 877-1201 X1242
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