[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]

Re: ~ long fogging and AC (was: Re: 1st Snow in My A4 (1/2 long))


> On Dec 11, 10:16am, marriott@Summa4.COM wrote:
> > > Had I been allowed (by Hans und Klaus) to use the AC, the same
> > > air would've been dry.
> >
> > If the outside temp was below ~40F and you were heating the air
> > before you blew it into the car, the air _was_ dry.
> No!  "Dry" is a very relative term here.

Hence the term "relative humidity."

> First, let's get rid
> of the "heating the air" part,

If you're not going to heat _something_ you'll have little chance of evaporating 
anything, especially if you're running the compressor.

> because though heating changes
> the relative humidity, the absolute humitidy (i.e. the amount
> of water in a given volume of air) isn't changed by heating.
Did you mean "given Masi" perhaps? ;-) That given volume of air will change by 
heating/cooling if the pressure remains constant.

> What counts when worried about windshield fogging is the
> relative humidity in the air, but not relative to the air's
> temperature... relative to the *windshield* temp, which will
> be close to the outside air temp!
> OK.  Now, how about an outside air temp of 35F.  Dry?  What if
> it's raining out?  That's 100% relative humidity.  Suck in some
> air.  Call it 80% relative humidity.  At 35F, it takes *very*
> little moisture to reach high relative humidities.  Now breath
> out.  You just added moisture to the air, and since it doesn't
> take much to affect the relative humidity at this low temp,
> blammo, your windshield is fogged.

So, you want to run the compressor, remove the one molecule of vapor in X cm^3 of air, 
and hope that that cm^3 can then absorb another molecule of water? Then maybe that 
molecule will brush against the evaporator and condense out of solution? You have to run 
that cm^3 through there lots of times to clear the window . . .

The ability of air to hold water vapor is dependent on the air's temperature above 32 F/ 
0C _raised to the fourth power_. That relation's not a fundamental law, it's just a curve 
fit of experimentally measured values. If you can heat that cm^3 up 30 or 40 degrees, it 
will hold thousands of molecules of water. (An A4 cruising at 60 mph and 30 mpg has about 
215000 Btu/hr of waste heat. Maybe 70% goes out the exhaust, 10% to radiation, 10% to 
convection, and 10% to the heater. So 21500 Btu/hr for the heater.)

A small change in temperature makes a _huge_ change in spongeability. That's why the 
relative humidity in your house is so low in the winter. Outside, maybe it's 40 F and 
80%. By the time that same air comes in your house and gets heated to 70 F, the relative 
humidity has gone down to 0.16%. Yes, less than 1%.

It seems more effective to me to warm up the air, both because we have lots of waste heat 
from an IC engine--but not much waste cold--and because the results are better. Warm the 
air to lower the RH and warm the windshield to help drive off the moisture. How does an 
electrical (windshield) defroster work? It's certainly not dehumidifying the air...

A clothes dryer works by heating both the air, to lower the RH, and the clothes. I 
haven't seen one yet that worked with a condenser to dehumidify the incoming air. 
Granted, this AC discussion is totally invalid for temps above ~40F--at that point, the 
AC compressor will kick butt and really make a giant difference. Below that temperature, 
though, heating the air, and large air-exchange rates, is the way to go.

It certainly is crappy that your new cars can't _seem_ to defog the windows, but 
frustration isn't going to change the facts.

> > > In every air-conditioned car that I ever owned - a Volvo, a
> > > Nissan, an Isuzu and  my father's Oldsmobile - I could run the
> > > AC at any temperature.
> This is NOT your father's Oldsmobile... :-)
> > I'll bet you could _select_ AC at any temperature, but the
> > compressor wouldn't run at 5  F. That's the way the AC cars I've
> > had (two non-CC Audis, Subaru, Mercury) worked.
> My experience is that two Hondas, two Nissans, and a Toyota
> would all let me engage the compressor (not just "select" it)
> at low temps.

Your compressor would run at 5 F? I know mine ('87 4kcsq) won't run below about 36 F. 
Neither will the one on the '86 4ks.

> Now, will someone please find out what microcontroller is in
> the climate control so we can chip it?!?

2.2 bar max without heater-core milling! And infinite O2 sensor life. ;-)


James Marriott, BSME
work:  Manchester, NH, USA  marriott@Summa4.COM (603)625-4050 x 2539
home:  Auburn, NH, USA      marriott@grolen.com (603)483-8587

'64 Falcon Sprint Convertible--balanced & blueprinted 260 V-8
    GOES great. Stopping? 217k
'86 4000S        the better half's commuter (heated seats), 100k + ??
'87 4000CSQ      H-stock auto-x und daily driver, Kanc-1 veteran, 138k