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  #1  
Old 10-10-2000, 12:42 PM
Nukeman Nukeman is offline
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You know when its a cold morning (but not frosty) and you start the car up and you get condensation on the inside of the windscreen?
I had an argument with someone about whether you should blow cold air or hot air on the window. I thought it should be hot air because the hot air makes the condensation evaporate, but she disagreed, saying that the condensation was caused by the different temperatures in the car, and that you should use cold air to equalise them. So we kept changing it, and what we agree DOES happen is that when you switch between them, it makes it much worse. I dont see why we should sit and suffer in the cold - what do you think?
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  #2  
Old 10-10-2000, 01:05 PM
Podkayne Podkayne is offline
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God, you don't know how many fights I've had with my husband about this. I'm glad to hear we're not the only ones. His car seems particularly prone to condensation on the windshield, and for some reason is particularly bad at getting rid of it.

Here are my thoughts on the matter. (He'll tell you I'm crazy.)

Condensation is caused when the temperature of the windshield is below the dewpoint of the air. Dewpoint is determined by how humid it is: how much water vapor is in the air. (Side point that becomes important later: Warmer air can hold more water vapor.)

Therefore, to get rid of condensation, you should raise the temperature of the windshield by blowing hot air on it.

EXCEPT that if you have a cool windshield, and start blowing warm, moist air from the defogger/defroster (such as you'll get if it's a rainy day) onto the windshield, it will hit the windshield and moisture will condense, and you're worse off than when you started.

So sometimes blowing cool air (which can hold less moisture, and may be even dried because it's gone through the air conditioner) onto the windshield will help remove the condensation. However, if it's a really humid day out, and you're blowing cool air onto the windshield, it'll cool the windshield enough to cause condensation OUTSIDE. No prob if you just turn on the wipers, of course, except under less than ideal conditions (driving in the dark, or sitting in a parking lot yelling at your spouse to leave the damn climate control alone) sometimes you don't notice WHERE the fog is.

So which should you do? My feeling is that if you just blast the windshield with hot air long enough, eventually it'll clear up.

I don't think the equalization argument works, because if it's cold out and you get condensation, cooling the windshield MORE would just cause more condensation--I think cool works, as I said above, because the cool air is drier.
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  #3  
Old 10-10-2000, 01:14 PM
starfish starfish is offline
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Cold air has less moisture than warm air. If you blow warm air, the moisture in that air, if there is enough, can condense on the cold windows. Eventually you warm the window and the condensed moisture evaporates.

If you blow cold [dry] air on the window, the moisture will evaporate off the window.
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  #4  
Old 10-10-2000, 01:23 PM
Philster Philster is offline
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Easy

Nothing dries like warm DRY air, and to accomplish getting warm dry air to blow onto the windshield, you need to what kind of car you have....and this is important, so I'll explain:

1. American car: Set fan to Defrost, set Temp. control to warm. In an American car, the defroster will automatically prompt the A/C compressor to come on, thus removing moisture from the air, and clearing the windhshield asap. Even when blowing warm air in defrost mode, the A/C compressor comes on and you get the most effective warm dry air you need.

2. Non-american: Generally, the "defrost" setting just blows "fresh" air on the window, and whether warm or cool, it doesn't do a whole helluva lot. Set to "Defrost", set Temp. control to Warm AND turn on the A/C, it's usually a button or seperate selector.

Any discussion about inside/outside temp is useless since the most effective way to dry something is to blow warm dry air onto it. You'll be filling the rest of the cabin with warm dry air too, and hence, that's less moisture that can get on the window via condensation. Cool dry air works, but warm air holds moisture better. That is, it doesn't make it condense on the glass.

Generally, my statements about American and non-American cars are true, but check your manuals. Most will explain that if defrost alone is insufficient, turn on the A/C.

Nothing like a good ol' disclaimer...
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  #5  
Old 10-10-2000, 01:32 PM
Philster Philster is offline
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Defrost problems? Usually a non-American car or one with busted A/C.

Quote:
Originally posted by Podkayne
God, you don't know how many fights I've had with my husband about this. I'm glad to hear we're not the only ones. His car seems particularly prone to condensation on the windshield, and for some reason is particularly bad at getting rid of it.

Here are my thoughts on the matter. (He'll tell you I'm crazy.)

Condensation is caused when the temperature of the windshield is below the dewpoint of the air. Dewpoint is determined by how humid it is: how much water vapor is in the air. (Side point that becomes important later: Warmer air can hold more water vapor.)

Therefore, to get rid of condensation, you should raise the temperature of the windshield by blowing hot air on it.

EXCEPT that if you have a cool windshield, and start blowing warm, moist air from the defogger/defroster (such as you'll get if it's a rainy day) onto the windshield, it will hit the windshield and moisture will condense, and you're worse off than when you started.

So sometimes blowing cool air (which can hold less moisture, and may be even dried because it's gone through the air conditioner) onto the windshield will help remove the condensation. However, if it's a really humid day out, and you're blowing cool air onto the windshield, it'll cool the windshield enough to cause condensation OUTSIDE. No prob if you just turn on the wipers, of course, except under less than ideal conditions (driving in the dark, or sitting in a parking lot yelling at your spouse to leave the damn climate control alone) sometimes you don't notice WHERE the fog is.

So which should you do? My feeling is that if you just blast the windshield with hot air long enough, eventually it'll clear up.

I don't think the equalization argument works, because if it's cold out and you get condensation, cooling the windshield MORE would just cause more condensation--I think cool works, as I said above, because the cool air is drier.
Let's clear this up...pun intended:

The most effective wat to dry something is to blow warm dry air onto it. The most effective warm dry air comes from the use of the warm setting and turning on the A/C, which happens automatically in most American cars.

Defrost problems? Chances are it is a non-American car and you need to turn on the A/C.

WARM DRY air dries surfaces the fastest. Period.

How does you back glass get defrosted? Most cars use heat- generating defroster strips. Heat dries. Dry heat dries faster. Ever notice how effective a hot blow dryer is? Or a hot curling iron?
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  #6  
Old 10-10-2000, 01:55 PM
matt matt is offline
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Your friend is wrong in her reasoning, but may be right about which is faster - it depends on the car, the weather and the windows.

The condensation is caused by increased humidity inside the car, generally because of all the moist air you're breathing out.

The heater can be slow because it can increase the humidity of the air it delivers to your windscreen. The heater should just take the outside air, warm it up and send it to you dry. But if it's raining or foggy or there's dew inside the ducts of your car, this might not be the case - the hot air can increase the humidity inside your car. Eventually the windscreen will warm up and clear the condensation anyway, but it takes a while.

Cold outside air removes the condensation by a different mechanism - it reduces the humidity by getting some airflow through your car, and prevents the moist air you're breathing out from contacting the windscreen by providing a "shield" of drier air. (You can also increase the airflow by opening the windows, which you tend not to do with the heater on.)

I see on preview that our American friends have cars which will cool the air with the A/C to dry it, then heat it up again before it blows it onto the windscreen! Greenpeace must love you guys!

I was going to suggest that you test the theory in dry weather, but don't bother - get an American car.
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  #7  
Old 10-10-2000, 03:02 PM
RESOL RESOL is offline
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Ok, water condenses on the window for two reasons.

1) The temperature of the window is cold enough to cause a phase change from the gas pahase to water phase, duh we all knew that

2) The amount of water outside the car is less then the amount of water inside your car. (I'm talkin winter here, I live in northern NY) The natural tendancy is for the water to diffuse to the air on the outside using the window as a membrane. Since the window is not as permiable as the water would like it doesn't work to well and the water collects on the window. Your car has more water vapor in the air due to your breath.

Now, where does this put us? Simple, To REALLY clear out the window you need to replace the air in the car with the dry air from the outside. The air has very little moisture and well absorbe the water from the windshield. So make sure that your car is using outside air and not recycled inside air. Cold or hot it doesn't really matter. Unless it's really cold out and the inside of the window freezes over, then you need to scrape the inside....
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  #8  
Old 10-10-2000, 04:31 PM
Podkayne Podkayne is offline
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Ah, a moment of revelation! I've finally connected the dots! Not long ago, I realized that the defroster on my truck kicks in the AC, even if you have it all the way to hot. I noticed it because the AC really puts a heavy load on the engine, and I was annoyed by it.

However, my husband's car (a Toyota) has a separate button for the AC.

Now I can't wait for a cool, moist day to try it out!

My eternal gratitude to all of you!
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  #9  
Old 10-10-2000, 04:45 PM
Caldazar Caldazar is offline
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I've tried both hot air and cold air to defrost the inside of my windshield, and after much experimenting, I'd have to say that hot air defrosts the inside of the glass more quickly.

As others have pointed out, though, it must be hot, DRY air. Hot, moist air just gives rise to more fogging of the windows (e.g. the stereotypical fogging of windows caused by two people making out in the back seat).

By increasing the temperature of the air near the windshield, you increase the ability of the surrounding air to hold moisture. The condensation on the inside of the glass evaporates, leaving you with a clear windshield.

I've always assumed that this is how the rear window heating strips work to defrost the window.
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  #10  
Old 10-10-2000, 05:13 PM
Nukeman Nukeman is offline
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Thanks guys, I tend to agree with Caldazar. I would much rather use the heat technique on a cold day.
A slightly related tip, but have you ever seen those people on monday mornings furiously scraping the frost off their windshield? Why not just use hot water? I do that all the time.
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  #11  
Old 10-10-2000, 06:38 PM
RufusLeaking RufusLeaking is offline
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I'd be careful about pouring hot water on your windshield to clear the frost off. I've heard that this can crack your windshield! When the windshield is really cold and you pour hot water on it, it does not expand uniformly and if the stress is too much it will crack. If you continue doing this I suggest warm water instead of hot water and don't do it in really cold weather.
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  #12  
Old 10-11-2000, 12:55 AM
labdude labdude is offline
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Re: Easy

Quote:
Originally posted by Philster

1. American car: Set fan to Defrost, set Temp. control to warm. In an American car, the defroster will automatically prompt the A/C compressor to come on, thus removing moisture from the air, and clearing the windhshield asap. Even when blowing warm air in defrost mode, the A/C compressor comes on and you get the most effective warm dry air you need.

2. Non-american: Generally, the "defrost" setting just blows "fresh" air on the window, and whether warm or cool, it doesn't do a whole helluva lot. Set to "Defrost", set Temp. control to Warm AND turn on the A/C, it's usually a button or seperate selector.
This is the correct method. You don't know how many fights I've had with my girlfriend over this.
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  #13  
Old 10-11-2000, 01:25 AM
honkytonkwillie honkytonkwillie is offline
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Philster beat me to it explaining the A/C-defrost connection in american vehicles. Boy did that piss me off when I figured it out several years ago. When it's -20F I don't want my defrost handicapped with A/C. I want the truck warm NOW!

And I don't get frost or condensation on the inside of the windshield. Just the outside. And after the A/C broke 6 years ago, the frost melts much faster.

If I wasn't so lazy, I'd actually scrape the frost off manually. But then I'd be late for work.
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  #14  
Old 10-11-2000, 08:08 AM
Mousseduck Mousseduck is offline
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I think hot air gets rid of the condensation faster, but the problem is waiting for the hot air to get hot. My old car (Toyota) took at least 5 minutes of idling for the heater to blow air that was warmer than the outside air. The a/c gets cold straight away, so I think the fastest way to clear the windscreen after you jump in is to use the a/c then turn the heater on once the car is warm. It works for me, anyway.
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  #15  
Old 10-11-2000, 10:09 AM
Feynn Feynn is offline
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Hot air works best period. Warm air can carry more moisture laden air to a cold window which will then fog or frost up even more.

Perhaps instead of rushing around and trying to find ways to make things defrost faster we should be starting our cars a few minutes earlier to allow the engine to warm and have the windows clear. When you start the car and get cool air from the defroster it will have some effect but as soon as the temperature rises to where the thermostat circulates hot coolant to the heater core you get the best results. My car has an automatic system with a thermostat inside the car. As soon as the engine temperature is sufficient the defroster turns itself on.

There are some products on the market that will prevent condensation from forming on your windows, they are similar to what I use on my helmet visor. We know that having your motorcycle helmet fog up is a bad thing.

It drives me nuts to see people here doing the "Canadian Crouch" on winter days... this is where one scrapes the ice off the windows which invariably then fog up. Said person then drives away peering through the small clear spot on the windshield where the defrost is beginning to have some effect.

Drive safe, you are sharing the road with the rest of us.
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  #16  
Old 10-11-2000, 10:58 AM
Nukeman Nukeman is offline
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Okay Rufus, im not stupid, i dont use BOILING water or anything. warm water is what I mean
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  #17  
Old 10-11-2000, 11:02 AM
inertia inertia is offline
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matt: "The heater can be slow because it can increase the humidity of the air it delivers to your windscreen." Humidity is the measure of the amount of water vapor in the air. Simply adding kinetic energy (heating) is not going to just create water. As for the sources of water you've cited - dew in the ducts, fog, rainy day - these same sources would affect cold air intake also. Let's make some sense please.

RESOL: "Ok, water condenses on the window for two reasons. 1) ... 2) The amount of water outside the car is less then the amount of water inside your car. [snip] The natural tendancy is for the water to diffuse to the air on the outside using the window as a membrane" What the hell are you talking about? Are you suggesting that glass is permeable? Get a grip.

Nukeman: "Why not just use hot water? I do that all the time." Because if it is a particularly cold day and your water is more than tepid your windshield is going to shatter. WARNING: This can also happen with just the defroster if it is a VERY cold day.

honkytonkwillie: "When it's -20F I don't want my defrost handicapped with A/C. I want the truck warm NOW!" Perhaps you should get a foreign car that expects people to think about whether or not they want the A/C on.
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  #18  
Old 10-11-2000, 11:28 AM
Nukeman Nukeman is offline
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inertia: matt is saying that the condensation will evaporate into the air making it more humid. (it wont be very much though) Also, the sources of water will not affect cold air as much as it holds less water than hot air. Go and look it up.

I agree that resol is talking bollocks. resol, the air in the car should have the same humidity as the air outside.

No, it really wont. I do it all the time and warm water is not going to have an effect. Plus, windscreen glass is stronger and different from normal glass. What do you mean by 'really cold day'? a really cold day is only minus a few degrees, ie not much different than 5 degrees, so not as big a difference as between warm water and boiling water, so a really cold day wont have that affect. Have you ever heard of an instance when the defroster has broken the rear windshield? I havent.

Kinetic engergy is not heat. Why dont YOU make some sense.
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  #19  
Old 10-11-2000, 01:35 PM
RESOL RESOL is offline
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OK, the air inside has more humidity in the winter. First off it's warmer (or it should be) inside your car than outside. Also since the interior of your car can be considered air tight (it's realy not of course but for all paractical purposes) and since every time you breath you are realising moisture into the air, the air stays in the car, hence your car air has more moisture then the outside.

Secondly, if the problem is taken from the engineering point of view on the molecular level (the whole diffusion thing) you could say glass is permiable but it's diffusion constant would be very low (everything is permiable not just noticibley, how do you think your car rusts? O2 has to get into the iron somehow) Remeber glass is technically a liquid (which is why it sags in old houses)
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  #20  
Old 10-11-2000, 01:55 PM
Kyberneticist Kyberneticist is offline
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Some slight inaccuracies there.

Regarding glass and old houses.
http://www.straightdope.com/columns/000211.html
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  #21  
Old 10-11-2000, 02:02 PM
Kyberneticist Kyberneticist is offline
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Hm. A chance to correct Cecil?

A thread on UrbanLegends.com
http://www.urbanlegends.com/science/...he_thread.html
Quote:
Newsgroups: alt.folklore.urban
From: jg94au@badger.ac.BrockU.CA (JASON J GILL)
Subject: Re: Old windows thickening at the base?
Date: Thu, 18 May 1995 00:48:06 GMT


So, then, to recap:
  • Glass does not so much flow as gradually bend under the influence of gravity, being, as it is, an amorphous solid.
  • Because of this, larger pieces of glass (such as window panes) will distort over time more rapidly than small pieces of glass, such as small mirrors, vases, etc.
  • Not all glass is created equal. Metallic or other additives can reinforce the glass by inserting themselves into the crystal lattice and strengthening the bonds.
  • Leaded glass, for instance (which is what fine crystal is) is much more rigid, and so does not exhibit the distortion found in plain old window glass as readily.
  • Glass is very old. The Egyptians did indeed produce glass, but the process was so energy intensive that it was only used for small ornaments and such. The glass museum maintained by the manufacturers of Corningware (in Corning, NY) has a few Egyptian glass artifacts on display, such as small scarabs, etc.
Hope this clears everything up,
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  #22  
Old 10-11-2000, 02:12 PM
barbitu8 barbitu8 is offline
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I don't know the scientifc reasons, but I know the following about my Honda. If the condensation is on the outside of the windshield, I use the defogger. The AC won't help. If the condensation is on the inside, I use the AC until the heater warms up sufficiently, at which time I don't need either. Condensation on the outside is no big deal. That's why we have wipers. But I don't have wipers on the inside of my windshield. The defogger alone won't clear that up quickly. I use the AC, and if it is a cold day, I'll use both. There's no harm blowing the hot air through the AC. The AC dries out the air inside the car, so the inside condensation disappears. The defogger emits moist air and makes the situation worse until the air gets sufficiently hot, unless you run it thru the AC first.
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  #23  
Old 10-11-2000, 02:38 PM
Nukeman Nukeman is offline
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Look RESOL
YOU
ARE
WRONG
Cars are NOT AIR TIGHT. Otherwise you would suffocate. (what does 'for all practical purpose' mean? thats like when people say 'with all due respect...' or 'no offence but...' before insulting someone. It doesnt mean anything.)


Glass isnt a liquid (thank you Kyberneticist)

Even if it were slightly permiable, it wouldnt be enough to let in enough moisture to cause condensation, but its not. The metal on cars rusts because it reacts with the oxygen. The oxygen doesnt 'get into it'. Paint isnt permiable is it? why do we paint stuff to stop it rusting?
Go back to school, learn physics, and then come back and agrue with me.
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  #24  
Old 10-11-2000, 02:41 PM
Nukeman Nukeman is offline
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sorry, PS
Barbitu8
hot air does not dry out the air in the car. The water doesnt go away, it just evaporates so it is still in the air. If you put dry air into a car, it will eventually push out the humid air, but this is not what you are suggesting.
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  #25  
Old 10-11-2000, 03:10 PM
RESOL RESOL is offline
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well Kyberneticist I think you need to look at what a amorphous solid is. This comes from Material Science and engineering (fifth edition) by William D Callister Jr. copyrighted year 2000

Sometimes such materials are called amorphous solids or supercooled liquids, inasmuch as thier atomic sturcture represents that of a liquid

This is refering to glass

next from
Elliott, S. R. (1994) Amorphous Solids: An Introduction.

we have

An amorphous (or synonymously, non-crystalline) material can be defined as one which is topologically disordered and which does not exhibit either the long-range translational order (periodicity) characteristic of single crystals, or the long-range orientational order characteristic of quasicrystals. Within this definition, such materials could be either solid or liquid

[...] Amorphous solids are characterized by a topological disorder, so there is no long-range order (or periodicity) in their structure. However, this does not mean that amorphous solids are structurally completely random (i.e., gas-like) at all length scales. In fact, covalent materials, in particular, exhibit a rather high degree of structure organization at length scales corresponding to several atomic separations [...]. On the other hand, materials characterized by non-directional centro-symmetric interatomic interactions, e.g. metals or completely ionic materials, are intrinsically much more disordered even at short length scales.
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  #26  
Old 10-11-2000, 03:44 PM
Nukeman Nukeman is offline
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Jesus RESOL, this is a thread about condensation in cars, not whether glass is a liquid or a solid or whatever. This doesnt even need to be discussed, as its been flogged to death in discussion before, and Cecil has cleared it up anyway.
Maybe you do know about glass (or maybe you just copied it all out of a book) but everything youve written so far has been crap.
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  #27  
Old 10-11-2000, 05:43 PM
Engineer Don Engineer Don is offline
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I think there is something missing in this disscussion. There are several things going on, and people are getting bogged down on one detail while ignoring others.

First, as stated in the OP, condensation occurs when moist air comes in contact with a surface with a temperature below the air's dewpoint. There are several ways to change this.
(a) decrease the available moisture in the air using the A/C. (actually reduces the grams of water per pound of air, but leaves the air saturated and cold with a low dewpoint)
(b) increase the dewpoint of the air by increasing the air temperature, using the heater. (Dewpoint goes up, grams H2O per lb. of air stays constant)
(c) increase surface temperature until above dewpoint using in glass defogger or heater.

Just the A/C isn't all that effective. You need the heater for reheating the air after dropping some of the moisture out. If it isn't wet outside having the A/C on won't do anything useful and could be slowing down the process of heating the air and glass.

Just the heater/defrost is best if the air (inside and out) is fairly dry, but if it is really moist outside (not just cold), or there are a lot of people inside the car breathing heavy then it probably will take a long while to work, whereas with the A/C (and recirc to minimize outside air) it will probably work faster.

Slowly heating the glass (only rear windows in most cars I have seen) will get rid of the condensation inside and out by raising the glass's temp above the dewpoint. It is slow to allow the heat to spread out, and prevent stress from thermal expansion. I have heard of car windows and windshields having lots of internal stresses in cold weather, and being more subject to shattering from incidental contact.
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  #28  
Old 10-11-2000, 06:51 PM
matt matt is offline
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Inertia:

"Humidity is the measure of the amount of water vapor in the air. Simply adding kinetic energy (heating) is not going to just create water. As for the sources of water you've cited - dew in the ducts, fog, rainy day - these same sources would affect cold air intake also. Let's make some sense please."

Perhaps I wasn't very clear. You're right of course, simply heating the air up can't create water out of nowhere.

But if you have liquid water present in contact with air, e.g. a pool in the bottom of a sealed tupperware box, then that water will evaporate into the air in the box until the equilibrium partial pressure of water vapour is reached. Such air is often said to be "saturated" with water.

(Or if you leave the box open, the air never becomes saturated and all the water will eventually evaporate.)

The equilibrium partial pressure is temperature dependent - raise the temperature and it goes up. More water will evaporate into the air in the box until it is saturated again.

Now, if you have rain, fog or dew, the air is going to contain enough water vapour to be already saturated or close to it. If you suck this air into your car heater (excluding liquid water) and heat this air up, the amount of water vapour it contains will be unchanged but it will no longer be close to saturated.

However, if your ducts contain liquid water because of rain or dew, pushing warm air through them will raise the amount of water vapour in that air, possibly to its new, higher equilibrium partial pressure.

Pushing cold air through them won't raise its humidity if that air is already saturated - it can't.
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  #29  
Old 10-12-2000, 04:36 AM
Nukeman Nukeman is offline
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Don is right, if the air is hot, it holds more water so the water will not condense in the first place. If it cools down, the air can hold less water, so the excess water will condense.
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  #30  
Old 10-12-2000, 06:34 AM
RM Mentock RM Mentock is offline
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Some cars have a "recycle" or "recirculate" button. This causes the interior air to be recirculated instead in the heater, instead of having the heater take in outside air. The stated purpose of the rec button is to warm the inside of the car faster (since it is not taking in cold outside air).

It may warm up the inside of the car a smidge faster in cold weather, but its major effect is to fog up the windows. All other suggestions are fine, but make sure the rec button, if you have one, is not mashed.
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  #31  
Old 10-12-2000, 08:58 AM
barbitu8 barbitu8 is offline
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Quote:
Originally posted by Nukeman
sorry, PS
Barbitu8
hot air does not dry out the air in the car. The water doesnt go away, it just evaporates so it is still in the air. If you put dry air into a car, it will eventually push out the humid air, but this is not what you are suggesting.
I never said that the hot air will dry the car out. It works because, as you said, warmer air can contain more moisture (higher dewpoint).

Inspite of what Engineer Don posted, I know from experience that the AC alone will eliminate compensation inside the car, and, I might add, instantly.
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