So i was fussing with my air conditioning controls today and got to wondering…
Does a car cool faster if the AC is on and the fan is set to low, or if the fan is on high? The way I figure, if its on low the air has more of a chance to cool before moving into the passenger area, but if its on high, you’re moving cool air into the p.a. faster.
I also figure that there is an optimal speed for the fan, for max cooling efficency, because if the fan doesn’t move (or better, moves slowly in reverse), then nothing gets cooled, and if it accelerates the air too much, it’ll actually heat the air up. So somewhere inbetween those extremes the car cools fastest.
When you first start the car up and it is hot inside, put the A/C on Max, high blower. When it is on Max, the air inside of the car is re-circulated and cools faster. After it is cool inside the car, switch it to normal. This brings in outside air and cools it. Set the fan speed to wherever keeps it comfortable for you. But it will cool faster with the fan on HIGH.
As a side note: Air conditioning does not actually “cool” the air. It actually “removes” the heat from the air that is already there. If, after you have been driving with the A/C on, you shut off the engine, look under the car. You will probably see water dripping on the ground. Moisture removed from the air.
Grossly over simplified, but hey – I’m simple. :D:D
Regardless of all the other factors, in practice setting the AC to Max (or inside air) at maximum fan speed will cool the car the fastest.
For the fan to work-heat the air appreciably, it would have to be a damn big fan. A 5000 hp coal power plant Forced Draft fan moving 400,000 acfm of air at 21 in w.g. pressure rise will often only raise the temperature of the air by 5-7 degrees F.
The water that drips is that which condenses from the evaporator in your dash. The water condenses out of the air as it’s temperature decreases below the dew point of the air. Look on a pyshchrometric chart to see what I mean.
At heart, your question is really one regarding the efficiency of the heat exchanger (the evaporator) in your dash. It cannot be easily answered at heart without digging out my heat transfer books, so I’ll let someone else with more energy do it.
I would have to disagree with you two about the ‘Max’ setting – when you first start your car on a sunny day, the interior temperature of the car is often 100 F or higher. Using the ‘inside air’ option at first will cause your car to cool more slowly, since you’re cooling air that was well above outside temperature to begin with. You’re better off to crack your windows, turn on the A/C, and use your highest fan speed with outside air for a minute or two, to clear out the heated air from the cabin. Once the interior of the car is no hotter than outside, switch to ‘Max’ cooling until you reach your desired temperature.
“When you first start the car up and it is hot inside, put the A/C on Max, high blower. When it is on Max, the air inside of the car is re-circulated and cools faster. After it is cool inside the car, switch it to normal. This brings in outside air and cools it.”
Wouldn’t this only be the case when the air inside the car is the same temp or cooler than the air outside the car? When I go to my car after work the air inside is considerably hotter than the air outside thanks to the greenhouse effect. In this situation I would rather bring in fresh cooler air and cool it further with the AC then try to cool the hotter air to the same temperature. Then when the air inside is the same temp as outside I can switch to recirculate until I can see my breath and icicles form on the rear view.
I’d like to address Hunsecker’s original question regarding fan speed and cooling capacity.
It’s really a trade-off: A higher fan speed will circulate (and cool) more air. A lower speed will not circulate as much air, but the temperature of the air as it exits the duct work will be lower. Assuming, of course, it is set to “max” (i.e. recirculating).
Right, thats what i figured, but I remember way back when I was learning thermo. in one of my physics classes about efficency of heat transfer varying with the temperature difference in your system. And I was wondering how that applied to the car situation.
Anthracite:
Thanks; I figured that it would take one damn fast fan to actually manage to heat the air. I’m surprised at just how fast though. Wow. I just wanted to make sure that this wasn’t just some “turn the fan on as fast as humanly possible” thing. If there is a limit at which the fan starts heating the air, then there must be an optimum speed for the fan somewhere below that speed.
It’s getting to sound like car fans aren’t powerful enough to get to that optimum speed, so cranking it up to max is the best way to go. But why is that better than a slow fan that gives the air a longer chance to cool before pushing it into the passenger area?
Well, it’s a problem that can only be answered in a general sense IMO. Heat transfer to a working fluid (the air) works on a limiting function, that is the rate of cooling of the air will slow as the temperature of the air approaches that of the coils. I believe you have 2 competing methods at work - a smaller amount of air that is cooled a bit more, or a larger amount of air that is cooled less. What is likely is that at the low fan speed in most cars, the cooling of the air has slowed greatly, as the temperature of the air is approaching that of the coils.
Like I said, it is not something one can actually calculate on paper unless they know a bit about the heat transfer surface, airflow and conditions, etc.
I seem to recall that Cecil himself actually tested fuel consumption with closed windows w/AC on vs. open windows w/AC off, and found no significant difference. Another tester I have read said about as much also. - MC