Help settle this argument - Hot A/C

[d10]

[ QUOTE ]
well ... i was arguing the side of "decrease" saying that the air goes through the same process regardless of the temperature setting, so logically it should have some decrease in temperature.

my friend argued that there were two steps, dehumidifying and cooling, and that when the temp setting was high, it skips the cooling step.

wish i knew enough about cars to figure this out.

[/ QUOTE ]

In that case your friend is wrong regardless of whether AC makes a difference in temperature with the heat on. Dehumidifying happens as a result of cooling, so it's the same step. I would listen to an argument that says there is some system that will return the heat back into the air after it's cooled, but dehumidifying without cooling is not even possible.

[rp2]

Remember grade-school chemistry?

Gases, in this case freon, behave according to the following formula: PV=nRT, where p=pressure, v=volume, n=equals the moles of the gas, and R=a constant (~0.81 or something close to it).

Solving for temperature gives us T=PV/nR


So with the AC being a closed system, the volume, moles, and the constant relating to the freon all stay the same, leaving the temperature of the freon to vary only according to the pressure. As the freon comes out of the compressor, it is hot beacuse it has had its pressure increased. But as it passes through the evapaorator, the line cools because the pressure has been relieved. Now even though the heater core coolant bypass heats the incoming air, the air still passes over the cool AC refrigerant evaporator line which cools it down some, but not a whole lot. This is also where the humidity condenses out of the air onto the refrigerant line and drips onto the ground.

Best I can come up with.

[econophile]

[ QUOTE ]
Remember grade-school chemistry?

[/ QUOTE ]

i don't think most ppl learn the ideal gas law in grade school

[gusmahler]

While it doesn't surprise me that not everyone knows what's going on, it does surprise me that so many people with obviously no idea how a car's heating & A/C system work, would make posts that merely guess how the systems work.

Here's the deal. Cars have an A/C system that includes, inter alia, an evaporator. All air that passes over the evaporator gets cooled and dehumidified.

Cars also have a heater core. This core takes coolant from the car's cooling system (the stuff you put in your radiator). Air that passes over the heater core gets heated. See http://auto.howstuffworks.com/cooling-system10.htm.

When you have the A/C on and the heating knob up, air passes over both the evaporator and the heater core. So the air is cooled and dehumidied, then heated.

You may not feel an effect because the heater core is so hot (about 180 to 190 degrees) that the difference between passing cold outside air over the core and passing even colder air can't be felt.