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BMW X3 Windows Fog Instantly

16 messages,  Last post on Feb 02, 2010 at 11:58 AM

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What is this discussion about? BMW X3, BMW X5, Safe Driving, Safety Technology, Heating / Cooling, SUV

Messages Page 3 of 3

#15 of 16 Re: BMW X3 Windows Fog Instantly [wwest] by pp2009pp

Jan 28, 2010 (4:17 pm)

Replying to: wwest (Jan 21, 2010 5:53 pm)
If heating the cabin reduces humidity, why is there more fogging with a cabin full of people exhaling warm air?
 
From what I have read:
 
- cold air is drier which is why cracking a front window to pass colder outside air across the inside of the windshield reduces the moisture of fogging.
 
- A/C compressor apparently quits around freezing but above freezing removes moisture from the HVAC system.
 
- I thought the AC dumps moisture out of the car - there is a pool of water - when it is turned off. It doesn't get stuck in the system.
 
I will try it with the snowflake button off and the recirc off and see if that snowflake makes much of a difference. Since the front windshield defogger is a big blast of warm air, I see what you are saying. I find the entire thing confusing.
 

#16 of 16 Re: BMW X3 Windows Fog Instantly [pp2009pp] by wwest

Feb 02, 2010 (11:58 am)

Replying to: pp2009pp (Jan 28, 2010 4:17 pm)
In a "static" situation heating the atmosphere will result in lowering the Rh. Put a steaming teakettle (four humans) in the mix and you will have to continually raise the air temperature to keep the Rh constant.
 
Yes, cold air is consistently drier air. That's what makes this situation even MORE dangerous. That COLD and DRY FRESH airflow coming from the outside and flowing through an A/C evaporator covered with moisture due to just previous dehumidification capability will often result in SUPER-SATURATION of the cabin.
 
Dehumidification of the system airflow can be efficiently done ONLY if chilling that airflow to ~35F will bring it to dewpoint. If the incoming airflow is both warm and humid then well and good. If the incoming air is fairly humid but already very near that 35F temperature then the efficiency of the A/C for dehumidification might be nil.
 
"..It doesn't get stuck in the system.."
 
But yes, it does..!
 
Assuming climatic conditions allow, and the A/C system is operating, as more and more condensate accumulates on the roughly 10,000 square inches of evaporator vane surface area it will begin to form into droplets to the point wherein gravity will overcome viscosity and those droplets will now flow down the evaporator and eventually exit via the drain hose provided.
 
As long as the evaporator is being cooled down to the level wherein dehumidification will occur this is a continuous, ongoing process.
 
 But.
 
The instant you switch off the A/C compressor, or it is switched off automatically, the continual gathering of additional moisture soon stops. And now you might well have those 10,000 square inches of evaporator vane surface area thoroughly coated with condensate, WATER.
 
Now it might well remain there until climatic conditions change to the point wherein the evaporation of the condensate will begin. Climatic conditions solely within the A/C plenum, within the plenum AND passenger cabin, or if you leave the windows down inclusive of outside climatic conditions.
 
The 2010 RX350 has a new feature added to the climate control system. If climatic conditions warrant (I assume sub-45F OAT) it will automatically switch the system into combined footwell and windshield outflow. That's presumably such that the interior windshield surface gets continually warmed, hopefully keeping that surface well above the dewpoint of the cabin atmosphere.

Messages Page 3 of 3

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