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I have a question related to Air conditioning in the air craft. While boarding the aircraft it is observed sometimes that lot of fog kind of air comes from the AC ducts whic disappears after some time. Mostly this happens in rainy season. Is this due to excess humidity or is there any spray used to customers comfort?
No, its not an added spray for pax comfort, its just fog from the rapid change in air temp. that will occur as the air for cabin is drawn off the engines and ran through an expansion turbine in the ac "pack" system to cool it down to a comfortable temp. before it is finally pumped into the cabin. If you turn the temp setting all the way down (from the cockpit controller) it will actually snow in the airplane cabin!...its pretty cool.
In addition to above, Warm air holds more moisture than cool air. When warm, moist air is cooled, it becomes visible or it drops(like rain). Snowing in the aircraft sound interesting.
Many aircraft systems use a sock to attempt to dry the moist air, as this sock gets dirty, a type of filter for moisture, it is less effective and allows more moisture to get to the cabin.
If you put your car tire on a tire machine to break the bead to prepare the tire for removal from the wheel (rim) and if there was still some pressure in the tire, when the bead broke, there would be a sudden rush of air from the bead area and it would look like a puff of white smoke. This sudden change in air pressure causes a sudden change in air temperature and makes the moist air visible to the eye.
Cars and refrigerators use freon gas to cool the air through a heat exchanger, many aircraft use a pressure expansion system to force the air to cool. Other types of acft use freon also. Both types and all systems have a drain for condensation, water drains from the bottom of your car and all acft also.
Duane is correct. Air is compressed by the compressors (in a jet engine). As air is compressed it heats up in accordance with a principle called Chales' Law. Typically the temperatures of the compressed air is in the hundreds of degrees (I seem to recall about 300 degree Farenheit).
This hot air is used for anti-icing and sometimes even fuel heat (jet fuel tends to turn to jello if it gets cold enough).
Because there is such a large difference in temperature between the compressed air and the surrounding air (especially if it is "blowing" on it at hundreds of miles per hour), the compressed air cools off significantly.
When the gas is allowed to expand again, it loses a lot of the heat it has left, and cooled to freezing temperatures. The cold air is re-mixed with hot air to regulate temperature.
The jets in a jet airliner actually take air from outside, and bring it inside the cabin, this might be why.
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You are correct that the air coming through the aircraft air conditioning system is from the outside. But this conditioned air is not where the fog comes from. The fog/mist is most noticeable on the ground when the airplane cabin doors are open for servicing and boarding. If it's hot and humid outside, the humid air coming through the cabin doors gets hit with the chilly air coming out of the air conditioner. This causes the moisture to instantly condense into fog. Once the cabin doors are closed, the mist goes away (or greatly reduces) quickly as then only conditioned air is coming into the cabin and it has much less moisture after passing through the air conditioning "pack," also called an "air cycle machine."
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