Originally Posted by
YVR Cockroach
Not surprising. Relative humidity (how saturated air is at a given temperature) in east Asia this time of the year is usually close to 100%. Any drop in temperature e.g., by cooling devices, drops the capacity of air to hold water and hence you get condensation and accumulation of water (ever noticed window and automobile air conditioners have drips at their evaporators - these work by transferring energy (heat) from the hot air inside and transfers the energy to the coolant which expands). Most aircraft sit with doors open between flights and get warm quickly. A/C is turned on just before boarding and when you try to cool the hot and humid cabin quickly with no forced ventilation to the outside (a/c are not really equipped to do this), all that water is going to have to go somewhere.
It's just that in flight, the air is so dry (I've seen references of 5-10% relative humidity - the deserts of the S.W. U.S. are around 10-20% RH by comparison) that it pretty much dries out the whole a/c (and not to mention humans onboard).
Just to clarify....airplane AC systems are not coolant type systems i.e. freon etc, they are air-cycle machines. They compress the air to squeeze out the moisture but that actually heats the air...so afterwards the air expands which cools it down (and usually some of the water is injected back into the air to humidify it).
Downstream of the FCV is the CAU (Cold Air Unit), also referred to as the refrigeration unit. There are many various types of CAu however they all use typical fundamentals. The bleed air enters the primary "Ram Air Heat Exchanger", where it is cooled by either ram air, expansion or a combination of both. The cold air then enters the compressor, where it is re-pressurized, which reheats the air. A pass through the secondary "Ram Air Heat Exchanger" cools the air while maintaining the high pressure. The air then passes through a turbine which expands the air to fuyrther reduce heat. Similar in operation to a turbo-charger unit, the compressor and turbine are on a single shaft. The energy extracted from the air passing through the turbine is used to power the compressor.
The air is then sent through a Water Separator, where the air is forced to spiral along its length and centrifugal forces cause the moisture to be flung through a sieve and toward the outer walls where it is channeled toward a drain and sent overboard. Then, the air usually will pass through a Water Separator Coalescer or, The Sock. The Sock retains the dirt and oil from the engine bleed air to keep the cabin air cleaner. This water removal process prevents ice from forming and clogging the system, and keeps the cockpit and cabin from fogging on ground operation and low altitudes.
http://en.wikipedia.org/wiki/Environ...Unit_.28CAU.29