Re: Static from Cross-feed

Date:         13 Jan 97 18:35:32 
From: (Simon Craig)
Organization: OzEmail Ltd - Australia
References:   1 2 3
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In article <airliners.1997.127@ohare.Chicago.COM>, wrote:

>Bilal Yousuf wrote:
>> B) The B747 can be ground conditioned, but if ground AC is not available the
>> APU is used to drive the aircraft airconditioning system, which does create
>> a lot of heat in the area fwd and under the wing.
>When a plane is at a gate, I assume those yellow/orange flexible pipes
>are the ones used to supply air to the plane. But I would also assume
>that electrical power is also supplied from the ground so that the
>aircraft need not rely on its APU. Is this a correct assumption ?

>So, if the airport does not supply forced air, wouldn't the aircraft at
>least use ground electrical power to run its fans and airconditioners ?

A/C can be supplied with external elec power, but this same APU can also
supply full elec power.  If the APU is supplying elec power, normal pack
(air con) operation is possible.

>Furthermore, I am somewhat puzzled at the though that the air
>conditioners under the fuel tanks would have caused the tanks to heat up
>so much.
>Where/how do air conditioning units on an aircraft dump their excess
>heat ?
>(at ground and during flight). Do they just dump in in the cargo hold ?

A/C packs in the 747 use heat to produce cold air (may sound strange but
true).  The heat is transferred into energy to drive a turbine, thus the
air that originally had the heat becomes very cold (all the energy is
extracted).  This basically means they do not really have "excess heat"  to
dump.  They do use ambient air for some cooling and this can become quite
hot on the ground.  This is drawn in (ambient temperature) and then dumped
overboard (a fair bit hotter than ambient) just aft of the a/conditioning
pack.  These packs are all found in the lower fuselage area underneath the
centre wing section.  I really wouldn't consider them a fire hazard.


Simon Craig

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