Re: Power Hungry air conditioning ?

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         22 Mar 95 02:34:22 
References:   1 2
Next article
View raw article
  or MIME structure

In article <airliners.1995.302@ohare.Chicago.COM> (Sense) writes:
>The APU is used to power the AC is correct, but it also powers all of the
>plane's electrical systems, and I think the hydraulic as well.  When the
>engines are running, the APU is turned off and engines power the entire
>plane.  In order to keep maximum power to the engines, the APU can be used
>as a "battery" to run the other systems of the plane (since the AC, like
>in your car, eats plenty of juice), and relieve some power from the
>engines from having to run 10 or 15 percent of the electrical motors and
>part of the AC.  Some airlines run the APU a couple of miles pre-arrival. 
>This also acts like a battery and gives a little of boost in handeling
>performance.  On a 767, it also stresses the APU Air Inlet Door Actuator
>which my company manufactures.

Normally, any engine can completely satisfy electrical loads, including 
a trickle-charger for the battery (ies).

The APU is primarily used for:
  1.  Pneumatics pressure for engine start and ground air conditioning.
  2.  Electrical power for ground operation.

>From (2), hydraulics and other electrically dependent systems may be operated.
Typically, APU generators are identical in capacity to engine-driven generat-
ors.  In some cases, cooling capabilities are better than engine generators
(e.g., 727 on ground), so the capacity is a bit greater.

The APU serves two other purposes, if it's certified for in-flight use:
  1.  Emergency source of electrical power in the case of total engine failure
      or CSD shutdown.
  2.  Regulatory redundancy if the airplane is permitted to dispatch with
      a CSD inoperative.  In this case, the APU is operated throughout the
      flight regime.  The 737-200, for example, can dispatch with one gener-
      ator inoperative; the APU has to be run for the entire flight.

The "pre-landing" operation you're referring to is probably to alleviate 
cold soak.  In long-distance, high-altitude flying, the APU can get extreme-
ly cold, and can be difficult to start after landing.  There are restrictions
to how many attempts can be made to start the APU, so if its services are
required, it'll need to be warmed up.

There is no need to use the APU to "save power" from the engines.  The
constant-speed drives, which power the engine-generated generators, run 
whether the generator is being used or not, so the work load is there.  The
airflow requirements are better satisfied by the engines: that's what they're
designed for.

As for the air conditioning system, it is normally powered from a pneumatics 
air source.  In the case of the 727, there are two air conditioning packs 
on the bottom of the airplane.  They may be "fed" by engine bleed air or APU 
bleed air.  

Each pack is independently comprised of a series of heat exchangers, an air 
cycle machine (which is a combination of compressor and turbine) and an air 
mix valve, which combines air of varying temperatures to come out with an
output temperature.  On this airplane, the packs' heat exchanger is in the 
belly of the airplane; airflow is controlled via a ram air door.

The packs must be operational for the pressurization system to work.  One-pack
operation can limit altitude capability.

If desired, an external *air conditioning* feed can be plugged downstream in 
the system to provide conditioned air on the ground.  A normal external 
pneumatics hookup will not provide enough airflow to satisfactorily run the


Robert Dorsett                         Moderator, sci.aeronautics.simulation