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which is mixed with ram air
through simple control valves.
The air itself is scooped in
through an inlet in the nose or
the dorsal fin.
·  Engine Bleed Air systems, for
large, pressurised aircraft, tap a
supply from the high pressure
(i.e. hot) end of the compressor
of an engine or the APU. Being
hot (200-400°C), it needs to be
cooled, so the air passes
through a shutoff valve (used
for isolation purposes), then to
a pressure-reducing valve to
calm things down a bit. A flow
rate valve keeps the pressure in
the cabin constant, and some air
is made to cool down through a
heat exchanger, then a water
226 Canadian Professional Pilot Studies
separator (to reduce the
moisture content) before being
remixed with the hot air again,
according to the temperature
required. The other benefit of
using compressor air is that it is
relatively uncontaminated, from
either dirt or fumes, although
filters are used anyway.
·  Compressors (or Blowers) are
found on large, unpressurised
aircraft, used in conjunction
with ram air. They can be driven
by the engine, to pump ram air
from the wing leading edge or
engine fairing into a system
similar to the Bleed Air system,
above, but there will be filters
and silencers as well. These can
be pump-type compressors, on
the basis that air heats up when
compressed. Alternatively, with
a blower, a butterfly valve can
cause enough back pressure to
warm the air up.
Heating
The simplest system uses a muff
round the exhaust that makes air
flowing through it warmer, on its
way to the cabin. As a result, there is
a danger of carbon monoxide
poisoning if the exhaust is faulty.
Slightly more complicated is what
can best be described as a flame in a
tube, under some control, of course,
around which air is forced. These
need a rundown period which
should be observed, otherwise
carbon will form on the igniter and
stop it from firing up again.
As mentioned above, a compressor
heats air up through pressure, and a
butterfly valve can restrict airflow
for the same result (choke valve).
Electrical Heaters are normally used
for the flight deck, as they would be
very heavy if used for the whole
cabin. The normal air supply is
passed over heating elements.
However, because of the availability
of cold air, it is easier to cool down
hot air than the other way round. A
turbine powered machine can use
bleed air from an engine, which can
be mixed with outside air to regulate
the temperature.
Cooling
This works on the basis of (latent)
heat absorption or release when a
liquid changes to or from a vapour
state. On small aircraft, ram air can
cool cabin air directly, being allowed
in through control valves.
Otherwise, systems can be either air
or vapour cycle. The former turns
heat energy into work. The latter
uses a refrigerant. Both can be used by
themselves, but a vapour cycle can
supplement an air cycle, as it can be
used without the engines running (it
has its own compressor).
With the air cycle, air is
compressed, and the heat created is
removed through a heat exchanger,
which is device that transfers heat,
before being expanded through a
turbine to do some work, such as
drive the compressor. Expansion
cools the air. There are three main
types, the turbofan, bootstrap or three
wheel systems.
The turbofan uses a turbine to drive
a fan that boosts the air through a
heat exchanger. It can also drive a
compressor for the same effect.
The bootstrap uses a heat exchanger
(for pre-cooling), the compressor
Airframes, Engines & Systems 227
section of the cooling turbine,
another heat exchanger and the
turbine. The maximum temperature
drop is in the second exchanger.
Both are cooled by ram air. Because
the turbine drives the compressor,
the air pressure is further increased
to create a higher pressure, and
hence temperature, drop.
The three wheel system uses two
heat exchangers and a compressor.
Air cycle systems are light and
inexpensive, but they do not provide
enough mass flow for machines with
high cooling needs.
With the vapour cycle, a gas
refrigerant (usually Freon) is pumped
round the system, collecting heat at
one end, which is removed when the
vapour is converted to liquid under
pressure at the other end. When the
　
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