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gas is pressurised, it condenses into a
liquid, which is why another name
for the heat exchanger in this case is
a condenser. It therefore works by
"moving" heat from one place to
another. The power required
depends on the difference in
temperatures and the amount of air
to be treated. The temperature
change depends on the rate of
compression or expansion.
In an open circuit vapour cycle system,
evaporated refrigerant is lost to
atmosphere, resulting in wastage. Air
is passed through a coil surrounded
by the fluid, which is evaporated by
the heat.
In a closed circuit system, as found in
motels, it is recycled by evaporation
and condensation. The refrigerant
passes from its storage tank through
an expansion valve into an evaporator at
a very low pressure, so it boils (and
evaporates) at a low temperature -
Freon normally boils at 4°C (water is
actually the lightest refrigerant, with
the highest latent heat value, but its
boiling point is too high for best
use). The evaporator absorbs heat
from the cabin air as it passes by. In
fact, it gets so cool that you need
water traps downstream to stop the
system freezing from the moisture.
The vapour is then compressed, so
its temperature rises to its saturation
temperature (dewpoint), to convert it
into liquid, going from the compressor
to the condenser, where it is cooled by
ram air. Then it goes back to its
storage tank.
An evaporator is actually a long, thin
tube, coiled as appropriate to save
space. At the inlet, refrigerant will
mostly be liquid, but, when it gets to
the outlet, it is a vapour. A separate
fan blows the air to be cooled
through it. The expansion valve is a
metering device that responds to
temperature to increase or decrease
the flow of refrigerant as required.
The compressor keeps a difference
going between the low and high
pressure sides of the system by
increasing the pressure of the
refrigerant going into the condenser,
and reducing the pressure in the
evaporator, to aid expansion.
Vapour cycle systems use the least
power, and can place cool air
accurately, but they can also reduce
humidity, which is why they might
be chosen over other systems.
Deicing
On some aircraft, a second inner
skin is used on the leading edges of
wings and tail sections to provide a
small gap between it and the leading
228 Canadian Professional Pilot Studies
edge, through which heated air is
ducted to warm everything up. The
hot air itself can come from
compressor bleeds, or heating ram
air through a heat exchanger in an
engine exhaust, as found on many
turbo-props. Air can also be blown
past a cylinder in which a fuel/air
mixture is burned. Whatever is used,
they still take a lot of engine power,
so are used in flight only.
Others may use a weeping wing, which
allows deicing fluid to creep out of
tiny holes in the leading edge of the
surface affected. However, if the
aircraft is too small, the supplies you
need to carry can easily make you
overweight.
Smaller aircraft use rubber deicing
boots, already discussed under Icing.
Electrical systems are generally used
on propellers, as they can take a lot
of power to operate.
Fire Detection
Fire has three elements—fuel,
oxygen and the heat. Take one away
and it stops. With dangerous goods,
you can get fire from the reaction of
flammable materials with an
oxidising agent – you don’t
necessarily need a source of ignition.
A Class A fire is an ordinary one,
that is, of normal combustible
material on which water is most
effective. A Class B fire is in a
flammable liquid, such as oil or
grease, where you would probably
use a blanket. A Class C fire is
electrical, for which you need a nonconducting
extinguisher. For the
latter two, you could use either CO2
or Dry Powder (which ruins the
avionics), but the fumes may be
toxic, so you will need plenty of
ventilation afterwards. You can use
Halon on anything, if you're allowed
to use it. A Class D covers other
materials, like metals, that may burn
if persuaded.
To help you identify the source,
smoke associated with electrical fires
is usually grey or tan and very
irritating to the nose or eyes (it
doesn't smell too good, either).
Anything else (say from the heater)
tends to be white, but you may get
some black from upholstery.
　
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