Essentials
SELELE MASHILO
Selele Mashilo has a mechanical engineering diploma from Tshwane University of Technology and
a refrigeration and air-conditioning diploma from Unisa. His experience includes over a decade in
government as deputy-director building services before rejoining the private sector in 1998 as HVAC&R
project engineer. He is the former chairperson of the Refrigeration and Air Conditioning Empowerment
Forum of SA (RAEFSA), the Air Conditioning and Refrigeration Industrial Council of SA (ACRICSA), and
Black Energy Services Companies (BESCO).
UNDERSTANDING
AIRCRAFT AIR CONDITIONING
By Selele Mashilo
Air conditioning for comfort is required in aeroplanes at various altitudes
especially for temperature and cabin pressure control.
A
dditional equipment requirements must be met, over
and above conventional air conditioning systems. The
equipment must be compact, lightweight, easily accessible
for maintenance, vibration free and unaffected by aeroplane
vibration and landing impacts.
CABIN AIR SOURCES AND DISTRIBUTION
Air entering the cabin is for pressurisation and ventilation. The
air is from compressors which are engine or air turbine driven.
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DESIGN CONDITIONS
Design conditions vary according to altitude for hot, standard
and cold days. We are used to heat load calculations based on a
particular area for summer and winter conditions. The difference
with aeroplanes is that they experience various temperatures
and pressures at various altitudes within short spaces of time.
Normal temperature rate of change during climb and descent
can be about 0.09⁰C/s and might be as high as 0.28⁰C/s.
Moisture control is critically important for the occupants of the
cabin and crew areas. Remember also that the moisture can add
to aeroplane weight. In a pressurised aeroplane, the amount of
air flowing into the cabin should be equal to air leakages arising
from structural leakages like door seals, skin joints, toilets and
electronic equipment vents.
Cooling and heating loads are determined by a heat transfer
study and analysis of the solar and the internal heat transfer from
occupants and electrical equipment. Air film coefficients which
vary with altitude must also be taken into account considered.
It needs to be considered that the higher you go the colder it
gets and therefore the lower the atmospheric pressure design
conditions become.
Where gas turbines engines are used, air may be bled directly
from engine compressors. The recommended ventilation rate
is 2.4ℓ/s per passenger and 14.2ℓ/s for cockpit crew members.
REFRIGERATION SYSTEM
Refrigeration systems of aeroplanes use air as the refrigerant.
Commonly used are air cycle systems, vapour cycle systems or a
combination of the two.
Air cycle system:
Refrigeration in a simple air cycle is achieved by three steps:
• compression
• heat transfer
• expansion by the turbine
Air is first compressed by the compressor to a pressure
higher than the space to be cooled. The heat of compression
is removed from the air by a heat exchanger rejecting it to a
suitable heat sink such as air. In the third step, air is cooled by
extracting work from it as it expands through a turbine. RACA
RACA Journal I March 2020
65