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ABSTRACT
In
this lecture we will be looking at the refrigeration of an aircraft otherwise known
as air conditioning .the factors considered in air conditioning of an aircraft
will be looked at and the types and advantages of one over another.
INTRODUCTION
Air-conditioning
of any given object is made as a means to an end and this is comfortability.
Comfortability of humans is very necessary because if this is not done, it can
lead to brain damage and side effect such as uncontrolled behavior laxity and
uncomfortability of humans. Air conditioning of an aircraft is of uttermost
importance because at the height.
NECESSITY
OF COOLING THE AIRCRAFT
It
is a common assumption that the aeroplane does not require any cooling or
air-conditioning because the temperature is low at a higher altitude compared
on the ground. But there are many external and internal heat sources which add
the heat in the passenger cabin and increase the temperature of air in the
cabin. For the comfort feeling, the temperature of air should not exceed 22ocand
it is necessary to maintain this temperature in the passenger compartment and
still lower the temperature (-50oc) is also required for the
preservation of foods and drinks.
The
following are the major sources which add the heat in the compartment
EXTERNAL
SOURCES
1. Sun pour heavy quantity of
heat in the passenger’s cabin through the windows and also heats the top
surface of the plane which further transfers the heat to the cabin.
2. The pressure required in
the passenger cabin is atmospheric pressure at sea level so that it is
necessary to compress the ambient air from the ambient pressure to the cabin
pressure because the pressures of the atmosphere decrease with an increase in
altitude. Due to this compression, the temperature of the air entering the
cabin will be higher than the ambient temperature.
3. When the fast moving plane
passes through air, it compresses a layer of air and raises its temperature.
Owing to the skin friction this compression occurs all over the aircraft and
high temperature air layer will be always in contact with the external surface
of the aircraft, so that the heat will flow from the surface of the air plane
to the cabin. The temperature rise of the air is same as would occur if the
stream of air moving at the speed of the plane suddenly struck a barrier. If
the conversion of the K.E into enthalpy is isentropic , then the temperature
rise can be calculated as follows,
V1 =VA=Speed of Aeroplane
or Speed of air in opposite in opposite direction.
hi = Enthalpy of air before
compression.
INTERNAL
SOURCES
1. Human body continuously
generates heat at a rate of 400KJ per hour at rest and this heat is dissipated
to the surrounding or to the cabin air. The passengers and pilot gives off this
heat to the cabin air.
2. The electrical and
electronic equipments used for the control system generates heat and this heat
should be taken out for the efficient working of the control system. An
aeroplane of 3000KW capacity carrying 50 -75 passengers require control
equipment of 10KW capacity and it requires nearly 3tons of refrigeration or
cooling capacity.
3. The engine parts of the
aeroplane are subjected to the high temperature throughout the flight and heat
from these parts comes to the cabin through conduction, convection and
radiation. All these above sources which are adding heat to the cabin will
increase the cabin temperature and it becomes uncomfortable for the passengers.
In the actual design, proper care is taken to reduce the heat flow from
different sources mentioned above. Nowadays it has become a common trend to
equip aeroplane with the refrigeration plant as the man require more comfort.
FACTORS
CONSIDERED IN SELECTING THE REFRIGERATION SYSTEM FOR AEROPLANE
The
advent of high speed aircraft, jet aircraft and missiles has introduced the
need for compact, simple refrigeration system, capable of high capacity with
minimum payload. The cooling demands per unit volume of space are quite heavy
due to compactness of all equipments and engine. Ordinary passenger aeroplanes
require a cooling system of 8 tons capacity and jet fighters need a cooling
system of 20 tons capacity. The miniaturization of electronic equipments
concentrates a heavy cooling load in a small area and creates difficulties in
transferring heat to air at high altitude and low pressure of air further
complicates the refrigeration design and requirements.
The
application of refrigeration system to military aircraft is more complicated
than the cooling system used for passenger’s aircraft, because refrigeration
cycle is used for cooling or heating purposes as per requirement of the pilot.
The pilot of military jet plane must wear special clothing in case he is forced
to bail out at high altitude where the temperature may be -45oc or lower. Under
such condition, it is desirable to maintain the cockpit at 20oc. For jet
fighter plane as much as 1 ton of refrigeration per person is required which is
25 times refrigeration load required per person in cinema hall. The control
system must be efficient to keep the machine on cooling cycle or cooling cycle
according to the requirement. All these requirement complicate the design of
cooling system used for aircraft.
Nowadays
air cycle refrigeration system are commonly used in air conditioning of
both commercial and military aircraft principally because of
extremely light weight , compact equipment which is typical of air cycle
system. The air cycle air conditioning has not been found economical in
residential in residential and commercial buildings because higher power is
required to operate an air cycle system than a conventional vapour cycle
system.
The
running cost of air cycle refrigeration system is not the only consideration
for the selection of refrigeration system used in aeroplane.
In designing an aeroplane, the following loads are
considered,
1. Engine load known as alive
load. (W E). 2. Fuel load (Wf). 3. Dead load or payload
(Wp).
In
the passenger plane ,the payload is the human load and aeroplane structure
load. An aeroplane is designed for minimum WE with the
minimum consumption of WF for the given Wp. Adoption of refrigeration
system is nothing but increase in Wp which will have direct
bearing on We and Wf. Increase in We means more investment cost and
increase in Wf means running cost. So the adoption of any refrigeration plant
depends upon the following three factors
1. Running cost of the
refrigeration plant itself.
2. Investment cost due to
increase in engine weight to carry the refrigeration plant.
3. Fuel consumption cost
(running cost) to carry the plant.
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REFERENCES
Eitel, Elisabeth. CFD Software Models How Moving Parts Affect
Aircraft-Cabin Airflow | Machine Design Magazine, 6 May 2014.
Niren Laxmichand Nagda (Ed): Air Quality and
Comfort in Airliner Cabins. ASTM International (2000) ISBN 978-0-8031-2866-8.
"Cabin Air Quality." Statement of
William W. Nazaroff, Ph.D. Professor of Environmental Engineering, University
of California, Berkeley and Member, Committee on Air Quality in Passenger
Cabins of Commercial Aircraft. (June 5, 2003)
"CTT Systems AB receives cabin humidity control
system order from Jet Aviation AG". Airline Industry
Information, (March 5, 2007)
