Refrigeration and Air
Conditioning
By
Dr. Sukanta Roy
Associate Professor, Department of Mechanical Engineering, National Institute of Technology Mizoram
Adjunct Associate Professor, Faculty of Engineering and Science, Curtin University Malaysia
PhD (IIT Guwahati), MTech (NIT Silchar), BEng (NIT Agartala) , MASME, MIE CEng (India), SFHEA (UK)
Erasmus Mundus Post-Doctoral Fellow (ECN, France)
Post-Doctoral Fellow (IRPHE/AMU, France)
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Engineering Inspection and Quality Control | Dr. Sukanta Roy | National Institute of Technology Mizoram
AIRCRAFT REFRIGERATION SYSTEMS
Necessity of Cooling the Aeroplane
The invention of high-speed passenger aircrafts, jet aircrafts,
and missiles has introduced the need for compact, single
refrigeration systems capable of high capacity with minimum
reduction of pay load.
Misconception:
In case of an aircraft, it is a common assumption there is no need of cooling or air-conditioning since the
temperature is low at higher altitude as compared to ground.
Fact:
There are several external and internal heat sources which add heat in the passenger cabin resulting in the
increase of air temperature in the cabin.
Normally, the temperature of air should not exceed 22°C in the passenger compartment and the temperature
to the tune of - 5°C is required for preserving drinks and foods.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
AIRCRAFT REFRIGERATION SYSTEMS
The following sources add heat in the compartment:
External heat sources:
• Heat from the sun.
• The temperature of air entering the cab in is higher than the ambient temperature (due to compression
of air from ambient pressure to the cab in pressure since the pressure of the atmosphere decreases with
increase in altitude).
• The increase in temperature of air, due to skin friction, when the fast-moving aircraft passes through air.
➢The actual rise in temperature will be 80 to 90% of the rise calculated theoretically because of heat
transfer from hot compressed air to the colder surroundings .
➢An aeroplane moving with 1000 km/h will experience 50°C rise in temperature of the surface.
Internal heat sources:
• Heat flow from the occupants (Human body continuously generates heat at a rate of 400 kJ/h at rest).
• Heat generated by the electrical and electronic equipments.
• Heat flow from the engine parts of the aircraft.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
AIRCRAFT REFRIGERATION SYSTEMS
Air Refrigeration is used, why?
When the power requirements needed to transport the additional weight (Pay Load) of
the refrigerating systems are considered, air cycle systems usually prove to be most
efficient.
In an aircraft application, the main consideration involved in order of importance are:
Weight, space and operating power.
Although the power per tonne of refrigeration is considerably more for air-cycle
refrigeration than for a vapour-compression system, yet the bulk and weight
advantages of air cycle system, due to the heat exchanger at the cold end and a
common turbo-compressor for both the engine and refrigeration plant, result in a
greater overall power saving in the aircraft.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
AIRCRAFT REFRIGERATION SYSTEMS
Air Refrigeration is used, why?
Following are the advantages of air cycle with regard to its application in aircraft refrigeration:
• Air is non-toxic and non-inflammable (desirable properties of refrigerant). With air as the refrigerant, leakage in small
amounts in tolerable.
• Quite cheap (no cost of refrigerant in involved since air is used as a cooling medium). Refrigerant's availability in mid air is
also an important consideration.
• The pressure in the whole system is quite Iow; thus, the piping, ducting, etc., are quite simple to design, fabricate and
maintain.
• In an air cycle (in its simplest form) with an open system, only one heat exchanger is required.
• Low maintenance cost (since no complicated extra parts are involved).
• Light in weight comparatively.
• No additional vibrational problem (as envisaged from the use of a vapour-compression system).
• Cabin pressurization and air-conditioning can be carried out in one operation.
• The ram effect (due to high kinetic energy of the ambient air relative to the aircraft) accomplishes the initial compression of
the air. The necessary power required for this, however, is derived from the aircraft engine.
• As the main compressor is employed for compressed air source, there is no problem for extra space, extra fixation for the
compressor etc. In some cases, however, separate compressors are used due to various seasons, since the air gets
contaminated due to undesirable gases etc.
• The chilled air is directly used for cooling, thereby eliminating the costs of separator evaporator and its weight.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
AIRCRAFT REFRIGERATION SYSTEMS
Aircycle refrigeration systems are commonly used now-a-days 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 and commercial buildings because higher power is required to operate
an air cycle system than a conventional vapour cycle system.
In designing an aeroplane, the following loads are considered :
• Engine load (known as a live load).
• Fuel load .
• Dead load (or pay load).
In the passenger plane, the pay load is human load and aeroplane structural load.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
AIRCRAFT REFRIGERATION SYSTEMS
Dry Air Rated Temperature ( DART)
The dry air rated temperature is the temperature in terms of which the comparison of different
types of air-refrigeration systems is made.
This is the temperature of the discharged air from the expander if there is no condensed moisture
present and is calculated by the usual procedure, (assuming, cp for air = 1 .005 kJ/kg).
Thus, DART will be essentially that which results from a calculation which assumes that air at a
humidity so low that the expanded discharge is unsatured as it enters the system.
• The rating of the aircraft units is given in terms of kg of air per unit time at the design DART
Thus, the machine's capacity giving m kg/s of air at a DART of t0 to maintain a cabin at a
temperature ti is given by:
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
AIRCRAFT REFRIGERATION SYSTEMS
Types of Aircraft Refrigeration Systems
Following are the various types of air-refrigeration systems employed for aircrafts these days:
1 . Basic or simple air-refrigeration system
(a) Without evaporative cooling, (b) With evaporative cooling.
2. Boot strap air-refrigeration system
(a) Without evaporative cooling, (b) With evaporative cooling.
3. Regenerative air-refrigeration system.
4. Reduced ambient air-refrigeration system.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Basic or simple air-refrigeration system: Without evaporative cooling
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Basic or simple air-refrigeration system: With evaporative cooling
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Basic or simple air-refrigeration system: Without evaporative cooling
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Basic or simple air-refrigeration system: Without evaporative cooling
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Basic or simple air-refrigeration system: Without evaporative cooling
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Basic or simple air-refrigeration system: Without evaporative cooling
If Q tonnes of refrigeration is the cooling load in the cabin, then the air
required for the refrigeration purpose,
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Basic or simple air-refrigeration system: With evaporative cooling
If Q tonnes of refrigeration is the cooling load in the cabin,
then the air required for the refrigeration purpose,
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Basic or simple air-refrigeration system: With evaporative cooling
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
A simple air-cooled system is used for an aeroplane having a load of 9 tonnes. The atmospheric pressure and
temperature are 0. 9 bar and 10°C respectively. During ramming pressure increases to 1.013 bar. In the heat
exchanger, the temperature of air is reduced by 5 5 °C. The pressure in the cabin is 1. 01 bar and the
temperature of air leaving the cabin is 2 5 °C. Determine: (i) Power required to take the load of cooling in the
cabin ; (ii) C. O.P. of the system.
Assume that all the expansions and compressions are isentropic. The pressure of the compressed air is 4 bar.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
An aircraft refrigeration plant has to handle a cabin load of 25 tonnes. The atmospheric temperature is
16 °C. The atmospheric air is compressed to a pressure of 0. 96 bar and temperature of 29°C due to ram
action. This air is then further compressed in a compressor to 4.8 bar, cooled in a heat exchanger to 66
°C, expanded in a turbine to 1 bar pressure and supplied to the cabin. The air leaves the cabin at a
temperature of 26 °C. The isentropic efficiencies of both compressor and turbine are 0. 9. Calculate:
(i) The mass of air circulated per minute;
(ii) C O P.;
Take for air: cp = 1. 005 kJ/kg K and y = 1. 4.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
An air-conditioning system unit of a pressurized aircraft receives its air from the jet engine
compressor at a pressure of 1.25 bar. The ambient pressure and temperature are 0. 2 bar and 237 K
respectively. The air-conditioning unit consists of a free-wheeling compressor and turbine mounted on
one shaft. The work produced by the turbine is sufficient to drive the compressor. The compressed air
is then cooled in the cooler at constant pressure and then expanded in the turbine to the cabin
pressure of I. 0 bar and temperature of 280 K. Draw the schematic diagram and T-s diagram of the
air-conditioning system. Calculate:
(i) The compressor discharge pressure; and
(ii) The cooler exit temperature.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Ambient conditions for an aircraft cruising at 1000 km/h are 0.35 bar and -15 °C. The cabin temperature
is 25 °C and turbine exit pressure is 1.06 bar. The pressure ratio of compressor is 3.0. Assuming 100 per
cent efficiency for ram effect, compressor and turbine and ideal heat exchanger. Draw the schematic
diagram and T-s diagram of the system.
Determine for simple gas refrigeration cycle of 18 tonnes capacity:
(i) Temperatures and pressures at all points of cycle,
(ii) Mass flow rate and volume flow rates at compressor inlet and turbine outlet,
(iii) Work requirement, and
(iv) C.O.P. Assume: cp = 1.005 kJ/kg K; Rair = 0.286 kJ/kg K; γ = 1. 4.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
A simple evaporative air-refrigeration system is employed for an aeroplane to take 18 tonnes of
refrigeration load. The ambient conditions are 0.9 bar and 23 °C. The ambient air is rammed
isentropically to a pressure of 1 bar. The air leaving the main compressor at pressure 3.6 bar is first
cooled in the heat exchanger having effectiveness of 0.64 and then in the evaporator where its
temperature is reduced by 6.2°C. The air from the evaporator is passed through the cooling turbine and
then it is supplied to the cabin which is to be maintained at a temperature of 22°C and a pressure of
1.05 bar. If the internal efficiency of the compressor is 82% and that of cooling turbine is 78%. Draw the
T-s diagram, and determine the following :
(1) Mass of air bled off the main compressor;
(ii) Power required for the refrigerating system;
(iii) C. 0. P. of the refrigerating system.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Homework-1
The speed of an aircraft flying at an altitude of 8000 metres, where
the ambient air is at 0,341 bar pressure and 263 K temperature, is
900 km/h. The compression ratio of the air compressor is 5. The
cabin pressure is 1.013 bar, and the temperature is 27°C.
Draw the schematic diagram and T-s diagram, and determine:
a) The power requirement of the aircraft for pressurization
(excluding the ram work);
b) The additional power required for refrigeration; and
c) The refrigerating capacity for simple aircraft refrigeration cycle
on the basis of 1 kg/s flow of air.
Assume: cp = 1.005 kJ/kg K; Rair = 0.286 kJ/kg K; γ = 1. 4.
d) Also find the solution of the above problem (a, b, c), if the
following data is to be taken into consideration:
Compressor efficiency = 82%; Expander or turbine efficiency = 77%;
Effectiveness of heat exchanger = 80%; and Ram efficiency = 84%.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Boot Strap aircraft refrigeration system: Without evaporative cooling
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
mCP (T6 – T7)
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mCP (T5 – T4) Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Boot Strap aircraft refrigeration system: Without evaporative cooling
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Since 45% of the enthalpy of air discharged
from the main compressor is removed in the
first heat exchanger (i.e. , during the process
3'-4), therefore temperature of air leaving
the first heat exchanger,
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Homework-2
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Boot Strap aircraft refrigeration system: With evaporative cooling
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
A boot-strap evaporative air-refrigeration system is employed in an aeroplane, moving with a Mach
number of 1.3, to take 18 tonnes of refrigeration load of the cabin. The cooling air is taken from the main
compressor at 4.2 bar and further compressed to 5.2 bar which is run by cooling turbine.
The following data are given:
The ambient conditions 0.52 bar, -12 °C
Isentropic efficiency of main compressor and secondary compressor 88% and 82 %, respectively
Isentropic efficiency of cooling turbine 80 %
Effectiveness of both heat exchangers 0.42
Ramming efficiency 90 %
The temperature of air coming out of evaporator cooler 100 °C
The pressure and temperature maintained in the cabin 1 bar, 27 °C
Determine the following :
(1) Mass taken from the main compressor for cabin cooling,
(2) Power required to run the refrigeration system, and
(3) C.O.P. of the system.
Assume: cp = 1.005 kJ/kg K; Rair = 0.286 kJ/kg K; γ = 1. 4.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Regenerative aircraft refrigeration system
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Regenerative aircraft refrigeration system
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
A regenerative air-refrigeration system is designed to take
18 tonnes of refrigeration load of an air-craft cabin. The
ambient air at pressure 0.82 bar and temperature 12°C is
rammed isentropically till the pressure rises to 1.22 bar.
The air bled off the main compressor at 4.4 bar is cooled by the ram air
in the heat exchanger whose effectiveness is 0.62. The air from the heat
exchanger is further cooled to 64°C in the regenerative heat exchanger
with a portion of the air bled after expansion in the cooling turbine. The
cabin is to be maintained at a temperature of 22°C and a pressure of 1
bar. If the isentropic efficiencies of the compressor and turbine are 88%
and 82%, respectively.
Determine the following :
(1) Mass of the air bled from cooling turbine to be used for regenerative
cooling,
(ii) Power required for maintaining the cabin at required condition, and
(iii) C.O.P. of the system.
Assume that the temperature of air coming out of regenerative heat
exchanger is 100°C
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Reduced ambient aircraft refrigeration system
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Reduced ambient aircraft refrigeration system
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
The reduced ambient air-refrigeration system used for an aircraft consists of two cooling turbines
with heat exchanger in between. The output of both the turbines is used to run the fan. The speed of
the aircraft is 1500 km/h. The ambient air conditions are 0.24 bar and - 38°C. The air passing through
the heat exchanger is discharged to the atmosphere with the help of fan. The compressed air is cooled
to 48°C in the heat exchanger. The pressure ratio of the main compressor run by a turbine from which
air is bled off for the refrigeration purposes is 4.2. There is a pressure loss of 0.06 bar at the supply air
nozzle to the cabin. The cabin condition is maintained at 1 bar pressure and 22°C. If ramming
efficiency = 88%, isentropic efficiency of compressor = 82% and isentropic efficiency of each turbine =
80%, determine the following:
a) Mass flow rate of air supplied to the cabin if the cooling load in the cabin is 22 tonnes;
b) Air flow rate of ram air passed over the heat exchanger, if the maximum rise in temperature is
limited to 120 K;
c) Power used to drive the fan; and
d) C. O.P. of the system.
Assume: cp = 1 kJ/kg K; γ = 1. 4.
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
Thank you
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Refrigeration and Air Conditioning| Dr. Sukanta Roy | National Institute of Technology Mizoram
