Basics of Air Conditioning System (ATA 21)
- Subhajit Debnath
- Nov 14, 2024
- 7 min read
Updated: Nov 15, 2024
Commercial passenger aircraft typically fly at altitudes ranging from 25,000 ft to 41,000 ft, where temperatures can fluctuate between -30 to -69 degrees Celsius. At such heights, the air is thin and frigid, posing a significant challenge to human survival. The extreme cold temperatures can quickly lead to frostbite or hypothermia if not properly managed.
To address this issue and ensure a comfortable and safe environment onboard, modern aircraft are equipped with sophisticated air conditioning systems. These systems play a crucial role in maintaining a suitable temperature and pressure inside the cabin throughout the flight duration.
The aircraft's Air Conditioning System is meticulously designed to provide comfort for both the flight crew and passengers. It regulates the temperature within the cabin, ensuring that it remains within a safe and pleasant range despite the harsh conditions outside. Additionally, the system is responsible for cooling specific areas and systems within the aircraft, such as avionics, electronics, and cargo compartments, to prevent overheating and ensure optimal performance.
Moreover, the air conditioning system plays a vital role in managing the pressurization of the cabin. As the aircraft ascends to higher altitudes, the air pressure decreases significantly, which can lead to discomfort and even health issues for those onboard. The system works to maintain a stable and safe cabin pressure, mimicking conditions found at lower altitudes to prevent altitude-related problems like hypoxia.
It is comprised of 5 principle sections:
Air Supply
Heating
Cooling
Temperature Control
Distribution
There are basically 2 types of Air Conditioning systems:
Air Cycle Air Conditioning System
Turbo Compressor/Bootstrap
Brake Turbine
Turbofan/Fan Turbine/Turbine Fan
Vapour Cycle Air Conditioning System
Air Cycle ACS is used is mostly used in aircrafts with:
High engine bypass ratio
Small turboproller aircrafts.
Vapour Cycle ACS is mostly used:
Domestic Appliances
Small reciprocating aircrafts
AIR CYCLE AIR CONDITIONING SYSTEM
TURBO COMPRESSOR/BOOTSTRAP ACS
COMPOSITION
Air Cycle Machine (ACM)
2 heat exchangers
Primary Heat Exchanger
Secondary Heat Exchanger
Rotating Turbine (also called Turbo refrigerator)
Compressor
Cabin fan
Cooling Medium for heat exchangers
RAM air
Points to note:
To assist ram air recovery, some aircraft use modulating vanes on the ram air exhaust.
The RAM air inlet and outlet doors temporarily closes during takeoff and landing
Power for the air conditioning pack is obtained by reducing the pressure of the incoming bleed air relative to the cooled air output of the system.
AIR SUPPLY
When it comes to aircraft systems, the extraction of the main air supply is a crucial process that ensures the proper functioning of various components. This supply is typically sourced from the hot bleed air of specific compressor stages within the engines. In most cases, this extraction occurs around the 5th and 8th or 9th stages of the compressor, although the exact stages involved can vary depending on the engine variants in use.
Connecting these essential sources of air is the bleed air manifold, a key component that plays a vital role in distributing air to different parts of the aircraft.
This manifold serves to link the Air Cycle Machine (ACM) units, which are typically isolated from each other to prevent any potential issues from affecting the entire system. This isolation is achieved through the use of dedicated isolation valves, which help maintain the integrity and efficiency of the air distribution network within the aircraft.
Basic flow of AIR CYCLE MACHINE (ACM)
The hot bleed air is sent to the primary heat exchanger where the air is cooled down before sending to the compressor.
Pre-cooling through the primary heat exchanger increases the efficiency, i.e. less work is required to compress a given air mass.
The compression heats up the air before sending to the secondary heat exchanger where it is again cooled down by the RAM air
The secondary heat exchanger further cools down the air and removes certain heat produced during compression in the compressor.
The compressed and cooled air from the Secondary heat exchanger is then directed into the expansion turbine controlled by a motorized bypass valve to rotate it along with the compressor that is connected together via a shaft. This leads to an extraction of energy from the air which further expands and cools the air between 20°C and 30°C.
The shaft that connects the Compressor and the turbine is also connected with a RAM air inlet fan operated electrically/pneumatically that draws in the external air during ground running.
WATER SEPARATOR
With such cooling, its water vapor condenses and to remove this, the moist air output of the expansion turbine is passed through a water separator, using the principle of cyclonic separator(Centrifugal force).
The water Separator consists of Coalescer and Collector tubes with a spring shaped swirl in between.
Coalescer
consists of filter of course wire gauge
Increases the size of water droplets
Collector Tubes
Centrifugal force sends the water outwards hitting the collector tubes which run down to the bottom and exit via water drain
Relief Valve
In case of accumulation of ice/debris, the valve will open to safeguard the airflow
This condensate is sometimes fed back into the RAM air entering the secondary heat exchanger to improve its performance.
HUMIDIFIER
The needs for humidifier are:
The passage of air through water separator completely dries up the air
At such high altitudes, humidity is considerably way less.
COMPOSITION
Potable Water tank
Pressurized by bleed air
Venturi Tube
Acts as a mixing chamber
Water Jet
Fitted after the water Separator which connects to the Venturi Tube
Bleed air is fed in it to build up the pressure.
Air Jet
To ensure the continuous flow of dry and cooled air from the water Separator.
High Pressure air supply to Venturi
Swirl Vanes
Ensures mixture of water from water spray inside the venturi tube to humidify the air
Consists of a fabric which absorbs extra moisture which can be utilized if humidifier is switched off.
Diffuser block
A block where all the components brought together, i.e. it behaves like a mixing unit.
Humidistat
Water Supply valve between potable water tank to the diffuser block is controlled by it
It measures the humidity in the air conditioning duct
PACKS
ACM along with its components are termed as PACKS.
A Bypass Valve is fitted between the hot bleed air duct and the cool conditioned air to adjust the temperature output. It is electrically controlled and operated for short duration
Pack flow Control Valve is electrically controlled and pneumatically operated.
It has 2 modes:
Electrical Mode- automatically controlled
Pneumatic Mode– operated in case of emergency
The bypass valve and the pack flow control valve are controlled by pack controller.
The pack inlet doors are electrically controlled and operated by pack controller for long duration.
Various pack parameters are monitored by ECAM. Some of them are:
Compressor outlet temperature
Pack flow
Bypass valve position
Pack outlet temperature
PACK OUTLET TEMPERATURE CONTROL
Controls the pack outlet temp by 2 modes
Auto mode: Works by comparing pilot selected temperature with actual air temperature
Manual mode: Pilot actually controls the hot air valve to regulate temperature.
MASS FLOW CONTROLLER
It may be incorporated along with pack flow valve or might function as a separate unit.
It ensures that a constant mass flow of air is supplied to Air Conditioning packs regardless of the engine RPM/air intake pressure.
It consists of:
Spill valve which dumps the excess air to the atmosphere
Piston in a barrel
If the inlet pressure in the piston is low, the piston will compress towards the inlet side and the cutouts of the barrel will be fully aligned allowing maximum airflow.
If the inlet pressure in the piston is high, the piston will compress towards the outlet side, reducing the airflow.
MIXER UNIT
Each pack in an aircraft operates independently which unites to a mixer unit.
In ground, Low Pressure Ground Air Conditioning cart is directly attached to the mixer unit.
Incase of emergency, i.e. if one or more packs fail, direct RAM air is fed into it.
CABIN FAN
Reduces the requirement of bleed air and recycle the existing air by sending it to the mixer unit again to cool the hot cabin air.
It does not have any ECAM indication.
ZONES
The mixer unit connects with the zones:
Cockpit
FWD Cabin
AFT Cabin
The number of zones might vary according to different types of aircraft. Generally, an A320 has 3 zones, whereas B747 has 5 zones.
The Zone Controller controls the zone temperature by regulating the trim air valves and hot air valves. It send signals to the Pack Controller to control the air delivered by the packs.
Incase of zone controller signal loss pack will regulate output at a fixed value.
Incase of an overheat, both the trim air and hot air valves close which can be reset if it functions normally again.
Since different zones might require different cooling, hot bleed air is directly connected with the outlet of the mixer unit to the zones via a Trim Air Valve. This connection got a hot air valve to regulate the hot air (from the inlet of the packs) supplied to the trim air valves.
The Zone Controller is controlled by a computer named as Zone Control Computer which monitors:
Inlet of zone duct temperature
Actual Zone Temperature
AIR DISTRIBUTION
COMPOSITION
Upper air outlet
Lower air outlet
Main supply duct
Riser duct
Connects the main supply with the upper and lower air outlet.
The air distribution is done through the floor and wall passages by supplying warm air that maintains the interior surfaces at cabin temperature reducing draughts and direct heat loss.
The upper and lower air outlet further divides into:
Recirculation fans
Reduces the number of pack usage during cruise
Draws cabin air through filters from underfloor area and introduces the air in the air conditioning distribution system
Air from galleys and toilets are recirculated but vented out.
Gasper air
Individual passenger conditioned air supply
Comes from one of the zone supply duct upstream where trim air is added
also maintains the zone temperature
Flight Deck air
Flow direction and quantity can be adjusted by crew
Also supplied to windows for demisting purpose
COCKPIT POV
Overhead panel consists the ACS
Ventilation Panel
Blower switch
Extract switch
Cabin fan switch
RAM air switch
It is guarded for operation during emergency
Hot air switch
Controls the hot air valve
Zone Temperature selector dials
Air Conditioning Panel
Pack flow switch (A320)
LO- 80% air
NORM- 100% air
HI- 120%
ECON FLOW switch (A321)
Pressed IN: 80-100% air
Pressed OUT: 100-120% air
In both the cases, HI flow is automatically controlled during single pack operation or APU bleed supply.
BRAKE TURBINE ACM
Engine Bleed air pressure should be high
Turbine rotates compressor to seek the Low Pressure air sent to heat exchanger which creates a back pressure on the compressor applying a brake on the turbine due to the control valve/restriction.
Jet pump increases the flow of air in the compressor.
It is lighter than bootstrap ACM but cannot be used when aircraft is stationary.
FAN TURBINE
Similar construction like Brake Turbine ACM with a single difference of installation of a fan in place of the compressor.
Not dependent on RAM air, therefore can be operated on ground.
Light and Compact
Engine bleed air pressure should be high.
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