Vapour Cycyle Machine Air Conditioning System (ATA 21)

Generally, reciprocating engine aircraft cruise at around 14,000 ft altitude, a critical phase where the temperature can fluctuate significantly. These variations in temperature can pose challenges for pilots, affecting their comfort and performance during the flight.

Recognizing the importance of maintaining optimal conditions for the flight crew and passengers, as well as ensuring the proper functioning of essential systems, aircraft are equipped with a sophisticated vapour cycle air conditioning system.

This air conditioning system plays a crucial role in creating a comfortable and safe environment inside the aircraft. It helps regulate the temperature, humidity, and air quality, providing a more pleasant experience for everyone on board.

Moreover, the vapour cycle air conditioning system contributes to the overall efficiency of the aircraft by cooling specific areas and systems that are essential for the smooth operation of the flight.

By effectively managing the temperature and humidity levels inside the aircraft, the vapour cycle air conditioning system ensures that both the flight crew and passengers are comfortable throughout the journey. This not only enhances the overall travel experience but also plays a vital role in maintaining the well-being and performance of the individuals on board.

Furthermore, the air conditioning system helps prevent the risk of overheating in critical components of the aircraft, such as avionics and other electronic systems, which are sensitive to temperature fluctuations. By maintaining a controlled environment, the system contributes to the longevity and reliability of these essential systems, ultimately ensuring the safety and efficiency of the flight.

In addition to passenger comfort and system reliability, the vapour cycle air conditioning system also plays a key role in optimizing fuel efficiency. By cooling specific areas and equipment, the system reduces the overall workload on the aircraft's cooling systems, resulting in lower energy consumption and improved performance. This not only benefits the environment by reducing emissions but also contributes to cost savings for the airline operators.

It is comprised of 5 principle sections:

1. Air Supply

2. Heating

3. Cooling

4. Temperature Control

5. Distribution

   
   

COMPOSITION

1. FREON

   • Refrigerant medium

   • Alternates between vapour and liquid state.

   • Boiling point: 3.5 degree celcius at STD sea level pressure.

     • If pressure increases, b.p. increases.

2. Evaporator

3. Thermostatic Expansion Valve (TEV)

4. Compressor

5. Condensor

6. Expansion Valve

7. Reservoir/Receiver (stores FREON)

8. ELEC. driven Blowers

   • Extracts side air across the evaporator

     
     

WORKING

Reservoir

The compressor is the heart of an air conditioning system. It moves the refrigerant through the system, and it divides the system into its high side and low side.

Expansion valve

It increases and decreases the pressure and temperature before transferring it to the evaporator.

Evaporator

The evaporator is the part of the air conditioning system where the cold air is produced.

It is made of a series of tubes over which thin sheet aluminium fins have been pressed. The area provided by the fins allows a maximum amount of heat to be picked up from the air inside the cabin and transferred into the refrigerant inside the evaporator tubing. The evaporator is usually mounted inside a shroud in such a way that a blower can pull hot air from inside the cabin and force it through the evaporator fins.

After the air leaves the evaporator, it blows over the occupants of the cabin.

The blower is equipped with a speed control that allows the pilot to vary the amount of air blowing across the evaporator coils. The thermostatic expansion valve is mounted at the inlet of the evaporator, and it breaks the refrigerant up into a fine mist and sprays it out into the coils. The refrigerant flowing through the coils picks up heat from the fins, is warmed, and turns into a gas. The air passing through the fins loses some of its heat and is cooled. The temperature-sensing bulb of the TEV is clamped to the discharge line of the evaporator, and it is insulated with tape so it is not affected by any temperature except that caused by the refrigerant vapors inside the evaporator.

In simple words, it takes heat from the room/cabin air, i.e. cooling the room/cabin air which boiks the FREON turns it to Low Pressure vapour before sending to compressor.

Thermostatic Expansion Valve (TEV)

The thermostatic expansion valve (TEV) is a metering device that measures the temperature of the discharge end of the evaporator to allow the correct amount of refrigerant to flow into the evaporator.

All of the liquid refrigerant should be turned into a gas (it should evaporate) by the time it gets to the end of the evaporator coil. Several types of thermostatic expansion valves are installed in aircraft air conditioning systems.

This section discusses both internally and externally equalized TEVs. Before discussing these valves, we must understand the term “superheat.” Superheat is heat energy added to a refrigerant after it has changed from a liquid into a vapor.

Refrigerant that has superheat in it is not hot, it is very cold.

Compressor

The compressor pulls the low-pressure refrigerant vapour from the evaporator and compresses it.

When the vapour is compressed, its pressure and temperature both go up. The compressor carries a specified amount of special moisture-free refrigeration oil that lubricates and seals the compressor, and circulates through the system with the refrigerant.

It sends the compressed high temperature and pressurised air to the condensor.

Condensor

The refrigerant leaves the compressor as a hot, high-pressure gas, and flows to the condenser mounted where outside air can pass through its fins.

The condenser is made of high-pressure tubing wound back and forth, with thin sheet metal fins pressed over the tubes. The hot refrigerant gas enters one side of the condenser and gives up some of its heat to the air flowing through the condenser fins. When the system is working properly, about two thirds of the condenser is filled with refrigerant gas, and the rest contains liquid refrigerant.

In simple words, it lowers the heat using RAM air and turns it to high pressure liquid.

Condensor again sends it back to the Reservoir and hence, the reservoir is also called receiver.

This cycle keeps on going and therefore, the vapour cycle ACS operates in a continuous cycle.