There is no better example to start a topic about cooling fluids than aircrafts. We all have today the option to travel by plane anytime and anywhere in the world. When you do that, of course maybe the only inconvenient you might have is that you must sit and keep the seat belt tighten during the flight and probably you get bored when the flight is on long distance and it takes couple of hours. Sometimes you get up from your seat because you need to go to the toilet, but in rest you don’t have other discomfort being in the cabin. You can still enjoy the voyage. You can enjoy listening to music, reading something or even watching a movie provided by the aircraft screen placed on the backside of the seat in front of you. But for these things to occur very normal and pleasant, there is a lot of research and development behind all that and obviously a lot of material science.
In case of domestic uses for refrigerators and air-conditioning systems, the most frequently used cooling fluid is butane. Even if this is a highly flammable substance, the risk of using butane may be small enough for household purposes.Yet the risk of generating hazard is still too high for aircraft engineering. These days liquid refrigerants aren’t used in aircraft air-conditioning systems. Instead, air is actually sucked in from outside the plane, and through a series of compression and expansion cycles is used to cool the interior – it’s very cold out there, after all.
The downside of this, though, is that when the plane is on the tarmac, the air conditioning doesn’t work very well because the air on the ground is warmer. Which is why, adding to the general pleasures of sitting on a delayed flight, when you’re stuck on a plane on the tarmac, waiting for takeoff, it can be sweltering. So a plane’s air-conditioning system does more than just regulate temperature and humidity, though; it’s also set to equilibrate the air pressure inside the cabin.
Let’s briefly recall how a plane travel goes. Whenever you travel by plane, The first thing to observe while you are inside the cabin is how the exit doors of the aircraft are made. You will notice that they all have a porthole with a large red handle on it (as shown in Fig.1). This is one of the main element responsible for your comfort during the flight, because it completely seals the inside of aircraft from the outdoor environment. This red handle must stay locked for the entire journey. If you are stupid enough to pull this handle open, then what will happen next is that the air inside the cabin would be sucked out, along with you and anyone else not wearing a seat belt. Everyone who is strapped in would stay put, but the air temperature on the plane would drop to approximately -50°C, and the air pressure would also drop, making it very difficult to breathe. At this point, as we know from the pre-flight safety briefing, the oxygen masks would fall from their overhead lockers.
COMPOSITION OF ATMOSPHERE
The atmosphere of Earth is the layer of gases, commonly known as air, that surrounds the planet Earth and is retained by Earth‘s gravity. The mixture of gases that make up the earth’s atmosphere is composed principally of :
- 78% nitrogen (N)
- 21% oxygen (O), and
- 1% remaining various gases in smaller quantities.
Structurally the Earth’s atmosphere has 5 main layers as follows:
- THE TROPOSPHERE = The air we breathe exists here. Almost all the weather phenomena like rainfall, fog & hailstorm occur in this layer.
- THE STRATOSPHERE = This layer is almost free from clouds and associated weather phenomenon, making it ideal for Flying Aeroplanes. It contains a layer of Ozone Gas, which protects us from the harmful effect of the sun rays.
- THE MESOSPHERE= Meteorites burn up in this layer on entering from space.It is the Coldest Layer of the atmosphere.
- THE THERMOSPHERE = It plays a crucial role in Earth’s climate system by acting as a barrier that absorbs a significant amount of solar radiation, preventing it from reaching the surface. The thermosphere houses the ionosphere, spanning it and the mesosphere below. This layer is vital for radio and GPS technology, created by ionisation where solar rays strip electrons from atoms, forming a zone of charged particles.Radio waves transmitted from the Earth are reflected back to the Earth by this layer.
- THE EXOSPHERE= The exosphere is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density is so low that molecules are essentially collisionless.Light gases like helium and hydrogen float into space from here.
As altitude increases, the total quantity of all the atmospheric gases reduces rapidly. However, the relative proportions of nitrogen (N) and oxygen (O) remain unchanged up to about 80km above the surface of the Earth. Atmospheric pressure at sea level is 1 atm (14.7 psi) because if a 2,5 cm2 (one square inch) column of air stretching from sea level into space weighs around 6,6kg. Atmospheric pressure decreases with increasing altitude. Most civilian aviation takes place in the troposphere in which temperature decreases as altitude increases.
At 12.000 meters altitude (12km), the air outside doesn’t have enough oxygen for people to breathe easily – or at all. So the air pressure inside the plane cabin has to be a lot higher than the air pressure outside.This puts the skin of the fuselage in essentially the same stress state as a balloon, causing the aircraft to bulge. The bulging can lead to cracks, so to minimize the chances of their formation the air-conditioning system makes a compromise: the pressure is set to be high enough to allow people to breathe normally, but not so high that the aircraft skin is put under undue stress. As the plane descends, the air-conditioning systems pump more air into the cabin to equilibrate to pressure levels on the ground, which is why your ears pop.
The low air pressure at altitude is, of course, the very reason why we fly so high; the lower density of the air provides less resistance to our passage, making the aircraft more fuel-efficient and allowing it to fly further. Nevertheless it presents a dual problem for aircraft engineers: they have to find ways of keeping their passengers from asphyxiating and developing hypothermia. They’ve achieved this through air conditioning, the history of which involves some of the most dangerous liquids on the planet. The turbulence at high altitudes is caused by changes in the density of the air we are flying through; because of weather patterns below, we are passing through a mixture of low- and high-density air. As the plane hit pockets of high-density air, it slows, because of the increased drag on the aeroplane.Then when it comes upon the low-density pockets, it will drop ‘suddenly, as lower-density air provides less lift to the wings.
But despite the rapid changes in air pressure outside, our breathing is fairly normal; the cabin pressure, though lower than we are used to, is not fluctuating. This is thanks to the air conditioning, a field of engineering so specialized that even Einstein became interested in it, in his day, and was awarded several patents for his innovations, although at the time he was more interested in saving lives on the ground, rather than allowing people to breathe during long-haul flights.
Therefore Planes don’t carry liquid oxygen on board for emergencies. In case of a loss of cabin pressure, the masks that drop from the overhead locker will supply you with oxygen made by a chemical oxygen generator – they create oxygen gas through a chemical reaction, allowing them to be very compact and lightweight, both essential features for anything carried on board an aircraft. Personally I’ve never been on a flight where the oxygen masks have been deployed, and I’m fascinated by how well those systems are hidden.
THE WORKING PRINCIPLE OF MODERN AIRCRAFT AIR CONDITIONING SYSTEMS
Air conditioning is provided in aircraft for the comfort of passengers. The same conditioned air is also used for cabin pressurization in the airplane. The most used type of air conditioning system for turbine passenger jets is called Air Cycle Air Conditioning (ACAC).
The air for an air cycle system is supplied by the engines. Excess air or bleed air is bled off from the engines and this hot air is routed through a primary heat exchanger to cool slightly then into the Air Cycle Machine (ACM). In the Air Cycle Machine, the air is compressed which raises its temperature again. So at any given time, about 50% of the air in the cabin is freshly pumped in and 50% is recirculated. The recirculated air has been drawn out of the cabin by fans and routed through a series of High-Efficiency Particulate Air (HEPA) filters. The air conditioning system is supplied by air processed through two packs that regulate airflow and temperature as required. Airplane air conditioning system mix hot and cold air to achieve the desired temperature. Aircraft types vary, but the principles and operations of the air conditioning system are the same in all aircraft. Vapor Cycle Air Conditioning (VCAC) is another AC system type that is more common with reciprocating aircraft.
The primary parts of the air conditioning system have these 3 main functions, namely to:
- Control the fresh air flow for airplane pressurization and ventilation
- Control the flight compartment and passenger cabin temperature
- Recirculate the cabin air for ventilation.
Recirculation of cabin air goes like this: The recirculation system uses two fans to move air from the passenger compartment to the mix manifold. This system reduces the amount of air that the packs need to supply. This part of the A/C system recycles approximately 50% of the cabin air for ventilation purposes. This reduces the quantity of fresh air from the pneumatic system for ventilation. On the plane there are recirculation fans and filters on each side, both on the left and on right side of the fuselage. They are the primary components. High-efficiency air filters are used in modern aircraft, which capture more than 99.9% of airborne in filtered air. They are similar to those used in hospital operating rooms.
Computer Aided Design & The 118 Elements 
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