THE ENVIRONMENT CONTROL SYSTEM (ECS)

The higher you fly in the zone of the atmosphere known as the troposphere, the drier, colder and thinner the air becomes. For these reasons, when flying at high altitudes, it becomes necessary to have an air conditioning system capable of maintaining humidity, temperature and pressure values acceptable to the human body.
Air is said to be 'dry' when it is devoid of water vapour, i.e. when it is devoid of humidity. On the other hand, air is said to be 'rarefied' when it has a lower density due to a decrease in pressure. The troposphere, moreover, is that layer of atmosphere extended in height for about 10-15 kilometres from sea level, in which clouds and atmospheric precipitation such as rain, snow, hail, dew and frost can be found.
The cruising altitudes reached by the most common airliners are around 42000 feet, more or less 12500 metres. At these altitudes the outside pressure can be as much as one fifth of that at sea level, while the outside temperature can often reach -55 degrees Celsius.
To ensure comfort on board, it is necessary to create climatic conditions inside the cabin that are pleasant for the passengers. For this purpose, temperatures between 20 and 25 degrees Celsius and pressures no more than 25 per cent below sea level are generally set. An adequate exchange of air must also be ensured, and to establish the right level of humidification, drinks should be offered frequently.
To ensure that the interior of the aircraft does not become too cold due to the cold outside temperatures and that the air in the cabin remains at the desired pressure, the entire cabin, the animal hold and often also the baggage hold are pressurised and kept at a controlled temperature. For the entire duration of the flight, it will therefore be necessary to ensure that new air is sent into the cabin at pressures and temperatures higher than those outside. This also compensates for the consumption of oxygen due to the presence of passengers, the diffusion of odours that may be unpleasant and the presence of any leaks from the fuselage.
An adequate level of air purity must also be ensured, eliminating carbon dioxide due to breathing, microorganisms such as bacteria and viruses, dust, the presence of ozone and all the various impurities that may be present.
Even when the aircraft is at low altitudes, where temperatures are higher, or on the ground, it is necessary to ensure adequate comfort for the passengers on board by cooling the interior of the aircraft, via the cooling system, or heating it, via the heating system, as required. Therefore, all functions related to pressurisation, air conditioning and also the supply of oxygen during emergencies are managed by a single environmental control system, better known as the 'Enviromental Control System (ECS)'.
The cooling system, also known as the 'Cooling System', is based on a refrigeration cycle that can be either open or closed loop.
The heating system, on the other hand, known as the 'Heating System', can make use of exhaust gases, special burners, compressed air tapped or, as they say, 'bleed-dried' from turbojets, or it can adopt an electric resistance heating system.
The operation of the ECS is governed by an automatic electronic system, which becomes manual in an emergency. Temperature conditions are also manageable by the crew or passengers themselves, so as to ensure individual comfort as far as possible. A series of alarm sensors also warns the crew in the event of an excessive rise or fall in temperature inside the aircraft or a rapid depressurisation of the cabin. In the latter eventuality, the oxygen system comes into operation, providing oxygen at the right pressure to each passenger.
Air from the galleys, areas set aside for food and drink preparation, and from the toilets is drawn in by forced ventilation and discharged near the outflow valves to the outside. The pressurisation system, also known as the 'Pressurisation Control System', in fact, also manages the discharge of air by means of valves that allow its exchange and, at the same time, regulate the pressure inside the fuselage.