By Mohamed Anas Maaz

The air conditioning system is possibly one of the most complex systems in an aircraft. There are a lot of moving parts, lots of computer logic, and many other systems that depend on it.

Airplane air distribution is by means of pressurisation air conditioning kits. Image credit: bottlein | Pixabay

Airplane air distribution is by means of pressurisation air conditioning kits. Image credit: bottlein | Pixabay

The air for the system is generally taken from the engine. Hot air from the compressor stages of the engine passes through a bleed valve. This air is known as the bleed air. The bleed air is usually tapped from either the low-pressure stages or from an intermediate stage (depending on the aircraft type) of the high-pressure compressor. In some conditions, the air can also be bled from the high-pressure stage of the compressor when for example, the air conditioning demand cannot be maintained by a lower stage bleed air.

As the bleed air that is passed from the compressor stages is very hot, this air initially goes through a pre-cooling stage. On ground, the air is provided by the APU (auxiliary power unit) or by connecting a ground air conditioning unit to the aircraft.

The packs or the Air Cycle Machine (ACM)

Packs or pressurisation air conditioning kits (PACKS) consist of a heat exchanger and a compressor turbine unit. The purpose of the pack is to reduce the engine bleed air temperature to an acceptable level. The air from the engines is fed to the pack via a one-way check valve. This valve is simply a flow control valve, and some aircraft manufacturers call it the pack flow control valve. The packs are also referred to as air cycle machines.

In the pack, there is a heat exchanger. It consists of a primary and a secondary heat exchanger. When the bleed air is passed through these heat exchangers, the temperature is lowered by a certain amount. The air is drawn into the heat exchangers through ram air inlets. These inlets are placed on the belly of the aircraft.

The air initially passes through the primary heat exchanger where it gets cooled. Then it is passed through a compressor. This compressor increases the pressure of the air, which also heats it a little. Then it is passed into the secondary heat exchanger, where the temperature is reduced a second time. From the secondary heat exchanger, it is then passed into a turbine. In the turbine, the air expands and cools. This drops the temperature of the air by a significant amount.

The computers can also control the opening and closing of the ram air inlets. When heating is required, the inlet can be closed as necessary so that less cooling air passes through the heat exchangers. In some aircraft, the ram air inlets are closed for take-offs and landings to prevent debris entry.

The hot air that builds up in the heat exchangers is continuously sent overboard by a cooling fan which is run by the compressor of the pack. The air is exhausted out of the aircraft through ram air outlets, which are also placed on the belly of the aircraft.

How cabin temperature is regulated

The temperature of the air that is given out by the pack is very cold. Sometimes it is too cold for normal passenger and crew comfort. So, there needs to be a way to control the temperature of the air. To control the temperature, a bypass valve in the pack can be controlled by the pilots. When the bypass valve opens, hot air from the primary heat exchanger is tapped and mixed with the normal pack outflow air.

In the cockpit, the pilots use knobs or switches pretty much like on a car to demand a certain temperature. This temperature is compared to the current temperature of the air in the aircraft cabin. Using this information, the pack control computers adjust the position of the bypass valve to meet the pilot’s demanded temperature value.

The air that is passed through the cabin is also recirculated. This increases the efficiency of the system as recycling of the air reduces the bleed air demand on the engines. The air is recirculated using recirculation fans or cabin fans, which are then passed into a mixer unit, where the pack air and recirculated air mix together before being sent into the cabin. To prevent foul smell in the cabin, the air that is circulated through the lavatory and galley areas is drawn by an extraction fan, which then removes it from the aircraft through outflow valves. Airplanes also use high efficiency particulate air (HEPA) filters that remove more than 99.9% of particulates such bacteria and viruses from the air before it is recirculated to the cabin.

The cabin air distribution

In smaller aircraft, such as turboprops, the packs divide their load. For instance, in some aircraft, one of the packs supply the cockpit, and the other feeds the cabin. If one were to fail, the remaining pack can supply both the compartments at reduced efficiency.

In larger aircraft, with bigger cabins, more control over the temperature regulation becomes necessary. For this, these aircraft utilise something called trim air valves. The trim air valves can directly manipulate the air sent by the packs.

To achieve this, the trim valves have a direct engine compressor bleed air feed controlled by a hot bleed valve. When the pilot, for instance, demands to heat the aft cabin, the aft cabin trim valve opens, adding hot air into the air sent by the packs and the mixer unit. The aft trim valve is operated such that demanded temperature is met.

There can be several trim valves depending on how large the aircraft is. The larger it is, the more zones the cabin is divided into, which requires more trim valves. For an aircraft like the Airbus A320, the cabin is divided into the cockpit, forward, and aft cabin. For an A330, the cabin is divided into the cockpit, forward, mid, and aft cabin.

Each of these zones has its trim air valve, which can be used to individually control the temperature of the zone.

What happens if the air conditioning system fails?

When the air conditioning system fails, the aircraft can no longer maintain a pressurised cabin as conditioned air is used for the purpose. This requires a descent to a safe altitude (10,000 ft). Once fully depressurised and with no conditioned air, it can be quite uncomfortable for passengers and crew alike, even when below 10,000 ft. So, to make the conditions more bearable, airplanes have an emergency ram air inlet. The air from the outside environment can be fed to the cabin through this inlet. Once this valve is opened, the pressurisation outflow valve automatically opens to prevent any pressure build-up in the cabin.

In older aircraft, this outflow valve must be opened manually. But in newer aircraft, once the emergency inlet is open, the outflow operates automatically as required to maintain a comfortable cabin environment.

Article Source: Simple Flying