AC system on the airliner

[Old Fella]

How does Airconditioning work in an airliner.

If all you want to know is what your post asks, then this is a simple answer. Bleed air, taken from the Bleed Air Manifold is passed through the Air Conditioner Pack Valve (or Valves) and initially passes through a Primary Heat Exchanger (Air -to - Air) before passing through a Secondary Heat Exchanger (Air-to-Air) where it by-passes the ACM before entering a Plenum, or Conditioned Air Manifold, from where it is distributed to the various "Zones" of the aircraft. Under conditions of high demand for Cold Air the air is directed through the Air Cycle Machine where the extraction of energy by the turbine in the ACM causes the air temperature to drop. This air is then directed into the Conditioned Air Manifold and distributed as previously described. Depending on requirement in individual Zones, bleed air is added to each individual zone outlet to set the desired temperature in that Zone. (Zone temperatures can vary greatly depending on passenger numbers within the Zone) As the demand for Cold air is reduced, e.g. as the aircraft climbs to colder ambient air conditions, first the ACM will eventually be "by-passed" and put into what is virtually an "idle" circuit. As the demand for Cool air reduces even more the Inlet and Exhaust doors on the Ram Air ducts, which allow air flow over the heat exchangers, will move towards the "Closed" position to reduce the cooling effect of the heat exchangers. (Also reduces drag and therefore helps reduce fuel consumption to a small extent) The Inlet can fully close, but not the Exhaust. Pressurisation is simply a function of controlling the "rate" at which the Conditioned Air, introduced into the pressurised hull, is allowed to escape via Outflow Valves resulting in the aircraft becoming pressurised. To guard against structural damage due to "over-pressurisation" Safety Valves are fitted to relieve the pressure if the Max Differential is exceeded. Hope this explains what you originally asked.