An ECS is mainly used to control the temperature of the cabin of an air vehicle to provide the optimal conditions for the crew to operate and the passengers to have a comfortable flight.
AC Systems receive high pressure, high temperature fresh air from the engines, APU, ground carts or fixed supplies. AC Systems also recirculate and receive fresh air for ventilation in which demoisturizing will also be a part of. Another function of AC systems is to cool the cabin when needed.
The flow supply of hot/cold air needs to be controlled and monitored for optimal performance and sufficient redundancies used for failures.
To achieve this complex systems using multiple valves, fans, sensors&switches and other components are required.