What is ECS and PACK in aircraft?

Environmental Control System (ECS) is one of the largest power consumption and non-propulsive systems for civil aircraft. It provides air supply, thermal control, and cabin pressurization for the crew and passengers. ECS comprises various systems performing the following functions: bleed air supply, bleed leak detection, air conditioning, distribution, avionics cooling, cabin pressurization control, oxygen supply.

Functions, Working Principle and Components of ECS and PACK: -

The Environmental Control System (ECS) of an aircraft provides conditioned air to pressurise the cabin and regulate the temperature and humidity throughout cabin. It also supplies the cooling air into the avionics bay to prevent the electronic equipment from overheating.

ECS contains two Passenger Air Conditioners (PACK), which are the primary system for conditioning the airflow within the ECS. The overall environmental control system of a typical civil aircraft is composed of several subsystems, which are the Bleed Air System (BAS), the Anti-Icing System (AIS), the Pressurized Air Conditioner (PACK) and the Cabin Pressure Control System (CPCS).

The bleed system provides pressurized air to the PACK for conditioning. The Bleed Air System is also known as the Pneumatic System. Bleed air is supplied by the engines or the Auxiliary Power Unit (APU). Engine bleed is typically used in flight whereas APU bleed is typically used on the ground and permitted in flight up to a certain altitude

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Part of the bleed mass flow before entering the PACK is taken out to drive the Ram turbofan to drive cold ambient air over the heat exchangers in the PACK to enable pre-cooling of the bleed air. After conditioning of the bleed air in the PACK, the air is mixed with trim and recirculating air from the cabin in the mixing manifold before it is distributed to different zones of the aircraft.

The PACK is the primary system for conditioning the airflow within the ECS. It consists of Valves, Heat Exchangers, an Air-Cycle Machine (ACM) and a High-Pressure Water Separator (HPWS).

The PACK consists of a PACK Valve (PV) and Temperature Control Valves (TCV) which regulate the hot mass flow through the system and the core. The Primary Heat Exchanger (PHX) and Secondary Heat Exchanger (SHX) use the cold Ram air as a heat sink. In between the heat exchangers sits the refrigeration unit of the PACK. The ACM drives the air through the core and contributes to cabin pressurization. Towards the end of the PACK sits the HPWS consisting of Reheater (RHX), Condenser (CHX) and Water Separator (WS) which regulates air temperature to enable condensation and extraction of water from the air to regulate humidity. The PACK has a control system that governs the opening of the valve by using temperature sensors at the WS outlet (to regulate the TCV opening) and a Compressor outlet (to regulate the Ram air mass flow modulating door) to meet cabin demand temperature. In addition to that, there are three temperature switches: at the compressor outlet, WS outlet and PACK outlet, which shut down the PV to prevent any damage to the system in the case of overheating.

Types of ECS

There are other ECS systems, such as electrically driven ECS (used in modern aircraft), pneumatic ECS with low-pressure water separator (LPWS) (generally used in older generation aircraft), vapour cycle ECS (used for light turboprop aircraft application) and membrane-based dehumidifier (used in spacecraft).

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