UNIT IV AUXILIARY SYSTEM Basic air cycle systems – vapour cycle systems, boot-strap air cycle system – evaporative vapour cycle systems – evaporation air cycle systems – oxygen systems – fire protection systems, deicing and anti icing system.

Basic air cycle systems Some of the systems are important for the safe operation of the aircraft under a variety of conditions, and some are designed to provide for emergencies. Systems not essential to the actual operation of the aircraft are commonly called as Auxiliary systems. Such systems are 1.Fire Protection system or Fire warning and Fire Extinguishing systems 2. Ice and Rain Protection system 3. Oxygen system 4. Water and waste system 5. Cabin Air pressurization system or cabin air conditioning system 6. Position and Warning system 7. Auxiliary Power system

ALTITUDE vs AIR DENSITY ALTITUDE vs AIR PRESSURE

Basic air cycle systems Pressurization systems constantly pump fresh, outside air into the fuselage. To control the interior pressure, and allow old, stinky air to exit, there is a motorized door called an outflow valve located near the tail of the aircraft. It’s about the size of a briefcase and located on the side or bottom of the fuselage. If higher pressure is needed inside the cabin, the door closes. To reduce cabin pressure, the door slowly opens, allowing more air to escape.

Basic air cycle systems

Basic air cycle systems Pressurization systems are designed to keep the interior cabin pressure between 12 and 11 psi at cruise altitude. On a typical flight, as the aircraft climbs to 36,000 feet, the interior of the plane “climbs” to between 6000-8000 feet.

Basic air cycle systems There are two main types of systems in use: • Air cycle systems • Vapour cycle systems

Air cycle systems Air cycle systems are used to cool engine bleed air down to temperatures required for cabin and equipment conditioning. Air cycle system is used because it is the simplest solution to the cooling problem, fulfilling both cooling and cabin pressurization requirements in an integrated system. However, although lighter and more compact than vapour cycle, air cycle systems have their limitations. Very large air flows are required in high heat load applications which require Large diameter ducts with the corresponding problems of installation in the limited space on board an aircraft.

Vapour cycle systems The vapour cycle system is a closed loop system where the heat load is absorbed by the evaporation of a liquid refrigerant such as Freon® in an evaporator. The refrigerant then passes through a compressor with a corresponding increase in pressure and temperature, before being cooled in a condenser where the heat is rejected to a heat sink. The refrigerant flows back to the evaporator via an expansion valve.

Vapour cycle systems Although vapour cycle systems are very efficient, with a coefficient of performance typically five times that of a comparable closed loop air cycle system. Applications are limited due to problems such as their limited temperature range and heavy weight compared to air cycle systems. The maximum operating temperatures of many refrigerants are too low, typically between 65 °C and 70 °C, significantly less than the temperatures which are required for worldwide operation.