Auxiliary Power Unit is an essential part of an aircraft. The primary function of this unit is to start the engine. In smaller aircraft the APU used is an electric motor. However, in the modern aircraft the APU used is an air turbine motor.
Basic parts of an air turbine motor APU are:
1. Power section
Power section like other gas turbine engine consist of compressor, combustion chamber and turbine.
It consist of a single stage centrifugal compressor, combustion chamber and 2 stage axial flow turbine.
As the APU main switch is turned on, the inlet from the right side of the APU system allows the atm air to enter to the system. The inlet air is compressed by the centrifugal compressor which is turned through a shaft coupled with a starter/ generator. The starter/ generator is powered by GPU or Battery cart.
The input from a GPU - 115V of 400kh AC current or input of 28V DC current is converted into 270V DC current using Start power Unit and finally the start convert Unit provides the necessary power to the Starter/generator to turn the APU shaft.
Now the compressed air from the Centrifugal pump enters the combustion chamber, and hot exhaust gas turn the 2 stage axial flow turbine and escape through the tail cone. The shaft of the turbine and the compressor is connected so that as the necessary pressure builds up the system becomes self sustaining. The shaft is also coupled to a Load compressor section.
2. Load Compressor section
The load Compressor section is directly coupled with the Power section so they both have the same RPM. The compressor provides the necessary bleed air to the main engine and other various mechanisms. This bleed is controlled by a valve which is controlled be ECU. This valve can turn from 15 to 110 degrees automatically.
The bleed air from the Load compressor section has following main fuctions:
1. ice prevention system
2. engine start
3. cabin pressurization
4. Air conditioning system
3. Gear box
The Gear box connected to the shaft reduces the High RPM to considerably low RPM.
Other than that it also consist of a starter generator which uses electric power or hydraulic accumulator to turn the APU shaft.
In the diagram 1,2,3 and 4 combine to become the power section of the APU.
1- inlet
2- centrifugal compressor
3- combustion chamber
4- 2 stage axial flow turbine
5- load compressor section
6- gear box
7- Starter/generator
The main manufacturer of APU system are Pratt and whitney or Honeywell.
Inlet of the APU at the right side of the tail cone.
Exhaust of the tail cone.
2nd hole of the tail cone.
Due to high pressure exhaust there is a vacuum inside the tail cone. The 2nd hole allows atm air to enter to the APU with the vacuum created by the exhaust. This atm air helps to cool the APU system without the use of any moving parts.
Basic parts of an air turbine motor APU are:
1. Power section
Power section like other gas turbine engine consist of compressor, combustion chamber and turbine.
It consist of a single stage centrifugal compressor, combustion chamber and 2 stage axial flow turbine.
As the APU main switch is turned on, the inlet from the right side of the APU system allows the atm air to enter to the system. The inlet air is compressed by the centrifugal compressor which is turned through a shaft coupled with a starter/ generator. The starter/ generator is powered by GPU or Battery cart.
The input from a GPU - 115V of 400kh AC current or input of 28V DC current is converted into 270V DC current using Start power Unit and finally the start convert Unit provides the necessary power to the Starter/generator to turn the APU shaft.
Now the compressed air from the Centrifugal pump enters the combustion chamber, and hot exhaust gas turn the 2 stage axial flow turbine and escape through the tail cone. The shaft of the turbine and the compressor is connected so that as the necessary pressure builds up the system becomes self sustaining. The shaft is also coupled to a Load compressor section.
2. Load Compressor section
The load Compressor section is directly coupled with the Power section so they both have the same RPM. The compressor provides the necessary bleed air to the main engine and other various mechanisms. This bleed is controlled by a valve which is controlled be ECU. This valve can turn from 15 to 110 degrees automatically.
The bleed air from the Load compressor section has following main fuctions:
1. ice prevention system
2. engine start
3. cabin pressurization
4. Air conditioning system
3. Gear box
The Gear box connected to the shaft reduces the High RPM to considerably low RPM.
Other than that it also consist of a starter generator which uses electric power or hydraulic accumulator to turn the APU shaft.
In the diagram 1,2,3 and 4 combine to become the power section of the APU.
1- inlet
2- centrifugal compressor
3- combustion chamber
4- 2 stage axial flow turbine
5- load compressor section
6- gear box
7- Starter/generator
The main manufacturer of APU system are Pratt and whitney or Honeywell.
Inlet of the APU at the right side of the tail cone.
Exhaust of the tail cone.
2nd hole of the tail cone.
Due to high pressure exhaust there is a vacuum inside the tail cone. The 2nd hole allows atm air to enter to the APU with the vacuum created by the exhaust. This atm air helps to cool the APU system without the use of any moving parts.
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