*
IP air is the primary source. HP air adds air as necessary
*
IP air is the only source.
The primary mode uses the IP sensor to get data about the pressure downstream of the HPSOV. The IP data is used to calculate the position of the HPSOV necessary to keep the setpoint pressure. These are the duct pressure setpoints:
*
40 psig when the altitude is less than or equal to 27,000 feet (8230m)
*
Decreasing value, 40 through 32.5 pisg when the altitude is more than 27,000 feet (8230m) and is less than or equal to 43,000 feet (13,106m)
*
32.5 psig when the altitude is more than 43,000 feet (13,103m).
The value of the setpoint pressure is biased by any of these conditions:
*
CTC request for more pressure -6 psig bias
*
Engine cross bleed start -9 psig bias.
The setpoint pressure increases by the amount of the highest request.
If the quantity of the intermediate pressure air is enough to supply the user systems, the HPSOV closes.
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ENGINE AIR SUPPLY SYSTEM -FUNCTIONAL DESCRIPTION -2
See the functional description part of the air conditioning -pack cooling and mix manifold temperature control section for the conditions that cause the CTC to request more pressure (SECTION 21-52).
Bleed Source Selection -Backup and Pneumatic Modes
The pneumatic mode automatically gives bleed source selection when the backup mode or the pneumatic mode controls the engine air supply.
Similar to the primary mode, the pneumatic mode provides three combinations for the source of engine bleed air:
*
HP air alone
*
IP air with HP air added as necessary
*
IP air alone.
The pneumatic mode uses the HPFAC internal reference pressure regulators (not shown) to control HPSOV position. The regulators are set to provide the HPSOV with the necessary control pressure so that the valve regulates to a setpoint of 55 +/-15 psig. This setpoint is designed to make sure pressure downstream of the HPSOV is more than sufficient for the systems that use engine bleed air.
If IP air pressure is less than the setpoint, then HP is the only source for engine bleed air.
If IP air pressure is more than the setpoint but can not provide the necessary air flow to keep pressure downstream of the HPSOV above the setpoint, the HPSOV adds HP air as necessary.
If IP air provides the necessary air flow and keeps pressure downstream of the HPSOV to a value more than the setpoint, the HPSOV closes.
HPSOV Clamp Logic
Primary mode provides HPSOV clamp logic. HPSOV clamp logic closes the HPSOV when the HP air source pressure is high. The primary reason for the HPSOV clamp logic is to prevent too much engine bleed if a pneumatic distribution duct fails. The logic gets pressure information about the HP air source from the EEC. The set pressure changes with airplane altitude.
This is an example of the airplane altitudes and the HP air pressure values that cause the HPSOV to close:
* -2,000 feet (-610m) through 20,000 feet (6,100m) the set
pressure is 220 psia
*
> 20,000 feet (6,100m) through 43,000 feet (13,115m) the value decreases with altitude
*
> 43,000 feet (13,115m) the value is 115 psia.
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ENGINE AIR SUPPLY SYSTEM -FUNCTIONAL DESCRIPTION -2
Pressure Regulation -Primary Mode
The pressure regulation function uses the HPSOV and PRSOV to control duct pressure. The HPSOV gives pressure regulation when the HP source gives air. The PRSOV gives pressure regulation when the HPSOV is closed. The HPSOV and the PRSOV regulate to different values.
Primary mode control of the HPSOV is the same as for the bleed source selection function.
The position of the PRSOV controls duct pressure to the usual setpoint value of 50 +/-5 psig.
Pressure Regulation -Backup and Pneumatic Modes
The pneumatic mode automatically gives pressure regulation when the backup mode or the pneumatic mode controls the engine air supply.
The pneumatic mode uses the reference pressure regulators (not shown) in the PRSOVC to give pressure regulation. The regulators are set to give the PRSOV the necessary control pressure so that the valve regulates to a usual setpoint of 60 +/-15 psig. This setpoint is designed to make sure pressure downstream of the PRSOV is more than sufficient for the systems that use engine bleed air.
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