Bleed air is supplied by the last compressor section of each engine (fig. 3-1, a foldout at the end of this chapter). This bleed air flows from the engines through two engine bleed-air pressure regulation and shutoff valves. The valves are spring-loaded closed when the system is not in use. When air conditioning is selected, the valves open and regulate bleed air to a predetermined pressure. The bleed air then passes through two engine bleed-air check valves, which prevent reverse flow from one engine to the other. At this point the bleed air from both engines enters a common duct and flows through the engine bleed-air secondary
pressure-regulating and shutoff valve. This valve is spring-loaded open and regulates the pressure of the combined flow of bleed air from both engines. The regulated bleed air then flows into the primary heat exchanger of the ACS. There are two overpressure switches (primary and secondary) incorporated in the system to prevent overpressure damage to system com-ponents in case of a pressure regulator mal-function. An air isolation valve is located in the system to provide a means of providing bleed air to the ACS when required and during cross starting of engines. These bleed-air components are discussed in the following paragraphs.
Engine Bleed-Air Pressure Regulation and Shutoff Valve
These two valves (fig. 3-1) act as system shutoff valves when air conditioning is not required. They are spring-loaded closed. When air conditioning is selected, an electric solenoid is energized, which unseats a poppet from the vent line. As air flows from the engine, a line downstream of the butterfly valve routes a small amount of the bleed air to the butterfly diaphragm. This air is called control air since its action on the diaphragm is the controlling force for the valve. As pressure builds on the diaphragm, it overcomes spring pressure holding the butterfly closed and the valve opens. As the bleed air passes through the valve, another line upstream of the butterfly routes bleed air to the regulator portion of the valve. As pressure builds and overcomes spring pressure, a poppet is reseated, allowing some of the control air pressure from the open side of the butterfly diaphragm to bleed off. Spring pressure can now start closing the butterfly, thus lowering the bleed-air pressure downstream of the butterfly. In this manner bleed-air pressure is controlled to 75 ± 15 psi.
Engine Bleed-Air Check Valve
These dual-flapper check valves are located downstream of the pressure regulator and shutoff valves (fig. 3-1). They prevent cross-flow of bleed air from one engine to the opposite engine in the event of single engine operation.
Engine Bleed-Air Secondary Pressure-Regulating And Shutoff Valve
This valve is located in the common ducting upstream of the bleed-air check valves (fig. 3-1).
It operates in the same manner as the two engine pressure regulation and shutoff valves with the following exceptions. The valve is normally spring-loaded open and regulates at a set pressure of 110 ± 5 psi, thus acting as a safety regulator in the event one or both engine regulators fail and allow pressure to build up in excess of system design.
Primary Bleed-Air Overpressure Switch
This switch is located downstream of the bleed-air check valves (fig. 3-l). This switch activates at 250 psi and provides a signal to the Digital Display Indicator (DDI). The DDI is located in the nosewheel well of the aircraft and stores failed systems/ component code numbers. These code numbers are used in troubleshooting the aircraft after flight, and aids in pinpointing malfunctions rapidly.
Secondary Bleed-Air Overpressure Switch
The pressure switch is mounted in the ducting downstream of the secondary bleed-air regulator. If bleed-air pressure at this point exceeds 150± 10 psi, the overpressure switch provides a signal to close the three pressure regulator and shutoff valves as well as store a failed system code number in the DDI.
Air Isolation Valve
The sir isolation valve serves two purposes. First, it is used to cross start engines. After starting one engine on the auxiliary power unit (APU) or ground air, with the APU switch in the off position, the engine crank switch will automatically open the air isolation valve when starting the other engine. Bleed air from the engine running is routed through the air isolation valve to the engine starter control valve (fig. 3-1) of the engine to be started. As the engine accelerates to a self-sustaining speed, the switch automatically returns to the off position. The air isolation valve is then closed by spring pressure. The air isolation valve can also be used to route APU air to augment the bleed-air supplv to the air-conditioning system at times when engine output is low. This could be when waiting to launch, with engines at idle power and air temperatures high and humid.