1. General
A. The air conditioning systems provide conditioned air to the control cabin, passenger cabin, electronic equipment compartment, forward cargo compartment, air conditioning distribution bay and aft cargo compartment.Air supply to the air conditioning system is furnished by the pneumatic system from either engine bleed air or the auxiliary power unit (APU) in flight; from engine bleed air, APU bleed air, ground pneumatic supply cart, or from a ground conditioned air supply cart during ground operation. Part of the warm air supply from the engines or pneumatic cart is passed through air conditioning packs to be cooled. The cold air is then mixed with the remainder of the warm air as required to obtain the conditioned air temperature called for by the temperature control system. This conditioned air then passes into the control and passenger cabins through the distribution system. Galley and toilet vents, and equipment cooling on the ground exhaust cabin air without regard for cabin pressurization control requirements. The combined flow from all outlets is limited to a value less than that which enters the cabin from the air conditioning system. The outflow valves are regulated to exhaust only that additional quantity of air required to maintain the desired pressure in the cabin. B. This chapter considers the air conditioning system as four subsystems:distribution, pressurization control, cooling, and temperature control.
2. Distribution
A. The conditioned air distribution system routes temperature controlled air to the passenger and control cabins. One duct system supplies the control cabin and a separate duct system supplies the passenger cabin. Both systems originate at the main distribution manifold. Riser ducts connect to an overhead duct extending fore and aft the full length of the passenger cabin. A nozzle at the bottom of the duct and outlets at every other window bay releases a balanced flow of air directly into the cabin. Air for the control cabin is extracted from the LH air conditioning pack outlet before it enters the main distribution manifold.
B. A gasper outlet provides each crewmember and passenger a method for ventilating his local area. An adjustable nozzle at each station allows the individual a choice from no airflow to maximum airflow from the distribution system.
C. A recirculation fan attached to the main distribution manifold recirculates conditioned air from around the forward cargo compartment into the distribution system manifold where the air mixes with the air supplied by the air conditioning packs.
3. Pressurization Control System
A. The pressurization control system includes pressurization control,pressurization relief valves, and pressurization indicating and warning.
B. AIRPLANES WITH DIGITAL PRESSURE CONTROLLERS;
The pressurization control system is an electrically operated and electronically controlled system. It controls the exhaust of air to pressurize the passenger and control cabins, the electronic compartment,the two cargo compartments, and the lower nose compartment. The pressurization control system has a pressure control panel, two pressure controllers, and a cabin pressurization outflow valve.(1) The system has three modes: automatic, alternate, and manual. Automatic is the mode for normal operation. The alternate mode and the automatic mode are fully redundant. If one of these two channels fail, the system will automatically switch to the other.Normal operation requires no adjustment by the flight crew during the flight except for barometric correction.(2) If the two automatic channels do not operate (AUTO and ALTN), the crew can set the mode selector to manual mode. In the manual mode the outflow valve is controlled directly by the toggle switch on the selector panel.
C. AIRPLANES WITH AN ANALOG PRESSURE CONTROLLER;
Cabin pressure is controlled by positioning a cabin pressurization outflow valve to meter cabin air exhaust. The valve operates electrically from three control systems. The control systems are AUTO, STANDBY and MANUAL. AUTO mode provides automatic pressure control of cabin altitude and rate of altitude change based on settings made to the pressurization control panel (P5) of flight and landing altitudes.STANDBY mode provides semi-automatic control of cabin altitude and rate of altitude change based on settings of the pressurization control panel(P5) of cabin altitude and rate selected. MANUAL mode provides control of cabin altitude and rate of altitude change by the selection of MAN AC or MAN DC and operation of a three position (CLOSE-OFF-OPEN) manually operated toggle switch. Normal control is from the AUTO system with the STANDBY system acting as a backup. The MANUAL system functions as an override over the AUTO and STANDBY systems.D. Pressurization relief valves include two safety relief valves, which prevent overpressurizing the airplane, and a vacuum relief valve which prevents pressure inside the airplane becoming appreciably less than ambient. In addition to the relief valves, pressure equalization valves are installed in both cargo compartments to provide a quick method of allowing cargo compartment pressure to vary, within limits, with cabin pressure. Blowout panels are used in both cargo compartments to protect against a sudden differential in pressure between the cargo compartment interior and exterior.E. Indicators are provided to allow monitoring cabin altitude, differential pressure, and rate of pressure change A cabin altitude warning system sounds a horn if cabin altitude exceeds approximately 10,000 feet.
4. Cooling
A. All cold air required for air conditioning is provided by air conditioning packs. Passing bleed air through a primary heat exchanger,an air cycle machine, and a secondary heat exchanger cools the air sufficiently to handle any cooling situation required. A ram air system provides coolant air for the heat exchangers. A ram air modulation control system automatically regulates the supply of coolant air during flight to obtain required cooling with minimum aerodynamic drag from the system. A turbofan draws air through the system for ground operation and in flight when flaps are extended. A water separator removes excess moisture from the cooled air which is then passed through a water injector and sprayed on the secondary heat exchanger core to aid in cooling bleed air. Various thermal switches, thermostats, sensors, and valves are included to provide automatic protection and warning of an air malfunction.
5. Temperature Control
A. Temperature of the air entering the distribution systems to the passenger
and control cabins is regulated by positioning mix valves. The mix valves proportion hot bleed air with cold air from the air conditioning packs to provide air conditioned comfort in the passenger and control cabin. There is one mix valve for each air cycle system. Two sets of controls on the forward overhead panel provide automatic or manual control and monitoring for each system. The control cabin controls the left air cycle system and the passenger cabin controls the right air cycle system. Each control system consists of a temperature controller,temperature selector, temperature indicator, temperature sensors, overheat control and warning light, mix valve, and mix valve position indicator.
