Unformatted text preview: ATA-21 A/C Pressurization and Air-conditioning System
The purpose of pressurization is to maintain a safe cabin altitude relative to the aircraft altitude and in association with an environmental air conditioning system provide a comfortable environment when flying at the higher altitudes. The pressurization artificially creates a lower altitude with in the cabin (cabin altitude) relative to the aircraft altitude using pressurized air. The main components source of pressurized air Pneumatic/Electronic pressure controller cabin air pressure regulator with various valves, ROCI, Cabin altimeter and differential pressure gauge and warning systems. The pressurization is achieved by the pressure controller passing a pneumatic or electrical command signals to the discharge valves (out flow valves) which respond by increasing or decreasing the restriction the flow of air from the cabin to atmosphere. Modes of pressurization: The Pressurized mode in which the out flow valve remains open and the cabin pressure is the same as the ambient pressure. The Isobaric mode in which the cabin pressure is maintained at a specific cabin altitude as flight altitude changes the controller will maintain the selected cabin altitude up to the flight altitude that produces the maximum differential pressure for which the aircraft structure is rated. The Constant Differential mode in which the cabin pressure is maintained a constant amount higher than the outside air pressure the maximum differential pressure is determined by the structural strength of aircraft cabin (greater stress). Sources of pressurizing air: Small reciprocating engine powered aircraft from the compressor of the engine turbo charger. Large reciprocating aircrafts have engine driven air compressors. For jet engines from the engine compressor bleed air or APU compressor. Cabin pressurization controller: It provides the control signals for pressurization system. The cabin altitude dial is graduated up to 10,000 feet with 3 knobs first knob for desired cabin altitude second for correcting the barometric scale third for cabin rate of climb. Cabin air pressure regulator uses cabin altitude for its isobaric control, Barometric pressure for differential range of control, Cabin rate of climb controller controls rate of pressure change inside the cabin. Electronic pressurization controller: Electronic pressurization controls are used in new generation aircrafts to set the position of the out flow valves which are operated by AC or DC actuators. The controller has got 3 modes Auto, Standby and Manual. This controller is a shaping and summing network with information derived from the air data computer cabin altitude control and various systems within the aircraft and a reference signal is produced. This reference signal will be compared by cabin altitude controller to the signal produced by cabin altitude monitor if any disagreement exits between two signal a correcting error signal is produced, applied and modulates the out flow valve. Over ride circuits constantly monitor the performance of the system for the cabin altitude or rate of change exceeding defined limits within the pressurization schedule. If any deviation occurs it changes to Standby mode (Auto1 to Auto2) or deactivates the system completely with appropriate flight desk indication and aural warnings. The ascent or decent of aircraft altitude 500 feet subjected to the following criteria (a) (b) (c) (d) Valves: Outflow valve controls the cabin pressure by opening to relieve all pressure above that for which the cabin pressure control is set. This valve is controlled by cabin pressure controller and it maintains the desired cabin pressure. Negative pressure relief valve opens any time the outside air pressure is greater than the cabin pressure. It prevents the cabin altitude ever becoming greater than the aircraft flight altitude. The relative aircraft altitude The auto rate selected The landing field height (QFE) The maximum cabin to ambient to differential pressure limit. Cabin air pressure safety valves are combination pressure relief, vacuum relief and dump valve. The pressure relief valve prevents cabin pressure from exceeding a predetermined differential pressure above the ambient pressure. The vacuum relief valve prevents ambient pressure from exceeding cabin pressure by allowing external air to enter the cabin when the ambient pressure is greater than the cabin pressure. The dump valve removes all positive pressure from the cabin and prevents the cabin from being pressurized when the aircraft is on the ground. When the dump valve switch in ram or auxiliary ventilation position the solenoid valve opens dumping the cabin air to the atmosphere. Air-Conditioning System
It provides a comfortable air temperature (between 700F 210c & 800f 270c) within the cabin of the fuselage of aircraft in addition it controls humidity, fogging of windows and maintenance temperature at wall panels and flows at a comfortable level or provide total environment for the comfort of crew and passengers. Air conditioning system controls temperature as well as humidity Air-cycle cooling system: Compressed bleed air passes through two heat exchangers where it gives up some of its heat then it drives an expansion turbine where it losses still more of its heat energy as the turbine drives a compressor. When the air leaves the turbine it expands and its pressure and temperature both are low. Heat exchanger is a device used to exchange heat from one medium to another. radiators, condensers and evaporators are the examples. Heat always moves from the object or medium having the greatest level of heat energy to a medium or object having a lower level. Vapor-cycle cooling system: This system comprises of compressor, condenser, receiver-drier, thermostatic expansion valve and evaporator Compressor in which the low pressure refrigerant vapors after they leave the evaporator or compressed to increase both the temperature and pressure before they pass into the condenser. Some compressors are driven by the electric motors or by hydraulic motors. for light aircrafts are belt driven from the engines. Condenser in which the heat taken from the aircraft cabin is given up to the ambient air outside the aircraft Receiver-drier serves as a reservoir for the liquid refrigerant. It contains a desiccant that absorbs any moisture that may be in the system. Desiccant is a drying agent used in a refrigeration system to remove water from refrigerant. It is made of silica gel or some similar material. Thermostatic expansion valve (TEV) that meters the refrigerant into evaporator. The amount of refrigerant metered by the TEV is determined by the temperature and the pressure of the refrigerant as it leaves the evaporated coils. It changes refrigerant from high pressure liquid to low pressure. Evaporator in which heat from the aircraft cabin is absorbed into the refrigerant. As the heat is absorbed the refrigerant evaporates or changes from liquid into the vapor and its function is to lower the cabin air temperature Air-conditioning system: These systems are designed to maintain selected air temperature conditions within flight crew, passenger & other compartments In general they are comprised of five principle sections, air supply, heating, cooling, temp control, & distribution. The operation of system varies depending on the size & type of a/c for which they are designed .As in the case of hot air bleed anti-icing system air is supplied from stages of main engine compressors & serves not only to provide, air conditioning but also pressurization of the cabin. The air from the compressor stages is it has to be mixed with some cold air in order to attain pre selected temperature conditions. This is effected by directing some of the bleed air through a cooling pack consisting of a heat exchanger system & a cooling turbine or air cycle machine. The control of the bleed air flow is accomplished by an electrically controlled pack valve, which is energized by a s/w on the system control panel in the cockpit. Downstream of the pack valve is a mix valve which has the function of proportionally dividing the hot air flow from the pack valve & the cold air flow from the air cycle machine into a mixing chamber. The mix valve is of the duel type both valves being positioned by a common 115 volt A.C. Actuator motor the valves are monitored by signal from the temperature control system such that as one valve moves towards its close positions the other valve moves towards its open position. The temperature control system is comprised principally of a selector s/w regulator & temp sensors located at selected point in the system. The whole system operates automatically & continuously monitors the mix valve position but in the event of failure of the regulator mix valve position may be carried out manually from the selector s/w. When the selector s/w s in the auto position & at a described cabin temp a potentiometer within the s/w establishes a reference resistance value in an arm of a control bridge circuit of the regulator. A cabin temp sensor is in the other arm of the control bridge circuit so that if the cabin" temperature is at a level other than that selected thin the bridge will be unbalanced as a result, a signal is developed in the circuit of the mix valve motor so that it will drive the valve to either a hot or cold position as required to attain the selected cabin temperature. At the same time conditioned air sensed by an anticipator sensor & a limit sensor both of which are located in the ducting to the cabin & are connected in an electrical bridge configuration. The purpose of the sensors is to modulate any rapid change demanded by an unbalanced control bridge so that when the actuator control moves the mix valve it will produce cabin temp change without sudden blasts of hot or cold air & without raising duct temp above limits. To prevent the mix valve staying at a too hot position a thermal s/w which is set at a particular level [e.g. 90°C (195°F) ] is located in the ducting to complete a circuit to mix valve so that its motor will shift in the valve to the full cold position at the same item a duct overheat light is illuminated after the overheat condition has been corrected, the system may be returned to normal by means of as reset s/w. Another thermals s/w set to close at a higher level (Eg. 120°C 250°F) protects against duct overheat should power control be lost. It completes a circuit which closes the pack valve & illuminates a pack trip off light the system may be returned to normal after the trip condition has been corrected by operating the reset s/w referred to above. Manual control of the system is effected by moving the selector s/w to cool or warm to directly actuate the max valve as approximate. Satisfactory performance of an aircraft depends up on the continued reliability of electrical system. Reliability is proportional to amount of maintenance received and knowledge of men who performed such maintenance. a. Damaged or over heated equipment, connections, wiring and installation. b. Excessive resistance at high current carrying connections as determined by milli-volt drop test. c. Misalignment of electrically driven equipment d. Poor electrical bonding.
e. Dirty equipment and connections. f. Improper support of wiring and conduit. g. Loose connections, terminals, and ferrules. h. Continuity of fuses. i. Condition of electric lamps. j. Insufficient clearance or poor insulation of exposed terminals. k. Broken or missing safety wire, cotter pins, etc. I. Operational check of electrically operated equipment such as motors, inverters, generators, batteries, lights, etc. m. Voltage check of electrical system with portable precision voltmeter. n. Miscellaneous irregularities such as poorly soldered or loose swaged terminals, loose quick disconnects, broken wire bundle lacing, broken or inadequate clamps, and insufficient clearance between exposed current-carrying parts and ground. CLEANING AND PRESERVATION. Frequent cleaning of electrical equipment to remove dust, dirt, and grime is recommended. Fine emery cloth may be used to clean terminals and mating surfaces if they are corroded or dirty. Crocus cloth or very fine sandpaper may be used to polish commutators or slip rings. Do not use emery cloth on commutators since particles from the cloth may cause shorting and burning. ADJUSTMENT. Accomplish adjustments to items of equipment such as regulators, generators, contactors, control devices, inverters, and relays outside the airplane on a test stand or test bench where all necessary instruments and test equipment are at hand. Follow the adjustment procedures outlined by the equipment manufacturer.
INSULATION OF ELECTRICAL EQUIPMENT. In some cases, a unit of electrical equipment is connected into a heavy current circuit, perhaps as a control device or relay. Such equipment is normally insulated from the mounting structure, since grounding the frame of the equipment may result in a serious ground fault in the event of equipment internal failure. If a ground connection for a control coil must be provided, use a separate small gauge wire. Bus bar maintenance. Periodically check bus bars used in aircraft electrical systems for general condition and cleanliness. Grease, oxide, or dirt on any electrical junction may cause the connectors to overheat and eventually fail. Clean bus bars by wiping with a clean soft cloth saturated with Stoddard solvent and drying with a clean soft cloth.
JUNCTION BOXES. a. Junction Box Construction. Fabricate replacement junction boxes using the same material as the original or from a fire-resistant, nonabsorbent material, such as aluminum alloy or an acceptable plastic material. Where fire-proofing is necessary, a stainless steel junction box is recommended. Rigid construction will prevent "oil-canning" of the box sides, which could result in internal short circuits, ^n all cases, provide drain holes in the lowest portion of the box. b. Internal Arrangement. The junction box arrangement should permit easy access to all installed items of equipment, terminals, and wires. Where marginal clearances are unavoidable, insert an insulating material between current-carrying parts and any grounded surface. It is not good practice to mount equipment on the covers or doors of junction boxes, since inspection for internal clearance is impossible when the door or cover is in the closed position. c. Junction Box Installation. Securely mount junction boxes to the aircraft structure in such a manner that the contents are readily accessible for inspection. When possible, face the open side downward or at an angle so that loose metallic objects, such as washers or nuts, will tend to fall out of the junction box rather than wedge between terminals. d. Junction Box Wiring. Junction box layouts must take into consideration the necessity for adequate wiring space and possible future additions. Lace or clamp electric wire bundles inside the box in a manner that terminals are not hidden, relay armatures are not fouled, and motion relative to any equipment is prevented. Protect cable at entrance openings against chafing using grommets or other suitable means. In the constant voltage charging system the maximum permissible voltage for lead acid battery is 2.35V per cell for Ni-Cd 1.42-1.5V per cell. The KOH will freeze at approximately -75F or -59C. The serviceability state of the Ni-Cd battery can be checked measured discharge. ...
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- Cabin pressurization, cabin pressure, cabin altitude, cabin air pressure, a. Junction Box Construction