Head
- Base Diaphragm P Middle compartment Inner compartment Outer compartment. 00000 Outflow valve Metering valve
- Pilot
• Filter Calibration screw Calibration spring
• Diaphragms Filter- Reference chamber P Solenoid valve Ball check valve Control chamber 190 Cabin pressure Decreasing control chamber pressure Atmospheric pressure Pressure bulkhead Diaphragm Pressure relief adjust
• Filter Vent port Bleed Cabin pressure Pressure relief being dumped Normal flight above (5.4 p.s.i.) (solenoid energized) FIcURE 14-15. Cabin air pressure safety valve.
exceeding cabin pressure by allowing external air to enter the cabin when ambient pressure exceeds cabin pressure. The dump valve illustrated is actuated by the cockpit control switch. When this switch is positioned to "ram," a solenoid valve opens, causing the valve to dump cabin air to atmosphere. On some installations a manual system, using cables and bellcranks, is provided to actuate the dump valve.
The safety valve consists of an outflow valve section and a control chamber. The outflow valve section and the control chamber are separated by a flexible pressure-tight diaphragm. The diaphragm is exposed to cabin pressure on the outflow valve side 555
and control chamber pressure on the opposite side.
Movement of the diaphragm causes the outflow valve to open or close. A filtered opening in the outflow valve allows cabin air to enter the reference chamber. The outflow valve pilot extends into this opening to limit the flow of air into the chamber.
Air pressure inside the reference chamber exerts a force against the inner face of the outflow valve to aid spring tension in holding the valve closed. The pressure of cabin air against the outer face of the outflow valve provides a force opposing spring tension to open the valve. Under normal conditions, the combined forces within the reference chamber are able to hold the outflow valve in the "closed" position. The movement of the outflow valve from closed to open allows cabin air to escape to atmosphere.
The head incorporates an inner chamber, called the pressure relief control chamber. Within the con trol chamber are located the two pressure relief diaphragms, the calibration spring, the calibration screw, and the spring-loaded metering valve. The action of these components within the chamber controls the movement of the outflow valve during normal operation.
The two diaphragms form three pneumatic compartments within the control chamber. The inner compartment is open to cabin pressure through a passage in the outflow valve pilot. The middle compartment is open to the reference chamber and is vented to the outer compartment through a bleed hole in the metering valve. The flow of reference chamber air from the middle compartment to the outer compartment is controlled by the position of the metering valve, which is spring-loaded to a normally closed position. The outer compartment, in which the calibration spring and screw are located, is opened to atmosphere through a passage in the head. Atmospheric pressure, reflected against the diaphragms, aids the calibration spring in keeping the metering valve closed. Cabin pressure, acting, on the diaphragms through the inner compartment, tries to open the metering valve by moving it back against the calibration screw. Under normal conditions, the combined forces of atmospheric pressure and the calibration spring hold the metering valve away from the calibration screw, keeping it closed.
Pressure relief occurs when the cabin pressure exceeds atmospheric pressure by a predetermined value. At this point, cabin pressure overcomes the combined forces of atmospheric pressure and spring tension in the control chamber, moving the metering valve back against the calibration screw, opening the metering valve. With the valve open, reference chamber air can escape through the outer compartment to the atmosphere. As the reference chamber air pressure is reduced, the force of cabin pressure against the outflow valve overcomes spring tension and opens the valve, allowing cabin air to flow to atmosphere. The rate-of-flow of cabin air to atmosphere is determined by the amount the cabinto-atmosphere pressure differential exceeded the calibration point. As cabin pressure is reduced, the forces opening the valve will be proportionately reduced, allowing the valve to return to the normally closed position as the forces become balanced.
In addition to the sutomatic operating provisions just described, the valve includes provisions for electrical activation to the dump position. This is accomplished by a passage in the head that allows reference chamber air to vent directly to atmosphere. The flow of air through the passage is controlled by a ball-check valve and an air solenoid valve. The solenoid valve is spring-loaded to a normally closed position. When the solenoid valve is opened by positioning the cockpit pressure switch to "ram," air flows from the reference chamber, decreasing the reference pressure and allowing the outflow valve to open and dump cabin air.
It should be remembered that the foregoing description of a pressure control system is for illustrative purposes and should not be construed to represent any particular make or model aircraft. Always refer to the applicable manufacturer's manual for the system details and limitations for the aircraft with which you are concerned.
AIR DISTRIBUTION The cabin air distribution system includes: (1) Air ducts, (2) filters, (3) heat exchangers, (4) silencers, (5) nonreturn (check) valves, (6) humidifiers, (7) mass flow control sensors, and (8) mass flow meters. The distribution system shown in figure 14-16 is typical of the system used on small turboprop aircraft.
Air enters the cabin supercharger through a screen-covered opening in the left engine oil cooler airscoop. If the air inlet screen ices over, a springloaded door beside the screen opens allowing air to bypass the screen. From the cabin supercharger, the air passes through a firewall shutoff valve, a pressure relief valve, and a silencer which dampens the supercharger noise and pulsations. The air then 556
Combustion heater Main duct Condenser • Valve Controller Electric motor Refrigerant compressor Fan Blower Evaporator Ram air- Return air Venturi Check valve Check and shutoff valve Air-to-air heat exchanger Hydraulic pump. Ram air-
- Overboard Ram air
- Flow control valve
- Silencer - Dump to ambient air Hydraulic motor gear box Pressure relief valve Supercharger firewall shutoff valve FICURE 14-16. Typical air distribution system.
passes through a flow control valve which governs the airflow rate to maintain the maximum pounds per minute airflow.
Air Ducts Ducts having circular or rectangular cross sections are most frequently used in air distribution systems. Circular ducts are used wherever possible.
Rectangular ducts are generally used where circular ducts cannot be used because of installation or space limitations. Rectangular ducts may be used in the cabin where a more pleasing appearance is desired.
Distribution ducts for various cabin zones, individual air outlets for passengers, and window demisters can have various shapes. Examples of circular, rectangular, elliptical and profiled ducts are illustrated in figure 14-17.
Cabin air supply ducts are usually made from aluminum alloys, stainless steel, or plastic. Main ducts for air temperatures over 200° C. are made from stainless steel. Those parts of the ducting where the air temperature does not exceed 100° C.
are usually constructed from soft aluminum. Plastic ducts, both rigid and flexible are used as outlet ducts to distribute the conditioned air.
Since heated air is routed throughout the duct system, it is important that the ducts be permitted to grow (expand through heating) and to shrink FIGURE 14-17. Cross sections of air distribution ducts.
again when the air cools down. This expansion and contraction must take place without loss of the pressure-tight integrity of the ducts. Expansion bellows (figure 14-18) are incorporated at various places throughout the duct system to permit the ducts to expand or contract.
In general, supports are necessary on both sides of a connecting bellows, a fixed support on one side to prevent duct movement, and a sliding support plus a fixed support on the other side. The sliding 557
Solid supports Fixed clamp Bellows Roller-fitted sliding clamp Fitting with fixed clamp and roller clamp (or sliding support)
- Swinging support FIcURE 14-18. Expansion bellows and duct supports.
support permits movement of the bellows while the duct section is under pressure. Typical duct support systems are illustrated in figure 14-18.
Whenever a duct is angled, means are provided to take care of the end forces which tend to push the duct sections apart. This can be accomplished with external swinging supports which attach the duct to rigid airframe structure (figure 14-19).
In some instances a connecting link is incorporated within the duct itself to transmit end loads.
The tension link within the bellows resembles a single link of chain that joins two segments of ducts. Figure 14-20 illustrates one such connecting link.
Filters The air delivered to a pressurized cabin from a supercharger or engine compressor may contain dust particles, oil mist, or other impurities. Unfiltered air which contains a considerable amount of impurities usually has an offensive odor and causes headache and nausea. Filters are generally incorporated into the ducting to clean the air.
AIR CONDITIONING SYSTEM The function of an air conditioning system is to maintain a comfortable air temperature within the aircraft fuselage. The system will increase or decrease the temperature of the air as needed to obtain the desired value. Most systems are capable of producing an air temperature of 70° to 80° F. with normally anticipated outside air temperatures. This temperature-conditioned air is then distributed so that there is a minimum of stratification (hot and cold layers). The system, in addition, must provide for the control of humidity, it must prevent the fogging of windows, and it must maintain the temperature of wall panels and floors at a comfortable level.
In a typical system the air temperature is measured and compared to the desired setting of the temperature controls. Then, if the temperature is 558
L- Rigid support Clamp Swinging support Combination solid & swinging support FIGURE 14-19. Typical supports for angled ducts.
FIGURE 14-20. Connecting link inside of bellows.
not correct, heaters or coolers are set into operation to change the air temperature, and the air is mixed together to create a uniform temperature in the cabin. In summary, an air conditioning system is designed to perform any or all of the following functions: (1) Supply ventilation air, (2) supply heated air, and (3) supply cooling air.
Ventilation Air Ventilation air is obtained through ram air ducts installed in the leading, lower, or upper surfaces of the aircraft or through other vents in the aircraft skin. Air entering these openings usually passes into and through the same duct system that is used for heating and cooling. On some aircraft, recirculating fans or blowers are present in the system to assist in circulating the air. Many aircraft have ground connections for receiving heated, cooled, or ventilating air from ground servicing equipment.
HEATING SYSTEMS A large part of the heating requirements for the conditioned air is accomplished automatically when the air is compressed by the cabin superchargers.
In many cases additional heat need not be added.
Compression of the air often provides more than the necessary heating. Consequently, cooling, to some degree, is required even when the outside air temperature is not high.
When a degree of heating in addition to that obtained from the "heat of compression" is needed, one of the following types of systems is put into operation: (1) Gasoline combustion heaters, (2) electric heaters, (3) re-cycling of compressed air, and (4) exhaust gas air-to-air heat exchanger.
Combustion Heater Combustion heaters operate similarly to the burner section of a turbojet engine. Gasoline is injected into the burner area under a pressure which breaks up the fuel into a fine mist. Combustion air is supplied to the burner by means of a ram air scoop or an electric motor driven fan. Ignition is supplied by continuous sparking of a special 559
spark plug. The combustion of fuel and air takes place continuously. The temperature output of the heater is controlled by a cycling process whereby combustion is turned on and off for short periods of time depending upon the heating required. The air which eventually mixes with the cabin air is routed around the burner section in a separate air passage. This ventilating air picks up heat from the burner by convection through the metal walls of the burner. The burner combustion gasses are exhausted overboard to prevent carbon monoxide contamination of the cabin.
Various automatic combustion heater controls prevent operation of the heater when dangerous conditions exist. As examples, the flow of fuel is cut off if there is insufficient combustion air, insuffcient ventilating air, and in some cases if the ignition system is not operating. Other controls prevent too rapid heating of the combustion chamber and prevent exceeding a maximum output temperature.
Electric heaters may be in the form of air duct heaters or electric radiant panels. The duct heater incorporates a series of high-resistant wire coils located in an air supply duct. When electric power is applied to the coils, they become hot. The air flowing through the duct carries the heat to the area where it is needed. Most duct heaters require the use of a fan to ensure sufficient airflow over the coils. Without the aid of the fan-produced airflow, the coils would fail due to overheating. Usually an electrical circuit is used which prevents heater operation unless the fan is in operation.
Air vent Conditioned air inlet Air vent Windshield defrosting blower Blower control Emergency spill valve Cold air scoop Main cabin heat control Main cabin air scoop Air scoop Heat exchanger Critical temperature waming lights.
FIGURE 14-21. Engine exhaust heater system.
Radiant Panels Radiant panels consist of wall and floor surfaces which have electric wires embedded in the panel material. When electric power is spplied to the wires, the wires and panel surface become hot. This type panel supplies heat to the cabin air principally by radiation.
Electric Heaters Electric heating systems require the expenditure of large amounts of electrical power. They cannot be used if the electrical system has limited capacity.
Electric heating systems, however, are quick acting and can be used to preheat the aircraft on the ground before the engines are started if an adequate electric ground power source is available.
Compressed Air Heating Some turbojet aircraft we a heating system in which the hot compressed air output of the cabin compressor is re-routed back into the compressor inlet. This double compression raises the temperature of the air to a sufficiently high degree so that other types of heating are usually not necessary.
Exhaust Gas Heaters A relatively simple heating system used on a few large aircraft utilizes the engine exhaust gases, figure 14-21, as a heat source. This system is particularly effective on aircraft where the engine exhaust is ejected through a long tailpipe. A hot air muff or jacket is installed around the tailpipe. Air Aircraft skin routed through the hot air muff picke up heat by convection through the tailpipe material. This heated air is then routed to an air-to-air heat exchanger, where its heat is given up to the air going to the cabin. By using the air-to-air heat exchanger in addition to the hot sir muff, the danger of carbon monoxide entering the cabin is minimized.
Regardless of the type, heating systems provide heated air for comfort and furnish heat for defrosting, deicing, and anti-icing of aircraft components and equipment. Nearly all types of heating systems use the forward motion of the aircraft to force conditioned air to various points on the aircraft. A heating system consists of a heating unit and the necessary ducting and controls. The units, ducts, and controls used will vary considerably from sys.
tem to system.
COMBUSTION HEATERS The number and size of combustion heaters used in a particular aircraft depend upon its size and its heating demands. These heaters are installed singly or in combination to fit the heating requirements of specific aircraft. A large single heater or several smaller heaters may be used. Regardless of size, every combustion heater needs four things for operation: (1) Fuel to burn, (2) ignition to ignite the fuel, (3) combustion air to provide the oxygen required to support the flame, and (4) ventilating air to carry the heat to the places where it is needed.
Aircraft fuel tank Combustion air scoop - Fuel fülter Sealed fuel control assy. Pamp Solenoid valve Regulator assembly Combustion air ground blower Combustion chamber Relief Thermal switches valve overheat and cycling Differential air pressure switch Filter Damper Line of flight -Spray nozzle Plenum Heated Damper air Exhaust Ignition unit • shroud Ventilating Drainair scoop Ventilating air Drain Aircraft skin Exhaust ground blower FIcuRE. 14-22. Heater installation schematic.
All combustion heaters are similar in operation and construction features. The major differences are in the methods of introducing fuel and in the location of units and accessories. The various components that comprise a complete aircraft heating system are shown schematically in figure 14-22.
Heater Fuel System The fuel used in the heaters is supplied in most cases from the same fuel tanks which supply the engines. Fuel flows from the tank to the heater by gravity or is pumped there by a fuel pump. Like the fuel which flows to the aircraft engine, heater fuel must first pass through a filter to remove impurities.
If foreign particles are not removed, they may eventually clog heater system units and prevent heater operation.
After the fuel is filtered, it flows through a fuel solenoid valve and metering nozzle. There are seyeral types of these valves and metering nozzles.
tained in a vertical standpipe. The latter type is illustrated in figure 14-24.
A preheater, in the form of an electrical wire coiled around the fuel line, Is used with some heaters having a vapor wick. It warms the fuel to speed vaporization and aids ignition when the outside temperature is below zero. Its use is usually limited to 2 min., because longer operation would damage the wire coil.
Fuel nozzle retainer Fuel screen Fuel line Air line Regardless of type, they usually have the same purpose, to maintain a constant volume at the fuel outlet to the combustion chamber. This uniform volume, in combination with a fixed combustion airflow, ensures a relatively constant fuel/air ratio to the heater. The result is a steady heater output. Fuel nozzle Tip core To increase or decrease the cabin temperature, the heaters are permitted to operate longer when Whirling Combustion more heat is needed and for shorter periods of time fuel spray chamber head when less heat is desired. On most heater systems this is accomplished automatically by an amplifier FIcURE 14-23. Typical heater spray nozzle assembly.
connected to temperature-sensing devices or by cycling switches which open and close the circuit to the fuel solenoid valve. Thus the heater cycles on Standpipe.
and off to maintain the temperature selected on a temperature control rheostat located in the cabin.
Most heater systems also include overheat Resistor (heater) switches in each heater outlet to automatically turn off the heater fuel supply when the temperature reaches about 350° F. It can be seen that control of the heater fuel supply is necessary, not only for normal operation of the heater but also for stopping it when overheated.
Another essential unit of the heater fuel system is the one that feeds fuel into the combustion chamber. Depending on the installation, it may be either Stainless steel a spray nozzle or a vapor wick. The spray nozzle wick steady spray into the stream of combustion air, (figure 14-23) is shaped so as to inject a fine, • Igniter assembly where it is ignited by a spark plug.
The vapor wick is made of asbestos contained in a circular hanged casting or of stainless steel con- FICURE 14-24. Stainless steel vapor wick.
Ignition System High voltage for heaters using spark plug igniters is supplied either by a high-potential ignition unit operating from the 28-v. d.c. aircraft supply or by ignition transformers operating from a 115-v.
a.c. aircraft source. The 28-v. d.c. ignition unit consists chiefly of a vibrator and step-up coil which produces a high-voltage spark at high frequency. A shielded lead is used to connect the step-up coil to the spark plug. The spark is produced between the center spark plug electrode and a ground electrode.
About the same result is obtained where ignition transformers are used. Here, however, power is supplied by the 115.v., 400-Hz main inverter a.c. sys.
tem. This is routed to the transformers, where it is stepped up to the very high voltage required to jump the spark gap at the spark plug. But whether a d.c. or an a.c. source is used to fire the spark plug, ignition is continuous during heater operation. This continuous operation prevents fouling of the spark plug electrodes.
It is the arrangement of the electrodes that makes the difference in the types of spark plugs used in aircraft combustion heaters. One type of spark plug is shown in figure 14-25A. This is known as a dual-electrode spark plug. Another type of plug to be found on combustion heaters is the shielded elec- 1/16" (A) Ground electrode Electrode (B) Shield (C) FIGURE 14-25. Heater ignition plugs.
trode plug (figure 14-25B). In this plug, the ground electrode forms a shield around the center electrode.
Although spark plug igniters differ somewhat in appearance, most glow coil igniters look similar to that shown in figure 14-25C. They consist of a resistance wire wound into a coil around a pin extending from the body of the igniter. The outer end of the coil is connected to the pin, thus providing both support and electrical continuity. The body of the igniter is fitted with two terminals, which are connected across the coil, and is threaded to provide for installation. The glow coil operates from the 24 or 28.v. d.c. power supply on the aircraft. The direct current causes the coil to become red hot, thereby igniting the fuel/air mixture until the heater is operating at a temperature sufficient to maintain the flame after the glow coil is turned off. A thermal cutout switch breaks the circuit to 'the' glow coil when this temperature is reached. This prolongs the life of the igniter.
Another type of plug used is a single electrode type (not shown). The ground electrode used with this type of plug is a separate installation attached to the heater at an angle that will provide an airgap between the plug's electrode and ground.
Combustion Air System Combustion air for each cabin heater is received through either the main air intake or through a separate outside scoop. On both pressurized and unpressurized aircraft it is provided by ram pressure during flight and by ground blowers during ground operation. To prevent too much air from entering the heaters as air pressure increases, either a combustion air relief valve or a differential pressure regulator is provided. The air relief valve is located in the line leading from the ram-air intake duct and is spring-loaded to dump excess air into the heater exhaust gas stream. The differential pressure regulator is also located in the combustion air intake line, but it controls the amount of air reaching the combustion chamber in a slightly different manner.
While the relief valve takes a large volume of air and bypasses the amount not needed, the pressure regulator allows only the needed amount of air to enter its intake in the first place. It does this by the use of a diaphragm and spring type control mechanism. One side of the diaphragm is vented to the heater intake air line and the other side to the heater exhaust gas line. Any change in the pressure 563
drop between these points is corrected at the regulator by letting in more or less air as required. Thus, a constant combustion eir pressure is provided to the heater. Coupled with a steady fuel flow, this constant air pressure makes possible a regulated flow of combustion gases through the combustion chamber and the connecting radiator. If a fire breaks out near the heater, a fire valve automatically stops the supply of combustion air to prevent spreading of the fire to the heating system.
A damper-type combustion air fire valve (figure 14-26) is located in the combustion air inlet of some heaters. It has two semicircular, spring-loaded segments soldered together to permit maximum air- How through the combustion air duct. The segments will release to seal the duct when the solder melts at about 400° F.
Fire valve torsion spring Fire valve Heater fuel inlet Fire valve fuse media Ignition cable FICURE 14-26. Cabin heater combustion air fire valve.
Ventilating Air Ventilating air may come from one of three sources: (1) A blower for air circulation and heater operation on the ground, (2) a ram-air inlet, or (3) the cabin compressors on pressurized air- Ventilating air, ram or blower, enters at the burner head end of the heater and, passing over the heated radiator surfaces, becomes heated and passes through the outlet end into the plenum assembly and into the distribution system ductwork.
MAINTENANCE OF COMBUSTION HEATER SYS- TEMS Combustion heater components are subject to wear and damage which can result in system failure. When this occurs, troubleshooting procedures must be followed to isolate component failures.
Then all damaged or excessively worn components must be replaced. During the replacement of the components, adjustments must be made to assure proper operation of the combustion heater system.
Always follow the manufacturer's instructions when making any adjustments to a heater or heating system.
In this section, heater system adjustments which are representative of those performed by the aircraft mechanic are discussed. Keep in mind that the components of the system vary with the types of aircraft, and so do the adjustment procedures.
On some aircraft careful adjustment of cabin heat outlets is necessary to obtain uniform heat distribution. Some of the factors which cause the variation in distribution are: (1) The distance of the outlet from the source of heated air, (2) the cross sectional area of the outlet, (3) the space serviced by the outlet, and (4) any restrictions to airflow caused by duct size and routing.
Air mixing valves are installed in airborne beating systems so that hot and cold air can be mixed in the required proportions to maintain adequate heat.
Some air mixing valves are preset on the ground and cannot be actuated during flight. External adjustments are provided on these valves to permit seasonal adjustment. During adjustment, the valves are set to a specified number of degrees from the fully closed position.
To assure the proper mixing of hot and cold air in motorized air mixing valves, adjustments are provided on each valve. The adjustments regulate the opening and closing positions of the valves.
Heater System Inspection The inspection of combustion heater systems includes checking the air openings and outlets for obstructions. All controls are checked for freedom of operation. Turn on the fuel pump so that the fuel lines, solenoids, and valves can be checked for leakage. The heater unit is inspected for proper operation by turning it on and observing whether or not hot air comes out of the outlets. The outside of the heater unit is checked for signs of overheating. Any burned or darkened areas usually indicate a burned-through combustion chamber. Heaters dam- 564
aged by overheating should be replaced. When replacing a heater due to overheating, always determine the cause of the trouble. The faulty operation of some part of the system, such as stopped-up heater air ilet ducts or improperly operating switches, regulators, valves, or other units, is the most likely cause of damage. The automatic and overheat control devices should be operationally checked. The cabin heating ducts should be examined for tears, breaks, and ballooning. To guarantee fuel flow, the heater fuel filter element should be inspected for cleanliness and the fuel injection nozze or glow coil for freedom from carbon deposits.
To obtain proper operation of combustion heaters under freezing conditions, a special winterization inspection should be performed. Check heater drain lines regularly for restrictions caused by ice formation. During low temperature operation below 0° C. (32° F.), water vapor in the combustion gases flowing through drain lines may condense and form ice. Under changing temperature conditions, water condenses and freezes in the ram and heater combustion sensing lines. Water produced during combustion may collect on the fuel nozzles and spark plug and form ice after the heater is turned off.
This ice may be sufficient to make it difficult, if not impossible, to start the heater without preheating.
COOLING SYSTEMS Air cooling systems are installed to provide a comfortable atmosphere within the aircraft both on the ground and at all altitudes. These systems keep the correct amount of air flowing through the interior of the aircraft at the right temperature and moisture content. Since the fuselage is a huge cavity, the capacity of the cooling system must be quite large. Several types of systems can be used to meet these requirements. Two of the more common types, air cycle and vapor cycle, are discussed in this section.
AIR CYCLE COOLING SYSTEM An air cycle cooling system consists of an expansion turbine (cooling turbine), an air-to-air heat exchanger, and various valves which control airflow through the system. The expansion turbine incorporates an impeller and a turbine on a common shaft.
High-pressure air from the cabin compressor is routed through the turbine section. As the air passes through the turbine, it rotates the turbine and the impeller. When the compressed air performs the work of turning the turbine, it undergoes a pressure and temperature drop. It is this temperature drop which produces the cold air used for air conditioning.
Before entering the expansion turbine, the pressurized air is directed through an air-to-air heat exchanger. This unit utilizes outside sir at ambient temperature to cool the compressed air. It should be evident that the heat exchanger can only cool the compressed air to the temperature of the ambient air temperature. The primary purpose of the heat exchanger is to remove the heat of compression so that the expansion turbine receives relatively cool air on which to start its own cooling process.
The impeller part of the expansion turbine can perform several functions. In some installations the impeller is used to force ambient air through the heat exchanger. In this manner, the efficiency of the heat exchanger is increased whenever the speed of the expansion turbine is increased. Other installations use the impeller to further compress the cabin supercharger air as an aid to forcing it through the heat exchanger and the turbine.
A valve controls the compressed airflow through the expansion turbine. To increase cooling, the valve is opened to direct a greater amount of the compressed air to the turbine. When no cooling is required, the turbine air is shut off. Other valves, operated in conjunction with the turbine air valve, control the flow of ambient air through the heat exchanger. The overall control effect of these valves is to increase the heat exchanger cooling airflow at the same time increased cooling is obtained at the turbine.
The power required to drive the air cycle system is derived entirely from the cabin supercharger compressed air. Use of the air cycle system, therefore, imposes an increased load on the superchargers. As more cooling is demanded from the turbine, a greater back pressure is placed on the superchargers, which must work harder to supply the air demands. It is often necessary to make a choice between the desired amount of cooling and the de sired degree of cabin pressurization, and a compromise is made by reducing the demand for one or the other. Maximum cooling and maximum pressurization cannot be obtained at the same time. Attempts to obtain both will cause the supercharger to surge or operate in an otherwise unsatisfactory manner.
System Operation This description of the operation of an air conditioning system is intended to provide an under- 565
Cooling air Cooling air - intake duct intake duct Ram-air valve Primary heat Secondary heat exchanger exchanger Primary heat exchanger Bleed air from bypass valve right engine Compressor Overboard exhaust Expansion • turbine Flow limiter Refrigeration unit shutoff valve Main To cabin heat exchanger relay Engine bleed Water crossseparator Refrigeration over duct unit bypass valve To cabin temperature To No. 1 Flow control Auxiliary heat control alternate control valve Temperature sensor cool relay
- To cabin temperature controller To defrost and anti-ice system Bleed air from shutoff and regulator valves left engine Cabin air Engine bleed air / Primary heat exchanger output pressure Ram air Secondary heat exchanger output regulator Conditioned air Regulated 300° F. air (defrost and anti-ice) Controlled outflow
• 0 O Conditioned air outlets Cockpit Cabin Cabin air pressure dump valve FIcURE 14-27. Cabin air conditioning and pressurization system flow schematic.