Flight deck Foot warmers supply Floor outlets Roof outlet External conditioned Conditioned air to cabin Regulating valve air supply through wall outets Temperature Ram Temperature control control valve To cold air louvers air valve valve Ram air V Ram air Water Water separators: separators Refrigeration Refrigeration unit unit Heat exchangers Refrigeration bypass Refrigeration valve bypass valve Flow control valve Automatic shutters Flow control valve To wing anti-icing To tail anti-icing Heat exchanger Heat exchanger Crossfeed valve / Ground connection Pressure Pressure relief Pressure reducing reducing Pressure relief valve valve valve valve To water injection To water injection turbopump turbopump Isolation valve Isolation valve Air delivery No. 1 valve No. 2 Engine Engine A.P.U.
Constant speed Constant speer drive and starter drive and starter Hot air Conditioned air Cold air Cooling air FIGURE 14-4. Typical pressurization and air conditioning system.
hold up a column of mercury 760 milimeters high (approximately 30 in.) at sea level (14.7 p.s.i.). This pressure decreases with altitude.
(8) Cabin altitude. Used to express cabin pressure in terms of equivalent altitude above sea level.
(9) Differential pressure. The difference in pressure between the pressure acting on one side of a wall and the pressure acting on the other side of the wall. In aircraft air conditioning and pressurizing systems, it is the difference between cabin pressure and atmospheric pressure.
(10) Gage pressure. A measure of the pressure in a vessel, container, or line, as compared to ambient pressure.
(11) Ram-air temperature rise. The increase in temperature created by the ram compression on the surface of an aircraft traveling at a high rate of speed through the atmosphere. The rate of increase is proportional to the square of the speed of the object.
(12) Temperature scales.
(a) Centrigrade. A scale on which 0° C.
represents the freezing point of water, and 100° C. is equivalent to the boiling point of water at sea level.
(b) Fahrenheit. A scale on which 32° F.
represents the freezing point of water, and 212° F. is equivalent to the boiling point of water at sea level.
BASIC REQUIREMENTS Five basic requirements for the successful functioning of a cabin pressurization and air conditioning system are:
(1) A source of compressed air for pressurization and ventilation. Cabin pressurization sources can be either engine-driven compressors, independent cabin superchargers, or air bled directly from the engine,
(2) A means of controlling cabin pressure by regulating the outflow of air from the cabin. This is accomplished by a cabin pressure regulator and an outflow valve.
(3) A method of limiting the maximum pressure differential to which the cabin pressurized area will be subjected. Pressure relief valves, negative (vacuum) relief valves, and dump valves are used to accomplish this.
(4) A means of regulating (in most cases cooling) the temperature of the air being distributed to the pressurized section of the airplane. This is accomplished by the refrigeration system, heat exchangers, control valves, electrical heating elements, and a cabin temperature control system.
(5) The sections of the aircraft which are to be pressurized must be sealed to reduce inadvertent leakage of air to a minimum.
This area must also be capable of safely withstanding the maximum pressure dif ferential between cabin and atmosphere to which it will be subjected.
Designing the cabin to withstand the pressure differential and hold leakage of air within the limits of the pressurization system is primarily an airframe engineering and manufacturing problem.
In addition to the components just discussed, various valves, controls, and allied units are necessary to complete a cabin pressurizing and air conditioning system. When auxiliary systems such as windshield rain-clearing devices, pressurized fuel tanks, and pressurized hydraulie tanks are required, additional shutoff valves and control units are neces.
sary.
Figure 14-4 shows a schematic diagram of a pressurization and air conditioning system. The exact details of this system are peculiar to only one model of aircraft, but the general concept is similar to that found in the majority of aircraft.
SOURCES OF CABIN PRESSURE Reciprocating engine internal superchargers provide the simplest means of cabin pressurization.
This is accomplished by ducting air from a manifold which supplies compressed air from a supercharger to the pistons. This arrangement can be used only when the engine carburetor is downstream of the supercharger. When the carburetor is upstream of the supercharger, as is often the case, this method cannot be used since the compressed air contains fuel. Air for cabin pressurization can also be ducted from a turbocharger used with a reciprocating engine.
There are several disadvantages in using these two methods. The cabin air becomes contaminated with fumes from lubricating oil, exhaust gases, and fuel. Also, cabin pressurization at high altitude be- 545
comes impossible as the discharge pressure of the supercharger decreases to nearly ambient. A third disadvantage is the decrease in engine performance near its design ceiling due to the air loss for cabin pressurization.
With gas turbine engines the cabin can be pressurized by bleeding air from the engine compressor.
Usually the air bled from an engine compressor is sufficiently free from contamination and can be used safely for cabin pressurization. Even so, there are several disadvantages when using bleed air from turbine engine compressors. Among these disadvantages are: (1) The possibility of contamination of the air from lubricants or fuel in the event of leakage, and (2) dependence of the air supply on the engine performance.
Because of the many disadvantages associated with the pressurizing sources previously described, independent cabin compressors have been designed.
These compressors can be engine driven through accessory drive gearing or can be powered by bleed air from a turbine engine compressor.
Generally, the compressors can be separated into two groups, (1) positive-displacement compressors and (2) centrifugal compressors.
Positive-Displacement Cabin Compressors (Superchargers) Included in this group are reciprocating compres sors, vane-type compressors, and Roots blowers.
The first two are not very suitable for aircraft cabin pressurization because of the large quantity of oil present in the air delivered to the cabin.
The action of a Roots-type blower (figure 14-5) is based on the intake of a predetermined volume of air, which is subsequently compressed and delivered to the cabin duct.
The rotors are mounted in an airtight casing on two parallel shafts. The lobes do not touch each other or the casing, and both rotors turn at the same speed. Air enters the spaces between the lobes, is compressed, and is delivered to the cabin air duct.
A cutaway view of a cabin supercharger is shown in figure 14-6. The supercharger housing is usually finned on the external surface to increase its cooling area. The cooling effect is sometimes further increased by shrouding the supercharger housing and passing a stream of air through it. Air cooling is also used to reduce the temperature of internal parts. The cooling air is ducted through drilled FIGURE 14-5. Schematic Roots-type cabin compressor.
passageways into the rotor cavities and is expelled at the inlet side of the supercharger cover.
To achieve an oil-free delivery of air, the supercharger bearings are contained in separate chambers. The rotor shafts can be fitted with seals made of oil-resistant rubber which prevents any lubricant from entering the compressor casing. The use of labyrinth seals permits a small amount of air leakage to ambient. Any drops of oil which may have passed the rubber seal are thus blown back.
Positive displacement compressors emit a shrill noise during their operation, because of the air pulsations caused by the rotors. Silencers are used with this type compressor to reduce the noise level.
Centrifugal Cabin Compressors The operating principle of a centrifugal compressor is based on increasing the kinetic energy of the air passing through the impeller. With compressor impeller rotation, the induced air is not only accelerated, but it is also compressed because of the action of centrifugal force. The kinetic energy in the air is then converted into pressure in the diffuser. There are two basic types of diffusers: (1) Vaneless, where the air enters the diffuser space directly on leaving the impeller, and (2) those having guide vanes. A schematic of a centrifugal cabin compressor is shown in figure 14-7.
The cabin supercharger shown in figure 14-8 is essentially an air pump. It incorporates a centrifugal impeller similar to the supercharger in the induction system of a reciprocating engine. Outside 546
Air inlet port Driving rotor Rotor case Drive end cover Driving gear Extractor seal shell Rotary shell Extractor scroll Driven rotor Stub shaft Adjusting ring Abutment collar Driven gear Ball bearing FIcURE 14-6. Cutaway view of a Roots-type cabin supercharger.
air at atmospheric pressure is admitted to the supercharger by suitable scoops and ducts. This air is then compressed by the high-speed impeller and delivered to the fuselage. The superchargers are usually driven by the engine through appropriate gearing; however, turbojet aircraft utilize superchargers (turbocompressors) which are pneumatically driven.
Engine-driven cabin superchargers are generally mounted in the engine nacelle. The supercharger is either splined directly to the engine accessory drive or is connected to an accessory drive by a suitable drive shaft. A mechanical disengaging mechanism is usually incorporated in the drive system to permit disconnecting the supercharger if it malfunctions.
The disengaging mechanism can be operated from the flight deck by the crew. In most aircraft it is not possible or permissible to re-engage the supercharger in flight once it has been disconnected.
FIGURE 14-7. Centrifugal cabin compressor. Engine-driven superchargers used on reciprocat- 547
FIGURE 14-8. Pictorial view of a centrifugal cabin supercharger.
ing engine aircraft require a variable-ratio drive mechanism. The gear ratio of these superchargers is automatically adjusted to compensate for changes of engine r.p.m. or outside atmospheric pressure. Normally, the gear ratio is eight to 10 times engine speed when operating under cruising conditions.
The drive ratio is at a maximum when operating at high altitude with low engine r.p.m.
Turbocompressors used on turbojet aircraft may be located in the engine nacelles or in the fuselage.
There may be as many as four turbocompressors in an aircraft. Turbocompressors consist of a turbine rotated by air pressure which in turn rotates an impeller. The compressed air supply used to operate the turbocompressor is taken from the aircraft's pneumatic system. Speed of the turbocompressor is controlled by varying the supply of compressed air to its turbine.
Cabin superchargers of all types contain their own lubrication system. The lubricant used may be the same type oil used for the engine, or it may be a special oil similar to hydraulic fluid. Supercharger bearings and gears are lubricated by pressure and spray. Oil pressure is also used to operate the control system for the supercharger. The typical lubrication system incorporates a pump, relief valve, sump, cooling system, and sometimes a separate oil tank.
High impeller speed is an important limitation in all superchargers. When the tip speed of the impeller approaches the speed of sound, the impeller rapidly loses its efficiency as an air pump. An equally important limitation involves the back pressure created in the outlet air ducts. If the back pressure is excessive, the impeller may stall or surge.
Supercharger Control The function of the supercharger control system is to maintain a fairly constant volume of air output from the supercharger. This is accomplished in the system used on reciprocating engine aircraft by varying the drive ratio of the supercharger. The drive ratio between the supercharger impeller and the engine is varied to compensate for changes in engine r.p.m. or atmospheric pressure. This is achieved by an automatic mechanism which samples the airflow output of the supercharger and, through a variable-speed drive gearbox, adjusts the impeller speed whenever the airflow output varies from a preset value.
The amount of f.hp. (friction horsepower) taken from the engine to drive the supercharger is dependent upon the drive ratio. Losses are lowest during low-ratio operation when the energy required to rotate the impeller is at a minimum.
Losses are approximately 75 f.hp. in high ratio and 25 f.hp. in low ratio. This loss is indicated at high altitudes, where, the engines which drive the cabin superchargers may require 3 or 4 in. Hg additional manifold pressure to produce the same b.hp. (brake horsepower) as that of other engines.
The speed of the supercharger impeller is therefore adjusted by the control system to maintain a constant mass airflow output. If variables such as altitude tend to increase or decrease the output, the control mechanism causes a correction of the drive ratio. Changes of drive ratio are furthermore dampened by various system refinements to prevent rapid acceleration or deceleration which may result in uncomfortable surges of pressurization.
Serious consequences may occur if the impeller speed becomes higher than its designed maximum.
To protect the supercharger against such an occurrence, the typical system has an overspeed governor.
This unit is similar to a propeller flyweight governor. The overspeed governor actuates a valve to position the control mechanism to the low-ratio po- 548
sition. It works automatically to reduce impeller
r.p.m. when an overspeed occurs.
Some installations also have an electrically operated valve which positions the control mechanism to the low-speed position. This minimum speed valve may be operated manually from the flight deck or automatically by a landing gear strut switch. It is used primarily to reduce supercharger drive ratio when pressurization is not being used or when emergencies occur.
SUPERCHARGER INSTRUMENTS The principal instrument associated with the supercharger is an airflow gage. This gage usually measures the differential air pressure between the input and the output of the supercharger. In some cases there are two needles on the gage to indicate input and output pressures on the same scale. The airflow (or input and output pressure) gage indicates the proper operation of the supercharger.
High readings, low readings, or fluctuating readings indicate various types of malfunctions.
Oil pressure and oil temperature indications are also made available by suitable instruments on the fight deck. In some cases warning lights may be used instead of, or in addition to, the actual gages.
Engine-driven cabin compressors are used on turboprop aircraft. These compressors do not have a variable-speed drive because the turboprop engine operates at a relatively constant speed. The output of this type compressor is controlled by automatically varying the inlet airflow through an airflowsensing mechanism and a suitable inlet valve which maintains a constant compressor airflow output.
Ordinarily a surge and dump valve is used at the outlet of the compressor. In some systems this is the only type of control employed for the compressor:
The surge and dump valve prevents surging of the compressor by partially reducing output pressure when system demands are heavy. The valve can also completely dump output pressure when the enginedriven compressor output is not needed. This valve can be operated from the flight deck and is also operated by various automatic control systems.
When the surge and dump valve is opened, the engine-driven cabin compressor output is dumped overboard through suitable ducts.
Instruments used in conjunction with the enginedriven compressor are similar to those used with the variable-speed supercharger. An inlet and discharge pressure gage measures compressor pressures. Compressor high oil temperature and low oil pressure are usually indicated by warning lights.
Turbocompressors used on turbojet aircraft are similar in operation to the exhaust-driven turbochargers used with some reciprocating engines.
Power derived from the aircraft's pneumatic system is used to drive the turbine of the unit. Since the turbocompressors do not rely upon direct engine drive shafts, they can be placed either in the engine nacelles or in the fuselage. Ordinarily multiple turbocompressor units are used to provide the high airflow needed by the large turbojet aircraft. The output of the turbocompressor units is usually controlled by varying the pneumatic supply to the turbine.
The pneumatic air supply is obtained from the compressor section of the turbojet engine. This air supply is regulated to a constant pressure of approximately 45 p.s.i. to 75 p.s.i. Pneumatic system air pressure is also used to operate anti-icing and other aircraft systems; therefore, various shutoff valves and check valves are used to isolate inoperative units of the turbocompressor system.
The turbocompressor output is controlled automatically by an airflow control valve and servo-operated inlet vanes. The inlet vanes control the pneumatic system air supply to the turbocompressor turbine. The vanes open or close according to the air pressure signal sensed at the airflow control valve, and turbocompressor speed is increased or decreased to maintain a relatively constant output air volume. Turbocompressor speed will therefore increase with altitude.
The principal turbocompressor control is a simple "on/off" valve. This valve is located in the pneumatic air duct. In the "off" position it completely closes off the pneumatic supply to the turbine. Various special circuits may also actuate this shutoff valve when operation of the turbocompressor is not desired.
Most turbocompressor units incorporate an overspecd control. A typical overspeed control unit is a simple flyweight governor which causes the turbocompressor to completely shut down when a certain limiting r.p.m. is reached. Usually the pneumatic duct shutoff valve is closed by the overspeed control. The turbocompressor system also uses a surge and dump valve similar to that used for enginedriven compressors.
The flight deck instruments are the same as those used on engine-driven systems with the addition of 549
a tachometer which measures turbocompressor speed. Turbocompressor speed on a typical aircraft varies from approximately 20,000 r.p.m. at sea level to 50,000 r.p.m. at 40,000 ft. The overspeed control may be set at about 55,000 r.p.m.
PRESSURIZATION VALVES The principal control of the pressurization system is the outflow valve. This valve is placed in a pressurized portion of the fuselage, usually underneath the lower compartments. The purpose of the valve is to vent cabin air overboard through suitable openings in the wing fillet or the fuselage skin. Small aircraft use one outflow valve; large aircraft may use as many as three valves which work in unison to provide the required volume outflow.
One type of outflow valve is a simple butterfly which is opened or closed by an electric motor. The motor receives amplified electrical signals from the pressurization controller to vary the valve position required for pressurized flight.
Some aircraft use a pneumatic outflow valve (figure 14-9). This valve receives signals from the pressurization controller in the form of controlled air pressures. The air pressures which operate the valve are obtained from the high pressure inside the cabin, with assistance from the pneumatic system pressure in turbine-powered aircraft.
FIGURE 14-9. Typical pneumatic outflow valve.
In many aircraft, the outflow valve(s) will be held fully open on the ground by a landing gear operated switch. During flight, as altitude is gained, the valve(s) close (s) gradually to make a greater restriction to the outfow of cabin air. The cabin rate of climb or descent is determined by the rate of closing or opening of the outflow valve (s). During cruising flight the cabin altitude is directly related to the degree of outflow valve opening.
In addition to the controllable outflow valve (s), an automatic cabin pressure relief valve is used on all pressurized aircraft. This valve may actually be built into the outflow valve or may be an entirely separate unit. The pressure relief valve automatically opens when the cabin differential pressure reaches a preset value.
All pressurized aircraft require some form of a negative pressure relief valve. This valve may also be incorporated into the outflow valve or may be an individual unit. A common form of negative pressure relief valve is a simple hinged flap on the rear wall (pressure dome) of the cabin. This valve opens when outside air pressure is greater than cabin pressure. During pressurized flight the internal cabin pressure holds the flap closed. The negative pressure relief valve prevents accidentally obtaining a cabin altitude which is higher than the aircraft altitude.
The outflow of air from the cabin can also be accomplished through a manually operated valve.
This valve may be called a safety relief valve, a manual depressurization valve, or some other similar term. The manual valve is used to control pressurization when all other means of control fail. It is primarily intended to permit rapid depressurization during fires or emergency descent.
Pressurization Controls The pressurization controller (figure 14-10) is the source of control signals for the pressurization system. The controller provides adjustments to obtain the desired type of pressurized condition. Most operators specify standard operating procedures for the controller which have proven best for their particular type of operation.
The controller looks very much like an altimeter which has several added adjustment knobs. The dial is graduated in cabin altitude increments up to approximately 10,000 ft. Usually there is one pointer which can be adjusted to the desired cabin altitude by the cabir altitude set knob. In some cases there is another pointer or a rotating scale which also 550
Barometric pressure indicator Indicates maximum altitude before differential operation 2 CABIN
PRESSUR
CONTROL CABIN RATE Rate selector knob Cabin altitude selector knob (Selects isobaric setting) - Barometric pressure correction knob FIGURE 14-10. Pressurization controller.
indicates the corresponding aircraft pressure altitude. A separate knob adjusts the controller to the existing altimeter setting (or sea level barometric pressure). The barometric setting selected is indicated on a separate dial segment. The third knob on the controller adjusts the cabin rate of altitude change. This adjustment can be made on a separate control in some installations.
When the controller knobs are set, adjustments are made on either an electric or a pneumatic signaling device inside the controller. The settings are compared to the existing cabin pressure by an aneroid or evacuated bellows. If the cabin altitude does not correspond to that which is set by the knobs, the bellows causes the appropriate signal to go to the outflow valve. When the bellows determines that the cabin altitude has reached that which has been set, the signals to the outflow valve are stopped. As long as other factors do not change, the outfow valve is held at the setting to maintain desired cabin pressure. The controller can sense any change, such as variance of aircraft altitude or loss of one supercharger, and re-adjust the outflow valve as necessary.
The rate control determines how fast the controller sends signals to the outflow valve. In some controllers the rate signal is partially automatic. The barometric setting compensates the controller for the normal errors in altimetry which are encountered on most flights. This setting improves the accuracy of the controller and, as an example, protects the cabin from being partially pressurized while a landing is being made.
The signals which originate in the controller are very weak. This is because it is a delicate instrument and cannot handle high electric voltages or pneumatic forces. These weak signals are amplified, either electrically or pneumatically, to operate the outflow valve.
Several instruments are used in conjunction with the pressurization controller. The cabin differential pressure gage indicates the difference between inside and outside pressure. This gage should be monitored to assure that the cabin is not approaching the maximum allowable differential pressure. A cabin altimeter is also provided as a check on the performance of the system. In some cases, these two instruments are combined into one. A third instrument indicates the cabin rate of climb or descent. A cabin rate of climb instrument and a cabin altimeter are illustrated in figure 14-11.
трі вад ~ 1000 FT. I FIGURE 14-11.. Instruments for pressurization control.
CABIN PRESSURE CONTROL SYSTEM The cabin pressure control system is designed to provide cabin pressure regulation, pressure relet, vacuum relief, and the means for selecting the desired cabin altitude in the isobaric and differential range. In addition, dumping of the cabin pressure is a function of the pressure control system. A cabin pressure regulator, an outflow valve, and a safety valve are used to accomplish these functions.
Cabin Pressure Regulator The cabir pressure regulator controls cabin pressure to a selected value in the isobaric range and limits cabin pressure to a preset differential value in the differential range. The isobaric range maintains 551
the cabin at constant-pressure altitude during flight at various levels. It is used until the aircraft reaches the altitude at which the difference between the pressure inside and outside the cabin is equal to the highest differential pressure for which the fuselage structure is designed. Differential control is used to prevent the maximum differential pressure, for which the fuselage was designed, from being exceeded. This differential pressure is determined by the structural strength of the cabin and often by the relationship of the cabin size to the probable areas of rupture, such as window areas and doors.
The cabin pressure regulator is designed to control cabin pressure by regulating the position of the outflow valve. The regulator usually provides either fully automatic or manual control of pressure within the aircraft. Normal operation is automatic, requiring only the selection of the desired cabin altitude and rate-of-change of cabin pressure.
Atmospheric vent ports Dump solenoid The cabin pressure regulator may be constructed integral with the outflow valve or may be mounted remote from the outflow valve and connected to it by external plumbing. In either instance the principle of operation is similar.
The regulator illustrated in figure 14-12 is integral with the outflow valve. This regulator is a differential pressure type, normally closed, pneumatically controlled and operated. This type regulator consists of two principal sections: (1) The head and reference chamber section, and (2) the outflow valve and diaphragm section.
The outflow valve and diaphragm section contains a base, a spring-loaded outflow valve, an actuator diaphragm, a balance diaphragm, and a baffle plate. The baffle plate is attached to the end of a pilot which extends from the center of the cover assembly. The outflow valve rides on the pilot Ground test valve Cabin air inlet True static atmosphere Isobaric control system Differential. connection control system . Filter
- Cabin air orifice Head -
• Solenoid Reference chamber_ air valve Cover -
- Flight Pilot Differential "on" Outflow - diaphragm Actuator valve — All "off" Base - C Baffle plate Balance diaphragm FIcURE 14-12. Cabin air pressure regulator.
between the cover and the baffle plate and is spring loaded to a closed position against the base.
The balance diaphragm extends outward from the baffle plate to the outflow valve, creating a pneumatic chamber between the fixed baffle plate and the inner face of the outflow valve. Cabin air flows into this chamber through holes in the side of the outflow valve to exert a force against the inner face, opposing spring tension, to open the valve. The actuator diaphragm extends outward from the outflow valve to the cover assembly, creating a pneumatic chamber between the cover and the outer face of the outflow valve. Air from the head and reference chamber section flows through holes in the cover, filling this chamber and exerting a force against the outflow valve's outer face to aid the spring tension in holding the valve closed. The position of the outflow valve controls the flow of cabin air to atmosphere for cabin pressure control. The action of components in the head and reference chamber section controls the movements of the outfow valve by varying the pressure of reference chamber air being exerted against the outer face of the valve.
The head and reference chamber section includes an isobaric control system, a differential control system, a filter, a ground test valve, a true static atmosphere connection, and a solenoid air valve.
The area inside the head is called the reference chamber.
The isobaric control system incorporates an evacuated bellows, a rocker arm, a follower spring, and an isobaric metering valve. One end of the rocker arm is connected to the head by the evacuated bellows. The other end of the arm positions the metering valve to a normally closed position against a passage in the head. A follower spring between the metering valve seat and a retainer on the valve causes the valve to move away from its seat as the rocker arm permits.
Whenever the reference chamber air pressure is great enough to compress the bellows the rocker arm pivots about its fulcrum. This allows the metering valve to move from its seat an amount proportionate to the amount of compression in the bellows.
When the metering valve is open, reference chamber air flows to atmosphere through the true static atmosphere connection.
The differential control system incorporates a diaphragm, a rocker arm, a metering valve, and a follower spring. One end of the rocker arm is attached to the head by the diaphragm. The diaphragm forms a pressure-sensitive face between the reference chamber and a small chamber in the head.
This small chamber is opened to atmosphere through a passage to the true static atmosphere connection. Atmospheric pressure acts on one side of the diaphragm and reference chamber pressure acts on the other. The opposite end of the rocker arm positions the metering valve to a normally closed position against a passage in the head. A follower spring between the metering valve seat and a retainer on the valve causes the valve to move away from its seat as the rocker arm permits.
When reference chamber pressure exceeds atmospheric pressure sufficiently to move the diaphragm, the metering valve is allowed to move from its seat an amount proportionate to the movement of the diaphragm. When the metering valve is open, reference chamber air flows to atmosphere through the true static atmosphere connection.
By regulating reference chamber air pressure, the isobaric and differential control systems control the actions of the outflow valve to provide for three modes of operation called unpressurized, isobaric, and differential.
During unpressurized operation, figure 14-13, reference chamber pressure is sufficient to compress the isobaric bellows and open the metering valve.
Cabin air entering the reference chamber through the cabin air orifice flows to the atmosphere through the isobaric metering valve. Since the cabin air orifice is smaller than the orifice formed by the metering valve, reference chamber pressure is maintained at a value slightly less than cabin pressure.
As pressure increases in the cabin, the differential pressure between the outflow valve inner and outer face increases. This unseats the outflow valve and allows cabin air to flow to the atmosphere.
As the isobaric range (figure 14-14) is approached reference chamber pressure, which has been decreasing at the same rate as atmospheric pressure, will have decreased enough to allow the isobaric bellows to expand and move the metering valve toward its seat. As a result, the flow of reference chamber air through the metering valve is reduced, preventing further decrease in reference pressure. In response to slight changes in reference chamber pressure, the isobaric control system modulates to maintain a substantially constant reference pressure in the chamber throughout the isobaric range of operation. Responding to the differential 553
Isobaric metering As the differential range is approached, the presvalve and bellows sure differential between the constant reference Cabin air pressure and the decreasing atmospheric pressure becomes sufficient to move the diaphragm and open the differential metering valve. As a result, reference chamber air flows to atmosphere through the differential metering valve, reducing the reference pressure. Responding to the decreased reference pressure, the isobaric bellows expands and closes the isobaric metering valve completely. Reference chamber pressure is now controlled, through the differential metering valve, by atmospheric pressure being reflected against the differential diaphragm.
Outlet As atmospheric pressure decreases, the metering valve valve opens more and allows reference pressure to decrease proportionately. Responding to the pressure differential between cabin and reference pressures, the outflow valve opens or closes as required Cabin air pressure to meter air from the cabin and maintain a prede- Control pressure termined differential pressure value.
- Atmospheric pressure In addition to the automatic control features just described, the regulator incorporates a ground test FIGURE 14-13. Cabin air pressure regulator in valve and a solenoid air valve, both of which are the unpressurized mode. located in the head and reference chamber section.
The solenoid air valve is an electrically activated Differential diaphragm valve spring-loaded to a normally closed position and metering valve True atmospheric vent against a passage through the head that opens the Cabin air Orifice reference chamber to atmosphere. When the cockpit Atmospheric pressure switch is positioned to "ram" the regulator vent solenoid opens, causing the regulator to dump cabin air to the atmosphere.
The ground test valve (see figure 14-12) is a three-position, manually operated control that allows for performance checks of the regulator and cabin pressurization system. In the test only-all off" position the valve renders the regulator completely inoperative. In the test only-differential on" position, the valve renders the isobaric control system inoperative so that the operation of the differential control system can be checked. In the "flight" position, the valve allows the regulator to a702? Cabin air pressure function normally. The ground test valve should Control pressure always be lockwired in the "flight" position unless Atmospheric pressure being tested.
FIGURE 14-14. Cabin air pressure regulator in the isobaric range.
between the constant reference chamber pressure and the variable cabin pressure, the outflow valve opens or closes, metering air from the cabin, as required, to maintain a constant cabin pressure.
Cabin Air Pressure Safety Valve The cabin air pressure safety valve (figure 14-15) is a combination pressure relief, vacuum relief, and dump valve. The pressure relief valve prevents cabin pressure from exceeding a predetermined differential pressure above ambient pressure.
The vacuum relief prevents ambient pressure from 554
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