apillary tube Power head diaphragm
- Superheat spring
• External equalizer line port
- Valve pin Outlet ports Inlet port FIGURE 14-39. Schematic of thermostatic expansion valve.
If the temperature of the gas leaving the evaporator increases above the desired superheat valve, it will be sensed by the remote bulb. The pressure generated in the bulb is transmitted to the diaphragm in the power section of the valve, causing the valve pin to open. A decrease in the tempera ture of the gas leaving the evaporator will cause the pressure in the remote bulb to decrease, and the valve pin will move toward the "closed" position.
The superheat spring is designed to control the amount of superheat in the gas leaving the evaporator. A vapor is said to be superheated when its temperature is higher than that necessary to change it from a liquid to a gas at a certain pressure. This ensures that the freon returning to the compressor is in the gaseous state.
The equalizer port is provided to compensate for the effect the inherent evaporator pressure drop has on the superheat setting. The equalizer senses evaporator discharge pressure and reflects it back to the power head diaphragm, adjusting the expansion valve pin position to hold the desired superheat value.
Evaporator The next unit in the line of cooling flow after the expansion valve is the evaporator, which is a heat exchanger forming passages for cooling air flow and for freon refrigerant. Air to be cooled flows through the evaporator.
The freon changes from a liquid to a vapor at the evaporator. In effect, the freon boils in the evaporator, and the pressure of the freon is controlled to the point where the boiling (evaporation) takes place at a temperature which is lower than the cabin air temperature: The pressure (saturated pressure) necessary to produce the correct boiling temperature must not be too low; otherwise, freezing of the moisture in the cabin air will block the air passages of the evaporator. As the freon passes through the evaporator, it is entirely converted to the gaseous state.
This is essential to obtain the maximum cooling and also to prevent liquid freon from reaching the compressor. The evaporator is designed so that heat is taken from the cabin air; therefore, the cabin air is cooled. All the other components in the freon system are designed to support the evaporator, where the actual cooling is done.
After leaving the evaporator, the vaporized refrigerant flows to the compressor and is compressed.
Heat is being withdrawn through the walls of the condenser and carried away by air circulating around the outside of the condenser. As the vapor condenses to a liquid it gives up the heat which was absorbed when the liquid changed to a vapor in the evaporator. From the condenser the liquid refrigerant flows back to the receiver, and the cycle is repeated.
DESCRIPTION OF A TYPICAL SYSTEM Since the vapor cycle system used in Boeing aircraft models 707 and 720 are typical of most vapor cycle systems, they are used here to describe the operation of such systems.
The major components of the vapor cycle air conditioning system are the: (1) Air turbine centri:
fugal compressors, (2) primary heat exchangers,
(3) refrigeration units, (4) heaters, and (5) necessary valves to control the airflow.
The vapor cycle system shown schematically in figure 14-40 is divided into a left-hand and a right-hand installation. Both installations are functionally identical.
Air Turbine Compressor The cabin and flight compartments are pressurized by using two air turbine centrifugal compressors (turbo-compressors). Each compressor consists of a turbine section and a compressor section as shown in figure 14-41.
Flight station Main cabin - Flight station electric heater Main •Pressure regulator cabin Left wing isolation valve heater electric •Flight station air mixing valve Right wing isolation valve Recirculation fan Recirculation fan -Gasper Gasper system system Pressure reguiator -Cabin Cabin recirculation recirculation valve valve From L/H refrigeration unit gang valve 3-port From R/H refrigeration Pack unit
- shutoff - Cooling valve Cooling air inlet air indet Leit primary heat exchanger. L Right primary / heat air outlet Cooling To L/H To R/H exchanger air outlet Cooling refrigeration unit refrigeration unit Engine starter Air turbine No. 3 pressor No. 1 centrifugal com- Air turbine centrifugal compressor No. 2 Engine No. 2 Engine No. 3 Pressure Pressure regulator Engine regulator starter No. 2 → Cooling air Cabin air Shutoff valve Check valve FIGURE 14-40. Schematic of vapor cycle air conditioning system on Boeing 707 and 720 airplanes.
Shaft Turbine scroll Compressor scroll Compressor impeller Sixteenth stage compressed air from engine bleed supply Turbine wheel Compressor inlet air Compressor discharge air Turbine inlet air 2353 Turbine discharge air FIGURE 14 41. Schematic of air turbine centrifugal compressor.
The turbine section inlet duct is connected to the sixteenth stage compressed air from the engine bleed air manifold. The bleed air is under a pressure of approximately 170 p.s.i. This high-pressure, high-velocity air is reduced to approximately 76
p.s.i. by a differential pressure regulator located in the air duct leading to the turbine inlet. This regulated air pressure turns the turbine at about 49,000
r.p.m.
Since the compressor is connected directly to the turbine, it also turns at the same r.p.m. The compressor output is approximately 1,070 cu. ft. of air per minute at a maximum of 50 p.s.i. The compressor section inlet is connected to a ram-air scoop and the outlet is connected through ducts into the air conditioning system.
Air flows through the ducts, through a wing isolation valve, past the shutoff valve, and through the primary heat exchanger.
Primary Heat Exchangers The two primary (air-to-air) heat exchangers are located in the left- and right-hand installation of the vapor cycle system as shown in figure 14-40.
Each primary heat exchanger consists of a duct assembly, a core assembly, and a pan assembly. The welded duct assembly contains both the inlet and outlet passages. The tube-type core assembly forms Cooling FIcURE 14-42. Schematic of primary heat exchanger.
the center portion of the unit. The pan assembly completes the enclosure of the tubes. Cabin ventilating air flows through the inside of the tubes of the core assembly. Ram air is forced around and between the outside of the tubes. Figure 14-42 shows a schematic diagram of the primary heat exchanger.
The primary heat exchangers remove about 10% of the heat of compression from the cabin ventilating air as it comes from the turbo-compressors, thus cooling the air to about 10° to 25° above outside air temperature.
Refrigeration Units From the primary heat exchangers the ventilating air is ducted to the refrigeration units. The two refrigeration units are located in the left- and right-hand installation of the vapor cycle system as shown in figure 14-40. Each refrigeration unit consists of an electric motor driven freon compressor, an air-cooled refrigerant condenser, a receiver (freon container), an evaporator heat exchanger, a dual control valve, a heat exchanger (liquid-to-gas), and the necessary electrical components to assure proper operation of the unit. The refrigerant used in the system is freon 114. Lubricating oil is added to the freon each time the refrigeration unit is charged to provide lubrication for the compressor After the air is cooled to the desired temperature, it is ducted into the cabin and flight deck.
The main cabin ventilating air and the flight compartment ventilating air are heated separately and independently by two electric heaters, one The flight compartment heater consists of a core which is made up of nine electrical heater elements mounted in a rectangular aluminum shell assembly, three protectors, a.c. power connection to the elements, and a control circuit to the thermal protectors.
The main cabin heater is similar but has a greater output capacity since it provides heat for a larger compartment and a greater volume of air.
Air Routing/Valves The solid black arrows in figure 14-40 indicate the flow route of the ventilating air from the turbocompressor, through the air conditioning units to the cabin and flight compartment. The three-port gang valve regulates the flow of hot or cold air to the cabin in response to the selected temperature.
AIR CONDITIONING AND PRESSURIZATION
SYSTEM MAINTENANCE The maintenance required on air conditioning and pressurization systems varies with each aircraft model. This maintenance follows procedures given in the appropriate aircraft manufacturer or equipment manufacturer's maintenance manuals. It usually consists of inspections, servicing, removing, and installing components, performing operational checks, and troubleshooting for the isolation and correction of troubles within the system.
Inspections Periodically inspect the system for component security and visible defects. Particular attention should be paid to the heat exchangers for signs of structural fatigue adjacent to welds. The ducting should be securely attached and adequately supported. Insulating blankets must be in good repair and secured around the ducting.
Servicing Each refrigeration unit contains freon for absorbing heat, plus oil mixed with the freon for lubricating the compressor motor bearings, If there is insuficient freon in the unit, it is incapable of absorbing heat from the air going to the cabin. If there is insufficient oil, the motor bearings will ov.
erheat and eventually cause unsatisfactory compressor operation. It is important that sufficient amounts of freon and oil be in the unit at all times.
In contrast to a hydraulic system where the circuits consist of closed loops containing fluid at all times, a freon loop contains quantities of both liquid and vapor. This, in addition to the fact that it is unpredictable exactly where in the system the liquid will be at any one instant, makes it difficult to check the quantity of freon in the system.
Regardless of the amount of freon in the complete system, the liquid level can vary significantly, depending on the operating conditions. For this reason, a standard set of conditions should be obtained when checking the freon level. These conditions are specified by the manufacturer and, as mentioned previously, vary from aircraft to aircraft.
To check the freon level, it is necessary to operate the refrigeration unit for approximately 5 min.
to reach a stable condition. If the system uses a sight glass, observe the flow of freon through the sight glass. A steady flow indicates that a sufficient charge is present. If the freon charge is low, bubbles will appear in the sight glass.
When adding freon to a system, add as much oil as is felt was lost with the freon being replaced. It is impossible to determine accurately the amount of oil left in a freon system after partial or complete loss of the freon charge. However, based on experience, most manufacturers have established procedures for adding oil. The amount of oil to be added is governed by: (1) The amount of freon to be added, (2) whether the system has lost all of its charge and has been purged and evacuated, (3) whether a topping charge is to be added, or (4) 583
whether major components of the system have been changed.
Usually one-fourth-ounce of oil is added for each pound of freon added to the system. When changing a component, an additional amount of oil is added to replace that which is trapped in the replaced component.
Oil for lubrication of the compressor expansion valve, and associated seals must be sealed in the system. The oil used is a special highly refined mineral oil free from wax, water and sulfur. Always use the oil specified in the manufacturer's maintenance manual for a specific system.
Freon-12 Freon-12 is the most commonly used refrigerant.
It is a fluorinated hydrocarbon similar to carbon tetrachloride with 2 of the chlorine atoms replaced by 2 fluorine atoms. It is stable at low or high temperatures, does not react with any of the materials or seals used in an air conditioning system, and is non-flammable.
Freon-12 will boil at sea level pressure at -21.6° F. Any freon-12 dropped on the skin will result in frostbite. Even a trace in the eye can cause damage. If this should occur, treat the eye with clean mineral oil, or petroleum jelly followed with a boric acid wash. GET TO A PHYSICIAN OR HOS PITAL AS SOON AS POSSIBLE.
Freon is colorless, odorless, and non-toxic; however, being heavier than air, it will displace oxygen and can cause suffocation. When heated over an open flame, it converts to phosgene gas which is deadly! Avoid inhaling freon or contact with this gas.
Wear a face shield, gloves and protective clothing when working with freon.
Compound Gage Manifold Set Whenever a freon system is opened for maintenance, a portion of the freon and oil will be lost.
Replenishment of the freon and oil is a must for efficient system operation. This requires the use of a special set of gages and interconnected hoses.
The manifold set (figure 14-43) consists of a manifold with three fittings to which refrigerant service hoses are attached: two hand valves with "O" ring type seals, two gages, one for the low pressure side of the system and one for the high pressure side of the system.
The low pressure gage is a compound gage, meaning it will read pressures either side of atmospheric. It will indicate to about 30 inches of mercury, gage pressure (below atmospheric) to about 60 psi gage pressure above atmospheric.
The high pressure gages usually have a range from zero up to about 600 p.s.i. gage pressure.
The low pressure gage is connected on the manifold directly to the low side fitting. The high pressure gage likewise connects directly to the high side fitting. The center fitting of the manifold can be isolated from either of the gages or the high and low service fittings by the hand valves. When these valves are turned fully clockwise, the center fitting is isolated. If the low pressure valve is opened (turned counter-clockwise), the center fitting is opened to the low pressure gage and the low side service line. The same is true for the high side when the high pressure valve is opened.
Special hoses are attached to the fittings of the manifold valve for servicing the system. The high pressure charging hose attaches to the service valve in the high side, either at the compressor discharge, the receiver dryer, or on the inlet side of the expansion valve. The low pressure hose attaches to High Pressure Gage Low Side Valve High Side Valve FIGURE 14-43. Freon manifold set.
the service valve at the compressor inlet, or at the discharge side of the expansion valve. The center hose attaches to the vacuum pump for evacuating the system, or to the refrigerant supply for charging the system. Charging hoses used with Schrader valves must have a pin to depress the valve.
When not using the manifold set, be sure the hoses are capped to prevent moisture contaminating the valves.
Purging the System Whenever a freon system is to be opened for maintenance it is necessary to purge the system.
The manifold set is connected as previously described except the center hose is not yet connected to the vacuum pump. Cover the center hose with a clean shop towel, open both valves slowly. This will permit the gas to escape but will not blow the oil from the system. When both gages read zero, the system may be opened.
the low pressure valve to allow vapors to enter the system. Put as many pounds of freon into the system as called for by the specifications. Close all valves, remove manifold set and perform an operational check.
Checking Compressor Oil The compressor is a sealed unit in the refrigeration system. Any time the system is evacuated the oil quantity must be checked. Remove the filler plug and using the proper type dip stick, check the oil quantity. It should be maintained in the proper range using oil recommended by the manufacturer.
After adding oil, replace the filler plug and recharge the system.
CABIN PRESSURIZATION OPERATIONAL CHECKS Two operational checks can be performed on a cabin air conditioning and pressurization system.
The first is a general operational check of the complete system, designed to ensure the proper operathe System tion of each major system component as well as the few drops of moisture will contami- complete system. The second is a cabin pressurizacompletely block an air conditioning sys- tion check designed to check the cabin for airtight- If this moisture freezes in the expansion ness.
action stops. Water is removed from To operationally check the air conditioning sysevacuation. Anytime the system has tem, either operate the engines or provide the necessary ground support equipment recommended by ing. the aircraft manufacturer.
With system controls positioned to provide cold air, ensure that cold air is flowing from the cabin distribution outlets. Position the system controls to provide heated air and check to see that there is an increase in the temperature of the airflow from the distribution outlets.
Checking the cabin pressurization system consists of the following: (1) A check of pressure regulator With the system under vacuum from evacuation, close all valves, connect the center hose to a refrigerant supply. Open the container valve, loosen the high side hose at its connection to the system, and allow some freon to escape. This purges the manifold set. Tighten the hose.
Open the high pressure valve, this will permit freon to flow into the system. The low pressure gage should begin to indicate that the system is coming out of the vacuum. Close both valves.
Start the engine, set the rpm at about 1250. Set the controls for full cooling. With the freon container upright to allow vapor to come out, open operation, (2) a check of pressure relief and dump valve operation, (3) a cabin static pressure test, and (4) a cabin dynamic pressure test.
To check the pressure regulator, connect an air test stand and a monometer (a gage for measuring pressure, usually in inches of Hg) to the appropriate test adapter fittings. With an external source of electrical power connected, position the system controls as required. Then pressurize the cabin to 7.13 in. Hg, which is equivalent to 3.5 p.s.i. The pressurization settings and tolerances presented here are for illustrative purposes only. Consult the applicable maintenance manual for the settings for a particular make and model aircraft. Continue to pressurize the cabin, checking to see that the cabin pressure regulator maintains this pressure.
The complete check of this pressure relief and dump valve consists of three individual checks.
First, with the air test stand connected to pressurize the cabin, position the cabin pressure selector switch to dump the cabin air. If cabin pressure decreases to less than 0.3 in. Hg (0.15 p.s.i.)
through both the pressure relief and dump valves, the valves are dumping pressure properly. Second, using the air test stand, re-pressurize the cabin.
Then position the manual dump valve to "dump." A lowering of the cabin pressure to 0.3 in. Hg (0.15
p.s.i.) and an airflow through the pressure relief and dump valve indicate that the manual dumping function of this valve is satisfactory. Third, position the master pressure regulator shutoff valve to iall off." (This position is used for ground testing only.) Then, using the air test stand, pressurize the cabin to 7.64 in. Hg (3.75 p.s.i.). Operation of the pressure relief and dump valves to maintain this pressure indicates that the relief function of the cabin pressure relief and dump valves is satisfactory.
The cabin static pressure test checks the fuselage for structural integrity. To perform this test, connect the air test stand and pressurize the fuselage to 10.20 in. Hg (5.0 p.s.i.). Check the aircraft skin exterior for cracks, distortion, bulging, and rivet condition.
pressure using an air test stand. Then with a monometer, determine the rate of air pressure leakage within a certain time limit specified in the aircraft maintenance manual. If leakage is excessive, large leaks can be located by sound or by feel. Small leaks can be detected using a bubble solution or a cabin leakage tester.
A careful observation of the fuselage exterior, prior to its being washed, may also reveal small leaks around rivets, seams, or minute skin cracks. A telltale stain will be visible at the leak area.
CABIN PRESSURIZATION TROUBLESHOOTING Troubleshooting consists of three steps: (1) Establishing the existence of trouble, (2) determining all possible causes of the trouble, and (3) identifying or isolating the specific cause of the trouble.
Troubleshooting charts are frequently provided in aircraft maintenance manuals for use in determining the cause, isolation procedure, and remedy for the more common malfunctions which cause the cabin air conditioning and pressurization systems to become inoperative or uncontrollable. These charts usually list the most common system failures.
Troubleshooting charts are organized in a definite sequence under each trouble, according to the probability of failure and ease of investigation. To obtain maximum value, the following procedures are Possible Cause Isolation Procedure Correction
(1) Trouble: Cabin temperature too high or too low (will not respond to control during "auto" operation).
Defective temperature sensor. Place system in manual operation If system operates correctly, reand rotate air temperature con- place temperature sensor with trol knob manually. one known to be operative and check system again in "auto" operation.
(2) Trouble: Cabin temperature too high or too low (will not respond to control during "auto" or manual operation):
Defective temperature control- With system being operated in If the valve is not opening and ler or refrigeration bypass valve manual position and the cabin air closing according to control setinoperative. temperature control knob being tings, disconnect electrical plug cycled between "cold" and "hot" from valve solenoid and check observe the valve position indica- the power source. If the valve tor (located on the valve). position indicates that the valve is opening and closing according to control settings, continue with the next troubleshooting item.
FiGURE 14-44. Troubleshooting an air cycle system.
recommended when applying a troubleshooting chart to system failures:
(1) Determine which trouble listed in the table most closely resembles the actual failure being experienced in the system.
(2) Eliminate the possible causes listed under the trouble selected, in the order in which they are listed, by performing the isolation procedure for each until the malfunction is discovered.
(3) Correct the malfunction by following the instructions listed in the correction column of the troubleshooting chart.
Figure 14-44 is an example of the type of troubleshooting chart provided in the maintenance manual for an aircraft that uses an air cycle system.
OXYGEN SYSTEMS GENERAL The design of the various oxygen systems used in aircraft depends largely on the type of aircraft, its operational requirements, and, where applicable, the pressurization system. In some aircraft a continuous-flow oxygen system is installed for both passengers and crew. The pressure demand system is widely used as a crew system, especially on the larger transport aircraft. Many aircraft have a combination of both systems which may be augmented by portable equipment.
Continuous Flow System In simple form a basic continuous-flow oxygen system is illustrated in figure 14-45. As shown in the illustration, with the line valve turned "on", oxygen will fow from the charged cylinder through the high-pressure line to the pressure-reducing valve, which reduces the pressure to that required at the mask outlets. A calibrated orifice in the outlets will control the amount of oxygen delivered to the mask.
The passenger system may consist of a series of plug-in supply sockets fitted to the cabin walls adjacent to the passenger seats to which oxygen masks can be connected, or it may be the "drop out" mask arrangement where individual masks are presented automatically to each passenger if pressurization fails. In both cases oxygen is supplied, often automatically, from a manifold. Any automatic control
(e.g. barometric control valve) in the system can be overridden manually by a member of the crew.
40,000 ft.
the cabin pressure altitude between 8,000 and 15,000 ft, regardless of the actual altitude of the aircraft. Under such conditions, oxygen is not needed for the comfort of the passengers and crew.
However, as a precaution, oxygen equipment is installed for use if cabin pressurization fails. Portable oxygen equipment may also be aboard for first-aid purposes.
With some of the smaller and medium size aircraft designed without cabin pressurization, oxygen equipment may be installed for use by passengers and crew when the aircraft is flown at high altitudes. In other instances where there is no installed oxygen system, passengers and crew depend on portable oxygen equipment stowed in convenient Pressure-Demand System A simple pressure-demand oxygen system is illustrated in figure 14-46. Note that there is a pressure-demand regulator for each crewmember, who can adjust the regulator according to his requirements.
PORTABLE OXYGEN EQUIPMENT Typical portable oxygen equipment consists of a lightweight steel alloy oxygen cylinder fitted with a combined flow control/reducing valve and a pressure gage. A breathing mask, with connecting flexible tube and a carrying bag with the necessary straps for attachment to the wearer, completes the set.
The charged cylinder pressure is usually 1,800
p.s.i.; however, the cylinder capacities vary. A popular size for portable equipment is the 120-liter capacity cylinder.
Pressure Mask outlets High Low 70 ° • o Cylinder Charging
- Pressure gage connection Pressure Check reducing valve Charging valve valve Line valve Filter Filter Shutoff valve FIGURE 14-45. Continuous-fow oxygen system.
Depending on the type of equipment used, it is normally possible to select at least two rates of flow, normal or high. With some equipment three flow rate selections are possible, i.e., normal, high, and emergency, which would correspond to 2, 4, and 10 liters per minute. With these flow rates a 120-liter cylinder would last for 60, 30, and 12 min., respectively.
SMOKE PROTECTION EQUIPMENT In some instances there is a requirement to carry smoke protection equipment for use by a member of the crew in a smoke or fume-laden atmosphere.
Smoke protection equipment consists of a special smoke protection facial mask with eye protection in the form of a clear-vision visor, together with the necessary oxygen supply hose and head straps.
Some are designed for use with oxygen from the aircraft oxygen system, and others are self-contained portable equipment.
OXYGEN CYLINDERS The oxygen supply is contained in either highpressure or low-pressure oxygen cylinders. The high-pressure cylinders are manufactured from heat-treated alloy, or are wire wrapped on the outside surface, to provide resistance to shattering. All high-pressure cylinders are identified by their green color and have the words AVIATORS' BREATH- ING OXYGEN" stenciled lengthwise in white, 1-in.
letters.
High-pressure cylinders are manufactured in a variety of capacities and shapes. These cylinders can carry a maximum charge of 2,000 p.s.l., but are normally filled to a pressure of 1,800 to 1,850 p.s.i.
There are two basic types of low-pressure oxygen cylinders. One is made of stainless steel; the other, of heat-treated, low-alloy steel. Stainless steel cylinders are made nonshatterable by the addition of narrow stainless-steel bands that are seam-welded to the body of the cylinder. Low-alloy steel cylinders do not have the reinforcing bands but are subjected to a heat treatment process to make them nonshatterable. They have a smooth body with the word "NONSHATTERABLE" stenciled on them.
Both types of low-pressure cylinders come in different sizes and are painted light yellow. This color indicates that they are used for low-pressure oxygen only. The cylinders may carry a maximum charge of 450 p.s.i., but are normally filled to a pressure of 588
regulator Copilot's regulator Engineer's Pressure gage Shutoff valve Pressure gage Cylinder, Pressure reducing valve Pressure reducing valve Check valve Line valve Filter - Filter Pilot's Observer's regulator regulator Pressure „Charging valve Charging connection Low C FIcURE 14-46. Typical pressure-demand oxygen system.
from 400 to 425 p.s.i. When the pressure drops to 50 p.s.i., the cylinders are considered empty.
The cylinders may be equipped with either of two types of valves. One type used is a self-opening valve which is automatically opened when the selfopening valve coupling assembly attached to the oxygen tubing is connected to the valve outlet. This coupling unseats a check valve, allowing oxygen from the cylinder to fill the oxygen system under high pressure. the other type is a hand-wheel, manually operated valve. This valve should be safety wired in the "full on" position when the cylinder is installed in the aircraft. This valve should be closed when removing or replacing parts of the oxygen system and when the cylinder is to be removed from the aircraft. Cylinders are often provided with a disk designed to rupture if cylinder pressure rises to an unsafe value. The disk is usually fitted in the valve body and vents the cylinder contents to the outside of the aircraft in the event of a dangerous pressure rise.
SOLID STATE OXYGEN SYSTEMS Emergency supplemented oxygen is a necessity in any pressurized aircraft flying above 25,000 ft.
Chemical oxygen generators can be used to fulfill the new requirements. The chemical oxygen generator differs from the compressed oxygen cylinder and the liquid oxygen conveter in that the oxygen is actually produced at the time of delivery.
Solid-state oxygen generators have been in use for a number of years, dating back to 1920, when it was first used in mine rescues. During World War Il the Japanese, British and Americans, all worked to develop oxygen generators for aircratt and submarines.
In figure 14-47, 120 standard cubic feet of oxy.
gen (10 Ibs.) is shown schematically in the number of cubic inches of space it would occupy as a gas, a liquid or a solid. In figure 14-48, the necessary hardware to install and operate the system has been included in the size and weight measurements. A close comparison of these values makes it apparent that the solid state oxygen generator system is the most efficient space wise. Likewise less equipment and maintenance is required for solid state oxygen converters. Integrity inspection is the only requirement until actual use is implemented.
Solid state describes the chemical source, sodium chlorate, formula NaCIO,. When heated to 478° F., sodium chlorate releases up to 45% of its weight as gaseous oxygen. The necessary heat for de- 589
→
44.5 Ibs GAS DIA 2160 in
- 10"
10. Ibs LIQUID 1000 in* LIQUID 295°F
6.9~ CUBE SOLID SOLID · 400 in ¡" 28 lbs
SODIUM CHLORATE CANDLE DENSITY OF 22 GRM/CC 26 LB.
FIGURE 14-48. Weight and volume comparison-gas, liquid FIGURE 14-47. Volume comparison. and solid oxygen storage.
composition of the sodium chlorate is supplied by iron which is mixed with the chlorate.
The Oxygen Generator Figure 14-49 illustrates a schematic representation of a basic oxygen generator. The center axial one end of the core is larger than the other. The purpose of this is to program a high oxygen evolu.
tion rate during the initial few minutes of burning such as is required for an emergency descent supply. Burning continues until the core is expended.
position is occupied by a core of sodium chlorate, BREATHING TUBE CONNECTION iron, and some other ingredients mixed together HERMETIC SEAI.
and either pressed or cast into a cylindrical shape.
This item has been popularly referred to as an oxygen candle, because when it is ignited at one end, it burns progressively in much the same manner - HAXDGRENADE as a candle or flare. Surrounding the core is por- BOLCHON ous packing. It supports the core and filters salt - GLASS WOOL particles from the gas as it flows toward the outlet.
A chemical filter and particulate filter at the outlet IGNITION CONE end of the container provide final clean up of the gas so that the oxygen delivered is medically pure 010" COPPER breathing oxygen. An initiation device is an in- I/I6 TK. MICARTA tegral part of the package. This may be either a LOCK FINISH mechanical percussion device or an electric squib. The choice depends on the application. The entire GLASS CLOTH NSULATION GLASS WOOL assembly is housed in a thin shelled vessel. Often CANDLE included is a layer of thermal insulation on the inside shell; a check valve seal on the outlet, and a KC10, pressure condition. relief valve to protect against an inadvertent over- •GLASS WOOL HOPCALITE In operation, the burning is initiated at one end of the core by activating the squib or percussion device. Oxygen evolution rate is proportional to the cross sectional area of the core and the burn OXYGEN OUTLET rate. The burn rate is determined by the concentration of fuel in the chlorate. In certain cases, FiGURE 14-49. Apparatus for burning chlorate candles.
The simplicity of the process should be readily apparent; likewise, the limitations. There are no on/off valves and no mechanical controllers. Refill is accomplished by simply replacing, in total, the entire device. A limitation is that once the generator is initiated, flow is delivered at a predetermined rate, thus demand use is not very efficient.
In order to keep the process from consuming a great quantity of the oxygen, the quantity of iron is kept to a minimum. There is a tendency toward liberation of small amounts of chlorine. Barium peroxide, or barium dioxide, may be added by the manufacturer to provide an alkaline medium for removing the trace amounts of chlorine that may On a volume basis, which is extremely important in the aircraft installation, the storage capacity of oxygen in candles is about three times that of compressed gas.
A typical three outlet module for a 15 minute decompression emergency descent supply for a supersonic transport (25000 foot max. cabin altitude) weighs less than 0.9 pound and consists simply of a 2.1 inch diameter by 3.55 inch long stainless steel cylinder attached to three manifolded hose nipples. The cylinder contains the generator, initiator, salt, fume filter, enough insulation to keep the cylinder surface below 250° F. during burning, a pressure relief plug, and a temperature indicating paint spot for generator status visual inspection.
The nipples contain orifices just small enough to assure essentially equal flow to all three masks.
The generators are inert below 400° F. even under severe impact. While reaction temperature is high and considerable heat is produced, the generators are insulated so that the outer surface of the cylinder is cool enough to avoid any fire hazard. The portable units may be held comfortably throughout the entire operation, as the 'heat generated is dissipated steadily over a long period of time. The same insulation works in reverse to delay initiation should a unit be subjected to an external fire. If such a fire is sufficiently prolonged to ignite the chlorate generator, oxygen production will be at a relatively low and continuous rate. In the simple continuous flow systems no pressure would be generated as all outlets would permit unrestricted flow of the oxygen, eliminating the intense jet torch effect of pressurized oxygen
• Elimination of distribution and regulation components-saves weight and maintenance.
• Simplification of individual distribution manifolds and drop-out mechanisms by the use of modular chlorate candle units.
• Improved reliability, hence safety by design of initiation circuitry, such that, an individual malfunction would not make other units inoperative (comparison here would be to ruptured lines, or high leakage in gaseous distribution systems).
• Simple, visual surveillance of each unit for condition of chlorate candle within the sealed container, by use of inspection window.
• Simple replacement of any unit, should it show any sign of deterioration, by plug-in cartridge, by relatively unskilled services crew; easily checked for installation and readiness for functioning from flight deck.
• Programmed oxygen release rates irrespective of the type of emergency.
OXYGEN PLUMBING Tubing and fittings make up most of the oxygen system plumbing and connect the various components. All lines are metal except where flexibility is required. Where flexibility is needea, rubber hose is used.
There are several different sizes and types of oxygen tubing. The one most frequently used in low-pressure gaseous systems is made of aluminum alloy. Tubing made of this material resists corrosion and fatigue, is light in weight, and is easily formed. High-pressure gaseous supply lines are Installed oxygen tubing is usually identified with color-coded tape applied to each end of the tubing, and at specified intervals along its length. The tape coding consists of a green band overprinted with the words "BREATHING OXYGEN" and a black rectangular symbol overprinted on a white background.
Solid State vs. High Pressure Gaseous Oxygen
• Elimination of high pressure storage containers-saves weight.
Oxygen System Fittings Tube segments are interconnected or connected to system components by fittings. Tubing-to-tubing fit- 591