standing of the manner in which the system is controlled, the functions of the various components and subassemblies, and their effect on total system operation. Figure 14-27 is a schematic of a typical system. Frequent reference to the schematic should be made during study of the following operational description.
The system is composed of a primary heat exchanger, primary heat exchanger bypass valve, fow limiters, refrigeration unit, main shutoff valve, secondary heat exchanger, refrigeration unit bypass valve, ram-air shutoff valve, and an air temperature control system. A cabin pressure regulator and a dump valve are included in the pressurization system.
Air for the cabin air conditioning and pressurization system is bled from the compressors of both engines. The engine bleed lines are cross-connected and equipped with check valves to ensure a supply of air from either engine.
A flow-limiting nozzle is incorporated in each supply line to prevent the complete loss of pressure in the remaining system if a line ruptures, and to prevent excessive hot air bleed through the rupture.
In reading the schematic, in figure 14-27, the initial input of hot air is indicated on the righthand side. The flow is depicted across the page through each unit, in turn, and back to the squares on the lower right-hand side which represent the cockpit and cabin.
Air from the engine manifolds is ducted through a flow limiter to the primary heat exchanger and its bypass valve simultaneously. Cooling air for the heat exchanger is obtained from an inlet duct and is exhausted overboard.
The air supply from the primary heat exchanger is controlled to a constant temperature of 300° F.
by the heat exchanger bypass valve. The bypass valve is automatically controlled by upstream air pressure and a downstream temperature-sensing element. These provide temperature data to cause the valve to maintain the constant temperature by mixing hot engine bleed air with the cooled air from the heat exchanger.
The cabin air is next routed through another flow limiter and a shutoff valve. The shutoff valve is the main shutoff valve for the system and is controlled from the cockpit.
From the shutoff valve, the air is routed to the refrigeration unit bypass valve, to the compressor section of the refrigeration unit, and to the secondary heat exchanger. The bypass valve automatically maintains compartment air at any preselected temperature between 60° F. and 125° F. by controlling the amount of hot air which bypasses the refrigeration unit and mixes with the refrigeration unit outpur.
Cooling air for the secondary heat exchanger core is obtained from an inlet duct. Some installations use a turbine-driven fan to draw air through the heat exchanger; others use a hydraulically driven blower. After cooling the cabin air, the cooling air is exhausted overboard.
As the cabin air leaves the secondary heat exchanger, it is routed to the expansion turbine, which is rotated by the air pressure exerted on it.
In performing this function, the air is further cooled before entering the water separator, where the moisture content of the air is reduced. From the water separator the air is routed through the temperature sensor to the cabin.
Air enters the cabin spaces through a network of ducts and diffusers and is distributed evenly throughout the spaces. Some systems incorporate directional vents that can be rotated by the cabin occupants to provide additional comfort.
An alternate ram-air system is provided to supply the cabin with ventilating air if the normal system is inoperative or to rid the cabin areas of smoke, foul odors, or fumes which might threaten comfort, visibility, or safety.
The air conditioning and the ram-air systems are controlled from a single switch in the cockpit. This switch is a three-position switch for selecting off, normal, and ram. In the "off" position (under normal conditions) all cabin air conditioning, pressurization, and ventilating equipment is off. In the "normal" position (under normal conditions) the air conditioning and pressurization equipment is functioning normally and ram air is off. In the "ram" position (under normal conditions) the main shutoff valve closes, and the cabin air pressure regulator and the cabin safety dump valve are opened.
This allows ram air from the secondary heat exchanger cooling air inlet duct to be routed into the cabin air supply duct for cabin cooling and ventilation.
With the air pressure regulator and the safety dump valve energized open, existing cabin air and incoming ram air are constantly being dumped overboard, ensuring a steady flow of fresh air through the cabin.
A duct incorporated in the air conditioning system between the constant-temperature line downstream from the primary heat exchanger bypass valve and the cabin compartment supplies hot air for supplemental heating. Control of this air is provided by the auxiliary heat control valve, which is a butterfly type valve. The heat control valve is controlled by a manually operated heat control handle, which is connected by cable to a control arm mounted on the valve.
The temperature control system consists of a cabin temperature controller, a temperature selector knob, a two-position temperature control switch, a modulating bypass valve, and a control network.
When the temperature control switch is in the "auto" position, the bypass valve will seek a valve gate position which will result in a duct temperature corresponding to the temperature controller setting.
This is accomplished through the control network, which transmits signals from the sensing element to the cabin temperature controller, which then electrically positions the valve in relation to the settings of the temperature control knob. With the temperature control switch in the "man." position, the controller will control the bypass valve directly, without reference to the duct temperature. In this mode of operation the desired temperatures are maintained by monitoring the air temperature knob as varying conditions alter cabin temperature.
AIR CYCLE SYSTEM COMPONENT OPERATION Primary Heat Exchanger This unit, illustrated in figure 14-28, reduces the temperature of engine bleed air or supercharger discharge air by routing it through the veins in the core of the heat exchanger. During flight the core is cooled by ram air. The amount of air to be cooled in the primary heat exchanger is controlled by the primary heat exchanger bypass valve.
Primary Heat Exchanger Bypass Valve The primary heat exchanger bypass valve (figure 14-29) is located in the high-pressure duct at the primary heat exchanger outlet. As previously stated, it modulates and controls the flow of primary heat exchanger air and primary heat exchanger bypass air to maintain a constant output air temperature of 300° F. The unit consists essentially of a regulator body assembly which contains a pressure regulator, a temperature control actuator, a solenoid valve, and a pneumatic thermostat. The body assembly of the unit contains two inlet ports marked "hot" and FIGURE 14-28. Primary heat exchanger.
"cold" and one outlet port. The two inlet ports incorporate butterfly valves, which are mounted on serrated shafts that extend across the width of the housing assembly and are attached to a common actuator control arm. The butterflies are positioned at 90° to each other and operate in such a manner that when one moves toward the "open" position the other moves toward the "closed" position. The actuator shaft contains an adjustable stop screw, which limits actuator travel, and pointers for indicating the position of the butterflies.
The temperature control actuator is mounted on the bypass valve body and consists of a housing and cover containing a spring-loaded diaphragm assembly. The diaphragm assembly is attached to the butterfly control arm and divides the actuator into a control pressure chamber and an ambient sensing chamber. The ambient chamber contains the diaphragm spring and the actuator rod.
As shown in the schematic in figure 14-29, pressure from the primary heat exchanger is routed through a filter and on through the pressure regulator into the control pressure chamber of the temperature control actuator. This internal pressure is called reference pressure. The reference pressure applied against the actuator diaphragm controls the position of the butterflies, which in turn controls the proportion of hot air from the bypass line and cooled air from the heat exchanger. The entire operation of the bypass valve is centered about the 568
Override solenoid Override solenoid (energized) Vent (de-energized) Reference pressure regulator Vent Vent Vent Control Thermostat actuator - valve Filter • Vent Primary heat Aluminum exchanger output Engine bleed air Full cold Modulating Engine bleed air CaME Regulator reference pressure Primary heat exchanger output Regulated pressure B8 Temperature control modulating pressure FIcURE 14-29. Primary heat exchanger bypass valve.
proportion of reference air pressure to heat. The greater the reference pressure that is supplied to the control actuator, the higher will be the temperature of the output air.
A pressure regulator is provided in the bypass valve to ensure a supply of reference air pressure to the control actuator on a schedule that eliminates the effect of altitude on the controlled temperature.
As the aircraft altitude increases, the constant reference pressure in the control actuator tends to move the control actuator diaphragm even further toward the ambient side. This moves the butterflies in a direction that causes the outlet temperature to rise.
The pressure regulator offsets this condition with the help of the pneumatic thermostat.
The variable-orifice type thermostat consists of a spring-loaded, ball-type valve and seat assembly housed in a core assembly. The core assembly is composed of a high-expansion element (aluminum) and a low-expansion element (Invar). As can be seen in the diagram (figure 14-29), the aluminum housing and the end of the Invar core extend into the outlet duct. Linear expansion of the aluminum housing moves the Invar core and ball-type valve assembly from the valve seat. This movement vents reference air pressure to the atmosphere. The resulting pressure applied against the temperature control actuator diaphragm controls the position of the butterflies.
The bypass valve regulating mechanism may be set to deliver cold air only by energizing the electromagnetic valve (override solenoid valve). The electromagnetic valve vents all reference air pressure to atmosphere when energized. With no reference air pressure, the spring-loaded diaphragm in the temperature control actuator returns the butterflies to the "full cold" position.
The electrical circuitry is so arranged that the solenoid can be energized only if the windshield anti-ice control switch is in the off" position. This ensures a supply of hot air for anti-ice operation.
Shutoff Valve The shutoff valve (figure 14-30), located in the air supply duct to the refrigeration unit, controls the air pressure to that unit. It is also the main 569
Solenoid valve Dump air Bleed orifice Pressure Diaphragm regulator Primary piston Flapper 0000 Spring Filter Spring Actuator Secondary piston - Linkage Duct Airflow Butterfly FICURE 14-30. Shutoff valve.
shutoff valve for the cabin air conditioning and pressurization systems.
The valve requires electrical power and a minimum of 15 p.s.i. upstream pressure to function. It will regulate the downstream pressure to 115 p.s.1.
Although this is an open/close valve, its major open function is to regulate. This is accomplished by a spring-loaded valve in the airflow line which is controlled by a primary piston. Upstream air pressure (if above 15 p.s.i.) bleeds through a filter and regulating mechanism to act on the primary piston, thereby opening the valve. After the downstream pressure rises to 115 p.s.i., it acts on a secondary piston which, through mechanical linkage, opens a bleed orifce to limit the amount of air acting on the primary piston. Since the primary piston is spring loaded to the "closed" position, it will then partially close, limiting the downstream pressure to 115 p.s.i.
The shutoff valve is operated by a solenoid valve that is spring loaded to off. In the "off" position, the control air from upstream is vented to atmos.
phere before it can operate the primary piston.
When the cockpit switch is actuated, the solenoid is energized and the vent closes, allowing pressure to build up to operate the primary piston.
Refrigeration Bypass Valve The refrigeration bypass valve (figure 14-31) operates in conjunction with the temperature control system to modulate and control the flow of bypass air to the refrigeration unit. This action automatically maintains the cabin air at the temperature selected through the temperature controller.
The valve is electrically controlled and pneumatically operated. Its operation relies on a signal from the downstream temperature-sensing element, which is controlled through the temperature control system, for an "open" position, but utilizes upstream pneumatic pressure to open the valve.
When electrical power is applied, a current-carrying coil and armature (transducer) is energized, closing a bleed port in the pressure chamber of the valve. The resulting pressure buildup in the chamber forces a piston to rotate a butterfly valve in the 570
Valve position окною Line pressure indicator x*1-5a Regulated pressure Encoma Modulating pressure Pressure regulator • Orifice Close To temperature controller Transducer Closed Modulating (no electric power) FIcURE 14-31. Refrigeration bypass valve.
cabin air duct to an "open" position. As the temperature varies or a new temperature is selected, the valve is re-positioned accordingly. Re-positioning is accomplished by action of the transducer in varying the amount of pressure allowed to bleed from the pressure chamber. Failure of the bypass valve or its components will cause the valve to move to the fail-safe (closed) position.
Secondary Heat Exchanger The function of the secondary heat exchanger is to partially cool the air for cabin pressurization and air conditioning to a temperature which makes possible the efficient operation of the refrigeration unit.
The heat exchanger assembly consists mainly of dimpled aluminum alloy tubes. The tubes are arranged so that pressurized cabin air can flow through them and cooling air can flow across them.
The secondary heat exchanger operates in essentially the same manner as the primary heat exchanger. Cabin air that is to be further cooled is routed through the tubes in the heat exchanger core. Cooling, air is forced through the secondary heat exchanger and returned to an engine air inlet or can be exhausted directly to the atmosphere.
Cabin air is regulated by the refrigeration bypass valve where it is directed to the secondary heat exchanger or to the refrigeration unit bypass line in metered quantities as required to meet the demands of the temperature control system.
Refrigeration Unit The refrigeration unit or turbine is used in the air conditioning system to cool the pressurized air for the cabin. Operation of the unit is entirely automatic, the power being derived from the pressure and temperature of the compressed air passing through the turbine wheel. The refrigeration cycle is modulated to meet varying cabin cooling demands by a refrigeration bypass valve which pro- 571
•Turbine scroll Compressor.
scroll
- Turbine wheel Compressor impeller Turbine Compressor inlet outlet
- Slinger Turbine inlet Compressor outlet Sump - Main housing assembly Sump drain FICURE 14-32. Schematic of a refrigeration turbine.
vides for a bypass of the entire refrigeration unit.
Thus, cabin temperature is regulated by mixing by.
passed air with that which has passed through the refrigeration unit.
The refrigeration turbine (figure 14-32) consists of three major sections: (1) Main housing assembly, (2) turbine scroll assembly, and (3) compressor scroll assembly.
The main housing assembly provides mounting for the two scroll assemblies and provides support for the two shaft bearings. It also serves as the oil reservoir from which the oil is supplied to the bearings by wicks. A dipstick for checking oil level is attached to the filler cap. The turbine scroll assembly is composed of two halves which confine the turbine nozzle within which the turbine wheel rotates. The compressor scroll assembly is composed of two halves which confine the diffuser within which the compressor wheel rotates.
A common shaft carries both assemblies and is supported by bearings in the housing assembly. An oil slinger is mounted outboard of each of the bearings which carry the shaft. These slingers pump an oil/air mist through the bearings to provide for lubrication. Air/oil seals are provided between each slinger and the adjacent wheel.
The supply air which is being cooled drives the refrigeration turbine. An impeller, driven by this turbine, forces the cooling air through the refrigeration unit.
The refrigeration process takes place as the hot compressed air expands through the turbine wheel of the air expansion turbine. This results in a reduction in the temperature and pressure of the air.
As this hot compressed air expands, it releases energy to the turbine wheel causing it to rotate at high speed.
Since the turbine wheel and compressor wheel are on opposite ends of a common shaft, the turbine wheel rotation results in a corresponding rotation of the compressor wheel. Thus, the energy released from the high temperature compressed air to the 572
turbine wheel provides the energy required by the compressor wheel to further compress the incoming air. The load imposed on the turbine by the compressor holds the speed of rotation within the range of maximum efficiency. Reduction of the air temperature assists in maintaining the cabin temperature within desired limits.
Water Separators Water separators (figure 14-33) are used in the cabin air conditioning system to remove excessive moisture from the air. In most refrigeration systems a water separator is installed in the discharge duct of the cooling turbine.
The water separator removes excess moisture from the conditioned air by passing the air through a coalescent bag or condenser. Very small water particles in the form of fog or mist in the air are formed into larger particles in passing through the condenser. As the moisture laden air passes through the vanes of the coalescent support, the water particles are carried with the swirling air and are thrown outward against the walls of the collector.
Air inlet The water then drains into a collector sump and is drained overboard.
Some water separators also contain a pressure-relief and altitude-sensitive bypass valve. Since very little moisture is present in the air at high altitudes, the bypass valve in the water separator opens at a predetermined altitude, generally 20,000 ft., to permit cold air to pass directly through the water separator, bypassing the coalescent bag and reducing system back pressure. The bypass valve will also open if, for some reason, the coalescent bag should become obstructed.
A coalescent bag condition indicator is provided on some water separators to indicate when the bag is dirty. The indicator senses a pressure drop across the bag and indicates when the pressure drop is excessive. Since the indicator is pressure sensitive, the condition of the bag can be determined only while the system is in operation.
Ram-Air Valve The ram-air valve is always closed during normal operations. It is energized to open when the cockpit Bypass valve Condenser Support Water collector Air outlet FIGURE 14-33. Water separator.
switch is placed in the "ram" position. With the ram-air valve open, air from the air inlet duct is admitted through the valve and directly into the cabin air supply duct.
ELECTRONIC CABIN TEMPERATURE CONTROL SYSTEM The operation of the electronic temperature control system is based primarily on the balanced.
bridge circuit principle. When any of the units which compose the "legs" of the bridge circuit change resistance value because of a temperature change, the bridge circuit becomes unbalanced. An electronic regulator receives an electrical signal as a result of this unbalance and amplifies this signal to control the mixing valve actuator.
In a typical application of the electronic temperature control system, three units are utilized: (1) Cabin temperature pickup (thermistor), (2) manual temperature selector, and (3) electronic regulator.
Figure 14-34 shows a simplified schematic diagram of an electronic temperature control system.
— Bridge circuit Cabin temperature regulator Cabin air Cabin temperature temperature selector pickup unit (thermistor) Mixing valve FIGURE 14-34. Electronic air temperature control system (simplified).
Cabin Temperature Pickup Unit The cabin temperature pickup unit (temperature sensing unit) consists of a resistor that is highly sensitive to temperature changes. The temperature pickup unit is usually located in the cabin or cabin air supply duct. As the temperature of the air supply changes, the resistance value of the pickup unit also changes, thus causing the voltage drop across the pickup to change. The cabin temperature pickup is a thermistor type unit (figure 14-35). As the ambient temperature of the resistance bulb increases, the resistance of the bulb decreases.
Connector - Resistance - element FICURE 14-35. Thermistor.
Cabin Air Temperature Selector The air temperature selector (see figure 14-34) is a rheostat located in the cabin. It permits selective temperature control by varying the effective temperature control point of the cabin air temperature pickup unit. The rheostat causes the cabin temperature pickup unit to demand a specific temperature of the supply air.
Cabin Air Temperature Control Regulator The cabin air temperature control regulator, in conjunction with the air temperature selector rheostat and the air duct temperature pickup unit, automatically maintains the temperature of the air entering the cabin at a preselected value. The temperature regulator is an electronic device with a temperature regulating range. In some installations, this range may extend from as low as 32° F, to as high as 117° F.
The output of the regulator controls the position of the butterfly in the mixing valve, thus controlling the temperature of the inlet air to the cabin.
Typical System Operation Figure 14-36 shows an electrical schematic of a typical air temperature control system. In most air temperature control systems, there is one switch to select the mode of temperature control. Usually, this switch will have four positions: "off," "auto," "man. hot," and "man. cold." In the "off" position, the air temperature control system is inoperative.
With the switch in the "auto" range, the air temperature control system is in the automatic mode. With the switch in either the "man. hot" or "man, cold" position, the air temperature control system is in the manual mode.
Cabin air temp control switch - d.c. bus X+ Off • • Man.
Heating hot element Auto Mang cold Cabin air B+ B -+ pickup unit Control (thermistor) thermistor : V3 field 3 Mixing valve No. 1 V1 motor Cabin Control Hot rheostat selector thermistor 1 No. 2 _ Bias field 3 Regulator T3
- X- FICURE 14-36. Air temperature control system (simplified).
ELECTRONIC TEMPERATURE CONTROL REGULA- TOR The cabin selector rheostat and the cabin air pickup unit (thermistor) determine the direction and amount of rotation of the mixing valve motor.
This function is controlled in the cabin air temperature regulator. The cabin selector rheostat and the cabin air pickup unit (see figure 14-36) are connected into a bridge circuit which also includes two thermistors that are located in the regulator.
The bridge circuit is energized by an a.c. source (T1). If the resistance of the cabin air pickup unit and the cabin selector rheostat were equal, then points A and B would have no potential difference.
Note that points A and B are the signal reference points for V1 (grid and cathode). If the cabin air temperature increases, the resistance value of the cabin air temperature pickup unit decreases, since the fiow of the air passes over the pickup unit. This decrease in resistance of the pickup unit causes the voltage developed across the pickup unit to decrease, resulting in a potential difference between points A and B.
This signal, which is impressed on the grid of Vl, goes through two stages of voltage amplification (Vl and V2). The amplified signal is applied to the grids of the two thyratron tubes (V3 and V4). The thyratron tubes (gas filled triode or tetrode) are used for signal phase detection. For example, if the signal on the grid of V3 is in phase with the signal on the plate, V3 will conduct, causing current to flow through the coil of relay K1 and close its contacts.
One set of contacts completes a circuit for direct current flow to the cold-field coil of the mixing valve motor. This directs more hot air into the refrigeration unit, thereby cooling the cabin air.
At the same time, the remaining set of contacts of K1 completes a source of a.c. power (T3) to the heating element of thermistor No. 1 of the bridge circuit, causing the resistance of thermistor No. I to decrease. (Remember that a thermistor's resistance decreases as the temperature rises.) The resultant change in the voltage drop across thermistor No. 1 results in a balanced bridge across points A and B.
This, in turn, causes relay K1 to become de-energized, stopping the rotation of the mixing valve motor.
At this point, heater voltage is removed from thermistor No. 1 and it cools, again unbalancing the bridge. This causes the mixing valve motor to drive farther towards the cool position, allowing still more refrigerated air to enter the cabin.
Cycling continues until the drops in voltage across the pickup unit and the selector rheostat are equal.
Had the cabin air temperature been colder than the selected setting, the bridge would have become 575
unbalanced in the opposite direction. This would have caused relay K2 on the regulator to become energized, thus energizing the hot-field coil of the mixing valve motor.
The bridge may also be unbalanced by another method, i.e., by re-positioning the cabin selector rheostat. Again the mixing valve moves to regulate the temperature of the air until the bridge is re-balanced.
VAPOR CYCLE SYSTEM (FREON) Vapor cycle cooling systems are used on several large transport aircraft. This system usually has a greater cooling capacity than an air cycle system, and in addition, can usually be used for cooling on the ground when the engines are not operating.
An aircraft freon system is basically similar in principle to the kitchen refrigerator or the home air conditioner. It uses similar components and operating principles and in most cases depends upon the electrical system for power.
Vapor cycle systems make use of the scientific fact that a liquid can be vaporized at any temperature by changing the pressure acting on it. Water, at sea level barometric pressure of 14.7 p.s.i.a. will boil if its temperature is raised to 212° F. The same water in a closed tank under a pressure of 90
p.s.i.a. will not boil at less than 320° F. If the pressure is reduced to 0.95 p.s.i.a. by a vacuum pump, the water will boil at 100° F. If the pressure is reduced further, the water will boil at a still lower temperature; for instance, at O.12 p.s.i.a., water will boil at 40° F. Water can be made to boil at any temperature if the pressure corresponding to the desired boiling temperature can be maintained.
Refrigeration Cycle The basic laws of thermodynamics state that heat will flow from a point of higher temperature to a point of lower temperature. If heat is to be made to flow in the opposite direction, some energy must be supplied. The method used to accomplish this in an air conditioner is based on the fact that when a gas is compressed, its temperature is raised, and, similarly, when a compressed gas is allowed to expand, its temperature is lowered.
To achieve the required "reverse" flow of heat, a gas is compressed to a pressure high enough so that its temperature is raised above that of the outside air. Heat will now fow from the higher temperature gas to the lower temperature surrounding air (heat sink), thus lowering the heat content of the gas.
The gas is now allowed to expand to a lower pressure; this causes a drop in temperature that makes it cooler than the air in the space to be cooled (heat source).
Heat will now flow from the heat source to the gas, which is then compressed again, beginning a new cycle. The mechanical energy required to cause this apparent reverse flow of heat is supplied by a compressor. A typical refrigeration cycle is illustrated in figure 14-37.
Gaseous refrigerant under low pressure Compressor under increasing pressure Gaseous refrigerant Evaporator Hea Cooled space Condenser coil
- Expansion valve Receiver Liquid refrigerant FICURE 14-37. Refrigeration cycle.
This refrigeration cycle is based on the principle that the boiling point of a liquid is raised when the pressure of the vapor around the liquid is raised.
The cycle operates as follows: A liquid refrigerant confined in the receiver at a high pressure is allowed to flow through the expansion valve into the evaporator. The pressure in the evaporator is low enough so that the boiling point of the liquid reftigerant is below the temperature of the air to be cooled, or heat source. Heat flows from the space to be cooled to the liquid refrigerant, causing it to boil (to be converted from liquid to a vapor). Cold vapor from the evaporator enters the compressor, where its pressure is raised, thereby raising the boiling point. The refrigerant at a high pressure and high temperature flows into the condenser.
Here heat flows from the refrigerant to the outside air, condensing the vapor into a liquid. The cycle is repeated to maintain the cooled space at the selected temperature.
Liquids that will boil at low temperatures are the most desirable for use as refrigerants. Comparatively large quantities of heat are absorbed when liquids are changed to a vapor. For this reason, liquid freon is used in most vapor cycle refrigeration units whether used in aircraft or in home air conditioners and refrigerators.
Freon is a fluid which boils at a temperature of approximately 39° F. under atmospheric pressure.
Similar to other fluids, the boiling point may be raised to approximately 150° F. under a pressure of 96 p.s.i.g. These pressures and temperatures are representative of one type of freon. Actual values will vary slightly with different types of freon. The type of freon selected for a particular aircraft will depend upon the design of the freon system components installed.
Freon, similar to other fluids, has the characteristic of absorbing heat when it changes from a liquid to a vapor. Conversely, the fluid releases heat when it changes from a vapor to a liquid. In the freon cooling system, the change from liquid to vapor (evaporation or boiling) takes place at a location where heat can be absorbed from the cabin air, and the change from vapor to liquid (condensation) takes place at a point where the released heat can be ejected to the outside of the aircraft. The pressure of the vapor is raised prior to the condensation process so that the condensation temperature is relatively high. Therefore, the freon, condensing at approximately 150° F., will lose heat to the outside air which may be as hot as 100° F.
The quantity of heat that each pound of refrigerant liquid absorbs while flowing through the evaporator is known as the "refrigeration effect." Each pound flowing through the evaporator is able to absorb only the heat needed to vaporize it, if no superheating (raising the temperature of a gas above that of the boiling point of its liquid state) takes place. If the liquid approaching the expansion valve were at exactly the temperature at which it was vaporizing in the evaporator, the quantity of heat that the refrigerant could absorb would be equal to its latent heat. That is the amount of heat required to change the state of a liquid, at the boiling point, to a gas at the same temperature.
When liquid refrigerant is admitted to the evaporator, it is completely vaporized before it reaches the outlet. Since the liquid is vaporized at a low temperature, the vapor is still cold after the liquid has completely evaporated. As the cold vapor flows through the balance of the evaporator, it continues to absorb heat and becomes superheated.
The vapor absorbs sensible heat (heat which causes a temperature change when added to, or removed from matter) in the evaporator as it becomes superheated. This, in effect, increases the refrigerating effect of each pound of refrigerant. This means that each pound of refrigerant absorbs not only the heat required to vaporize it, but also an additional amount of sensible heat which superheats it.
FREON SYSTEM COMPONENTS The major components of a typical freon system are the evaporator, compressor, condenser, and expansion valve (figure 14-38). Other minor items may include the condenser fan, receiver freon storage), dryer, surge valve, and temperature controls. These items are interconnected by appropriate tubing to form a closed loop in which the freon is circulated during operation.
Freon System Operational Cycle Compressor The principle of operation of the system can be explained by starting with the function of the compressor. The compressor increases the pressure of the freon when it is in vapor form. This high pressure raises the condensation temperature of the freon and produces the force necessary to circulate the freon through the system.
The compressor is driven either by an electric motor or by an air turbine drive mechanism. The compressor may be a centrifugal type or a piston type. The compressor is designed to act upon freon in a gaseous state and in conjunction with the expansion valve, maintains a difference in pressure between the evaporator and the condenser.
If the liquid refrigerant were to enter the compressor, improper operation would occur. This type of malfunction is called "slugging." Automatic controls and proper operating procedures must be used to prevent slugging.
Condenser The freon gas is pumped to the condenser for the next step in the cycle. At the condenser the gas passes through a heat exchanger where outside (ambient) air removes heat from the freon. When heat is removed from the high-pressure freon gas, a change of state takes place and the freon condenses to a liquid. It is this condensation process which releases the heat the freon picks up from 577
Condenser pressure switch Ram air out Electronic Ram air in equipment _ Ejector assembly Aircraft skin Ejector air Condenser shutoff valve cooling Ground Charge valve assembly Engines Sub Thermostatic connection assembly Header cooler expansion valve Filter/drier Air filter High pressure cutout switch Receiver Sight Oil glass Evaporator separator Compressor assembly Pressure Charge switch valve Low temperature Relief Oil filter cutout switch valve Evaporator fan Refrigerant vapor Lubricating oil and refrigerant vapor | Refrigerant liquid Lubricating oil
••o∞ Engine bleed air Refrigerant 3 Evaporator discharge airflow superheated vapor Flow direction Refrigerant liquid and vapor FIGURE 14-38. Vapor cycle system flow schematic.
the cabin air. The flow of ambient air through the condenser unit is ordinarily modulated by controllable inlet or outlet doors according to cooling requirements. A condenser cooling air fan or air ejector is often used to help force the ambient air through the condenser; this item is important for operation of the system on the ground.
Receiver From the condenser the liquid freon flows to the receiver which acts as a reservoir for the liquid refrigerant. The fluid level in the receiver varies with system demands. During peak cooling periods, there will be less liquid than when the load is light. The prime function of the receiver is to ensure that the thermostatic expansion valve is not starved for refrigerant under heavy cooling load conditions.
Subcooler Some vapor cycle systems use a subcooler to reduce the temperature of the liquid refrigerant after it leaves the receiver. By cooling the refrigerant premature vaporization (flash-off) can be prevented. Maximum cooling takes place when the refrigerant changes from a liquid to a gaseous state.
For efficient system operation this must occur in the evaporator. If the refrigerant vaporizes before it reaches the evaporator the cooling efficiency of the system is reduced.