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.
The subcooler is a heat exchanger containing passages for liquid freon from the receiver on its.
way to the evaporator and cold freon gas leaving the evaporator on its way to the compressor. The.
liquid on the way to the evaporator is relatively warm in comparison to the cold gas leaving the During refrigeration unit operation, a steady flow of freon refrigerant observed through the sight glass indicates that sufficient charge is present. If the unit requires additional refrigerant, bubbles evaporator. Although the gas leaving the evaporator has absorbed heat from the air being cir- ve for culated through the evaporator, its temperature is ming still in the vicinity of 40° F. This cool gas is fed rethrough the subcooler where it picks up additional freon heat from the relatively warm liquid freon that is the flowing from the receiver. This heat exchange ces subcools the liquid freon to a level that ensures the little or no flash-off (premature vaporization) on its way to the evaporator: evap- Subcooling is a term used to describe the cooling the of a liquid refrigerant at constant pressure to a pends point below the temperature at which it was con- into densed. At 117 p.s.i.g., freon vapor condenses at loads a temperature of 100° F. If, after the vapor has size been completely condensed, the liquid is cooled restill further to a temperature of 76° F., it will have vever, been subcooled 24°: Through subcooling, the reliquid delivered to the expansion valve is cool event enough to prevent most of the flash-off that would which normally result, thereby making the system more iency.
efficient. the evap- Filter/Drier The system illustrated in figure 14-38 has a satisfy presfilter/drier unit installed between the subcooler xpanand the sight glass. The filter/drier is essentially the a sheet-metal housing with inlet and outlet connec- the tions and contains alumina desiccant, a filter screen, moisture absorbent so that dry freon flows to the and a filter pad. The alumina dseiccant acts as a ed inlet in expansion valve. A conical screen and fiber glass the pad act as a filtering device, removing contami- tering nants. by valve is a must because of the critical clearances Scrupulously clean refrigerant at the expansion buib, orator involved. Moisture may freeze at the expansion ternal valve, causing it to hang up with a resulting starvation or flooding of the evaporator. is closed system filled bulb attached to the Sight Glass ithin the bulb corresponds To aid in determining whether servicing of the re leaving the evaporator.
refrigerating unit is required, a liquid line sight the diaphragm in the glass or liquid level gage is installed in the line le valve, an any increase between the filter/drier and the thermostatic ex- valve to move towards pansion valve. The sight glass consists of a htting bottom side of the diawith windows on both sides, permitting a view of the superheat spring and fluid passage through the line. In some systems ssure acting in direction the sight glass is constructed as a part of the filter/ The valve position at any drier. ese three forces.
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.