Air-Cycle Air Conditioning

Forward air outlet Overhead supply ducts Flexible duct panel AFT air outlet Main distribution manifold Mixing chambers Fan Control cabin distribution duct Main distribution manifold relief valve 250° F duct overheat switch 190° F duct overheat switch Duct temperature anticipator sensor Duct temperature limit sensor Mixing chamber Gasper fan Cold air supply Hot air supply Drain hose Hot air supply Ground service connection Gasper fan bypass duct To control cabin Muffler To passenger cabin FWD Main distribution manifold floor. Riser ducts run horizontally then vertically from the manifold to supply ducts, which follow the curvature of the fuselage carrying conditioned air to be released in the cabin.

16-35 or cold air from a ground-based source throughout the cabin using the aircraft’ s own air distribution system ducting. the aircraft is stationary on the ground. In normal flight operations, a pneumatic manifold is supplied by the engine bleed air through the use of valves, regulators, and ducting. The air conditioning packs are supplied by this manifold as are other critical airframe systems, such as the anti-ice and hydraulic pressurization system. Component Operation Pack Valve The pack valve is the valve that regulates bleed air from the pneumatic manifold into the air cycle air conditioning system. It is controlled with a switch from the air conditioning panel in the flight deck. Many pack valves are electrically controlled and pneumatically operated. Also known as the supply shutoff valve, the pack valve opens, closes, and modulates to allow the air cycle air conditioning system to be supplied with a designed volume of hot, pressurized air. [Figure 16-64] When an overheat or other abnormal condition requires that the air conditioning package be shut down, a signal is sent to the pack valve to close.

Bleed Air Bypass A means for bypassing some of the pneumatic air supplied to the air cycle air conditioning system around the system is present on all aircraft. This warm bypassed air must be mixed with the cold air produced by the air cycle system so the air delivered to the cabin is a comfortable temperature. In the system shown in Figure 16-58, this is accomplished by the mixing valve. It simultaneously controls the flow of bypassed air and air to be cooled to meet the requirements of the auto temperature controller. It can also be controlled manually with the cabin temperature selector in manual mode. Other air cycle systems may refer to the valve that controls the air bypassed around the air cycle cooling system as a temperature control valve, trim air pressure regulating valve, or something similar.

Primary Heat Exchanger Generally, the warm air dedicated to pass through the air cycle system first passes through a primary heat exchanger. It acts similarly to the radiator in an automobile. A controlled flow of ram air is ducted over and through the exchanger, which reduces the temperature of the air inside the system. [Figure 16-65] A fan draws air through the ram air duct when the aircraft is on the ground so that the heat exchange is possible when the aircraft is stationary. In flight, ram air doors are modulated to increase or decrease ram air flow to the exchanger according to the position of the wing flaps.

During slow flight, when the flaps are extended, the doors are open. At higher speeds, with the flaps retracted, the doors move toward the closed position reducing the amount of ram air to the exchanger. Similar operation is accomplished with a valve on smaller aircraft. [Figure 16-66] Refrigeration bypass valve sensor Refrigeration bypass 350° valve Primary heat exchanger Water separator drain Water separator ACM turbine expansion outlet Turbine Compressor Mixing valve Bypass bleed air Air cycle machine Primary heat exchanger Secondary heat exchanger Refrigeation byp ass air FWD looking up into the air conditioning bay located in the lower fuselage on each side of the aircraft.

16-36 SW 365 °F Secondary heat exchanger Air mix valve Water separator anti-ice valve Override Pack valve Turbofan valve Primary heat exchanger PASS CABIN Air mix valve Hot Cold Auto normal Manual WarmCool R Pack Ice Auto Off Wind Manual Condition: on ground pack on Auto On 2 Off Auto High Pack Trip off Wing body overheat Bleed trip off SE SW Ram air controller Auto temp controller 230 °F 35 °F Mixing chamber 210 °F Ram air Water separator Air cycle machine To mix manifold To wing TAI Pneumatic duct (Manual) Engine 2 Isolation valve Passenger cabin Ram air door actuator Flap position Power A Air mix valve Hot air Heat exchanger—cooler air Cold air Conditioned air Ram air 16-37 Manual override Valve actuator Cabin pressure sensor To A/C pack APU supply solenoid to 13th stage valve reg Shutoff solenoidFrom aircraft pneumatic manifold from the flight deck and automatically responds to supply and air cycle system parameter inputs.

Refrigeration Turbine Unit or Air Cycle Machine & Secondary Heat Exchanger The heart of the air cycle air conditioning system is the refrigeration turbine unit, also known as the air cycle machine (ACM). It is comprised of a compressor that is driven by a turbine on a common shaft. System air flows from the primary heat exchanger into the compressor side of the ACM. As the air is compressed, its temperature rises. It is then sent to a secondary heat exchanger, similar to the primary heat exchanger located in the ram air duct. The elevated temperature of the ACM compressed air facilitates an easy exchange of heat energy to the ram air. The cooled system air, still under pressure from the continuous system air flow and the ACM compressor, exits the secondary heat exchanger. It is directed into the turbine side of the ACM. The steep blade pitch angle of the ACM turbine extracts more energy from the air as it passes through and drives the turbine. Once through, the air is allowed to expand at the ACM outlet, cooling even further. The combined energy loss from the air first driving cycle air conditioning system are of similar construction. They both Figure 16-66. A ram air door controls the flow of air through the cool bleed air when ram air passes over the exchanger coils and fins. primary and secondary heat exchangers.

16-38 the turbine and then expanding at the turbine outlet lowers the system air temperature to near freezing. [Figure 16-67] Water Separator The cool air from the air cycle machine can no longer hold the quantity of water it could when it was warm. A water separator is used to remove the water from the saturated air before it is sent to the aircraft cabin. The separator operates with no moving parts. Foggy air from the ACM enters and is forced through a fiberglass sock that condenses and coalesces the mist into larger water drops. The convoluted interior structure of the separator swirls the air and water. The water collects on the sides of the separator and drains down and out of the unit, while the dry air passes through. A bypass valve is incorporated in case of a blockage. [Figure 16-68] Refrigeration Bypass Valve As mentioned, air exiting the ACM turbine expands and cools.

It becomes so cold, it could freeze the water in the water separator, thus inhibiting or blocking airflow. A temperature sensor in the separator controls a refrigeration bypass valve designed to keep the air flowing through the water separator above freezing temperature. The valve is also identified by other names such as a temperature control valve, 35° valve, anti-ice valve, and similar. It bypasses warm air around the ACM when opened. The air is introduced into the expansion ducting, just upstream of the water separator, where it heats the air just enough to keep it from freezing. Thus, the refrigeration bypass valve regulates the temperature of the ACM discharge air so it does not freeze when passing through the water separator. This valve is visible in Figure 16-62 and is diagrammed in the system in Figure 16-63.

All air cycle air conditioning systems use at least one ram air heat exchanger and an air cycle machine with expansion turbine to remove heat energy from the bleed air, but variations exist. An example of a system different from that described above is found on the McDonnell Douglas DC-10. Bleed air from the pneumatic manifold is compressed by the air cycle machine compressor before it flows to a single heat exchanger. Condensed water from the water separator is sprayed into the ram air at its entrance to the exchanger to draw additional heat Compressor scroll Turbine scroll Compressor impeller Compressor inlet Compressor outlet to secondary heat exchanger Sump drain Main housing assembly Slinger Turbine inlet from secondary heat exchanger Turbine outlet Turbine wheel Sump turbine are attached. Oil lubricates and cools the shaft bearings.

16-39 Coalescer bag Bag condition indicator Outlet shell Collector Inlet shell Bypass valve Coalescer supportNormal airflow Bypass airflow Drain orifice force sends the water to the walls of the collector where it drains from the unit. from the compressed bleed air as the water evaporates. A trim Cabin Temperature Control System air valve for each cabin zone mixes bypassed bleed air with Typical System Operation conditioned air in response to individual temperature selectors Most cabin temperature control systems operate in a similarfor each zone. When cooling air demands are low, a turbine manner. Temperature is monitored in the cabin, flight deck, bypass valve routes some heat exchanger air directly to the conditioned air ducts, and distribution air ducts. These conditioned air manifold. [Figure 16-69] values are input into a temperature controller, or temperature control regulator, normally located in the electronics bay. A 16-40 Flight Deck Galley FWD Compt MID Compt AFT Compt Turbine Compressor Water separator Anti-ice screen Turbine bypass valve Pack anti- ice valve Trim air check valve Trim air pressure regulating valve Trim air check valve Trim air pressure regulating valve Flow control valve Ground pneumatic connector Engine No. 1 bleed air Engine No. 3 bleed air Heat to FWD cargo compt and galley ventilation Conditioned air check valve Trim air valve Trim air valve Trim air valve Ground conditioned air connector Trim air valve Trim air valve Ground conditioned air connector Overboard Overboard Water injector Fan Ram air Turbine Compressor Anti-ice screen Turbine bypass valve Pack anti-ice valve

Pressure

relief Flow control valve Conditioned air check valve Potable water Fan Ram air

Heat

exchanger No. 1 Pack No. 3 PackNo. 2 Pack

Heat

exchanger Turbine Compressor Water separator Water separator Anti-ice screen Turbine bypass valve Pack anti-ice valve Isolation valves Flow control valve Ground pneumatic connector Heat to center cargo compt Engine No. 2 / APU bleed air Heat to AFT bulk cargo compt and AFT LAV ventilation Conditioned air check valve Overboard Fan Ram air

Heat

exchanger 16-41 temperature selector in the flight deck can be adjusted to input the desired temperature. [Figure 16-70] The temperature controller compares the actual temperature signals received from the various sensors with the desired temperature input. Circuit logic for the selected mode processes these input signals. An output signal is sent to a valve in the air cycle air conditioning system. This valve has different names depending on the aircraft manufacturer and design of the environmental control systems (i.e., mixing valve, temperature control valve, trim air valve). It mixes warm bleed air that bypassed the air cycle cooling process with the cold air produced by it.

By modulating the valve in response to the signal from the temperature controller, air of the selected temperature is sent to the cabin through the air distribution system. Cabin temperature pickup units and duct temperature sensors used in the temperature control system are thermistors. Their resistance changes as temperature changes. The temperature selector is a rheostat that varies its resistance as the knob is turned. In the temperature controller, resistances are compared in a bridge circuit. The bridge output feeds a temperature regulating function. An electric signal output is prepared and sent to the valve that mixes hot and cold air. On large aircraft with separate temperature zones, trim air modulating valves for each zone are used. The valves modulate to provide the correct mix required to match the selected temperature. Cabin, flight deck, and duct temperature sensors are strategically located to provide useful information to control cabin temperature. [Figure 16-71] Vapor Cycle Air Conditioning The absence of a bleed air source on reciprocating engine aircraft makes the use of an air cycle system impractical for conditioning cabin air. Vapor cycle air conditioning is used on most nonturbine aircraft that are equipped with air conditioning. However, it is not a source of pressurizing air as the air cycle system conditioned air is on turbine powered aircraft. The vapor cycle system only cools the cabin. If an aircraft equipped with a vapor cycle air conditioning system is pressurized, it uses one of the sources discussed in the pressurization section above. Vapor cycle air conditioning is a closed system used solely for the transfer of heat from inside the cabin to outside of the cabin. It can operate on the ground and in flight.

Theory of Refrigeration Energy can be neither created nor destroyed; however, it can be transformed and moved. This is what occurs during vapor cycle air conditioning. Heat energy is moved from the cabin air into a liquid refrigerant. Due to the additional energy, the liquid changes into a vapor. The vapor is compressed and becomes very hot. It is removed from the cabin where the very hot vapor refrigerant transfers its heat energy to the outside air. In doing so, the refrigerant cools and condenses back into a liquid. The refrigerant returns to the cabin to repeat the cycle of energy transfer. [Figure 16-72] Heat is an expression of energy, typically measured by temperature. The higher the temperature of a substance, the more energy it contains. Heat always flows from hot to cold.

These terms express the relative amount of energy present in two substances. They do not measure the absolute amount of heat present. Without a difference in energy levels, there is no transfer of energy (heat). Adding heat to a substance does not always raise its temperature. When a substance changes state, such as when a liquid changes into a vapor, heat energy is absorbed. This is called latent heat. When a vapor condenses into a liquid, this heat energy is given off. The temperature of a substance remains constant during its change of state. All energy absorbed or given off, the latent heat, is used for the change NORMAL MAN FLIGHT DECK STANDBY NORMAL MAN CABIN STANDBY COLD HOT TEMP COLD HOT TEMP TEMPERATURE CONTROL 0 20 40 −40 −20 CABIN TEMP 0° C jet (right). On large aircraft, temperature selectors may be located on control panels located in a particular cabin air distribution zone.

16-42 16-43 AUTO OVRD EQUP COOLING ON RECIRC UPPER AUTO OFF L PACK AUTO OFF R PACK ON FANS LOWER ON L TRIM AIR R ON FAULT FAULT MAN TEMP AUTO FLT DECK WC TEMP AIR COND RESET AIR CONDITIONING CABIN WC LEFT CABIN TEMPERATURE CONTROLLER RIGHT CABIN TEMPERATURE CONTROLLER AIMS ASCPC F/D A B C D E LEFT AC PACK RIGHT AC PACK ARINC 629 Flight deck zone Passenger cabin zone Air conditioning panel Zone air temperature sensors Zone duct temperature sensors AFT upper recirculation fan Trim air modulating valves (6) Forward upper recirculation fan Trim air pressure regulating and shutoff valve Ozone converter Pneumatic system Ozone converter Flow control and shutoff valves Flow control and shutoff valves Lower recirculation fans Mix manifold Trim air pressure regulating and shutoff valves modulating valves for each zone. Redundant digital left and right cabin temperature controllers process temperature input signals from the sensors and temperature selectors on the flight deck panel and throughout the aircraft to modulate the valves.

process. Once the change of state is complete, heat added to a substance raises the temperature of the substance. After a substance changes state into a vapor, the rise in temperature of the vapor caused by the addition of still more heat is called superheat. The temperature at which a substance changes from a liquid into a vapor when heat is added is known as its boiling point. This is the same temperature at which a vapor condenses into a liquid when heat is removed. The boiling point of any substance varies directly with pressure. When pressure on a liquid is increased, its boiling point increases, and when pressure on a liquid is decreased, its boiling point also decreases. For example, water boils at 212 °F at normal atmospheric pressure (14.7 psi). When pressure on liquid water is increased to 20 psi, it does not boil at 212 °F. More energy is required to overcome the increase in pressure. It boils at approximately 226.4 °F. The converse is also true.

Water can also boil at a much lower temperature simply by reducing the pressure upon it. With only 10 psi of pressure upon liquid water, it boils at 194 °F. [Figure 16-73] Vapor pressure is the pressure of the vapor that exists above a liquid that is in an enclosed container at any given temperature. The vapor pressure developed by various substances is unique to each substance. A substance that is said to be volatile, develops high vapor pressure at standard day temperature (59 °F). This is because the boiling point of the substance is much lower. The boiling point of tetrafluoroethane (R134a), the refrigerant used in most aircraft vapor cycle air conditioning systems, is approximately –15 °F. Its vapor pressure at 59 °F is about 71 psi. The vapor pressure of any substance varies directly with temperature.

Liquid refrigerant absorbs cabin heat and changes into a vapor. Refrigerant is compressed and becomes hot. Heat from hot refrigerant vapor is given off to the outside air which is not as hot. The refrigerator changes into a liquid. Cabin air is cool after heat is given to evaporate the refrigerant. OUTSIDE AIR CABIN AIR Low temperature low pressure vapor High-temperature high-pressure vapor High-pressure liquid Fan the cabin to the outside air by a refrigerant which changes from a liquid to a vapor and back again. Basic Vapor Cycle Vapor cycle air conditioning is a closed system in which a refrigerant is circulated through tubing and a variety of components. The purpose is to remove heat from the aircraft cabin. While circulating, the refrigerant changes state. By manipulating the latent heat required to do so, hot air is replaced with cool air in the aircraft cabin.

To begin, R134a is filtered and stored under pressure in a reservoir known as a receiver dryer. The refrigerant is in liquid form. It flows from the receiver dryer through tubing to an expansion valve. Inside the valve, a restriction in the form of a small orifice blocks most of the refrigerant. Since 10 psi 14.7 psi 20 psi 194°F 212°F 226°F it is under pressure, some of the refrigerant is forced through the orifice. It emerges as a spray of tiny droplets in the tubing downstream of the valve. The tubing is coiled into a radiator- type assembly known as an evaporator. A fan is positioned to blow cabin air over the surface of the evaporator. As it does, the heat in the cabin air is absorbed by the refrigerant, which uses it to change state from a liquid to a vapor. So much heat is absorbed that the cabin air blown by the fan across the evaporator cools significantly. This is the vapor cycle conditioned air that lowers the temperature in the cabin.

The gaseous refrigerant exiting the evaporator is drawn into a compressor. There, the pressure and the temperature of the refrigerant are increased. The high-pressure high-temperature gaseous refrigerant flows through tubing to a condenser. The condenser is like a radiator comprised of a great length of tubing with fins attached to promote heat transfer. Outside air is directed over the condenser. The temperature of the refrigerant inside is higher than the ambient air temperature, so heat is transferred from the refrigerant to the outside air. The amount of heat given off is enough to cool the refrigerant and to condense it back to a high-pressure liquid. It flows through tubing and back into the receiver dryer, completing the vapor cycle.

There are two sides to the vapor cycle air conditioning system. One accepts heat and is known as the low side. The other gives up heat and is known as the high side. The low and high refer to the temperature and pressure of the refrigerant. As such, the compressor and the expansion valve are the two components that separate the low side from the high side of the cycle. [Figure 16-74] Refrigerant on the low side is characterized as having low pressure and temperature. Refrigerant on the high side has high pressure and temperature. Vapor Cycle Air Conditioning System Components By examining each component in the vapor cycle air conditioning system, greater insight into its function can be gained.

Refrigerant For many years, dichlorodifluoromethane (R12) was the standard refrigerant used in aircraft vapor cycle air conditioning systems. Some of these systems remain in use today. R12 was found to have a negative effect on the environment; in particular, it degraded the earth’s protective ozone layer. In most cases, it has been replaced by tetrafluoroethane (R134a), which is safer for the environment. R12 and R134a should not be mixed, nor should one be used in a system designed for the other. Possible damage to soft components, such as hoses and seals, could result causing leaks and or malfunction. Use only the specified refrigerant when servicing vapor cycle air conditioning systems.

16-44 Cool air to cabinWarm air from cabin Low-pressure liquid Blower High side Low side Evaporator Receiver dryer High-pressure/high-temperature vapor Low-pressure/low-temperature vapor Thermal expansion valve High-pressure liquid Condenser Compressor Return air to outside of aircraft Ambient ram air from outside of aircraft the low side from the high side of the cycle. This figure illustrates this division. Refrigerant on the low side is characterized as having low pressure and temperature. Refrigerant on the high side has high pressure and temperature. [Figure 16-75] R12 and R134a behave so similarly that the descriptions of the R134a vapor cycle air conditioning system and components in the following paragraphs also apply to an R12 system and its components.

R134a is a halogen compound (CF3CFH2). As mentioned, it has a boiling point of approximately –15 °F. It is not poisonous to inhale in small quantities, but it does displace oxygen. Suffocation is possible if breathed in mass quantity. Regardless of manufacturer, refrigerants are sometimes called Freon®, which is a trade name owned by the Dupont Company. Caution should be used when handling any refrigerant. Because of the low boiling points, liquid refrigerants boil violently at typical atmospheric temperatures and pressure. They rapidly absorb heat energy from all surrounding matter. If a drop lands on skin, it freezes, resulting in a burn. Similar tissue damage can result if a drop gets in one’s eye. Gloves and other skin protection, as well as safety goggles, are required when working with refrigerant.

Receiver Dryer The receiver dryer acts as the reservoir of the vapor cycle system. It is located downstream of the condenser and upstream of the expansion valve. When it is very hot, more refrigerant is used by the system than when temperatures are moderate. Extra refrigerant is stored in the receiver dryer for this purpose. Liquid refrigerant from the condenser flows into the receiver dryer. Inside, it passes through filters and a desiccant material. The filters remove any foreign particles that might be in the system. The desiccant captures any water in the refrigerant. Water in the refrigerant causes two major problems. First, the refrigerant and water combine to form an acid. If left in contact with the inside of the components 16-45