Aviation Australia CASA B1-11d Aeroplane Systems - Pneumatics PDF

MODULE 11A Category B1 Licence CASA B1-11d Aeroplane Systems - Pneumatics  Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold or other...

MODULE 11A Category B1 Licence CASA B1-11d Aeroplane Systems - Pneumatics  Copyright © 2020 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold or otherwise disposed of, without the written permission of Aviation Australia. CONTROLLED DOCUMENT 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 2 of 222 CASA Part Part 66 - Training Materials Only  Knowledge Levels Category A, B1, B2 and C Aircraft Maintenance Licence Basic knowledge for categories A, B1 and B2 are indicated by the allocation of knowledge levels indicators (1, 2 or 3) against each applicable subject. Category C applicants must meet either the category B1 or the category B2 basic knowledge levels. The knowledge level indicators are defined as follows: LEVEL 1 Objectives: The applicant should be familiar with the basic elements of the subject. The applicant should be able to give a simple description of the whole subject, using common words and examples. The applicant should be able to use typical terms. LEVEL 2 A general knowledge of the theoretical and practical aspects of the subject. An ability to apply that knowledge. Objectives: The applicant should be able to understand the theoretical fundamentals of the subject. The applicant should be able to give a general description of the subject using, as appropriate, typical examples. The applicant should be able to use mathematical formulae in conjunction with physical laws describing the subject. The applicant should be able to read and understand sketches, drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using detailed procedures. LEVEL 3 A detailed knowledge of the theoretical and practical aspects of the subject. A capacity to combine and apply the separate elements of knowledge in a logical and comprehensive manner. Objectives: The applicant should know the theory of the subject and interrelationships with other subjects. The applicant should be able to give a detailed description of the subject using theoretical fundamentals and specific examples. The applicant should understand and be able to use mathematical formulae related to the subject. The applicant should be able to read, understand and prepare sketches, simple drawings and schematics describing the subject. The applicant should be able to apply his knowledge in a practical manner using manufacturer's instructions. The applicant should be able to interpret results from various sources and measurements and apply corrective action where appropriate. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 3 of 222 CASA Part Part 66 - Training Materials Only  Table of Contents Air Supply (11.4.1) 9 Learning Objectives 9 Aeroplane Air Supply 10 Pneumatic System 10 Air Supply Sources 10 Engine Bleed Air 11 Auxiliary Power Unit 13 Emergency Ram Air 15 Electrically Driven Compressors 16 Smaller Turbo-Prop Aircraft 18 Jet Pumps 19 Ground Supply Cart 20 Air Conditioning (11.4.2) 22 Learning Objectives 22 Air Cycle System 23 Air Conditioning Systems 23 Air-Cycle System – Location 23 Air-Cycle System – Operation 25 Air-Cycle System – Temperature Control 27 Air Cycle System Components 29 Flow Control Shut-Off Valve 29 Primary and Secondary Heat Exchangers 30 Primary Heat Exchanger 31 Secondary Heat Exchanger 32 Air Cycle Machine Turbine Unit 33 Air-Cycle Machine Construction 36 Low-Limit Anti-Ice Valve 38 Cabin Temperature Control Valves 40 Air-Cycle Machine Temperature Control Valve 40 Air-Mixing Valve 42 Water Separator 45 High-Pressure Water Extractor 48 Mixing Chamber 49 Ram Air Door 50 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 4 of 222 CASA Part Part 66 - Training Materials Only  Vapour Cycle Machine 55 Vapour Cycle 55 Vapour Cycle Machine Compressor 57 Vapour Cycle Machine Condenser 59 Vapour Cycle Machine Receiver Dryer 60 Vapour Cycle Machine Thermal Expansion Valves 61 Vapour Cycle Machine Evaporator 64 Vapour Cycle Machine Refrigerant 65 Air Distribution Systems 67 Cabin Air Supply Sources 67 Cabin Air Distribution 68 Cabin Air Filters 71 Cabin Airflow Control 72 Cabin Air Recirculation System 73 Gasper Air 75 Temperature and Humidity Control System 77 Temperature Control System Operation 77 Temperature Control System Sensors 78 Cabin Temperature Controller 79 Cabin Heating Systems 84 Electric Heating Systems 84 Exhaust Shroud Heaters 85 Combustion Heaters 86 Humidity Control 88 Pressurisation (11.4.3) 90 Learning Objectives 90 Pressurisation Systems 91 Aeroplane Pressurisation 91 Pressurisation Terms 92 Pressurisation Design Considerations 97 Pressurisation Modes of Operation 98 Cabin Pressurisation Control 100 Aeroplane Pressurisation Control 100 Pressurisation Air Sources 100 Pressurisation Controller Indications 101 Cabin Altimeters 102 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 5 of 222 CASA Part Part 66 - Training Materials Only  Rate-Of-Change (Vertical Speed) Indicators 103 Differential Pressure Indicator 104 Cabin Rate of Change Control 105 Pressurisation Digital Indication 106 Cabin Pressure Controller Modes 107 Pneumatic Cabin Pressurisation System 109 Pneumatic Pressure Controller 109 Pneumatic System Operation 109 Pressurisation Safety Valve 113 Safety Valve Operation 113 Pressurisation System Maintenance 118 Pneumatic System Fault Finding 120 Electronic Cabin Pressurisation System 122 Electronic Cabin Pressurisation System Operation 122 Electronic Pressure Controllers 122 Electronic Control Operation 123 Negative Pressure Relief Valve 125 Pressurisation Indication and Warning Systems 127 Pressurisation Indication and Warning Systems Operation 127 Electronic Pressurisation System Fault Finding 128 Video Resources 132 Pressurisation Videos 132 Safety and Warning Devices (11.4.4) 133 Learning Objectives 133 Safety and Warning Systems 134 Introduction to Warning and Safety Systems 134 Bleed Air Supply and Air Conditioning System Protection 134 Temperature Controller 134 Multi-Zone Climate Controlled Indication 135 Cabin Pressurisation Safety and Warning Systems 136 Pressurisation System Protection 137 Cabin Pressurisation Controls 138 Emergency Air Supply During Unpressurised Flight 138 Ditching Protection 139 Fire Protection (11.8) 141 Learning Objectives 141 Aircraft Fire Protection 142 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 6 of 222 CASA Part Part 66 - Training Materials Only  Introduction to Aircraft Fire Protection 142 Fire and Smoke Detection and Warning Systems 143 Fire/Overheat Detection Systems 143 Spot-Type Fire/Overheat Detection Systems 144 Continuous-Loop (Fire Wire) Fire/Overheat System 147 Overheat Detection System Operation 155 Smoke Detection Systems 156 Fire Extinguishing Systems 166 Fire Extinguishing Systems 166 Engine Fixed Fire Extinguishers 167 Lavatory Fire Extinguishing 176 APU Fire Extinguishing System 177 Fire Extinguishing System - Precautions 179 Portable Fire Extinguishers 181 Portable Fire Extinguishers – Cabin and Cockpit Area 181 Classes of Fire 183 Pneumatic and Vacuum (11.16) 185 Learning Objectives 185 System Layout and Sources 186 Aircraft Pressure and Vacuum Systems 186 Venturi System Vacuum Supply 186 Vacuum Systems and Pumps 187 Low-Pressure Systems 189 Vane-Type Air Pump 190 Combined Vacuum and Low-Pressure Systems 191 Medium-Pressure Systems 192 High-Pressure Systems 193 Sources 195 Introduction 195 Engine Bleed Air Supply 195 APU Supply 196 Ground Supply (External Supply) 198 Compressors 199 Pressure Control 206 Vacuum and Low-Pressure Control 206 Engine Bleed Air Pressure Control 210 APU Bleed Air Pressure Control 211 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 7 of 222 CASA Part Part 66 - Training Materials Only  Distribution 214 Bleed Air Distribution 214 Ducts 214 High-Pressure Distribution System 215 Emergency Backup Distribution Systems 216 Indications and Warnings 218 Sensors and Indicators – Example 218 Typical Pneumatic Bleed Air Control Panel 218 Duct Pressure Indication 219 Overheat Detectors 220 Interface With Other Systems 222 Introduction to Interface with Other Systems 222 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 8 of 222 CASA Part Part 66 - Training Materials Only  Air Supply (11.4.1) Learning Objectives 11.4.1 Describe the purpose and operation of sources of air supply including engine bleed, APU, and ground cart (Level 2). 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 9 of 222 CASA Part Part 66 - Training Materials Only  Aeroplane Air Supply Pneumatic System The pneumatic system supplies compressed air (low to medium pressure – high flow) to many aircraft user systems. The following aircraft systems use pneumatic power: Engine start systems Air conditioning and pressurisation systems Nitrogen generation systems Engine inlet cowl and wing anti-ice systems Water tank pressurisation system Hydraulic reservoir pressurisation system. Aircraft system users 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 10 of 222 CASA Part Part 66 - Training Materials Only  Air Supply Sources Engine bleed air Auxiliary power unit (APU) Electric cabin air compressors Centrifugal cabin compressor Turbo charger Jet pump Ground supply cart. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 11 of 222 CASA Part Part 66 - Training Materials Only  Engine Bleed Air With aircraft that use turbine engines, air is taken from the engine compressor to be used for air conditioning and cabin pressurisation. It is bled off the compressor in one or two places at the Low- Pressure (LP) and High-Pressure (HP) stage. Low-Pressure (LP) bleed air is used at medium to high engine power settings (i.e., during take-off, climb and cruise conditions). High-Pressure (HP) bleed air is used at low engine rpm during descent when the low-pressure air supply is inadequate. The switch to HP air at low engine rpm ensures adequate pressure and flow to airframe systems during low-power engine operation. The changeover is automatic, and both bleed ports are never open at the same time. The air is supplied from the low-pressure bleed when the High-Pressure (HP) bleed valve is closed. When the HP bleed valve opens, a check valve downstream of the LP port closes automatically to prevent HP air from flowing back into the LP stages. A Pressure Regulating and Shut Off Valve (PRSOV) is installed downstream of the LP and HP bleeds. This valve can also act as a shutoff valve to prevent overpressure or overheat in the system. The valve is controlled by a switch in the cockpit. Jet transport aircraft also require large quantities of pneumatic air for engine starting. The APU compressor can supply the air to start the first engine. The APU itself is started electrically from the battery. Engine bleed air 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 12 of 222 CASA Part Part 66 - Training Materials Only  Engine bleed air system 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 13 of 222 CASA Part Part 66 - Training Materials Only  Auxiliary Power Unit Turbine-powered transport aircraft require large amounts of power for starting and operation. Engine-starting and ground air conditioning requires a medium-pressure, high-volume pneumatic air source that is not available at remote airports. To meet these demands for ground power when the engines are not running, most large turbine aircraft are equipped with APUs. A typical APU consists of a small turbine powerplant driving an electric generator. It is commonly used to supply bleed air to all the pneumatic systems. As with any other gas turbine engine, bleed air loads generally place the greatest demand on an APU. Most APUs are designed for bleed air extraction on the ground only. If an APU is designed for bleed air extraction in flight, then the bleed air extraction is normally limited up to a specific flight altitude only. As an example, the Honeywell 131-9B can only supply pneumatic power alone up to 17 000 feet Auxiliary power unit (APU) The APU is located in the tail section of most aircraft and provides air supply through the bleed air duct. The APU bleed air duct runs under the passenger floor from the the tail section to the wheel well and then to the air conditioning bay where it joins the crossover duct on the pneumatic manifold. A check valve prevents reverse flow from engine bleed air into the APU. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 14 of 222 CASA Part Part 66 - Training Materials Only  APU bleed air supply system 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 15 of 222 CASA Part Part 66 - Training Materials Only  Emergency Ram Air In case of failure of both packs, an emergency ram air inlet flap can be opened for A/C ventilation or smoke removal. In case of smoke removal or loss of both packs, the ram air selector switch must be set to on. When set to on, and if ditching is not selected, the emergency ram air inlet flap opens. The flap installed between the low-pressure ground connection and ram air inlet closes one side of the duct when air is supplied from the other side. The check valve stays closed. Low pressure ground and emergency ram air connection Pneumatic ground cart 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 16 of 222 CASA Part Part 66 - Training Materials Only  Electrically Driven Compressors Recent improvements in electrical technology enable the latest generation of jet transport (e.g., Boeing 787) to save fuel by minimising the quantity of air bled from the engines. Bleed air is only used for engine cowl ice protection and pressurisation of hydraulic reservoirs. Improved efficiency is obtained by feeding ram air to electric motor driven cabin air compressors. The advantages of this technique lie in improved fuel consumption (engines are not required to provide large quantities of bleed air) because it avoids excessive energy extraction from engines with the associated energy waste by pre-coolers and modulating valves. There is no need to regulate down the supplied compressed air. Instead, the compressed air is produced by adjustable speed motor cabin air compressors at the required pressure without significant energy waste. Moreover, the no-bleed architecture allows significant simplification in engine build-up due to the elimination of the pneumatic system and associated pre-coolers, control valves and required pneumatic ducting. The fewer installed parts reduce maintenance and aircraft weight significantly. Example of an air conditioning system 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 17 of 222 CASA Part Part 66 - Training Materials Only  Cabin air compressors diagram 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 18 of 222 CASA Part Part 66 - Training Materials Only  Smaller Turbo-Prop Aircraft Aircraft with modern bypass and fan engines and larger turboprops use air bled directly from the engine compressor. Smaller turboprop aircraft use a combination of engine bleed air and turbo compressors or jet pumps. Turbo Compressors Engine compressor bleed air may be used directly to pressurise aircraft or alternatively to drive a turbo compressor. By driving a turbo-compressor, a smaller amount of high-pressure air is drawn from the engine compressor stage (bleed air). The turbo compressor then provides the compressed air for cabin pressurisation. This means that not as much engine power is removed from the engine compressor. In the turbo-compressor system, bleed air from the engine drives a turbine, which directly drives a compressor. The hot compressed bleed air driving the turbine cools dramatically (due to the work performed and the de-compression of the bleed air), so the air at the output of the turbine is actually quite cold. The compressor of the turbo-compressor takes in outside air and compresses it. The act of compressing the air heats it significantly. The heated, compressed outside air is blended with the cooled, decompressed bleed air to achieve the correct temperature and pressure before it enters the cabin or environmental system. Aviation Australia Turbo compressor system 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 19 of 222 CASA Part Part 66 - Training Materials Only  Jet Pumps Some aircraft use a jet pump flow multiplier to increase the amount of air taken into the cabin. The jet pump is essentially a special venturi mounted inside a duct connected to ram air. A nozzle blows a stream of high-velocity compressor bleed air into the throat of the venturi, which induces a low pressure that draws in a much larger mass of ram air. This is mixed with the compressor bleed air and carried into the aircraft cabin at the correct temperature and pressure before it enters the cabin or environmental system. Jet pump system 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 20 of 222 CASA Part Part 66 - Training Materials Only  Ground Supply Cart High-Pressure Ground Connection An aircraft without a serviceable APU will require a source of pneumatics for ground operations. A ground cart may supply medium-pressure compressed air to the entire aircraft pneumatic system and connects directly to the pneumatic manifold. The medium-pressure supply is not normally designed to cope with the demands of the air conditioning system. Use is normally restricted to main engine- starting and short-duration maintenance checks. Ground pneumatic connection Low-Pressure Ground Connection A low-pressure ground connector is connected to the ram air ducting for a low-pressure ground air conditioning supply. The ground conditioned air connector lets an external source of conditioned air supply the airplane air conditioning ducting system. Low pressure ground connection 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 21 of 222 CASA Part Part 66 - Training Materials Only  Air Conditioning (11.4.2) Learning Objectives 11.4.2.1 Explain the purpose and describe common configurations of aeroplane air conditioning systems (Level 3). 11.4.2.2.1 Explain the characteristics and operation an aeroplane air cycle machine and its components (Level 3). 11.4.2.2.2 Explain the characteristics and operation of an aeroplane vapour cycle machines and its components (Level 3). 11.4.2.3 Explain the purpose and operation of aeroplane air-conditioning distribution systems and components (Level 3). 11.4.2.4 Explain the purpose and operation of aeroplane air-conditioning flow, temperature and humidity control systems (Level 3). 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 22 of 222 CASA Part Part 66 - Training Materials Only  Air Cycle System Air Conditioning Systems Aircraft fly in a wide variety of climate conditions. Flights might begin on the ramp at 35 °C and then climb to cruise at a temperature of up to 56 °C. Climate control systems then must be able to provide comfortable cabin temperatures, regardless of the outside air temperature. The quality of the air supply is also important: it must be free of contaminants, fumes, odours or other factors that might affect the health or comfort of the passengers or crew. The function of an air conditioning system is to maintain a comfortable air temperature within the aircraft cabin. The system will increase or decrease the temperature of the supply air as needed to obtain the desired value. Most systems are capable of producing an air temperature of 21° to 27 °C with normally anticipated outside air temperatures. This temperature-conditioned air is then distributed so that there is a minimum of stratification (hot and cold layers). The system, in addition, must provide for the control of humidity, it must prevent fogging of windows and it must maintain the temperature of wall panels and floors at a comfortable level. An air conditioning system is designed to supply ventilation and heated or cool air. In a typical system, the air temperature is measured and compared to the desired setting of the temperature controls. Then, if the temperature is not correct, heaters or coolers are set into operation to change the air temperature, and the air is mixed to create a uniform temperature in the cabin. There are basically two types of air conditioning systems: Air cycle machine (used mainly in larger passenger aircraft). Vapour cycle machines (used mainly in small and medium-sized aircraft). 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 23 of 222 CASA Part Part 66 - Training Materials Only  Air-Cycle System – Location Most commercial aircraft have two to three air cycle system or air conditioning packs installed in the wing root area, forward of the landing gear bay. The packs supply conditioned air to the cabin for air conditioning, ventilation and pressurisation. The primary component of each pack assembly is the air cycle machine. An air cycle cooling system consists of an expansion turbine (cooling turbine), air-to- air heat exchangers and various valves which control airflow through the system. Air conditioning pack air intake and exhaust locations (left and right systems) 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 24 of 222 CASA Part Part 66 - Training Materials Only  Air conditioning pack access door open 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 25 of 222 CASA Part Part 66 - Training Materials Only  Air-Cycle System – Operation The Flow Control Shut-Off Valve (FCSOV) gets hot bleed air from the pneumatic manifold. The FCSOV controls the flow of hot bleed air to the primary heat exchanger and the hot side of the air mixing valve. A hot air connection downstream of the FCSOV supplies hot bleed air to the turbine case. This prevents ice in the turbine case. When bleed air goes through the primary heat exchanger, ram air removes some of the heat. This partially cooled bleed air goes to the cold side of the air mix valve. The air loses some of its heat but almost none of its pressure. From the air mixing valve, partially cooled bleed air goes to the compressor section of the air cycle machine (ACM). The compressor section increases the pressure and temperature of the partially cooled air. The compressed air goes through the secondary heat exchanger where ram air removes some of the heat. This bleed air goes to the turbine section of the ACM. The turbine section uses rapid expansion to decrease the temperature of the bleed air. The cold bleed air then goes into the water separator. The water separator removes condensed moisture. The water spray nozzle sprays this water into the ram air duct. If the temperature of the cold bleed air in the water separator decreases towards freezing, the low limit system adds partially cooled bleed air to cold bleed air via the low limit valve, upstream of the water separator. This heats the air and prevents ice in the water separator. After the water separator, the cold bleed air goes to the mix chamber. The mix chamber adds hot bleed air from the hot side of the air mixing valve. Various cabin sensors give signals back to position the mixing valve, which will vary the mix of hot or cold air to regulate temperature throughout the cabin. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 26 of 222 CASA Part Part 66 - Training Materials Only  Air cycle system schematic diagram 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 27 of 222 CASA Part Part 66 - Training Materials Only  Air-Cycle System – Temperature Control The temperature control valve is the primary valve to control the discharge temperature of the air conditioning pack. The systems incorporate temperature control valves to mix hot or cold air to regulate temperature throughout the cabin. Temperature controllers are used to keep the desired setting by receiving signals from various sensors and giving signals back to open or close the temperature control valves. The system also incorporates the standby temperature control valve. This valve gives backup control for the discharge temperature of the air conditioning pack if the normal temperature control system fails. It also increases the temperature of pack discharge air to melt ice in the condenser. Air-cycle system (temperature control valve) 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 28 of 222 CASA Part Part 66 - Training Materials Only  Air Cycle System Components Flow Control Shut-Off Valve The Flow Control Shut-Off Valve (FCSOV), often called the pack valve, is used to control and adjust the flow of air into the air conditioning pack. It can either shut off the air flow or modulate the flow of air to provide that which is needed to operate the air conditioning package. The flow control shut-off valve is normally situated in the air conditioning compartment. The flow control shut-off valve requires electrical power and upstream pneumatic pressure to open; therefore, the valve is electrically controlled and pneumatically actuated. It also regulates downstream pressure and flow. Flow control shut-off valve (FCSOV) 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 29 of 222 CASA Part Part 66 - Training Materials Only  FCSOV Valve Operation Upstream bleed air pressure through a filter acts on the primary piston to open the valve. Downstream overpressure will act on the secondary piston to release control pressure and modulate the valve to the closed position. When the dump valve solenoid is energised, it opens to vent control pressure to ambient, and this ensures that the valve closes under action of the spring acting on the piston. When the dump valve solenoid is de-energised, it closes and pneumatic pressure acting on the piston will cause the valve to open. The pack valve’s failsafe is in the open position in the event of electrical power failure, maintaining air supply for pressurisation. If the downstream pressure is too high, the secondary piston will move and, via the lever mechanism, will open the bleed orifice. Opening this orifice will allow air in the primary piston to escape, and it will then, via its lever mechanism, move the shut-off valve towards the closed position. Restricting the airflow to the downstream line will lower the pressure, and the system will go about balancing itself to maintain a constant downstream pressure. Pack valve 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 30 of 222 CASA Part Part 66 - Training Materials Only  Primary and Secondary Heat Exchangers Primary and secondary heat exchangers are air-to-air plate fin cross-flow-type heat exchangers. Two isolated airstreams flow through thin-walled channels. The channel walls are made up of plates and fins that increase surface area. Primary and secondary heat exchangers 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 31 of 222 CASA Part Part 66 - Training Materials Only  Primary Heat Exchanger The primary heat exchanger is a radiator through which cold ram air passes to cool the hot bleed air from the engines. Air from the flow control shut-off valve flows through the primary heat exchanger. A crossflow of ram air removes heat before the air enters the ACM compressor inlet. As the cold ram air passes over the radiator’s fin-like tubes, bleed air passing through the tubes is cooled. The flow of ram air through the heat exchangers is controlled by moveable inlet and exit doors, which modulate in flight to provide the required cooling. On many aircraft, the heat exchangers are sized to provide most, if not all, of the necessary cooling in flight. The air supply from the primary heat exchanger is controlled to a constant temperature by the heat exchanger mixing valve. Bleed air is cooled to about 150 °C. On the ground, there is not enough air passing through the cooling doors, so fans called pack fans provide adequate air-flow to cool the heat exchangers. These fans may be driven by engine bleed air, mechanically by the air cycle machine drive shaft or by an electric motor. When the airplane is in flight, ram air pressure opens the fan bypass check valve and provides all the cooling. Primary heat exchanger 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 32 of 222 CASA Part Part 66 - Training Materials Only  Secondary Heat Exchanger The function of the secondary heat exchanger is to remove heat from bleed air that comes from the compressor section of the air cycle machine (ACM) and to partially cool the air for the cabin pressurisation and air conditioning to a temperature which makes possible the efficient operation of the air conditioning pack. As cooling requirements increase, air exiting the primary heat exchanger is routed to the compressor side of the ACM. The compressor raises both the pressure and temperature of the air passing through it. The warmer, high-pressure air is then directed to the secondary heat exchanger. 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 exhausted directly to the atmosphere. Air from the heat exchanger has been further cooled. Exiting air is about 50 °C. Secondary heat exchanger 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 33 of 222 CASA Part Part 66 - Training Materials Only  Air Cycle Machine Turbine Unit The heart of the air-cycle air conditioning system is the refrigeration turbine unit, also known as the ACM. The air-cycle machine is a high-speed rotating assembly and consists of a centrifugal compressor driven by an expansion turbine on a common shaft. Air is passed from the primary heat exchanger to the compressor, where it is reheated. It then flows through to the secondary heat exchanger for further cooling. After this, air flows through the turbine, where it is cooled significantly. From here the cool air passes through to the water separator and then onto the mixing chamber before entering the cabin. View Interactive Flash Animation: Air cycle machine turbine assembly 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 34 of 222 CASA Part Part 66 - Training Materials Only  Air-Cycle Machine Turbine Operation Air-cycle machine turbines run at very high speeds (20 000–50 000 rpm), and their shafts are normally supported by air bearings or oil lubricated roller bearings (using an independent oil system). The Air-Cycle Machine (ACM) consists of a centrifugal air compressor and an expansion turbine that drives the compressor. When the heated bleed air passes through the primary heat exchanger, it loses some of its heat but almost none of its pressure. This air then enters the compressor of the air-cycle machine, and its pressure is further increased. With the increase in pressure, there is some increase in its temperature, but this is removed by the secondary heat exchanger. Now the somewhat-cooled high-pressure air flows into the expansion turbine, where a large percentage of its remaining energy is used to drive the compressor. As this air expands across the turbine, there is a large decrease in pressure. The decrease in pressure, coupled with the energy extracted to drive the compressor, results in a very large decrease in temperature. Due to this, the turbine outlet temperature can theoretically decrease to below the freezing point. There are two forms of cooling used in this system. Some is done by transferring heat to the ram air, but most of the heat is removed by expansion and converting it into work to drive the compressor. Air cycle machine 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 35 of 222 CASA Part Part 66 - Training Materials Only  Cutaway diagram of an air cycle machine 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 36 of 222 CASA Part Part 66 - Training Materials Only  Air-Cycle Machine Construction Oil-Lubricated ACM 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 bearings by wicks. A dipstick for checking the oil level is attached to the filler cap. 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 lubrication. Air/oil seals are provided between each slinger and the adjacent wheel. Oil lubricated air cycle machine 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 37 of 222 CASA Part Part 66 - Training Materials Only  Air-Bearing Air Cycle Machine Air-bearing ACMs are oil free, using foil air bearings to support the shaft. The air bearings let the ACM rotate at a high speed with little friction and are becoming more common. They eliminate oil contamination of the air conditioning system. It is not necessary to service air-cycle machines that have air bearings. Air-bearing air cycle machine Air cycle machine 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 38 of 222 CASA Part Part 66 - Training Materials Only  Low-Limit Anti-Ice Valve Some systems use a low-limit valve to keep the temperature of the air exiting the ACM from becoming too cold. Generally, this air is kept at about 2 °C (35 °F) by passing warm bleed air around the ACM and mixing it with the output air of the ACM. The purpose of this valve is to monitor and adjust the temperature in the water separator to prevent water-freezing and ice-formation at the outlet of the ACM, which would collect in and clog the water separator. ACM schematic diagram with low-limit anti-ice valve 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 39 of 222 CASA Part Part 66 - Training Materials Only  Low limit anti-ice valve Cabin Temperature Control Valves The cabin temperature control valves are the primary valves to control the discharge temperature of the air conditioning pack. The hot and cold air are mixed proportionally to satisfy cabin temperature requirements. The temperature control valve and the air mix valve are two valves used to control the pack outlet temperature. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 40 of 222 CASA Part Part 66 - Training Materials Only  Air-Cycle Machine Temperature Control Valve When cooling requirements are low, some or all of the hot bleed air from the engines can be bypassed around the ACM (the compressor and turbine) if warm air is needed in the cabin. This outlet air from the inlet into the air conditioning pack may be routed directly to the mix chamber. ACM schematic diagram with temperature control valve 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 41 of 222 CASA Part Part 66 - Training Materials Only  Temperature Control Valve Operation 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 uses upstream pressure to open the valve. When electrical power is applied, a coil is energised, closing a bleed port in the pressure chamber of the valve. The resulting pressure build-up forces a piston to rotate a butterfly valve in the cabin air duct to an open position. Repositioning is accomplished by action of the coil in varying the amount of pressure to bleed from the pressure chamber. Failure of the valve will cause the valve to move to the spring-loaded closed position (fail safe closed). Temperature control valve 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 42 of 222 CASA Part Part 66 - Training Materials Only  Air-Mixing Valve The air-mixing valve adjusts the proportions of hot and cold air that goes into the pack and the distribution system. Air-mixing valve installation 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 43 of 222 CASA Part Part 66 - Training Materials Only  Air-Mixing Valve Operation The valve controls pack output temperature by directing airflow through the cooling pack or around the cooling pack to the mix chamber. The valve is a dual housing assembly with two disc plates mounted on a common shaft 90° opposed. As one disc moves from open toward close, the other moves from closed toward open. The common disc shaft is driven by a 115-V AC motor. This is controlled by the cabin temperature controller or the manual control switch on the flight deck. Air-mixing valve Air-Mixing Valve Visual Position Indicator There is a visual position indicator on the valve. It shows the valve in the cold or the hot range. The valve also incorporates a position transmitter. It sends signals to the air-mixing valve position indicator on the cabin temperature control panel in the flight deck. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 44 of 222 CASA Part Part 66 - Training Materials Only  Air-mixing valve position indication 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 45 of 222 CASA Part Part 66 - Training Materials Only  Water Separator The large reduction in temperature causes the moisture in the air to condense. The water separator is used to separate, collect and remove the excess moisture from the air before it enters the cabin. In this way, the humidity can be controlled. In most packs, a water separator is installed in the discharge duct of the cooling turbine. Water separator Water Separator Operation Cold air leaving the air cycle machine turbine passes through to the water separator. Moisture in the air at this reduced temperature begins to condense. The condensate is so finely atomised, however, that it will follow along in the air stream unless a suitable method is used to collect it. The water separator is used to collect and remove the excess moisture from the air before it enters the distribution system. The water separator is a cylindrical chamber consisting of an inlet and outlet shell assembly which houses a polyester coalescer (bag), a conical-shaped metal coalescer support, a bypass valve assembly and a valve support guide. A coupling joins the inlet and outlet shell assemblies and secures the coalescer support. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 46 of 222 CASA Part Part 66 - Training Materials Only  The outlet shell assembly contains a collection chamber, a baffle, a water spray extractor boss and an overboard water drain. A boss is provided for the installation of the 1.7 °C (35 °F) sensor for anti-ice operation. A bag condition indicator is also fitted, which consists of a spring-loaded piston and disk enclosed in a housing and a colour coded cap. The coalescer bag and its support are conically shaped with the small diameter at the upstream end. The support fits inside the bag and has louvers shaped to impart a whirling motion to the air as it passes through. Air enters the separator around the outside of the bag, passes through the bag, then through the louvers. As the damp air passes through the bag, the bag is wetted, and larger droplets of water are formed. These droplets along with the air are caused to whirl by the louvers of the support. As the air and moisture pass through the separator, the centrifugal force keeps the heavier moisture close to the inside of the support until it reaches the collection chamber. The water and air whirling in a greater diameter than the baffle must make a double reverse turn in order to leave the separator. The turning does not appreciably affect airflow, but the much heavier water cannot make the turn and remains in the collection chamber. The water collected in the water separator is injected in the ram air duct and evaporated in the heat exchanger. This results in more heat being removed from the heat exchanger. Air from the air cycle machine turbine inlet is used to force the water through the spray injector. Water separator and coalescer bag 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 47 of 222 CASA Part Part 66 - Training Materials Only  A bypass valve allows air to pass through the water separator to the distribution system without first passing through the coalescer bag. The valve opens as a result of increased pressure differential should the coalescer bag become clogged or frozen. The bypass valve opens only when the coalescer bag becomes excessively clogged to allow passage of sufficient air for proper ventilation and cabin pressure. The bypass valve assembly is secured to a mounting ring within the inlet shell assembly. Water separator bypass valve 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 48 of 222 CASA Part Part 66 - Training Materials Only  High-Pressure Water Extractor This is an alternative type of water extractor fitted to some newer types of aircraft. It has the advantage that it requires no routine maintenance, such as changing the coalesce bag, as is required of the low-pressure water extractor. The water extractors are inertial-type centrifugal flow fluid separators. The inlet of the water extractor has a swirl chamber to create a vortex airflow. The water part of the airstream goes into the outer shell of the extractor by centrifugal force. A sump collects the water, and pressure in the extractor forces the water out of the sump into the drain nipples. Lines connect the drain nipples to the water spray nozzle. High-pressure water extractor 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 49 of 222 CASA Part Part 66 - Training Materials Only  Mixing Chamber A mixing chamber (or plenum) allows any hot air that has bypassed the air cycle machine to mix with cool air before the air goes into the cabin and flight deck distribution system. This allows unlimited temperature increments to be selected from the cockpit. Recirculation air from the recirculation fans also mixes with fresh air in the mixing chamber. Mixing chamber 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 50 of 222 CASA Part Part 66 - Training Materials Only  Ram Air Door The ram-air system controls the quantity of outer ambient air that flows through the heat exchangers. The ram-air inlet ducts let cooling air flow from the ram-air inlet to the heat exchangers. The ram-air exhaust ducts let air flow from the heat exchangers discharge overboard. The ram-air actuator moves the ram-air inlet deflector door and the ram-air inlet modulation panel (ram-air inlet door). The ram-air inlet modulation panel has two panel sections: the forward and aft panel. The two panels attach with hinges. The ram air actuator moves the modulation panels. The modulation panel and the ram air inlet deflector door are mechanically connected. When the system senses the aircraft is on the ground, the actuator moves the modulation panels to the fully open position and the deflector door extends and prevents ice, rocks and other unwanted material from entering the ram-air inlet. At take-off, when the aircraft goes from ground to air and the flaps are not fully retracted, the modulation panels remain fully open and the deflector door retracts, allowing maximum ram air flow. When the flaps are fully retracted, the actuator moves the modulation panels towards the closed position. This is to decrease drag during cruise conditions If the aircon packs fail in flight, the ram air modulation panel on the side of the fuselage for the appropriate pack moves to the closed position to reduce drag. Ram air door on a 737NG 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 51 of 222 CASA Part Part 66 - Training Materials Only  Some aircraft have ram-air exhaust ducts to let air flow from the heat exchangers to discharge overboard. Other aircraft are equipped with ram-air outlet doors. The ram-air exhaust ducts or doors are aft of the air conditioning compartments. Ram-air inlet/outlet doors 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 52 of 222 CASA Part Part 66 - Training Materials Only  Emergency Ram-Air Valve or Door/Flap Emergency ram-air valves or doors can be opened to allow the outside air to enter the cabin. The emergency ram air from outside supplies fresh air for ventilation of the aircraft cabin during unpressurised flight in the event of a double pack failure (all air conditioning packs fail), or it can be used to remove smoke. The aircraft must descend to 10 000 ft. The ram-air valve or door is always closed during normal operations. It is energised open when the cockpit switch is placed in the on position. An electric heater may be provided to warm this ram air and the check valves in the ducts prevent airflow in the opposite direction. ACM schematic diagram with ram air valve circled 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 53 of 222 CASA Part Part 66 - Training Materials Only  Ram air door Relevant Youtube link: Air conditioning part 1 - pneumatics - airframes and aircraft systems #39 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 54 of 222 CASA Part Part 66 - Training Materials Only  Vapour Cycle Machine Vapour Cycle This is also known as a refrigeration system or Freon system. Vapour cycle cooling systems are installed in older piston or jet aircraft to cool the cockpit and passenger cabin. This method consists of a continuously repeating set of processes (known as loops or cycles). Vapour air conditioning systems use the flow of a sealed refrigerant that converts between a liquid and a gas (called phase changes) in a cycle that transports heat from lower temperatures to higher ones. This cyclic (repeating) set of processes is known as the vapour cycle. Vapour cycle heat transfer The basic vapour cycle air conditioning system uses a refrigerant to transports the heat and five physical components. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 55 of 222 CASA Part Part 66 - Training Materials Only  A compressor that compresses the low-pressure refrigerant vapour from the evaporator. A condenser that condenses the hot refrigerant gas coming from the compressor. A receiver-dryer serves as a reservoir for the liquid refrigerant. A thermal expansion valve that regulates the amount of liquid refrigerant flowing into the evaporator. The evaporator absorbs heat from the aircraft cabin ambient air into the refrigerant. Refrigerant liquid must be highly volatile (high vapour pressure and a low boiling temperature). There are many types of refrigerants. However, exclusively R134a is now used in aircraft applications due to its low environmental impact. For servicing a vapour cycle system, it is important that only the refrigerant specified in the aircraft manufacture’s maintenance manual is used. Vapour cycle air conditioning The system is divided into two sides: one that accepts the heat and the other that disposes of it. Both the low and high side of the compressor are pressurised; therefore, the ‘suction’ or compressor inlet side is called the low side. The compressor outlet or discharge has a high pressure and is called the high side. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 56 of 222 CASA Part Part 66 - Training Materials Only  The side that accepts the heat is called the low side because here the refrigerant has a low temperature and is under a low pressure. The heat is given up on the high side, where the refrigerant is under high pressure and has a high temperature. The system is divided at the compressor where the refrigerant vapour is compressed, increasing both its pressure and temperature, and at the expansion valve where both pressure and temperature drop. The refrigeration cycle starts at the receiver-dryer which acts as a reservoir to store any of the liquid refrigerant that is not passing through the system at any given time. If any refrigerant is lost from the system, it is replaced from the receiver-dryer. Liquid refrigerant leaves the receiver-dryer and flows under pressure to the expansion valve, where it sprays out through a tiny metering orifice into the coils of the evaporator. The refrigerant is still a liquid, but it is in the form of tiny droplets, affording the maximum amount of surface area so the maximum amount of heat can be absorbed. The evaporator is the unit in an air-conditioning system that produces the cold air. Warm air is blown through the thin metal fins that fit over the evaporator coils. This heat is absorbed by the refrigerant, and when the air emerges from the evaporator, it is cool. When heat is absorbed by the refrigerant, it changes from a liquid into a gas without increasing its temperature. The heat remains in the refrigerant in the form of latent heat. The refrigerant vapour that has the heat from the cabin is taken into the compressor, where additional energy is added to it to increase both its pressure and temperature. Refrigerant leaves the compressor as a hot, high-pressure vapour. The heat trapped in the refrigerant vapours in the condenser escapes into the walls of the coil and then into the fins that are pressed onto these coils. Relatively cool air from outside the aircraft flows through these fins and picks up the heat that is given up by the refrigerant. When it loses its heat energy, the refrigerant vapour condenses back into a liquid and then flows into the receiver-dryer where it is held until it passes through the system for another cycle. View Interactive Flash Animation: Vapour cycle air conditioning operation 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 57 of 222 CASA Part Part 66 - Training Materials Only  Vapour Cycle Machine Compressor The compressor circulates the refrigerant through the system. Refrigerant leaves the evaporator as a low-pressure, low-temperature vapour and enters the compressor. The compressor provides the energy necessary to operate the system. The gas leaving the compressor is at a high temperature and pressure. Aircraft air-conditioning systems usually use reciprocating-type compressors, which have reed valves and a lubricating system that uses crankcase pressure to force oil into its vital parts. On small aircraft, these compressors are usually belt driven by the engine, very similar to the arrangement used in an motor vehicle. The compressors in systems used on larger aircraft are driven by electric or hydraulic motors or by compressor bleed air powered turbines. Engine-driven compressors are single-speed pumps whose output is controlled by a magnetically actuated clutch in the compressor drive pulley. When no cooling is needed, the clutch is de-energised, and the compressor does not operate. When the air conditioner is turned on and the thermostat calls for cooling, the magnetic clutch is energised, causing the drive pulley to turn the compressor and pump refrigerant through the system. Electric motor-driven compressors are controlled by a thermostat that turns the compressor motor on and off as required. Hydraulic motors are turned off and on by solenoid valves controlled by the thermostat. When the valve is opened, hydraulic fluid is directed under pressure to the motor. When the motor is not being driven, the output of the engine-driven hydraulic pump is returned to the reservoir. In all of these systems, the cabin blower operates continually, forcing the cabin air over the evaporator so heat from cabin air can be transferred into the refrigerant. Aeroplane vapour cycle machine compressor 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 58 of 222 CASA Part Part 66 - Training Materials Only  Electric Motor-Driven Compressors Dedicated electric motor-driven compressors are also used on aircraft. Use of an electric motor allows the compressor to be located nearly anywhere on the aircraft since wires can be run from the appropriate bus to the control panel and to the compressor. Hydraulically driven compressors are also able to be remotely located. Hydraulic lines from the hydraulic manifold are run through a switch-activated solenoid to the compressor. The solenoid allows fluid to the compressor or bypasses it. This controls the operation of the hydraulically driven compressor. Electric motor-driven compressors 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 59 of 222 CASA Part Part 66 - Training Materials Only  Vapour Cycle Machine Condenser The condenser is the radiator-like component that receives hot, high-pressure vapours from the compressor and transfers the heat from the refrigerant vapours to the cooler air flowing over the condenser coils. When heat is removed from the vapour, the refrigerant returns to a liquid state. The condenser is made of copper tubing with aluminium fins pressed onto it, formed into a set of coils and mounted in a housing. The condenser and the evaporator are similar in both construction and appearance, differing primarily in strength. Since the condenser is in the high side of the system, it must be capable of withstanding the high pressure found there. Condensers normally operate at a pressure of about 300 psi and have a burst pressure in excess of 1500 psi. In larger aircraft, the condenser is mounted in an air duct where cooling air can be drawn in from the outside and blown over the coils. In flight, ram air usually provides sufficient airflow over the condenser for proper operation. For ground operation, a fan must be used to supply the necessary cooling airflow. Aeroplane vapour cycle machine condenser 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 60 of 222 CASA Part Part 66 - Training Materials Only  Vapour Cycle Machine Receiver Dryer The receiver-dryer is the reservoir for the system and is located in the high side between the condenser and the expansion valve. Liquid refrigerant enters from the condenser and is filtered and passed through a desiccant such as silica-gel to absorb any moisture that might be in the system. A sight glass is normally installed in the outlet tube to indicate the amount of charge in the system. Bubbles can be seen in the glass when the charge is low. A pickup tube extends from the top of the receiver-dryer to near the bottom where the liquid refrigerant is picked up. A filter is installed either on the end of the pickup tube or between the tube and the desiccant to prevent any particles getting into the expansion valve. It is very important that all moisture be removed from the system, as a single drop can freeze in the expansion valve and stop the entire air conditioning process. Water will also react with the refrigerant to form hydrochloric acid which is highly corrosive to the metal in the system. Aeroplane vapour cycle machine receiver dryer 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 61 of 222 CASA Part Part 66 - Training Materials Only  Vapour Cycle Machine Thermal Expansion Valves There are two types of thermal expansion valves: Internally equalised valve Externally equalised valve. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 62 of 222 CASA Part Part 66 - Training Materials Only  Internally Balanced Thermal Expansion Valve The thermal expansion valve is the control device which meters the correct amount of refrigerant into the evaporator. The refrigerant should evaporate completely by the time it reaches the end of the coils. The heat load in the aircraft cabin controls the opening, or orifice, in the valve. The internally equalised thermal expansion valve is controlled by the amount of heat in the evaporator. A capillary tube to the evaporator connects the diaphragm chamber of the valve. The end of the capillary is coiled into a bulb and is held tightly against the discharge tube of the evaporator. Coiling this tube allows a greater area to be held in intimate contact with the tube, allowing for a more accurate temperature measurement. If the liquid refrigerant completely evaporates before it reaches the end of the evaporator, it will continue to absorb heat and become superheated. It is still very cold to touch, but it is considerably warmer than it would be if it had not absorbed this additional heat. The expansion valve is adjusted to a given amount of superheat. When the pressure of the refrigerant vapour reaches this value, the diaphragm pushes down against the superheat spring and opens the valve, allowing more refrigerant to enter the evaporator. A balance between the vapour pressure on the diaphragm and the superheat spring controls the amount of refrigerant flow. Aeroplane vapour cycle machine thermal expansion valve 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 63 of 222 CASA Part Part 66 - Training Materials Only  Externally Balanced Thermal Expansion Valve Vapor cycle air conditioning systems that have large evaporators experience significant pressure drops while refrigerant is flowing through them. Externally equalised expansion valves use a pressure tap from the outlet of the evaporator to help the superheat spring balance the diaphragm. This type of expansion valve is easily recognisable by the additional small-diameter line that comes from the evaporator into the valve (2 total). Better control of the proper amount of refrigerant allowed through the valve is attained by considering both the temperature and pressure of the evaporator refrigerant. Externally balanced thermal expansion valve 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 64 of 222 CASA Part Part 66 - Training Materials Only  Vapour Cycle Machine Evaporator The evaporator is the actual cooling unit in a vapour-cycle air-conditioning system. An evaporator consists of one or more circuits of copper tubing arranged in parallel between the expansion valve and the compressor. These tubes are silver-soldered into a compact unit, with thin aluminium fins pressed onto their surface. The evaporator is usually mounted in a housing with a blower. The blower forces cabin air over the evaporator coils. The refrigerant absorbs heat from the cabin air, thereby cooling it before it returns to the cabin. A drip pan is mounted below the evaporator to catch water that condenses out of the air as it cools. The capillary of the thermostat is placed between the fins of the evaporator core to sense the temperature of the coil, and it is this temperature that controls the cycling of the system. The evaporator is similar to the condenser in construction, and somewhat similar in appearance, but since it is in the low side of the system, the evaporator is not subject to such high pressures as the condenser. The refrigerant should use the entire length of the evaporator when changing from a liquid into a vapour. Aeroplane vapour cycle machine evaporator 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 65 of 222 CASA Part Part 66 - Training Materials Only  Vapour Cycle Machine Refrigerant Any liquid chosen for use as a refrigerant must be highly volatile. At any given pressure, a volatile liquid will boil at a low temperature compared to a less volatile liquid such as water. A volatile liquid has a high vapour pressure. A volatile liquid is one which readily evaporates (vapourises) at normal temperature. The vapour pressure of a liquid is the pressure that will exist above a liquid in an enclosed container at any given temperature. For example, a particular liquid refrigerant in an open container boils vigorously as the liquid turns into a gas at a temperature of 21 °C. If the container is closed, the liquid will continue to change into a vapour and the pressure of the vapour will increase. When the pressure reaches 70.1 psi, no more vapour can be released from the liquid. The vapour pressure of this material is then said to be 70.1 psi at 21 °C. Many different materials have been used as refrigerants in commercial systems, but for aircraft air- conditioning systems, an environmentally friendly refrigerant called R134A is almost universally used. It is a stable compound at both high and low temperatures and does not react with any of the materials in an air-conditioning system. It will not attack the rubber used for hoses and seals and is colourless and practically odourless. R134A has a boiling point of -26.6 °C (1.013 bar) and its vapour pressure is 4.9 bar (at 15 °C). Aeroplane vapour cycle machine refrigerant 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 66 of 222 CASA Part Part 66 - Training Materials Only  Air Distribution Systems Cabin Air Supply Sources Distribution of cabin air on pressurised aircraft is managed with a system of air ducts leading from the pressurisation source into and throughout the cabin. Typically, air is ducted to and released from ceiling vents, where it circulates and flows out floor-level vents. The air then flows aft through the baggage compartments and under the floor area. It exits the pressure vessel through the outflow valve(s) mounted low, on or near the aft pressure bulkhead. The flow of air is nearly imperceptible. Ducting is hidden below the cabin floor and behind walls and ceiling panels depending on the aircraft and system design. The cabin air distribution system includes ducts, filters, heat exchangers, silencers, non-return check valves, humidifiers and mass-flow sensors and meters. The flight compartment conditioned air distribution system supplies the flight crew with conditioned air. The left air conditioning pack supplies the conditioned air for the flight compartment. The usual control for the left pack makes sure that it supplies air at a temperature that gives the necessary cooling for the flight compartment. The air flows through ducts that go forward along the left side of the airplane. The flight compartment distribution uses different ducts than the passenger compartment distribution. The flight compartment receives conditioned air from the right pack if the left pack is not operational. The flight compartment distribution lets the flight crew select a different air temperature than the other areas of the airplane. The control for the right pack makes sure that it supplies air at a temperature that gives the necessary cooling for the passenger compartment. The passenger cabin conditioned air distribution system divides the flow of conditioned air to the passenger cabin. The temperature control system controls the temperature in the two zones: the flight compartment and the passenger cabin. The temperature changes are made by the mixing valve with signals from the temperature controller. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 67 of 222 CASA Part Part 66 - Training Materials Only  Air conditioning distribution system 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 68 of 222 CASA Part Part 66 - Training Materials Only  Cabin Air Distribution Conditioned air from the main distribution manifold flows through sidewall riser ducts. The riser ducts follow the airplane contour along the right and left fuselage. The overhead distribution ducts run along the top centre of the passenger cabin. The air distribution supply ducts include metering orifices and mufflers. The metering orifices control flow. The mufflers decrease air noise. Conditioned air from the overhead distribution ducts goes to the sidewall diffusers. It also goes to nozzle assemblies through diffuser/hoses and mufflers. The overhead distribution duct connects to flexible hoses to supply the galleys and the lavatories in the front and aft cabin areas. The passenger cabin exhaust air goes through floor grilles to the recirculation system or overboard. Main distribution manifold 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 69 of 222 CASA Part Part 66 - Training Materials Only  Passenger cabin conditioned air distribution 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 70 of 222 CASA Part Part 66 - Training Materials Only  Distribution Ducts Cabin air supply ducts are usually made from aluminium alloys, stainless steel or plastic. Main ducts for air temperatures over 200 °C are made from stainless steel. Those parts of the ducting where the air temperature does not exceed 100 °C are usually constructed from soft aluminium. Plastic ducts, both rigid and flexible, are used as outlet ducts to distribute the conditioned air. Since heated air is routed throughout the duct system, it is important that the ducts be permitted to grow (expand through heating) and to shrink again when the air cools down. This expansion and contraction must take place without loss of the pressure-tight integrity of the ducts. Expansion bellows are incorporated at various places throughout the duct system to permit the ducts to expand or contract. In general, supports are necessary on both sides of a connecting bellows, a fixed support on one side to prevent duct movement and a sliding support plus a fixed support on the other side. The sliding support permits movement of the bellows while the duct section is under pressure Whenever a duct is angled, means are provided to take care of the end forces which tend to push the duct section apart. This can be accomplished with external swinging supports. Air distribution ducts, expansion bellows and duct support 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 71 of 222 CASA Part Part 66 - Training Materials Only  Cabin Air Filters Air delivered to a pressurised cabin from a supercharger or engine compressor may contain dust particles, oil mist or other impurities. Unfiltered air which contains a considerable amount of impurities usually has an offensive odour and causes headaches and nausea. Filters are generally incorporated into the ducting and recirculation system to clean the air. The recirculation air filters remove small particles of material from the air that flows back to the passenger cabin. The recirculation fan pulls air from the passenger compartment through a high efficiency particulate air (HEPA) filter to remove very small particles at the bacteria and microorganism level. Recirculation air filter 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 72 of 222 CASA Part Part 66 - Training Materials Only  Cabin Airflow Control Air enters the passenger cabin from overhead distribution outlets that run the length of the cabin. Outlets are designed to create carefully controlled circular airflow patterns in the cabin. Air is supplied and exhausted from the cabin on a continuous basis. The exhaust air leaves the cabin through return air grilles located in the sidewalls near the floor and running the length of the cabin on both sides. The exhaust air is continuously extracted from below the cabin floor (lower lobes), creating a pressure differential which moves more exhaust air through the return air grilles to the lower compartments. The passenger cabin exhaust air goes through floor grilles to the recirculation system or overboard. The cabin ventilation system is designed and balanced so that air supplied at one seat row leaves at approximately the same seat row, minimizing airflow in the fore and aft directions. By controlling fore and aft airflow, the potential for spreading passenger generated contaminants is minimised. Airflow control 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 73 of 222 CASA Part Part 66 - Training Materials Only  Cabin Air Recirculation System The recirculation system supplies air for ventilation. The use of cabin air for ventilation decreases the use of air from the engine bleed system. The recirculation fans pull air in from the passenger cabin and supplies it into the mixing chamber (main distribution manifold). This part of the air conditioning system puts 50 percent of the cabin air back into the system for ventilation. This decreases the quantity of fresh air necessary from the pneumatic system for ventilation. Less engine compressor bleed means more power; therefore, the engines can be throttled back to maintain cruise speed, thereby saving fuel. Recirculation fans are electric motor driven. The recirculation fan and filter are the primary components. The recirculation fans pull air from the passenger compartment through a high-efficiency particulate air filter to remove very small particles at the bacteria and microorganism level. Cabin air recirculation system 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 74 of 222 CASA Part Part 66 - Training Materials Only  Recirculation fans and filters Gasper Air A gasper refers to the adjustable air outlet situated above each passenger seat. These outlets are part of the aircraft's air conditioning and cabin air recirculation system and may feature adjustments for both direction and strength of air flow. They are typically round vents situated above passenger seating with a flow control dial and, in some cases, a directional nozzle. In older aircraft, these outlets were fed directly from the aircraft’s packs or air conditioners. In newer planes, the gaspers are fed from the air recirculation system. These vents are located in the Passenger Service Units (PSUs). Gasper air 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 75 of 222 CASA Part Part 66 - Training Materials Only  Christopher Doyle from Horley, United Kingdom, CC BY-SA 2.0, via Wikimedia Commons Passenger service unit (PSU) 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 76 of 222 CASA Part Part 66 - Training Materials Only  Temperature and Humidity Control System Temperature Control System Operation For temperature control, the aircraft cabin is normally divided in different zones. The actual temperature in these zones may vary by different factors: The temperature of the air delivered by the air conditioning system The temperature of surrounding furnishings and structures Solar radiation during daytime The outside air temperature The thermal conductivity of furnishings, structures and windows The number of passengers in the immediate vicinity. Cabin temperature control must deal with both steady state and dynamic conditions. In a steady state, to maintain a constant temperature, the amount of heat delivered to the cabin must be equal to the heat lost. Other than the air conditioning supply, the main source of heat on board is the passengers themselves. So with a full passenger load, the air conditioning system will be working continuously to cool the cabin, that is, to supply cool air to remove heat, even when the exterior temperature may be as low as minus 57 °C. With lower passenger numbers, the cabin may have to be heated to maintain a constant temperature. Aircraft may have more than one seating layout, typically first, business and economy classes. In each of these areas, the seating density (the number of passengers per square meter) changes, and consequently, the heat load also changes. So throughout the cabin, some areas may need to be heated while other ones are cooled simultaneously. Dynamic temperature regulation is necessary during: 1. Aircraft Power-Up – The cabin temperature will have to be increased (in cool climates) or decreased (in warm climates) to achieve comfortable temperatures preferably prior to passenger boarding. 2. Boarding – When the passengers do board, a significant heat load arrives in the cabin. 3. Altitude Changes – During climb/descent where the exterior temperature changes significantly and the aircraft structure and furnishing temperatures follow. So the air conditioning system design incorporates the ability to precisely regulate the temperature to selectable levels with different selections in the different cabin zones. 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 77 of 222 CASA Part Part 66 - Training Materials Only  The following are the components of a temperature control system: Cabin and flight deck and duct temperature sensors Temperature controller Cabin selector Duct temperature anticipator. Temperature sensing and control 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 78 of 222 CASA Part Part 66 - Training Materials Only  Temperature Control System Sensors The temperature pickup unit (temperature sensor) consists of a resistor that is highly sensitive to temperature changes. The unit is usually located in the cabin and 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 unit to change. The cabin sensor is a thermistor-type unit. A thermistor is a resistor that changes with its exposure to heat. It can act like an on/off switch at a specified temperature. It can also provide a resistance that varies inversely with heat. High-limit sensors are used to indicate overheat situations which will limit auto control and drive mix valves toward the cold position. Temperature control system sensor 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 79 of 222 CASA Part Part 66 - Training Materials Only  Cabin Temperature Controller Most cabin temperature systems operate in a similar manner. Temperature is monitored in the cabin, cockpit, conditioned air ducts and distribution air ducts. These values are sent to a temperature controller, or temperature control regulator, normally located in the electronics bay. A temperature selector in the cockpit can be adjusted to input the desired temperature. The temperature controller compares the actual temperature signals received from 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 air conditioning system. (i.e., mixing valve, temperature control valve or trim valve). It mixes warm bleed air that has bypassed the air cycle cooling process with 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. The temperature is normally automatically controlled by the temperature controller. In the event of automatic control failure, the temperature controller has an additional manual selector to control the mixing valve motor manually. Cabin temperature controller 2023-07-26 B1-11d Turbine Aeroplane Aerodynamics, Structures and Systems Page 80 of 222 CASA Part Part 66 - Training Materials Only  Cabin Air Temperature Selector The air temperature selector is a rheostat located in the cabin. It permits selective temperature control by varying the effective temperature control point of the cabin air pickup unit. The rheostat causes the cabin pickup unit to demand a specific temperature of the air supply. The temperature-sensing element is located in the cabin or at the air conditioning outlet. The cabin selector knob is an adjustable leg of a Wheatstone bridge (adjustable resistance). The remaining two resistors are of a fixed value. The Wheatstone bridge remains balanced whenever the temperature-sensing element and the cabin selector KNOB resistance values are proportional. If they are not proportional, a voltage imbalance across the bridge (between points B and D) creates an input to the amplifier. The amplified output controls the position of the mixing valve, which varies the proportion of cold/hot air provided to the aircon system mixing chamber (it adds or restricts the flow of hot air to the mixing chamber), thereby controlling the temperature of the output air by the air conditioning system. If the temperature in the cabin goes up or down and the selector hasn’t been moved, the mixer valve is modulated to maintain the selected temperature. If the cabin se