AS Chpt 12

• Overview of the vapour cycle air conditioning:

  • Energy can be neither created nor destroyed; however, it can be transformed and moved.
  • This is what occurs during vapour cycle air conditioning
• Overview of the vapour 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 vapour.
  • The vapour is compressed and becomes very hot
• Overview of the vapour cycle air conditioning:

  • It is removed from the cabin where the very hot vapour 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
• Overview of the vapour cycle air conditioning:

  • In vapour cycle air conditioning, heat is carried from the cabin to the outside air by a refrigerant which changes from a liquid to a vapour and back again
• Theory of refrigeration:

  • 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)
• Theory of refrigeration:

  • Adding heat to a substance does not always raise its temperature
  • When a substance changes state, such as when a liquid changes into a vapour, heat energy is absorbed
  • This is called latent heat
• Theory of refrigeration:

  • When a vapour condenses into a liquid, this heat energy is given off
  • The temperature of a substance remains constant during its change of state
• Theory of refrigeration:

  • All energy absorbed or given off, the latent heat, is used for the change process
  • Once the change of state is complete, heat added to a substance raises the temperature of the substance
• Theory of refrigeration:

  • The temperature at which a substance changes from a liquid into a vapour when heat is added is known as its boiling point
• Theory of refrigeration:

  • This is the same temperature at which vapour condenses into a liquid when heat is removed
  • The boiling point of any substance varies directly with pressure
• Theory of refrigeration:

  • When pressure on a liquid is increased, its boiling point increases, and when pressure on a liquid is decreased, its boiling point also decreases
• Theory of refrigeration:

  • For example, water boils at 212 °F (100 °C) at normal atmospheric temperature (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
• Theory of refrigeration:

  • It boils at approximately 226.4 °F. (108 °C) 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 (90 °C)
• Theory of refrigeration:

  • Vapour pressure is the pressure of the vapour that exists above a liquid that is in an enclosed container at any given temperature
  • The vapour pressure developed by various substances is unique to each substance
• Theory of refrigeration:

  • A substance that is said to be volatile, develops high vapour pressure at standard day temperature (59 °F) (15 °C)
  • This is because the boiling point of the substance is much lower
• Theory of refrigeration:

  • The boiling point of tetrafluoroethane (R134a), the refrigerant used in most aircraft vapour cycle air conditioning systems, is approximately –15 °F (-26.1 °C)
  • Its vapour pressure at 59 °F (15 °C) is about 71 psi
  • The vapour pressure of any substance varies directly with temperature
• Basic vapour cycle air conditioning system - Overview:

  • Is a closed system in which a refrigerant is circulated through tubing & a variety of components
  • Purpose - 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
• Basic vapour cycle air conditioning system - The 2 sides:

  • The high & low refer to the temperature & pressure of the refrigerant
  • The compressor & the expansion valve are 2 components that separate the low side from the high side of the cycle
  • Low side - low pressure & temperature
  • High side - high pressure & temperature
• Basic vapour cycle air conditioning system - Refrigerant flows to expansion valve & evaporator:

  • To begin, R134a is filtered & store under pressure in a reservoir known as 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
• Basic vapour cycle air conditioning system - Refrigerant flows to expansion valve & evaporator:

  • Since it is under pressure, some of the refrigerant is forced through the orifice
  • It emerges as a spray of tiny droplets in the downstream of the valve
  • The tubing is coiled into a radiator-type assembly known as evaporator
• Basic vapour cycle air conditioning system - 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 vapour
  • So much heat is absorbed that the cabin air blown by the fan across the evaporator cools significantly
  • This is the vapour cycle conditioned air that lowers the temperature in the cabin
• Basic vapour cycle air conditioning system - Compressor:

  • The gaseous refrigerant exiting the evaporator is drawn into a compressor
  • There, the pressure & the temperature of the refrigerant are increased
  • The high-pressure high-temperature gaseous refrigerant flows through tubing to a condenser
• Basic vapour cycle air conditioning system - 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 ambient air temperature, so heat is transferred from the refrigerant to the outside air
• Basic vapour cycle air conditioning system - Condenser:

  • The amount of heat given off is enough to cool the refrigerant & to condense it back to a high-pressure liquid
  • It flows through tubing & back into the receiver dryer, completing the vapour cycle
• Refrigerant:

  • Tetrafluoroethene (R134a) is commonly used in Vapour Cycle Air Conditioning system
  • Do not mixed different types of refrigerant as this may lead to possible damage of to components such as hoses and seals
• Refrigerant:

  • R134a has a boiling point of approximately –15 °F (-26.1 °C).
  • It is not poisonous to inhale in small quantities, but it does displace oxygen.
  • Suffocation is possible if breathed in mass quantity
• Refrigerant:

  • 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
  • Gloves and other skin protection, as well as safety goggles, are required when working with refrigerant
• Receiver Dryer:

  • It acts as the reservoir of the vapour cycle system
  • 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
• Receiver Dryer:

  • 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
• Receiver Dryer:

  • 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 and tubing, the acid deteriorates the materials from which these are made
• Receiver Dryer:

  • The second problem with water is that it could form ice and block the flow of refrigerant around the system, rendering it inoperative.
  • Ice is particularly a problem if it forms at the orifice in the expansion valve, which is the coldest point in the cycle
• Receiver Dryer:

  • A stand tube is used to remove refrigerant from the receiver dryer.
  • It runs to the bottom of the unit to ensure liquid is withdrawn and forwarded to the expansion valve.
  • At the top of the stand tube, a sight glass allows the technician to see the refrigerant
  • When enough refrigerant is present in the system, liquid flows in the sight glass
• Receiver Dryer:

  • If low on refrigerant, any vapour present in the receiver dryer may be sucked up the stand tube causing bubbles to be visible in the sight glass.
  • Therefore, bubbles in the sight glass indicate that the system needs to have more refrigerant added
• Thermal Expansion Valve:

  • Refrigerant exits the receiver dryer and flows to the expansion valve.
  • The thermostatic expansion valve has an adjustable orifice through which the correct amount of refrigerant is metered to obtain optimal cooling.
  • This is accomplished by monitoring the temperature of the gaseous refrigerant at the outlet of the evaporator
• Thermal Expansion Valve:

  • The temperature of the cabin air to be cooled determines the amount of refrigerant the expansion valve should spray into the evaporator.
  • Too little causes the gaseous refrigerant to be superheated by the time it exits the evaporator
• Thermal Expansion Valve:

  • Changing the state of the refrigerant from liquid to vapour absorbs much more heat than adding heat to already converted vapour (superheat).
  • The cabin air blowing over the evaporator will not be cooled sufficiently if superheated vapour is flowing through the evaporator
• Thermal Expansion Valve:

  • If too much refrigerant is released by the expansion valve into the evaporator, some of it remains liquid when it exits the evaporator.
  • It will flows to the compressor which is designed to compress only vapour.
  • The compressor will break down as it compress liquid since liquids are essentially incompressible
• Evaporator:

  • The evaporator is situated in such a way that cabin air is pulled to it by a fan.
  • The fan blows the air over the evaporator and discharges the cooled air back into the cabin.
  • This discharge can be direct when the evaporator is located in a cabin wall.
  • A remotely located evaporator may require ducting from the cabin to the evaporator and from the evaporator back into the cabin.
• Evaporator:

  • The construction of an evaporators consists of copper or aluminum tubing coiled into a compact unit.
  • Fins are attached to increase surface area, facilitating rapid heat transfer between the cabin air blown over the outside of the evaporator with a fan and the refrigerant inside
• Evaporator:

  • As the refrigerant absorbs heat from the cabin air, it changes into a low-pressure vapour
  • This is discharged from the evaporator outlet to the next component in the vapour cycle system, the compressor