Purpose: maintain the atmosphere of an enclosed space at a required temp, humidity and purity
Refrigeration system is at heart of AC system
Heaters in ventilation system Types Used:
Self-contained Refrigerant circulating Chill water circulating
Air Conditioning
Air in passenger compartment is cooled, dried, and circulated◦ Heat is removed from inside to outside
Modern cars get better freeway fuel economy with windows up and air-conditioning on◦ Above 40 mph: more gas used with windows
down
Air Conditioning
Must be a transfer of heat for refrigerant to change state◦ Liquid absorbs heat as
changes to gas◦ Vapor releases heat as
changes to liquid
Air-Conditioning Principles
Self-Contained System◦ Add-on to ships that originally did not have AC
plants◦ Not located in ventilation system (window unit)
Refrigerant circulating system◦ Hot air passed over refrigerant cooling coils
directly Chilled water circulating system
◦ Refrigerant cools chill water◦ Hot air passes over chill water cooling coils
AC System Types
Split System an outdoor metal cabinet contains the condenser and compressor, and an indoor cabinet contains the evaporator Packaged
◦ the evaporator, condenser, and compressor are all located in one cabinet
Type of Central AC
Arrangement
Heat flows to anything with less heat◦ Convection
Heat rises Heat always flows from hot to cold
◦ Radiation Example: heat from the sun
◦ Evaporation Moisture absorbs heat as it is vaporized Example: perspiration
Heat Transfer
Low humidity ◦ Permits heat to be taken away from the human
body Evaporation and perspiration
High humidity ◦ Makes evaporation difficult
People feel as comfortable at 79°F with 30% humidity as at 72°F at 90% humidity
Humidity
Common matter exists in three states depending on temperature◦ Solid ◦ Liquid◦ Gas
Solid heated above freezing melts◦ Becomes a liquid
States of Matter
Closed system ◦ Four major devices
Compressor Condenser Evaporator Metering device
◦ Refrigerant circulates among devices◦ Changing pressure and state of refrigerant
regulates cooling cycle operation◦ Four stages: compression, condensation,
expansion, and vaporization
Air-Conditioning System Operation
Compressor designs◦ Several types
Axial compressor has four or more cylinders◦ Swash plate: axial plate connected to drive shaft◦ Wobble plate: wobbles in place
Variable displacement compressors: no clutch cycling
Radial compressor: multiple cylinders with pistons and a scotch yoke
Air Conditioning Compressors
Scroll compressor ◦ Moveable scroll oscillates around a fixed scroll
Rotary vane ◦ Blades like power steering pump
Electric compressors◦ Pressurize the air-conditioning system so the
engine does not have to be run Used in hybrids
Air-Conditioner Compressors (cont'd.)
Similar to two-stroke gasoline engines◦ Lubricated by oil carried in refrigerant◦ Heat transfer diminished when oil flows with
refrigerant◦ Newer designs keep oil within the compressor
Compressor Lubrication
Technology for High Efficiency and Reliability in Compressors
Some systems have a muffler installed on the compressor outlet◦ Due to pumping
noises due to high or low side pressure vibrations from multiple pistons
Muffler
Thermostatic switch◦ Tube containing mercury or CO2 senses
temperature Turns off compressor clutch when temperature drops
to 32°F Pressure cycling switch
◦ Temperature predicted by pressure of refrigerant◦ Mounted on accumulator at evaporator outlet
Evaporator Icing Control
Sensor inputs maximize system performance◦ Outside air temperature (OAT) sensor denies
compressor operation between 35°F and 45°F◦ Sunload sensor opens blend door and spins the
blower faster in response to sunlight intensity◦ Discharge duct temperature sensor senses the
temperature of air leaving the duct◦ Interior temperature sensor is sometimes used for
the first few minutes of vehicle operation
Automatic Air-Conditioning Sensors
30% of released CFCs are from mobile air-conditioning sources◦ Most of the problem from repair and service
Older cars use R-12 refrigerants◦ Recovered and recycled
Cars today use R-134A refrigerant◦ Does not deplete ozone layer
Still a greenhouse gas◦ 40 million pounds per year leak into atmosphere
Refrigerants and the Environment