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