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► The vehicle heating system and automotive air conditioning system were developed in the 1930s.

► The Environmental Protection Agency regulates automotive air conditioning via Section 609 of the Clean Air Act.

► Automotive HVAC technicians must obtain a license to verify that they are familiar with federal rules and regulations.

► HVAC systems use heat transfer to ensure temperature comfort inside a vehicle.

► Heat transfer can take place via conduction, convection, or radiation.

► A vehicle’s air conditioner transforms the refrigerant from a liquid to a gas and back to liquid.

► Heat energy (the amount of heat needed to move a temperature up or down by degrees) is measured in British thermal units (BTUs).

► The temperature at which a liquid changes to gas is called latent heat of evaporation; likewise, the temperature at which a liquid forms a solid (ice) is called latent heat of freezing.

► Vehicle air conditioners use a refrigerant for the vaporization and condensation process.

► HVAC systems have four major components: evaporator, compressor, condenser, and the restriction.

► The compressor is designed to increase the pressure of the refrigerant (in gas form) and push it into the condenser.

► A vehicle’s air conditioner is a closed loop system.

► The condenser is designed to move gas through a series of coils and cool it into a liquid.

► The restriction works to transform high pressure liquids into low pressure liquids.

► The evaporator vaporizes liquid into gas as it removes heat energy from the passenger compartment.

► Each HVAC system component requires the refrigerant to be delivered in the proper state and at the correct level.

► Components of a compressor include: pulley, clutch, axial plate, piston, reed valves, and body.

► One of the most common types of compressor is an axial piston compressor.

► Refrigerant moves through the air conditioning system because of the high pressure and volume of gas leaving the compressor.

► The condenser uses ram air flow and the engine cooling fan to cool the refrigerant.

► A restriction can be a fixed orifice tube or a thermostatic expansion valve.

► The velocity and pressure of the refrigerant changes when it moves through the restriction.

► Bernoulli’s principle states that the speed/flow of a liquid changes in opposition to the liquid’s pressure (when one increases, the other decreases).

► Pascal’s law states that liquid cannot be compressed.

► The evaporator is a one-piece unit of copper or aluminum tubes, with no serviceable parts.

► Automotive air conditioners can be fixed orifice tube systems or thermal expansion valve (TXV) systems.

► Fixed orifice tube systems use an accumulator to prevent liquid from entering the compressor.

► TXV systems use a receiver filter dryer to draw in and store liquid refrigerant.

► Air doors are controlled by cables, vacuum servos, or electric servos.