Automotive Air Conditioning: How It Works and Key Components
Air Conditioning System Basics
The primary goal of an air conditioning system is to reduce and control the temperature, humidity, and purity of the air within a vehicle’s cabin. The ideal cabin temperature ranges from 21°C to 24°C, with relative humidity between 40% and 70%. The systems commonly used are of the ‘continuous cycle’ type.
These systems aim to achieve and maintain a comfortable interior temperature quickly and consistently. They offer a range of temperature selections to ensure comfort regardless of external weather conditions or the driver’s physiology. Additionally, they maintain appropriate cabin humidity to prevent windshield condensation.
Understanding State Changes
State changes refer to the transition of a physical substance between solid, liquid, and gas states. The key changes relevant to air conditioning are:
- Fusion: Solid to liquid state.
- Solidification: Liquid to solid state.
- Evaporation: Liquid to gas state.
- Condensation: Gas to liquid state.
- Sublimation: Solid to gas state.
Air conditioning systems operate based on heat exchange, utilizing the state changes of evaporation and condensation to cool the cabin. Evaporation absorbs heat, while condensation releases heat.
Ideal Gas Conditions
The conditions of an ideal gas are determined by pressure, volume, and temperature. Varying these quantities causes the gas to undergo transformations.
Stages of Cold Production
The cooling process involves several key stages:
- Compressor: The compressor increases the pressure and temperature of the refrigerant gas (vapor). It exits at approximately 12 bars, with pressure rising from low to high and temperature increasing significantly (around 80°C).
- Condenser: The high-pressure, high-temperature gas enters the condenser. Here, heat is transferred to the outside environment, causing the refrigerant to change from a gas to a high-pressure liquid. This liquefaction is aided by airflow from fan motors.
- Filter Drier: The liquid refrigerant then passes through a filter drier to remove moisture and impurities.
- Expansion Valve: The refrigerant flows through the expansion valve, where its pressure is significantly reduced (from about 12 bar to 3.5 bar). This pressure drop causes a decrease in temperature (to around 5°C) and initiates the change from liquid to vapor (approximately 20% vaporized).
- Evaporator: The low-pressure, low-temperature mixture of liquid and vapor enters the evaporator. Here, the refrigerant absorbs heat from the cabin air, causing the remaining liquid to evaporate into a gas. This absorption of heat cools the air that is then circulated into the cabin. The air temperature exiting the evaporator is typically around 10°C.
System Summary
The cycle can be summarized as: Compressor ˃ Condenser ˃ Filter Drier ˃ Expansion Valve ˃ Evaporator ˃ Compressor.
Component Functions:
- Compressor: Increases refrigerant pressure and circulates it.
- Condenser: Facilitates the phase change of the refrigerant from gas to liquid.
- Drier Filter: Removes contaminants and moisture.
- Expansion Valve: Reduces refrigerant pressure, lowering its boiling point for efficient evaporation.
- Evaporator: Absorbs heat from the cabin air, thus cooling it.
Refrigerant
Key features of a refrigerant include:
- Low freezing point.
- Low boiling point.
- Low flammability.
- Non-corrosive and non-oxidizing properties.
- Compatibility with specialized lubricants.
- Environmental compliance.
The most common refrigerant used today is R134a, with a boiling point of -26.5°C at atmospheric pressure and a freezing point of -101°C.
Lubricating Oil
The oil used in air conditioning systems must:
- Not foam.
- Not freeze.
- Be miscible with the refrigerant.
- Be purified and dried to prevent ice formation.
Synthetic PAG (polyalkyl glycol) oils are commonly used.
Handling Rules:
- Always keep containers closed.
- Dispose of used oil as hazardous waste.
- Do not use expired oil.
- Respect expiry dates.
- Do not mix mineral and synthetic oils.