An air conditioner does not create cold air by itself. It moves thermal energy from an indoor room to the outdoors using a repeating refrigeration cycle. This matters because the same engineering idea is used in refrigerators, heat pumps, cars, and many industrial cooling systems.
A split-system air conditioner separates the indoor evaporator unit from the outdoor condenser unit, then connects them with refrigerant lines.
Key Facts
- Heat is absorbed indoors at the evaporator coil when low-pressure refrigerant boils into a gas.
- The compressor raises the refrigerant pressure and temperature so it can release heat outdoors.
- Heat is rejected outdoors at the condenser coil when hot refrigerant condenses into a liquid.
- The expansion valve lowers refrigerant pressure and temperature before it returns to the evaporator.
- Cooling capacity can be estimated by Q = m c ΔT for air passing over the indoor coil.
- Coefficient of performance for cooling is COP = Q_cold / W_in.
Vocabulary
- Refrigerant
- A working fluid that carries heat by changing pressure, temperature, and phase inside the air-conditioning cycle.
- Evaporator
- The indoor coil where cold, low-pressure refrigerant absorbs heat from room air and evaporates.
- Condenser
- The outdoor coil where hot, high-pressure refrigerant releases heat to outdoor air and condenses.
- Compressor
- A motor-driven pump that compresses refrigerant vapor to a higher pressure and temperature.
- Expansion valve
- A restriction that drops the pressure of liquid refrigerant so it becomes cold enough to absorb indoor heat.
Common Mistakes to Avoid
- Thinking the air conditioner makes cold from nothing. It actually uses work to move heat from inside the building to the outside air.
- Reversing the roles of the coils. The evaporator is the indoor heat-absorbing coil, while the condenser is the outdoor heat-releasing coil during cooling mode.
- Assuming the refrigerant stays the same phase everywhere. Phase changes are central to the cycle because boiling absorbs large amounts of energy and condensing releases it.
- Ignoring airflow across the coils. Even if the refrigerant cycle is working, dirty filters or blocked fins reduce heat transfer and lower cooling performance.
Practice Questions
- 1 Indoor air flows across an evaporator at 0.50 kg/s and cools from 26°C to 14°C. Using c = 1000 J/(kg°C), calculate the cooling rate Q = m c ΔT.
- 2 An air conditioner removes 3600 W of heat from a room while using 1200 W of electrical power. Calculate its cooling COP using COP = Q_cold / W_in.
- 3 Explain why the outdoor condenser can release heat to warm outdoor air even though the system is cooling the indoor room.