Auto A/C Terms - Glossary
Latent Heat

The term latent is a Latin term meaning hidden. It is used to describe the heat that is consumed or released when a substance changes state. In an A/C system, when the liquid refrigerant in the evaporator absorbs heat and boils or changes state, the heat it absorbs is known as latent heat because it can not be measured with a thermometer.

Understand that whatever the temperature of the liquid refrigerant is in the bottom of the evaporator, when it boils and changes state to vapor refrigerant, the vapor will be the same temperature. The heat that the liquid absorbed was latent heat and can not be measured.

When asking what happened to the heat, understand that the heat was used or consumed to cause the change of state. Just like when water boils, the steam or vapor rising from the water is the same temperature because the heat absorbed was consumed to cause the change of state.

In the evaporator, when the liquid changes state to vapor, it remains the same temperature. Only when the vapor refrigerant continues to absorb heat, after it has changed state, does it change temperature and become hotter. At that point, the refrigerant is superheated.

In the A/C condenser, the vapor refrigerant is cooled. When the vapor is cooled, it releases the latent heat that was absorbed in the evaporator. Once again, the liquid refrigerant will be the same temperature as the vapor because the only heat released technically was the latent heat, which can’t be measured.

However, understand that in automotive A/C systems, the condenser is an efficient heat exchanger. When the vapor refrigerant is cooled and changes state to a liquid, the condenser continues to cool the liquid causing it to reach a temperature lower than the refrigerants saturation point. That additional cooling is known as sub cooling.