# Heat Transfer

In the AC system, refrigerant is the substance that is used to move the heat out of the cabin. In a similar way, coolant is the substance that is used to move heat away from the engine. A main difference between these two systems is that the refrigerant changes from a liquid to a gas during the process of moving the heat, but the engine coolant maintains its liquid state when the system is operating normally.

To understand how the heat is moved and how these state changes take place, more explanation is needed in the area of sensible heat and latent heat.

Sensible Heat
Sensible heat is the measure of heat that can be felt and measured on a thermometer. When a thermometer gives a reading of 75°F, sensible heat is being measured.

Latent Heat
Latent heat is different from sensible heat because it is not able to be measured on a thermometer. Latent heat is a hidden heat that causes a substance to change states. For example, when water changes from a liquid to a gas, the latent heat of evaporation takes place. When water vapor or steam is cooled down and changes from a gas back to a liquid, the latent heat of condensation takes place.

These two processes are continually happening with the refrigerant in a functioning AC system. The last type of latent heat is when water turns into a solid and this is called the latent heat of fusion. This is not supposed to happen in a normally functioning AC system. However, if the AC system malfunctions, then ice may be seen forming on some of the low side components while in operation.

Heat quantity is a unit of measure for heat and there are two terms that are commonly used to measure this: the British thermal unit (BTU) and the calorie. The BTU is the amount of heat it takes to change 1 pound of water 1 degree Fahrenheit. The calorie is the amount of heat it takes to change 1 gram of water 1 degree Celsius.

As stated above, the refrigerant is continually moving between the liquid and vaporous states when the AC system is functioning. A key point to remember is that large quantities of heat are transferred during the changes of state of the refrigerant. It takes 180 BTU of heat to change 1 pound of water from 32°F to 212°F. It then takes 970 BTU to change the 212°F water into a vapor. On the contrary, 970 BTU of heat must be released from the water vapor to change it back to a liquid. The processes of great heat absorption during evaporation and great heat released during condensation is why the AC system is so efficient at moving heat out of the cabin.