Refrigeration Principles: 2 Kinds of Heat

In the study of refrigeration and air conditioning, it is required to understand the two kinds of heat that affects the behavior of a liquid substance when heat is added. This is to understand how the refrigerant works during the heat absorption and extraction process in all refrigeration and air conditioning units.

Before discussing the 2 kinds of heat, it is necessary to recall the effect of heat on water.  As presented on page I labeled "principles" in this website, water evaporates when heat is added and when the temperature reaches  to 100⁰C  (boiling point). When heat is removed, it solidifies into ice at temperature of 0⁰C.

Before evaporation occurs when water is heated, the only thing that would change is the water temperature.  It increases from its initial state, which must be above 0⁰C until it reaches its boiling point.  In the same way, when water is put in a freezer, its temperature decreases until it reaches to its freezing point. Beyond this level, water must undergo change in state.  The 2 kinds of heat are responsible when changes in temperature and state occurs. These are the latent heat and sensible heat.

 

Latent Heat

Latent heat causes the water to change its phase from liquid to vapor when it is added. Similarly, it causes the water vapor to change its phase into liquid when it is removed without having the temperature to change. The word "latent" means concealed or hidden, or something that our physical senses could not detect.

In the context of refrigeration and air conditioning, air temperature is always measured to identify the efficiency of the equipment with the use of thermometer. But when talking about latent heat, its presence can never be measured by any temperature measuring device, only by its effect.

Sensible Heat

Another kind of heat is called sensible heat.  This is what we always experience in our daily lives when we feel being uncomfortable in the presence of heat. When we touched something hot, we know that  it is hot because we sensed it. Sensible heat causes the change in temperature of any given substance when added or removed, without changing the state.This is the heat that we can measure using thermometer.

Sensible heat is limited with respect to the kind of substance being subjected to it. In the case of water, sensible heat is only limited to 99 ⁰C.  Beyond this point, latent heat  comes into play to break the chemical composition of water and turn into vapor. On lower temperatures, sensible heat for water is only limited to temperature above 0 ⁰C. Beyond this point, latent heat must be taken away from water to turn into ice. In some other liquid substance like oil, this might not be the case.

The amount of sensible heat that a substance can handle identifies its freezing and boiling point. For refrigerants, sensible heat is limited to temperature below the boiling point of water. This makes refrigerants highly reactive to even slightest amount of heat at negative temperatures.

For example, R-12 evaporates at temperature of -29 degrees Celsius while water evaporates at 100 degrees Celsius, which is quite hot for all living organisms.  Boiling point is the maximum temperature of sensible heat to be held upon in that particular liquid substance. This property of the substance called refrigerant is the reason why it is the best medium for heat absorption used in refrigeration and air conditioning.

As being mentioned earlier, rain occurs as a result of condensation of water vapor in the atmosphere. This is due to the effect when latent heat leaves the water vapor collected in the air. When sea water evaporates, latent heat has been absorbed by the body of water.

Sensible heat and latent heat goes together when heat is added or removed in a substance. When sensible heat reaches its equilibrium point, latent heat will become noticeable as it causes chemical change to occur.