Sea Frost BG 2000 - Manual (Page 8)

8
HOW REFRIGERATION WORKS
There are two important concepts to understand in order to learn about refrigeration.
They are latent heat and phase changes.
A great deal of heat is required to change a solid to a liquid, and a liquid to a gas. A
great deal of heat must be removed to reverse these changes. These changes are
called phase changes, or changes of state. The heat removed or added at these phase
changes has no effect on the temperature of the substances until the change is
complete. For instance, ice melts at 32 degrees F, water freezes at 32 degrees F also.
Ice and water will remain at 32 degrees F until the freezing or melting process is
complete. Latent heat is this hidden energy required to make or break the bonds in a
phase change.
By evaporating liquid to a vapor, we can absorb heat. By condensing a vapor to a
liquid, we give up heat. Refrigeration is the use of these phase changes to move heat
out of the icebox (cooling it).
We all know that cold is the absence of heat. A practical example of heat absorption by
evaporation is that of rubbing alcohol evaporating in your hand and cooling it. The
alcohol is actually using the heat from your hand to boil. The absorption of heat cools
your hand.
Pressure affects the temperature at which a gas phase change will occur. Using water
as an example, water boils at sea level at 212 degrees F. On top of Mt. Everest it boils
at a much lower temperature. The air pressure is lower allowing the water-to-steam
phase change to occur more easily. A pressure cooker increases the pressure on water
to restrict boiling to a higher temperature. A pressure cooker will cook food faster
because the temperature is higher. Remember that a phase change involves latent
heat. The temperature of boiling water is only 212 degrees F. at sea level. The
evaporation action is absorbing heat at a rate equal to the rate of heat applied,
preventing further temperature rise.
Let's look at Refrigerant-404-a (R-404-a). R-404-a will boil at minus 57 degrees F. at
sea level. By evaporating liquid R-404a in the SEA FROST plates heat is absorbed
making refrigerant vapor. To dispose of this heat, a condensing phase change is
necessary. By increasing the pressure (compressing) we can raise the boiling point of
the gas vapor. Heat is removed from the pressurized gas vapor at the condenser.
Seawater passing the condenser coils removes the heat, forcing the vapor to a liquid
state again. Pressure, therefore, is the key that allows passing the heat we have taken
from the icebox to a warmer place (the seawater) and converting the gas to a liquid to
be re-evaporated again.