That's an good question, and the answer lies in a concept called latent heating.
Let's start with nice familiar example. As we all know, water exists in three
phases (as all molecules do). A water molecule has a really high amount of energy
as a gas particle (or water vapor) because it's moving at high speeds (i.e. high
kinetic energy) in all sorts of random directions. When that same particle is in
liquid form, it's confined within the particles surrounding it, and it's not moving
nearly as fast as it was as a gas particle. Therefore, it has LESS energy as a
liquid particle than it did as a gas particle.
According to the law of conservation of energy, energy cannot be created nor
destroyed. So where'd did the particles energy go when it changed from a gas phase
to a liquid phase?
The energy went to the particles around it through latent heat exchange. The
particle went from a high energy gas state to a low energy liquid state, and had to
give up some of that energy to it's surroundings causing the other particles to
gain energy and 'heat up'.
The same thing goes when a liquid particle goes to a gas particle. It's going from
a low energy liquid state to a high energy gas state. The increase in energy has
to come from somewhere, and that place is from the surrounding particles. The
molecule sucks energy from surrounding molecules when it goes from a liquid to a
gas causing the surrounding particles to lose energy and 'cool off'.
The change from a liquid state to a gas state is evaporation. Evaporative coolers
simply use the evaporation of a liquid to suck energy from whatever you put in
them, and therefore keep things cool!!!
Now, you still might be wondering how evaporative coolers can have evaporation at
the low temperatures you need in a 'cooler'. Well, as you decrease the air
pressure above a liquid, there's less force pushing down on the surface of the
liquid, and surface particles can exit the surface of the liquid and become gas
particles much more easily. Therefore, by creating a REALLY LOW pressure above a
liquid, that liquid will evaporate at much LOWER temperatures. Then those
particles in turn suck heat out of the system in order to be at a gaseous state of
Matthew Norman, Undergrad Student, NCSU, Raleigh, North Carolina
Evaporative coolers work by the evaporation of the cooling medium (usually water).
Water is a very effective coolant medium because it has a large heat of
vaporization -- about 10 Kcal/mol. or about 42 KJ/mol. When you consider that 1 mol of water is only 18 gm, or 18 cm3 that is a lot of energy absorbed on a mass basis.
However, evaporative coolers also
require that the surrounding air have a low relative humidity (R.H.) because the
rate of evaporation is roughly inversely proportional to the relative humidity. At
100% R.H. no water would evaporate and the efficiency of the cooler would be
Vince Calder, Ph.D., Physical Chemist, Retired
'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'