In the refrigeration cycle, the refrigerant must undergo a change
in state in order for the heat to be absorbed from the environment.
The refrigerant has the property of changing its state from liquid
to vapor at normal room temperatures.
Have you ever felt your skin becoming cool whenever you put alcohol
on it? Or even when a bit of petrol spilled onto your hands? It's
because, the alcohol or petrol has evaporated when it comes in contact
with your skin.
The result - heat is removed from your skin, and it feels cool!
In a closed-loop refrigeration circuit, the change of state from
liquid to vapor is achieved mechanically by pressurizing the liquid
at one end and forcing it through a small opening at another end.
Once the liquid comes out from the small opening, it expands into
vapor. The effect is similar to the spray you get when you restrict
the outlet of your garden hose. But water is not a refrigerant. It
does not feel cold because it does not change to a vapor.
Bad example. But you get the idea? Pressurized restriction causes
spray at the other end.
In many refrigeration circuits, the small opening is made in the
form of a thermostatic expansion valve. Many other designs make
use of small capillary tubes. In big chillers, orifice plates are
common.
All of them achieve the same purpose - to expand the liquid to
become vapor and to cause the evaporator tubes to become cold.
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