The Expansion Valve: You Have To Expand That Liquid
The expansion valve is where the cooling actually takes place. As the refrigerant travels through the valve, it expands. The ideal gas law defines why the fluid cools while it expands. The ideal gas law states:
This equation can be rearranged to make:
Let's imagine two systems. System one has a pressure of 10 psi and a volume of 10 cubic feet. System two has a pressure of 10 psi and a volume of 20 cubic feet. With the ideal gas constant, the number of moles, and the pressure between the two systems being the same, we can use the rearranged ideal gas law to compare the temperati of the two systems.
Substituting in values:
This shows that as the liquid expands to twice it's volume the temperature is halved. This means that as the refrigerant passes through the expansion valve in our refrigerators it's temperature is drastically reduced. When the liquid refrigerant hits the low pressure side of the expansion valve it passes through the evaporator starting at the freezer, where it’s coldest, then through the refrigerator. While this liquid is passing through the evaporator it absorbs heat and begins to boil, changing phase to gas. It is because of this phase change that the insulated compartments are cooled.
The equation above can be used to calculate the efficiency of the refrigeration system and is a ratio of the heat absorbed to the inlet pressure.
