Refrigerator is one invention that has certainly changed the way we live. It has made it possible for everyone to preserve food for days together. The cold temperature inside the refrigerator slows down the bacterial growth in food and thus preserves the food for longer. But how exactly does the inside of the inside of the refrigerator remain cold?
The principle on how does a refrigerator work is pretty simple. Gases get hotter when you compress them into less volume because you have to work to push their energetic molecules together. When you expand a gas, it is suddenly able to occupy much more volume. The heat energy its molecules contain is now divided over a much bigger volume of space, so the temperature of the gas falls (it gets cooler). The other principle at work in a refrigerator is that when you have two things that are different temperatures that touch or are near each other, the hotter surface cools and the colder surface warms up. This is a law of physics called the Second Law of Thermodynamics.
Fig. 1 Vapour Compression Cycle – A: hot compartment (kitchen), B: cold compartment (refrigerator box), I: insulation, 1: Condenser, 2: Expansion valve, 3: Evaporator unit, 4: Compressor
A special gas called the refrigerant is used in the vapour compression cycle of a refrigerator. It used to be CFC (chlorofluorocarbon). But due to environmental concerns with CFC, namely, the depletion of ozone layer, the gas used nowadays is HFC-134a, also called tetrafluoroethane. The refrigerant passes through the various components of the refrigeration cycle listed in the figures 1 and 2.
Fig. 2 Vapour compression cycle or refrigeration cycle
Here is how the components of the refrigeration cycle interact to cool your food:
1. The compressor constricts the refrigerant vapor, raising its pressure and temperature, and pushes it into the coils of the condenser on the outside of the refrigerator.
2. When the hot gas in the coils of the condenser meets the cooler air temperature of the kitchen, it becomes a liquid.
3. Now in liquid form at high pressure, the refrigerant cools down as it flows through the expansion valve into the evaporator coils inside the freezer and the fridge.
4. The refrigerant absorbs the heat inside the fridge when it flows through the evaporator coils, cooling down the air inside the fridge.
5. Last, the refrigerant evaporates to a gas due to raised temperature, and then flows back to the compressor, where the cycle starts all over again.
The main component of a refrigerator that needs power is the compressor. It is essentially a pump which is driven by a motor. The hum you hear when the fridge is on is that of the compressor working. The thermostat controls the temperature of the fridge by switching on-and-off the compressor.