How Fridges Work

The fridge is an incredible invention which completely transformed the way we live. Instead of relying solely on fresh ingredients, the fridge has made it possible to preserve food for days — some even for weeks or months. This is all thanks to the low and controllable temperature.

By keeping food chilled, the fridge slows down bacterial growth, keeping food fresh and safe for longer than it ever could be at room temperature. But how exactly does the inside of the fridge stay cold? The principle behind how fridges work is actually pretty straightforward.

Fridge vapour compression cycle
The primary foundation of how fridges work is based on gases.

When compressed into a small space, gases get hotter. This is because there is work involved in pushing their energy molecules together. When expanded into a larger space, gases get cooler. This is because the gas is able to occupy more volume and the energy molecules are spread out over a wider area.

The other principle at work involves the Second Law of Thermodynamics. When two things with different temperatures are close to each other or touch, the hotter surface cools down, while the cooler surface warms up. This creates a vapour compression cycle.

The image above is a visual example of how this works.

A: hot kitchen
B: cold fridge
I: insulation
1: condenser
2: expansion valve
3. evaporator unit
4. compressor
Fridge cycle

In the vapour compression cycle of a fridge, a special gas known as a “refrigerant” is used. Years ago, it used to be CFC (chlorofluorocarbon). But we now know this gas is potentially harmful to the ozone layer. Today we use a more environmentally-friendly gas known as HFC-134a (tetrafluoroethane).

The refrigerant gas passes through the fridge cycle components named in the two images on this page.

Although it’s incredibly effective and efficient, the principle is remarkably simple. It was first used by Oliver Evans in the 1800s and is the same refrigeration standard we use today.

This is how the components of the refrigeration cycle work to cool your food:

1. The compressor restricts the refrigerant vapor. This raises its pressure and temperature, driving it into the coils of the condenser on the outside of the fridge.

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 fridge.

4. The refrigerant absorbs the heat inside the fridge as it flows through the evaporator coils, cooling down the air inside the fridge.

5. Finally, the refrigerant evaporates and becomes a gas due to raised temperature. It then flows back to the compressor, where the cycle begins again.

The main component of a refrigerator which requires power is the compressor. This element is essentially a pump driven by a motor. The hum you hear when your fridge is plugged in is the sound of the compressor working. The thermostat controls the temperature of the fridge by switching the compressor on and off.