What goes on inside a refrigerator?

Have you tried closing the door of your refrigerator slowly just to see exactly when the light goes off? We have all been curious about this one thing. Although I was never able to watch until it was dark inside the fridge, I made many attempts at trying to understand how this was done. The secret behind this piece of sorcery is very simple. A push button switch is placed near the hinge of the fridge door. The button gets released when the door is opened and the light goes on. Now when you close the door, the button is pushed back to the off position only when the door closes totally. This simple piece of trickery is only a small part of the brilliant technology employed in a refrigerator.

Refrigerators are devices found in every modern household. They store food for days and weeks together. The principle behind food staying fresh in a refrigerator is that chemical reactions are slowed down in a cool environment. When food is left out in the open, it gets spoilt within a few hours because of bacteria acting upon it. However, inside a refrigerator, the temperature is maintained at around 50C which is much lower than the outside temperature (25-350C). Bacterial decomposition of food is essentially a chemical reaction. The speed of a chemical reaction almost doubles with every 100C rise in temperature! Thus food degrades at a much slower rate inside a refrigerator and can be safe for consumption over some days.

The question that now comes to our mind is: How do we keep a container at such a low temperature?

The whole idea of refrigeration is based upon the simple fact that when a liquid evaporates, it brings about cooling. If you have ever accidentally dropped aftershave lotion on your skin, you would have felt a chilling sensation while it evaporated. The aftershave lotion contains spirit which evaporates quickly off your arm taking away heat from your body  and making you feel a momentary chill. The same phenomenon is repeated when water evaporates, although at a slower speed than spirit. The simplest example of a refrigerator is the earthen pot one uses to store water at home. Water seeps out of the pores of the pot and evaporates from the surface eventually cooling the water inside.

In a refrigerator too, a liquid is evaporated and condensed over and over again to bring about cooling of the air inside. The liquid used is called the refrigerant. Traditionally liquid ammonia (NH3) was used as a refrigerant. It has been replaced by chloro-flouro carbons (CFC) over the years because of the potential hazard that ammonia can cause if inhaled in large quantities. The use of CFC as a refrigerant is also on the decline since it is one of the gases that deplete the ozone layer.

The refrigerant moves through a cycle of operations when it passes through a number of bent coils.

First the liquid is made to evaporate in a coil which takes away heat from the air inside the refrigerator. The air inside is cooled. Now the gaseous refrigerant has to be compressed into a liquid once again. This  is achieved through an electrically run compressor.

The temperature rises due to this operation and the hot gaseous refrigerant needs to be cooled in coils running at the back of the unit. As it cools the refrigerant condenses back to its liquid state. The refrigerant passes through a valve known as the expander and once again reaches the cooling coils at the cooling temperature (around -150C).

And that is how the air inside the refrigerator remains cool.

This piece of cooling technology has become indispensable for us. Feeling thirsty? Go grab a lemonade from the refrigerator then!