Fridges and freezers are some of the highest energy consumers in the average household. Day and night, they work to keep tiny, separate rooms below freezing.
To make these mini rooms cold, they use energy not to cool anything down – but instead to heat something up. In some portable and older versions of fridge technology, this is literally true: they use actual fire. But modern household cooling appliances go about it in a much more interesting way…
Vapour Compression Cooling
All modern static coolers operate on the vapour compression refrigeration cycle. Before we look at just what that means, let’s see where it comes from.
• In 1805, Oliver Evans designed the first of its kind
• In 1834, Jacob Perkins obtained the first patent
• In 1840, 35 years after the foundation of modern refrigeration was developed, crude, basic Iceboxes became popular – keeping their contents cool by supplying chunks of ice.
• In 1856, James Harrison patented a specific variant, and put it into commercial use for the first time: full rooms for brewing and meat-packing
• In 1867, Thaddeus S. C. Lowe used another variant for transportation, keeping frozen meat frozen in steamship cargo.
• In 1913, the first electric fridge for household use entered the market
• In 1926, General Electric Company introduced a hermetic (completely airtight components) compressor fridge, the first of its kind.
• In 1929, after multiple deaths as a result of leaking compression vapour, research began to develop safer vapour, leading to the discovery of Freon.
• In 1950, over 90% of American homes contained an electric fridge, while only 2% of UK homes could say the same. The UK didn’t catch up until a few decades later.
• In 1970, while Freon saved many lives, it posed a significant, direct threat to the atmosphere. New, more energy efficient materials (like R134a) were developed, with even better alternatives being introduced to this day and the future.
It took around 150 years for this technology to become commonplace.
So how do they work?
The physics behind them are deliciously simple. First, the ingredients:
• Two condensers: coiled or snaking ‘S’ pipes – essentially radiator panels.
• A pin-hole connecting both condensers together.
• A refrigerant with a low boiling point to push through these pipes.
• A compressor that takes in low-pressure vapour and outputs high-pressure vapour.
Next, some preparation:
• One condenser is placed on the inside of the cool box. The other is placed on the outside, connected to the inside through that pin-hole.
• Connecting the condensers on their other ends is the compressor.
• Fill the system with the refrigerant.
Turn it on, wait a bit, et voila – you now have a refrigerator!
Okay, okay, how does it actually work, though?
The first thing to keep in mind is that this is a closed system. That is, in normal operation, nothing gets in and nothing gets out. The only thing being put into the system is the energy going into the compressor.
This also means that it’s kind of impossible to say where the process ‘starts’; it’s a circular process – does a circle really begin anywhere? So we’ll start at the business end: the compressor.
The compressor doesn’t push cold vapour into the fridge, it pushes cold vapour out of the fridge by compressing it into super heated vapour.
The process is analogous to fire. Imagine having 10 bonfires scattered around a field. The compressor brings all of these bonfires together into one tight pyre. The ‘pressure’ of the fires is now 10 times greater, and the fire rages brighter.
Now lets suppose we want to extinguish this fire. It’s much easier to quench 1 than 10.
The principle here is similar. As this super heated vapour passes through the outside condenser it radiates the heat away. The hotter it is compared to the outside air, the quicker the vapour cools. By the same principle literally, you’ll notice that a hot tea will fall to a lower temperature if you wait 10 minutes before adding the cold milk. And the hotter the water passing through a radiator, the quicker the room will heat up.
By the time the vapour reaches the end of the condenser, it liquefies. It’s near or at room temperature now. But that’s no good. If we let this into the next condenser now, it will just make the fridge room temperature. Not useful.
Thankfully, when this high-pressure liquid/vapour hits that tiny pin-hole between them, it explodes (technically speaking) that high pressure into much lower pressure. This very rapidly cools the vapour to below freezing.
Similar analogously to the difference between a hot tap and the same hot tap spraying out a mist instead of a stream.
So in summary: it pressurises refrigerant to increase its temperature before cooling it down to room temperature, then depressurises it rapidly to cool it drastically below room temperature. Cool and simple!
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