It is a marvel of our times, and yet it is
so easily overlooked. There’s hardly a house or dwelling without one, and when
you walk into stores, the walls are lined with them. It’s almost the first
thing a new homeowner buys, and yet its only noticed when its noisy, or it
stops working. It’s that wonderful white slab that adorns the wall – the
refrigerator.
Refrigeration changed the world, and yet we
hardly give it a second thought. Before refrigeration, or about 100 years ago,
we had to make do with ice. Since 1100 BCE, the Chinese used underground
icehouses. Donkeys transported ice from the Alps to the Roman emperors. Boats
shipped snow to Istanbul during the 16th century. By the 19th
century, snow was cut from the lakes in North America for commercial use, and
transported to places as far away as India.
Ice production became an industry unto
itself. Railroads with refrigerated cars crossed the wealthy nations, and ice
wagons were a familiar sight on the streets. But ice had a fundamental problem
– it could be polluted, and that posed health hazards. That meant the time was
right for someone to discover mechanically manufactured ice – or mechanical
refrigeration. And by applying the discoveries made by Robert Boyle, and tested
by Michael Faraday, inventors across the globe set themselves to the task. And
succeeded.
Our lives would be much different if not
for refrigeration. Before refrigeration, everyone had to rely on local foods
produced – and dried meats. A steady diet of fruit would be all but impossible,
after all – fruit is kept chilled though all through its myriad supply chains
on the way to your home. Fish? Inland? Impossible without being frozen just
after being caught. Butter? You’d just have a sloppy mess to spread on your
bread. Refrigeration supresses the growth of food-borne bacteria, so you are
unlikely today to get as sick from food as you would even a few generations
ago.
The availability of refrigeration means
fresh meat and produce is available almost across the globe, and is a lifesaver
to farmers, who can sell their produce quite far afield. We have a much greater
variety of food in our diet, and yet so few people understand the basic
principles on which it operates.
It’s well-known that a calorie is the
energy needed to raise the temperature of 1 gram of water by 1°C. (The calorie
used in food is actually a kilocalorie, just so you know.) And it makes sense
that if you mix 100g of liquid water at 100°C and 100g liquid water at 0°C,
that you’d have a mixture of water at 50°C after you stir it. However, if you
mix 100g of liquid water at 100°C and 100g of solid water (ice) at 0°C, and
stir it, you’d find to your surprise that the temperature of the mixture after
is only 10°C. In fact, you’d find that if you heat a mixture of ice and water
at 0°C, the temperature would not rise above 0°C until all the ice is melted.
The water molecules in ice are, in fact,
bound together by strong attractive forces (electromagnetic, or electro-nuclear
forces) that keep it a solid. Energy is required to counter these forces, and
thus extra energy is needed to break apart those bonds. For water to turn into
ice, it needs to lose 80 calories of energy per gram, and conversely, for ice
to melt, it needs to absorb 80 calories per gram. This is called the ‘latent
heat of fusion,’ as there is another type of latent heat as well. When water
turns to steam, a similar effect occurs – water remains at 100°C until 539
calories of energy per gram is added, and then it turns to steam. This is
called the ‘latent heat of vaporisation.’
Thus the ‘latent heat of vaporisation’ uses
or releases quite a bit more energy in the conversion between liquids and gas.
Gases, however, have another property that is quite useful. Boyle’s Law states
that the volume of a gas is inversely proportional to the pressure. When enough
pressure is applied to a gas, the molecules come in closer and closer contact,
until they revert to a liquid.
Suppose a gas, such as ammonia, or isobutane,
is placed under pressure in a closed container. If the pressure is high enough,
it will turn into a liquid and lose the latent heat of vaporisation. If this
ammonia is in a free-flow environment, with liquid water or simply air flowing
around it, the heat will be absorbed by the water or air, and the liquid
ammonia would be no warmer than the gas had been.
Now suppose this ammonia or isobutane flows
from this location to another one, and the pressure is decreased so that the
ammonia boils and again becomes a gas. It needs to absorb an amount of energy,
or heat, in the same amount it had lost before. It will absorb this heat from
the nearest source – itself and its neighbours. The temperature would drop! Now
imagine this cycle is made part of some sort of device that alternately
compresses it and allows it to evaporate – you would have developed a heat
pump! Finally, you place one part of the machine inside a heat-isolated box,
and voila! You have a refrigerator.
Refrigeration is not only important in
farming, to prevent post-harvest and post-slaughter losses, or for maintaining
food safety. It has spread to many other industries. Air conditioning works on
the same principles, and is vital in hot countries such as ours. Medicines and
vaccines are kept refrigerated, and treatments such as cryosurgery and
cryotherapy depend on it. It is vital to keep machines cool during
manufacturing, and all computer systems depend on heat pumps to stay operational.
CERN’s Large Hadron Collider, which is investigating the sub-atomic particles,
would be impossible to operate without it.
So next time you walk into the kitchen, and
open that door, feeling the wash of cold air over your face, take a moment. Remember
you are standing in front of a marvel of the modern age. Refrigeration has
enabled humanity to solve large problems in food distribution, and the billions
of people who can eat every day depend on it. And yet it is so ubiquitous that
you hardly give it a second thought.
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