I was going to write this about x-rays, but I started reading those Wikipedia pages and quickly became overwhelmed. I mean, goodness gracious, who knew things taking pictures of the insides of our bodies could be so complicated?
Anyway, refrigerators are more my speed—x-rays for the common man, if you will.
I knew people used to just use blocks of ice in boxes to cool things—my hometown was an ice-mining town back in the day. Before they figured out how to do that, I’m assuming everything was just warm? No way to know.
Anyway, it turns out refrigerators don’t work how I thought they did. Meaning, they are not, in fact, magic.
The first guy to figure out the process of refrigeration was a Scottish chap named William Cullen. Cullen was not primarily a refrigerator. Instead, he was a doctor and an academic—his writings on chemistry made him well-known in his day. Still, in 1755, Cullen designed the first machine capable of refrigerating, using principles he demonstrated in 1756.
There’s a law out there (don’t worry, you aren’t breaking this law, because you literally can’t break this law) named after a guy named Boyle. Boyle’s Law says that if you decrease the volume of a quantity of a gas, the gas’ pressure will increase. In other words, if you squeeze a gas, it will push back—its pressure will rise. This is why you can pop a balloon by sitting on it: As you decrease the volume the gas inside can occupy, its pressure increases, until it pushes hard enough on the balloon itself to break through.
The same law states that if you decrease pressure on a gas, the gas’ volume will increase, which was important to what Cullen did.
Cullen took some liquid (diethyl ether, which is something I’m assuming we shouldn’t drink) and created a partial vacuum above it using a pump. Don’t ask me how this thing was set up. Wikipedia doesn’t have the answers to that. But that’s what the man did, and when he did it, the liquid started boiling. The lower pressure above the liquid led the liquid to try to expand, which required turning into a gas.
When the liquid boiled, it drew in energy from the world around it in the form of heat, dropping temperatures so low that some ice appeared. Why did this happen? Well, it turns out that substances take some energy to boil—decreasing the pressure isn’t enough. To get that energy, the substance takes heat from whatever surrounds it. In Cullen’s case, that heat came from the air surrounding his now-boiling liquid.
Refrigerators today, for the most part, use the same principles. A substance—called the refrigerant—has its pressure manipulated by a pair of valves: the compressor and the expansion valve. Let’s talk through what they do, referencing this handy diagram:
The compressor takes the refrigerant, at this point a gas, and (true to its name) compresses it, raising its pressure.
The refrigerant then passes through a device called the condenser, where the refrigerant gives off energy—so much energy that it turns into a liquid. The energy, released in the form of heat, comes out in some way through the outside of the refrigerator, which is why your fridge might have a place on the back that feels warm to the touch.
Once liquid, the refrigerant travels through the expansion valve, where pressure rapidly decreases and a portion of the liquid “flash evaporates,” meaning it boils instantly, sending the refrigerant to the final stage in the cycle as a mixture of liquid and gas (“final” given where we’ve chosen to start—the cycle continues as long as power is supplied to the system’s pump, which I’m under the impression is also the compressor).
On its way back to the compressor, the liquid/gas mixture passes through the evaporator, where it fully evaporates back into a gas. The energy it needs for this to happen—the same energy Cullen’s device took from the surrounding air—is taken from the air inside the refrigerator, which is being blown over the evaporator by a fan.
In short:
Compressor: Takes the refrigerant, at this point a gas, and raises the pressure.
Condenser: Lets the refrigerant release some energy, turning from a gas to a liquid and releasing heat.
Expansion Valve: Takes the refrigerant, at this point a liquid, and lowers the pressure.
Evaporator: Lets the refrigerant absorb energy, turning from a liquid to a gas and absorbing heat, thereby cooling the air inside the refrigerator.
This isn’t the only type of refrigerator, but we have rules here, and one of those rules involves not writing about multiple refrigeration systems in the same blog post. Also, this is how a lot of refrigerators work, so if yours is different and you must know how it works, ask someone else.
Wikipedia articles enlisted in this search for knowledge:
Refrigerator
Refrigeration
Timeline
of low-temperature technology
William Cullen
Boyle’s law
Enthalpy of
vaporization
