Years ago I read the book World War Z: An Oral History of the Zombie War by Max Brooks. (It was a great book – much better than the movie.) In the book, the front lines were staffed with all the expendable people: the investment bankers, hedge fund managers, lawyers and the like. The important people were kept far from the front lines: the carpenters, electricians, plumbers and engineers. They were important because they could keep the country running and rebuild when and if the zombies were defeated. They knew how stuff worked. That concept has stuck with me over the years. I would be on the front lines – an expendable person – because I don’t know how things work.

So, how does refrigeration work? It cools air inside by creating hot air on the outside. Crazy! More on how it does that below.

A few basic concepts first

• Heat: occurs when molecules move around and bump into each other. More molecular movement equals more heat.
• Cold: is the absence of heat – molecules are moving slower than something hotter
• Pressure:  At a steady volume, rising heat creates more pressure. The converse is true as well, increasing pressure creates heat. This is called Gay-Lussac’s Law. Thus expanding gas cools and compressing gas heats up.
• Heat Transfer: The Second Law of Thermodynamics provides that heat will spontaneously flow from hot to cold. Over time something hot and something colder within proximity will approach the same temperature as heat transfers from the hot substance.
• Evaporation: Liquids, as they heat, evaporate into gas.
• Condensation: Gases, as they cool, condense into liquids.
• Refrigerant: is a fluid used in a refrigeration cycle which has some desirable thermodynamic properties such as the ability to absorb heat by evaporating at a low temperature and which gives up heat by condensing at high temperature.

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The refrigeration cycle

The refrigeration cycle works pretty simply using the concepts outlined above. Basically, in refrigeration the system removes heat from the desired area through heat transfer and deposits it elsewhere.

A key point: the term “cooling” is a misnomer because in order to cool something, it must actually be “de-hotted” – cooling occurs by removing or transferring heat away – not by moving “cold” to the hot item. Thus, when you put a warm beer in a cooler of ice the ice does not cool the beer, instead, the bottle of warm beer transfers heat to the ice which causes the beer to “de-heat” and cool down. The ice, of course, warms and melts.

The refrigeration system removes heat from an area that is low-pressure, low temperature (evaporator) into an area of high-pressure, high temperature (condenser).

1. Compressor. Starting at the bottom of the above diagram: The refrigerant, at this point a vapor, goes through the compressor which uses a motor to compress the refrigerant into a smaller volume. Compressing a substance increases pressure which increases heat. Thus, after moving through the compressor the refrigerant is very hot – usually at least 20 degrees hotter than the air that surrounds the condenser.

2. Condenser. The hot, pressurized refrigerant moves into the condensing unit. Because the refrigerant is hotter than the surrounding air, heat moves from the refrigerant to the air. As the heat leaves the refrigerant, it cools and condenses back into a liquid.

Home air conditioner condenser units (also contains the compressor)

Back of refrigerator condensing unit

If the air behind your refrigerator is 75 degrees and the refrigerant comes into the condensing unit as a hot vapor at 110 degrees, heat will move to air from the hot refrigerant, which will cool and condense into a liquid. Note a few things – first the refrigerant is still quite warm, it is not anywhere near the temperature needed to cool your food or your house – second, heat has been removed from the system by the refrigerant cooling from really hot to less hot.

3. Expansion Valve. The first two steps above have been at high pressure. Next the condensed liquid will move to an expansion valve that causes the pressure of the refrigerant to drop. As mentioned in concepts above, as a substance goes from high pressure to low pressure it cools. This is the most amazing part of refrigeration: the liquid refrigerant entering the valve is quite hot and when it leaves the valve it is cold. This occurs merely by changing the pressure of the refrigerant.

4. Evaporator. The cold liquid refrigerant then flows into the evaporator at low pressure. Then the relatively warm air inside the refrigerator or air conditioner (or water if its an ice maker) flows past the evaporator and again heat transfer occurs: the heat moves from the air (or water) to the refrigerant which causes the refrigerant to heat up and evaporate back into a vapor. At this point it enters the condenser to be pressurized which heats the refrigerant and the whole cycle starts over.

Evaporator coil for a home central air conditioning unit

An evaporator coil in a freezer