Why We Always Leave Some Room Between the Fridge and the Wall

The Second Law of Thermodynamics says that heat will always flow from a warm object to a cold object. So how do we keep the inside of the refrigerator cool? As usual, our first thought was gnomes. We don't know how they'd do, just that they would. Here in reality, though, we need to use a heat pump to take heat out.

Refrigerators move heat from the inside of the refrigerator and dump it outside into the kitchen. Of course, the universe doesn't favor this course of action, since heat spontaneously flows from warm to cold (and not the other way around). So how do we appease the universe into keeping our drinks cold?

Instead moving heat from a warm reservoir to a cold reservoir to do work (like a heat engine does), work has to be put into a refrigerator to move heat from a cold reservoir to a warm reservoir. What kind of work can a refrigerator do, though, to keep our food cold? It's not holding a 9-to-5 job and bribing the universe with its pay…is it?

Nah. What it actually does is circulate a coolant between the inside and the outside of the refrigerator. If you circulate a gas that is 5 °C from the inside of the fridge to the outside, it will be able to absorb excess heat from inside the fridge, but it won't dump it outside of the fridge—because again, heat doesn't move from a 5 °C gas to the 20 °C air in the kitchen willingly.

To transfer heat from the inside of the fridge to the kitchen air, the temperature of the circulating gas must be cooler than the inside of the fridge when it is inside the fridge, and then hotter than the kitchen air when it is outside of the fridge. That's the heat pump's job.

So how does the heat pump do that? By compressing and decompressing the gas. When a gas is compressed, it heats up. When allowed to expand, it cools down. PV = nRT rears its head again. Now let's take a tour of what's happening inside of a refrigerator to see how it all works.

Inside of the refrigerator, the coolant is under low pressure. This causes the gas to contract and cool down. It cools down enough so that the inside of the refrigerator acts as a hot reservoir. The excess heat in the refrigerator packs its bags and moves into the coolant, cooling down the refrigerator.

The coolant next travels through the coils to the outside of the refrigerator. But the air outside of the fridge is warmer (at least, if it's doing its job). If we just let the coolant circulate, the kitchen air would heat the coolant, instead of the coolant dumping heat from inside the fridge into the kitchen air. That sounds like a one step backwards, two steps even more backwards kind of situation.

To get the coolant to release the extra heat from the fridge rather than circulate it right back into the fridge, we have to do work on the coolant. This work is done by the refrigerator's compressor. It takes the coolant gas that is carrying the excess fridge heat and just smashes and smooshes and compresses it down. The compressor is in the business of compressing, and business is good.

This compression does more than provide a sense of fulfillment for the compressor; it also heats up the coolant. The compressor compresses the coolant so much that it becomes hotter than the kitchen air. Heat then moves from the coolant into the kitchen. Mission accomplished.

Then the coolant is pumped back into the fridge, under low pressure, expanding it, which in turn cools it back down below the interior of the fridge temperature and the cycle repeats.

Thermodynamics—keeping things frosty.