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Physics of the Everyday

# How does a refrigerator work?

Prior to the invention of refrigerators, food preservation imposed serious constraints. The best options were to suspend perishable items in rivers, wells, or at the bottom of lakes. Absent these tactics, staple foods were heavily salted, spiced, pickled, canned, or dried to prevent bacterial growth.

With the industrial age, we began to understand thermodynamic cycles and—through an interplay of pumps, valves, and heat exchange—learned to cool food to arbitrary temperatures using the refrigerator cycle.

In this quiz, we’ll learn about its three crucial components and understand how they come together to produce the refrigerator’s cooling cycle.

What is the basic idea behind a refrigerator?

We just saw that the refrigerator carries out a cycle to move heat out from the food compartment and into the room.

This occurs via three main components:

• a pump that compresses gas
• a valve that expands gas
• radiators that allow gas to exchange heat with the surroundings.

We’ll now understand the behavior of each of these components by analogy with everyday phenomena, starting with the compressor pump.

An analogy for the action of the compressor pump is the act of pumping up a basketball. Suppose you use a hand pump to quickly fill a basketball with air.

Just after you finish, how does the temperature of the air in the basketball compare to that of the outside?

An analogy for what happens to gas at the expansion valve is what happens to air in an expanding piston.

Suppose you set up a piston that is filled with dense gas and that the piston arm can expand into a bed of soft clay. You observe the the piston arm expanding into the clay, quickly at first, before coming to rest.

How does the temperature of the gas in the piston before the expansion, $$T_\textrm{before}$$, compare to the temperature after, $$T_\textrm{after}?$$

Finally, we consider a hot balloon in a cold room, an analogy for what happens to gas in a radiator. Suppose you fill a balloon with air that is $$\SI{40}{\celsius}$$ and place it in a room that's $$\SI{30}{\celsius}$$.

Approximately what temperature will the air in the balloon have after a long time?

Assume that the volume of the room is far greater than that of the balloon.

We’ve seen how the basic devices of the refrigerator work in analogy with common experience.

• The compressor pump heats gas and raises its pressure.
• The expansion valve drops the temperature of gas as well as its pressure.
• The radiator allows gas to come to thermal equilibrium with the surrounding environment.

We'll now show how this set of phenomena can be used in conjunction to cool the contents of a refrigerator.

Which of the following arrangements would make the air at the outlet colder than the air at the inlet?

We now have all the pieces in place to understand how the refrigerator's cooling cycle works.

• When we put food into a refrigerator for the first time it is at a greater temperature than the air inside—this raises the temperature of the air inside the refrigerator.
• The air inside the refrigerator donates heat to the cold gas in the internal radiator, which carries the heat to the outside.
• This gas is then compressed at the pump, raising its temperature and pressure.
• After exiting the compressor, this gas cools to room temperature in the external radiator, maintaining its high pressure.
• At the expansion valve, this high pressure gas is allowed to expand, lowering its temperature significantly (below that of the refrigerator compartment).

By repeating this cycle, heat is iteratively removed from the inside of the refrigerator.

Note that the compressor pump isn't running all the time. Instead, it's controlled by a thermostat.

A thermostat turns the pump on when the temperature inside the refrigerator rises above a specific threshold level and turns the pump back off once the temperature drops below a target level.

Thus, the pump is engaged as needed.

Suppose a running, closed refrigerator is sitting in a room for a long time, so that the temperature of the room has settled. What would happen to the temperature of the room (after a long time) if the refrigerator door were left open?

In this quiz, we worked through the fundamental phenomena that underlie the workings of a refrigerator. We then applied this knowledge to understand the core components of a refrigerator and learned how they conspire to cool food.

In doing so, we've encountered some aspects of thermodynamics:

• The fast compression or expansion of gases is known as adiabatic compression and expansion.
• The process of taking a system through a series of steps in a loop is called a thermodynamic cycle.

These and related principles are the basis for a variety of applications, including the combustion engine, cooking, and maintaining a livable state on the International Space Station.

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