Prior to the invention of refrigerators, food preservation imposed serious constraints. The best options were to suspend food in rivers, wells, or at the bottom of lakes, and absent these tactics, stable foods were heavily salted, spiced, pickled, 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 with the refrigerator cycle.
[spoiler title=’Question #1′ style=’blue’ collapse_link=’no’]
What is the basic idea behind a refrigerator?
- It makes cold air and injects it into the compartment.
- It sucks in cold air from the outside and pumps it into the compartment.
- It removes heat from the compartment and pumps it to the outside.
[spoiler2 title=’Answer’ style=’red’ collapse_link=’no’]It removes heat from the compartment and pumps it to the outside[/spoiler2]
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 behaviour 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.
[spoiler title=’Question #2′ style=’blue’ collapse_link=’no’]
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? Did it take work to operate the pump? Where did that energy go?
- It’s colder.
- It’s the same temperature.
- It’s warmer.
[spoiler2 title=’Answer’ style=’red’ collapse_link=’no’]It’s warmer[/spoiler2]
An analogy for what happens to gas at the expansion valve is what happens to air that leaks from a balloon.
[spoiler title=’Question #3′ style=’blue’ collapse_link=’no’]
Suppose you poke a small hole in a filled balloon of temperature [math]$T$[/math] giving rise to a stream of escaping air. How does the temperature of the airstream [math]$T_s$[/math] compare to [math]$T$[/math]?
- [math]$T < T_s$[/math]
- [math]$T = T_s$[/math]
- [math]$T > T_s$[/math]
[spoiler2 title=’Answer’ style=’red’ collapse_link=’no’][math]$T < T_s$[/math][/spoiler2]
[spoiler title=’Question #4′ style=’blue’ collapse_link=’no’]
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 40°C and place it in a room that’s 30°C, approximately what temperature will the air in the balloon have after a long time?
[spoiler2 title=’Answer’ style=’red’ collapse_link=’no’]30°C[/spoiler2]
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.
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 at 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.
[spoiler title=’Question #5′ style=’blue’ collapse_link=’no’]
Suppose a closed refrigerator is sitting in a room for a long time, and the temperature of the room has settled. What would happen to the temperature of the room—in the long run—if the refrigerator door were left open?
- It would go down
- It would stay the same
- It would go up
[spoiler2 title=’Answer’ style=’red’ collapse_link=’no’]It would stay the same[/spoiler2]
In story above, 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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