Lets face it. There's probably not a day that goes by that you don't venture to the frig. It's an essential to everyday life that we all take for granted. But have you ever thought how exactly your frig works? You might have thought 'oh it just works the same as my AC to cool the room down'. NO! Your frig does not behave the same way an AC does. In fact, if you were to leave the frig door open for a long time your frig would actually start to heat up and in result act as a heater for your apartment.
So? How does it work? I think another interesting question is how were people able to preserve food prior to the invention of refrigerators. Here are two of the best options people had back in the day: 1) They would take the food item most likely to go bad quickly and preserve them in wells, rivers, bottom of lakes ,etc. 2) Foods eaten routinely were spiced, heavily salted(not the healthiest I know), canned or dried to prevent bacterial growth.
Luckily, as be began to understand the thermodynamics cycle of heat exchange, pumps and valves we the earthlings learned to keep food at an arbitrary temperature inside a container. So lets begin.
What is the basic idea of a refrigerator? It removes heat from a compartment and pumps it to the outside. I want you to take a look at figures 1 and 2 below and study them for a little as I will be referring to them in my explanation. Notice the four main devices: compressor, external radiator, expansion valve, and an internal radiator.
Here's how they work:
- A pump that compresses gas
- Valve that expands gas
- An external and internal radiator that for the gas to exchange heat with its respected environment
First, lets talk about the compressor pump. Lets take pumping up a soccer ball as an example. Say the compressor is a normal pump and the external(outside) radiator is the inside of a soccer ball. What are the two main things that happen to a soccer ball just after you finish? 1) The pressure of the inside air will keep the ball inflated and 2) The temperature of the air inside the ball will be greater than compared to outside. The compressor does exactly that but it pumps gas through pipes producing a high gas pressure which lead to high temperature gas.
Second, lets discuss the expander valve that expands the gas to cool it down. This is a tricky concept so put your thinking cap on. Take a look below at figure 3.
Notice the top valve is compressed with high pressure high temperature while the bottom valve expands lowering the gas pressure and temperature. In general, with thermodynamics when gas is compressed the gas pressure goes up resulting in the temperature raising and when gas expands the pressure lowers resulting in the temperature lowering.
Third is the external and internal radiators. You can see above in figures 1 and 2 the gas in the internal radiator absorbs the heat up to a certain point before the compressor is turned on and the gas is pumped out. At the external radiator and hot and high pressured gas from the compressor cools down to room temperature.
Awesome so now we under the different components to a refrigerator lets put it all together. But first go ahead and observe figure 4 below to refresh your memory.
Here is how the cooling cycle works when all put together
You come back from the grocery store and place the food inside the frig. As a result the temperature inside the frig raises since the food is a greater temperature.
The raising air temperature inside the frig then donates heat to the cold gas inside the internal radiator. Since the gas is getting warmer the thermostat turns on the compressor which pumps the gas outside and into the compressor.
The gas is then compressed and pumped outwards at a high temperature and high pressure.
After exiting the compressor the gas then cools down at the external radiator to room temperature while maintaining its high pressure.
The gas will then flow to the expansion valve where the high pressured gas is allowed to expand(lowers gas pressure) thus lowering its temperature significantly.
By repeating this thermodynamic cycle we are able to consistently remove heat from inside the refrigerator. This process is turned off/on by a thermostat monitoring the temperature inside.
Hope you enjoyed this article and stay tuned for more!
Quick challenge: Can you relate the first law of thermodynamics to this article?