Topic 4: Refrigeration
Refrigerators are so much a part of our everyday lives -- some 99.5% of American homes have a refrigerator -- that it's hard to remember they're a relatively new technology. The first home refrigerators appeared in 1911, and it was only after World War II that mass production of refrigerators really took off. This commonplace appliance owes its existence to centuries of low-temperature physics.
Refrigeration, of course, doesn't require temperatures near absolute zero, but the methods used to keep food cold rely on a detailed understanding of how to remove heat from any given substance. That last sentence is important -- refrigerators keep your food cold not because there is a mechanism to pump cold air in but because there is a mechanism to take hot air out.
To understand how a refrigerator works, one has to understand several important things about temperature. For one thing, if two areas of different temperature are in contact with each other, warmth from the hotter area will always flow towards the colder area -- which means the hot area will get colder and the cold area will get hotter. (This concept is known as the Second Law of Thermodynamics.)
The second thing you need to know is that there are other things that can cause changes in temperature -- evaporation and compression, for example. You've probably noticed the cooling effect of evaporation when you get out of a hot shower. The water on your body evaporates into the air and since evaporation requires heat, it takes the heat from your body -- making you feel colder. You can experience temperature change due to compression
by examining a can of aerosol deodorant. The contents of the bottle are under pressure, or compressed. When you spray some of the deodorant onto your body it feels very cold because the compressed gas expands and loses heat.
A refrigerator uses evaporation and compression of special liquids to cool your food. The liquid -- or refrigerant -- starts its journey going through an expansion valve into the evaporator coil. Here the liquid expands dramatically and evaporates into a gas, becoming icy cold. This cold refrigerant then travels through a set of coils in the back of the refrigerator. Heat from the inside of the fridge travels into the cold coils, which makes the refrigerator -- and the food -- colder and the refrigerant gas warmer.
Now the refrigerant needs to expel its heat into the outside air, but that's a problem since even though the gas is now warmer than the inside of the refrigerator it's still colder than room temperature. Since heat only ever travels toward an area that's colder, there's no way for the refrigerant to transfer its heat to the outside, warmer world. So the refrigerator has to make the gas even hotter, which it does by compressing the gas in something called a compressor. Just like the pressurized contents of an aerosol can, compressing the refrigerant makes it even hotter -- hotter than the outside air temperature. Last, the hot gas funnels its way to a condenser, which is in the back of the refrigerator and in contact with the outside air. Here, finally, the heat flows away from the refrigerator into the surrounding room. (This explains why the back of the refrigerator always feels warm!) When the refrigerant gas loses heat, it becomes cold again, condensing back into a liquid. Once in liquid form, it moves back to the expansion valve and starts the whole process over again.
Additional hands on activities
The role of gases in refrigeration
Two really good and comprehensive refrigeration lesson plans:
- Topic 1: Measuring the Cold - Thermometers
- Topic 2: Understanding Heat and Energy
- Topic 3: States of Matter
- Topic 4: Refrigeration
- Topic 5: Cryogenics
- Topic 6: The Quest for Absolute Zero
- Topic 7: How Animals Survive the Cold
- Topic 8: Superconductivity
- Topic 9: Astronomy
- Topic 10: Spaceflight
- Topic 11: Agriculture
- Topic 12: Cold Medicine