How Does Your Refrigerator Refrigerate? - Part One

By Earl Hunsinger

How does your refrigerator work? How does your air conditioner work? Although these common household appliances look different, they operate in the same way. The operation of most modern refrigeration and air conditioning equipment is based on three basic scientific principles. Wait! Don’t be afraid because I used the word scientific, these are things that you probably already understand.

The first principle involves the transfer of heat. We could discuss entropy and the laws of thermodynamics, but we’re not going to do that. That isn’t really necessary to understand this concept. It is one that we are all familiar with, whether we have any interest in science or not. If you take your supper off the stove but don’t eat it right away, it gets cold. If you leave the milk out on the counter, it gets warm. Actually, your supper and your milk would become the same temperature, the temperature of the room. Because your supper is hotter than the room, heat energy moves from it into the room. Because you milk is colder than the room, heat energy moves from the room into the milk. This movement of heat energy affects the objects involved, your supper or milk, changing their temperatures. This concept of moving heat has a direct bearing on our lives. In the winter, we move heat from a fire, or a radiator, or an electric heater into our house, changing its temperature. In summer, we want to do the opposite, move heat from our house to somewhere else (we don’t really care where), again changing the temperature of our house.

The second principle that we need to understand is that it takes heat to change something from a liquid to a gas (to make it boil or evaporate). Technically this is called the latent heat of vaporization, but it is a principle that anyone that has ever boiled water understands. When you put a pot of water on the stove and turn on the burner, two things happen. First, as you add heat to the water, the water gets hotter. Eventually, the water reaches its boiling point and begins to boil. Now, no matter how much heat we add, the water will not get any hotter. It can’t get hotter because any heat that we add is being used to change it from a liquid to a gas (water to steam). The same principle also applies to other liquids. Although they may have a boiling point that is higher or lower than water, when they reach that boiling point, they boil or evaporate when we add more heat.

The third principle is related to the second. The boiling point of water, or any liquid, changes with a change in pressure. Many cooks have learned this the hard way. For example, water boils at 212 degrees Fahrenheit (100 degrees Celsius), at sea level. However, on the top of Mount Everest, water will boil at 156.2 degrees Fahrenheit (69 degrees Celsius), making it harder to cook something in boiling water. The opposite is also true; an increase in pressure will increase the boiling point. This is why cooks use pressure cookers. As the water in the pressure cooker boils, the steam created has nowhere to go. As more heat is added, more steam is created and a larger quantity of steam has to squeeze into the limited volume of the pressure cooker. The increased pressure created by the captured steam increases the boiling point of the remaining water. Of course, the good news for the cook (and those waiting for supper), is that the higher cooking temperature in the pot causes the contents to cook faster, which was the point of using the pressure cooker in the first place. The point to remember is that the boiling, or evaporation, point of a liquid is not fixed at an unchangeable temperature. It increases with an increase in pressure and decreases with a decrease in pressure.

Modern refrigeration equipment makes use of these principles. It could use water to cool things off. If you ever ran through the sprinklers as a child or went for a swim as an adult (or vice versa) you know how well this works. As the water evaporates from your skin on a hot summer day, you feel cooler. This is because it takes heat to change the water from a liquid to a gas. This heat comes from your skin. (Perspiration is supposed to work the same way, but somehow swimming pool water seems to work better.)

Instead of using water, most refrigeration equipment uses a chemical with a very low boiling point. This chemical, or refrigerant, passes through four basic components, moving heat from one place to another along the way. These basic components are the condenser, metering device, evaporator, and compressor. Auxiliary devices may also be added for safety, control, or to increase efficiency, but these four form a basic refrigerator or air conditioner.

In a future article, I’ll describe how each of these components functions and how they make use of the three basic principles discussed here to cool your room or the food in your refrigerator.