How Does Your Refrigerator Refrigerate? - Part Two

By Earl Hunsinger

In a previous article, I discussed the basic scientific principles used for a refrigerator or air conditioner. We’re all familiar with these principles, whether we have an interest in science or not. For example, we know that we can make our drink colder by adding something cold, like ice cubes. We know that it takes a lot of heat to change something from a liquid to a gas, like making steam by boiling water. And we know that if we increase the pressure, the boiling point will increase, which is what a pressure cooker does. How can we use these principles to cool our house, or our food?

Just as running through a garden hose would cool us off on a hot summer day, we could cool our house off in the same way, by just spraying water on the roof. As the water evaporated, it would cool the roof, which hopefully would make the inside of the house cooler, depending in large part on how much insulation is in the attic. A more efficient way of using water would be to blow hot air from the house through a water spray. The water would use some of the heat in the air to evaporate, cooling the air. Because it increases the amount of humidity in the air, this kind of air conditioning is sometimes called a swamp cooler. It is also called an evaporative cooler. It is most effective, and most often used, in areas with very low humidity, like deserts.

Instead of using water, most of us use air conditioners (and refrigerators, which are just air conditioned boxes for your food) that contain a chemical refrigerant. A refrigerant is just a fancy name for a substance that is used to provide cooling. Most people think of Freon when they think of refrigerants. This probably makes Dupont feel good, since Freon is just their trade name for the chemical refrigerants that they sell. The same chemicals can be purchased from other companies under the names Genetron, Isotron, Arcton, etc. Regardless of who makes it, each specific chemical refrigerant is given a number to identify it, R-12, R-22, R-502, etc. (water is even given a number, R-718). In contrast to water, most of the chemicals used as refrigerants have a very low boiling point. As an example, at atmospheric pressure R-22 has a boiling point of -41 degrees Fahrenheit (coincidentally, this is also about -41 degrees Celsius).

Whatever chemical, or refrigerant, is used in your air conditioner or refrigerator, it passes through four basic components, moving heat from one place to another along the way. These basic components are the evaporator, compressor, condenser, and metering device. Auxiliary devices may also be added for safety, control, or to increase efficiency, but these four components form a basic refrigerator or air conditioner. What do they do?

Let’s start with the part we care the most about, the part where the actual cooling is done. I’m sure you’ve already guessed that this is the evaporator. Just as the name implies, this is where liquid refrigerant evaporates. Like the evaporation of water from our skin, the evaporation of this refrigerant requires heat. This heat comes from whatever it is we are trying to cool, whether ourselves or the six pack we just put in the refrigerator. The evaporator usually consists of several rows of pipes, or one very long pipe that loops back and forth. For this reason it is often referred to as the evaporator coil. A fan is normally used to blow air across the evaporator coil, and flat sheets of metal, or fins, are usually attached to it to assist in the transfer of heat. So here’s how our first scientific principle is used. Because our bodies are hotter than the air, when the fan blows air across us, heat leaves our bodies and enters the air. Because the air is hotter than the evaporator, heat leaves the air and enters the evaporator.

In our second principle, we discussed the fact that after water reaches its boiling point, the temperature stops rising and begins to boil when more heat is added. The same thing happens here, the heat that enters the evaporator cannot make the temperature rise above the boiling point. Instead, this heat causes the refrigerant to boil, or evaporate. Just as a pot of boiling water stays at the same temperature until all the water boils away, the evaporator stays at the same temperature (cold) until all the refrigerant boils away (evaporates). This means that we can keep blowing warm air across the evaporator and it will keep absorbing heat from it. The effect is air conditioning, warm air goes in and cold (or at least cool) air comes out.

Of course, you might have seen a flaw in what we have so far. Eventually all the refrigerant will have evaporated and we won’t be able to cool any more air. Somehow we have to get rid of the heat that our refrigerant has absorbed. Ordinarily this might be tricky, since our refrigerant is still pretty cold (remember that all the heat from the air blowing past the evaporator was used to evaporate the refrigerant). This is where our compressor comes in. The compressor is like a pump for refrigerant. It creates a suction that draws the refrigerant out of the evaporator, keeping the low pressure in the evaporator needed to boil more refrigerant. There are different types of compressors (reciprocating, rotary, centrifugal, etc.), but they’re all designed to do the same thing, compress refrigerant. As the refrigerant is compressed, two things happen. Because it’s being squeezed into a smaller space, its pressure goes up. This process also increases its temperature. So while the compressor sucks in cold, low pressure refrigerant vapor, it pumps out hot, high pressure refrigerant vapor. This high pressure refrigerant vapor is pumped into a condenser, where it (logically) condenses. This happens because of the third principle that we discussed.

Remember our pressure cooker? You can use your pressure cooker for an hour at a temperature well above the normal boiling point of water, and yet when you open it, there is still water inside. Why? Because 212 degrees Fahrenheit (100 degrees Celsius) is only the boiling point at sea level. At a higher pressure (such as found inside a pressure cooker), the boiling point goes up. You might say that the opposite happens in our condenser. Because the refrigerant vapor is at a high pressure, its boiling point has increased. Just as steam can turn back into water if it’s cooled, refrigerant vapor can turn back into liquid refrigerant if it’s cooled below its boiling point (or condensation point). The condenser looks very similar to the evaporator, and with good reason, they perform the same function. Both are designed to move heat; the evaporator moves heat from the air to the refrigerant and the condenser moves heat from the refrigerant to the air blowing past its coils. This is why the condenser for an air conditioner is usually outside, you don’t want to put that heat back in your house (this is what a refrigerator does, notice the condenser coil on the back).

After the refrigerant turns back into a liquid, we’re ready to send it back to the evaporator and use it for more cooling. Remember though that it is still at a high pressure. You might say that this is the big difference between the evaporator and condenser, the difference in pressure. Just connecting the condenser and evaporator together won’t work because the high pressure liquid won’t boil in the evaporator. This is where our metering device comes in. There are various kinds of metering devices. The simplest is just an extremely thin piece of tubing. The idea is to just let a tiny trickle of refrigerant into the evaporator, just enough to replace what the compressor is sucking out without raising the pressure.

That’s the entire thing, a basic refrigeration system. The basic components are the same for everything from a dorm room refrigerator to a walk in freezer, from a window air conditioner to a system designed to cool a football stadium. And they’re all based on some simple scientific principles that we use every day without thinking about them.