Cold and Dust-Free

At any hardware store or electronics store you can buy a can of electronics duster. This is just a spray can that blasts a stream of gas when you pull the trigger. You use it to blow the dust out of tiny crevices in electronic circuits. Your teacher is going to bring some cans of electronics duster to class and you can feel for yourself what happens when you spray this stuff. If you spray it long enough, the can will get very cold. It can even get cold enough to give you frostbite.

If you're inquisitive, you may be asking "what gas does the electronics duster shoot out?" It so happens that the gas is a hydrofluorocarbon, or an HFC. Remember that HFCs are the family of compounds used to replace chlorofluorocarbons as refrigerants. It's obvious from feeling the can that HFCs can make things cold, but just how do they do that?

The can of electronics duster is not very complicated. It's just a can filled with a liquid HFC under pressure. At the top of the can is a small valve. When you pull the trigger of the can, you open that valve. Because the HFC is under pressure, it rushes out of the valve.

When the HFC escapes from the can, something interesting happens. Remember that the boiling point of a liquid changes with pressure. The higher the pressure is, the higher the boiling point of a liquid will be. Likewise, the lower the pressure is, the lower the boiling point of a liquid will be. If you have ever been camping in high mountains you may have seen this in action. At sea level, where atmospheric pressure is 14.7 pounds per square inch (or 1 atmosphere), water boils at 100°C. However, the higher you go above the earth's surface, the lower the pressure is. So on a high mountain, the atmospheric pressure is lower. Because of this water boils at lower temperatures. Cooking a hard boiled egg takes longer for campers in high mountains, because boiling water is not as hot as it is at sea level! This is why recipes sometimes have separate instructions for cooking at high altitude.

When the HFC emerges from its steel prison into the freedom of the open air, it moves from a place where the pressure is very high to a place where the pressure is much lower. Since the pressure drops, so does the boiling point of the HFC. In fact it drops so much it that becomes lower than room temperature. So as soon as the HFC leaves the can, it boils and becomes a gas.

Heat and Changes of State

This is a change of state, of course. As you may remember, for every change of state there is a heat of transition. When a solid becomes a liquid, it absorbs heat in the process of melting. This is called the heat of melting. When a liquid becomes a gas, it absorbs heat in the process. This is called the heat of vaporization. This works backward, too. When gases condense to become liquids they give off heat, and when liquids freeze to become solids, they give off heat as well.

Right now we're talking about the heat that a liquid absorbs when it becomes a gas. When the HFC liquid in the can of electronics duster escapes and becomes a gas, it absorbs a lot of heat. It has to get that heat from somewhere, of course. (It's all about that law of the conservation of matter and energy.) So, to get the heat it needs to absorb while becoming a gas, the HFC steals heat away from its surroundings. This is why the can gets so cold in your hands while you're spraying it.

Heat of Vaporization and Your Refrigerator

A refrigerator works in the same way. In a refrigerator, an HFC is pumped through a tube called a coil, like you see in the animation below. In the coil, there is a plug with a small hole in it called a throttle valve. Because this opening is so small, pressure builds up behind the throttle valve, enough for the HFC to become a liquid. Slowly, the HFC passes through the throttle valve. On the other side of the throttle valve, the pressure is not as high. So the boiling point of the HFC drops low enough for the HFC to evaporate. As it evaporates, the HFC absorbs heat from its surroundings, specifically the inside of the refrigerator. The inside of the refrigerator then gets cold.

But there's more to this story. The HFC keeps moving through the coil. The coil passes to the outside of the refrigerator and to the compressor. The compressor puts pressure on the HFC, which condenses back into a liquid, and the whole process can start all over again.

The Heat Sponge

Now some of you with very inquisitive minds may be wondering, "what happens to the heat that the refrigerant absorbs from the air inside the refrigerator?" The answer involves the very first step of the cycle. Remember, that once the refrigerant passes through the coil, and through the inside of the refrigerator, it returns to the compressor. The compressor once again compresses the refrigerant back into a liquid. As we said earlier, when a gas condenses to form a liquid, it gives off heat, the heat of condensation. In this way, the refrigerant gets rid of the heat it absorbed from the inside of the refrigerator. (This is why the coil in back of a refrigerator feels warm.) Thus rid of this heat, the refrigerant is ready to cycle through the coil again and remove more heat from the inside of the refrigerator.

In a way, you might think of the refrigerant as a sort of sponge, that soaks up heat from the inside of the refrigerator. When the refrigerant returns to the compressor, the heat is "squeezed out" into the surrounding air.

There's a lot more going on than we've explained here, but these are the basics. For more information, visit the Web site listed at the end of this page.

What Makes a Good Refrigerant?

Why is it so hard to find a good refrigerant? To be a good refrigerant, a compound has to live up to a few requirements. Obviously, we want something that is nontoxic. We also want something that is unreactive. The refrigerant has to be stable for the lifetime of the refrigerator. In addition, we want a compound that is ozone-safe. But on top of all these criteria, we need a compound that has a low boiling point. So why not use nitrogen (N₂)? It's nontoxic (we breathe it all the time), the atmosphere is already full of it, and its boiling point is way down at -196°C. That's a little too low, as we'll soon see. We want a refrigerant to a have a low boiling point but not too low, because when a refrigerator is running, the refrigerant is constantly being boiled from a liquid to a gas, and then being condensed back into a liquid again. If the boiling point is too low, it will be hard to condense back into a liquid.

Since the refrigeration cycle involves evaporating and liquefying the refrigerant over and over again, the boiling point of the refrigerant has to be at a convenient temperature. CFCs boiled at just the right temperature, and this along with the fact that they weren't poisonous made them very attractive at first as refrigerants. Fortunately, we have learned that HFCs can be used to replace CFCs as refrigerants and won't damage ozone. To read about this new class of low-boiling compounds, read Swapping Atoms.

Next: The Two Faces of Ozone

How Refrigerators Work — from Marshall Brain's HowStuffWorks.