|MadSci Network: Engineering|
Scientists use many different types of refrigerators, depending upon the temperature that is needed. Some really exotic refrigerators have been in the news lately. These use laser cooling (after a bunch of other refrigerators have gotten things fairly cool) to obtain temperatures of about 10 nanokelvin, only 10 billionths of a degree above absolute zero! At these temperatures some strange properties of matter are being explored. No one knows what this work might lead to, so scientists are very excited about it. I think your question is asking about the refrigerators we use in our homes for keeping food cold. This same basic type of refrigerator is used in air conditioners to supply cool air to keep homes and offices cool. Refrigerators work by taking energy and using it to pump heat from a low temperature reservoir, the thing you are trying to cool, to a high temperature reservoir. On your refrigerator at home you might have noticed that the coils in back, or underneath, the refrigerator get very hot. This heat is dumped into the room, which serves as the high temperature reservoir. Incidentally, this means that you can’t cool your kitchen on a hot day by leaving the refrigerator door open. The extra heat the coils pass to the room air will more than make up for the cool air coming out of the refrigerator. In order to cool your room you need to hang the high temperatures coils outside, as an air conditioner does. This means that you are actually heating up the outdoors a bit when you run your air conditioner. Now we will consider the details of how a refrigerator cools something. Home refrigerators use the fact that a phase change from liquid to gas requires the input of a lot of energy. You have probably noticed that if you are wet and it is windy you can get cold very quickly. This is due to the evaporation of water from your skin, which cools your skin a lot. This is also the basis for how sweating allows us to cool ourselves on hot days. If you have a droplet of liquid and some of the droplet evaporates (changing from a liquid to a gas), it gets the extra energy needed to boost the molecules from the liquid to gaseous phase from the rest of the droplet. This loss of energy shows up as a reduction in temperature of the droplet. The cold liquid can cool the surrounding material, which in turn cools the gas. Inside the refrigerator the refrigerant starts out as a liquid. In order for it to be a liquid at room temperature it needs to be pressurized. The liquid refrigerant is pumped into another section where its pressure is reduced and it then evaporates. It is at this stage that the refrigerant becomes very cold, due to the effect described above. This cold gas is pumped through coils that run through the freezer and refrigerator sections. Next a compressor pressurizes the gas, which heats it up. The hot gas circulates through a condenser. The condenser is a coiled set of tubes that are outside the refrigerator. The condenser gives up the heat of the gas to room air which circulates over the condenser tubes. When the temperature in the condenser is low enough the gas condenses back to the liquid phase. Then the cycle can start all over. Refrigerators that have perfect insulation would only need to run long enough to cool the food down to the set temperature, then it could turn off. However, refrigerator insulation is imperfect, so heat leaks into the food chamber necessitating that the refrigerator run from time to time to maintain the desired temperature. Next we will talk about why it takes energy to boost a molecule from the liquid phase into the gaseous phase. In a liquid there are weak bonds between neighboring molecules. These bonds are not strong enough to hold the molecules rigidly in place, as in a solid. The molecules can sort of roll around one another, but always keeping some nearby neighbors. If you try to separate a single molecule from a bunch of its buddies and let it wander around as a single molecule, in the gaseous phase, it will take energy to pull it away from the other molecules. This energy can only come from the other molecules, and they all wind up slowing down just a little, which is equivalent to a reduction in temperature. Think of it as a sort of lottery. Say it takes $100.00 in order to buy your way out of a particularly bad class. All of the people in the class contribute a dollar and then the lucky winner is chosen. They take the money and run, leaving the rest of the class slightly poorer. Every time a student evaporates from class they leave the students that are left somewhat poorer, analogous to the liquid molecules being at at lower temperature. There is another effect known as the Joule-Thomson effect which I think contributes slightly to the cooling of the gas. If gas is pressurized and then allowed to expand so that the gas stream has some bulk velocity, then the gas cools down. You may have noticed this when you let the air out of a tire through a valve very quickly. The escaping gas is so cold that frost can form on the valve. If you can get the gas to perform some net work while it expands, all the better, and the gas will cool further. There is more to be said about equilibrium states where vapor and liquid are sealed up together in a container. In such a situation evaporation is balanced by condensation, and there is no net cooling. For this reason, sweating does not work on a hot day if the humidity is 100%, since water will not show any net evaporation. I’ll close by saying that it is the energy that you pump into the system that keeps the system from being in a state of equilibrium. The playwright Tom Stoppard paraphrased the Second Law of Thermodynamics as saying, roughly, that in a sealed system, everything comes to room temperature. Fortunately in our universe there is a lot of energy available that is not in thermal equilibrium with us, or things would get very boring. References: “The Feynman Lectures on Physics” (look up evaporation in Volume I), “The Second Law”, P.M. Atkins, Scientific American Books, 1984.
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