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Refrigerators Posted June 24, 2001 Refrigerators are extremely popular these days. We have a beautiful old fridge down here at the radio station, with rounded corners and a huge spring-loaded latch. I even heard a story about a bunch of well-meaning English people who sent a shipment of refrigerators to some impoverished African country during a famine. Evidently they figured the reason why they didn't already have fridges was that they couldn't afford them. Of course, the other reason was that fridges run on electricity, and they had no electricity. But I suppose it's the thought that counts. But you probably already knew that fridges run on electricity, so you won't make a mistake like that. You might have also noticed that fridges have some very strange looking parts on the back. If you've looked around back there, you've probably noticed some similarity between the coils on the back of the fridge and the fins in an electric baseboard heater, and the loops of pipe on a hot water radiator. If you're one of those brave people who look inside their computers and stereo systems, you've probably seen a few heat sinks, which are just big pieces of metal with lots of little fins or spikes. All these things are designed to expose as much surface area as possible. In the language of thermal physics, the surface of a radiator is the interface between hot metal and cold air. Heat is transferred from the hot metal to the cold air in proportion to the size of the interface. If you want to transfer heat faster, then you increase the size of the interface. Coils, fins, and spikes are just different ways of increasing the size of the interface. Electric baseboard heaters have fins so they can release heat into the room faster. And fridges have coils for exactly the same reason. The main difference between a baseboard heater and a fridge is where the heat comes from. A fridge has to somehow cause heat to transfer from the food on the inside to the coils on the outside. This is a difficult proposition. In order for the coils to work, the air outside the fridge has to be colder than the coils. In order for the fridge to be useful, the stuff inside the fridge has to be colder than the air outside. If the stuff inside the fridge is colder than the coils, it's going to require some clever trick to get heat to transfer from the cold food to the hot coils. Surely you remember the law of thermodynamics that says you can't pass heat from the cooler to the hotter. Which would suggest that fridges are impossible. But then there's the other law of thermodynamics which says that heat is work and work is heat. In other words, heat is a form of energy. And that means it can be converted into other forms of energy. The trick is to transfer heat from the air inside the fridge into a pipe containing a cold vapour; then pressurize the slightly warmed vapour on its way out of the fridge. If you've heard of the ideal gas law in chemistry class, you know that PV=nRT; increasing pressure amounts to increasing temperature. By the time it gets out of the fridge and through the compressor, the vapour is hot, so it transfers some of its heat into the outside air. The greater the surface area of exposed pipe, the greater the heat transfer. At the end of the pipe, when the vapour is ready to go back into the fridge, it has lost a lot of its heat energy. Ideally, it's the same temperature as the outside air. But it gets even colder if we expand it. Reducing pressure amounts to reducing temperature; and if we expand the gas on the way in just as much as we compressed it on the way out, which is the only possible way to do it, then the temperature drop should be about the same. Which means the gas is colder when it goes into the fridge than it was when it came out. And if you've got cold gas going in, and hot gas coming out, then you're effectively sucking heat out of the fridge. Even better, if you're pushing hot vapour through exposed coils on the back of your fridge in order to cool it down, that means some heat is being released into your kitchen. There's even more heat than that, though. Another law of thermodynamics says that it's impossible to extract heat from a reservoir and release an equivalent amount of energy. For example, let's say you captured the heat from the coils on the back of your fridge, converted it into electricity, and used the resulting power to run the compressor. Well, it's a good idea, sustainability-wise, but it wouldn't work. You can use this refrigeration idea to transfer heat from colder things to hotter things, but not without using some external source of energy to help you. There are several popular applications of the refrigeration principle. One is the common air conditioner, which turns a room or even a whole house or grocery store into a giant fridge. The heated air from around the coils is simply blown out the window, sometimes directly into the faces of incoming customers attracted by the sign that says "air conditioned". Which seems to mean that the air is dry as well as cold. As evidenced by the water dripping from the air conditioning unit. Another application is the heat pump. A heat pump is just an outdoor fridge that extracts heat from the ground and releases it into your house. It compresses gas on the way into the house, so it gets hotter than the air in the house; then it expands it on the way out, so it's colder than the ground. In the summer, you can just reverse the flow of gas, so it expands and cools on the way in, absorbs heat from the house, and gets compressed on the way out so it can release heat into the ground. In case you're wondering, no chlorofluorocarbons were produced to make this show. So the ozone layer is probably in roughly the same condition it was in ten minutes ago. Common refrigerants include R-22, chlorodifluoromethane, which is a hydrochlorofluorocarbon, or HCFC, with low ozone-depletion potential. R-11, R-12, R-502, R-113 have been out of production since the US clean air act of 1996, and most of the good recycled stuff was used up in the next couple of years. R-134a is a good replacement for R-12, and apparently it's reasonable to use R-123 instead of R-11 in low pressure centrifugal chillers. You can get truckloads of any of these strange chemicals at r-22.com. I hope you find that information useful. |