Applications
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Chapter 12: Thermodynamics Applications |
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Keeping things cool is one of the basic needs in our society. Keeping ourselves cool in summer allows us to be more productive, keeping our food cool allows us to store it longer and to enjoy foods grown in other parts of the world. Furthermore, many important medicines must be kept cool. Relief agencies must keep careful track of the availability of refrigerators and refrigerated trucks in order to ensure that medical supplies can reach remote parts of world intact. This system is known as a "cold chain".
The central element in the operation of a refrigerator (or a freezer) is a fluid, called a refrigerant or, occasionally, a working fluid.Of course, some refrigerants work better than others. The most efficient refrigerants that have been found to date belong to the Freon family which includes CCl2F2 (FREON-12), CClF3 (FREON-13), and many others. These materials are known as chloroflorocarbons (CFC's). Unfortunately, it has recently become apparent that these chemicals damage the Earth's ozone layer.
In a refrigerator (or air conditioner) the refrigerant forms the "circuit" that allows heat to be "pumped" from the cooler region (inside of the 'fridge) to the warmer region (the kitchen).
To see how this works we start by looking into this "circuit". The basic parts are a compressor, the condenser coils, the expansion valve and the refrigerant.
We begin with the refrigerant in the gaseous state as it leavesthe cooler interior of the 'fridge. A compressor, using energyfrom the wall outlet, compresses the gas adiabatically. This raises its temperature and forces it into the condensing coils (located on the back of most refrigerators). For an adiabatic process, the change in temperature can be calculated using
where T1 and V1 are the initial temperature and pressure, and T2, V2 are the final temperature and pressure and the exponent z is approximately 0.5.
As the compressed gas moves into the condenser coils,its temperature is higher than that of the surrounding air (i.e. your kitchen). Thus, heat will flow out of the refrigerant and be taken away by the air. (This is the step in which running the 'fridge heats the kitchen). As the name suggests, the refrigerant is condensed during this process, so it is in its liquid state as it moves on the next stage.
The liquid is forced back into the refrigerator through an expansion valve. Here, a rapid drop in pressure cools the refrigerant and allows it to return to the gaseous state.
Now, the temperature of the refrigerant is lower than that of the interior of the refrigerator. Heat will flow from the air inside the compartmentinto the much cooler substance flowing through the coil. Finally, the "circuit" is closed; the refrigerant enters the compressor again.
This process shows the importance of choosing the proper refrigerantfor the job: it must be able to maintain a liquid state whiledropping to temperatures much lower than the freezing point ofwater. FREON-12 (the refrigerant used most commonly from the Freonfamily) has a freezing temperature of -158°C. It must also have a boiling point somewhat below room temperature and a heat of vaporization low enough to allow the partial phase change to occur in the expansion valve. It must also have low toxicity (since they may accidentally be released in the home or car) and it must have a low viscosity in order to move freely through the system.
As we mentioned above the members of the Freon family do damage the ozone layer in the Earth's upper atmosphere; so why don't we find another refrigerant that can do the same job without damaging the environment?The effort to accomplish this is ongoing, and viable alternatives have been found for many applications. However, many others have been rejected either because the do not provide enough cooling capacity or because their environmental effects are as bad or worse than those of Freon. A few refrigerants, such as R-409A and EC-12a, have been approvedby the EPA (Environmental Protection Agency), and although notas efficient as CFC-12 and its brethren, do not pose such a longterm threat to the world in which we live.
In addition to Altavistaand Lycos, you can find out more by following these links:
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