|MLA Citation:||Bloomfield, Louis A. "Air Conditioners" How Everything Works 20 Jul 2018. Page 1 of 2. 20 Jul 2018 <http://www.howeverythingworks.org/prints.php?topic=air_conditioners&page=1>.|
A heat pump is a device that transfers heat against its natural direction of flow. If you use one to heat your home, the heat pump uses electricity to transfer heat from the colder outside air to the hotter inside air, so that the inside air becomes even hotter and the outside air becomes even colder. The electricity that the heat pump uses also becomes thermal energy inside your home. Since both the electric energy and the thermal energy pumped from the air outside end up inside your home, a heat pump provides more heat than a simple space heater can provide with the same electricity. The energy efficiency of a heat pump decreases as the temperature difference between inside and outside becomes greater, but it typically provides 4 or more times as much heat to your home as a normal electric space heater would provide with the same amount of electricity. Incidentally, when the heat pump is reversed, so that it pumps heat out of your home, it is then an air conditioner.
Electric discharge lamps are between 2 and 5 times as energy efficient as normal incandescent light bulbs. The hot filament of an incandescent lamp delivers only about 10% of its electric power as visible light. In contrast, a fluorescent lamp delivers about 25% of its electric power as visible light and some gas discharge lamps (particularly low-pressure sodium vapor) deliver as much as 50% of their electric powers as visible light.
A microwave oven transfers about 50% of its electric power directly into the water molecules of the food that you are cooking. Cooking occurs quickly and because the cooking chamber doesn't get hot, there is no power wasted in heating the oven itself or the room surrounding the oven. Depending on how large an object you are cooking, a microwave oven probably uses between 5 and 20 percent of the electricity it would take you to cook the same food in a standard oven.
The working fluid leaves the condenser as a high-pressure liquid at roughly room temperature. It then flows into the refrigerator, then through a narrowing in the pipe, and then into the evaporator, which is another snake-like pipe that's wrapped around the freezing compartment (in a non-frostfree refrigerator) or hidden in the back of the food compartment (in a frostfree refrigerator). When the fluid goes through the narrowing in the pipe, it's pressure drops and it enters the evaporator as a low-pressure liquid at roughly room temperature. It immediately begins to evaporate and expands into a gas. In doing so, it uses its thermal energy to separate its molecules from one another and it becomes very cold. Heat flows from the food to this cold gas. The working fluid leaves the evaporator as a low-pressure gas a little below room temperature and heads off toward the compressor to begin the cycle again. Overall, heat has been extracted from the food and delivered to the room air. The compressor consumed electric energy during this process and that energy has become thermal energy in the room air.
So as the hot air rises upward, the air pressure around it gradually diminishes and the hot air expands. It has to expand because whenever its pressure is higher than the surrounding pressure, its molecules experience outward forces that cause them to spread out. But this expansion process uses some of the hot air's thermal energy—the hot air must push the surrounding air out of the way as it expands. With less thermal energy in it, the hot air becomes cooler. Dry air loses about 10° C for every kilometer it rises, while moist air loses about 6° or 7° C per kilometer. This cooling effect explains why air at higher altitudes, such as the air on mountains, is colder than the air at lower altitudes, such as the air in valleys.
Furthermore, whenever cold air descends through the atmosphere, it is compressed and its temperature rises! This warming process also increases the air's water-carrying ability so that it becomes relatively dry. That effect explains the special "Katabatic" winds that blow warm and dry out of the mountains—including the Santa Ana winds near Los Angeles, the Chinook in the Rocky Mountains, the Foehn in the Alps, and the Zonda in Argentina.
But there is a second type of ammonia refrigerator that makes use of an absorption cycle—ammonia dissolves extremely well in cool water but not so well in hot water. In an absorption cycle refrigerator, a concentrated solution of ammonia in water is heated in a boiler until most of the ammonia is driven out of the water as a high-pressure gas. This hot, dense ammonia gas then enters a condenser, where it gives up heat to its surroundings and becomes a cooler liquid. The liquid ammonia then enters a low-pressure evaporator, where it evaporates into a cold gas. This evaporation process draws heat out the evaporator and refrigerates everything nearby. Finally, the ammonia gas must be returned to the boiler to begin the process again. That return step makes use of the absorption process, in which the ammonia gas is allowed to dissolve in relatively pure, cool water. The gas dissolves easily in this water and thus maintains the low pressure needed for evaporation to continue in the evaporator. The now concentrated ammonia solution flows to the boiler where the ammonia is driven back out of the water and everything repeats.
As for the creation of life, that could have been a random event and thermodynamics permits random events. Improbable events do occur—people win the lottery, lightning strikes twice, two snowflakes are occasionally alike—and the creation of life could have been one of those unlikely but not impossible events. Once the simplest organism had assembled itself by chance, it could then begin the process of consuming order and exporting disorder.
The working fluid arrives at the compressor as a cool, low pressure gas. The compressor squeezes this working fluid, packing its molecules more tightly together so that their density and pressure increase. The squeezing process also does work on the working fluid, increasing its energy and therefore its temperature. The working fluid leaves the compressor as a hot, high-pressure gas and flows into the condenser. The condenser has metal fins all around it that assist the working fluid in transferring heat to the surrounding outdoor air. As this transfer takes place, the closely spaced molecules of the working fluid begin to stick to one another, releasing additional thermal energy into the surrounding air and causing the working fluid to transform into a liquid. By the time the working fluid leaves the condenser, its temperature has almost dropped back down to the outdoor temperature but it is now a liquid rather than a gas.
This high pressure liquid then flows into the evaporator through a narrow orifice. This orifice allows the liquid's pressure to drop so that it begins to evaporate into a gas. As it evaporates, it extracts heat from the air around the evaporator because that heat is needed to separate the molecules of the working fluid. Like the condenser, the evaporator has metal fins to assist it in exchanging thermal energy with the surrounding air. By the time the working fluid leaves the evaporator, it is a cool, low-pressure gas. It then returns to the compressor to begin its trip all over again.
Overall, the working fluid releases heat into the outside air and absorbs heat from the inside air. The direction of heat transfer, from a cooler region to a hotter region, is the reverse of normal and requires an input of ordered energy so that it doesn't violate the second law of thermodynamics (the disorder of an isolated system can never decrease). This ordered energy is used to operate the compressor and is converted into thermal energy in the process. This additional disordered thermal energy enters the outside air and makes up for the additional order that's given to the indoor air as that air is cooled.
In a sense, probability is destiny. Thermodynamics observes that vast systems tend to evolve toward the mostly likely configurations. To understand this process, consider what happens when you mix hot and cold water. The most likely final configuration for the mixed water is for it to reach a uniform temperature about half way in between the two original temperatures. While it's possible for the water to end up extremely hot in one place and extremely cold in another, that outcome is extremely unlikely. It's so unlikely that it never happens.
So in what sense does thermodynamics overwhelm things? The world is filled with relatively ordered arrangements and these ordered arrangements are unlikely by themselves (how they came to be ordered in the first place is another matter for another questions). If you take a crystal vase and drop it on the floor, it's going to evolve toward a more likely arrangement of atoms and dropping it a second time isn't going to return it toward its original unlikely state. In short, ordered systems naturally drift toward disorder when given a chance. How quickly they drift depends on their situation. A coffee cup will remain a nicely ordered object for thousands or millions of years if you don't disturb it. But in a hot environment, or one that is chemically aggressive, it may not last very long.
One last thought: how do living organisms maintain their order in the face of this tendency to disorder? They do it by consuming order and exporting disorder—they eat ordered foods and release disordered wastes to their surroundings.
In that respect, people are much more vigorous producers of disorder than most other living organisms. People seek order not only in their bodies, but also in the objects around them and they achieve this ordering by consuming order in their environment—fossil fuels, minerals, pure water—at a furious pace and producing disorder in its place—burned gases, garbage, polluted water. Fortunately, sunlight is a tremendous source of order for our earth and it undoes some of the disordering caused by living organisms. However, we are consuming much of the order that sunlight stored on earth over millions of years in only a few generations. At this pace, we're destined to have troubles with the disorder we're creating. Many of the environmental issues that face us today can be viewed from this order/disorder perspective: we have to learn how to create less disorder.
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