Heat Energy and Thermal Efficiency
Thermal efficiency is the relationship between the potential for power contained in a specific heat source, and how much usable power is actually created when that heat source is used. The formula for calculating thermal efficiency is:
Thermal Efficiency = Horsepower Produced ÷ Potential Horsepower in Fuel
For example, consider the piston engine used in a small general aviation airplane, which typically consumes 0.5 lb of fuel per hour for each horsepower it creates. Imagine that the engine is creating 200 hp. If we multiply 0.5 by the horsepower of 200, we find the engine is consuming 100 lb of fuel per hour, or 1.67 lb per minute. Earlier in this chapter, one horsepower was found to be 33,000 ft-lb of work per minute. The potential horsepower in the fuel burned for this example engine would be:
Hp =1.67 lb per minute × 18,900 BTU per lb × 778 ft lb per BTU/33,000 ft-lb/min
The example engine is burning enough fuel that it has the potential to create 744 horsepower, but it is only creating 200. The thermal efficiency of the engine would be:
Thermal Efficiency = Hp Produced ÷ Hp in Fuel
More than 70 percent of the energy in the fuel is not being used to create usable horsepower. The wasted energy is in the form of friction and heat. A tremendous amount of heat is given up to the atmosphere and not used inside the engine to create power.
There are three methods by which heat is transferred from one location to another or from one substance to another. These three methods are conduction, convection, and radiation.
Heat transfer always takes place by areas of high heat energy migrating to areas of low heat energy. Heat transfer by conduction requires that there be physical contact between an object that has a large amount of heat energy and one that has a smaller amount of heat energy.
Everyone knows from experience that the metal handle of a heated pan can burn the hand. A plastic or wood handle, however, remains relatively cool even though it is in direct contact with the pan. The metal transmits the heat more easily than the wood because it is a better conductor of heat. Different materials conduct heat at different rates. Some metals are much better conductors of heat than others. Aluminum and copper are used in pots and pans because they conduct heat very rapidly. Woods and plastics are used for handles because they conduct heat very slowly.
Figure 3-27 illustrates the different rates of conduction of various metals. Of those listed, silver is the best conductor and lead is the poorest. As previously mentioned, copper and aluminum are used in pots and pans because they are good conductors. It is interesting to note that silver, copper, and aluminum are also excellent conductors of electricity.
Liquids are poorer conductors of heat than metals. Notice that the ice in the test tube shown in Figure 3- 28 is not melting rapidly even though the water at the top is boiling. The water conducts heat so poorly that not enough heat reaches the ice to melt it.
Gases are even poorer conductors of heat than liquids. It is possible to stand quite close to a stove without being burned because air is such a poor conductor. Since conduction is a process whereby the increase in molecular energy is passed along by actual contact, gases are poor conductors.
At the point of application of the heat source, the molecules become violently agitated. These molecules strike adjacent molecules causing them to become agitated. This process continues until the heat energy is distributed evenly throughout the substance. Because molecules are farther apart in gases than in solids, the gases are much poorer conductors of heat.
Materials that are poor conductors are used to prevent the transfer of heat and are called heat insulators. A wooden handle on a pot or a soldering iron serves as a heat insulator. Certain materials, such as finely spun glass or asbestos, are particularly poor heat conductors. These materials are therefore used for many types of insulation.
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