Читать книгу Heat-Its Role in Wildland Fire - Clive M. Countryman - Страница 11

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Thus far in our discussion of heat and energy, we have been concerned with the addition and subtraction of heat that does not result in a change in the physical structure or "state" of a substance—the kind of change that occurs when ice melts or water is turned to vapor. But heating or cooling if carried far enough can cause such a change, and considerable amounts of heat are frequently associated with this change. In the combustion of wildland fuels, for example, there is a change in state of the fuel being burned, during which large quantities of energy are released.

The heat associated with the changes of state of water is of major importance in our daily lives, and also affects wildland fire, both directly and indirectly. If a pan of ice water at 32°F is heated, the temperature of the water rises until it reaches 212°F, and 180 Btu per pound of water are absorbed in the heating. If the heating is continued, the water begins to boil and to vaporize, but there is no further increase in the water temperature. Thus, a quantity of heat in addition to that required to raise the temperature of the water is needed to change the state of liquid water to vapor. At sea level, this additional heat amounts to 972 Btu per pound of water—more than 5 times the amount required to raise the temperature from 32° to 212° F. However, this thermal energy is not lost, for when the water vapor condenses back to liquid the 972 Btu is released as heat. Water also changes to vapor at temperatures below 212°, but the amount of heat required increases as the temperature decreases. At 104°F, about 1035 Btu/lb are needed, and this increases to 1066 Btu/lb at 50°F. The heat required to change a liquid to vapor, or that is released when the vapor changes back to a liquid is the heat of vaporization.

Heat of vaporization is used in many of the activities of man. For example, the evaporative type air conditioner, or "swamp cooler," depends directly on the heat of vaporization of water for its operation. As warm outside air flows through water soaked excelsior pads, some of the water vaporizes, and much of the heat needed for this vaporization is drawn from the incoming air, thus cooling it. Cooling towers used to cool water in industrial processes operate in much the same way.

The heat of vaporization of water is important in natural processes also. Many living creatures, including man, depend on the cooling effect of water evaporating from the skin or lungs to regulate their internal body temperature. The evaporation of water into vapor and the condensation of the vapor back into water in the form of clouds or precipitation has a major effect on our weather and climate. Because of the large volumes of water and vapor involved, enormous amounts of thermal energy can be released or stored. Lightning storms depend greatly on the heat released from condensing water vapor for their development, and much of the tremendous energy of a hurricane comes from the same source.

In addition to indirectly affecting wildland fire through its influence on weather, the heat of vaporization of water also has a more direct bearing on fire. Water vapor is formed in the combustion process and from the moisture in the burning fuel. This vapor is carried aloft in the smoke or convection column of the fire, and frequently condenses at the top of the column to form a white "cap". The heat released when the vapor condenses can add significant amounts of energy to the convection column, increasing its strength and adding to the fire activity. The condensation of enough water vapor to form 8 pounds of liquid water releases about the same amount of heat as burning 1 pound of wildland fuel. As we shall see in a future report, the heat needed to vaporize the moisture in fuels has an important effect on the ignition of fuel and the rate at which it burns.