Heat-Its Role in Wildland Fire. Clive M. Countryman
temperature changes with the initial temperature of the water. The amount of heat needed to raise water temperature from 40° to 41° for example, is not the same as that needed to raise it from 90° to 91°. Because standards of electrical voltage and of resistance are maintained at national standardizing laboratories throughout the world, scientists agreed internationally to use the heat produced in one second by a current of one ampere through a resistance of one ohm as the standard unit of heat. This heat unit is called the joule, in honor of the early English physicist, James Joule.
Although the joule has been used as the standard unit of heat in most scientific work for a long time, the terms "calorie" and "Btu" have continued to be used in industrial and other practical applications, particularly in countries where the metric system of measurement has not yet been adopted. Because of the continued use of these terms, the calorie has now been arbitrarily defined as equal to 4.1840 joules, which is nearly equivalent to the quantity of heat needed to raise the temperature of a gram of water from 14.5° to 15.5° C. One Btu equals 1055 joules or 252 calories. But by definition and derivation, the calorie and Btu are no longer connected in any way with the properties of water.
Molecular structure affects temperature change
Heat measurement units, as we have seen, are defined in terms of a temperature change brought about by heating. But the same amount of heat applied to different materials does not necessarily bring about the same temperature change—it is well known that some materials can be heated or cooled more quickly than others. These differences are the result of variations in the number of molecules in different substances and the way these building blocks of matter are put together. The variation in temperature change in different substances that accompanies the addition or subtraction of a given quantity of heat can be quite large. For example, the amount of heat needed to raise the temperature of a pound of water by 10° will increase the temperature of a pound of granite by 32° and that of a pound of iron by about 94°. Because of the variation in the amount of temperature rise when the same amount of heat is applied to different substances, a warm object may actually have less heat than a cooler one. Temperature indicates only the average molecular activity in a substance-its relative degree of hotness or coldness-and heat units must be used to measure the amount of heat it contains or can absorb.
The quantity of heat required to increase a unit weight of a substance by one degree is the specific heat of the substance, and is usually given as Btu per pound per degree Fahrenheit (Btu/lb/°F) or as calories per gram per degree Celsius (cal/g/°C). Materials with high specific heats require a large amount of heat to increase their temperature, and hence contain more heat at a given temperature than do materials with low specific heats. The specific heat of many materials varies considerably with temperature, so the temperature at which the specific heat was determined is usually specified. Water, with a specific heat of 1.0 Btu/lb/°F at ordinary atmospheric temperatures has one of the highest specific heats of common substances. The specific heat of most metals is low. Lead, for example, has a specific heat of only 0.031 Btu/lb/°F at 32°F, and requires very little heat to increase its temperature. Most wildland fuels have specific heats in the range of 0.45 to 0.65 Btu/lb/°F.
Often information on the amount of heat needed to produce a given temperature change in some volume of a material is needed. In wildland fire, for example, we are interested in the amount of heat needed to raise the temperature of a layer or volume of the fuels, because this helps determine the characteristics of a firebrand that can ignite a particular fuel. The amount of heat required to raise the temperature of a unit volume of a substance by one degree is its heat capacity. In English units, this is given in Btu per cubic foot per degree Fahrenheit (Btu/ft³/°F).
Heat capacity varies with density and specific heat
Heat capacity is calculated from the density and the specific heat of a substance. Density is the weight of a unit of volume whereas specific heat indicates how much heat is required to increase the temperature of each unit of weight by one degree. Both density and specific heat vary widely in different substances; hence the heat capacities of these substances also vary widely. And since specific heat often varies with temperature, so also does heat capacity.
Materials with high heat capacities can absorb and lose large quantities of heat without much temperature change. Conversely, relatively little heat is needed to change the temperature of materials with low heat capacity. At ordinary temperatures, the heat capacity of air is about 0.017 Btu/ ft³/°F—little heat is needed to change its temperature. Under similar conditions, the heat capacity of dry soil and rock is 19 to 20 Btu/ft³/°F, and that of water is about 62 Btu/ ft³/°F. The high heat capacity of water is one of the reasons why the climate near oceans and large lakes is often more moderate than that of the climate further inland. The water absorbs and stores large quantities of heat during the summer without much change in temperature, and this tends to keep the air over the adjacent land relatively cool. In the winter the stored heat is released and warms the air over nearby land areas.
Because the specific heat of most wildland fuels varies over a relatively narrow span, differences in heat capacity of the fuels depend chiefly on their density. The differences in density of wildland fuels are quite large, and hence variations in heat capacity are also. Solid oak wood, for example, has a density of about 48 lb/ft³, while that of punky and decayed wood may be only 6 or 7 lb/ft³. The oak wood, then, requires a considerable amount of heat to raise its temperature to the ignition point, but decayed wood requires a relatively small amount. Thus, heat capacity is important in the ignition of wildland fuels.
Much energy is involved in changes of state
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.