Countryman 1977 Part1 Nature of Heat Yellow Ocr

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HEAT AND WILDLAND FIRE-Part 1

THE NATURE OFHEATClive M. Countryman

1977 FOilES,. SERVICEu.s DEPi\RTMEt\:T To A IUCULTUREP.O nox :l45, I3ERKELEY. CALIFOR;\llA 9 ~ 7 0 1

PACIFIC SOUTHWESTForest and RangeExperiment Station

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, - -------------TheAuthor-------------,CLIVE M. COUNTRYMAN was, until his retirement in 1977, in charge ofthe Station's fire behavior studies, with headquarters at the Fores t Fi reLaboratory, Riverside, Calif . He earned a bachelor' s degree in forestry atthe University of Washington in 1940, and joined the Forest Service thefollowing year.

NOTETI1is publication is part of a group designed to acquaint fire control personnel , wildlandmanagers, and forestry students with important concepts of fire behavior and the application ofthese concepts to wildland fire problems. The level of difficulty of the treatment of topics inthese publications varies, as signaled by the color of the cover: the blue cover group is generallyelementary and the yellow cover group is intermediate. The following publications, by CliveCountryman, are available on request to:

DirectorPacific Southwest Forest and Range Experiment StationP.O. Box 245Berkeley, California 9470 IAttention: Publication Distribution

This Humidity Business: What It Is All About and Its Use in Fire Control . 1971 (blue)Fire Whirls . . . Why, When, and Where. 1971 (blue)Carbon Monoxide: A Firefighting Hazard. 1971 (yellow)The Fire Environment Concept. 1972 (blue)Heat-Its Role in Wildland Fire (blue)

Part I-The Nature of Heat. 1975Part 2-Heat Conduction. 1976Part 3-Heat Conduction and Wildland Fire. 1976Part 4-Radiation. 1976Par t 5 -Radiation and Wildland Fire. 1976Heat and Wildland Fire (yellow)Part I-The Nature of Heat. 1977


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THE NATURE OF HEATThree ingredients are essential for a wildland fire to start

and to burn. First, there must be burnable fuel available.Then enough heat must be applied to the fuel to raise i tstemperature to the ignition point. And finally, there must beenough air to supply the oxygen needed to keep the combustion process going and thus maintain the heat supply forignition of unburned fuel. These three essential ingredientsfuel, heat, and oxygen-make up the fire triangle. All must bepresent if there is to be fire. In the following discussion, wewill examine some of the basic characteristics of the heatsegment of the triangle-the nature of heat itself.

Heat is indispensableEveryone knows how heat feels, and is well aware of its

many uses and applications in our lives. Heat is used to warmour houses and to cook our food. Heat is necessary in manyindustrial and manufacturing processes-most products weutilize or consume involve the use of heat somewhere alongthe line in their production. Modern power sources oftendepend on heat; internal combustion and steam operatedengines, for example . And heat is often used to genera te theelectrical power so important in the present way of life. Heatreceived at the earth's surface f rom the sun is the basiccontrol of our weather, and this heat is also essential ingrowing food crops and other vegetation, including wildlandfuels. Without the heat received from the sun, life could no texist on the earth.

Heat science is relatively newThe science of heat ( thermodynamics) has brought an

understanding of many of the physical laws governing heatand heat phenomena . But as recently as 200 years ago, thetme nature of heat was no t understood. In the early days ofscience, the phenomena associated with hea t were ascribed toan int angible and mysti ca l fluid cal led "caloric." This f1uidwas believed to have the power of penetrating and expandingmaterials, sometimes melting or dissolving them, and converting some substances to vapor. The caloric fluid was con-


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sidered intangible, since it could not be seen and even themost careful experiments failed to show that non-destructiveheating or cooling of a substance would produce any changesin weight.The early theory of heat as a fluid was simple in application and difficult to refute, for in some respects heat doesbehave like a fluid. The theory satisfactorily explained theresults of elementary experiments of the time in heat, andmany of the manifestations of heat in daily life. Heatproduced by friction or the compression of gases, forexample, was attributed to latent or stored caloric that wassqueezed or ground out of the substance. The abradedmaterial produced in drilling or grinding was assumed to benot capable of holding as much caloric as the parent material,thus accounting for the heat produced in such operations.The idea of heat as an intangible fluid was useful in helpingto conceive of an abstract phenomenon that makes itselfapparent in concrete ways. But in many respects, the ideawas misleading, and the persistence of the caloric theory formany years severely retarded the understanding of the truenature of heat and the science of thermodynamics.

Heat is energyWe know now that heat is not a fluid, nor a substance atall, bu t is really a form of energy called thermal energy.

Other common forms of energy are electrical energy, radiantenergy, chemical energy, and the mechanical or kineticenergy possessed by moving materials and objects, such asfalling water or a rotating wheel. Atomic or nuclear energy isstill another form. Simply defined, energy is the capacity todo work, and in the science of physics "work" is the transferof energy from one object to another that results in themotion of displacement of the object acted upon.Energy cannot be created or destroyed in any way that weknow. But it can be changed from one form to another, andalso transferred from one substance to another. And thistransformation and exchange of energy is constantly goingon-both in nature and through human activities. Consider,for example, the operation of a coal-burning steam piledriver. Radiant energy from the sun is transformed by theprocess of photosynthesis into stored chemical energy invegetative material. The vegetation is converted by naturalprocesses into coal, which is another type of stored chemicalenergy. When burned in a pile driver firebox, the storedchemical energy is converted to heat, which is transferred tothe water in the boiler. thereby changing it into steam. Whilestored under pressure, the steam has stored or potentialenergy. When released into the steam cylinder, the expandingsteam does work on the piston and gives kinetic energy to it.

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This kinet ic energy is t ransferred through the connecting rodand gear train to the hoisting drum, allowing work to be donein raising the driving weigh 1. While poised in the air, theenergy used in raising the weight appears as potential energy,and this is transformed into kinetic energy when the weight isreleased and permits work to be done in driving the pile intothe ground.

Not all of the energy of the coal is applied toward drivingthe pile. Some of the thermal energy escapes through thefirebox, boiler, and steam cylinder, and is absorbed by the airand surrounding objects, increasing their thermal energy. Partof the kinet ic energy is changed back to thermal energy byfriction in the machinery. The impact of the driving weighton the pile changes some of the kinetic energy to thermalenergy that increases the temperature of both the weight andthe pile and the amounts of their thermal energy. Some ofthe kinet ic energy of the moving pile is also conver ted tothermal energy by frict ion with the ground, and potentialenergy is imparted to the earth displaced by the pile. In all ofthis complex transformation and transfer, none of the original radiant energy received from the sun is destroyed-itsimply appears in different forms.According to molecular theory, all substances are made upof molecules , and as long as the temperature of the substanceis above absolu te zero (- 469 0 F) these molecules are in somedegree of motion. When heat is applied to a substancewithout changing its state-such as occurs when ice ischanged to water or water to vapor-the molecular activityincreases and the temperature rises. Conversely, if a substanceloses heat without change of s ta te , the molecular activitydecreases and so does the temperature. We can think of heat,then, as the energy of molecular motion.

The jou Ie is the standard un it of heatBecause water is one of the most abundant compounds on

earth, it is perhaps no t surprising that early scientists frequently compared the physical characteristics of varioussubstances with those of water, and often used the attributesof water as a standard of measurement. And the standardunit of heat was first based on the amount of heat needed toincrease the temperature of water. In the metric system ofmeasurement, the unit of heat was the calorie and wasdefined as the amount of heat required to increase thetemperature of 1 gram of water by 10 C. The British ThermalUnit (Btu) was the hea t uni t in the English system, and wasdefined as the quantity of heat needed to increase thetemperature of a pound of water (about a pint) by 10 F.

Water did no t prove to be a good standard for themeasurement of heat quantity, however, for it was soonfound that the amount of heat required to raise its tempera-

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ture varied with the initial temperature of the water. Theamount of heat needed to change the temperature by 1degree at 40, for example, is not the same as when the watertemperature is 90. This characteristic of water led to several"kinds" of calories. For example, if the water temperaturechange is from 3.5 to 4.5 0 C-the temperature at which waterhas its greatest density-the unit of heat was the small calorie.The amount of heat required to change the temperature of agram of water from 14.5 to 15.5 0 C was the normal calorie,and was the unit of heat most commonly used. The meancalorie was l/ l 00 of the amount of heat needed to raise thetemperature of a gram of water from 0 0 to 1000 C. And alarge calorie was equal to 1000 small calories.The numerous heat units, each representing a differentquantity of heat, caused a great deal of confusion andrequired that the heat unit be defined each time it was used.To correct this problem, it was agreed internationally todefine the standard unit of heat as the amount of heatproduced when an electrical current of one ampere flowsthrough a resistance of 1 ohm for 1 second. Standards ofelectromotive force (voltage) and resistance are maintained atthe national s tandardizing laboratories, and the mean solarsecond is used as the time standard. This new heat unit iscalled the joule in honor of James Joule, an early Englishphysicist.Although most theoretical and experimental work in heatsince 1910 has been based on the joule, the terms "calorie"and "Btu" continued to be used in industrial and otherpractical applications requir ing the measurement of heat.Results of research were usually converted from joules tosome kind of calorie, giving the incorrect impression that theunit of heat was still in some way connected with water , andrequired that the kind of calorie still be specified. To solvethis difficulty, it was agreed internationally to arbitrarilydefine the calorie as equivalent to 4.1840 joules. This calorie,sometimes known as the thermochemical calorie, is verynearly identical in size with the normal calorie. The thennochemical calorie has been used in nearly all thennodynamicaland thermochemical work in the United States since 1930.By definition and derivation, the calorie is not connected inany way with the properties of water and does not vary insize.Because the term "Btu" is so commonly used in practicalapplications of heat units in the Uni ted Sta tes, i t will be usedin this discussion. One Btu equals about 1055 joules or 255calories.

Molecular structure affects temperature changeHeat measurement units, as we have seen, are defined interms of a temperature change brought about by heating. But

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Temperature increaseSpecific heat =

the same amount of heat applied to different substances doesnot necessarily bring about the same temperature change-itis well known that some materials can be heated or cooledmore easily than others. These differences are the result ofvariations in the number of molecules in different substancesand the way these building blocks of matter are put together.The variation in temperature change in different substancesthat accompanies the addition or subtraction of a givenquantity of heat can be qui te large. Adding 1 Btu to a poundof water will increase its temperature by only 1F. Butadding 1 Btu to a pound of iron will increase its temperatureby about 9 , and the same amount of heat added to a poundof lead will cause a 32 F increase.

For a unit weight of a substance, the ratio of added hea t tothe temperature rise is the specific heat of the substance:Added heat

In English units, specific heat is expressed as Btu per poundper degree F of temperature change (Btu/lbt F) .Substances with high specific heats require more heat toincrease their temperatures than those with low speci ficheats. Hence, a substance with a high specific heat wilI

contain more heat at a given temperatu re than wilI asubstance with a low specific heat. Specific heats for manysubstances can be found in engineering and other references.The specific heat often varies with temperature, so thattemperature at which the specific heat was determined isusually specified. Water, with a specific heat of 1.0 Btu/lb/ F at ordinary atmospheric temperatures, has one of thehighest specific heats of common substances. Dry air at aconstant pressure has a specific heat of 0.24-the sameamount of heat added to a pound of air and a pound of waterwill increase the temperature of th e water only 24 percent asmuch as the air. Some metals have very low specific heats.Copper, for example , has a specific heat of 0.0919 Btu/lb/ F, and that of gold is 0.0305. Such metals need little heatto increase their temperature. Most wildland fuels havespecific heats in the range of 0.45 to 0.65 Btu/lbt F.

Water

TEMPERATURE INCREASE PER POUND FOR i BTU ADDED HEAT

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Specific heat gives th e amount of heat needed to raise thetemperature of a unit weight of a substance by I degree.Often, however, information on the amount of heat neededto produce a given temperature change in some volume of asubstance is needed. In wildland fuel, fo r example, we areinterested in th e amount of heat needed to raise the tempera-ture of a layer or volume of fuel because this helps determinethe characteristics of a firebrand that can ignite it. Theamount of heat required to raise the temperature of a unitvolume of a substance by 1 degree is th e heat capacity of thesubstance. In English units, th is is expressed in Btu per cubicfoot per degree Fahrenheit of temperature rise (Btu/ft3 ;o F).

Heat capacity varies with density and specific heatThe density of a substance indicates its weight per unit of

volume, whjJe its specific heat establishes how much heat isrequired to increase th e temperature of each uni t of weightby I degree . Therefore, the heat capacity of a substance can

SUMMER

v.:AAM AlA

WINTER

COOL AIR

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be calculated from the simple equation:Heat capacity = Density x Specific heat

If the densi ty and specific heat of a substance is known, itsheat capacity can readily be determined. Solid oak wood, forexample, has a specific heat of 0.57 Btu/lbt F and a densityof 48 lbs/f t3 . It s heat capacity is then 27 Btu/ft 3 t F (0.57 x48 = 27)-it will absorb 27 Btu per cubic foot for each degreeof temperature rise.

Both the density and specific heat of different substanceshave a wide variation, and consequently, their heat capacitiesalso vary widely. And since specific hea t o ften varies withtemperature, so does the heat capacity. Substances with highheat capacit ies can absorb and lose large quantities of heatwithout much temperature change. Conversely, relativelylittle heat is needed to change the temperature of a substancewith a low heat capacity. At ordinary temperatures, the heatcapacity of air is about 0.017 Btu/ft3 t F-little heat isneeded to change it s temperature. With similar temperatures,the heat capacity of dry soil and rock is 19 to 20 Btu/ft3 t F,and that of water is about 62 Btu/ft3 t F.

The high heat capacity of water is one of the reasons whythe climate near oceans and large lakes is often moremoderate than that of the climate further inland. The waterabsorbs and stores large quantities of heat during the summerwithout much change in temperature, and this keeps the airover the water and nearby land areas relatively cool. In thewinter the stored heat is released and warms the air over thewater and adjacent land. The ability of water to absorb andtransport large quantities of heat also makes it useful inheating and cooling systems-hot water heating systems inbuildings and cooling systems in automobiles, for example.Because the specific heat of most wildland fuels varies overonly a relatively narrow span, differences in heat capacity ofthe fuels depend chiefly on their density. The difference inthe density of wildland fuels is quite large, and hencevariations in heat capacity are also. Solid oak wood has adensity of about 48 Ib/ft 3 , while that of punky and decayedwood may be only 6 or 7 Ib/ft 3 . The oak wood, then, willrequire considerably more heat to raise its temperature to theignition point than will the decayed wood. Thus, heatcapacity is an important factor in the ignition of wildlandfuels.Much energy is involved in changes of state

Thus far, our discussion of heat and energy has beenconcerned with the addition and subtraction of heat thatdoes not change the physical st ructure or "state" of asubstance. But heating or cooling if carried far enough cancause a change in state of a substance, and considerable

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amounts of energy are frequently associated with this change.The combust ion of wildland fuels, for example, involves achange of state in the fuel being burned during which largequantities of thennal energy are released.

If a block of ice is heated, its temperature will rise until itreaches the melting point of 32 0 F. Continued heating willcause the ice to melt, but there will be no further increase intemperature until all of the ice is melted. Thus, addit ionalheat is requi red to change the state of ice to water. Heatingof the water from the melted ice will result in a temperaturerise of the water until it reaches 212 0 F. The water will thenbegin to boil and to vaporize, bu t there will be no furtherincrease in its temperature with continued heating. Again,addit ional heat is needed to change the state of water tovapor.

To change ice to water , or water to vapor, heat is neededfrom an outside source. Bu t if the process is reversed and thevapor is changed to water or water to ice, an equivalentamount of heat is released. The heat required to change asolid to a l iquid, or that released when a liquid is convertedto a solid, is the heat of fusion. And the hea t needed tochange a liquid to vapor and that is released in changing avapor to a liquid is the heat of vaporization.

Both the heat of fusion and heat of vaporization ofdifferent substances vary over a wide range. The heat offusion of aluminum is 170 Btu/lb and its heat of vaporiza-tion is 3591 Btu/lb. The heats of fusion and vaporization ofnitrogen are 11 and 86 Btu/lb respectively, while those ofbu tane are 34 and 166. The heat of fusion of wa ter is 144Btu/lb, and at sea level the heat of vaporization of water is972 Btu/lb. Water will change to vapor at temperatures below212 0 F, but the heat of vaporization increases as the tempera-

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Il a f l. 111 W"11" I lS WAfER-~ JI '" 00--- ----t 170 1!1U , '00 BTl; e71I1TU'

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ture decreases. At 104 F it is 1035 Btu/lb, and this increasesto 1066 Btu at 50.The heat involved in changes of state of water is of major

importance in daily life, and also affects wildland fire, bothdirectly and indirectly. Many living creatures, includinghumans, regulate their internal body temperature through theheat used in evaporating water from the skin or lungs. Heatof vaporization is important in many industrial processes andin air conditioning. The evaporative type air conditioner, or"swamp cooler" depends on the heat needed for watervaporization for its operation. As warm outside air flowsthrough water soaked excelsior pads, some of the watervaporizes, and part of the heat needed for this vaporization isdrawn from the incoming air. The evaporation of water intovapor, and the condensation of the vapor back to water inthe form of clouds and precipitation, has a major effect onour weather and climate. Because of the large volumes ofwater and vapor involved, enormous amounts of energy canbe released or stored. Thunderstorms depend greatly on theenergy released by condensing water vapor for their develop-ment, and much of the tremendous energy of a hurricanecomes from the same source.In addit ion to indirectly affecting wildland fire through itseffect on weather, the heat of vaporization of water also hasa more direct bearing on fire. Frequently, large fires developa white "cap" at the top of the convection column. This iscondensed water vapor, produced from vapor created in thecombustion process and from moisture in the fuel, andcarried aloft in the convection column. The heat releasedwhen the vapor condenses can add significant amounts ofenergy to the convection column, increasing its strength andadding to the fire activity. The condensation of enough watervapor to form 8 pounds of liquid water will release about thesame amount of heat as burning 1 pound of wildland fuel. Aswe shall see in a future report , the heat needed to vaporizemoisture in fuels has an important effect on the ignition offuel and the rate at which it burns.Heat in wildland fuel can be measuredThe amount of heat that can be obtained from fuel when

it burns is known variously as its caloric value, heat ofcombustion, heating value, or sometimes heat content. Fueloil will produce 18,000 to 22,000 Btu per pound when it isburned, and gasoline about 21,000 Btu per pound. Theheating value of most wildland fuels falls wi thin a range of8000 to 9500 Btu per pound, but there are some exceptions.Pitch pine produces more than 11,000 Btu per pound, whileplants with high mineral content, such as some of thesaltbushes, produce only 6500 to 7000 Btu per pound.

Some of the constituents of wildland fuels, such as resins,waxes, and oils, have a much higher heating value than the

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rest of the fuel. In chamise, for example, these high energyconstituents have heating values ranging from 17,000 to24,500 Btu per pound-about the same as fuel oil andgasoline. Consequently, the parts of a plant that contain mostof the resins, waxes, and oils will have a higher heating valuethan the rest of the plant. For example, the needles and smalltwigs of pinyon pine have a heating value of more than 9500Btu per pound, while that of the tree trunk is about 8800Btu per pound.The heating value for various fuels is determined throughl aboratory procedures for dry fuels and complete combustion. This value represents the maximum amount of heat thatcan be obtained from the fuel. Under wildfire conditions, theactual heat production is likely to be considerably less. Someflammable gases produced from the fuel may be carried intothe convection column unburned, and complete oxidation ofall the fuel chemicals may not occur. This is likely to be ofmost consequence in intense fires in which the convectiveactivity is strong and the air flow turbulent-in very intensefires only 50 to 60 percent of the potential heat from thefuel may actually be realized.Moisture in the fuel also reduces the heat production. Theheating value for fuel containing moisture can be calculatedfrom an equation developed by the U.S. Forest ProductsLaboratory:

where Hw=

M.e.100 - 7Hd x 100 +M.e.

Heating value of moist fuelHd Heating value of dry fuelM.C. Moisture content in percent

For a fuel having a heating value of 8500 Btu per poundwhen dry, a moisture content of 5 percent will reduce thisvalue to 8274. At 20 percent moisture content, the heatingvalue will be reduced to 6881 Btu, and at 80 percentmoisture to 4183 Btu per pound. Since living fuels often havemoisture contents exceeding 80 percent, they produce muchless heat than dead and dry fuels.SUMMARY

Heat is a form of energy called thermal energy. Energycannot be created or destroyed, but i t can be conver ted fromone form to another, and transferred from one place toanother and between substances. This conversion and transferof energy is constantly going on, both in nature and throughthe activities of man.

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The standard unit of heat is the joule, which is based onthe amount of heat produced by a current of one standardampere through a standard resistance of 1 ohm in 1 second.The thermochemical calorie is arbitrarily defined as equivalent to 4.1840 joules. The thermochemical calorie is nearlythe same in size to the normal calorie which is the amount ofheat required to raise th e temperature of 1 gram of waterfrom 14.5 to 15.5 C. One British Thermal Unit (Btu) isequal to 1055 joules or 25 2 thermochemical calories.For a unit weight of a substance, the ratio of added heat tothe temperature rise is the specific heat of the substance. InEnglish units specific heat is given as Btu per pound pe rdegree Fahrenheit of temperature rise (Btu/lb;oF). The heatcapacity of a substance is defined as th e ratio of added heatto the temperature increase of a unit volume of the substance(Btu/ft3 ;o F). Heat capacity is strongly affected by thedensity of a substance, and is related to specific heat by theequation:

Heat capacity = Density x specific heatMuch energy is involved in th e change of state of asubstance, such as occurs when ice is changed to water, orwater to vapor. The heat absorbed when a solid is changed toa liquid, or that is released when a liquid is changed to a solid

is the hea t of fusion. The heat required to change a liquid toa vapor and that is released when a vapor changes to a liquidis the heat of vaporization. In wildland fire, the heat ofvaporization of water is of major importance through itseffect on water processes, heat needed for ignition, and theburning rate of fuels.The maximum amount of heat that can be obta ined from afuel when it burns is i ts heating value. For most wildlandfuels the heating value ranges from 8000 to 9500 Btu pe rpound. However, fuels with large amounts of resins, oils, orwaxes may have higher heating values, while those with highmineral content can be lower. Only part of the heating valueof fuels is realized from a wildland fire, and the heating valuedecreases as increasing moisture content increases.

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Countryman 1977 Part1 Nature of Heat Yellow Ocr

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