Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGamount of internal kinetic energy possessed byan object or a substance, and it is therefore anattribute or property of the substance. The movementor flow of thermal energy— or, in otherwords, heat— is an attribute of the energy systemrather than of any one component of it."^Units of MeasurementIn engineering, heat is commonly measuredin the unit called the British thermal unit (Btu).Originally, 1 Btu was defined as the quantity ofheat required to raise the temperature of 1 poundof water through 1 degree on the Fahrenheitscale. A similar unit called the calorie (cal) wasoriginally defined asthequantity of heat requiredto raise the temperature of 1 gram of waterthrough 1 degree on the Celsius scale. Theseunits are still in use, but the original definitionshave been abandoned by international agreement.The Btu and the calorie are now defined in termsof the unit of energy called the joule. ^ TheThe correct definition of heat is emphasized here inorder to avoid subsequent misunderstanding in thestudy of thermodynamic processes. It is obvious that"heat" and related words are sometimes used in ageneral way to indicate temperature. For example,we have no simple way of referring to an object witha large amount of internal kinetic energy except to saythat it is "hot." Similarly, a reference to "the heat ofthe sun" may meaneither the temperatureof the sun orthe amount of heat being radiated by the sun. Even"heat flow" or "heat transfer"—the terms quite properlyused to describe the flow of thermal energy— aresometimes used in such a way as to imply that heat isa property of one object or substance rather than anattribute of an energy system. To a certain extent,such inaccurate use of "heat" and related words isreally unavoidable; we must continue to "add heat"and "remove heat" and perform other impossibleoperations, verbally, unless we wish to adopt a verystuffy and long-winded form of speech. It is essential,however, that we maintain a clear understanding ofthe true nature of heat and of the distinction betweenheat and the stored forms of thermal energy.Several reasons contributed to the abandonment of theoriginal definitions of the Btu and the calorie. For onething, precise measurements indicated that the quantityof heat required to raise a specified amount of waterthrough 1 degree on the appropriate scale was not constantat all temperatures. Second— and perhaps evenmore important—the recognition of heat as a form ofenergy makes the Btu and the calorie unnecessary. Indeed,it has been suggested that the calorie and the Btucould be given up entirely and that heat could be expresseddirectly in joules, ergs, foot-pounds, or otherestablished energy units. Some progress has been madein this direction, but not much; the Btu and the calorieare still theunitsof heat most widely used in engineeringand in the physical sciences generally.following relationships have thus been establishedby definition or derived from the establisheddefinitions:1 calorie = -^^ watt-hour= 4.18605 joules= 3.0883 foot-pounds1 Btu = 251.996 calories= 778.26 foot-pounds= 1054.886 joulesThe values given here are, of course, considerablymore precise than those normally requiredin engineering calculations.When large amounts of thermal energy areinvolved, it is often more convenient to use multiplesof the Btu or the calorie. For example, wemay wish to refer to thousands or millions ofBtu, in which case we would use the unit kB(1 kB = 1000 Btu) or the unit mB (1 mB =1,000,000 Btu). Similarly, the kilocalorie maybe used when we wish to express calories inthousands (1 kilocalorie = 1000 calories). Thekilocalorie. also called the "large calorie," isthe unit normally used for indicating the thermalenergies of various foods. Thus a portion offood which contains "100 calories" actually contains100 kilocalories or 100,000 ordinary calories.Heat TransferHeat flow, or the transfer of thermal energyfrom one body, substance, or region to another,takes place always from a region of higher temperatureto a region of lower temperature.^ Inthermodynamics, the high temperature regionmay be called the source or the emitting region;the low temperature region may be called thesink, the receiver , or the receiving region.This statement, although entirely true for all practicalengineering applications, should perhaps be qualified.Energy exchanges between molecules may bethought of as being random, in the statistical sense;therefore, some exchanges of thermal energy may indeed"go in the wrong direction"—that is, from acolder region to a warmer region. On the average,however, the flow of heat is always from the higher tothe lower temperature.162