PRINCIPLES OF NAVAL ENGINEERING 



amount of internal kinetic energy possessed by 

 an object or a substance, and it is therefore an 

 attribute or property of the substance. The move- 

 ment or flow of thermal energy— or, in other 

 words, heat— is an attribute of the energy system 

 rather than of any one component of it."^ 



Units of Measurement 



In engineering, heat is commonly measured 

 in the unit called the British thermal unit (Btu). 

 Originally, 1 Btu was defined as the quantity of 

 heat required to raise the temperature of 1 pound 

 of water through 1 degree on the Fahrenheit 

 scale. A similar unit called the calorie (cal) was 

 originally defined asthequantity of heat required 

 to raise the temperature of 1 gram of water 

 through 1 degree on the Celsius scale. These 

 units are still in use, but the original definitions 

 have been abandoned by international agreement. 

 The Btu and the calorie are now defined in terms 

 of the unit of energy called the joule. ^ The 



The correct definition of heat is emphasized here in 

 order to avoid subsequent misunderstanding in the 

 study of thermodynamic processes. It is obvious that 

 "heat" and related words are sometimes used in a 

 general way to indicate temperature. For example, 

 we have no simple way of referring to an object with 

 a large amount of internal kinetic energy except to say 

 that it is "hot." Similarly, a reference to "the heat of 

 the sun" may meaneither the temperatureof the sun or 

 the amount of heat being radiated by the sun. Even 

 "heat flow" or "heat transfer"— the terms quite prop- 

 erly used to describe the flow of thermal energy— are 

 sometimes used in such a way as to imply that heat is 

 a property of one object or substance rather than an 

 attribute of an energy system. To a certain extent, 

 such inaccurate use of "heat" and related words is 

 really unavoidable; we must continue to "add heat" 

 and "remove heat" and perform other impossible 

 operations, verbally, unless we wish to adopt a very 

 stuffy and long-winded form of speech. It is essential, 

 however, that we maintain a clear understanding of 

 the true nature of heat and of the distinction between 

 heat and the stored forms of thermal energy. 



Several reasons contributed to the abandonment of the 

 original definitions of the Btu and the calorie. For one 

 thing, precise measurements indicated that the quantity 

 of heat required to raise a specified amount of water 

 through 1 degree on the appropriate scale was not con- 

 stant at all temperatures. Second— and perhaps even 

 more important— the recognition of heat as a form of 

 energy makes the Btu and the calorie unnecessary. In- 

 deed, it has been suggested that the calorie and the Btu 

 could be given up entirely and that heat could be ex- 

 pressed directly in joules, ergs, foot-pounds, or other 

 established energy units. Some progress has been made 

 in this direction, but not much; the Btu and the calorie 

 are still theunitsof heat most widely used in engineer- 

 ing and in the physical sciences generally. 



following relationships have thus been estab- 

 lished by definition or derived from the estab- 

 lished definitions: 



1 calorie = -^^ watt-hour 



= 4.18605 joules 



= 3.0883 foot-pounds 



1 Btu = 251.996 calories 



= 778.26 foot-pounds 



= 1054.886 joules 



The values given here are, of course, con- 

 siderably more precise than those normally re- 

 quired in engineering calculations. 



When large amounts of thermal energy are 

 involved, it is often more convenient to use mul- 

 tiples of the Btu or the calorie. For example, we 

 may wish to refer to thousands or millions of 

 Btu, 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 may 

 be used when we wish to express calories in 

 thousands (1 kilocalorie = 1000 calories). The 

 kilocalorie. also called the "large calorie," is 

 the unit normally used for indicating the thermal 

 energies of various foods. Thus a portion of 

 food which contains "100 calories" actually con- 

 tains 100 kilocalories or 100,000 ordinary calo- 

 ries. 



Heat Transfer 



Heat flow, or the transfer of thermal energy 

 from one body, substance, or region to another, 

 takes place always from a region of higher tem- 

 perature to a region of lower temperature.^ In 

 thermodynamics, the high temperature region 

 may be called the source or the emitting region; 

 the low temperature region may be called the 

 sink, the receiver , or the receiving region. 



This statement, although entirely true for all practi- 

 cal engineering applications, should perhaps be quali- 

 fied. Energy exchanges between molecules may be 

 thought of as being random, in the statistical sense; 

 therefore, some exchanges of thermal energy may in- 

 deed "go in the wrong direction"— that is, from a 

 colder region to a warmer region. On the average, 

 however, the flow of heat is always from the higher to 

 the lower temperature. 



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