PRINCIPLES OF NAVAL ENGINEERING 



heat) from one substance to another is normal- 

 ly reflected in a temperature change in each 

 substance— that is, the hotter substance be- 

 comes cooler and the cooler substance becomes 

 hotter. However, the flow of heat is not re- 

 flected in a temperature change in a substance 

 which is in process of changing from one physi- 

 cal state ' to another. When the flow of heat is 

 reflected in a temperature change, we say that 

 sensible heat has been added to or removed 

 from a substance. When the flow of heat is not 

 reflected in a temperature change but is re- 

 flected in the changing physical state of a sub- 

 stance, we say that latent heat has been added 

 or removed. 



Since heat is defined as thermal energy in 

 transition, we must not infer that sensible heat 

 and latent heat are really two different kinds of 

 heat. Instead, the terms serve to distinguish 

 between two different kinds of effects produced 

 by the transfer of heat; and, at a more funda- 

 mental level, they indicate something about the 

 manner in which the thermal energy was or will 

 be stored. Sensible heat involves internal kinetic 

 energy and latent heat involves internal potential 

 energy. 



The three fundamental physical states of all 

 matter are solid, liquid, and gas (or vapor). The 

 physical state of a substance is closely re- 

 lated to the distance between molecules. The 

 molecules are closest together in solids, farther 

 apart in liquids, and farthest apart in gases. When 

 the flow of heat to a substance is not reflected in 

 a temperature change, we know that the energy 

 is being used to increase the distance between 

 the molecules of the substance and thus change 

 it from a solid to a liquid or from a liquid to a 

 gas. In other words, the addition of heat to a 

 substance that is in process of changing from 

 solid to liquid or from liquid to gas results in 

 an increase in the amount of internal potential 



In thermodynamics, the physical state of a substance 

 (solid, liquid, or gas) is usually described by the term 

 phase, while the term state is used to describe the 

 substance with respect to all of its properties— phase, 

 pressure, temperature, specific volume, and so forth. 

 Thus the phase of a substance may be considered as 

 merely one of the several properties that fix the state 

 of the substance. While the precision of this usage 

 has some obvious advantages, it is not in standard use 

 among engineers. In this text, therefore, the term 

 physical state (or sometimes state) is used to denote 

 the molecular condition of a substance that determines 

 whether the substance is a solid, a liquid, or a gas. 



energy stored in the substance, but it does not 

 result in an increase in the amount of internal 

 kinetic energy. Only after the change of state 

 has been fully accomplished does the addition of 

 heat result in a change in the amount of internal 

 kinetic energy stored in the substance; hence, 

 there is no temperature change until after the 

 change of state is complete. 



In a sense, we may think of latent heat as 

 the energy price that must be paid for a change 

 of state from solid to liquid or from liquid to 

 gas. But the energy is not lost; rather, it is 

 stored in the substance as internal potential 

 energy. The energy price is "repaid," so to 

 speak, when the substance changes back from 

 gas to liquid or from liquid to solid; during 

 these changes of state, the substance gives off 

 heat without any change in temperature. 



The amount of latent heat required to cause 

 a change of state— or, on the other hand, the 

 amount of latent heat given off during a change 

 of state— varies according to the pressure under 

 which the process takes place. For example, it 

 takes about 970 Btu to change 1 pound of water 

 to steam at atmospheric pressure (14.7 psia) 

 but it takes only 62 Btu to change 1 pound of 

 water to steam at 3200 psia. 



Figure 8-2 shows the relationship between 

 sensible heat and latent heat for one substance, 

 water, at atmospheric pressure." If we start 

 with 1 pound of ice at 0°F,we must add 16 Btu 

 to raise the temperature of the ice to 32° F. We 

 call this adding sensible heat. To change the 

 pound of ice at 32°F to a pound of water at 32°F, 

 we must add 144 Btu (the latent heat of fusion ). 

 There will be no change in temperature while 

 the ice is melting. After all the ice has melted, 

 however, the temperature of the water will be 

 raised as additional heat is supplied. Again, we 

 are adding sensible heat. If we add 180 Btu— 

 that is, 1 Btu for each degree of temperature 

 between 32° F and 212°F-the temperature of the 

 water will be raised to the boiling point. To 

 change the pound of water at 212°F to a pound 

 of steam at 212°F, we must add 970 Btu (the 

 latent heat of vaporization ). After all the water 

 has been converted to steam, the addition of 

 more heat will cause an increase in the tem- 

 perature of the steam. If we add 42 Btu to the 



The same kind of chart could be drawn up for other 

 substances, but different amounts of thermal energy 

 would of course be required for each change of tem- 

 perature or of physical state. 



168 



