Chapter 8- INTRODUCTION TO THERMODYNAMICS 



pound of steam which is at 212°F,wecan super- 

 heat^ it to 300° F. 



The same relationships apply when heat is 

 being removed. The removal of 42 Btu from the 

 pound of steam which is at 300° F will cause 

 the temperature to drop to212°F. As the pound 

 of steam at 212°F changes to a pound of water 

 at 212°F, 970 Btu are given off. When a gas or 

 vapor is changing to a liquid, we usually use the 

 term latent heat of condensation; numerically, 

 of course, the latent heat of condensation is 

 exactly the same as the latent heat of vaporiza- 

 tion. The removal of another 180 Btu will lower 

 the temperature of thepoundof water from 2 12°F 

 to 32° F. As the pound of water at 32° F changes 

 to a pound of ice at 32° F, 144 Btu are given off 

 without any accompanying change in tempera- 

 ture. Further removal of heat causes the tem- 

 perature of the ice to decrease. 



Heat Transfer Apparatus 



Any device or apparatus designed to allow 

 the flow of thermal energy from one fluid to 

 another is called a heat exchanger. The ship- 

 board engineering plant contains an enormous 

 number and variety of heat exchangers, ranging 

 from large items such as boilers and main con- 

 densers to relatively small items such as fuel 

 oil heaters and lubricating oil coolers. 



As a basis for understanding something about 

 heat transfer in real heat exchangers, it is nec- 

 essary to visualize the general configuration of 

 the most commonly used type of heat exchanger. 

 With few exceptions, 1^ heat exchangers used 

 aboard ship are of the indirect or surface type— 

 that is, heat flows from one fluid to another 

 through some kind of tube, plate, or other "sur- 

 face" that separates the two fluids and keeps 

 them from mixing. Most surface heat exchangers 



A vapor or gas is said to be superheated when its 

 temperature has been raised above the temperature 

 of the liquid from which the vapor or gas is being 

 generated. As may be inferred from the discussion, 

 it is impossible to superheat a vapor or gas as long 

 as it is in contact with the liquid from which it is being 

 generated. 



A notable exception is the deaerating feed tank, 

 discussed in chapter 13 of this text. Deaerating feed 

 tanks are basically described as direct-contact heat 

 exchangers,' rather than surface heat exchangers, be- 

 cause heat transfer is accomplished by the actual 

 mixing of the hotter and the colder fluids. 



are of the shell-and-tube types, consisting of a 

 bundle of metal tubes that fit inside a shell. 

 One fluid flows through the inside of the tubes 

 and the other flows through the shell, around 

 the outside of the tubes. 



The exchanges of thermal energy that take 

 place in even a simple heat exchanger are really 

 quite complex. The processes of conduction, 

 radiation, and convection are involved in practi- 

 cally all heat exchangers. Processes involving 

 latent heat— that is, the processes of evapora- 

 tion, condensation, melting, and solidification- 

 may contribute to the heat transfer problem. In 

 all cases, heat transfer is affected by physical 

 properties of the fluids which are exchanging 

 thermal energy and by physical properties of the 

 metal through which the change is being ef- 

 fected. The temperature differences involved, 

 the extent and nature of the fluid films, the 

 thickness and nature of the metals through which 

 heat transfer takes place, the length and area 

 of the path of heat flow, the types of surfaces 

 involved, the velocity of flow, and other factors 

 also determine the amount of heat transferred 

 in any heat exchanger. 



Because heat transfer is such a complex 

 phenomenon, heat transfer calculations are nec- 

 essarily complex. For some purposes, heat 

 transfer problems are simplified by the use of 

 an overall coefficient of heat transfer (U) which 

 may be determined experimentally for any 

 specific set of conditions. Tabulated values of 

 y are available for various kinds of heat ex- 

 changer metal tubes, for building materials, and 

 for other materials; in most cases the values of 

 U. are approximate, since various conditions 

 such as temperature, velocity of flow, condition 

 of the heat transfer surfaces, and the physical 

 properties of the fluids have a profound effect 

 upon the amount of heat transferred. 



The transfer of heat in a heat exchanger 

 involves the flow of heat from the hot fluid to 

 the tube metal and from the tube metal to the 

 cold fluid. In addition, heat must also be trans- 

 ferred through two layers of fluid (one on the 

 inside and one on the outside of the tube) which 

 are not flowing with the remainder of the fluid 

 but are almost motionless. These relatively 

 stagnant layers, known as boundary layers or 

 fluid films , are extremely small in size but 

 have an extremely important effect on heat 

 transfer. 



As previously noted, most fluids are very 

 poor transferrers of heat. As a fluid is flowing, 

 however, convection and mechanical mixing of 



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