PRINCIPLES OF NAVAL ENGINEERING 



LATENT HEAT 

 OF VAPORIZATION 



(OR LATENT HEAT 

 OF CONDENSATION) 



BTU PER POUND OF WATER 



38.1 

 Figure 8-2.— Relationship between sensible heat and latent heat for water at atmospheric pressure. 



the fluid bring the molecules into such intimate 

 contact that heat transfer can and does occur. 

 Other things being equal, increasingthe velocity 

 of fluid flow increases heat transfer. ^^ 



Since the fluid film is almost motionless, 

 heat transfer through the film is very poor. The 

 effect of fluid films on heat transfer is shown in 

 figure 8-3. The temperature line indicates the 

 changes in temperature that occur as heat is 

 transferred from the hot fluid to the fluid film, 



It is important here to maintain the distinction, 

 previously established, between heat and temperature. 

 Increasing the velocity of flow increases the amount 

 of heat that is transferred, but decreasing the velocity 

 increases the temperature of the fluid. This fact is of 

 considerable practical importance in the design and 

 operation of heat exchangers. In a heat exchanger 

 designed for high velocity flow, stagnation of the flow 

 is likely to cause severe overheating of the heat ex- 

 changer metal. 



from this fluid film to the tube metal, from the 

 tube metal to the other fluid film, and from this 

 fluid film to the cold fluid. As may be seen, the 

 major part of the temperature drop occurs in the 

 fluid films rather than in the tube metal. Note, 

 also, that the thicker fluid film is more resistant 

 to heat transfer than the thinner fluid film. 



The velocity of flow and the amount of tur- 

 bulence in the flow affect heat transfer by alter- 

 ing the thickness of the fluidfilm. Increasingthe 

 velocity of flow diminishes the thickness of the 

 fluid film and thus increases heat transfer. 

 Turbulent flow breaks up the fluid film and thus 

 increases heat transfer. Although there are some 

 obvious disadvantages to excessive turbulence, 

 many heat exchangers are designed to operate 

 with a certain amount of turbulence so that the 

 fluid films will be kept to a minimum. 



In real heat exchangers, the accumulation of 

 deposits of scale, soot, or dirt on the inside or 

 the outside of the tubes has a profound and 



170 



