Chapter 8-INTRODUCTION TO THERMODYNAMICS 



FLUID 

 FILM 



FLUID 

 FILM 



tr 



< 



s 



111 



I- 



147.61 

 Figure 8-3. — Effect of fluid film on heat transfer. 



detrimental effect upon heat transfer. Such de- 

 posits not only reduce the efficiency of the heat 

 exchanger but also tend to cause overheating of 

 the tube metal. 



In surface heat exchangers, the components 

 may be arranged so as to provide parallel flow, 

 counter flow, or cross flow of the two fluids. In 

 parallel flow (fig. 8-4) both fluids flow in the 



same direction. Parallel-flow heat exchangers 

 are rarely used for naval service, largely be- 

 cause they would require an impossibly long 

 heat transfer surface to achieve the required 

 amount of heat transfer. In counter flow (fig. 

 8-5) the two fluids flow in opposite directions. 

 Many heat exchangers used aboard ship are of 

 the counter-flow type. In cross flow (fig. 8-6) 

 one fluid flows at right angles to the other. Cross 

 flow is used particularly where the purpose of 

 the heat exchanger is to remove latent heat and 

 thus change the physical state of a substance. 

 Main and auxiliary condensers are typically of 

 the cross-flow type, as are several other small- 

 er shipboard condensers. 



Surface heat exchangers are referred to as 

 single-pass u nits, if each fluid passes the other 

 only once, or as multipass units, if one fluid 

 passes the other more than once. Multipass flow 

 may be obtained by the arrangement of the tubes 

 and of the fluid inlets and outlets, or it may be 

 obtained by using baffles to guide a fluid so that 

 it passes the other fluid more than once before 

 it leaves the heat exchanger. 



THE FIRST LAW OF THERMODYNAMICS 



In the previous discussion of energy, we have 

 occasionally assumed a general principle which 

 must now be stated. This principle is called the 

 principle of the conservation of energy. The 

 principle may be stated in several ways. Most 

 commonly, perhaps, it is stated as energy can 

 be neither destroyed nor created, but only 

 transformed. Another statement is that energy 

 may be transformed from one form to another, 

 but the total energy of any body or system of 

 bodies is a quantity that can neither be increased 

 nor diminished by the action of the body or bod- 

 ies. Still another way of stating this principle is 

 by saying that the total quantity of energy in the 



■ SECOND FLUID 



FIRST FLUID - 



- SECOND FLUID 



98.30 



Figure 8-4.— Parallel flow in heat exchanger. 



98.31 

 Figure 8-5. — Counter flow in heat exchanger. 



171 



