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HEAT TRANSFER 817

(b) Recuperators : ‘Recuperator’ is the most important type of heat exchanger in which the
flowing fluids exchanging heat are on either side of dividing wall (in the form of pipes or tubes
generally). These heat exchangers are used when two fluids cannot be allowed to mix i.e., when the
mixing is undesirable.
Examples : (i) Automobile radiators, (ii) Oil coolers, intercoolers, air preheaters, economisers,
superheaters, condensers and surface feed heaters of a steam power plant, (iii) Milk chiller of pas-
teurising plant, (iv) Evaporator of an ice plant :
Advantages :

1. Easy construction ; 2. More economical ;


2. More surface area for heat transfer ; 4. Much suitable for stationary plants.
   Disadvantages :


3. Less heat transfer coefficient ; 2. Less generating capacity ;


4. Heavy and sooting problems.
   The flow through direct heat exchangers and recuperators may be treated as steady state
   while through regenerators the flow is essentially transient.


5. Relative direction of fluid motion
   According to the relative directions of two fluid streams the heat exchangers are classified
   into the following three categories :
   (i) Parallel-flow or unidirection flow (ii) Counte-flow (iii) Cross-flow.
   (i) Parallel-flow heat exchangers. In a parallel-flow exchanger, as the name suggests, the
   two fluid streams (hot and cold) travel in the same direction. The two streams enter at one end and
   leave at the other end. The flow arrangement and variation of temperatures of the fluid streams in
   case of parallel flow heat exchangers, are shown in Fig. 15.31. It is evident from the Fig. 15.31. (b)
   that the temperature difference between the hot and cold fluids goes on decreasing from inlet to
   outlet. Since this type of heat exchanger needs a large area of heat transfer, therefore, it is rarely
   used in practice.

t (Temp.)

L (Length)

th 1

tc 2

th 2

tc 1

Hot fluid

Cold fluid

(b)

tc 2

tc 1

Cold

Cold

Hot th 2

(a)

th 1

Fig. 15.31. Parallel-flow heat exchanger.

Examples : Oil coolers, oil heaters, water heaters etc.

As the two fluids are separated by a wall, this type of heat exchanger may be called parallel-

flow recuperator or surface heat exchanger.

(ii) Counter-flow heat exchangers. In a counter-flow heat exchanger, the two fluids flow in

opposite directions. The hot and cold fluids enter at the opposite ends. The flow arrangement and

temperature distribution for such a heat exchanger are shown schematically in Fig. 15.32. The