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816 ENGINEERING THERMODYNAMICS

(i) Direct contact heat exchangers. In a direct contact or open heat exchanger the ex-
change of heat takes place by direct mixing of hot and cold fluids and transfer of heat and mass takes
place simultaneously. The use of such units is made under conditions where mixing of two fluids is
either harmless or desirable. Examples : (i) Cooling towers ; (ii) Jet condensers ; (iii) Direct contact
feed heaters.
Fig. 15.30 shows a direct contact heat exchanger in which steam mixes with cold water, gives
its latent heat to water and gets condensed. Hot water and non-condensable gases leave the con-
tainer as shown in the figure.

Non-condensable

gas

Container

Steam

Cold water

Hot water

Fig. 15.30. Direct contact or open heat exchanger.
(ii) Indirect contact heat exchangers. In this type of heat exchanger, the heat transfer
between two fluids could be carried out by transmission through wall which separates the two flu-
ids. This type includes the following :
(a) Regenerators.
(b) Recuperators or surface exchangers.
(a) Regenerators : In a regenerator type of heat exchanger the hot and cold fluids pass alter-
nately through a space containing solid particles (matrix), these particles providing alternately a sink
and a source for heat flow.
Examples : (i) I.C. engines and gas turbines ; (ii) Open hearth and glass melting furnaces ;
(iii) Air heaters of blast furnaces.
A regenerator generally operates periodically (the solid matrix alternately stores heat ex-
tracted from the hot fluid and then delivers it to the cold fluid). However, in some regenerators the
matrix is made to rotate through the fluid passages arranged side by side which makes the heat
exchange process continuous.
The performance of these regenerators is affected by the following parameters :
(i) Heat capacity of regenerating material,
(ii) The rate of absorption, and (iii) The release of heat.
Advantages :

1. Higher heat transfer coefficient ; 2. Less weight per kW of the plant ;


2. Minimum pressure loss ; 4. Quick response to load variation ;


3. Small bulk weight ; 6. Efficiency quite high.
   Disadvantages :


4. Costlier compared to recuperative heat exchangers.


5. Leakage is the main trouble, therefore, perfect sealing is required.