Heat exchanger, Classification of heat exchanger

              -: Heat Exchanger :- 

A device in which heat transfer takes place between two fluids ( hot and cool ) at different temperature is known as heat exchanger . 

Ex.- Boilers, Condensers , Cooling towers , evaporators etc .  

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  -:Classification Of Heat Exchanger:-

The Heat Exchanger are mainly divided into three groups according to their working features-

a. Recuperative 

b. Regenerative 

c. Mixed Type 

(a)  Recuperative :- 
                                                In recuperative two fluids performing the exchange of heat in the exchanger can flow  :- 

1. With each other in the same direction (  parallel flow ) or in opposite the directions ( counter flow )
2. At right angles to me another cross flow with both types of flow a single or a multi pass arrangements is possible. 

Advantages - 

1. It is easy for construction. 
2. It is more economical.
3. It is much suitable for stationery plant.
4. It has more surface area for heat transfer.

Disadvantages- 

1. It has more weight and slotting problem.
2. It gas less heat transfer coefficient.
3. It's generating capacity is less.

(b) Regenerative Heat Exchanger:-  
                                                                              We have seen that in regenerative heat exchanger the hot and clod fluid flow through one the same passage and heated surface is alternatively exposed to the hot and cold fluid.
                Therefore, if the periods of heating and cooling are of equal duration continuous heating requires two apparatus, while the hot fluid is cooled in one apparatus is switched over and heat transfer process is reversed. In generation Exchanger, the process of heat transfer is transient. The temperature of the wall varies as it cools or heat variation in the wall temperature is of course; accompanied by change in the fluid temperature with tine and alone the heating surface as well owing to such complex temperature difference in time and space, precise calculation of regenerative exchanger very difficult and practically Insoluble problem.

                          Conductive, accumulating capacity, duration period and fluid temperature etc. Which in operating condition the heat transfer coefficient may vary due to the burning of gas in the regenerator, flying ash deposit etc.

Advantage and disadvantage:-

1. It has higher transfer coefficient.
2. It has less weight per H.P. of the plant.
3. It minimum pressure  loss.
4. It is self-cleaning apparatus so no shooting troubles.
5. A study distribution of temperature in space coordinates .
6. It gives quick response to the variation in load.
7. Small bulk weight and appreciably greater efficiency.
8. It is costlier compared with recuperative heat exchanger.
9. Leakage is the main trouble and requires perfect sealing.

(C) Mixed Type Heat Exchanger:- 

                                                                                    As the name implies, the direct contact type exchanger is one in which the two fluids are not reported from one another. If heat is to be transferred between a gas and liquid feed healers of the liquid is sprayed in the form of drops through the other.
It is a more complex phenomenon of heat transfer where the heat transfer takes place with mass transfer. The heat is mostly carried with the air it's performance not only depends upon the temperature difference by relative humidity of air. Until now develops and it's design is done more or less by trial and find error method.
                                              The common examples of this type are feed heater, cooling towers and evaporative condensers.

Advantage and disadvantage:-

1. Can be used in systems with higher operating temperature and pressures.
2. Pressure drops across a tube cooler is less.
3. Tube leaked are easily located and plugged since pressure test is comparatively easy.
4. Heat transfer efficiency is less compared to plate type cooler.
5. Required more space in comparison to plate cooler.