Heat Exchangers , Classification , Type of fluid flow arrangement

These are devices used for transferring heat energy from one fluid ( hot gas or stream ) to another fluid through a metal wall .

Classification :

- Type of fluid flow arrangement 

1. parallel flow heat exchangers .

2. Counter flow heat exchangers .

3. cross flow heat exchangers .

- Method of heat transfer .

1. indirect contact type 

2. Direct Contact type 

1. Parallel flow heat exchangers 

-  it is also known as counter current of parallel stream exchangers .

-  the hot and cold fluids streams enter together at one end , flow parallel to each other in same direction and leaves together at other end .

- with parallel flow the temperature difference between the two fluid is large at entrance end but it become small at the exit and as the two fluid temperature approaches each other .

-  although this flow arrangement is not used for the application requiring high temperature effectiveness it may be used in the following applications .

-  Temperature sensitive material , highly viscous liquids .

2. Counter Flow heat Exchangers .

- it is also known as counter current exchangers .

-  in this type the hot and cold fluid flow parallel but in opposite direction .

- the hot fluid entering at one end of the heat exchnagers flow path and cold fluid entering at the other end of the flow path .

-  it has three significance over the parallel flow design .

* the more uniform temperature difference between the two fluids minimizes the thermal stress .

* the outlet temperatures of the cold fluid can approaches the highest temperature of the hot fluid .

* the more uniform temperature difference produce a more uniform rate of heat transfer  .

- whether parallel or counter flow , heat transfer within the heat exchangers involves both conduction and convection .

3. Cross Flow heat exchangers

- in cross flow type , one fluid flow perpendicular to the second fluid , i.e. one fluid flows through tubes and second fluid passes around the tubes at right angle .

- in a cross flow management arrangement , mixing of either fluid stream may or may not occur depending on the design .

- these heat exchangers are typically used for heat transfer between a gas and a liquid .

- it is used in a cooling and ventilation system that requires heat to be transferred from one air stream to another .