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Heat exchange between flowing fluids is one of the most important physical process 
of concern, and a variety of heat exchangers are used in different type of installations, as in 
process industries, compact heat exchangers nuclear power plant, HVACs, food processing, 
refrigeration, etc. The purpose of constructing a heat exchanger is to get an efficient method 
of heat transfer from one fluid to another, by direct contact or by indirect contact. The heat 
transfer occurs by three principles: conduction, convection and radiation. In a heat exchanger 
the heat transfer through radiation is not taken into account as it is negligible in comparison 
to conduction and convection. Conduction takes place when the heat from the high 
temperature fluid flows through the surrounding solid wall. The conductive heat transfer can 
be maximised by selecting a minimum thickness of wall of a highly conductive material. But 
convection is plays the major role in the performance of a heat exchange

Tubular Heat Exchangers:
Tubular heat exchangers are built of mainly of circular tubes although some other 
geometry has also been used in different applications. This type of construction offers a large 
amount of flexibility in design as the designing parameters like the diameter, length and the 
arrangement can be easily modified. This type is used for liquid-to-liquid (phase changing 
like condensing or evaporation) heat transfer. Again this type is classified into shell and tube, 
double pipe and spiral tube heat exchangers.
Double Pipe Heat Exchanger:
The double pipe or the tube in tube type heat exchanger consists of one pipe placed 
concentrically inside another pipe having a greater diameter. The flow in this configuration 
cane be of two types: parallel flow and counter-flow. It can be arranged in a lot of series and 
parallel configurations to meet the different heat transfer requirements. Of this the helically 
arranged stands out as it has found its place in different industrial applications. As this 
configuration is widely used, knowledge about the heat transfer coefficient, pressure drop, 
and different flow patterns has been of much importance. The curvature in the tubes creates a 
secondary flow, which is normal to the primary axial direction of flow. This secondary flow 
increases the heat transfer between the wall and the flowing fluid. And they offer a greater 
heat transfer area within a small space, with greater heat transfer coefficients. Study has been 
done on the types of flows in the curved pipes, and the effect of Prandtl and Reynolds number 
on the flow patterns and on Nusselt numbers. The two basic boundary conditions that are 
faced in the applications are constant temperature and the constant heat flux of the wall.

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