1.3.1 Double pipe heat exchangers

A double-pipe heat exchanger consists of two concentric pipes or tubes. The outer tube is called the annulus. In one of the pipes a warmer fluid flows and in the other a colder one. Due to the temperature difference between the fluids heat is transferred. By the word ‘fluid’ all substances that can ‘flow’ is meant. So the word fluid means not only liquids but also gases. In this part there will be looked at a double-pipe heat exchanger with parallel flow. This means that the hot fluid and the cold fluid flow in the same directions. There are also counter flow heat exchangers. In this situation the hot fluid and the cold fluid flow in opposite directions.

Counter and parallel flow heat exchanger temperature profiles are as shown below. From this easily can be concluded that the counter flow is in any case more efficient than the parallel flow since the pipe fluid gets further cooled using this counter flow. While the temperatures T (of the cooled fluid) and t (of the warmed fluid) in the parallel flow heat exchanger can only approach each other, they can pass each other in the counter flow (Tout < tout) and in this case there has to be more heat been transferred. 

Figure.4 (double pipe heat exchanger)

This explains why in practice only counter flow will be seen in case of the double pipe heat exchangers. But there is one other advantage for the counter flow, since the maximum temperature differences between the two flows are much smaller, they suffer less thermal forces. Double pipe exchangers are mostly built of common water tubing. The use of two single flow areas leads to relatively low flow rates and moderate temperature differences. 

A straight double pipe heat exchanger as seen in the diagrams will not appear in practice. Most common are U-type or hairpin constructions. Due to the need of a removable bundle construction and the need for the ability to handle differential thermal expansions the exchanger is implemented in two parts.  The fluids enter and leave the exchanger by the four nozzles on the right while the exchanger can freely expand to the left which makes the of expansion joints to the other machinery superfluous and makes demounti.

1.4  DEFINITIONS

Nano = 10^-9

1nm  =10^-9 meter

Nano particle = particle with a size between 0.1-1000 nm

Nano fluid =nano particle mixed in a conventional fluid

Conventional Fluid = water, oil, ethylene glycol

Heating Element = converts electricity into heat

Convection = heat transfer between a solid and conventional fluid

Steady State = temperatures remain constant with time

Laminar = dominated by diffusion and velocity profile is nearly linear

Turbulent = dominated by turbulent mixing

Reynolds Number = transition between laminar and turbulent boundaries       

  

Criteria	Microparticles	Nanoparticles
stability	Settle	Stable(remainin suspention almost indefinitely)
Surface/volume ratio	1	1000 times larger than that of micro   particles
conductivity	Low	High
Clog in    microchannel	Yes	No
Erosion	Yes	No
Pumping          Power	Large	Small
Table-1              Nanofluid heat transfer enhancement                        ·        Thermal conductivity enhancement                        ·        Convective heat transfer enhancement                        ·        Critical heat flux enhancement.