Double pipe heat exchangers (Counter and parallel flow heat exchanger temperature profiles, U-type or hairpin constructions, design calculations)

The double pipe heat exchangers are quite simple exchangers to analyse.

There are two possibilities: the use of a counter flow or parallel flow.

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. 

 

Fig.1. counter flow verses parallel flow

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. In figure 2 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 demounting much easier.

. 2: U-type or hairpin construction for a double pipe heat exchanger

 

The Double-Pipe Heat exchanger Design Calculations formulas(Click Here)