An investigation on flow parameters and flow characteristics necessary for improving performance of heat exchangers
This study investigates the flow parameters and flow characteristics necessary for improving performance of heat exchangers.
For a certain heat exchanger, the heat exchanger duty is depending on flow characteristics (Reynolds number) and fluid characteristics (prandtl number). The straight forward method to improve heat duty is by increasing the velocity therefore increasing the Reynolds number, but if so, heat exchanger performance is not increased for sure, since the increase in velocity will result in increase in pressure as well.
On the other hand, by changing the flow pattern, to modify the laminar sub-layer near the surface, which has the largest thermal resistance, the flow would be guided to make remarkable increase in heat transfer coefficient (nusselt number) with small increase in pressure loss. This could be achieved by either affecting directly the flow near the surface, the core flow or make a swirl flow. Fortunately, for many situations, the modifications done to alter the flow increase the surface area of heat transfer as it works as fins.
The increase in heat transfer is a well-studied subject named heat transfer enhancement, augmentation or intensification. There are many fabricators for enhanced tubes. Many configurations are used and the decision of selection a certain type depends on many parameters such as mode of flow and mode of heat transfer. For an owner engineer perspective, there is another important decision to be taken, which exchanger has the priority to be enhanced in a plant for the purpose of retrofitting an existing heat exchanger network (HEN).
Retrofit of HEN in an existing plant is important in energy saving. In the conventional way, by implementing additional areas, the heat load of an exchanger will increase so that energy recovery will be increased. However, in practice, implementation of physical area increase is difficult due to topology, safety and downtime constraints. Moreover, the current retrofit designs normally include many topology modifications which will increase retrofit investment significantly.
Due to these problems in HEN, HTE can be very attractive in HEN retrofit design. Because the implementation of enhancement devices is relative simple, which means reduced civil and pipe work cost and a short modification duration.
The optimum exchangers to retrofit in heat exchanger network are obtained by step by step methodology; the example of retrofitting is the process of preheating crude oil in a distillation unit. This case study shows 3.4% energy saving accompanied with a small initial cost.
As a result of this study the statement “using heat transfer enhancement techniques in retrofitting an existing heat exchanger is the first choice from economic perspective” is proved and explained.