An Investigation on Flow Parameters and Flow Characteristics Necessary for Improving Performance of Heat Exchangers

Title             :   An Investigation on Flow Parameters and Flow Characteristics Necessary for Improving Performance of Heat Exchangers

Author        :  Karim Mahmoud Moustafa Saleh El-Shaarawy 

Collection   :  M.Eng. Mechanics 

Abstract:  

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 loss 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).
This study could be read as it is divided into two parts. Part one (the theory) an approach of study is used to dive into the flow characteristics and how to alter it to achieve higher increase in heat transfer with minimum increase in pressure loss. Part two (the application) is divided to two parts part one is a survey of manufacturer products and part two is a case study on how to effectively use these products to retrofit an existing heat exchanger in a heat exchanger network. As a result of this study the speech” using heat transfer enhancement techniques in retrofitting an existing heat exchanger is the first choice from economic perspective” is proved and explained.