Development and Evaluation of Enhanced Heat Transfer Tubes for Transitional Reynolds Number Flow

Abstract

Heat transfer enhancement plays an important role in improving energy efficiency and developing high performance thermal systems. A wide variety of industrial processes involve the transfer of heat energy and many of those processes employ old technology. These processes would be ideal candidates for a redesign that could achieve improved process performance. Increasing efficiency in process plant operations is always a priority with engineers constantly looking for new ways to reduce energy requirements in process plants. Additionally, there is pressure from the government to reduce energy usage to meet economic and environmental goals. Utilization of an enhanced heat transfer tube is an effective method to be utilized in the development of high performance thermal systems. In many areas of the world the availability of process water is scarce and the lack of abundant cooling water volume causes major problems in process design. Extreme water availability risks exist across the Middle East and North Africa. Many countries in this region have a growing population and ambitious economic development plans, creating additional demands on water. Of particular importance to the global and regional economy is the use of large quantities of water in the production of oil and chemical products. Water scarcity could also lead to further increases in global oil prices and heightened political tensions to protect water supplies in the future. Use of enhanced heat transfer tubes to decrease process water requirements while at the same time provide higher levels of heat transfer in energy conversion processes are important design considerations. These were some of the goals that were considered when the Vipertex™ EHT series of enhanced tubes were developed. Enhanced heat transfer tubes must be considered in the design of high efficiency heat exchangers. Their use will allow operations to decrease the required cooling water mass flow rate in order to obtain the required heat transfer rate; allowing the heat exchangers to operate in the transitional flow regime, at flowrates not previously considered with current designs, will save both energy and water. Transition from laminar to turbulent flow for smooth tubes typically is assumed to occur for a Reynolds Number of 2300. In reality, a transition point is not as well defined and for some process conditions actually could occur over a wider range of Reynolds Numbers, typically varying between 2300 and 10,000. Vipertex™ enhanced tubes allow transition to occur earlier than 2300, providing increased heat transfer while at the same time using a smaller volume of cooling fluid. Vipertex™ enhanced surfaces, have been designed and produced through material surface modifications, which result in flow optimized heat transfer tubes that increases heat transfer through a combination of factors that include: increasing fluid turbulence, secondary flow development, disruption of the thermal boundary layer and increasing the heat transfer surface area. Considerations in the Vipertube™ design (when compared to smooth tubes) include the maximization of heat transfer; minimization of operating costs; and/or minimization of the rate of surface fouling. Vipertex™ was able to develop several optimized enhanced heat transfer tubes. This study details the heat transfer and fluid flow results of the Vipertex 1EHT enhanced heat transfer tube for a range of Reynolds Numbers to approximately 18,000. Optimized Vipertex 1EHT tubes produces a region of operation for the inside tube heat transfer performance that is more than 5 times greater than smooth tubes and requires only modest increases in the friction factor. These enhanced tubes recover more energy and provide an opportunity to advance the design of many heat transfer products.