External flow heal transfer

Movement of the Huid may be generated by means external to the heat transfer process, us by fans, blowers, or pumps. It may also be created by density differences connected with the heat transfer process itself. The first mode is culled timet cniireeiirtn the second one natural or free t ttttveclion. Convection heal transfer may also be classified as heat transfer in iltni /fnn. or in interna flow (over cylinders, spheres, air foils, and similar objects). In ilie case of external flow, the heal transfer process is essentially concentrated in a thin fluid layer surrounding the object (boundary layer . 

The magnitude of the natural convection heal transfer between a surface and a fluid is directly related to the flow rate of the fluid. The higher the flow rate, tbe higher the heat transfer rate. In fact, it is the very high flow rales that increase the heat transfer coefficient by orders of magnitude when forced convection is used. In natural convection, no blowers are used, and therefore the flow rale cannot be controlled externally. The flow rale in this case is established by the dynamic balance of buoyancy and friction. 

Hot oil is to be cooled in a multipass shell-and tube heal exchanger by water. The oil flows through the shell, with a heat transfer coefficient of/i = 35 W/m - X, and the water flows through the tube with an average velocity of 3 in/s. The lube is made of brass (k = 110 W/m X) with Internal and external diameters of 1.3 cm and 1,5 cm, respectively. Using water properties at 25X, determine the overall heat transfer coefficient of this heat exchanger based on ihe inner surface. 

The concept of a circulating flow reactor was further developed in the Buss reactor technology (Figure 1.26). Large quantities of reaction gas are introduced via a mixer to create a well dispersed mixture. This mixture is rapidly circulated by a special pump at high gas/liquid ratios throughout the volume of the loop and permits the maximum possible mass transfer rates. A heal exchanger in the external loop allows for independent optimisation of heat transfer. For continuous operation, the product is separated by an in-line cross-flow filter which retains the suspended solid catalyst within the loop. Such a system can operate in batch, semi-continuous and continuous mode. 

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