Feedwater heaters have suffered premature failures in the desuperheating zone resulting from a combination of high cross flow velocities and relatively long baffle spacing. High steam flows coupled with longer unsupported tube spans necessary to keep pressure drops low and tube walls dry create an increased probability for flow induced vibration. This vibration is primarily the result of a fluidelastic whirling of the tube. The most significant factor in the calculation of the natural frequency of the tube is unsupported span length. The natural frequency varies as the square of the unsupported span. Keeping the span short is critical in avoiding flow induced vibration.rnNo-Tubes-in-the-Window (NTIW) baffling allows the use of intermediate support plates without affecting the mean flow velocity in the baffle space. This allows the thermal design engineer the distinct advantage of providing a design with a very high safety margin at any anticipated overload condition. The asymmetrical flow pattern necessitated by the U-tube design of high pressure feedwater heaters can be appropriately modeled to yield an overall design heat transfer coefficient.
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