Steam generator with axial flow preheater

Abstract

An improved heat exchanger utilizing a primary heated fluid to vaporize a secondary fluid flowing axially therethrough including a preheater assembly located within a cold leg side of the heat exchanger to allow axial flow of the secondary fluid through a preheater assembly to heat the secondary fluid to it boiling point temperature. The heat exchanger generally comprises a vertical shell portion with a tubesheet disposed adjacent a lower end thereof and a tube bundle which includes a plurality of U-shaped tubes through which the primary fluid flows. The preheater assembly is located in the vicinity of the tubesheet wherein the secondary fluid enters the preheater through a fluid inlet nozzle into a lower feedwater inlet box which is disposed around the inlet nozzle for dispersing the secondary fluid from the nozzle over a greater area. Specifically, the feedwater inlet box may include a double-perforated plate assembly whose flow resistance produces a low velocity uniform introduction of feedwater into the tube bundle. This water flows upward providing axial-counter flow and enhances heat transfer. This cold water is introduced near the coldest end of the tubes where it is most effective, while the design prevents thermal shock of the tubesheet with a layer of warm recirculating water providing a cushion between the tubesheet and the cold feedwater.