GB1037995A - Improvements in or relating to tubulous vapour generators of the forced flow, once through type - Google Patents

Abstract

1,037,995. Forced flow steam boilers. BABCOCK & WILCOX Ltd. June 17, 1963 [June 15, 1962], No. 23257/62. Heading F4A. In a forced flow, once through boiler heated by a fluid entering the boiler at a comparatively low temperature, for example the gas coolant from a nuclear reactor, first and second parallelconnected sections in which evaporation occurs are provided, the region of the second section in which evaporation commences being disposed downstream in the heating fluid flow path of the region of the first section in which evaporation commences and being arranged to operate with a lower pressure of said region of the second section than in said region of the first section, thereby increasing the efficiency of utilization of the available heat, reducing the overall pressure drop through the tube system and hence increasing the pressure at which steam is generated. In the form shown, the boiler consists of banks 4, 7 ... 11 and 13 of horizontal, return-bend tubes arranged one above another in a vertical pressure vessel 1 down through which the heating gas flows. Feedwater enters the lowest bank 4 which acts as an economizer and passes into a header 6 whence it flows in parallel streams through a first system comprising the tube banks 7, 9, 11 which constitute 90% of the heating surface of the two systems and a second system comprising the tube banks 8, 10 constituting the remaining 10% of the heating surface to a header 12 wherein the streams unite for flow through a common superheater 13. In the first system the bank 7 and the first part of the bank 9 are economizer sections. The remainder of the bank 9 is an evaporating section and the bank 11 is a superheater. The full load pressure drop through this sytem is considerable, for example, 80 lb. per sq. in. In the second system the first part of the bank 8 is an economizer section and the remainder an evaporating section. The bank 10 is a superheater and ensures, as does the superheater 11, that steam enters the header 12 with a small amount of superheat. The full load pressure drop in this system is small, for example, 15 lb. per sq. in. Flow restrictors 15, 16 giving respective pressure drops at full load of 40 lb. and 105 lb. are fitted at the inlets to the first and second systems respectively, their effect being that the pressure in the region where evaporation commences in the first system is higher than that in the region where evaporation commences in the second. Hence the evaporation temperature in said region of the first system is higher than that in said region of the second system where the heating gas temperature is lower and more efficient utilization of the heating gas is obtained. Moreover, the overall pressure drop in the boiler is reduced. In a modification, the economizer bank 7 is divided into two parts connected respectively to the banks 8, 9.