Patent GB-1089508-A (GB)
Title: Improvements in or relating to boilers
IPC: F22B35/00
Filed: 1966-04-05
Abstract: 1,089,508. Once through forced flow vapour generators; controlling steam reheat temperatures. BABCOCK & WILCOX CO. April 5, 1966 [April 5, 1965], No. 15222/66. Heading F4A.  In a forced flow once through vapour generator, feed water from economizer 34 flows in parallel through the wall tube circuits of independently operable cyclone furnaces 27, then through tubes 42 lining the floor of furnace chamber 10 and into lower ring header 47. From there the water flows upwardly in a first pass of parallel tubes (37A, 38A, 39A) lining the furnace chamber walls 11, 13, 14 respectively to upper ring header 49, down through conduit 51 to lower ring header 52 and up through a second pass of parallel wall lining tubes (37B, 38B, 39B) alternating with the tubes of the first pass to upper headers 53, 54 and 56 respectively and then into headers 67, 59 and 62 respectively. From these latter headers the water flows through a third pass of parallel wall tubes 37C, 38C, 39C to the headers 58, 61, 63 respectively. The tubes 38C are spaced to form an outlet for the furnace gases passing from furnace chamber 10 to horizontal gas pass 19. From the headers 58, 61, 63 the water flows through tubes 67 lining the roof 16 of the unit and then in parallel up through tubes 71, 74, 78, 82 and 86 lining the upright walls of convection pass 22 and the side walls and floor of horizontal gas pass 19 respectively. The flow then passes through primary superheater 33, secondary superheater 28, a high pressure turbine 99, initial high pressure reheater 32, the final high pressure reheater 29, a second turbine 106, and low pressure reheater 31 on its way to a low pressure turbine 109. The initial high pressure reheater section 32 provides the major part of the total heating surfaces of the high pressure reheater sections 32, 29. Flue gases from upright pass 22 are recirculated through ducts 112, 113 back into the furnace chamber 10 by fan 111. The gases are supplied into the furnace chamber 10 through upper and lower ports 119, 121 respectively under the control of linked dampers 117, 118 respectively. These dampers which cannot be completely closed, operate so that the gas recirculation through ports 119 decreases as that through ports 121 increases and vice versa. At full load the majority of the recirculated gas flow is through upper ports 119, but on a reduction in load, the steam reheat temperatures as measured by sensors 129, 131, begin to fall, so controller 128 actuates the dampers 117 and 118 to increase the proportion of recirculated gas entering through the lower ports 121 and so maintain the reheat steam temperatures. The sensors 129 and 131 also actuate spray attemperators 133, 136 by means of controllers 132, 134 in the event that the reheat steam temperatures become too high. The superheater steam outlet temperatures and pressures are maintained by a controller 123 utilizing signals from temperature and pressure sensors 122, 124 respectively, which regulate the fuel, combustion air and feed water supplies. The total volume of recirculated flue gases is regulated by controller 127, based on the feed water supply rate measured by sensor 126, which activates damper 115 such that the volume of gas recirculation decreases with increasing load.