Patent GB-758374-A (GB)
Title: Improved methods of and apparatus for controlling the temperature of superheated vapour
IPC: F22G5/12
Filed: 1953-11-25
Abstract: 758,374. Steam superheaters. BABCOCK & WILCOX, Ltd., and BAILEY METERS & CONTROLS, Ltd. Nov. 25, 1953 [Nov. 26, 1952], No. 32665/53. Class 123(3). [Also in Group XXIX] A method of operating a vapour generating, superheating, and reheating unit to give regulated superheated and reheated vapour temperatures comprises, over a lower load range, controlling recirculation of furnace gases in accordance with superheated vapour temperature and gas flow through the superheating and/or reheating passes in accordance with reheated vapour temperature and, over a higher load range, controlling an attemperator for superheated vapour in accordance with superheated vapour temperature and the proportioning of gases between the superheater and reheater in accordance with reheated vapour temperature. The invention is applied to a high pressure steam generating unit in which the walls of the furnace of the radiant section are lined with vertical tubes and the heating gases, after leaving the radiant section, pass in succession through a tube screen, a secondary superheater, two primary superheaters and a reheater in parallel passes, and an economiser. After the economiser, gases can be withdrawn and recirculated to the lower part of the furnace. Fuel and air are fed into the furnace through vertically spaced burners. The invention is also applicable to units having duplicated superheater and reheater sections with the elements of the individual sections connected in parallel. A control circuit for carrying out the invention is shown wherein dampers 68 in the flue gas recirculation duct are controlled by three variables ; demand or load on the boiler as measured by combustion air flow AF or superheated steam flow SF, final superheated steam temperature TS4, and recirculated gas flow RGF. The valve 34 of a spray attemperator for limiting superheat temperature is controlled by final superheat temperature, attemperator outlet temperature TS3, and boiler load. The regulation or proportioning of gas flow through the superheater and reheater passes is effected by dampers 9-11 and 10 in the superheater and reheater passes, respectively, under the control of the difference, TS4-TR4, between final superheat temperature and final reheat temperature. The valve 49 of a spray attemperator in the reheater circuit is controlled by final reheat temperature TR4 and the temperature TR3 at the outlet of the attemperator. The various instruments for measuring the variables operate pilot valves 81, as described in Specification 425,507 [Group XII] which control the motors operating the dampers, spray attemperator valves, &c. through relays, such as 88, which are interconnected to produce the desired sequence of operation. The relays are as described in Specifications 709,021 [Group XXIX] and 461,389 [Group XXIX] or modifications of the latter. Switching valves 90 allow of changeover from automatic to manual control. In modified manually-controlled systems, the motors actuating the regulating dampers, &c. are electrically powered. Where the unit has parallel-connected duplicated superheater and reheater actions, the controlling variables are measured in each section and applied to the control system so as to keep the unit in balance. In a modification, two of the variables, load index and recirculated gas flow are replaced by a comparison of the mass heat flow of gases through the primary superheater pass with the mass heat flow of steam being superheated or reheated. An index of the mass heat flow of gases is obtained from a measure of the average pressure difference across the parallel superheater passes compensated for average gas temperature. Specifications 711,457 and 750,092 also are referred to.