GB728838A - Improvements in or relating to vapour generating and superheating units - Google Patents

Abstract

728,838. -Fluid-pressure servomotor control systems. BAILEY METERS & CONTROLS, Ltd. July 4, 1952 [July 5, 1951], No. 16852/52. Class 135. In a vapour generator with a superheating surface, the temperature of the vapour which has left the superheater is controlled by regulating the quantity of gas flowing through a superheater by-pass, or the quantity of cooling fluid supplied to a vapour attemperator, in accordance with a temporarily availing control impulse derived from the gas temperature in the neighbourhood of the superheating surface, the quantity of gas by-passed or the quantity of cooling fluid supplied increasing on a rise in such heating gas temperature. The invention is shown applied to a steam generator comprising a furnace 1, Fig. 1, with walls lined by the water tubes, and a gas path consisting of a tube screen 3, secondary superheater 4, primary superheater 5, economizer 6 and air heater 7. A by-pass 9 around a portion of the superheating surface is provided, and the distribution of gases between the main and by-pass paths is controlled by dampers 10, 10<SP>1</SP>. The burners X, Y, Z are spaced along the length of the furnace, and each burner is fed with a separate supply of pulverized fuel entrained in air from an induced draught fan and with secondary air from a forced draught fan 40, this latter air flowing through the heater 7 and past regulating dampers situated just before the burner boxes. The temperature of the gas in the vicinity of the superheating surface is measured by bolometers 16, Fig. 5, or by thermo-couples and the ensuing signals are applied to a recorder-controller 20 which actuates a pilot valve 22 to apply an appropriate air-loading pressure to a relay 32. A measure of the temperature of the steam at the outlet is also obtained and, by means of a recorder-controller 25, a pilot valve 27, and a relay 30 giving a proportional and time-integral control, is applied to relay 32. The output of this latter relay is passed through a manual-automatic change-over panel 34 and applied to a power device 19 which positions a damper 10 in the gas by-pass 17. The temperature of the heating gases as detected by the bolometers 16 thus furnishes a coarse control of the final steam temperature, and the latter itself gives the fine control to its desired value. The power device 19 may consist of a diaphragm motor actuating a valve controlling the supply of cooling fluid to an attemperator positioned between the primary and secondary superheaters; The rate of firing by the individual burners X, Y, Z is selectively controlled and, as the boiler load decreases, burner X is the first to be shut-down to be followed later by burner Y. Thus the steam pressure exercises a control over all the burners, and burners Z are additionally controlled by the steam and air flow. All three of the above variables exercise a controlling influence on vanes situated in the inlet to, and on a valve which,controls the speed of the forced draught fan, and such speed is also correlated with that of a hydraulic tachometer. Vanes in the inlet of the induction draught fan, and also the speed of such fan, are controlled in a similar manner by the furnace draught. An arrangement having only two banks of burners is described. The various controls are all effected through relays as described in Specification 461,389, or modifications of such relays. Specifications 425,507, 656,551, [both in Group XII]; 707,225 and 707,226, [both in Group XL (b)], also are referred to.