GB884132A - Improvements in or relating to the control of forced-flow, once-through boilers - Google Patents

Abstract

884,132. Forced-flow steam boilers; superheaters. DURRWERKE A.G. March 20, 1958 [March 20, 1957], No. 8928/58. Classes 123(2) and 123(3) A forced-flow, once-through boiler comprising economizers 2, 3, evaporating surfaces 4, 5 and primary, secondary and final superheaters 6, 7, 8 has a control heating surface 32 connected in parallel with the economizers 2, 3 and located in the path of the combustion gases from the boiler furnace, the flow of working medium through the control surface being held constant by the automatic adjustment of a valve 33 in accordance with variations of the pressure difference across a flow resistance 42 in the control surface outlet connection. The working fluid temperature as it flows through the surface 32 is measured at points 43, 44 and the temperature difference which, since the flow of working medium is constant is indicative of the heating value of the furnace gases, is applied in the form of a control signal to a main fuel controller 13, this signal anticipating those derived from changes in steam outlet pressure and temperature caused by fluctuation in the rate of fuel supply or in its calorific value and thus allowing the boiler firing rate to be more quickly adjusted to the new conditions. The steam temperature is regulated by two spray-type attemperators 20, 21 inserted respectively before the superheaters 7, 8. The water flows to the attemperators 20, 21 are respectively controlled according to measures of steam temperature at the outlets of the superheaters 7, 8. A working fluid by-pass 50 is provided for the evaporating surface 4, the amount of working fluid flowing in the by-pass being controlled according to measures of steam temperature and pressure at the outlet of the surface 5 to maintain a small degree of superheat at the said outlet and so ensure that the transition zone is always located in the heating surface 5. In operation, a master controller 11 controlling a feedwater regulator 9, the main fuel controller 13 and a combustion air control means 15 is set so that the boiler yields the desired output pressure throughout its load range. The steam temperature is automatically regulated by the attemperators 20, 21 and the heating capabilities of the furnace gases are maintained by signals derived from the heating surface 32.