GB771888A - Improvements relating to vapour generating and superheating units and to a method ofoperation thereof - Google Patents

Abstract

771,888. Controlling steam temperature in water-tube boilers. BABCOCK & WILCOX, Ltd. May 18, 1955 [May 19, 1954], No. 14763/54. Classes 123(2) and 123(3) In a water-tube boiler having a convection superheater and arranged to supply both superheated and saturated steam, the demand for saturated steam, when the superheated steam demand falls below a preset value, is met partly by steam passed directly from the steam and water drum to the saturated steam main and partly by steam leaving the superheater and passing through a desuperheater, the proportion of the total steam passing through the superheater being automatically decreased as the superheated steam demand decreases so that the desired superheat is maintained at low loads whilst providing for a sufficient flow of steam through the superheater at the lowermost demand for superheated steam, which may be zero, to prevent overheating of the metal thereof. The invention is shown applied to a twodrum, integral furnace type, marine boiler supplying superheated steam to a ship's main propulsion turbines and saturated steam for auxiliary purposes, the latter demand persisting or arising when the demand for superheated steam is small or zero. The boiler comprises a steam generating tube lined oil-fired combustion chamber 1 bounded on the one side by a tube screen 14 behind which extend two lateral gas passes 7, 8, separated by a tubulous partition 9, the pass 7 locating a superheater 5 and spaced banks 12 of inclined steam generating tubes whilst pass 8 locates spaced banks 15 of inclined generating tubes, the banks 12, 13 being disposed in staggered relationship to allow of access to the whole of the partition 9. The flow of gases through the superheater. pass 7 and the superheater pass' 8 is controlled by dampers 10 (not shown in Fig. 1) and 11. The tubes lining chamber 1 and those of banks 12, 13 extend between an upper steam and water drum 3 and a lower water drum 4, the latter housing a nested U-tube desuperheater 25. As shown in Fig. 6, steam from drum 3 can pass directly to a saturated steam main 44 through a pipe 43 in which is disposed an orifice 45 in the form of an adjustable valve secured at a predetermined opening. It can also flow to superheater 5 via a line 41. From the superheater outlet the superheated steam can pass to the main turbines via a line 42. It can also flow through a pipe 46 to the desuperheater 25 and thence through a line 47 in which is disposed a non-return valve 48 into the saturated steam main 44. The flow resistances of pipe 43. with its orifice 45 and of pipe 46, the desuperheater coil and pipe 47 are so chosen, having regard to the relative demands for superheated and saturated steam, that the proportion of the total steam passing through the superheater is automatically decreased as the superheated steam demand falls, allowing the desired degree of superheat to be maintained at loads down to 10 to 20 per cent of full load. Furthermore, a sufficient flow of steam through the superheater is maintained at times of lowermost superheated steam demand, which may be zero, to prevent excessive superheat temperature or overheating of the superheater metal. At and above a predetermined demand for superheated steam in comparison with the demand for saturated steam, the flow resistance ratio automatically provides that the demand for the superheated steam absorbs the whole output of the superheater 5, and all the saturated steam demanded by main 44 passes thereto from drum 3 via the orifice 45. The superheat temperature is controlled by the setting of dampers 10, 11 and this control may be supplemented in the uppermost load ranges by spray attemperation. In a modification, the non-return valve 48 is omitted and the setting of orifice 45 adjusted so that at the highest demand for superheated steam, when its temperature may be too high, saturated steam flows into pipe 42 through pipes 47, the desuperheater 25 and pipe 46 to reduce the temperature to the required value.