Patent GB-738021-A (GB)
Title: Improved methods of and apparatus for controlling the temperature of superheated vapour
IPC: F22G5/20
Filed: 1952-12-04
Abstract: 738,021. Fluid-pressure servomotor-control systems. BAILEY METERS & CONTROLS, Ltd. Dec. 4,1952 [Dec. 7, 1951], No. 30751/52. Class 135. [Also in Groups XII and XIII] In a vapour generator of the kind provided with a vapour generating radiant section and a vapour heating convection section, gas which has been cooled by contact with the convection section is recirculated to the radiant section, such recirculation being controlled by the temperature of the gas, or its total amount of available heat, as measured in the neighbourhood of the convection section. The gas temperature in the region of the superheater of a steam generator is taken as a measure of the final steam temperature, and by adjustment of the gas recirculation in dependence on the gas temperature, the final steam temperature is raised to a constant value for loads on the generator below the rated load. For loads above the rated load, the final steam temperature is lowered to the same constant value by removing some of the heat from the steam by means of a spray attemperator or by by-passing some of the gas around a section of the superheater. The invention is shown applied to a steam generator having a radiant section 1 lined with water tubes, followed by a tube screen 3, secondary superheater 4, primary superheater 5, economiser 6, and air heater 7. A by-pass containing an economiser 8 is provided for sections 5, 6, and both the main gas passage and the by-pass can be controlled by dampers 14, 15, respectively. Secondary air passing through the heater 7 is supplied by a forced draft fan 13 and flows to a burner box 11, and the primary air and fuel for the rows of burners X, Y, Z is supplied by fans 10 and mills 9, respectively. After passing the dampers 14, 15, some of the gas is tapped off through a pass 16 and recirculated by a fan 17 to the bottom of the radiant section of the combustion chamber. The temperature of the gas is measured at one of the locations A ... D by means of bolometers, or bare metal or high-velocity thermocouples, and this indication is applied to a pilot valve controlling various fluid-pressure relays which supply an output pressure for application to a motor actuating regulating dampers 18 in the gas recirculation passage. The final steam temperature is also measured and applied through a pilot valve and various relays so that it can be combined with the control signal furnished by the determination of gas temperature. The control effect due to the steam temperature determination regulates the dampers 15 or a valve in the water flow line to the attemperator, and the control system may be adjusted so that the two effects due to gas temperature and final steam temperature overlap or act sequentially. Measurements of other conditions of the plant, e.g. recirculated gas flow, attemperator water flow, air flow, temperature of steam following attemperation, may be injected into, and influence, the regulation provided by the control system. In a modification wherein one of the control effects is due to the total heat availability of the gas, the air flow and gas temperature are first detected and then combined in a balanced bridge network and amplifier circuit as in Specification 654,976, [Group XXXVII], before being injected in the fluid-pressure control system. Various selector valves are incorporated in the system to allow the dampers &c. to be actuated manually instead of being automatically controlled by the operating variables of the plant. In another modification, the method of operation constituting the invention is carried out manually from a central station where the operator actuates switches controlling the operating motors of the various darmpers &c. Specifications 425,507, [Group XII], 461,389, 656,551, [Group XII], 707,225 and 707,226, [both in Group XL (b)], also are referred to.