Patent GB-758554-A (GB)
Title: Improvements in or relating to tubulous vapour generating and vapour heating units and an improved method of generating and superheating vapour
IPC: F22G5/06
Filed: 1954-03-29
Abstract: 758,554. Recirculating combustion products. BABCOCK & WILCOX, Ltd. March 29, 1954 [March 29, 1953], No. 9103/54. Class 51(1). [Also in Groups XIII and XXIX] In a furnace unit comprising a radiant vapour generating section, a convection vapour heating section, and gas recirculation means, the recirculated gases are introduced into the combustion chamber in such manner that, for high loads on the unit, the recirculated gases are mixed with they main combustion gas and, for low loads, they form a blanket over the heat exchange surfaces of the vapour generating section. In an embodiment, the walls of the combustion chamber 1, Fig. 1, are lined with water tubes and fuel is supplied through horizontal rows of burners R<SP>1</SP> .... <SP>3</SP> The combustion gases pass through a superheater and reheater screen 65, through parallel passes 42 housing superheater and reheater tubes respectively and past an economizer 48. From the screen 65, the gases flow in separate passes the outlets of which are controlled by separate sets of dampers E, F. Separate by-passes to the economizer are controlled by separate sets of dampers C, D. The recirculated gases pass through a fan 44, and flow past dampers G to horizontal rows of inlets 19 ... 22 alternating with the burner rows. The inlets 19 ... 22 are controlled by manually- or automatically-actuated dampers, and with the outer rows of dampers closed and the inner ones opened, the recirculated gases are mixed with the incoming fuel, and when the positions of the dampers are reversed, a blanket of the cooler recirculated gases will form over the tubes lining the walls of chamber 1. A control system for carrying out the invention is shown in Fig. 3 wherein the pilot valves employed are as in Specification 425,507 and the various relays are similar to or, as indicated below, modifications of the relay shown in Fig. 4 of Specification 461,389, [Group XXIX]. The dampers G determining the percentage of recirculated gas are controlled from instruments 70, 74 measuring, respectively, the vapour flow (load) and gas flow through the unit. The measuring instruments actuate pilot valves 71, 75 to give output pressures which are applied to the A and C chambers of a relay 73, and as there is no connection between the C and D chambers, the relay serves to add the effect of the two variables, and its output actuates the dampers G. Actuation of the recirculation proportioning dampers A, B regulates the final superheat temperature, and these dampers are controlled by the vapour flow, a variable 85 representative of the product of gas flow and gas temperature, and the final superheat temperature 88. The outputs of pilot valves 71, 86 are applied to the A and B chambers of a relay 81 which thus gives an effect proportional to the difference of the two variables, and the ouput of pilot valve 89 is applied to a relay 91 which has a proportional and integral action. The outputs of relays 81, 91 are combined in a relay 79 which is similar to relay 73 and controls the operation of dampers A, B. The desired reheat temperature is maintained by proportioning the gas flow between dampers C, E and D, F positioned in the reheater and superheater passes, respectively, the control system effecting actuation of dampers C, D together and dampers E, F together in sequence. In controlling the dampers, the ouput of relay 79 is applied to both chambers A and, C of relays 95, 96 which thus give a doubled output. The output from relay 95 is led to the A chamber of relays 97, 98 and the C chamber of those relays is supplied with a loading pressure obtained from a pilot valve 111 and representative of the difference of superheat and reheat temperatures. Relays 97, 98 serve to add the effects due to the two variables by which they are controlled. The input to relay 98 from pilot valve 111 is first passed to the B chamber of a relay 113 which thus reverses the sense of the pressure change in the signal. Relays 97, 98 control, respectively, the dampers C, D ; and dampers E, F are similarly controlled by further relays 102, 103 but in these latter relays the input signals are applied to the B and C chambers, so that the output is representative of the difference, instead of the sum, of the variables. Manualautomatic change-over valves may be inserted in the lines leading to the motors operating the members A ... G.