GB750092A - An improved method of and apparatus for regulating the temperature of a fluid - Google Patents

Abstract

750,092. Combustion apparatus. BAILEY METERS & CONTROLS, Ltd. April 21. 1953 [April 21, 1952], No. 10975/53. Class 51(1). [Also in Groups XIII and XXIX] In an indirect contact heat exchanger, the heat availability of the heating fluid, or the temperature of the heated fluid, is controlled by a factor representative of the mass heat flow rate (i.e. the combination of control impulses representative of mass flow and temperature) of the heating fluid. The invention is applicable to process heating but is shown applied to steam generators with superheating and also reheating of the steam. In an embodiment, a steam generator comprises a furnace lined with water tubes constituting the generating section and fired by rows of burners at different levels. The combustion gases from the generating section pass in succession through a tube screen, a secondary superheater, three damper-controlled parallel passes of which the outer ones house a primary superheater and the middle one houses a reheater, an economiser, and an air heater. Some of the combustion gases between the air heater and the stack can be diverted through a damper-controlled conduit and recirculated back into the furnace. A dampercontrolled by-pass around part of the superheater section is also provided. A spray attemperator is provided between the primary and secondary superheaters and a further spray attemperator, which is normally manually controlled, is provided for steam flowing to the heater. An average temperature of the combustion gas in the neighbourhood of the superheating and reheating sections is obtained by means of a gas-filled element, thermo-couples, a bolometer, &c., and the value of the gas flow is obtained from a measurement of the pressure drop across a section of the gas flow path. The values of temperature and flow so obtained are combined to give a measure of the mass heat flow. The mode of operation is shown in Fig. 10 where the various steam temperatures are plotted against boiler load. For loads below and near the control point (line TU), the uncontrolled temperatures M, P of the superheater and reheater, respectively, are raised in the direction of the desired final steam temperature X by the recirculation of furnace gases. Above the control point load, the temperature of superheat is lowered by spray attemperation (area SA) or by adjustment of the gas by-pass damper, and spray attemperation in the " overload " range (area RA) is also provided for the reheater. Differential operation of the dampers in the parallel superheater and reheater passes is provided throughout the total load range as indicated on the Figure. An automatic control system for operating the above is shown in Fig. 9. The various measuring devices 18, &c. actuate pilot valves 51, as described in Specification 425,507, and the output pressures from the pilot valves act through relays 56A, &c. and manual-automatic change-over valves 64 to control the motors actuating the various dampers, &c. The relays are similar in construction to that described in Specification 461,389 [Group XXIX], but in some instances the connections to the various chambers are modified as shown. The dampers 7 in the gas recirculation conduit are controlled by the final steam temperature 21, steam flow 18, and gas mass heat flow 30, the various control pressures being conbined in relay 60 to give an output pressure to which is added an integral control effect in relay 62. An integral control effect is also added by a relay 57 to the final steam temperature output pressure before the latter enters relay 60. In the control of the dampers 85 ... 87 in the superheater and reheater passes, a measure of the difference between the superheater and reheater final temperatures is applied through a relay 138, which adds an integral control effect, to a relay 140 where the measure of the mass heat flow 30 is added. The latter is first passed through a relay 141 in which the ratio of input to output can be adjusted. The superheater attemperator outlet temperature 107 and a final override is given by the final steam temperature 21. These two effects are combined in a relay 127 after an integral control effect has been added in a relay 56A to the final steam temperature signal. In this instance, the reheater attemperator valve 115 is hand controlled. In a further embodiment, the unit incorporates twin circuits for the superheater and reheater, the circuits operating in parallel with a cross-over between the primary and secondary superheaters. The various steam temperatures in the two circuits are taken separately and compared in a relay which selects the lower temperature or temperature difference to provide the appropriate control impulse. The method of control comprised in the invention may be carried out manually instead of automatically.