OCR Text |
Show 19 negat ive values) and, vi ce versa, superheater and reheater surfaces when TFE drops (tilt seeks positive angles). Only that limited amount of b 1 owi ng woul d be perfonned whi ch woul d suffi ce to keep TFE and the til t within the set limits (a fact which would also minimize control action within the automatic loops). Which surfaces to clean would be nominated by the cleanliness screens. The operator would be provided with a schedule for selecting blowers according to (i) the degree of loss of heat flux through particular surfaces, and (ii) intervals between their previous cleanings. To accomplish the above, the existing control panels for operating soot blowers would need modification. Their design, at least in the older units, reflects the philosophy that all tube surfaces are to be cleaned once, or perhaps several times per shift, but that this is best done all at once, or by using a rigidly defined cycle. This approach is of course not appropriate for the proposed control. Access to individual blowers from the control room ; s necessary. It should be stressed that the proposed control strategy offers a high likelihood of complete automation of boilers. There is no evident reason why the furnace-exit temperature (and, for a C.E. design boiler, the burner tilt) could not become set points in an automatic control loop. The cOrTJ>uter, rather than the operator, would be programmed to select the surfaces to be cleaned and would actuate soot blowers accordingly. CONCLUSIONS AND SIGNIFICANCE 1. A new boiler cleanliness monitoring systen has been developed. The cOrTJ>uter operated colour displays show the operator the degree of fouling in the furnace, primary superheater, both sections of the secondary superheater, the reheater and the economizer tube banks. 2. During bad fouling periods, wallblowers are often unable to renove slag. Heat absorption in the furnace is accommodated through dramatic increases in the heat absorption of the lower walls. This necessitates a furnace design with extra wall surface to allow operation during heavy foul i ng peri ods. 3. Instruments used to determine cleanliness of the convection banks include radi at ion pyrorreters and ei ghteen hanger tube thennocoupl es. The narrow band radiation pyrometers viewing particles in the flue gas have shown to be the best means of measuring gas temperature. A means of interpreting and calibrating signals has been incroporated into the softwa re. 4. The greatest loss of heat transfer due to fouling in a convection section was 40%. This occurred in the Radiant Superheater section during a period of heavy fouling. Local heat transfer losses as high as 80% were experienced at the furnace waterwalls. |