OCR Text |
Show 13 4.3.2 Control sequence As mentioned above, two modes are repeated alternately in two N 2 gas heaters with one half of phase difference. Fig.23 shows the mode change diagram in two heaters and in total system. A s clearly understood from the diagram, the "N2 Jet mode" in one heater is completed a short time after the start of "N2 Jet mode" in the other, so that, in the total system, the high-temperature nitrogen gas is ceaselessly supplied into tundish. Because of such control sequence, the inside pressure of tundish can be kept always higher than the atmosphere with no suction of surrounding air. Heater A Heater B System "•\condition mode^-\^ Combustion N? Jet Combustion Na Jet Combustion N2 Jet Condition.l! ! Condition.2! ! Condition.1 ' / ,V.'. • A••. f//\ VJJA fJJ/JJ, y/y/y/y/y/A y y y y y y A \\ ***** , Fis.23 Control of N2 Jet Heater 4.4 Effects of N 2 Jet Heater on operations 4.4.1 Thermal aspects Fig.24 shows the change of temperature in two combustion chambers. This vouches that the nitrogen gas is effectively heated up to 1500°C. The effectiveness of the N2 Jet Heater in preventing temperature drop at the inner surface of the tundish is shown in Fig.25. With the N 2 Jet Heater, the inner surface temperature of the tundish after 8 hours of waiting can be increased by approximately 180°C in comparison with the temperature when the conventional non-heating method is used. As a result, even after 20 hours of waiting time, the tundish temperature can be secured over 900°C which is a critical limit for the smooth cast start. r 16oo 2 1 1500 CD E 1400 £ 5 1300 I 1200 o §1100 i 1000 V -Heater A ^Heater B 10 :mm 20 3C Time (min) Fig.24 Change of combustion chamber temperature 2 3 4 5 6 7 Waiting time (hr) Fig.25 Effect of N2 Jet Heater on tundish temperature |