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Show 11 N2=120Nm3/h |Open the teeming hole- ' i rL / • r • *.. . • ^•""^ 0 40 80 120 160 Waiting time (min) Fig.20 O: content in tundish with sealing gas supply 4.2 Conventional non-oxidizing heating methods A couple of conventional methods were evaluated, turning out the unacceptable results summarized in Table I . Despite of its good operability, the electric heating system has an uneasy problem in its heater life. The reduction burner method, which is deemed the first candidate in the light of operational requirement, has a serious safety problem in C O gas emission and explosion, whose possibility cannot be neglected in the tundish heating operation. Even though such problems are solved, the expensive investment cost as well as high running cost are expected, resulting in the decline of further study on such conventional methods and a necessity for the different type of non-oxidizing heating method. 4.3 Basic construction and function of N2 Jet Heater 4.3.1 Principle and outline A pair of heat exchangers with regenerator are equipped onto the newly developed N2 Jet Heater so as to provide the heated up nitrogen gas constantly into the tundish. The two modes are cyclically repeated in each combustion chamber with one half of phase difference. In the "combustion mode", the regenerator is heated up by the combustion of fuel gas and used nitrogen gas exhaled out of tundish so as to maximize the thermal efficiency. In the following "N2 Jet mode", the thermal energy accumulated in the regenerator is transferred to the inhaled fresh nitrogen gas. which is exhaled out of chamber into tundish with sufficient heating capacity. Fig.21 schematically shows such mode change. In the upper illustration, the Table I Summarv of evaluation on conventional methods ~"~^--.__ Equipment Maximum temperature Reducing conditions Operability Safety Technical problem Running cost Equipment cost Electric heater j \ •®] AMJ N2 * Ca I300'C Good No problem 1) The life of heater is short (Several ten hours under the high temperature) 2)Prevennon of the heater from falling off Expensive Expensive Reducing burner COG or LPG I _ exhaust -Jr0: ft nCa. 2 000*C7 CO/CO: > 3 4 H2/H2OI.I Poor 1 (Possibility of explosion 2)Possibility of C O emission 1 (Combustion control (Excess air ratio .0 j~0.4) 2)Disposal of the unburn! gas 3 (Prevention of the C pick up Expensive Expensive |
