THERMAL CONDITIONS

IN HOT-BLAST

STOVES

A. T. Y a k o v e n k o

UDC

669.162.238.4

In the Ninth Hve-Year Plan it is the intention at this plant to raise blast temperature to ll00~ by redesigning old hot-blast stoves and by building new stoves with disintegration-resistant Dinas in the high-temperature zone, increasing the calorific value of the gas, automating valve and gate switchover, centralized air feed to the burners for the set of stoves, optimization of cycle frequency, and other measures, New equipment still does not completely solve the problem of achieving a high level of blast heating. Thermoeouples were built into various levels of the brickwork to study thermal conditions in the hot-blast stoves. The measurements showed that the static instruments used to measure the temperature of the dome and of the outgoing combustion products could be used for precise assessment of the amount of heat stored and given out by the brickwork and checkerwork in various periods of stove operation. The dome temperature curves during the gas period define the heating of the upper tiers of checkerwork, those during the blast period define the amount of heat stored by the whole of the brickwork and checkerwork. The smaller the reduction in dome and blast temperature in the period, the better the hot-blast stove heats up and the higher the blast temperature. The temperature curves for the outgoing combustion products must rise smoothly, reaching the maximum by the end of the period. In such a case the checkerwork accumulates the maximum amount of heat at the prescribed heat loading. By analyzing the rise and fail of temperatures in the cycle the furnace gas man can takes steps beforehand to achieve the necessary blast temperature. The duration of the stove operating periods affects the blast temperature and the efficiency of the stove. Thus a prolonged blast period cools the checkerwork to a great depth, consequently reducing the blast temperature, lengthening the gas period, and so reducing the efficiency of the stove. Both the maximum dome temperature and its reduction must therefore be indicated in the thermal conditions for a hot-blast stove. The processing of 2000 diagrams (dome, blast, and outgoing combustion product temperatures, consumption of gas and blast, duration of operating periods) on a Minsk-22 computer using a special program has shown that to achieve a blast temperature of 1100~ the optimum gas and blast periods (valve switchover 0.1-0.15 h) must be 2.1-2.5 and 1.1-1o25 h, respectively. The optimum gas period can be calculated by the experimental formula

~g p t = r p

V

fl2td

p.c. t;t. c.

s wr

'

where r is the coefficient of hydraulic resistance; t d c is the maximum temperature of the combustion products under the dome, ~ rsw is the switchover time, h; ~and t~tc is the minimum temperature of the combustion products at the start of the gas period, ~ The coefficient of hydraulic resistance r is in the following relationship to changes in hydraulic resistance of the hot-blast stove tl, mm water column:

Donetsk Metallurgical Plant. Translated from Metallurg, No. 9, pp. 12-13, September, 1973.

9 1974 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission of the publisher. A copy of this article is available from the publisher for $15.00.

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1,15 1,17 50 75 1,30 1,34 175 200

h h

1,20 100 1,40

1,23 125 1,45 250

225

1,27 150 1,50 300

The maximum temperature of the combustion products under the dome is

td p.c.

=

Ct.c *p

d

Q~ V~

or tp.c.

d

----tma x

p.c,

+ 170--200~ where Q~ is the calorific value of the fuel, k c a l / m a ; Cpt c is the specific heat of the combustion products, ~ c a J d e g C-m3; ~ . c is the mean volume of the combustion products at an excess air factor a > 1.0; t d a x is the maximum d permitted dome temperature, oC; 170-200 is the difference in temperatures (tp. c _ d tmax), oC. The optimum duration of the blast period according to hot-blast stove operating routine can be calculated by the following formulas: for consecutive operation of three stoves in the unit ~opt= bl

~pt ~ 2 -w Tsw, h,

for four stoves

~;ft= -5I (~gpt

1_2Tsw);

for paired parallel operation of four stoves opt opt Tbl - - - I g t 2 T s w " The consumption of gas for heating must be constant and must be maintained in accordance with the thermal conditions worked out by the laboratory and handed to the furnace gas men for their use. The fan louvers are opened to an angle sufficient for complete consumption of the f u e l The correctness of the control and measuring instrument readings and of the g a s - air ratio must be checked if the temperature of the dome or of the outgoing combustion products does not reach the prescribed value. The heat loading for a hot-blast stove should be selected to take account of conservation of the brickwork and the installation below the checkerwork, the draft- and blast-producing equipment, and the resistance of the stove. The tendency for certain gas men to boost the thermal conditions solely by increasing the consumption of gas and air doesnot lead to an increase in blast temperature; the opposite occurs. The heat loading can be increased only by eliminating all the limiting factors. Reducing the blast temperature in order to control the running of the furnace is undesirable; other technological factors should be used more fully for this purpose.

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