Process for operating a plurality of regenerative hot blast stoves for supplying hot blast to a blast furnace

A plurality of regenerative hot blast stoves are operated under a staggered-parallel process wherein each stove is alternately operated under a fixed heating cycle wherein no cold blast is supplied to the stove and no hot blast issues therefrom and a fixed blasting cycle wherein cold blast is supplied to the stove and issues therefrom as hot blast. The initiation of the cycles of the stoves is staggered in time with respect to each other such that at any given time hot blast issues from more than one of the stoves but at different temperatures. The temperature of the hot blast issuing from the stoves is regulated to a predetermined temperature required at the blast furnace in a two-stage operation including a first stage wherein relatively cooler and warmer hot blast issuing from those stoves operating under blasting is mixed and controlled as a function of a first reference temperature greater than the temperature required at the blast furnace, and a second stage wherein cold blast is admixed with the hot blast mixture and controlled as a function of a second reference temperature equal to the temperature required at the blast furnace. Upon a lowering of the second reference temperature due to an operating requirement of the blast furnace, the first reference temperature is raised, whereby the second stage temperature regulation overlaps the first stage temperature regulation.

BACKGROUND OF THE INVENTION 
The present invention relates to a process for operating a group of 
regenerative hot blast stoves for heating cold blast from a constant 
supply source and for supplying the thus formed hot blast at a selectively 
variable predetermined temperature and at a constant volume to a blast 
furnace. The present invention is particularly directed to such a process 
wherein the temperature of the hot blast is regulated to the temperature 
required at the blast furnace in a two-stage operation. In a first stage, 
hot blast issues from more than one of the stoves, but at relatively 
cooler and warmer temperatures. These different temperature blasts are 
mixed, and this mixing is regulated as a function of a first reference 
temperature which is greater than the temperature required at the blast 
furnace. In a second stage, this hot blast mixture is admixed with cold 
blast as a function of a second reference temperature which equals the 
temperature required at the blast furnace. 
As is well known, hot blast, i.e. air which is heated to a high temperature 
in hot blast stoves, is supplied to a blast furnace for the purpose of 
carrying out combustion and reduction operations within the blast furnace. 
Typically a plurality of hot blast stoves are alternately regenerated and 
employed for the purpose of supplying such hot blast. Conventionally such 
hot blast stoves are operated in accordance with three different 
processes. 
When the installation is operated in accordance with the "parallel" 
process, half of the hot blast stoves are operated simultaneously for a 
predetermined length of time to supply hot blast to the blast furnace, 
while the other half of the hot blast stoves are operated during the same 
predetermined length of time to be heated. When the first stoves are 
switched from blasting to heating, then the other half of the stoves are 
simultaneously switched from heating to blasting. During the operation of 
the hot blast stoves in accordance with the parallel process, the 
temperature of the hot blast which is supplied to the blast furnace is 
regulated by admixing therewith cold blast as a function of the 
temperature required at the blast furnace. 
The second known process is the "tandem" process, and when the installation 
is operated under this process, each hot blast stove is operated under 
blasting for a predetermined length of time and is then operated under 
heating for a further predetermined length of time. The cycling of the 
blast periods of the hot blast stoves is in a tandem manner with normally 
only one hot blast stove operating under blasting at a given time, or when 
a plurality of hot blast stoves are operating under blasting, their 
blasting cycles correspond in time with respect to each other. During the 
operation of the hot blast stoves in accordance with the tandem process, 
the hot blast supplied to the blast furnace is also admixed with cold 
blast as a function of the temperature required at the blast furnace. 
Both of the parallel and tandem processes are recognized to have the 
disadvantage in that each of the hot blast stoves is required to operate 
throughout the entire blasting cycle thereof at a temperature higher than 
the temperature required at the blast furnace. 
This disadvantge can however be overcome by operating the hot blast stoves 
in accordance with the third known type of process, i.e. the 
"staggered-parallel" process. When operating the installation in 
accordance with the staggered-parallel process, each of the hot blast 
stoves is operated under blasting for a predetermined length of time and 
then operated under heating for a predetermined length of time. However, 
the cycles of the hot blast stoves are staggered in time with respect to 
each other such that at any given time hot blast issues from more than one 
of the stoves but at different temperatures. The temperature of the hot 
blast issuing from the stoves is regulated to the temperature required at 
the blast furnace in a two-stage operation. In a first stage, the 
relatively cooler and warmer hot blasts issuing from those stoves 
operating under blasting are mixed, and this mixing is controlled as a 
function of a first reference temperature which is greater than the 
temperature required at the blast furnace. During a second stage, the hot 
blast mixture has admixed thereto cold blast, and this admixing operation 
is controlled as a function of a second reference temperature 
corresponding to the temperature required at the blast furnace. 
However, operation of the installation in accordance with the 
staggered-parallel process suffers from a further disadvantage which has 
not been solved in the art prior to the present invention. 
Specifically, when it is necessary to drastically reduce the second 
reference temperature over a short period of time, the second stage 
temperature regulation operation inherently requires a greater amount of 
cold blast. This results in the second stage temperature regulation 
operation using at least a portion of the cold blast which would otherwise 
be supplied to those hot blast stoves which are operating under blasting. 
Therefore, the hot blast which issues from the hot blast stoves is 
inherently at an increased temperature which is at a sufficiently high 
level that it is no longer possible to perform the first stage temperature 
regulation operation. That is, the relatively warmer and cooler hot blasts 
issuing from the hot blast stoves are all at a temperature such that it is 
impossible to regulate the mixture of such blasts to the first reference 
temperature. 
The result of this phenomenon is that the first stage temperature 
regulation operation becomes completely inoperative, and the entire 
temperature regulation must be achieved only by the second stage 
temperature regulation operation. Thus, when it becomes necessary due to 
the requirements of the blast furnace to drastically lower the second 
reference temperature, the hot blast stove installation cannot be operated 
under the staggered-parallel process, and thus the advantages thereof are 
lost. 
SUMMARY OF THE INVENTION 
With the above discussion in mind, it is the primary object of the present 
invention to provide an improvement of the above described 
staggered-parallel process such that, even when the second reference 
temperature is drastically lowered during a short period of time, the hot 
blast stove installation will still be operated under the above described 
two-stage temperature regulation. 
This object is achieved in accordance with the present invention, in that, 
when it becomes necessary to reduce the second reference temperature due 
to an operating requirement of the blast furnace, the first reference 
temperature is raised, with the result that the second stage temperature 
regulation operation overlaps the first stage temperature regulation, but 
that both stages remain operative. 
Specifically, when the second reference temperature is drastically lowered 
over a short period of time, with the result that at least a portion of 
the cold blast which would otherwise be supplied to the hot blast stoves 
is rather used for the second stage temperature regulation operation, with 
the result that the hot blast issuing from the hot blast stoves operating 
under blasting becomes increased, the first reference temperature is 
raised. Specifically, the first reference temperature is raised 
simultaneously with the lowering of the second reference temperature. 
Therefore, even though the blast issuing from the hot blast furnaces is at 
a relatively increased temperature, the hot blast stoves may still be 
operated such that the hot blast issuing therefrom is regulated as a 
function of the now increased first reference temperature.

DETAILED DESCRIPTION OF THE INVENTION 
Reference will initially be made to FIGS. 1a, 1b and 1c to illustrate the 
general sequence of operation of a plurality of regenerative hot blast 
stoves when operated in accordance with the known processes of parallel 
operation, tandem operation and staggered-parallel operation, 
respectively. In the following description and in the accompanying 
drawings reference is made to four hot blast stoves. However, it is to be 
understood that the present invention is applicable to the use of a 
plurality of hot blast stoves other than four to supply hot blast to a 
blast furnace. 
With reference to FIG. 1a, when operating hot blast stoves 1, 2, 3 and 4 in 
accordance with a parallel process, two of the hot blast stoves are 
operated simultaneously for a predetermined length of time to supply hot 
blast to the blast furnace, while the other two hot blast stoves are 
operated during the same predetermined length of time to be heated. When 
the first two stoves are switched from blasting to heating, then the 
second two stoves are simultaneously switched from heating to blasting. 
For instance, again with reference to FIG. 1a, during the time period 
prior to switching time X, hot blast stoves 1 and 2 are operated under a 
blasting cycle, i.e. they simultaneously supply hot blast to the blast 
furnace. During this same predetermined length of time, for example 
forty-five minutes, blast stoves 3 and 4 are being heated. At the end of 
this predetermined time, i.e. at point of time X, hot blast stoves 1 and 2 
are switched to heating, and hot blast stoves 3 and 4 are simultaneously 
switched to blasting. During the next predetermined time cycle, for 
example 45 minutes, the blast furnace is supplied with hot blast from hot 
blast stoves 3 and 4. During the operation of the hot blast stoves in 
accordance with the parallel process shown in FIG. 1a, the temperature of 
the hot blast which is supplied to the blast furnace is regulated by 
admixing therewith cold blast as a function of a predetermined required 
temperature of hot blast which is required at the blast furnace. 
With reference now to FIG. 1b of the drawings, when the regenerative hot 
blast stoves are operated in accordance with a tandem process, each hot 
blast stove is operated under blasting for a predetermined length of time, 
for example thirty minutes, and then operated under heating for a further 
predetermined length of time, for example sixty minutes. The cycling of 
the blast periods of the hot blast stoves is however cycled in a tandem 
manner as shown in FIG. 1b. More particularly, as shown in FIG. 1b, during 
a time period immediately preceding point in time Y, hot blast stove 1 is 
shown as blasting, while hot blast stoves 2 and 3 are shown as being 
heated. Hot blast stoves 4 is shown as being shut down for repair. 
However, it is to be understood that hot blast stove 4 could be in 
operation, and the timing sequence would then be in tandem with stoves 1, 
2 and 3. When point in time Y is reached, hot blast stove 1 is 
automatically shifted to heating and continues to heat for a predetermined 
time of for example 60 minutes. At point in time Y, hot blast stove 2 is 
automatically shifted to blasting and continues to blast for a further 
predetermined period of time, for example 30 minutes. Hot blast stove 3 
continues to be heated for the remaining 30 minutes of its 60 minute 
heating cycle, at which time hot blast stove 3 would be automatically 
switched to blasting, hot blast stove 2 would be automatically switched to 
heating, and hot blast stove 1 would continue to be heated. During the 
operation of the hot blast stoves in accordance with the tandem process 
shown in FIG. 1b, the hot blast supplied to the blast furnace is regulated 
in the same manner as described above with regard to the parallel process 
of FIG. 1a. That is, the hot blast supplied from the particular hot blast 
stoves is admixed with cold blast as a function of a hot blast temperature 
required in the blast furnace. 
The above described parallel and tandem processes are each recognized to 
have the disadvantage in that each of the hot blast stoves is required to 
operate throughout the entire blasting period thereof at a temperature 
higher than the temperature required at the blast furnace. That is, in 
both the parallel and tandem processes, each individual hot blast stove 
must be operated, during the entire blasting cycle thereof, at a 
temperature above the temperature of the hot blast required at the blast 
furnace. 
The above disadvantage can however be overcome by operating the hot blast 
stoves in accordance with the staggered-parallel process, and with 
reference now to FIG. 1c of the drawings, this process will be described. 
Specifically, in the staggered-parallel process, each of the regenerative 
hot blast stoves is operated under blasting for a predetermined length of 
time, for example 60 minutes, and then operated under heating for a 
predetermined period of time, also for example 60 minutes. However, the 
cycles of the hot blast stoves are staggered in a manner such that during 
any given period of time hot blast is supplied from more than one hot 
blast stove, but not in a simultaneous manner. Rather, hot blast is 
continually supplied from more than one hot blast stove, but the hot blast 
stoves which are supplying hot blast are staggered in time with respect to 
each other. Specifically with reference to FIG. 1c, before a point of time 
Z, hot blast stove 1 has operated a complete blasting cycle of 60 minutes, 
but at point of time Z is switched to a heating cycle and then is heated 
for a complete cycle of 60 minutes. Previous to point of time Z, hot blast 
stove 2 has completed one-half, i.e. 30 minutes, of a blasting cycle, and 
after point of time Z continues the remaining 30 minutes of such blasting 
cycle. Prior to point of time Z, hot blast stove 3 has completed an entire 
heating cycle, and at point of time Z is switched to blasting whereafter 
it completes an entire blasting cycle. Prior to point of time Z hot blast 
stove 4 has completed one-half of a heating cycle, and after point of time 
Z continues being heated to complete such heating cycle. Accordingly, it 
will be apparent that at any given time hot blast is supplied from two hot 
blast stoves. However, the switching is never simultaneous, and it will be 
apparent that the temperatures of the blasts supplied from two hot blast 
stoves at a given point of time will be different. For example, at a point 
of time immediately after point Z, the hot blast supplied from hot blast 
stove 3 will be at a maximum temperature but at a minimum quantity. On the 
other hand, the temperature of the hot blast being supplied from hot blast 
stove 2 will be at a lesser temperature, but will involve larger 
quantities. Accordingly, it will be apparent that when operating the hot 
blast stoves in accordance with the staggered-parallel process, the hot 
blast of each hot blast stove need not be at a temperature above the 
temperature required at the blast furnace throughout the entire blasting 
cycle of each blast furnace. This is due to the fact that hot blast is 
supplied from more than one hot blast stove in staggered temperature 
conditions and is then admixed. Thus, while hot blast is being supplied at 
a maximum temperature from one hot blast stove, further hot blast may be 
supplied at a minimum temperature from a second hot blast stove. 
Accordingly, during operation of the hot blast stoves in accordance with 
the staggered-parallel process, the temperature of the hot blast is 
regulated in two stages. In the first stage, hot blast at two different 
temperatures from two different hot blast stoves is mixed to form a first 
hot blast flow. The temperature of this first hot blast flow is regulated 
as a function of a first reference temperature value which is set at a 
level higher than the temperature of the hot blast required at the blast 
furnace. This regulated first hot blast flow is subsequently admixed with 
smaller amounts of cold blast as a function of a second reference value 
which corresponds to the required temperature of the hot blast at the 
blast furnace. That is, in the first stage of temperature regulation, the 
temperature of the mixed hot blasts from the hot blast stoves which are 
switched to blasting is adjusted to a level which is somewhat above the 
hot blast temperature which is required at the blast furnace. In the 
second stage of temperature regulation this first hot blast flow is then 
regulated to the final temperature required at the blast furnace, and this 
second hot blast flow is supplied to the blast furnace in a conventional 
manner. 
With reference now to FIG. 2 of the drawings, an installation operating 
under the staggered-parallel process will be described. Particularly, a 
blast furnace 100 is supplied with hot blast at a required temperature by 
means of hot blast stoves 1, 2, 3 and 4 operating in accordance with the 
staggered-parallel process. Cold blast is supplied through a cold blast 
pipe 13 in which there is provided a volume governor or regulator 5 to 
ensure the maintenance of a constant amount of cold blast flowing through 
pipe 13. Cold blast is supplied to hot blast stoves 1 through 4 during the 
respective blasting cycles thereof via feed pipes 1a, 2a, 3a, and 4a, 
respectively. Feed pipes 1a through 4a have therein adjustable cold blast 
throttling valves 1b, 2b, 3b and 4b. respectively. During the respective 
blasting cycles of hot blast stoves 1 through 4, hot blast exits therefrom 
through hot blast removal pipes 1c, 2c, 3c and 4c, respectively, into a 
hot blast pipe 7. Hot blast from the particular hot blast stoves operating 
under their respective blasting cycles is admixed in hot blast pipe 7 to 
form a first hot blast flow. 
The temperature of this first hot blast flow is regulated to be at a first 
reference value T1 (see FIG. 3), which is selected to be somewhat above 
the hot blast temperature T2 required at the blast furnace. This first 
temperature regulating stage is carried out in a known manner by measuring 
the temperature in hot blast pipe 7, for example by a temperature 
measuring device 18. The actual temperature measured in hot blast pipe 7 
is compared to first reference temperature T1 by temperature regulator 10 
which generates a signal representative of any difference therebetween. 
For instance, the greater the difference between the actual measured 
temperature and reference temperature T1, the greater the signal produced 
by temperature regulator 10. 
The specific operating construction and parameters of temperature measuring 
device 18 and temperature regulator 10 do not in and of themselves form 
any portion of the present invention. 
However, one specific type of temperature measuring device 18 may include a 
thermocouple and a millivolt to current converter. Specifically, 
temperature measuring device 18 may include a thermocouple attached to 
pipe 7 and capable of generating a millivolt signal representative of the 
temperature within pipe 7. Temperature measuring device 18 may 
additionally include a converter of the type manufactured by Honeywell 
Corporation under the name "Currentpak", Model No. NAX100, Spec. sheet 
YH-NA-ld, millivolt to current converter and capable of converting a 
millivolt signal from the thermocouple to a current signal ranging from, 
e.g., 4-20 mA, depending upon the magnitude of the millivolt signal 
received from the thermocouple. The linear millivolt input signal received 
from the thermocouple is passed through a measuring circuit and is 
converted into a millivoltage. A multi-section filter removes any 
prevailing a-c stray pickup, and the thus filtered millivolt signal is 
compared with a feedback voltage. The resultant d-c error signal enters a 
chopper inverter circuit where it is a-c to d-c converted and is fed to an 
amplifier. The amplified a-c signal is synchro-rectified and goes through 
an isolating transformer where it is diverged into two signals, i.e. the 
feedback signal and a signal to output circuits. Each diverged signal is 
then rectified and filtered, thereby producing the feedback voltage and an 
input voltage to a V/I converter which generates a proportional 4-20 mA 
d-c output signal. 
It is specifically to be understood that the above type of temperature 
measuring device is exemplary only, and that other conventional 
temperature measuring devices capable of producing a proportional output 
signal may be employed. 
A specific example of one type of temperature regulator 10 which may be 
employed is that manufactured by Yamatake-Honeywell Co., Ltd., under the 
brand name "Currentronik Vertical Scale Indicating Controller With Reset 
Limiter", Model No. NBL02-X-(YIA), Spec. sheet YH-NB-ld. This type of 
regulator device is capable of receiving the proportional d-c output from 
temperature measuring device 18, i.e. a signal of from 4-20 mA, and 
generating an output signal of from 4-20 mA, dependent upon the relative 
size of the input signal received from temperature measuring device 18. It 
is to be understood that the present invention is not limited to the use 
of a temperature regulator 10 of the above specifically described 
configuration. Rather, it is to be understood that any other known type of 
temperature regulator which is capable of generating a proportional signal 
may be employed. 
Accordingly, the greater the temperature difference between the actual 
measured temperature and reference temperature T1, the greater will be the 
value of the electric signal generated by electronic temperature regulator 
10. For example, if no difference exists between the actual measured 
temperature and reference temperature T1, regulator 10 will generate a 
signal of 4 mA. However, if a temperature difference does exist between 
the actual temperature and reference temperature T1, then a signal greater 
than 4 mA in proportion to the size of the detected temperature difference 
will be generated. 
The signal generated by regulator 10 is passed to a relay station 17 which 
controls the opening and closing of throttle valves 1b-4b through 
respective lines 1d, 2d, 3d and 4d. The specific structure of relay 
station 17 in and of itself forms no portion of the present invention and 
is known in the temperature control of hot blast stoves operating under 
the staggered-parallel process. Thus, the structure of relay station 17 
will not be described in detail. Relay station 17 is however of the type 
which initiates the opening and closing of valves 1b-4b at predetermined 
time intervals and in a predetermined sequence, as shown in FIG. 1c and in 
the lower portion of FIG. 3. Particularly, relay station 17 relays the 
signal from regulator 10 to the various valves 1b-1b at predetermined set 
time intervals. The magnitude of the signal from regulator 10, which is a 
function of the temperature difference between the actual measured 
temperature and reference temperature T1, is passed to the respective 
valves 1b-4b by relay station 17. The magnitude of the signal passed to 
respective valves 1b-4b may regulate the degree of opening and/or closing 
of such respective valves. In the lower portion of FIG. 3 of the drawings 
it is shown that the rate of opening of each of the respective valves is 
constant, with the rate of closing of the respective valves being varied 
dependent upon the magnitude of the signal passed thereto, to thereby 
regulate the amount of hot blast delivered from the respective hot blast 
stoves, and to thereby regulate the temperature of the hot blast mixture 
in hot blast line 7. However, it should be understood that the rate of 
opening of each of the valves could be similarly varied. 
With reference to FIG. 3 of the drawings, and particularly the right-hand 
portion thereof, simplified curves T.sub.C1, T.sub.C2, T.sub.C3 and 
T.sub.C4 illustrate the staggered time cycle as well as temperature drop 
of hot flow exiting from hot blast stoves 1-4, respectively. T1 represents 
the above discussed reference temperature which is somewhat higher than 
second reference T2 corresponding to the temperature of the hot blast 
required at the blast furnace. In the lower portion of FIG. 3, simplified 
curves K.sub.C1, K.sub.C2, K.sub.C3 and K.sub.C4 represent the open-closed 
positions of valves 1b-4b, respectively, during the respective blasting 
and heating cycles of hot blast stoves 1-4, respectively. It will be 
apparent from curves T.sub.C1 -T.sub.C4 that the temperature of the hot 
blasts from the individual hot blast stoves need not be maintained above 
the temperature T2 required at the blast furnace throughout the entire 
individual blasting cycles. Rather, and with reference to point of time A 
in FIG. 3, relay station 17 closes throttle valve 1b, thereby switching 
hot blast stove 1 from blasting to heating. Hot blast stove 4 continues in 
its heating cycle. Hot blast stove 2 continues in its blasting cycle, with 
the temperature of the hot blast issuing therefrom reducing to a level 
below reference value T1. Throttle valve 2b remains in the fully opened 
condition, as shown by curve K.sub.C2. Relay station 17 initiates opening 
of throttle valve 3b, thereby switching hot blast stove 3 from the heating 
cycle thereof to the blasting cycle thereof. At a point in time 
immediately after point A, hot blast will be supplied to hot blast line 7 
from both of hot blast stoves 2 and 3, while hot blast stoves 1 and 4 are 
in their heating cycles. The temperature of the hot blast from hot blast 
stove 3 will be at a maximum, whereas the temperature of the hot blast 
from hot blast stove 2 will be substantially lower. However, since 
throttle valve 3b is still opening, the quantity of the higher temperature 
hot blast from hot blast stove 3 will be less than the quantity of hot 
blast from the lower temperature hot blast stove 2. Accordingly, the 
mixture of hot blast in line 7 will substantially remain at temperature 
T1. Any variation of the hot blast in line 7 from temperature T1 will be 
detected by regulator 10. 
At a predetermined time B after the initiation of the opening of valve 3b, 
as determined by predetermined command signal 3' programmed into relay 
station 17, valve 3b will reach its fully opened position. The time at 
which valve 3b becomes fully opened is detected so that the signal from 
regulator 10 is switched from valve 3b to valve 2b to cause closing of 
valve 2b at a rate dependent upon the magnitude of the signal generated by 
regulator 10. This may be achieved in various known manners. However, in 
the embodiment illustrated in FIG. 2 of the drawings, the movement of 
valve 3b to its fully opened position causes a limit or end position 
switch 3e to close a respective circuit 3f which causes a supplemental 
relay station 17a to transfer the signal from regulator 10 from valve 3b 
to valve 2b, thereby commencing the initiation of closing of valve 2b. As 
mentioned previously, the rate at which valve 2b moves from its fully open 
to its fully closed position will be dependent upon the magnitude of the 
signal transferred from regulator 10 to valve 2b via relay station 17. 
This rate of closing of valve 2b will thus be regulated to control the 
amount of hot blast supplied to line 7 from hot blast stove 2 during the 
closing of valve 2b. Thereby, the temperature of the hot blast mixture in 
line 7 will be regulated. 
The further operation of the elements shown in FIG. 2 will continue in 
accordance with the staggered-parallel process in the above explained 
manner. For example, at point C shown in FIG. 3 the programmed signal 4' 
supplied to relay station 17 will transfer the signal from regulator 10 
from valve 2b to valve 4b to initiate opening thereof. At point of time D 
valve 4b will be in its fully opened position, thereby causing limit 
switch 4e to close circuit 4f, thus causing supplemental relay station 17a 
to activate relay station 17 to transfer the signal from regulator 10 from 
switch 4b to switch 3b to initiate the closing operation of switch 3b. The 
rate at which the closing of valve 3b will proceed will be dependent upon 
the magnitude of the signal transmitted thereto from regulator 10. 
The staggered-parallel operation of hot blast stoves 1-4 and the continued 
first stage temperature regulation of the hot blast mixture in line 7 will 
continue in the above manner. 
The second stage temperature regulation, i.e. the regulation to reduce the 
temperature of the hot blast from temperature T1 to temperature T2 
required at the blast furnace, is achieved by a second temperature 
regulator 11 in response to a temperature measurement by temperature 
measuring device 19 in a second hot blast pipe 9 which receives the hot 
blast from first hot blast pipe 7. Cold blast may be supplied from cold 
blast supply pipe 13 through cold blast admixing pipe 8 and/or cold blast 
by-pass pipe 16 to an admixing point 23 between hot blast pipes 7 and 9. 
That is, temperature regulator 11 controls the amount of cold blast 
supplied at point 23 to adjust the temperature of the hot blast from 
temperature T1 to the temperature T2 required at the blast furnace. 
It is to be understood that temperature measuring device 19 and temperature 
regulator 11 may be precisely the same type of elements as temperature 
measuring device 18 and temperature regulator 10, respectively, as 
discussed above. Alternatively, temperature measuring device 19 and 
temperature regulator 11 may be any known conventional such elements 
capable of producing necessary proportional signals. 
This second stage adjustment may be carried out in several known manners. 
In the specific arrangement shown in FIG. 2, this is achieved by a 
regulated control of valve 12 arranged in cold blast supply pipe 13, a 
rough control valve 14 in cold blast admixing pipe 8 and a precise control 
valve 15 in by-pass pipe 16. 
Temperature regulator 11 may be, as stated above, the same type of 
regulator as first temperature regulator 10. Thus, regulator 11 may be of 
the type which will always emit an electrical signal of from 4 to 20 mA, 
the precise magnitude of such signal being generally dependent on, for 
example proportional to, the difference between the temperature of the hot 
blast measured by temperature measuring device 19 and the temperature T2 
required at the blast furnace. The actual signal from regulator 11 will be 
supplied to one of three divided or split range relay devices 20, 21 or 
22, respectively operable to control the opening or closing of valves 12, 
14 and 15, which may be of any type of known variably controlled throttle 
valve. More particularly, split range devices 20, 21 and 22 may each be of 
the type which is responsive only to a predetermined magnitude of signal. 
By designing the signal responsiveness of each of devices 20, 21 and 22 so 
as to be responsive to separate magnitude ranges of the signal from 
regulator 11, it is possible to selectively control the open-closed 
position of valves 12, 14 and 15. 
One specific type of split range device which may be employed for devices 
20, 21 and 22 is that manufactured by Yamatake-Honeywell Co., Ltd., under 
the brand name of "Split Range Currentpak Unit", Model No. NAX511, Spec. 
sheet YH-NA-8c. In this type of split range device, each such device may 
be designed to be operable only upon the receipt of a particular portion 
or range of possible input signals received from an input source. 
Specifically, assume that regulator 11 will generate a signal of from 4-20 
mA as discussed above. Split range devices 20, 21 and 22 may be designed 
such that device 22 is actuatable only upon receipt of a signal of an 
amplitude of from 4 to 10 mA, device 21 may be designed to be operable 
only upon receipt of a signal of an amplitude of from 10 to 16 mA, and 
device 20 may be designed to be operable only upon receipt of a signal of 
an amplitude of from 16 to 20 mA. It is however to be understood that 
split range devices 20, 21 and 22 may be of a construction other than that 
specifically described above. 
Additionally, the valves 12, 14 and 15 may each be of the type which is 
closed by a power cylinder assembly which is operated by a motor driver in 
proportion to the signal generated by the respective split range device 
20, 21 or 22, to proportionally open or close respective valves 12, 14 and 
15. One possible such motor driver is that manufactured by 
Yamatake-Honeywell Co., Ltd., Model No. NAX170, Spec. sheet YH-NA-17. This 
type of device is operable to receive and convert the signal of 4-20 mA 
from the respective split range device 20, 21 or 22 and to operate the 
respective power cylinder assembly by a proportional amount dependent upon 
the amount of the input signal received from the respective split range 
device. It is however to be understood that valves 12, 14 and 15 may be of 
a configuration other than that specifically described above, the 
important feature being that they control the amount of flow through 
respective lines 13, 8, and 16 in proportion to a temperature regulation 
signal. 
The above operational characteristics of regulator 11 and split range 
devices 20, 21 and 22 will be explained in more detail below with 
reference to specific embodiments which are not intended to be limiting, 
but rather are exemplary only. 
Specifically, as stated above, temperature regulator 11 may be designed to 
emit a signal of from 4 to 20 mA, dependent upon the temperature 
difference between the actual temperature measured by temperature 
measuring device 19 and the temperature T2 required at the blast furnace 
100. That is, assume that the actual temperature measured by device 19 is 
1450.degree. C. Assume further that precise control valve 15 is closed 
except when opened by actuation of split range device 22, that split range 
device 22 is actuatable only upon receipt of a signal from regulator 11 of 
an amplitude of from 4 to 10 mA, and that split range device 22 generates 
a signal of a predetermined range, for example 4 to 20 mA, in proportion 
to the magnitude of the signal received. 
Assume further that rough control valve 14 is normally closed and is opened 
only when split range device 21 is actuated. Assume yet further that split 
range device 21 is activated upon the receipt of a signal from regulator 
11 of a particular range different from the range which actuates split 
range device 22. For example, split range device 21 may be operable when 
the signal from regulator 11 is from 10 to 16 mA. Assume yet further that 
split range device 21 always generates a signal within a predetermined 
range, for example 4 to 20 mA, and that the exact magnitude of this signal 
is proportionate to the magnitude of the actuating signal received from 
the regulator 11. 
Assume further that split range device 20 is operable to generate a signal 
of a specific range, for example from 4 to 20 mA. Assume that split range 
device 20 is operable to receive a signal from regulator 11 only when such 
signal is of a predetermined range different from the ranges receivable by 
split range devices 21 and 22. For example, split range device 20 may be 
designed and/or adjusted to receive only a signal of from 16 to 20 mA. 
When the signal from regulator 11 is without the range of from 16 to 20 
mA, then split range device 20 generates a signal equal to 4 mA, and 
control valve 12 remains fully open. When split range device 20 receives a 
signal of between 16 to 20 mA from regulator 11, then the signal generated 
by split range device 20 is proportionally increased to result in a 
proportional closing of control valve 12. However, control valve 12 must 
be set such that even at maximum signal generated by split range device 20 
control valve 12 remains open to supply at least some cold blast to 
respective of the hot blast stoves 1-4. For example, control valve 12 may 
be set so that even at a maximum signal of 20 mA from split range device 
20, control valve 12 still is opened at least a minimum amount, for 
example 30% opened. 
With the above discussion in mind, the following specific examples will 
illustrate the manner in which the temperature of the hot blast in line 7 
is reduced to a value corresponding to temperature T2 required at the 
blast furnace. 
EXAMPLE 1 
When T2 is 1350.degree. C. and the actual temperature measured by 
temperature measuring device 19 is 1450.degree. C., then the signal 
generated by temperature regulator 11 may be 9 mA. This size signal will 
not be received by split range devices 20 or 21, and thereby the signals 
generated by split range devices 20 and 21 will continue to be 4 mA. Thus, 
control valve 12 remains 100% open, and rough control valve 14 remains 
entirely closed. However, the signal of 9 mA from regulator 11 will 
activate split range device 22 and cause the signal generated thereby to 
be increased to about 17 mA. Thus, precise control valve 15 is activated 
to be opened to a position of approximately 83% open, whereby a relatively 
small amount of cold blast will be supplied from supply line 13, through 
cold blast admixing pipe 8 and by-pass pipe 16 to point 23 whereat such 
relatively small amount of cold blast is mixed with the hot blast from 
line 7 to reduce the temperature of such hot blast down to the required T2 
temperature of 1350.degree. C. 
EXAMPLE 2 
The actual temperature measured by temperature measuring device 19 remains 
1450.degree. C., but the temperature T2 required at the blast furnace is 
now reduced to 1250.degree. C. The signal generated by temperature 
regulator 11 is proportionally increased to 14 mA. This signal is not 
received by split range devices 20 or 22, and accordingly such devices 
continue to generate signals equal to 4 mA. Thus, control valve 12 remains 
100% open, and precise control valve 15 remains entirely closed. However, 
this signal of 14 mA from temperature regulator 11 is received by split 
range device 21 and causes the signal generated thereby to be increased to 
approximately 15 mA. This causes rough control valve 14 to be opened to a 
position of approximately 67% open. Accordingly, under the above 
circumstances, a relatively greater amount of cold blast is supplied from 
supply pipe 13 through cold blast admixing pipe 8 and valve 14 to point 23 
to mix such relatively larger amount of cold blast with the hot blast from 
line 7 to reduce the temperature of the hot blast supplied to the blast 
furance to the reduced T2 temperature of 1250.degree. C. 
EXAMPLE 3 
The actual temperature measured by temperature measuring device 19 remains 
1450.degree. C., however the T2 temperature required at the blast furnace 
is drastically reduced to 1150.degree. C. This causes temperature 
regulator 11 to generate a substantially larger signal of 19 mA which is 
received by split range device 20 and causes the signal generated thereby 
to be increased to approximately 16 mA. This causes control valve 12 to be 
closed to a position until it is approximately only 53% open. Split range 
device 22 is operable to receive no portion of the signal from regulator 
11, and thus precise control valve 15 remains closed. Split range device 
21 is designed such that at signals above 16 mA, it will receive the 
signal generated by regulator 11, but will generate only the maximum 
possible signal therefrom, i.e. 20 mA, whereby rough control valve 14 
remains 100% open. Thus, under this situation where the required 
temperature T2 of the blast furnace is drastically reduced, the cold blast 
necessary to achieve such a temperature is in substantial part taken from 
the cold blast which would otherwise be transferred to the hot blast 
stoves. 
When the temperature T2 required at the blast furnace is raised, the above 
described control system will still remain operable, merely by raising the 
first stage reference temperature T1. Furthermore, when the temperature T2 
required at the blast furnace is gradually reduced during a slow planned 
variation, no problems are encountered during the operation of the above 
system. 
However, a severe operational problem occurs when the temperature T2 
required at the blast furnace is drastically reduced for short periods of 
time, for example in the manner discussed above regarding Example 3. 
Specifically, when the temperature T2 required at the blast furnace is 
drastically reduced in a short amount of time by an amount such that 
control valve 12 is partially closed to supply a greater amount of cold 
blast to point 23, there of course inherently is less cold blast available 
to pass through the particular hot blast stoves which are operated under 
blasting. Thus, less heat is removed from the hot blast stoves operating 
under blasting. At such time as a given hot blast stove reaches the end of 
its blasting cycle, the temperature of the blast issuing therefrom is 
thereby made higher than would be the case if control valve 12 were fully 
open. When the next staggered hot blast stove is simultaneously switched 
to its blasting cycle, the hot blast supplied to line 7 is at such a 
temperature that it is impossible to reduce it to the previously set first 
reference temperature T1. 
Accordingly, in the past when operating hot blast stoves according to the 
staggered-parallel process, at any such time that the temperature required 
at the blast furnace is drastically reduced over a short period of time, 
it basically becomes impossible to achieve the first stage temperature 
regulation by means of temperature regulator 10. Therefore, in the past, 
such a circumstance has generally resulted in the entire temperature 
control being achieved by the second stage only, i.e. by temperature 
regulator 11, and the control of valves 1b-4b by regulator 10 has been 
impossible. This is particularly true when considering the fact that in 
the past it has been considered to be operationally necessary to 
simultaneously lower first reference temperature T1 when lowering 
temperature T2 required at the blast furnace. 
However, in accordance with the present invention the above severe 
operational problem is overcome by, rather than lowering first reference 
temperature T1 upon a lowering of temperature T2, the increasing of first 
reference temperature T1. The unique and unexpected advantage of this 
process operation will be described with reference to FIG. 3 of the 
drawings. 
Specifically, time period t.sub.b represents the blasting cycle for hot 
blast stove 1, and time period t.sub.h represents the heating cycle for 
hot blast stove 1. It is to be remembered that the blasting and heating 
cycles of each of the hot blast stoves are equal and constant. 
With regard to the left part of the upper portion of FIG. 3, hot blast 
stove 1 begins its blasting cycle at time period E under normal operating 
conditions whereat temperature T2 required by the blast furnace is set to 
be 1300.degree. C. and first reference temperature T1 is set somewhat 
higher, i.e. at 1350.degree. C. At this same time period E, hot blast 
stove 4 continues to be operated at its blasting cycle. At a later period 
in time, i.e. time period F, hot blast stove 2 is switched to blasting, 
and hot blast stove 4 is switched to heating, while hot blast stove 1 
continues with its blasting cycle, the operation of the installation still 
being normal. 
However, assume that at a later point in time, i.e. time period G, the 
temperature T2 required at the blast furnace is drastically reduced to 
1200.degree. C., and that this reduction is sufficient to cause control 
valve 12 to be at least somewhat closed, thereby restricting the amount of 
blast available for passage through hot blast stoves 1 and 2. Accordingly, 
upon the drastic reduction in a short amount of time of the temperature 
required at the blast furnace to the level T2', from the time period G the 
temperature reduction of the hot blast flows issuing from hot blast stoves 
1 and 2 do not follow the normal curves T.sub.C1 and T.sub.C2, but rather 
are cooled at a lesser rate as indicated by the curves T.sub.C1, and 
T.sub.C2'. 
That is, during the time period G-H, i.e. when hot blast stove 1 is being 
switched from blasting to heating by closing of valve 1b, the signal 
supplied to valve 1b from regulator 10 attempts to retard the closing of 
switch 1b so that a greater quantity of relatively lower temperature blast 
may be supplied to line 7. However, due to the fact that valve 12 has been 
partially closed, there is insufficient cold blast supplied to hot blast 
stove 1 and thus line 7. Further, at time period H, i.e. when hot blast 
stove 1 is switched to heating and hot blast stove 3 is switched to 
blasting, the combined temperature of the blast from hot blast stoves 2 
and 3 is so high that it is impossible for regulator 10, by regulation of 
valve 2b, to control the temperature of the hot blast in line 7 to be 
equal to original first reference temperature T1. Therefore, upon the 
drastic reduction of temperature T2 required by the blast furnace, in 
accordance with prior known processes, first temperature regulator 10 
would become incapable of performing the first stage temperature control. 
The result would be that the entire temperature control would be achieved 
only by regulator 11. 
However, it has totally uniquely been discovered in accordance with the 
process of the present invention that rather than leaving first reference 
temperature T1 unchanged, or even reducing such temperature as would be 
considered normal in the prior art, upon the drastic reduction of 
temperature T2 required by the blast furnace, the first reference 
temperature is simultaneously increased from value T1 to a value T1' at 
least as high as and preferably higher than the actual temperature L of 
the blast issuing from hot blast stove 2. Therefore, by raising the first 
reference value in the above manner, it again becomes possible for 
temperature regulator 10 to achieve a regulated exhausting of the hot 
blast stoves. Therefore, it still becomes possible to operate the 
installation in the staggered-parallel manner by a two-stage temperature 
control operation. Thereafter, at a later time period such as time period 
J, when the temperature required at the blast furnace is returned to its 
normal operating level, the first reference temperature may again readily 
be lowered from T1' to T1. 
In accordance with the above described process operation of the present 
invention, the heat demand can be brought to approximately the same value 
as prior to the lowering of the temperature required at the blast furnace, 
and the control system of temperature regulator 10 will continue to remain 
operable. 
It is of course to be understood that the manner of adjusting reference 
temperature T2 to T2', and similarly the manner of adjusting reference 
temperature T1 to level T1' would be readily understandable to those 
skilled in the art, and that temperature regulators 11 and 10, 
respectively, would clearly be designed to be capable of such adjustment. 
It is once again further emphasized that the specific devices employed to 
achieve the above described control features are not in and of themselves 
the present invention. Such specific structural devices are known to those 
skilled in the art. Rather, the present invention is directed to the 
process feature of raising the first reference temperature upon a 
necessary substantial reduction of the temperature of the hot blast 
required at the blast furnace in a short period of time, particularly such 
a reduction which would result in a partial closing of control valve 12, 
such that the amount of cold blast available to the hot blast stoves is 
reduced. Those skilled in the art will understand from the above 
discussion the specific electrical and mechanical components employable in 
carrying out the above described staggered-parallel operation of the hot 
blast stoves. It is further to be understood that other types of control 
devices other than those specifically described above and which are known 
in the art may be employed to achieve the staggered-parallel operation.