US3108790A

US3108790A - Method and apparatus for preheating air

Info

Publication number: US3108790A
Application number: US90427A
Authority: US
Inventors: Jagdish C Agarwal
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1961-02-20
Filing date: 1961-02-20
Publication date: 1963-10-29
Anticipated expiration: 1980-10-29

Description

Oct. 29, 1963 Jg c. AGARw/u.l 3,108,790

- METHOD AND APEARATUS FOR PREHEATING AIR FiledFeb, 20, 1961 Fuel INVENTOR A Harney JAGD/SH C. AGARWALv United States Patent O 3,108,790 METHD AND APPARATUS FR PREHEATING AIR Iagdish C. Agar-wai, Penn Hiiis Township, Ailegheny County, Pa., assigner to United States Steel Corporation, a corporation of New .Iersey Filed Feb. 20, 19431, Ser. No. 90,427 7 Claims. (Cl. 263-19) This invention relates to an improved method and apparatus for preheating air introduced to a blast furnace.

Conventionally a blast furnace is equipped with stoves (usually three) which act as regenerators for preheating the air. Normally one stove is on blast, that is, actually preheating air for the furnace, while the others are on gas, that is, gaseous fuel is burning in these stoves to raise their temperature. Periodically the stoves are changed over from one to the other. As a stove continues on blast, its temperature and the temperature to which it is capable of heating air of course progressively decrease. Desirably the blast reaches the furnace at a constant temperature; hence the usual practice is to bypass a decreasing portion of the air around the stove and mix the bypassed and heated air ahead of the furnace. Automatic regulators are known for controlling the portion bypassed to maintain a constant blast temperature. This arrangement has a disadvantage that the blast never is heated to the maximum possible temperature. The purpose in preheating the blast is to intensify and accelerate the burning of coke in the furnace with a consequent reduction of the quantity of coke required. Increasing the blast temperature saves more coke.

An object of my invention is to provide an improved method and apparatus for preheating air in which the emperature is maintained constant but at a higher level than Vwith conventional arrangements.

A further object is to provide an improved preheating method and apparatus which utilize a direct-fired recuperator in parallel with the usual stoves and bypasses for producing a blast of constant but higher temperature than that otherwise attained.

In the drawing:

FIGURE 1 is a schematic showing of a preheating apparatus constructed in accordance with my invention; and

FIGURE 2 is a similar schematic showing of a modification.

FIGURE l shows a blast furnace stove 10, a blower 12, a cold blast main 13 connecting the blower and stove, and a hot blast main 14 connecting the stove and a blast furnace, not shown. A bypass 16 is connected between the cold and

hot blast mains

13 and 14 around the stove. The bypass contains an adjustable valve 17 which has an operating means 18. An automatic temperature-responsive regulator 19 is connected into the hot blast main 14 and to the valve operating means 18 to adjust valve 17. The adjustment is such that when the stove is on blast the bypass always carries the proper quantity of air around the stove to maintain a constant hot blast temperature despite a continually dropping stove temperature. The structure thus far described is conventional; hence no more detailed showing is deemed necessary. This arrangement is duplicated for each of the several stoves associated with the furnace.

In accordance with my invention, I connect a line 20 into the cold blast main 13, preferably ahead of the connection of the bypass 16 to this main. Line 20 leads to a direct-fired recuperator 21. I connect another line 22 between the recuperator and the bypass 16. Thus the recuperator is connected in parallel with both the stove and bypass 16. Line contains an adjustable valve 23 which has an operating means 24. An automatic 3,108,700 Patented Oct. 29, 1963 ICC How-responsive regulator 25 is connected into line 20 and to the valve operating means 24 for maintaining a constant rate of flow through the line. I introduce gaseous or liquid fuel along with air for its combustion to recuperator 21, the fuel line being indicated at 26. Line 26 contains an adjustable valve 27 which has an operating means 28 for controlling the flow of fuel into the recuperator. An automatic temperature-responsive regulator 29 is connected into line 22. and to the operating means 2S for adjusting the quantity of fuel to maintain air leaving the recuperator at a predetermined temperature. With an appropriate valve arrangement, a single recuperator 21 can serve several stoves. The individual components of my apparatus are commercially available devices, per se not of my invention; hence no more detailed showing is deemed necessary.

FIGURE 2 shows a modification in which I place a second direct-fired recuperator 30 in the cold blast main 13 ahead of the connection with line 20. Recuperator 30 has a fuel line 31 which is equipped with a temperatureresponsive control similar to that of the other recuperator 21. In other respects this modification is similar to the apparatus shown in FIGURE 1; hence the description is not repeated.

According to my preheating method, I route a controiled constant first portion of the air through the recuperator 21. This portion constitutes about 20 to 55 percent of the total quantity of air which passes from blower 12 into the cold blast main 13. Valve 23, its operating mechanism 24, and the flow-responsive regulator 2S automatically maintain this portion at the desired level. I route a second portion of the air through stove 10. This second portion constitutes about to 80 percent of the total quantity of air. I route the remaining air, if any, through the bypass 16. Valve 17, its operating means 1S and the temperature-responsive regulator 19 control variations in flow of the second and third portions to maintain constant temperature. The maximum temperature of the first portion is limited only by materials available for construction of the direct-fired recuperator; in practice I control this temperature to the range 1300 to 1600 F. or preferably 1300 to 1500 F. When using the modication of FIGURE 2, I heat the cold blast in the recuperator 30 to temperature of 400 to 800 F. or preferably 500 to 700 F. Thus I reduce the heat duty of stove 10 and recuperator 21.

As a specific example of a conventional method of heating air, I heated air at a wind rate of 120,000 c.f.m. in the usual 3-stove arrangement. Each stove was on gas 118 minutes, and the burning rate in its combustion chamber was 24,000 c.f.m. of blast furnace gas having a heating value of 92 B.t.u. per cu. ft. Each stove was on blast for 64 minutes. The maximum constant blast temperature I could attain was 1700 F.

I then incorporated the apparatus of the present invention in the foregoinT arrangement and applied my heating method with the same wind rate. In contrast I obtained -a constant hot blast temperature of 1980 F. I achieved the higher temperature by routing 20 percent (24,000 c.f.m.) of the cold blast through recuperator 21, wherein I heated this air to 1400 F. I maintained a constant rate of iiow (through the recuperator, but I varied the cold blast passing through the stove from 86,000 c.f.m. at the beginning of the on-blast period to 93,400 c.f.m. at the end. `Only 10,000 c.f.m. of cold blast bypassed the stove and .recuperator at the beginning and 2000 c.f.m. at the end. The on-blast period lasted minutes.

In another example I used only two stoves. Each stove Was on lgas 56 minutes, and the burning rate in the combustion chamber and the wind rate Were the same as in sacarse the previous example. Each stove also was on blast 56 minutes. I controlled flow through reouperator 2t at a constant rate of 612,600 c.f.m. or 52.2 percent of total ilow, `and heated this ilow to a constant temperature of 1400 F. At the beginning of the on-blast period the ilow through the stove was 47,400 Cim. (39.5 percent), While .10,000 c.f.m. bypassed the recuperator and stove. Operating under these conditions, l was able to achieve a constant blast temperature of 1680 F., contrasted with a maximum constant temperature of only 1060l F. by operating the two stoves in the conventional manner.

In Ia third example I used the two-stove arrangement with a lower wind rate of 70,000 csfim. By operating in the conventional manner, I achieved a maximum constant blast temperature of only 1670 F., and by using my method a temperature of `1940 F. I maintained how through the recuperator at 2.0 percent or 14,000 c.f.m., wherein I heated the air to 1600 F. At the beginning of the ort-blast period 67,-8 percent of 47,1i00 c.f.-m, of the cold blast passed through the stove, while 12 percent bypassed the stove and recuperator.

From the foregoing description and examples, it is seen my invention affords a simple method and apparatus for raising the temperature of the hot blast to a blast furnace and yet maintaining this temperature constant. aware of previous arrangements which embody a recuperator in series `'with a stove or replace the stoves entirely with recuperators, but such arrangements have not accomplished comparable results.

While I have shown and described certain preferred embodiments of my invention, it is apparent that other modifications may arise. Therefore, I do not Wish to be limited to the disclosure set forth but only by the scope of the appended claims. l

I claim:

1. A method of prelieating an air blast comprising routing a constant first portion of a cold blast through a recuperator maintained at a constant temperature, routing variable second and third portions ot' the cold blast through a regenerator yand a bypass respectively, the recuperator, regenerator and bypass being arranged in parallel, combiningthe three portions to form a single hot blast, and relatively increasing the second portion land decreasing the third portion as the regenerator becomes progressively cooler to :maintain the hot blast at a constant temperature. f

2. A method of preheating an air blast comprising routing a constant rst portion of about to 55 percent of a cold blast through a recuperator maintained atk a constant temperature of about 1300 to 1600 F., routing l amvariable second and third portions ofthe cold blast through a relgenerator and a bypass respectively, the recuperator, regenerator and bypass being arranged in parallel, combining the three portions to'forrn a single hot blast, and

relatively increasing the second portion and Ydecreasing the third portion as the regenerator becomes progressively cooler to maintain the hot blast at a constanttemperature. u

3. A method as delined in claim 2 in which the hot blast temperature is in the range of 1800 to 2000 F.

4. A method as dened in claim 2 in which the cold blast is heated to a temperature of 400l to 800 F. before the different portions are routed therefrom.

5. An apparatus -for preheating an air blast to a constant temperature comprising a cold blast main, a recur erator connected to said main, means to deliver a constant first portion of the air `from said main to said recuperator to raise the temperature of said first portion to a l constant level, a regenerator and a bypass connected to said main and yadapted to receive variable second and third portions of the air carried thereby and raise the temperature of the combined second and third portions to a constant level, and a hot blast main connected to said recuperator, regenenator and bypass for receiving a combined constant temperature hot blast therefrom.

6. An apparatus for pre/heating an air blast to a constant temperature comprising a cold blast main, a direct-fired rccuperator connected to said main, Imeans to deliver a constant first portion of the air from saidmain to said recuperator, means for controlling the temperature of said recuperator to maintain at a constant level the temperature to which the tirst portion is raised, a regenerator and a bypass connected to said main in parallelwith said recuperator and ladapted to receive variable second and third portions of the air carried thereby and raise l the temperature of the combined second and third portions to a constant level, and a hot blast main connected to said rccuperator, regenerator and bypass -for receiving a combined constant temperature hot blast therefrom.

7. Anlapparatus as defined in claim 6 including anotl1- er direct-fired recuperator in said cold blast main for raising the temperature of the air before the iirst,-sec

ond and third portions are taken therefrom.

1,458,288 Gra-ham June l2, 1923 1,816,174

2,514,084 Mowat July 4,l 1950 Brown July 28, 1931`

Claims

1. A METHOD OF PREHEATING AN AIR BLAST COMPRISING ROUTING A CONSTANT FIRST PORTION OF A COLD BLAST THROUGH A RECUPERATOR MAINTAINED AT A CONSTANT TEMPERATURE, ROUTING VARIABLE SECOND AND THIRD PORTIONS OF THE COLD BLAST THROUGH A REGENERATOR AND A BYPASS RESPECTIVELY, THE RECUPERATOR, REGENERATOR AND BYPASS BEING ARRANGED IN PARALLEL, COMBINING THE THREE PORTIONS TO FORM A SINGLE HOT BLAST, AND RELATIVELY INCREASING THE SECOND PORTION AND DECREASING THE THIRD PORTION AS TTHE REGENERATOR BECOMES PROGRESSIVELY COOLER TO MAINTAIN THE HOT BLAST AT A CONSTANT TEMPERATURE.