CA1077467A - Heat reclaim system - Google Patents

Abstract

Application of Alan English For: Heat Reclaim System ABSTRACT OF THE DISCLOSURE
A heat reclaim system for reclaiming waste heat from a material processing line by bleeding a portion of the heated air from one or more points on the material heat processing line and directing such heated air through an economizer containing a bank of serpentine coils. Water is pumped through the economizer coils to extract heat from the air, and the heated water leaving the economizer is piped to another heat exchanger where the reclaimed heat is extracted from the water for use in other processes. More than one economizer may be provided in each heat reclaim system for receiving bleed-off air from different points on the material heat processing line, with the economizers connected in series to obtain the highest temperature difference between the air and water. Separate waste heat reclaim systems should be provided for each material heat processing line, with separate controls for each of the heat reclaim systems to permit the heat reclaim sys-tems to operate independently of the other systems so that any one line may be shut down without affecting the operation of the other lines.

Description

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DISCLOSURE
¦ This invention relates generally as indicated to a heat reclaim system for recuperating waste heat from various material , heat processing operations for use in other processes.
, In the heat processing of iron ore concentrates and ~`1 other materials, there is usually some form of heat recuperation , ~ ,~ , .
' to minimize fuel consumption, but the normal methods presently be-,` . ~! .
I ing used particularly in straight grate indurating equipment still permit large amounts of heat to be wasted to the atmosphere.
' 10 ~dditional heat recovery is possible from both the ¦ straight grate and grate-kiln indurating equipment used in pellet-izing operations and the like. Significant amounts of heat are lost both from the cooling and recuperation sections of such straight grate equipment, and also from the cooling section of such grate-kiln equipment, and it has been found that by bleeding off ~j air from these areas and directing such air to suitable heat ex-~¦ changers, the heat recovery is su~stantial and can be effectively used for other purposes such as building heating, concentrate dry-, ing, preheating Aerofall ~lill classification air to dispel mois- ' 20 ture from the ore grinding and classification circuit~ or any other process reyuiring sensi~le heat. Since building heating is season-able and concentrate drying is only needed during the freézing per-~,~, iod, preheating the air for the ore grinding and classification ;1 circuit provides maximum utilization of the reclaimed waste heat which can be utilized as the major source of classification air heat.
Concentrate drying also requires the use of steam, andaccordingly if the ~vailable waste heat is used for creating the steam needed for concentrate drying, the need for putting in a ~ 30 costly steam plant for that purpose is eliminated.

- In a typical pelletizing plant including severa~l straight grate indurating lines, tremendous quantities of waste heat are ex-hausted to the atmosphere. Not only LS thls costly from a fuel~

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; consumption standpoint, but the higher fuel consumption also greatly -~ increases the particulate load in the stack exit gases and the plant emissions of sulfur dioxide, which is objectionable from a pollution standpoint.
SUMMARY OF THE INYENTION
~; In accordance with the present invention, one or more heat reclaim systems are provided for reclaiming waste heat from a corresponding ... .
~` number of material heat processing lines. Within each heat reclaim system there is prov~sion for bleeding heated air from the corresponding heat , .
processing line and directing such heated bleed air through a first economlzer to permit extraction of heat from the air by water flowing through a bank of serpentine coils within the first economizer. A second '~ economizer may also be provided through which heated air bled from a plurality of other points on the mater~al heat processing line is directed .~ ' ~ to permit extraction of heat from the air by water passing thraugh a bank ; :
of serpentine coils in the second economizer. The coils of the first and . j ; second economizers are desirably in series to create the highest temperature , . ..
difference Between the air and water, and a drop-out box is provided for recelving the heated bleed air from such plurality of other points to mix the Bleed a~r prior to passage through the second economizer. An adjustable orif~ce is also provided for equalizing the pressure of the heate~ bleed air prior to entry into the drop-outbox,and heat extraction means may be provided for subsequently extracting the heat from the water after leaving -the economizers for use in other processes requiring sensible heat.

It is accordingly a principal object of this invention to provide a practical and economical system for reclaiming waste heat from material heat processing operations for effective use elsewhere.
Another oaject is to proyide such a system for reclaiming waste !. ~ ~ -.
;I heat especially for use in heating Aerofall Mill classification air.
Still another o~ject is to provide such a heat reclaim system with effective controls which permit reclamation of heat from plural -material Keat processing lines at the same time and shut-down of any one of the lines without affecting the rest of the system.
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~ Still another object is to provide a heat reclaim system `~ with effective dew point safety control and protection against overheating.
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To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the : annexed drawings setting forth in detail certain illustrative embodiments ':.'.:
;- of the invention, these being indicative, however, of but a few of the various .. . .
ways in which the principles of the invention may be employed.
BRIEF DESCRIPTION OF THE DRAWINGS
~; ! . In the annexed drawings:
Fig. 1 is a schematic diagram of a typical material heat processing system from which waste heat may be reclaimed utilizing the various heat reclaim systems of the present invention;

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, Fig. 2 is a schematic diagram illustrating a preferred ~` form of heat reclaim systeln in accordance with the present inven-tion for reclaiming waste heat from a single material processing line;
:j~ Fig. 3 is a schematic diagram similar to Fig. 1, but showing a modified form of heat reclaim system in accordance with this invention for use in reclaiming waste heat from two separate processing lines;
Fig. 4 is a schematic diagram of a heat reclaim system i ;, ~ 10 similar to Fig. 3 but showing the use of more than one heat ex- ;
' changer for recovering waste heat from the same processing line;
~' ~ and Fig. 5, appearing on the second sheet of drawing along ! ~ with Fig. 2, is a fragmentary schematic diagram of another typi-cal material heat processing system from which waste heat may be reclaimed utilizing the various heat reclaim systems of the pre-sent invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now in detail to the drawings and initially to Fig. 1 thereof, there is shown by way of example a material heat processing line 1 including a movable grate for moving the material M to be treated from left to right through a furnace 4 including ' ~
a series of stations or sections where the material is first dried ~ in sections 5 and 6, then preheated in section 7 and fired in 5i~ iZ~ ~ sections 8 and 9 and finally cooled in various stages in sections 10 and 11. A plurality of burners 12 direct a flame and hot gases into the preheat and firing sections 7, 8 and 9 of the furnace 4.
The cooling sections 10 and 11 are primary and secondary i~ cooling sections, respectlvely, which receive and cool in that ~` 30 order the material burned in the furnace. ~eneath the primary "t,~ ~ and secondary cooling sections 10 and 11 are a plural~ of wind-' boxes 13 which receive cold air from a blowing fan 14 an~ air . .: ~. .

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~`~. ~ 10774~7 -' distributing chamber 15 and direct the cold air upwardly through ,:' the traveling grate and the bed of material M on the grate in the primary and secondary caoling sections. As shown, the air ~,', which is forced upwardly into the primary cooling section 10 is 'tr'~' directed to an air header 16 above the furnace 4 for discharge ,,~ ' into the furnace to provide air for combustion of'the fuel intro-,,,,, duced by the burners 12. A hood recuperation fan 17 withdraws ~,',, - the air from the secondary cooling section 11 which has been forced upwardly through the bed of material by the blower fan 14, ~' 10 and a portion of this heated air is directed to the burners 12 tQ' ' provide additional air for combustion of the fuel. A portion of , the heated air leaving the secondary cooling section 11 is also directed to the downdraft drying section 6. The remainder of the i.! ~ heated air exhausted from the secondary cooling section 11 is ordinarily vented to the atmosphere through the stack S as by the ' exhaust fan 24.
A plurality of windboxes 18 are also positioned beneath the firing sections 8 and 9 of the furnace, and a suction fan 19 in communication with these windboxes through chamber 20 exhausts ~20 hot gases from the furnace through the bed of material. These hot gases from the furnace may be forced upwardly by an updraft drying ~' fan'21 into a chamber 22 for discharge through a plurality of windboxes'23 beneath the updraft drying section 5 upstream of and immediately adjacent to the downdraft drying section 6 for upward , passage of hot gases through the granular material on the grate prior to introduction into the preheat and firing sections of the furnace. A hood exhaust fan 24 exhausts the gases from the up-~ draft drying section 5 and vents them to atmosphere through the ,,, stac~ S.
~ , 30 Additional windboxes 25 are shown located beneath the ,.., ~, downdraft drying section 6, preheat section 7, and a po,,~rtion of ; the firing section 8 of the furnace. A waste gas fan 26~in ~:`' ~ '-' ' .
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`'`l ' . . ~ , ~ communication with these additional windboxes 25 through chamber ¦ 27 exhausts gases from the various sections 6, 7 and 8 of the ~¦ furnace through the bed of material and vents the gases to atmos-` ~I phere through the conduit 36 and stack S.
Suitable dampers 28 may be provided in the air conduits 29, 29' from the secondary cooling section 11 to the burners 12 and downdraft drying section 6 to control the flow of heated air in the conduits, and dust collectors, not shown, may also be pro-~` ~ vided wherever required. A bypass conduit 37 may also be provided .
`j I 10 between the suction fan 19 and updraft drying fan 21 to permit ..'~the fan 19 to operate independently of the fan 21 and bypass the ~i' updraft drying section 5 for venting of the exhaust gases from the furnace directly to atmosphere through conduit 36 and stack S
upon opening the damper 28 in conduit 37 if desired. --~~r ~ ~ Although the materiaI heat processing line 1 illus-^~ trated in Fig. 1 provides for some heat recuperation to minimize -fuel consumption, large amounts of heat are still wasted to the atmosphere, particularly from the cooling section 11 and firing .. ..
sections 8 and 9. The heat reclaim system of the present invention provides an effective means for recuperating such waste heat and ~' making good use of it, in a manner to be subsequently fully .i~ described.
i In Fig. 2 there is shown a preferred form of heat re-covery system 30 in accordance with this invention for use in recovering additional waste heat from a material processing line such as illustrated in Fig. 1. Included in the system is the hood recuperation fan 17 of the material processing line of Fig.
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; ~ 1 which directs a portion of the hot exhaust gases from the -~
secondary cooling section 11 to the burners 12 to provide com-bustion air and also to the downdraft drying section 6 for -5- ~ -; :, .' . !
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!. ': j ` --" 1077467 ~,- drying the material prior to entering the preheat and firing sections 7, 8 and 9. Also included in the system of Fig. 2 is ,, the updraft drying fan 21 of Fig. 1 which forces the hot exhaust gases from the furnace through the updraft drying section S.
To obtain additional heat recovery utilizing the heat recovery system 30 of Fig. 2, air is bled off from both the up-draft drying fan 21 and hood recuperation fan 17 and directed to a drop-out box 31 where the air is mixed and co,~bined and the pressure is equalized. The amount of air bled off may be varied as desired depending on requirements, and in fact all of the air from one or both of the fans 21 and 17 may be passed through the heat recovery system if desired. The mixed bleed-off gas temperatures from the cooling and recuperation fans 21, 17 of the material processing apparatus of Fig. 1 may range from 525 to 575F with an average temperature of approximately 550P.
Since these gas temperatures and volumes are variable, an exit temperature safely above apparatus dew point (approximately ; 180F) is advisable due to the sulfur dioxide content in the waste gas.
From the drop-out box 31 the bleed air is directed to ~t,~'' ~ ~ an economizer 32 which the air enters at approximately 550F.
A supply fan 33 may be required for supplying air to the econ-~, ~ o,mizer 32 depending upon whether there is sufficient positive `!`,;: V: ~' static pressure available at the two bleed-off points A and B
to overcome the added pressure drop through the heat reclaim system 30. If the lowest fan pressure is equal to or greater than the added pressure drop, no additional fan will be required.
; However, the positive pressure of the two bleed-off supplies :J, ~; must be equalized by adjustable orifices 34 prior to entry ; ~ 30 into the drop-out box 31.
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~ The economizer 32 consLsts of a single air pass through ; ~ a deep bank of serpentine coils 35, and water is supplied to the serpentine coils 35 counterflow to the air flow. The heat ab-- sorption rate of the water is desirably based on approximately a ;1~ 300F drop in gas temperature, which means that the leaving gas .
~j temperature will be in the range of approximately 250F. The exit ,) gases from the economizer 32 are then exhausted to the main stack .~.i: ' ' `,~.. ~ 5.
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aste heat recovery in exhaust gases containing products ; ;
j'~!.i 10 of fuel oil combustion and a relatively high particulate load ~ usually makes air to air heat exchangers best suited for this .~.,.,; .
"~i type of recovery due to the relatively low waste gas entry tem-perature (less than 600F) and its relatively high apparatus dew point (approximately 180F). However, because of the large amounts of waste heat to be recovered, it is physically and econ-; omically impractical to use air as the transfer media. Accord-. .
ngly, the heat recovery system of the present invention utilizes -~
water as the transfer media because it is economical to trans-port to and from the various points where it is to be used and is also compatible with the heat recovery system of the present invention. ~`
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~, A closed water system is provided for the water to the economizer 32 so that only a small amount of make-up water is required for water lost through leaks, packing seals, etc. Make- --up water may be supplied to the system from a make-up water .~ storage tank 38 by two supply pumps 39 and 40. One of the supply -pumps 39 is a small capacity pump which may be operated con-~ ~ stantly with a bypass pressure relief valve 41 on the discharge ,`, which returns the relieved water to the storage tank. The other ;~.,; j ~30 pump 40 may be much larger, having a capacity equal to ~pproxi- -mately 25% of the total supply pumping capacity, to provl~e for emergency make-up in case of a major line leak.
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After the water passes through the economizer 32, the water is piped to the suction side of one or more circulating - supply pumps 45 (two such pumps being shown) which pump the ;:
heated water to wherever desired to permit reclamation of the stored heat from the water for such useful purposes as building heating, production of low temperature steam, and so on. I~ow- --ever, it has been found that maximum utilization of the reclaimed waste heat can be obtained by pumping the heated water through ~`~ an air heating coil bank 46 for heating air by the water for use in other processes requiriny sensible heat such as an Aerofall Mill air classification system. The coils in the coil bank 46 should be sized to permit the air passing thereover to extract heat equal to approximately a 250F temperature drop in the supply water so ~' 20 that the supply water is returned to the heat reclaim economizer ~!:
s~j ~, ' 32 at approximately 100F for relleating back to approximately --~
350F.

Separate waste heat reclaim systems 30 such as shown ~, ~ in Fig. 2 should be provided for each material heat processing ~; ~ line and made to operate independently of the other systems to ,,~.,,, ~
permit shut-down of any one line without affecting the balance of the total system. Two such waste heat reclaim systems 30 and 30' are shown in Fig. 3 each associated with a different mater-ial heat processing line and each including updraft drying fans 21, 21' and hood recuperation fans 17, 17'. Of course, the coil ; banks 46, 46' in the ~erofall Mill air classification c~cuits ~: ' ,.~

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3~', and if any of the material processing lines are shut down, the corresponding air heating coil banks and pumping thereto must be reduced to prevent coil freeze-up. The flow of heated ;; water through the respective coil banks 46, 46' is separately i controlled by independent operation of the respective pow~er con- ~ -:~..
trol valves 47, 47'. -` Each of the various waste heat reclaim systems 30, 30' - ~i may be energized by the pressure and temperature of the bleed-off ~ ~ 10 air from each material heat processing line. The starting of the ;
~ fans for the material heat processing lines will energize the .., , water circulating pump circuits for the respective heat reclaim , systems, open the automatic shut-off valves 48, 48' and start water circulating to the economizers. Conversely, when the re-,,,. ~
~; spective circulating pumps 45, 45' stop, the automatic valves , 48, 48' will close on the water circuits, and if there is a sub-i stantial drop in bleed-off air temperature below a predetermined -I ~ temperature, for example 500F, the bypass air valves 49, 49' ~.:
i will open and the supply air valves 50, 50' will close thus stop-ping air flow through the economizers 32, 32' to provide auto-matic dew point safety control.
To prevent overheating of the water in the economizers :,s~
32, 32', a combination pressure-temperature device 51, 51' may 1~ be provided in the water line on the exit side of the economizers ;~ ~ 32, 32' for closing the air valve 50, 50' on the air supply to the economizers and opening the bleed-off air valve 49, 49' ...... .
to the stack S, S' to shut off the air supply. Pressure relief valves on the economizers' water supply drums also prevent over-.. ~
pressure.
Make-up water may be supplied to each of the heat x, ;
' ~ reclaim systems 30, 30' from the same water storage tan~ 38 and g _ : -.
,.. : - - . . . . . . , . . .. : -~i ` 1077~67 supply pumps 39 and 40 shown in Fig. 2.
~,, If the temperature of the waste gas exhaust from the downdraft drying and preheating sections 6 and 7 of any of the material heat processing lines 1 is sufficiently above the appar-atus dew point, for example, 350F, heat may also be reclaimed ,~ from such waste gas exhaust as by providing an additional econom- -izer 32" in the heat reclaim system 30" for that particular ~'1 ' material processing line. The waste gas exhaust from the waste gas exhaust fan 26 from which additional heat is to be reclaimed is supplied to an additional economizer 32" and then returned to ;~
the main stack S' as shown in Fig. ~. The water flowing through the additional economizer 32" is put in series with the associated bleed-off air economizer 32' to create the highest temperature ;~ difference between the air and water, but the water flow is ~, desirably increased to maintain the same temperature difference ';"j ~ between the supply and return water as in the heat reclaim systems 30 associated with the other material heat processing lines 1.
These additional economizers 32" may be automatically controlled with safety protection in the same manner as previously described ~'; ~,, 20 for the bleed-off air economizers 32, 32'.
~, From the foregoing, it will now be apparent that the ~ .
various heat reclaim systems of the present invention are both physically and economically practical for use in reclaiming waste heat from the exhaust gases,of various material heat processing lines by bleeding off air at different points along the lines.
These points will of course vary depending on tlle particular material heat processing,line. The material heat processing line shown in Fig. 1, for example, is of the straight qrate furnace type from which additional heat recovery is possible from both the cooling and firing sections and also possibly from the downdraft drying and preheating sections as previo~s~y described. Significant amounts of heat may also be ~ecov~ered ., ~ ,~ .

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from other material heat processing lines, for example, from the ~' air being discharged from the cooling section 55 of a grate-kiln 56 such as shown schematically in Fig. 5 and described in ..
greater detail in United States Patent No. 3,671,027, granted June 20, 1972. All or a portion of the cooling air which is heated by passing through the bed in the cooling section 55, -;~ rather than being vented directly to atmosphere through a stack, ~-may be directed to a heat recovery system 30 of the type pre-viously described and illustrated for recovery of heat therefrom 10 for other purposes. Moreover, the nature of such heat reclaim , systems is such that they have a relatively long life, and the ~; annual fuel savings resulting from such additional heat recovery is very substantial. There are also some side benefits obtained from such heat reclaim systems, including a reduction in the particulate load and sulfur dioxlde in the stack exit gases because of a reduction in the amount of fuel required and in-', creased operating efficiency.
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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A heat reclaim system for reclaiming waste heat from a material heat processing line comprising a first economizer containing a bank of serpentine coils for passage of water therethrough, means for bleeding heated air from one point on a material heat processing line and directing such heated air through said first economizer to permit extraction of heat from the air by the water passing therethrough, a second economizer containing a bank of serpentine coils for passage of water therethrough, means for bleeding heated air from a plurality of other points on such material heat processing line and directing such heated air through said second econo-mizer to permit extraction of heat from the air by the water passing there-through, said coils of said first and second economizers being in series to create the highest temperature difference between the air and water, a drop-out box for receiving the bleed air from such plurality of other points to mix the bleed air prior to passage through said second economizer, and adjust-able orifice means for equalizing the pressure of such heated bleed air prior to entry into said drop-out box.

2. The system of claim 1 wherein heated air is bled off from both the updraft drying fan, hood recuperation fan, and recuperation fan of a straight grate furnace and mixed in said drop-out box.

3. The system of claim 1 wherein each said means for bleeding heated air from the material heat processing line and for directing such heated air through the respective economizers-comprises a supply air conduit for passage of heated air from each point on the material heat processing line to the respective economizer, a supply air valve for selectively opening and closing said supply air conduit, a bypass conduit for passage of heated air from each such point on such material heat processing line to a stack, a bypass air valve for selectively opening and closing said bypass conduit, and means for closing said supply air valve and opening said bypass air valve for stopping air flow through the respective economizer in response to a drop in the bleed air temperature below a predetermined level to provide automatic dew point safety control.

4. A plurality of heat reclaim systems for reclaiming waste heat from a corresponding number of material heat processing lines, each of said heat reclaim systems comprising means for bleeding heated air from the corresponding heat processing line, a first economizer containing a bank of serpentine coils, means for circulating water through said coils, and means for directing such heated bleed air through said first economizer in a single pass to permit extraction of heat from the air by the water flowing through said coils, at least one of said material heat procesing lines also including a second economizer containing a bank of serpentine coils for passage of water therethrough, and means for bleeding heated air from a plurality of other points on such one material heat procesing line and directing such heated bleed air through said second economizer to permit extraction of heat from the air by the water passing therethrough, said coils of said first and second economizers of said one material heat processing line being in series to create the highest temperature difference between the air and water, and a drop-out box for receiving the heated bleed air from such plurality of other points on such one material heat processing line to mix the bleed air prior to passage through said second economizer, adjustable orifice means for equalizing the pressure of such heated bleed air prior to entry into said drop-out box, and heat extraction means for subsequently extracting the heat from the water after leaving said economizers for use in other processes requiring sensible heat.

5. The systems of claim 4 wherein each said means for bleeding heated air from the corresponding material heat processing line and for directing such heated bleed air through the respective economizers comprises a supply air conduit for passage of heated aiar from each point on the material heat procesing line to the respective economizer, a supply air valve for selectively opening and closing said supply air conduit, a bypass conduit for passage of heated air from such point on such material heat processing line to a stack, a bypass air valve for selectively opening and closing said bypass conduit, and means for closing said supply air valve and opening said bypass air valve for stopping air flow through the respective economizer in response to a drop in the bleed air temperature below a predetermined level to provide automatic dew point safety control.

6. The systems of claim 4 further comprising means providing a common closed water system between said heat extraction means and all of said economizers permitting a continuous flow of water therebetween.

7. The systems of claim 6 further comprising separate control means for each of said heat reclaim systems to permit said heat reclaim systems to operate independently of the other systems so that any one line may be shut down without affecting the balance of the total system.

8. The systems of claim 6 further comprising pump means for pro-viding make-up water to said common closed water system, said pump means including a small capacity pump which is operated constantly to supply such make-up water, and a larger capacity pump to provide for emergency make-up in case of a major line break.

9. The systems of claim 6 wherein means are provided for increasing the water flow through said first and second economizers of said one material heat processing line to maintain the same temperature difference between the supply and return water of said one material heat processing line as in the other material heat processing line.