US1867163A - Heat recuperation - Google Patents

Description

July 12, 1932. H. o. LOEBELL h HEAT RECUPERATION 2 Sheets-Sheei'l 1 Filed Jan. 26, 1925 @Miou/nasi HENBY 0. LOEBELL, 0F NEW YORK,

4Eatente July 12, 1932 Urso STATES n. Y.. AssraNonmo J. Louis cnavnmu, or rame,

PAT-sur OFI-"lcs HEAT BECUPWTIDN Appleman mea January 26.1925. semi no. 4,880.

The present invention relates to improvements in heat recuperation, and more particularly to a'method of and apparatus for effecting rapid and eiicient heat transfer from a bod of heating gas to a body of gas to .be heate through the medium of metal ldlaphragms. lMany industrial processes, particularly those involving heat treatment of materials at high temperatures', are concerned very d1- rectly with the problem of effecting the transmisslonvof heat from one bod. of gas to another through theY medium o refractory or metal walls. It is wellknown that this system oers a good deal of resistance to the passage of heat. As it is now-fairly Iwell established that'the greatest factorlin the resistance to the passage of heat from one body of gases to another through recuperator walls is not the resistance of the walls themselves, but is the resistance of the dead gas films which adhere very tenaciously to both sides thereof, very little' apparent increase in heat transfer can be obtained by simply replacing refractory sections by sections of high temperature metal in the ordinary type of recuperator commonly employed for preheating gas or air.

yIn using the ordinary type of metal or refractory recuperator for preheating air or gas by heat exchange with sensible heat in a gaseous heating medium, as for instance heat in waste products of combustion leaving a heat treating operation, very lowv rates of heat transfer must usually be accepted because all the heat given up by the gas must first pass by convection to the outside ofthe recuperator diaphragm, then by conduction thorough the diaphragm, and thence by convection to the gas or air which is being heated. ln general the conduction of the diaphra a: particularly when of metal construction, is

' very high as compared with the conduction of the films of dead gases which adhere 'to the surfacesof the diaphragm and produce what is called skin eiiect. .y

The primary object ofthe present invention is to provide-a method of and apparatus for effecting rapid and eilicient transfer of heat from one body of gases to another through the medium of'metal walls.

In accordance with this object one feature of the invention contemplates imoving the design and arrangement o f the or 'nary type of metal sectioned recuperator to minimize the skin effect and to decrease the resistance normally offered to heat transfer.

Another object of the present invention is to provide improvements in methodsv and apparatus for heat transfer by means of which the rate at which a current of air or other gas is normally preheated by indirect heat transfer with a heatin medium of metal walls 1s materially in creased.

gas through the 1 ISIS` It has been found that heat transferred by radiation is entirely independent of the skin effect of dead gas ihns adhering to recuperator walls and is only interfered with by solid bodies, which absorb or reflect this heat.

Accordinglyv another feature of the invention contemp tes4 materially increasing the ei'ectiveness of sectioned recuperator by employing relativemedium of metal walls, means for continuing the heating operation ,of the gaseous medium taking up heat during intervals in whichthe hcw of gas giving up heat is suspended.

the ordinary type of metal u ly inexpensive heat absorbing and heat radi- Q With these and other objects and features in view the invention consists in the method of and apparatus for eectingrapid and ecientl heat transfer from hot to cold gases through the medium of metal walls, hereinafter described and particularly dehned in the claim..

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* The various features ofl the invention lare illustrated' in the accompanying drawings, in which:

Fig. 1 is an elevation in diametrical section vof an upright lcylindrical recuperator` equipped with upright metal conduits of circular section having refractory cores and suri rounded by refractory' checker brick, in accordance with the 'preferred form of the in-` vention;

Fig. 2 is a horizontal sectionalview of one of the tubular metal conduits and its refractory core, taken on the. linev2--2`of 1; l ig. 3 is a sectional plan view of a similar type of tubular conduit, illustratnga` modi ied form of refractory core; Fig. 4 is a sectional plan view of a tubular metal conduit loosely filled' with broken lumps of refractory material; y Fig. 5 is a sectional lan view o'f one of the tubular conduits, ilustrating the preferred arrangement of refractory checker brick around the outside of the conduit ;v

Fig. 6 is a sectional plan view of metal f conduits 'of rectangular section equipped tangular recuperator illustrated in Fig. 7

taken on the line 8 8 of Fig. 7

Fig. `9 is a sectional plan enlargement of one end of one of the narrow metal conduits illustrated in Figs. 7 and 8, with its refractory core.

The methodI of effecting rapid and elli-A cient 'heat transfer from a heating gas to a gas to be heated through the medium of a metal wall or diaphragm which comprises the preferred form of the present invention is of particular advantage when applied to the continuous preheating of gas or air or steam `or to-two or more of these materials simultaneously or alternately by means of sensible heat recovered from products of combustion leaving a high temperature heat treating operation.

'The plan of operation normally employed improved upon according to the in such preheating processes, whereby heat first passes from the hot products of combustion by convection to the outside of the metal diaphragm, thence by conduction through the diaphragm and thence by convection to the as or air which is being heated, is varied and resent invention by placing a body of relatlvely inex- Kensive material having oodA heat radiating, eat resisting and-heat a sorbing properties on one or both sides of the metal diaphragml in the path of the hot or cold gases or both hot an cold gases and,k spaced from the surface of the diaphragm toabsorbl `additional heat from the heating As for-transmissiondirectly to the metal sur ace by radiation and to absorb heat radiated from the opposite surface of the diaphragm for transmission by convection from the enlarged heating surface thus exposed to the heat. .More specificall t is based on the use of rebrick, carborundumor other refractory material having good heat radiating and heat absorbing propertiesand gas undergoing pree present invention y n having a low cost per square foot of heating l surface as compared to high temperature metal,for economically increasing theefecpresent method, the amount of heat normally transferred directly from the heating gas to the metal by convection, thence through the metal by conduction and thence to the cold gas 'by convection, is supplemented by the in direct transfer of additional'heat from the heating gas to the metal, by convection to the refractory andthence by radiation to the metal, and by the transferof additional heat from the metal to the gasto be heated indirectly by radiation to refractory and thence by convection tothe'cold gas.

The apparatus elmployed'for carrying out the preferred method comprises a housing 10 having walls 12 of heat insulating material built inthe form of an upright cylinder (see Fig. 1), or of a prone hexahedron (see Figs. 7 and 8) or in almost any other shape desired, the exact form and design vof the housing being optional so 'far as the present invention is concerned. This housing 10 has an air or gas distributing chamber 14 in its lower portion above a base 16, preferably of refractory material, and an inlet 18 opening into chamber 14 for the admission of the steam,`

air or gas to be preheated. In the upper part of the housing there is a chamber 20, or, in the modified form of apparatus illustrated r in Fig. 8, horizontal liues20a, for collecting the hot air or gas after the preheating operation, such chamber 20 and fines 20a communicat ing with an oiitake 22.

The major portion of the interior of housing 10 is taken up-by a heating chamber 24 which is separated from the distributing chamber 14 by a flue sheet or header 26 and is also separated from the collecting chamber 20 at the top of the recuperator by a similar oiltake 34C. A bao plate l when a rectangular ber 2d from an external ported on usamos header 28s Extending between headers 26 and 28, or between header 26 and dues 20a (see Fig. 8), there are a plurality of metal conduits 30 having circular, rectangular, or other odd shaped cross sections (see Figs. 5, 6 and 7) .y Such conduits serve as externally heated preheating chambers for the air,- steam or other gas during its passage between chamber la and chamber 20 or fines 20a. Hot products of combustion or other hot gaseous heatingmedia enter the top of heating chamsource of heat (not an inlet 32, and after passing downwardly through the heating chamber and in direct heat'transferring relationship with the outer surface of conduits 30, and' thereby becoming substantially cooled by indirect heat transfer with the cool gas passing through 'the conduits, the gaseous'heating media exit from the recuperator through an 36 may be extended across part of the uppery part of heating chamber 24 (see Fig. 1) for the purpose of forcing the heating gas to follow a zigzag path during its passage through the heating .chamber of the recuperator, thereby insuring a thorough circulation of the gas around the outside ot conduits 30. However, this badle plate is n ot an essential part of the apparatus, for according to the preferred form. or the invention the heating chamber is substantially lled around and between the conshown) through duits with checker brick or partition walls 38.i or with both checker brick and parti-V tion walls of refractory or 'other good heat absorbing, heat radiatingxand heat resisting material having a relatively low cost per square toot of heating surface as compared with high temperature metal, and the presence ci' this re'ractory-iilling in the heating chamber serves to insure a thorough and uni- 'orin distribution oi: the heating'A gas and an intimate contacting of the hot gas andthe outer suriiace oi conduits 30 throughout each part ci chamber 2d. 'lhe refractory checker brick or partitions 38 are preferably supheader 26 (see Fig. l) or on auxiliary supporting girders d0 (see Fig. 8) and housing isemployed the preferably collected in heating gases are d2 in the lower portion transverse chambers of the heating chamber, between the metal conduits 30, and from chamber i2 are passed through a horizontal flue lid to the odtak'e Si v(see Fig. 8)

The edectiveness et the normal type of recuperator having metal sections or ldiaphragrns 30 is greatly increased by thus placing refractory material A38 around the outer or heating side et the metal walls in the path of the heating gases and spaced at a distance from the wall so that hot gases may circulate around theA refractory and between the reiractory and the metal wall. The refractory 38 preereblyarranged to present a maxiabove header 26 and mum surface directly facing lthe outer surface vof the conduit tor the transmission of heat absorbed from the gas as radiant ener directly from the refractory to the metal.

thus arranging the refractory 38 the current or` hot gases entering the recuperator is forced through the narrow paths d6 between two surfaces, one or metal and tory, each of which takes up sensible heat from the gas by convection. Both the refractory8 and the metal conduit 30 are def signed Ato present a maximum heat absorbing surface to the heating gas. Heat taken up the other ot retracby the metal surfaces passes rapidly through the metal to the cool gas side, due tothe high heat conductivity ot the metal and tothe temperature diderential which exists between the heating. gas on one side of the metalI and the gas taking up heat on the other. The temperature ota the heating gas is at all times higher than that of the refractory in its path for the reason that the refractory is surrounded on all sides by the heating gas, and the temperature of the refractory quickly rises,

4above the temperature of the metal because the metal will always conduct heat away from the heating surface many 'times faster than heat can be taken up from the gas. As the refractory reaches a higher tern erature than the metal surface exposed to t e heating gas, heat will be transmitted directly :troni the surface of the refractory to the metal surface opposite, and as, particularly at high temperatures, the rate oi heat transfer by radiation is very high as compared tothe rate at which itis transferred by convection,

it is possible with the. present apparatus to absorb and transmit much more heat from the heating gas to the metal surface ot the recuperator conduits per square foot of exposed surface than would be possibleit the refractory were not used,

The openings 46 between the metal and the refractory are so restricted that the velocity 'of the gas passing therethrough is necessarily much higher than it would b e ii the refractory were not used. This increased velocity of the heating gas reducing thethickness of the dead gas films which adhere to the surface oi' the refractory and to themetal surface of conduits 30 and thereby effect of these gas films to heat transfer. However itis not advisable to reduce the size of'openings 46 to such an extent as to the resistance o'ered to the flow of heating gas through the recuperator and the resultant pressure drop excessive, and accordingly, to

maintain proper spacing between the refracmake` bib.

has the eiiect of f still 'further reduces the resistance type of refractory According inventionmetal con uits 30 are equipped with cores 50 of relativel inexpensive material having good heat a sorbing and heat resistant properties, such as the ordinary frebrick. These cores may be built up 1 of blocks of odd cross Asectiling supported by tion resting on the foundation of the r'ecuperator (see Fig. 1) or they may consist of loose pieces of broken refractory, or thin brackets 52 (see Fig. 9) or by a erforated metal floor at the base of the' con uit,y as Where loose refractory'such as that illustrated in Fig. 4 is employed. These refractory cores 50 preferably extend through the whole length ofthe metal conduits and are centered in the conduit so as to` leave anannular passage 54 between the outer surface of the core and the inner metal surface of the conduit for the passage of air,

a steam or'other medium undergoing preheat from. distributing chamber 14 to collecting chambers 20 and 20a. Refractoryv material is readily molded into odd shapes such as those illustrated in Figs. 2, 3, and 4, and by building the refractory cores 50, as well as the refracto vchecker brick and partitions 38 on the outsi e of the metal conduits, in shapes of odd cross section so as to expose a large surface to the heating gas and to the air or gas to be heated as compared with the surface of exposed metal, the effectiveness of the metal surface is greatly increased, or in other words 'it is possible to extract much more heat from the heating' gas and to put much more heat into the gas being heated per square vfoot of metal surface than would e possible if the refractory were not used.

Various' shapes and arrangelrents of refractory such as those illustrated in the drawings 4particularly in Figs. 2 3 4 5 6 and 9 maybe adopted in accordanbe withthe tem-- peratures and volume of gas undergoing heat interchange. It is of obvious advantage to have the refractory on the outside of the metal diaphragms so arranged that radiant refractory energy will pass directly from the refractory to the metal; andthe refractory on the inside so arranged that radiant energy will pass directly from the metal to the refractory. Likewise it is obviously important to have the on the heating side of the metal so arranged that the heating gas will circu- .late in direct heat transferring relationship with the metal and the refractory; and to have the refractory on the inner or cold gas side of the metal diaphragm so arranged that the gas tobe heated willcirculate in direct heat transferring relationship with the surfaces of the metal and the refractory.

For preheaters in which there is a high pressure drop Lorturbulent flow of gas to be heated throu h the metal conduits, the conduits are pre era-bly constructed with a rectangular or other odd shaped cross section to the referred form hof the such as to provide a relatively large heating surface per unit volume displacement (s ee Figs. 6 and 9). In employing this type of narrow conduit the refractory cores on theY inside and the refractory 'partition walls around the outside of the light construction, flow of heating gas and the flow of gasto be heated is substantially continuous. For operations in which .it is necessary to impart heat continuously to a current of gas or air to be heated, and where it is occasionally necessary to cut off for short intervals theflowL of heating gas through the recuperator, refractory partition walls 38 or refractory cores 50, or both partition walls and cores of rela-l tively bulk construction, may be used (see Figs. 7 an y'8), Thus in cases in which the heat treating or other operation from which vthe supply of products of combustion or other heating gas is obtained is intermittent, and where it 1s at thesame time desired to continue the preheatin of air, steam or other gas, either b` passing t e gas undergoing preheatthroug the recuperator conduits as before or by passing the gas through the heating chamber passages of the recuperator in the Same direction or in a direction opposite to that normally taken by the products of combustion, it is obviously of advantage to use refracto of relatively large heat storage capacity 1n the passages surrounding the conduits may be of in operations in which the aol conduits or inside the? conduits or both inside y the `conduits and in the passages surrounding the conduits. By employing such bulky cording toits heat storage caipaclty to maintainlcontinuous VApreheating o the gas taking up heat during the interval in which 'no heating gas is passed through the recuperator.

One advantage obtained by the use of refractory bodies placed in the path of the heating gas and having a large heat radiating surface facing the outside heat absorbing surface of the metal recuperator diaphragm is that with this arrangement it is possible to transmittheat from the heating gas to the metal surface at a greatly increased rate which, while it will usually be well below the maximum heat conducting capacity of the metal, will nevertheless more than double the efectivehheat absorbing capacity of the metal surface, particularly athigh temperatures. The cost of refractory per unit of heating area is generally relatively low as compared v operations em I memes to the cost of a corresponding area of high temperature metal, and as it is possible by employing refractor to at least double the effective heat absorblng surface of the recuperator and to greatly increase the heat transmitting effectiveness of the recuperator at a relatively of the greatly increased rate at which heat can be transferred by radiation as compared to l the rate at which it is transferred by convection, the capacity of therecuperator can be at least doubled without the necessity of increasing the surface of high temperature metal or the size of the recuperator. By using refractory cores, i. e. cores of material capable of absorbing and reflecting radiant heat, positioned in t -e path of the air or gas taking up heat and preferably having a surface area at least as large as the inner surface area of the metal conduits, it is possible to transmit heat to the gas over substantially double the heating area normally presented by the metal alone, and by absorbing heat radiated from the metal to substantially double the volume of heat normally removed from the inner surface of the metal during a given period, thereby substantially increasing the heating. effectiveness of the recuperator per square foot of metal surface. Thus the rate at which heat is transferred from refractory to air is about the same as that at which it is transferred from metal to air, and although the differential temperature will be somewhat less for refractory to air than for metal to air on the inside of the conduits, it is possible and eX- tremely feasible to get more surface on the refractory than on the metal and thereby, with coresl suchl as those illustrated in the drawings, to at least double the heat transfer effectiveness of the recuperator.l The rate at which heat is transferred from metal to air by convection is relatively low as compared to the rate at which heat is transferred by conduction through the metal, due partly t0 the resistance to heat transfer offered by the dead gas films adhering to the metal surface. Consequently there isa lways a supply of heat radiating from the inner walls of the metal to take the place of the heat transmitted by convection from the surface of the refractory core to the gas or air taking up heat.

The apparatus constituting the present invention is designed primarily to make use of the advantages in heat transfer gained by Atransmitting heat in the form of radiant energy rather than solely by convection, in securing more effective heat recuperation in ploying gaseous fuel. However, while deslgnedprimarily for such recuperative service the present apparatus can be readilyad'apted to regenerative in addition to recuperative service by simply employing bulky refractory small expenditure, largely because through the medium of heat conducting walls which comprises a housing, heat conducting; conduits extendin therethrough and each comprising a flat, t in fiue, means for passing gas to be heated through said conduits, a at, thin plate of refractory material inside each iiue and having heat absorbing and heating surfaces spaced from and substantially parallel to the inner walls of said f'iues', means for passing a heating gas through-said housing 1n contact with the outer surfaces of said ilues, refractory material in said housing between adjacent flues and outside the same, said outside refractory comprising supporting posts and fiat plates. mounted therebetween and spaced from said heat conducting flue walls to form ues for said heating gas,

said heat conductin iue walls being pro- -vided at spaced pomts with protuberances engaging said su porting-posts to maintain sald outer fiat p ates and heat conducting lues in redetermined spaced relation.

In testimony vlieof I aiiix my signature.

and making use of the heat

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