CA1208140A - Exhaust gas treatment method and apparatus - Google Patents

Abstract

EXHAUST GAS TREATMENT METHOD AND APPARATUS

Abstract of the Disclosure: Exhaust gas treatment method and appara-tus extract heat from an exhaust gas by operating in a water-condensing mode which allows more heat to be recovered, removes particulate matter and condensed acid from the exhaust gas, and washes heat exchange sur-faces to keep them clean and wet to improve heat transfer. Systems for heat-ing water, air, and both water and air are disclosed. Methods of constructing and assembling improved heat exchangers are disclosed.

Description

~8~
My invention relates to exhaust gas treatment method and apparatus, and more particularly, to improved method and apparatus useful not only for recovering large amounts of heat from various industrial exhaust gases, but also for simultaneously removing substantial amounts of particulate matter and corrosive products of combustion from such exhaust gases, thereby to reduce air pollution from stack emissions. The invention is particularly di-rected toward such treatment of sulfur-containing exhaust gases, such as those typically produced by burning oil or coal in f'urnaces, though it will be-coms apparent that the invention will be useful in a wide variety of other ap-10 plications. A primary object of the invention is to provide method and appara-tu~ which are useful for recovering a substantially larger percentage of the heat contained in an exhaust gas than that recovered in typical prior system3~
which has very important economic implications, due to the high costs of fuels.
Another important object of the invention is to provide method and apparatus which are useful for removing substantial amounts of particulate matter and corrosive products OI combustion from exhaust gases, thereby decreasing pollution. Natural gas, #2 fuel oil, #6 fuel oil, and coal, generally ranked in that order, produce flue gases containing increasing amounts of sulfur dioxide and sulfur trioxide. and particulate matter, such as soot and silica products.
20 One object of the invention is to provide method and apparatus which IS useful in connection with flue gases produced by any of those ~uels.
In many applications it is desirable that waste heat be used to prehea t a liquid, such as boiler make-up water, or industrial process water as exam-ples, while in many other applications it may be preferred that waste heat be used to preheat a gas, such as air, and in some applications to heat both a li-quid and a gas. Another object of the invention is to provide a method which lends itself to preheating of either a liquid or a gas or both a liquid and a gas, and to provide apparatuses which preheat liquid or a gas or both a liquid and a gas.

-2-lZ~B~40 A very important object of the present inven~on is to provide method and apparatus which is rugged and reliable, and useful over long periods of time with minimum attention, and minimum requirements for "down-time"
for cleaning ~r repair.
Another more specific object of the invention is to provide a heat ex-changer which functions as a self cleaning gas scrubber as well as recover-ing increased amounts of heat from an exhaust gas, It long has been known that the thermal efficiency of a plant or process can be increased by recovering some of the heat energy contained in the ex-10 haust gas from a boiler furnace or the like. Flue gas commonly is directedthrough boiler economizers to preheat boiler feedwater, and commonly di-rected through air preheaters to preheat furnace combustion air, in each case providing some increase in thermal efficiency. The amount of heat which it has been possible to recover from flue gas ordinarily has been quite lirnited, due to serious corrosion problems which otherwise result. Combustion of oil, coal or natural gas producesflue gas having substantial moisture, sulfur di-oxide, sulfur trioxide, and particulate matter in the cases of oil and coal. If a hsat exchanger intended to recover heat from flue gas condenses appreciable amounts of suLfur trioxide, sulfuric acid is formed, resulting in severe cor-20 rosion. The condensed sulfur product can readily ruin usual economlzers andaîr preheaters, and the exhaust stacks associated with them. Thus prior art systems intended to recover heat from flue gas traditionally have been opera-ted with flue gas temperatures scrupulously maintained high enough to avoid condensation of sulfur products.
The temperature at and below which condensation will occur for a flue gas not containing any sulfur oxides, i. e., the dew point due to water vapor only, is usually within the range of tl6~F to 130F, depending on the partial pres-sure of water vapor. But the presence of sulfur trioxide even in small amounts, such as 5 to 100 parts per million, drastically increases the temp-30 erature at which condensation will occur, far above that for water vapor only.

- 3--For example) 50 to l00 parts per million of SO3 may raise the dew point temperature to value3 such as 250F to 280F, respectively. Thus it has been usual practice to make absolutely certain that flue gas is not cooled be-low a temperature of the order of 300F, in order to avoid condensation and corrosion. Such operation inherenMy results in an undesirably small portion of the sensible heat energy being extracted from the flue gas, and in absolute-ly no rer:overy of any latent heat energy contained in the flue gas. One conceptof the present invention is to provide method and apparatus Ior recovering heat from a potentially corrosive exhaust gas, such as flue gas, in a manner directlycontrary to prior art practices, using a heat exchanger which continuously operates in a "water condensing" mode, allowing substantial amounts of latent heat, as well as more sensible heat, to be recovered from the exhaust gas.
The term "water condensing" is meant to mean that the temperature of a large percentage (and ideally all) of the exhaust gas is lowered not only below the sulfuric acid condensation or saturation temperature, but even below the sat-uration temperature of water at the applicable pressure, i. e~ below the dew point, e. g. 120F, for water vapor only. In a typical operation of the invention where absolute pressure OI the flue gas within a heat exchanger is of the order of 2 to 5 inches of water (0. 07 psig. ), the temperature of large portions of the flue gas is lowered at least below 120F to a temperature of say 75F to 100F, by passing the flue gas through a heat exchanger scrubber unit. The unit con-tinuously condenses a large amount of water from the flue gas, as well as con-densing sulfuric acid. Parts within the heat exchanger-scrubber unit which would otherwise be exposed to the corrosive condensate are appropriatelylined or coated with corrosion-resistant materials, e. g. Teflon (trademark), to pre vent corrosion.
When prior art waste heat recovery systems have been operated with flue gas temperatures ~e.g.250F) too near the SO3 condensation temperature, whether by accident, or during startup, or in attempts to improve system thermal efficiency, the occasional condensation of SO3 tends to produce very 81'~0 strong or concentrated sulfuric acid. The sulfuric acid, is extremely corros-ive and can rapidly destroy an ordinary heat exchanger. The volume of sulfur-ic acid which c an be condensed is small, but sufficient to keep heat e~changer surfaces slightly moist. However, if one not only ignores prior art practice, but operates in direct contradiction thereto, and further lowers the flue gas temperature to a level markedly below the SO3 condensation temperature, to operate in the water condensing mode of the invention, the production of large amounts of water tends to substantially dilute the condensed sulfuricacid, making the resultant overall condensate much le ss corrosive, and le ss likely to 10 damage systemparts. Thus operationinthewatercondensingmode of the in-vention has an important tendency to le ssen corrosion of system parts, as well as allowing large amountsoflatentheatenergy to berecovered. Suchdilution of the sulfuric acid does notwhollyeliminate corrosion, however, so that it remains necessary to appropriately line or coat various surfaces within the heat exchanger-scrubber unit with protective materials.
I have discovered that if I pass flue gas through heat exchanger appara-tus which is operating in the water condensing mode, not only can much more heat energy be extracted from the flue gas, and not only can the condensate be made less corrosive, but in addition, large amounts of particulate matter 20 and SO3 simultaneously can be removed from the flue gas, thereby consider-ably reducing air pollution. I have observed that if flue gas is cooled slightly below the sulfuric acid condensation temperature, and if the flue gas contains a substantial amount of particulate matter, a soggy mass Oe sulfuric acid com-bined with particulate matter often will rapidly build up on heat exchange sur-faces, and indeed the build-up can clog the heat exchanger ina matter of a few hours. But if the system is operated in the water condensing mode in accor-dance with the present invention, the production Oe copious amounts of water continuously washes away sulfuric acid and particulate matter, preventing buildup of the soggy mess, In simple terms, the water condensing mode of 30 operation forms a "rain"within the heat exchanger. The "rain" not only en-~8~

traps part icles in the flue gas as it falls, ~ut it also washes aua~, down to adrain, particles which have lightly stuck to the wetted surfaces. Thus advan-tages akin to those of gas scrubbing are obtained without a need for the cont~u-ous supply of water required by most gas scrubbers, and with no need for mov-ing parts.
Yet, even in addition to recovering large amounts of latent heat energy, greatly dilut;ng and washing away sulfuric acid to minimize corrosion prob-lems, and removing substantial amounts of particulate matter from exhaust gases such as flue gas, operation in the "water condensing"mode also enhances 10 the heat transfer coefficient of heat exchanger units used to practice the inven-tion. Heavy condensation rnade to occur near the top of the hea$ exchangerunit runs or rains downwardly, maintaining heat exchange surfaces wetted in lower portions of the heat e~cchanger, even though condensation otherwise is not oc-curring on those surfaces. With substantially all heat exchange surface area maint~ined wetJ the heat transfer coefficient is markedly improved over that of a dry or non-condensing heat exchanger, thereby allowing units which are reasonably compact, and economical to ~abricate, to proyide sufficient heat transfer to recover large amounl:s of waste heat.
Heat transfer is improved for several separate but related reasons.
20 Heat transfer from ~e gas occurs better if a surface is wet from dropwise *
condensation. Use of Teflon promotes dropwise condensation. Further, ~e constant ra;n with;n the heat e~change keeps the tube surfaces clean, pre-venting thebuildup of deposits which would decrease heat t~ansfer. Thus an-o~er object of the invention is to provide improved heat recovery apparatus having Improved heat transfer. -While I have found various forms of ptfe (Teflon) to provide very effec-tive corrosion protection, and to have hydrophobic characteristics which co-operate with water vapor condensation to keep a water-condensing heat ex-changer cleanD currently available forms of those corrosion-protection rnater-30 ~als have deformation, meltin~ or destruction temperatures far below the tem-*Trademark - 6 -~Z~ 4~

peratures of some flue aases from which it is desirable to ex-tract waste heat. In accordance with a particular feature of the present invention, I propose to operate a water-condensing heat exchanger in cooperation with another heat exchanger of conventional type, which may be severely damaged if condensa--tion takes place in it. The conventional heat exchanger can initially cool an exhaust gas such as flue gas down to a temp-erature which is low enough that it will no-t damage -the cor-rosion-protection linings of the water-vapor condensing heat exchanger, yet high enough that sulfur trioxide cannot con-dense in the conventional heat exchanger so as to damage it.
Thus some added objects of the invention are to provide im-proved heat recovery systems which are useful with exhaust gases having a wide range of temperatures.
Some added objects of the invention are to provide heat exchanger modules suitable for use in the mentioned water-condensing mode which can be readily combined as needed to suit a wide variety of different flow rate, temperature and heat transfer requirements.
Other objects of the invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises the several steps and the relation of one or more of such s-teps with respect to each of the others, and the apparatus embodying features of construction, combination of elements and arrangement of parts which are adapted to effect such steps, all as exempli-fied in the following detai.led disclosure, and the scope of the invention will be indicated in the claims.
For a fuller understanding of the nature and objects of the invention reference should be had to the followina detailed description taken in connection with the accompanyinq drawings, in which:

Fig. 1 is a diagram of a heat exchanger system useful in understanding some basic principles of the present inven-tion and that of copending Canadian Patent Application No.
388,113 filed October 16, 1981 of which the present applica-tion is a divisional.
Fig. 2 is a set of graphs useful in understanding principles of the method of the present invention.
Fig. 3a is a schematic diagram of one form of water heating system according to the invention as described in the aforesaid application No. 388,113.
Fig. 3b is a diagram illustrating the use of a direct-fired paper dryer to save heat energy and remove particulate matter from the exhaust gas from the paper dryer.
Figs. 3c and 3d each is a diagram illustratinq a use of the water-condensing heat exchanger together with a prior art air preheater.
Fig. 4a is a plan view of an exemplary heat exchange module for use with the invention.
Fig. 4b is a partial cross-section elevational view taken at lines 4b-4b in Fig. 4a.
Fig. 4c is a diagram useful for understanding the heat exchange tube spacing in a preferred embodiment of the inven-tion.
Fig. 4d is a diagram illustrating one form of water manifolding which may be used in connection with the invention.
Figs. 5a and 5b are front elevation and side elevation views of an exemplary air heating form of wa-ter-vapor con densing heat exchanger.
Fig. 6 illustrates an exemplary heat recovery system utilizing a heat exchanger which heats both air and water from boiler flue gas.
FigsO 7a and 7b are partial cross-sec-tion views useful ~8:~L4(:~
for understanding a method of assembly of a heat exchanger and the nature of tube-to-tube sheet seals provided by use of such a method.
Some major principles of the present invention can be best understood by initial reference to Figs. 1 and 2.
In Fig. 1 a duct 10 conducts hot exhaust gas, such as flue gas drawn from a boiler stack (not shown) by blower B, to a bottom plenum or chamber 11. The flue gas passes upwardly through one or more hea-t exchange uni-ts, such as the four units indicated at 12, 13, 14 and 15, - 8a -8~

thence into an upper plenum or chamber 16 and out a stack 17. In typical operations the flue gas velocity is arranged to be 30-40 feet per second with-in the heat e~change units, and a gas pressure drop of the order of 1 1/2 to 2 inches of water is arranged to occur across the heat exchange units. A fluid to be heated, which typically will be water or air, is shown introduced into the uppermost heat exchange unil: at 20, understood to flow downwardly through successive ones of the heat exchange units, and to exit at 21. For simplicity of explanation it initially will be assumed that water i9 to be heated, and that the hot exhaust gas is flue gas from a boiler.
It will be apparent that the flue gas will be cooled to some extent as it travels upwardly through the unit, and that the water will be heated to some extent as it travels downwardly through the unit. To facilitate explanation, the range of elevations within which significant gas cooling and water heating oc-cur is shown divided into four zones Zl to Z4. The four zones are shown for simplicity of explanation as corresponding to the vertical ranges of the four heat recovery units, Fig. 2 illustrates the variations of flue gas and water temperatures in typical practice of the invention to heat water, with the temperatures plotted against vertical elevation. Thus the temperature of flue ga9 falls from an as-20 sumed input temperature Gl of 500F at the bottom of the heat exchanger sys-tem to an assumed output temperature Go of 90F at the top of the heat ex-changer system as the flue gas travels upwardly through the heat exchanger, as indicated by curve G. The gas temperature plot in Fig. 2 should be under-stood to be approximate, and in general to depict for any elevation the lowest temperature to which substantial portions of the gas are lowered at that e1eva-tion. At any elevation above the lowermost row of tubes there are temperature gradientsJ of course, and the average temperature, if averaged over the entire cross-sectional area of the heat exchanger at a given elevation, will be above that plotted as curve G. Viewed in another way, at a given elevation, such as 30 that bounding zones Z2 and Z3, some portions of the flue gas, such as portions ~L2~

near or on a tube may ha~e the temperature G2 at which sulfuric acid is forming, while other portions of the gas at the same elevation but at greater distances from any tube may be hotter and not yet experiencing condensation~
Simultaneously,water assumed to have an input temperature Wl of 55Fatthe top of the heat exchanger is heated to an output temperature Wo of 180F by the time it reaches the bottom of the heat exchanger, as indicated by curve W. The ordinate scale in Fig~l is shown divided into the four zones Zl to Z~. A verti-cal dashed lineFat250Findicates a typical temperature C~2 atwhich sulfuric acid forms in typical flue gas obtained from burning No. 6 fuel oil. The prior 10 art has taught that flue gas temperature always should be maintained amply above that level in order to avoid production of any sulfuric acid.
Following the flue gas temperature curve S~ for upward flue gas travel, it will be seen that the flue gas ternperature drops from 500 F at G 1 to 250 F
at G2 as the gas passes through the two lower zones, Zl and Z2. During that portion of its upward travel very little condensation is occurring from the gas, but sensible heat from the flue gas is being transferred to heat the water. More precisely, most of the gas in zones Zl and Z2 is still too hot for sulfuric acid to condensegbut some amounts of the gas in zones Zl and Z2 will condense while in ~ose zones,because condensate falling through those zones from 20 above will cool some amounts of gas in those zones to the condensation tem-perature. E~en though little or no condensation from the gas is occurring in lower zones Zl and Z2, the gas-side surfaces of heat exchanger tubes in those lower zones will continuously remain wet,by reason of copious condensation falling from above. The wet condition of those tubes causes particulate matter to tend to lightly and temporarily stick to them, but condensate Ealling from above acts to continually wash them, washing sulfuric acid and particulate mat-ter down to the drain(D, Fig.l) at the bottom of the heat exchanger unit.
As the flue gas is cooled below 250F, a typical dew point for sulfur tri-oxide, sulfuric acid forms in zone Z3. If zone Z3 were the uppermost zone in 30 the system, the condensed sulfuric acid would slightly wet heat exchange sur-~8~L4~

faces in zone Z3, and particulate matter would build upon those slightly wet-ted surfaces. The sulfur trioxide condenses to form sulfuric acid in the as-sumed exami~le, as gas travel through zone Z3 cools the gas substantially be-low the acid dew point. At some level within ~one Z3 most or all of the sulfur-ic acid which will be condensed, will have condensed. Just above that level there is believed to be a range of elevations, indicated generally by bracket R
in Fig. 2, in which one might deem little or no condensation to be occurring, the gas temperature being low enough that condensation of sulfuric acid has been largely completed,but being too high for water vapor to condense in sub-10 stantial quantities. However, as the gas is further cooled during further upwardtravel through zone Zl, condensation of water vapor occurs in an increasingly copious manner. Thus all of the tubes in the heat exchange units are continu-ously wet, which not only increases heat transfer, but which, at the mentioned gas flow rate, also causes particulate matter to tend to temporarily and lightly stick to the tubes, removing large amounts of particulate matter from the gas.
The water caused by continuous condensation acts to continuously wash the sur-faces OI the heat exchange ~besJ washing particulate matter and condensed sul-furic acid down to the drain at the bottom of the system. The tubes within the heat e~change UllitS are arranged in successi~e rows which are horizontally 20 staggered relative to each other, so that a drop of condensate falling from one tube tends to splash on a tube below. A double form of cleaning of the flue gas occurs, from the combined scrubbing of the gas as it passes through the falling rain of condensate, plus the tendency of the particulate matter to temporarily and lightly adhere to the wetted tubes until condensate washes it away. The thin coverings of Teflon on the heat exchange tubes not only prevent corrosion of those tubes while allowing good heat transfer,but because those coverings are hydrophobic, the falling condensate is readily able to wash away particulate matter from the tubes. Tests recently conducted by Brookhaven National Lab-oratory indicate that approximately 50% of the particulate matter and 20 to 30 25% of the SO3 in Elue gas from No. 6 fuel oil can be removed. In a typical ap-~Z~

plication of the invention, if say 9660 pounds per hour of flue gas from No. 6 oil are passed through the water condensing heat exchanger, approximately 250 pounds per hour of water mixed with sulfuric acid and particulate matter will be drained from the bottom of the unit. The particulate matter in the mix-ture has been about 3~ by weight in tests made to date.
The amount of sulfur trioxide in typical flue gas is measured in parts per million, so that the amount of sulfuric acid which even complete conden-sation of sulfur trioxide could produce i9 much, much less than the amount of water which can readily be condensed if the flue gas i9 cooled sufficiently.
10 The precise temperature at which water vapor in flue gas will condense varies to some extent in different flue gas mixtures,but it is approximately 120F
(49C). Thus in accordance with the present invention, water (or air~ at or be-low that temperature, and preferably well below that temperature, is maintained in a group of tubes near the top of the heat exchanger unit, insuring that large amounts of water vapor condense. In Fig. 2 the water temperature is assumed to rise from 55F to about 75F as tne water passes downwardly through zone Z4. Thus substantial tube volume at the upper portion of the heat exchanger system in Figs. 1 and 2 contains water below 120F, It may be noted that in the example of Fig. 2 the water temperature 20 reaches the water vapor dew point (120F) at substantially the same elevational level where the flue gas temperature reaches the sul~uric acid dew polnt(250F), and further, that the level occurs substantially at the vertical mid-level of the heat exchanger. Those precise relationships are by no means necessary.
Irl Fig. 2 curve D is a plot of the difference between flue gas temperature and water temperature versus elevational level in the heat exchanger unit. The temperature difference at a given level has an important bearing, of course, on the amount of heat transfer which occurs at that level. It is important to note that near the bottom portions of the heat exchanger unit, in zones Zl and Z2, where there is maximum difference between gas temperature and water 30 temperature and hence a maximum potential for effective heat transfer, the 8~L40 amount of heat transfer which actually occurs is further in-creased because the heat exchange tube surfaces in those zones are maintained wet by falling condensate.
It may be noted that in most systems constructed in accordance with the invention, the flue gas exit temperature, measured in chamber 16 or stack 17 in Fig. 1, for example, will be less than the flue gas water vapor dew point tempera-ture of say 120F, but in some systems the exit temperature at such a location may exceed that dew point to some extent, without departing from the invention. If the heat exchanger tube and housing yeometry allows some quantum of flue gas to pass through the unit with only modest cooling, the ~uan-tum will mix in chamber 16 with flue gas which has been cooled sufficiently to condense large amounts of water, and tend to raise the average or mixture temperature in chamber 16, in the same manner that by-passing some flue gas around the heat exchanger and admitting it to chamber 16 would raise the average temperature in that chamber.
From the above it will be seen that the method of the invention comprises simultaneous recovery of both sensible and latent heat from a hot exhaust gas containing water vapor, a condensable corrosive constituent (sulfur trioxide) and particulate matter, and removal of substantial amounts of the condensed corrosive constituent from the gas, by passiny the gas through a gas passage of a heat exchanger, simultaneously passing a fluid cooler than the exhaust gas through a second passage of the heat exchanger in heat exchange relationship with the gas passage, with the flow rates of the gas and fluid arranged in relation to the heat transfer characteris-tics of the heat exchanger so that continuous condensation ofwater vapor and the corrosive constituent occurs, providing falling droplets which capture and wash away portions of the ~L2~8~0 condensed corrosive constituent. Subs-tantlal amounts of par-ticulate matter may also be washed away.
While prior art economizers and air preheaters require ho-t input water or air, the invention advantageously can use cold water or air, and indeed, efficiency increases the cold-er the input fluid is, with more condensation oc-- 13a -curring and more latent heat being e.Ytracted from the e:;haust gas. The cooler the water at inlet 20, the lower the e~it temperature of the e.chaust gas will be, the more latent heat will be recovered, the more water vapor will be conden-sed from the exhaust gas, and the more effective particulate and SO3 removal will be, for given flow rates. Tests to date have indicated that with a gas vel-ocity of the order of 30-40 feet per second, provision of sufficient heat e~-change surface area will enable one to lower the temperature of the exiting flue gas to within about 8F of the input ternperature of the fluid being heated, be it water or air~ For example, with water being supplied to the heat exchan-ger at 55F, it has been possible to obtain flue gas exit temperatures of 63F.
In most applications of the inventi~n it will not be deemed necessary to lower the flue gas temperature to that extent, and the amount of heat exchange sur-face will be selected so that with the desired flow rates and fluid temperature the nue gas tempera ture will be lowered to within the range of 80 F- 100 F .
Typical flue gases contain 5% to 12% water vapor,, depending upon the ~rpe of fuel, so if a 10,000 pounds of flue gas are passed to a heat exchanger in an hour that represents 400-1200 pounds of water per hourO By operaffng in the water-condensing mode, several hundreds of pounds of water will condense per hour.
The advantageous effect which very substantial water condensation has on heat transfer has been illustrated by operati:ng a heat exchan~fe unit under two sets of operating conditions. The unit was first operated with flue gas produced by burning No. 2 fuel oil, wi~ particular inlet and outlet tempera-tures and flow rates of flue gas and water. The heat recovered was measured to be approximately 1,000,000 Btu per hour, and condensate flowed from the unit at approximately one half gallon per minute. Then later, flue gas produced by burning natural gas was used. The natural gas can be and was burned with less excess air, and due to the greater hydrogen content of the natural gas, the flue gas contained a greater amount of moisture. At the same gas and water flow rates as had been used with fuel oil operation, the amount of con-densate which flowed from the unit was essentially doubled, to approximately a full gallon per minute. However, the water outlet temperature increased and the gas exit temperature decreased, and the heat recovered had increased approximately 20%, to 1, 200, 000 Btu per hour.
In the application depicted in Fig.3a flue gas is drawn from the conven-tional stack 30 of boiler 31 past one or rnore damper valves by an induced draft blower B driven by blower motor BM, to supply the flue gas to the bottom plenum of the heat exchange unit E~X, and cooled flue gas exits via fiberglass hood 16 and fiberglass stack 17 to atmosphere. The use of a fiberglass stack 10 to re9ist corrosion is not per se new. Cool water from the bottom of hot water storage tank ST, froxn a cold water supply line SL and/or from a water main WM, is pumped through the heat exchange unit HX by circulator pump CP, and heated water from the heat exchanger is pumped into storage tank ST near its top. Hot water is drawn from the top of the storage tank via line HW for any of a variety of uses. Make-up water for boiler 31 can be supplied from tank ST, of course, or directly from unit HX.
If insufficient hot water is drawn via line HW, the contents of storage tank ST can rise in temperature enough that insufficient flue gas cooling be-gins to occur in the heat exchange unit, tending to endanger the Teflon corro-20 sion-protection coverings and liners in ~hat unit. A conventional thermal sen-sor TS senses outgoing -flue gas temperature and operates a conventional positioner PVl, which closes damper valve DVl to decrease or terminate passage of hot flue gas to the heat exchanger~ In Fig. 3a a second positioner P~2 also responsive to thermal sensor TS is shown connected to operate dam-per valve DV2, which can open when stack 17 temperature climbs too high, to mix cool ambient air with the flue gas, thereby to prevent temperature with-in the heat exchanger from exceeding a value (e.g. 550F) deemed dangerous for the corrosion-protective coatings. In many applications only one or the other of the two described heat limit protecting means will be deemed suf~ic-30 ient. As will become clear below, preventing a temperature rise which would damage the corrosion-protection coatings can instead be done by using a con-ventional non-condensing heat e~changer to cool the exhaust gas to a safe op-erating temperature before the exhaust gas is passed through the water-con-densing heat exchanger.
In Fig. 3a a spray manifold SP carrying a plurality of nozzles is opera-tive to spray water down through heat exchanger HX when valve V is opened, to wash away any deposits which might have built up on heat exchanger tubes.
Because operation in the water condensing mode ordinarily ~unctions to keep the tubes clean, operation of such a spray can be quite lnfrequent, and in some applications of the invention provision of such a spray means may be deemed wholly unnecessary. In certain applications, such as where particu-late removal is deemed particularly important, valYe V can be opened to per-mit a continuous spray, to augment ~e "rain" caused by water vapor conden-sation.
Practice of the invention may be carried out in order to heat air rather than water, utilizing heat exchanger apparatus which is extremely similar to that previously described for water heating. While use of copper tubes is pre-ferred for water heating, aluminum tubes are preferred for air heating. In either case corrosion-protection coatings such as Teflon are used.
Practice of the invention is not restricted to treatment of boiler or furnace flue gases, but readily applicable to a variety of other hot exhaust gases. In the papermaking industry it is common to provide direct-fired dry-ers in which large amounts of ambient air are heated, ordinarily by burning No.2 fuel oil, and applied by large blowers to dry webs of paper. The heated exhau~t air which has passed through the paper web contains substantial amounts of paper particles, as well as the usual constituents of flue gas, with more than usual amounts of moisture. Only a limited amount of the heated air can be recirculated, since its humidity must be kept low enough to effect dry-ing, so substantial amounts of make-up air are required. Heating outside or

4~
ambient air to the temperature desired for paper drying re-quires a large amount of fuel. Prior art heat recovery tech-niques have been clearly unsuitable in such an application.
Ambient air is always cool enough that passing such air through a usual air preheater would result in condensation of sul-furic acid from the exhaust gas, causing corrosion, and in the presence of condensation the paper particles rapidly stick to and build up on moist surfaces within the heat ex-changer, clogging it. I do not believe any technique for re-covering heat from a dryer has been successful. With the ap-paratus illustrated in Fig. 3b, hot (e.g. 540F) paper-laden exhaust air from a conventional paper dryer PD is passed through the gas passage of a water-condensing heat exchanger AH, to heat ambient air which passes into and through the tubes of the heat exchanger from an ambient air inlet duct ID, whereby the ambient air is heated, from an initial temp-erature of say 30F to 100F, up to a temperature of say 380F. The air leaving the heat exchanger is moved throuqh a duct by blower B to be used as make-up air in the dryer apparatus, and having been significantly pre-heated in heat exchanger unit AH, substantially less fuel is required to conduct the drying process. As in the case of the water heating sys-tems previously discussed, sulfuric acid condenses at one level within the heat exchanger. The condensation entraps paper particles as well as other particulate matter, with incipient tendencies of causing severe corrosion and clogging, but control of flow rates in relation to temperatures in accordance with the invention, so that large amounts of water vapor are condensed at a higher elevation within the 3C heat exchanger causes a rain to wash away the sulfuric acid-particulate matter composition. And as in the case of water heating applications, use of the water condensing mode increases O

the amount of sensible heat recovered, provides recovery of significant amounts of latent heat~ and maintains heat ex-change surfaces wet and clean to improve heat transfer.
The invention of this divisional application is particularly concerned with situa-tions where the hot exhaust gas supplied by a furnace or other device may have an initial tempera-ture substantially exceeding that (e.g. 550F) to which the Teflon protective coatings can safely be exposed; in such cases a first, conventional heat exchanger can be used to reduce the temperature. In Fig. 3c exhaust gas emanating from an industrial furnace IF and assumed to have a tempera-ture of 950F, is passed to a conventional prior art air pre-heater AP which need not have corrosion-pro-tective coatings.
The air preheater AP also receives air from a heat exchanger HXA operated according to the invention in the water conden-sing mode. Heat exchanger HX~ heats ambient air up to a tem-perature (e.g. 360F) substantially above that at which sul-furic acid will condense, and hence conventional preheater AP does not experience eorrosion or clogging. Preheater AP further raises the temperature of the 360F air up to a higher temperature, e.g. 550F, and that air is shown supplied to the burner of furnace IF. In heating the combustion air from 360F to 550F, the flue gas passing through unit AP
cools from 950F to ~25F, the latter being a temperature which the corrosion-protective coatings in heat exchanger HXA
can readily withstand. With ambient air entering uni-t HXA
at 60F, very substantial condensation of water vapor from the flue gas oeeurs in unit HXA, and the same advantages oE its water condensing mode of operation as previously discussed are obtained. In Fig. 3c it is not necessary that the heated air (shown at 550F) be supplied to the same device (shown as furnace IF) which produces the initial hot exhaust gas (shown at 950F); i.e. the heated air could be used for a ~ZB~
completely different industrial process, but the arrangement shown is believed to he an advantageous and natural use of the invention. While the description of Fig. 3c refers to sulfuric trioxide as a specific condensable corrosive con-stituent in the exhaust gas, it will be apparent that the pri-nciples of Fig. 3c well may find application with exhaust gases which contain other potentially corrosive cons-tituents which condense at temperatures in between the material limit operating temperature of the corrosion-protective coatings and the water vapor condensation temperature. ~ conventional non-condensing heat exchanger can be used to lower an exhaust gas temperature below the material limit operating tempera-ture of the condensing heat exchanger in numerous applica-- 18a -12~ 0 tions where, unlike Fig.3c, the fluid heated by the condensing heat exchanger does not pass through the conventional heat exchanger, and that fluid heated by the condensing heat exchanger can be water, of course, rather than air.
In Fig.3d exhaust gas from the stack of furnace F2 passes through a conven-tional air preheater AH, and thence through a water-condensing heat exchanger HXB operated according to the invention, to heat water circulated through unit HXB via lines 20, 21. A thermal sensor TS2 senses the gas temperature enter-ing unit HXB and controls the flow of the fluid being heated by the conventional heat exchanger AH, increasing that flow to decrease ~e temperature of the gas 10 entering unit HXB should it begin to rise above a desired value. The ~ermal sensor is depicted as controlling the speed of a blower motor BM via a motor controller MC, but it will be apparent that the thermal load imposed on unit AH may be varied in other ways in various applications, such as by positioning of a damper valve which controls flow of the cooler fluid through unit AH.
While Figs.3c and 3d illustrate uses of water-condensing heat exchangers with conventional or prior art non-condensing heat exchangers, it i9 to be under-stood that a water-condensing heat exchanger and a non-condensing heat ex-changer can be combined in the sense of being mounted adjacent each other or on a common support so as to shorten or eliminate ducting between the two 20 heat exchangers. ~The usual flue gas exit temperature from many convention-al economizers and air preheaters is maintained at about 350F to avoid con-densation of SO3. ) It is common at many steam generating plants to route boiler furnace gas successively through an economizer, an air preheater and a bag house to a stack. In order to avoid serious corrosion in the air pre-heater it has been necessary to preheat ambient air before it enters the con-ventional air preheater. Such preheating conventionally has been done by means of steam coils in the inlet air cluct. In accordance with the invention, such steam coils may be eliminated. A water-condensing heat exchanger in-stalled to receive flue gas which has passed through the conventional air pre-33 heater may be used to heat ambient air and supply it to the conventional air 4~
preheater at a sufficiently high temperature that no condensat:ion or corrosion will occur in the conventional air preheater. Many industrial boilers use econ-omizers to heat boiler feed water, and in many cases such boilers require 50-75% cold makeup water. The water-condensing heat exchanger of the invention may be installed to receive the flue gas exiting from the economizer at say 350F,which normally has been expelled through a stack, to heat the boiler makeup water before the water goes to the deaercLtor associated with the boiler.
It should be understood that while temperatures of the order of 500-10 550F have been mentioned as suitable upper limits with currently availableTeilon rnaterials, that such limits may rise as the thermal properties of Tef-lon are improved in the future, or as other materials having higher material limit operating temperatures become available.
Because the heat transfer surface area which is required varies widely between different applications, it is highly desirable that water-condensing heat exchan~ers be made in modular form. Figs~4a and 4b illustrate one ex-emplary module, Outwardly facing channel-shaped members 40,41 of formed sheet steel form rigid side members, and carry bolt holes 42,42 in upper and lower channel flanges, allowlng as many modules as may be needed to provide 20 a desired heat transfer surface area to be stacked vertically atop one another and bolted together. The unit includes two tube sheets or end plates 43,4~L
which are bolted to the side members 40,41, as by means of bolts 45, 45. Each tube sheet is provided with four outwardly-extending flanges along its four respective edges, such as flanges 43a to 43d shown for tube sheet 43 in Fig~4b.
In the module depicted each tube 5heet carries 8 rows of holes, with 18 holes in each row, with the holes in alternate rows staggered as shown, and 144 t~lbes 48,48 extend between the two twbe sheets~ extending about 2.81 inches outside each tube sheet. In one successful embodiment the outside diameter of each tube, with its corrosion-protection coating was 1.22 inches, the tubes in each 30 row were spaced on 1.75 inch centers, ~e tubes in successive rows horizon-tally staggered 0.875 inch, and ~e vertical distance between tube centers was 1.516 inch, as shown in Fig.4c,making the center-to-center distance between tubes in one row and tubes in an adjacent row also 1.75 inch. In Fig.4c the centers OI the three tubes shown, two of which are in one row and the other of which is in an adjacent lower row, lie on the vertices of an equilateral triangle.
With that tube size and horizontal spacing, it will be seen that a view vertically through the module is fully occluded by the tubes, e~cept for narrow ~e. g, 7/16 in. )strip spaces adjacent the channel side members 40,41, Except in those narrow strip spaces upward passage of gas necessarily requires the gas to be 10 deflected horizontally, promoting turbulence, and condensate which drips from a tube in any upper row tends to drip onto the center of a tube which is two rows lower. With the length of each tube inside the module equal to 54 in-ches, and the outside diameter of each tube equal to 1.125 inch, each tube has a surface area inside the module of 1.33 sq. ft., providing a heat transfer sur-face for all 144 tubes of 190,8 sq.ft.within a volume of approximately 13, 5 cu.
ft. Each tube comprised a Type L(0.050 in.wall thickness) copper water t~lbe of nominal 1 inch inside diameter having an actual inside diameter of 1.025 inch and an outside (uncoated) diameter of 1.125 inch. Each tube was covered with a 0.020 inch (20 mil. ) thick layer of fluorethylenepropylene (FEP) Teflon.
20 The inside wall of each channel member and the inside walls of ~e two tube ~heets were lined with 0.060 inch (60 mil. ) thick tetrafluorethylene (TFE) Tef-lon, and hence all surface area within the module is protected by a thin Teflon covering. Tube sheet 43 carries a 60 mil layer of TFE Teflon not only on its inside surface visible in Fig.4b,but also on the upper surface of its upper flange 42a, the lower surface of its lower flange 42d, and the leftside and right-side (in Fig.4b) surfaces of flanges 42b and 42c, and tube sheet 44 is covered with Teflon in the same manner. Channel shaped side members 40,41 carry a 60 mil layer of TFE Te-flon not only on their vertical (in Fig.4b) inside sur-faces 40a,41a, but also on the top surfaces 40b,41b of their upper flanges and 30 the lower surfaces 40c,41c of their lower flanges. Thus Teflon-to-Teflonj-)ints ~)8~

e~istbetween the channel-shaped side members 40,41 and the side flanges of the tube sheets where they are bolted together. The Teflon coverings for the tube sheets and side members are formed by cutting Teflon sheet material to size, and then heating it sufficiently to make the 90-degreebends necessary to cover the flanges. Three modules of the type described were vertically stacked, to provide 572 sq. ft.of heat transfer surface area, in a system intended to han dle a flue gas flow of 96601bs.per hour,with a gas mass flow of 0.762 lbs.per second per sq. ft. of open gas passage area. The velocity of the flue gas within the heat exchanger should be high enough to provide turbulent flow to insure 10 good heat transfer,but not so high a~ to cause abrasion of the Teflon coatings or to blow large amounts of condensate up the stack. Velocities within the range of 30-40 feet per second have proven suitable in ~e unit described, It should be recognized that smaller or larger velocities may be quite suitable in many applications OI the invention.
The corrosion-prevention covering has been applied to the heat exchan-ger tubes by heat-shrinkingJ a technique generally well known. After buffing a straight section of copper (or aluminum) tube to clean it and to remove any burrs, Teflon FEP tubing approximating the length of th~ metal is slid over the metal tube. Next, a short end length portion of several inches of the metal tube~
20 covered with Teflon tubing extending slightly beyond the end of the metal tubeJ
i5 imrnersed in a tank of propylene glycol heated to 330F. Only the short length is immersed and heated for several seconds, causing the Teflon tubing to shrink tightly about the short length of immersed metal tube. Only after the short end length of Teflon has shrunk, is it safe to further immerse the assem-bly; otherwise heated propylene glycol might enter between the Teflon tubing and the exterior surface of the metal tube~ Once the short end length of tubing has shrunk, the metal tube-Teflon tubing a~ssembly may be lowered further into the propylene glycol bath to its full length, typically at a rate of about 1 foot per second. Because the heated propylene glycol flows inside the metal tube as well 30 as surrounding the outside of the Teflon tubing, uniform heating and shrinking occurs. After the full length has been immersed for 2-4 seconds the assem-bly may be removed Erom the tank. The Teflon tubing increases in length as it shrinks in diameter, so that after heat shrinking it extends beyond the ends of the metal tubing. and may be cut off.
After a sheet of Teflon has been bent to surround the flanges of a hlbe sheet, holes are punched through the Teflon sheet concentric with the holes in the tube sheet,but with a smaller diameter. The holes in each steel tube sheet each have a diameter (eO g. 1. 28) inch which exceeds the outside diameter of the covered tubes by 0.060 in. (60 mils) the thickness of the Teflon covering 10 in the tubes, but the holes punched in the Teflon ltube sheet are each substanti-ally smaller, e. g. 0. 625 inch in diameter, as is shown in Fig. 7a, where portions of the TFE Teflon sheet 101 initially cover portions of a hole in tube sheet 43.
The edge of the hole in the tube sheet is preferably made slightly beveled on the inside of the tube sheet, as best seen at 102 in Fig. 7b, but flat or perpendi-cular on the outside of the tube sheet, as shown at 1û3. Next, a tapered elec-trically heated tool 104 is used to extrude portions of the Teflon sheet through the holes in the steel tube sheet. The metal tool 104 has a rounded nose small enough (e. g. 0. 375 inch) to enter a 0. 627 inch hole in the Teflon sheet 101, and rearwardly from the nose the tool tapers gradually upwardly to a constant di-20 ameter d equalllng the outside diameter of the TeElon-covered tuhes to be used.
The tool i9 heated to 780F. With the tool nose inserted into a hole in the Tef-lon sheet, as showrl in Fig. 7a, tool 104 is urged lightly against the Teflon sheet by an air cylinder (not shown). As the tool heats the edges of the hole in the Teflon, the tool is gradually advanced by the constant force from the air cylin-der. As the constant diameter portion of the tool nears the Teflon sheet, tool advancement tends to 910w or stop until the Teflon i~ further heated, and then the tool suddenly pushes through, after which further tool advancement i5 pre-vented by a stop (not shown). The Teflon then lines the hole in the metal tube sheet with a 30 mil thick lining, with some Teflon extending on the outside of 30 the tube sheet. The tool is held extending through the tube sheet for about 15 ~3_ ~ 8~4~
seconds while Teflon on the outside of the tube sheet further heats, and ~en the tool i5 rapidly retracted out of the tube sheet. Retraction causes a collar or bead having a diameter exceeding that of the hole in the tube sheet to be formed around the hole on the outside of the tube sheet, as indicated at 105 in Fig. 7b. The rounding or beveling of the tube sheet hole on its inside edge helps prevent the Teflon sheet frorn cracking as the tool is forced into the hole. The perpendicular edge of the hole on the outside of the sheet tends to impede inward flow of soft Teflon as the tool is retracted, and consequent forming of the collar or bead 105, Immediately (e.g.within 6 seconds) after the heated tool 104 is retrac-ted, a plug having the same outside diameter as the Teflon~covered tube to be used is inserted into the tube sheet hole through which Te~lon has been extru-ded, to prevent any decrease in diameter. Cylindrical plugs formed of many dif~erent materials can be used,but short lengths of Teflon-covered copper or aluminum tubing cut from a tube covered as previous~y described are pre-ferred, Thus Fig. 7b can be deemed to illustrate a tube sheet hole carrying such a plug, if item 48 is deemed to be a short length of Teflon-covered tube.
When all of the holes in two tube sheets have been proce~sed in such a manner, a pair of side members 40,41 and a pair of tube sheets are bolted to-20 gether into their fmal configuration. Then the Teflon-covered tubes are slid through mating pairs of holes in the two tube sheets in the following manner.
The plug is removed from a hole in tube sheet 43, one end o~ a tube 48 is promptly inserted into the hole from the outside of the tube sheet 43, ancd the tube promptly urged inwardly until its entry end nears tube sheet 44 at the other end of the module. The tube can be manually urged through the hole in tube sheet 43 if that is done within 2 or 3 minutes aIter the plug has been removed. As the entry encl oE the tube nears tube sheet 44~ the plug in the pro-per hole in tube sheet 44 is removed by another person at the tube sheet 44 end of the module, and the entry end of the tube can be pushed through the 30 hole in tube sheet 44 to its final position, with two persons at opposite ends -2ac-8~40 of the module pushing and pulling on the tube. That it generally requires two strong persons to slide the tube when it is installed in both tube sheets indi-cates the tightness of the fit which occurs. Hydraulic rams or like can be used, of course, to facilitate insertion of the tubes. While insertion of a tube can take place over a time period of several (e. g. 3) minutes, use of less time tends to be advantageous. But in any event, insertion of the plugs into the Teflon-lined holes in the tube sheets to prevent diametrical reduction until no more than a few minutes before a tube is installed, is deemed very important. With a plug maintained in each Teflon-lined tube sheet hole from the time when the lO Teflon is extruded through the hole until just before a tube i9 inserted in the hole, no diametrical reduction of the Teflon-lined hole begins until the plug is removed. Diametrical reduction occurs slowly enough after a plug ha~ been removed that there is time to urge a tube into place if it is done promptly enough, and then, importantly, after a tube is in place further diametrical re-duction occurs, so that the TFE collar and hole lining in a given tube sheet hole tightly grips the FEP layer on the tube, clamping the tube very tightly in place, so that it cannot be removed except with extreme force. No other mechanism or clamping devices are used to hold the tubes in place, so that the tubes can be deemed to mechanically float relative to the tube sheets, being clamped 20 only by the Teflon collars. Such an arrangement has proven to accommodate the expansions and contractions which heating and cooling cause to occur, with no loss of integrity in the Teflon-to-Teflon seal~.
In order to provide one water inlet and one water outlet for a heat e~-changer con~tructed of modules of the nature shown in Figs. 4a-4c, it is nec-essary, of course, to provide return bend connections on the ends of some of the tube ends extending out of the tube sheets. Conventional U-shaped copper return bends are sweated on the ends o~ the tubes to make such connections.
The equilateral triangle spacing of the tubes ad~antageously allows a single type of return bend connection to be used either to connect tubes in the same 30 row or to connect tubes in adjacent rows. While it might be possible in some ~2~8~

applications to provide water flow serially in one path through all of the water tubes, most applications will utilize manifolding into plural water paths, to provide better and more uniform heat transfer, and to avoid tube erosion from high water velocities. For example, in a system using three modules of the 18 by 8 tube matrix shown in Figs. 4a-4c, water was introduced into nine of the 18 tubes in the uppermost row and directed through the remaining nine tubes in that row through a series of ret~lrn bends, providing flow paths as diagram-matically indicated in F ig, 43, where arrows indicate inlet flow from a simple manifold MA ~e. g. a 3-inch pipe), and circles represenk connections to tubes 10 of the adjacent lower row. Nine separate flow paths through the heat exchanger were provided in such a manner to accommodate water flow of 70 gallons per minute at a water velocity kept under 4 -Eeet per second to avoid erosion, The overall water flow rate which is desired may vary widely in different applica-tions. Because the tubes are straight inside the module, with all return bends connected outside the tube sheets, modules fabricated to be identical ultimately rnay be used to accommodate Plow rates within a large range, which advantage-ously leads to economies in fabrication and stocking.
While most heat exchangers utilize return bends inside a heat exchanger chamber or housing, the use in~ide the module of Figs. 4a-4c of only straight 20 cylindrical sections of tubes has further significant advantages. Cleaning oE
the tube~ by the falling condensate i9 more thorough and unlform because no return bends or extended surfaces are used. The use of solely straight sec-tions also makes heat-shrinking of Teflon on the tubes practical.
In Figs. 5a and 5b exhaust gas is conducted via inlet duct 50 into the lower plenum 51 of a water-condensing heat exchanger for upward passage between nests of FEP Teflon-covered aluminum tubes into fiberglass upper plenum 56 and out fiberglass stack 57. Each tube extends through the two tube sheet~ supporting its ends. An inlet air duct 52 covers one end of an uppermost group of tubes 62. The other ends of tube group 62 and the ends 30 of a next lower group of tubes 63 are shown covered by hood or cover 63, so ~ 8~4(~ .
that air e~iting from tube group 62, rightwardly in Fig.5a, is returned through tube group 64, leftwardly in Fig. 5a~ Similar hood means 65-69 similarly re-verse the direction of air flow at opposite sides of the assembly. Air from the lowermost group of tubes 70 passes into outlet duct 71. Figs.5a and Sb illustrate a system in which air passes through the heat exchanger seven timesJ for sake of illustration. In actual practice one to five passes ordinarily has been deem-ed adequate. The insides of the tube sheets, slide members, and the covers, and the inside of plenum 51, are lined with corrosion-protection lining, as in the case of water-heating exchangers. Though not shown in Figs. Sa and 5b, it 10 will be apparent at this point, that if desired, spray nozzles can be provided in-side the air-heating heat exchanger of Figs. 5a and 5b for the same purposes as were mentioned in connection with the water-heater heat exchanger of Fig.
3a.
In the system shown in Fig. 6 a water-condensing heat exchanger BHX
arranged to heat both combustion air and boiler make-up water comprises six verticall~-stacked modules 61-66. Ambient or room air enters module 63 and the upper half of module 62 through an inlet duct 67, makes one pass across the heat exchanger, and i9 directed by return plenum or hood 68 back through the tubes in module 61 and the top half of module 62, into ducting 70 which connects 20 to the inlet of a forced draft fan FDF.
The upper three modules 64-66 of heat exchanger BHX preheat boiler makeup water. The water flows from a cold water main source 71 to an inlet manifold IM which distributes the water laterally across the top row of tubes in module 66 in seven water flow paths which progress horizontally and verti-cally through modules 64-66 to outlet manifold OM. Flue gas enters lower plenum 72 of heat exchanger BHX through duct 73, and passes upwardly through modules 61 to 66 in succession, into fiberglass upper plenum 74 and thence out fiberglass stack 75. The heat recovery system of Fig~ 6 is intended to accom-modate flue gas, combustion air and makeup water flow rates pre~railing in a 30 boiler system having two boilers Bl, B2, which system produces a maximum ~8~

of 50,000 pounds per hour of steam firing No.6 fuel oil atlSnlo excess air and with 67% makeup water, and an average of 30,000 pounds per hour. Flue gas is pulled from the stacks of the two boilers by a slngle induced-draft fan IDF, with the amount of flue gas being drawn from each boiler being controlled by a respective damper, ~6 or 779 which i9 controlled by a respective modulating positioner~ MPl or MP2, and the modulating positioners are each controlled by the load on a respective boiler using conventional pneumatic control signals from a conventional boiler control system. The modulating positioners are set so that under full load conditions dampers 76 and 77 are Pully open and all of 10 the flue gas produced by each boiler is directed to heat exchanger BHX. II fan IDF were to pull more flue gas than both boilers are producing, either the boiler excess air would increase or outside air would be drawn down the boiler stacks, in either case decreasing efficiency. To avoid l~ose problems, dam-pers 76 and 77 decrease the amount of flue gas drawn from each boiler when its load is decreased. The modulating positioners MPl and MP2 close their respective dampers when their associated boilers are shut down, and they are interlocked with induced-draft fan IDF to close their dampers if fan IDF is not running, and then all flue gas will exit through the boiler stacks~
The ducts 7B980 containing dampers 76,77 merge to a common duct 81.
20 Damper 82 in duct 81 regulates the total amount of flue gas going to heat ex-changer BHX depending upon the combustion air and makeup water demand rates. Damper 82 is modulated to control the temperature of the preheated makeup water exiting from exchanger BHX at a desired set point (180F),by sensing the water temperature at its exit from unit BHX to operate a propor-tional servomotor SMl. Damper 82 is normally fully open, allowing all of the flue gas being produced by the boilers to pass through heat exchanger BHX, but during rapid transient conditions, or during periods OI reduced makeup water requirements, more heat may be available from the flue gas than can be utilized to preheat the combustion air and makeup water, in which case the 30 temperature of the water exiting from heat exchanger BHX will start to rise, -2~-but servomotor SMl then will begin to close damper 82 to maintain the exit temperature of the water from heat exchanger BH~ at the desired setpoint.
Since the flue gas passes first through the lower air-heating section and then through the water-heating section, where the input water temperature (e. g, 46 F) is lower than the input air temperature (e. g.85F), maximum heat re~
covery i9 obtained. A further modulating damper 83 ahead of the induced draft fan operates to limit the temperature of flue gas by admitting sufficient room air to mix with the flue gas that the inlet temperature of the flue gas to heat exchanger BHX does not exceed the safe operating temperature (e. g.500F) 10 for the Teflon corrosion-prevention materials which line heat exchanger P~HX~
Temperature sensor TS3 senses the flue gas temperature at the exit of fan IDF
to control the position of damper 83 via servomotor SM2. If the temperahlre of the flue gas is 500F or less, damper 83 will be fully closed.
The flue gas passes from fan IDF through a short section of ducting 73 into bottom plenum 72, and ~ence upwardly through heat exchanger BHX, first heating combustion air and then heating boiler makeup water, and copious con-densation occurs, providing numerous advantageous effects heretofore describ-ed. Drain line D at the bottom of unit BEIX includes a transparent section of tubing 85 through which the color of the condensate may be observed. When No.
20 6 oil i9 used as boiler Euel, the condensate is black due to the large amount of particulate matter and SO3 removed from l~e flue gas, while use of natural gas as boiler fuel provides a virtually clear condensate due to the very small amounts of particulate matter in the flue gas.
A dial thermometer DTl located in stack 75 indicates flue gas exit tem-perature9 which typically varies between 90F and 200F, dep0nding upon boiler load. Temperature sensor TS4 also located at stack 75 serves to shut dow the heat recovery system by closing damper 82 if the flue gas exit temperature should exceed 200F, Simple BTU computers receive water and air temperature and flow rate 30 signals and comput2 the amounts of heat recovered. A t an average steam load .
_~9 ~ 2~8~
oî 30,000 pounds per llour, the combined amount of h~at recovered is 3,~8,000 Btu per }-our. Prior to use of the condensing heat e:~changer the average ste~m load of 30,000 pounds per hour required an average fuel consumption of 148.
gallons per hour of No.6 fuel (148,000 Btu per gallon) with a boiler efficiency of 80 %. Utilizing the wa ter - condensing hea t exchanger 2 9 . 3 gallons per hour of fuel are saved, a savings of 20%.
The lower housing (11 in Fig. 1, for example) comprises a simple steel sheet housing completely lined with 60 mil TFE Teflon, with its flanges also covered with Teflon in generally the same manner as in the tube modules.
10 The lower housing may take a variety of different shapes in various applica-tions. An upper section ~e. g. 1 foot) of drain D which connects to the bottom of the lower housing is also preferably formed of TFE Teflon tubing.
It is believed that ~lue gas flow velocities up to 6~ feet per second will be quite workable, and that flue gas pressure drops across a water-condensing heat exchanger of 3 inches of water should be workable.
It will thus be seen that the objects set forth abo te, among those made apparent from the preceding description, are efficiently attained. Since cer-tain changes may be made in carrying out the above method and in ~e con-structions set forth without departing from the scope of the invention, it is in-20 tended that all matter contained in ~e a~ove description or shown in the ac-companying drawings shall be interpreted as illustrative and not in a limiting sense .

Claims (19)

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

1. The method of recovering heat energy from an exhaust gas by heating a fluid wherein said exhaust gas includes water vapor condensable at and below a water vapor condensation temperature and a corrosive constituent condensable at and below a corrosive condensation temperature which is higher than said water vapor condensation temperature, said method comprising the steps of passing said exhaust gas successively through a first heat exchan-ger and a second heat exchanger, said second heat exchanger con-taining corrosion-protection material having a material limit operating temperature exceeding said corrosive condensation tem-perature, the flow rates and temperatures. of said exhaust gas and said fluid being arranged so that passage of said exhaust gas through said first heat exchanger cools said exhaust gas from a first temperature to a second temperature which is below said material limit operating temperature but above said corrosive condensation temperature, passage of said exhaust gas through said second heat exchanger cools said exhaust gas from a temper-ature above said corrosive condensation temperature to a third temperature below said water vapor condensation temperature, and passage of said fluid through said second heat exchanger heats said fluid from a temperature below said water vapor condensation temperature to a fourth temperature which is above said corrosive condensation temperature but below said material limit operating temperature, whereby condensation of both water vapor and said corrosive constituent continuously occur within said second heat exchanger and neither water vapor nor said corrosive constituent condense within said first heat exchanger.

2. The method of claim 1 wherein said first temperature exceeds said material limit operating temperature.

3. The method of claim 1 wherein passage of said fluid through said first heat exchanger heats said fluid to a temperature which is above said material limit operating temperature.

4. The method of claim 1 wherein said fluid is air and which method includes the step of applying said exhaust gas to said first heat exchanger from a fuel-burning means which produces said exhaust gas, and the step of applying air heated by first heat exchanger as combustion air to said fuel-burning means.

5. The method of claim 1 which includes passing said fluid up-wardly through said second heat exchanger, whereby condensation of water vapor occurs at a higher level within said second heat exchanger than condensation of said corrosive constituent and gravitational fall of condensed water vapor washes condensed corrosive constituent from lower heat exchange surfaces in said second heat exchanger.

6. The method of claim 1 which includes the step of draining condensation from said second heat exchanger.

7. The method of claim 1 wherein said fluid is water.

8. The method of claim 1 wherein said fluid is air.

9. The method of claim 1 which includes the steps of sensing the temperature of the gas entering the gas passage of said second heat exchanger, and controlling the thermal load imposed on said first heat exchanger in accordance with the sensed temperature.

10. The method of claim 1 wherein said corrosive constituent comprises sulfur trioxide.

11. Apparatus for recovering heat energy from an exhaust gas by heating a fluid, wherein said exhaust gas includes water vapor condensable at and below a water vapor condensation temperature and a corrosive constituent condensable at and below a corrosive constituent condensation temperature which is higher than said water vapor condensation temperature, comprising in combination:
a first heat exchanger having a gas passage lined with corrosion-resistant material having a deformation temperature which is less than the initial temperature of said exhaust gas but greater than said corrosive constituent condensation temperature, and a fluid passage in heat exchange relationship with said gas passage; a second heat exchanger susceptible to damage by the condensed cor-rosive constituent, said second heat exchanger being connected to receive said exhaust gas at said initial temperature, to cool said gas to a second temperature which is below said deformation temperature but above said corrosive constituent condensation temperature, and to supply said exhaust gas to said gas passage of said first heat exchanger; and means for establishing the flow rates of said exhaust gas through said first and second heat ex-changers and the flow rate of said fluid through said first heat exchanger so that condensation of water vapor occurs continuously in said first heat exchanger and condensation does not occur in said second heat exchanger.

12. Apparatus according to claim 11 wherein said second heat exchanger includes a fluid passage and a gas passage in heat ex-change relationship thereto, said apparatus including means for applying said fluid from said first heat exchanger to the fluid passage of said second heat exchanger, said first heat exchanger being operable to heat said fluid from a temperature which is below said water vapor condensation temperature to a third temper-ature which is above said corrosive constituent condensation temperature but below said deformation temperature.

13. Apparatus according to claim 12 wherein said second heat exchanger is operative to heat said fluid to a temperature exceed-ing said deformation temperature.

14. Apparatus according to claim 12 which includes a combustion source which burns combustion air to provide said exhaust gas, and means for conducting said fluid to said combustion source to supply combustion air to said combustion source.

15. Apparatus according to claim 11 wherein said fluid passage of said first heat exchanger comprises a plurality of tubes ex-tending generally horizontally through said gas passage of said first heat exchanger, the exterior surfaces of said tubes being covered with corrosion-resistant material having a deformation temperature which is less than said initial temperature of said exhaust gas but greater than said corrosive constituent condensa-tion temperature.

16. Apparatus according to claim 11 wherein said corrosion-resistant material comprises ptfe.

17. Apparatus according to claim 11 wherein said fluid is water

18. Apparatus according to claim 11 wherein said fluid is air.

19. Apparatus according to claim 12 having means for sensing the temperature of the gas being supplied to the gas passage of said first heat exchanger; and means responsive to the sensed temperature for controlling the flow of said fluid through said fluid passage of said second heat exchanger.