EXHAUST GAS TREATMENT METHOD AND APPARATUS

Abstract: Exhaust gas treatment method and apparatus 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 surfaces to keep them clean and wet to improve heat transfer. Systems for heating water, air, and both water and air are disclosed. Methods of constructing and assembling improved heat exchangers are disclosed.

Description: EXHAUST GAS TREATMENT METHOD AND APPARATUS 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 directed toward such treatment of sulfurcontaining exhaust gases, such as those typically produced by burning oil or coal in furnaces, though it will become apparent that the invention will be useful in a wide variety of other applications. A primary object of the invention is to provide method and apparatus which are useful for recovering a substantially larger percentage of the heat contained in an exhaust gas than that recovered in typical prior systems, 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 of 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. One object of the invention is to provide method and apparatus which is useful in connection with flue gases produced by any of those fuels. In many applications it is desirable that waste heat be used to preheat a liquid, such as boiler make-up water, or industrial process water as examples, 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 liquid 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. A very important object of the present invention 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't for cleaning or repair. Another more specific object of the invention is to provide a heat exchanger which functions as a self-cleaning gas scrubber as well as recovering 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 exhaust gas from a boiler furnace or the like. Flue gas commonly is directed through boiler economizers to preheat boiler feedwater, and commonly directed 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 limited, due to serious corrosion problems which otherwise result. Combustion of oil, coal or natural gas produces flue gas having substantial moisture, sulfur dioxide, sulfur trioxide, and particulate matter in the cases of oil and coal. If a heat exchanger intended to recover heat from flue gas condenses appreciable amounts of sulfur trioxide, sulfuric acid is formed, resulting in severe corrosion. The condensed sulfur product can readily ruin usual economizers and air preheaters, and the exhaust stacks associated with them. Thus prior art systems intended to recover heat from flue gas traditionally have been operated 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 1160F to 130IF, depending on the partial pressure of water vapor. But the presence of sulfur trioxide even in small amounts, such as 5 to 100 parts per million, drastically increases the temperatllre at which condensation will occur, far above that for water vapor only. For example, 50 to 100 parts per million of 503 may raise the dew point temperature to values such as 2500F to 2800F, resp#ectively. Thus it has been usual practice to make absolutely certain that flue gas is not cooled below a temperature of the order of 300IF, in order to avoid condensation and corrosion. Such operation inherently results in an undesirably small portion of the sensible heat energy being extracted from the flue gas, and in absolutely no recovery of any latent heat energy contained in the flue gas. One concept of the present invention is to provide method and apparatus for recovering heat from a potentially corrosive exhaust gas, such as flue gas, in a manner directly contrary 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 saturation temperature of water at the applicable pressure, i. e. below the dew point, e. g. 120IF, for water vapor only. In a typical operation of the invention where absolute pressure of 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 1200F to a temperature of say 750F to 100IF, by passing the flue gas through a heat exchanger scrubber unit. The unit continuously condenses a large amount of water from the flue gas, as well as condensing sulfuric acid. ] ;lrts within the heat exchanger-scrubber unit which would otherwise be exposed to the corrosive condensate are appropriately lined or coated with corrosion-resistant materials, e. g. Teflon (trademark), to prevent corrosion. When prior art waste heat recovery systems have been operated with flue gas temperatures (e. g. 250IF) too near the SO3 condensation temperature, whether by accident, or during startup, or in attempts to improve system thermal efficiency, the occasional condensationof SOB tends to produce very strong or concentrated sulfuric acid. The sulfuric acid is extremely corrosive and can rapidly destroy an ordinary heat exchanger. The volume of sulfuric acid which can be condensed is small, but sufficient to keep heat exchanger 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 sulfuric acid, making the resultant overall condensate much less corrosive, and less likely to damage system parts. Thus operation in the water condensing mode of toe invention has an important tendency to lessen corrosion of system parts, as well as allowing large amounts of latent heat energy to be recovered. Such dilu tion of the sulfuric acid does not wholly eliminate 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 apparatus 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 and SOQ simultaneously can be removed from the flue gas, thereby considerably 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 of sulfuric acid combined with particulate matter often will rapidly build up on heat exchange surfaces, and indeed the build-up can clog the heat exchanger in a matter of a few hours. But if the system is operated in the water condensing mode in accordance with the present invention, the production of 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 operation forms a "rain" within the heat exchanger. The "rain" not only entraps particles in the flue gas as it falls, but it also washes away, down to a drain, particles which have lightly stuck to the wetted surfaces. Thus advantages akin to those of gas scrubbing are obtained without a need for the continuous supply of water required by most gas scrubbers, and with no need for moving parts. Yet, even in addition to recovering large amounts of latent heat energy, greatly diluting and washing away sulfuric acid to minimize corrosion problems, and removing substantial amounts of particulate matter from exhaust gases such as flue gas, operation in the "water condensing" mode also enhances the heat transfer coefficient of heat exchanger units used to practice the invention. Heavy condensation made to occur near the top of the heat exchanger unit runs or rains downwardly, maintaining heat exchange surfaces wetted in lower portions of the heat exchanger, even though condensation otherwise is not occurring on those surfaces. With substantially all heat exchange surface area maintained wet, the heat transfercoefficientis markedly improved over that of a dry or non-condensing heat exchanger, thereby allowing units which are reasonably compact, and economical to fabricate, to provide sufficient heat transfer to recover large amounts of waste heat. Heat transfer is improved for several separate but related reasons. Heat transfer from the gas occurs better if a surface is wet from dropwise condensation. Use of Teflon promotes dropwise condensation. Further, the constant rain within the heat exchanger keeps the tube surfaces clean, preventing the buildup of deposits which would decrease heat transfer. Thus another 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 effective corrosion protection, and to have hydrophobic characteristics which cobperate with water vapor condensation to keep a watercondensing heat exchanger clean, currently available forms of those corrosion-protection materials have deformation, melting or destruction temperatures far below the temperatures of some flue gases from which it is desirable to extract waste heat. In accordance with another aspect 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 condensation takes place in it. The conventional heat exchanger can initially cool an exhaust gas such as flue gas down to a temperature which is low enough that it will not damage the corrosion-protection linings of the water-vapor condensing heat exchanger, yet high enough that sulfur trioxide cannot condense in the conventional heat exchanger so as to damage it. Thus some added objects of the invention are to provide improved 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. Another object of flue invention is to provide a satisfactory method of constructing and assembling a water-condensing heat exchanger system. 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 steps 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 exemplified in the following detailed 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 following detailed description taken in connection with the accompanying drawings, in which: Fig. 1 is a diagram of a heat exchanger system useful in understanding some basic principles of the present invention. 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. Fig. 3b is a diagram illustrating use of the invention in connection with a direct-fired paper dryer to save heat energy and remove particurate matter from the exhaust gas from the paper dryer. Figs. 3c and 3d each is a diagram illustrating a use of be watercondensing heat exchanger of the present invention together with a prior art air preheater. Fig. 4a is a plan view of an exemplary heat exchange module in accordance 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 invention. Fig. 4d is a diagram illustrating one form of water manifolding which may be used in connection with the invention. Figs. Sa and 5b are front elevation and side elevation views of an exemplary air heating form of water-vapor condensing heat exchanger. Fig. 6 illustrates an exemplary heat recovery system utilizing a heat exchanger which heats both air and water from boiler flue gas. Figs. 7a and 7b are partial cross section views useful for understanding a method of assembly according to the present invention 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 heat exchange units, such as the four units indicated at 12,13, 14 and 15, 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 within the heat exchange 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 unit 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 is 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 facilistate explanation, the range of elevations within which significant gas cooling and water hea#ting occur is shown divided into four zones Z1 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 gas falls from an assumed input temperature G1 of 5000F at the bottom of the heat exchanger 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 understood 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 elevation. At any elevation above the lowermost row of tubes there are temperature gradients, 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 that bounding zones Z2 and Z3, some portions of the flue gas, such as portions near or on a tube may have 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. Simul taneously, water assumed to have an input temperature W1 of 550F at the top of the heat exchanger is heated to an output temperature Wo of 18Q F by the time it reaches the bottom of the heat exchanger, as indicated by curve W. The ordinate scale in Fig.1 is shown divided into the four zones Z1 to Z4. A vertical dashed line F at 2500F indicates a typical temperature G2 at which sulfuric acid forms in typical flue gas obtained from burning No. 6 fuel oil. The prior 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 G forQaR fit gas travel, it will be seen that the flue gas temperature drops from 5000F at G1 to 250 F at G2 as the gas passes through the two lower zones, Z1 and 22. 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 Z1 and Z2 is still too hot for sulfuric acid to condense, but some amounts of the gas in zones Z1 and Z2 will condense while in those zones, because condensate falling through those zones from above will cool some amounts of gas in those zones to the condensation temperature. Even though little or no condensation from the gas is occurring in lower zones Z1 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 falling from above acts to continually wash them, washing sulfuric acid and particulate matter down to the drain (D, Fig. 1) at the bottom of the heat exchanger unit. As the flue gas is cooled below 250IF, a typical dew point for sulfur trioxide, sulfuric acid forms in zone Z3. If zone Z3 were the uppermost zone in the system, the condensed sulfuric acid would slightly wet heat exchange surfaces in zone Z3, and particulate matter would build upon those slightly wetted surfaces. The sulfur trioxide condenses to form sulfuric acid in the assumed example, as gas.travel through zone Z3 cools the gas substantially below the acid dew point. At some level within zone Z3 most or all of the sulfuric 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 substantial quantities. However, as the gas is further cooled during further upward travel through Zone Z1, condensation of water vapor occurs in an increasingly copious manner. Thus all of the tubes in the heat exchange units are continuously 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 surfaces of the heat exchange tubes, washing particulate matter and condensed sulfuric acid down to the drain at the bottom of the system. The tubes within the heat exchange units are arranged in successive rows which are horizontally 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 tender¯ cy 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 Laboratory indicate that approximately 50% of the particulate matter and 20 to 25% of the 503 in flue gas from No. 6 fuel oil can be removed. In a typical application 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 mixture 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 condensation of sulfur trioxide could produce is much, much less than the amount of water which can readily be #ondensed if the flue gas is cooled sufficiently. 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 120 F (49 C). Thus in accordance with the present invention, water (or air) at or below 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 550F to about 750F as the 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 120IF. It may be noted that in the example of Fig. 2, the water temperature reaches the water vapor dew point (120"F) at substantially the same elevational level where the flue gas temperature reaches the sulfuric acid dew point (250IF), and further, that the level occurs substantially at the vertical mid-level of the heat exchanger. Those precise relationships are by no means necessary. In 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 Z1 and Z21 where there is maximum difference between gas temperature and water temperature and hence a maximum potential for effective heat transfer, the amount of heat transfer which actually occurs is further increased because the heat exchange tube surfaces in those zones are maintained wet by falling condensate. It maa 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 telnperature of say 120IF, 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 geometry allows some quantum of flue gas to pass through the unit with only modest cooling, that quantum 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 mixed 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 particulate matter and condensed corrosive constituent from the gas, by passing 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 characteristics of the heat exchanger so that continuous condensation of water vapor and the corrosive constituent occurs, providing falling droplets which capture and wash away portions of the particulate matter and condensed corrosive constituent. While prior art economizers and air preheaters require hot input water or air, the invention advantageously can use cold water or air, and indeed, efficiency increases the colder the input fluid is, with more condensation occurring and more latent heat being extracted from the exhaust gas. The cooler the water at inlet 20, the lower the exit temperature of the exhaust gas will be, the more latent heat will be recovered, the more water vapor will be condensed from the exhaust gas, and the more effective particulate and 503 removal will be, for given flow rates. Tests to date have indicated that with a gas velocity of the order of 30-40 feet per second, provision of sufficient heat exchange surface area will enable one to lower the temperature of the fluid being heated, be it water or air. For example, with water being supplied to the heat exchanger at 55IF, it has been possible to obtain flue gas exit temperatures of 63IF. In most applications of the invention it will not be deemed necessary to lower the flue gas temperature to that extent, and the amount of heat exchange surface will be selected so that with the desired flow rates and fluid temperature the flue gas temperature will be lowered to within the range of 80 F-100 F. Typical flue gases contain 5% to 12% water vapor, depending upon the type 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 hour. By operating 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 operating one form of the invention under two sets of operating conditions. The unit was first operated with flue gas produced by burning No. 2 fuel oil, with particular inlet and outlet temperatures 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 condensate 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 conventional stack 30 of boiler 31 past one or more 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 HX, and cooled flue gas exits via fiberglass hood 16 and fiberglass stack 17 to atmosphere. The use of a fiberglass stack to resist corrosion is not per se new. Cool water from the bottom of hot water storage tank ST, from 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 begins to occur in the heat exchange unit, tending to endanger the Teflon corrosion-protection coverings and liners in that unit. A conventional thermal sensor TS senses outgoing flue gas temperature and operates a conventional positioner PV1, which closes damper valve DVI to decrease or terminate passage of hot flue gas to the heat exchanger. In Fig. 3a a second positioner PV2 also responsive to thermal sensor TS is shown connected to operate damper 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 within the heat exchanger from exceeding a value (e. g. 500IF) 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 sufficient. As will become clear below, preventing a temperature rise which would damage the corrosion-protection coatings can instead be done by using a conventional non-condensing heat exchanger to cool the exhaust gas to a safe operating temperature before the exhaust gas is passed through the water-condensing heat exchanger. In Fig. 3a a spray manifold SP carrying a plurality of nozzles is operative.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 functions to keep the tubes clean, operation of such a spray can be quite infrequent, and in some applications of the invention provision of such a spray means may be deemed wholly unnecessary. In certain applications, such as where particulate removal is deemed particularly important, valve V can be opened to permit a continuous spray, to augment the "rain" caused by water vapor condensation. 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 preferred 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 directfired dryers 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 exhaust 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 drying, so substantial amounts of makeup air are required. Heating outside or ambient air to the temperature desired for paper drying requires a large amount of fuel. Prior art heat recovery techniques 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 sulfuric 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 exchanger, clogging it. I do not believe any technique for recovering heat from a dryer has been successful. In accordance with the present invention, as illustrated in Fig. 3b, hot (e. g. 540oF) 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 temperature of say 300F to 100IF, up to a temperature of say 380IF. The air leaving the heat exchanger is moved through 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 systems previously discussed, sulfuric acid condenses at one level within the heat exchanger. The conden sation entraps paper particles as well as other particulate matter, with in cipient 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 heat exchanger causes a rain to wash away the sulfuric acidparticulate matter composition. And as in the case of water heating applications, use of the water condensing mode increases the amount of sensible heat recovered, provides recovery of significant amounts of latent heat, and maintains heat exchange surfaces wet and clean to improve heat transfer. In various applications, the hot exhaust gas supplied by a furnace or other device may have an initial temperature substantially exceeding that (e. g. 550IF) to which the Teflon protective coatings can safely be exposed, but that by no means rules out use of the invention in such applications. In Fig. 3c exhaust gas emanating from an industrial furnace IF and assumed to have a temperature of 950IF, is passed to a conventional prior art air preheater AP which need not have corrosion-protective coatings. The air preheater AP also receives air from a heat exchanger HXA operated according to the invention in the water condensing mode. Heat exchanger HXA heats ambient air up to a temperature (e. g. 3600F) substantially above that at which sulfuric acid will condense, and hence conventional preheater AP does not experience corrosion or clogging. Preheater AP further raises the temperature of the 3600F air up to a higher temperature, e. g. 550IF, and that air is shown supplied to the burner of furnace IF. In heating the combustion air from 3600F to 5500the flue gas passing through unit AP cools from 9500F to 4250F, the latter being a temperature which the corrosion-protective coatings in heat exchanger HXA can readily withstand. With ambient air entering unit HXA at 60IF, very substantial condensation of water vapor from the flue gas occurs in unit HXA, and the same advantages of its water condensing mode of operation as previously discussed are obtained. In Fig. 3c it is not necessary that the heated air (shown at 5500F) be supplied to the same device (shown as furnace IF) which produces the initial hot exhaust gas (shown at 950IF); i. e. the heated air could be used for a completely different industrial process,but the arrangement shown is believed to be an advantageous and natural use of the invention. While the description of Fig. 3c refers to sulfuric trioxide as a specific condensable corrosive constituent in the exhaust gas, it will be apparent that the principles of Fig. 3c well may find application with exhaust gases which contain other potentially corrosive constituents which condense at temperatures in between the material limit operating temperature of the corrosion-protective coatings and the water vapor condensation temperature. A conventional non-condensing heat ex changer can be used to lower an exhaust gas temperature below the material li#mit operating temperature of the condensing heat exchanger in numerous applications 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 conventional 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 entering unit HXB and controls the flow of the fluid being heated by the conventional heat exchanger AH, increasing that flow to decrease the temperature of the gas entering unit HXB should it begin to rise above a desired value. The thermal sensor is depicted as controlling he 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 is to be understood that a water-condensing heat exchanger and a non-condensing heat exchanger 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 heat exchangers. (The usual flue gas exit temperature from many conventional economizers and air preheaters is maintained at about 3500F to avoid condensation 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 preheater it has been necessary to preheat ambient air before it enters the conventional air preheater. Such preheating conventionally has been done by means of steam coils in the inlet air duct. In accordance with the invention, such steam coils may be eliminated. A water-condensing heat exchanger installed to receive flue gas which has passed through the conventional air preheater may be used to heat ambient air and supply it to the conventional air preheater at a sufficiently high temperature that no condensation or corrosion will occur in the conventional air preheater. Many industrial boilers use economizers to heat boiler feed water, and in many cases such boilers require 50-75a10 cold makeup water. The water-condensing heat exchanger of the invention may be installed to receive the flue gas exiting from the econo mizer at say 350IF, which normally has been expelled through a stack, to heat the boiler makeup water before the water goes to the deaerator associated with the boiler. It should be understood that while temperatures of the order of 500-550 F have been mentioned as suitable upper limits with currently available Teflon materials, that such limits may rise as the thermal properties of Teflon 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 watercondensing heat exchangers be made in modular form. Figs. 4a and 4b illustrate one exemplary 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, allowing as many modules as may be needed to provide 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, 44 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 sheet carries 8 rows of holes, with 18 holes in each row, with the holes in alternate rows staggered as shown, and 144 tubes 48, 48 extend between the two tube 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 row were spaced on 1. 75 inch centers, the tubes in successive rows horizontally staggered 0. 875 inch, and the vertical distance between tube centers was 1. 516 inch, as shown in Fig. 4c, making the centerto-center distance between tubes in one row and tubes in an adjacent row also 1. 75.inch. In Fig. 4c the centers of 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 7 fully occluded by the tubes, except for narrow (e. g. 16in.) 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 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 inches, 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 surface 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 tube 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. The inside wall of each channel member and the inside walls of the two tube sheets were lined with 0. 060 inch (60 mil. ) thick tetrafluorethylene (TFE) Teflon, 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 rightside (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 Teflon not only on their vertical (in Fig. 4b) inside surfaces 40a, 41a, but also on the top surfaces 40b, 41b of their upper flanges and the lower surfaces 40c, 41c of their lower flanges. Thus Teflon-to-Teflon joints exist between 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-degree bends 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 handle a flue gas flow of 9660 lbs. 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 good heat transfer, but not so high as 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 the unit described. It should be recognized that smaller or larger velocities may be quite suitable in many applications of the invention. The corrosion-prevention covering has been applied to the heat exchanger tubes by heat-shrinking, 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 the metal is slid over the metal tube. Next, a short end length portion of several in ches of the metal tube, covered with Teflon tubing extending slightly beyond the end of the metal tube, is immersed in a tank of propylene glycol heated to 3300F. 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 assembly; 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 assembly 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 as surrounding the outside of tie Teflon tubing, uniform heating and shrinking occurs. After the full length has been immersed for 2-4 seconds the assembly may be removed from 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 tube 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 (e.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 in the tubes, but the holes punched in the Teflon tube sheet are each substantially 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 perpendicular on the outside of the tube sheet, as shown at 103. Next, a tapered electrically 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 diameter d equalling the outside diameter of the Teflon-covered tubes to be used. The tool is heated to 780IF. With the tool nose inserted into a hole in the Teflon sheet, as shown 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 cylinder. As the constant diameter portion of the tool nears the Teflon sheet, tool advancement tends to slow or stop until the Teflon is further heated, and then the tool suddenly pushes through, after which a further tool advancement is prevented 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 the tube sheet. The tool is held extending through the tube sheet for about 15 seconds while Teflon on the outside of the tube sheet further heats, and then the tool is 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 from 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 retracted, a plug having the same outside diameter as the Teflon-covered tube to be used is inserted into the tube sheet hole through which Teflon has been extruded, to prevent any decrease in diameter. Cylindrical plugs formed of many different materials can be used, but short lengths of Teflon-covered copper or aluminum tubing cut from a tube covered as previ-. ously described are preferred. 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 processed in such a manner, a pair of side members 40,41 and a pair of tube sheets are bolted together into their final 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 of a tube 48 is promptly inserted into the hole from the outside of the tube 43, and 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 after the plug has been removed. As the entry end of the tube nears tube sheet 44, the plug in the proper 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 hole in tube sheet 44 to its final position, with two persons at opposite ends 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 indicates the tightness of the fit which occurs. Hy draulic rams or like can be used, of course, to facilitate insertion of tie 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 Teflon is extruded through the hole until just before a tube is 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 has 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 reduction occurs, so that the TFE collar and hole lining in a given tube sheet hole tightly grips the FE;Player 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 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 seals. In order to provide one water inlet and one water outlet for a heat exchanger constructed of modules of the nature shown in Figs. 4a-4c, it is necessary, of course, to provide return bend connections on the ends of some of the tube ends extending out of the tube sheets. Conventional Ushaped copper return bends are sweated on the ends of the tubes to make such connections. The equilateral triangle spacing of the tubes advantageously allows a single type of return bend connection to be used either to connect tubes in the same row or to connect tubes in adjacent rows. While it might be possible in some 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 return bends, providing flow paths as diagrammatically indicated in Fig. 4e, where arrows indicate inlet flow from a simple manifold MA (e. g. a 3-inch pipe), and circles represent connections to tubes of the adjacent lower row. Nine separate flow paths through the heat exchanger were pro vided in such a manner to accommodate water flow of 70 gallons per minute at a water velocity kept under 4 feet per second to avoid erosion. The overall water flow rate which is desired may vary widely in different applications. Because the tubes are straight inside the module, with all return bends connected outside the tube sheets, modules fabricated to be identical ultimately may be used to accommodate flow rates within a large range, which advantageously leads to economies in fabrication and stocking. While most heat exchangers utilize return bends inside a heat exchanger chamber or housing, the use inside the module of Figs. 4a-4c of only straight cylindrical sections of tubes has further significant advantages. Cleaning of the tubes by the falling condensate is more thorough and uniform because no return bends or extended surfaces are used. The use of solely straight sections 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 sheets 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 of a next lower group of tubes 63 are shown covered by hood or cover 63, so that air exiting from tube group 62, rightwardly in Fig. 5a, is returned through tube group 64, leftwardly in Fig. 5a. Similar hood means 65-69 similarly reverse 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 5b illustrate a system in which air passes through he heat exchanger seven times, for sake of illustration. In actual practice one to five passes ordinarily has been deemed 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. 5a and 5b, it will be apparent at this point, that if desired, spray nozzles can be provided inside 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 vertically-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 is 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 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 vertically 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 accommodate flue gas, combustion air and makeup water flow rates prevailing in a boiler system having two boilers B1, B2, which system produces a maximum of 50, 000 pounds per hour of steam firing No. 6 fuel oil at 15% 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 single induced-draft fan IDF, with the amount of flue gas being drawn from each boiler being controlled by a respective damper, 76 or 77, which is controlled by a respective modulating positioner, MPI 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 fully open and all of the flue gas produced by each boiler is directed to heat exchanger BHX. If 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 those problems, dampers 76 and 77 decrease the amount of flue gas drawn from each boiler when its load is decreased. The modulating positioners MP1 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 78, 80 containing dampers 76, 77 merge to a common duct 81. Damper 82 in duct 81 regulates the total amount of flue gas going to heat exchanger BHX depending upon the combustion air and makeup water demand rates. Damper 82 is modulated to control the temperature of the pre-heated makeup water exiting from exchanger BHX at a desired set point (180IF), by sensing the water temperature at its exit from unit BHX to operate a proportional servomotor SMI. 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 of 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 temperature of the water exiting from heat exchanger BHX will start to rise, but servomotor SMl then will begin to close damper 82 to maintain the exit temperature of the water from heat exchanger BHX at the desired setpoint. Since the flue gas passed first through the lower air-heating section and then through the water-heating section, where the input water temperature (e. g. 460F) is lower than the input air temperature (e. g. 850F), maximum heat recovery is 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. 5000F) for the Teflon corrosion-prevention materials which line heat exchanger BHX. 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 temperature of the flue gas is 500oF 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 thence upwardly through heat exchanger BHX, first heating combustion air and then heating boiler makeup water, and copious condensation occurs, providing numerous advantageous effects heretofore described. Drain line D at the bottom of unit BHX includes a transparent section of tubing 85 through which the color of the condensate may be observed. when No.6 oil is used as boiler fuel, the condensate is black due to the large amount of particulate matter and SO3 removed from the 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 DT1 located in stack 75 indicates flue gas exit temperature, which typically varies between 900F and 200 F, depending upon boiler load. Temperature sensor TS4 also located at stack 75 serves to shut down the heat recovery system by closing damper 82 if the flue gas exit temperature should exceed 200"F. Simple BTU computers receive water and air temperature and flow rate signals and compute the amounts of heat recovered. At an average steam load of 30,000 pounds per hour, the combined amount of heat recovered is 3,468,000 Btu per hour. Prior to use of the condensing heat exchanger the average steam load of 30,000 pounds per hour required an average fuel consumption of 148.4 gallons per hour of No. 6 fuel (148,000 Btu per gallon) with a boiler efficiency of 80%. Utilizing the water-condensing heat exchanger 29.3 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. The lower housing may take a variety of different shapes in various applications. 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 flue gas flow velocities up to 60 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 above, among those made apparent from the preceding description, are efficiently attained. Since certain changes may be made in carrying out the above method and in the constructions set forth without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

Claims: CLAIMS 1. A method of recovering heat energy contained in a hot exhaust gas which contains water vapor, sulfur trioxide and particulate matter, and of simultaneously removing substantial amounts of said particulate matter and sulfur trioxide from the exhaust gas which comprises the steps of: passing said exhaust gas in heat exchange relationship upwardly between tubes of a tube nest; and simultaneously passing a fluid downwardly through tubes of said nest, the temperature and flow rate of said fluid being selected in relation to the input temperature and flow rate of said exhaust gas and the heat exchange surface of said nest so that the temperature of said fluid within an upper group of said tubes remains below 1200P, thereby causing continuous substantial condensation of water from said exhaust gas as said gas passes said upper group of tubes and a continuous fall of condensate from said upper group of tubes to wash lower tubes of said nest, whereby a substantial portion of the latent heat energy in said hot exhaust gas is transferred to said fluid, the wetting of said tubes from condensation causes portions of said particulate matter to temporarily stick to tubes of said tube nest, and the continuous fall of condensation entraps portions of particulate matter, washes particulate matter and sulfuric acid from tubes, and dilutes sulfuric acid formed by condensation at lower portions of said tube nest. 2. A method as claimed in claim 1, wherein said exhaust gas is directed upwardly through said tube nest at a velocity within the range of 30 to 60 feet per second. 3. A method as claimed in claim 1 wherein said exhaust gas is passed upwardly through a housing containing said nest of tubes, with a pressure drop within the range of 1 to 3 inches of water occurring in said exhaust gas within said housing. 4. A method as claimed in claim 1 wherein said tubes of said tube nest are staggered, so that condensate dropping from upper tubes splashes on lower tubes. 5. A method as claimed in claim 1 which includes the step of continuously removing condensate mixed with particulate matter which has fallen below said tube nest. 6. A method as claimed in claim 1 which includes the steps of monitoring said input temperature of said exhaust gas and mixing a cooler gas with said hot exhaust gas to prevent said input temperature from exceeding a predetermined value. 7. A method as claimed in claim 1 which includes the step of periodically directing a spray of liquid onto said nest of tubes from above said nest of tubes. 8. A method as claimed in claim 1 wherein said fluid passed through tubes comprises water, and said method includes the step of supplying water to said tubes from a hot water storage means and the step of supplying water from said tubes to said storage means. 9. A method as claimed in claim 1 which includes the step of varying the amount of said exhaust gas being: passed between said tubes of said nest to maintain the temperature of gas exiting upwardly above said tube nest at a predetermined value. 10. A method as claimed in claim 1 which includes the steps of sensing the outlet temperature of said fluid, and varying the amount of said exhaust gas being passed between said tubes of said nest to maintain said outlet temperature of said fluid at a predetermined value. 11. Apparatus for recovering heat energy from an exhaust gas which contains water vapor and sulru#triox#tde, comprising in combinations means defining a gas passage housing; a plurality of tubes extending generally horizontw through said gas passage housing at a plurality of different levels, the inside of said gas passage and the exterior surfaces of said tubes within said gas passage housing carrying corrosionprotecting coverings; means for applying said exhaust gas to said gas passage housing for upward flow of said exhaust gas between said tubes and though said housing; and means for passing a fluid which is cooler than 900F downwardly through successive ones of said tubes, thereby causing continuous condensation of water vapor after condensation of sulfuric acid as said exhaust gas travels upwardly through said housing and a continuous washing of lower tubes in said housing by condensate falling from upper levels within said housing. 12. Apparatus as claimed in claim 11 which includes a stack, and upper housing means situated above said gas passage housing to convey said exhaust gas from the upper end of said gas passage housing to said stack, said upper housing means and said stack being formed of acid-resistant reinforced fiberglass. 13. Apparatus as claimed in claim 11 which includes lower housing means situated below said gas passage housing to collect condensate and to convey said exhaust gas into the lower end of said gas passage housing, said lower housing means including drain means for draining said condensate from said lower housing means. 14. Apparatus as claimed in claim 11 wherein said means for applying said exhaust gas to said gas passage housing includes a blower to provide a flow velocity of said exhaust gas inside said housing within the range of 30 to 60 feet per second. 15. Apparatus as claimed in claim 11 which includes means for mixing air with said exhaust gas prior to application of said exhaust gas to said gas passage housing to limit the temperature of gas passed into said gas passage housing to a temperature below a predetermined material limit temperature of one of said corroaioniprotecting coverings. 16. Apparatus as claimed in claim 11 which includes means for automatically terminating application of said exhaust gas to said gas passage housing if the temperature of said exhaust gas exceeds a predetermined value. 17. Apparatus as claimed in claim 11 which includes means for decreasing flue gas flow if the temperature of said fluid begins to exceed a predetermined value. 18. Apparatus as claimed in claim 11 wherein said tubes comprise copper tubes each having their exterior surface carrying a layer of Teflon less than 30 mils thick. 19. Apparatus as claimed in claim 11 wherein said tubes comprise aluminum tubes each having their exterior surface carrying a layer of Teflon less than 30 mile thick. 20. Apparatus as claimed in claim 11 which includes a heat exchaJ#r connected to conduct said exhaust gas to said gas passage housing, and means for controlling flow of a fluid through said heat exchanger to control the temperature of the exhaust gas conducted to said gas passage. 21. Ehe 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 excess per and a second heat exchanger, said second heat exchanger containing corrosion-potection material having a material limit operating temperature exceeding said corrosive condensation temperature, 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 exchange cools said exhaust gas from a temperature 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 temperatire, 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. 22. The method of claim 21 wherein said first temperature exceeds said material limit operating temperature. 23. The method of claim 21 wherein passage of said fluid through said first heat exchanger heats said fluid to a temperature which is above said material limit operating temperature. 24. The method of claim 21 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. 25. The method of claim 21 which includes passing said fluid upwardly 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. 26. A method as claimed in claim 21 which includes the step of draining condensation from said second heat exchanger. 27. 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 tesperature 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 corrosive 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 exchangers 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. 28. Apparatus as claimed in claim 27 wherein said second heat exchanger includes a fluid passage and a gas passage in heat exchange 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 temperature which is above said corrosive ccastituent condensation temperature but below said deformation temperature. 29. Apparatus as claimed in claim 28 wherein said second heat exchanger is operative to heat said fluid to a temperature exceeding said deformation temperature. 30. Apparatus as claimed in claim 28 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. 31. Apparatus for extracting heat energy from a hot exhaust gas containing water vapor to heat air and water comprising, in combination: means defining a gas passage housing; means for passing said exhaust gas upwardly through said gas passage housing; a first plurality of tubes extending through a lower portion of said gas passage housing; a second plurality of tubes extending through an upper portion of said gas passage housing, the interior of said housing and the exterior surfaces of said tubes within said housing carrying plastic corrosionprotection coverings; means for passing air to be heated through said first plurality of tubes; and means for passing water to be heated through said second plurality of tubes, the temperature of said water in a group of said tubes of said second plurality being maintained below the condensation temperature of said water vapor, whereby condensation occurring on tubes of said second plurality continuously falls within gas passage housing to wet the tubes of said first plurality. 32. Heat exchanger apparatus, comprising, in combination: a pair of side members and a pair of metal tube sheets defining a generally rectangular housing, each of said tube sheets having a plurality of holes; a pair of plastic sheets covering the inside surfaces of respective ones of said tube sheets with portions of said plastic sheets extending through each of said holes in said tube sheets to the outside of the, tube sheets; and a plurality of metal tubes having plastic exterior coverings, each of said metal tubes extending through respective holes in both of said tube sheets and being =echanically fixed in place by the gripping of its exterior plastic covering by the portions of the plastic sheets which extend th=-oagh the tube sheet holes which the metal tube extends. 33. Apparatus as claimed in claim 32 having a pair of plastic sheets covering the inside surfaces of respective ones of said side members and wherein said tube sheets include a pair of flanges, each of said plastic sheets covering the inside surface of one of said tube sheets being formed to extend from said inside surface to cover at least a portion of each flange, said side members and said tube sheets being fastened together so that portions of the plastic sheets covering said side members are seated against portions of the plastic sheets covering said flanges. 34. A method of conserving heat energy and removing particulate matter from exhaust gas produced by a direct-fired dryer apparatus which is operative to produce heated air from combustion of fuel with input air and operative to apply said heated air in a drying process, thereby providing an exhaust gas carrying particles and sulfur trioxide, said method comprising the steps of passing said exhaust gas upwardly between tubes of a nest of tubes in a heat exchanger, and passing ambient air into and through said tubes to provide input makeup air for said direct-fired dryer apparatus thereby causing condensation of sulfuric acid and entrapment of particles at one elevational level within said heat exchanger, and continuous condensation of water vapor at a higher elevational level within said heat exchanger. 35. A method of fabricating a heat exchanger which comprises the steps of: heat-forming edge portions of holes in a plastic sheet having a plurality of holes to extend through and line holes in an adjacent metal sheet having holes which concentrically register with respective holes in said plastic sheet, thereby to form a plastic-lined metal sheet assembly; inserting plng means in each hole before the plastic lining has cooled to limit dlnetrical reduction of the plastic-lined holes; spacing a pair of said plastic-lined metal sheet assemblies fixedly apart and parallel to each other; and entering an end of a plastic-covered tube successively through a plastic-lined hole in each of said plastic-lined metal sheet assemblies sufficiently shortly after removal of the plug means from said holes that the progressive diametrical reduction of said plastic-lined holes allows said end to be entered through said plastic-lined holes but then causes said plastic-lined holes to grip and fixedly locate said plastic-covered tube.

Inventors: WARNER, DONALD F.
Applicants: HEAT EXCHANGER INDUSTRIES, INC.
Application year: 1981
Main classification: B01D53
IPC codes: F28F19, F28F9, B01D53, B23P15, F28B1, B01D5, F23J15, F28D21, F28D7