APPARATUS FOR HEATING AIR

Abstract: A forced hot air furnace has a recuperative heat exchan­ ger (26) positioned in the air chamber(14) at an upward incline from a firebox (22) to an outlet (66) to enable condensate to drain counter to combusting products. Condensate is collec­ ted in a water reservoir (18) in the firebox (22). Combustion products from a burner (16) flow over the reservoir (18) thereby becoming elevated in dew point before entering the tubes (42) of the heat exchanger (26). Condensate forms along the entire length of the tubes (42) resulting in all wet oper­ ation. In an alternative embodiment, a continuously wet re­ cuperative heat exchanger is utilized to heat a liquid, con­ densate forming on the outer surface of fins attached to a tube carrying the liquid to be heated.

Description: The invention relates to an apparatus for heating air, comprising: means for providing combustion products; a recuperative heat exchanger having a plurality of conduits for receiving said combustion products; and means for moving air across said recuperative heat exchanger wherein said air is heated by both sensible heat and heat of condensation from said combustion products. Nonrecuperative furnaces only transfer sensible heat from combustion products as they are cooled. Condensation does not occur during the heating cycle since combustion products are exhausted at a temperature above their dew point. Heat transfer by nonrecuperative furnaces is, therefore, commonly referred to as a dry process. On the other hand, recuperative furnaces not only transfer heat by the dry process of nonrecuperative furnaces but also simultaneously transfer heat of condensation. A separate recuperative heat exchanger is commonly used to transfer additional heat from combustion products after they have passed through a dry process. Recuperative heat exchangers are herein defined as heat exchangers which transfer both sensible heat and heat of condensation from combustion products to another fluid. The recuperative heat exchanger cools combustion products sufficiently to condense them thereby transferring both condensate heat and sensible heat. The additional heat transfer by the recuperative heat exchanger increases overall furnace efficiency to approximately 95%, whereas nonrecuperative furnaces are limited to 85% efficiency. Besides providing high efficiencies, the lower exhaust temperatures of recuperative furnaces enable the use of inexpensive exhaust venting such as, for example, pvc pipe rather than conventional chimneys. Further, low exhaust temperatures eliminate the draft hood associated with nonrecuperative furnaces wherein heat is lost during the cool down period at the end of each heating cycle. Recuperative furnaces are subject to corrosive attack of the recuperative heat exchanger by acidic condensate formed therein. In combusting natural gas, and to a greater extent fuel oil, a number of potentially acid forming gases are produced. Although these gases are typically noncondensable at the operating temperatures of the recuperative heat exchanger, they are absorbed by water vapor condensate thereby forming acids. For example, carbon dioxide forms carbonic acid, nitrogen dioxide forms nitric acid, hydrogen chloride forms hydrochloric acid, and hydrogen fluoride forms hydrofluoric acid. In addition, sulphur dioxide will condense within a recuperative heat exchanger thereby forming sulphurous acid. The acidity of the condensate is further increased when water condensate evaporates leaving behind concentrated acids which corrosively attack the heat exchanger. Corrosive attack may also occur on heat exchange surface areas which are only exposed to combustion products that are above their dew point temperature. At the beginning of the heating cycle, incipient condensation may briefly form on initially cool surface areas. As these surfaces become heated during the heating cycle, the condensation evaporates and does not recur. Localized corrosion may therefore occur on these surfaces. Several prior art approaches attempted to prevent corrosive attack. First, stainless steel components were used. However, it was found that chlorides are often present in our environment at levels which produce sufficient hydrochloric acid to corrode stainless steel. For example, chlorides are commonly found in laundry room areas which are often in close proximity to residential furnaces. Second, hydrochloric acid resistant materials were proposed. A stainless steel molybdenum alloy may be effective but is prohibitively expensive for residential heat exchangers. Although polymer or ceramic coatings may also be effective, they would increase thermal resistance and may also be subject to thermal shock. See U.S. Dept. of Energy, BNL51770, Condensing Heat Exchanger Systems for Residential/Commercial Furnaces and Boilers, Phase III, February 1984. Third, U.S. Patent No. 4,449,511 shows an external water flush system which is activated after each heating cycle to flush condensate from the coupling between the recuperator and burner. A potential corrosive problem may still exist in high chloride environments for corrosive attack on areas not flushed. US-A-4069807 describes an apparatus of the kind defined hereinbefore, in which the conduits of the heat exchanger are horizontal, combustion products from a burner are drawn up to one end of the conduits and expelled upwards from the other end by an exhaust blower, and air is forced upwards through the heat exchanger by another blower installed below a chamber housing the burner. Combustion products from the burner are drawn through a bubble cap assembly before entering the exchanger. The bubble cap assembly has the top end of a chimney from the burner projecting up through a water reservoir to a cap with a skirt that extends below the water surface to provide bubble holes allowing the combustion products to escape through the water. Condensate from the exhaust end of the exchanger is returned to the water reservoir. GB-A-2031572 describes an apparatus for exchanging heat between combustion products and a fluid to be heated in which the combustion products from a burner pass through a horizontal water spray chamber before exiting through a vertical exhaust duct. Sulphur dioxide is thus removed from the exhaust gases. US-A-4478158 describes an apparatus of the kind defined hereinbefore in which the combustion products are drawn by an outlet blower through a plurality of finned tubes extending downwardly to a condensate collection manifold from which the condensate is withdrawn through a pipe. At the top end of the finned tubes a water spray chamber provides water vapour which mixes intimately with hot combustion gases to dilute the acidic combustion products and extract heat. Before entering the water spray chamber, the combustion products pass through a non-recuperative annular heat exchanger in the outlet duct for air to be heated, and through a conduit connecting the annular heat exchanger to the water spray chamber and mounted in the inlet duct for air to be heated. The primary object of the invention is to provide a high efficiency furnace have a recuperative heat exchanger wherein corrosive attack by acidic combustion product condensate is substantially lessened. According to the present invention, an apparatus of the kind defined hereinbefore is characterised in that said apparatus further comprises means for raising the dew point of combustion products from said providing means, and said conduits are upwardly inclined wherein said condensate flows through the entire length of said conduits, the direction of flow of the condensate being opposite to that of the upward flow of said combustion products. In a preferred embodiment of the invention, water vapor condensate continuously flushes away acidic condensate from the heat exchanger. The dew point of combustion products is elevated above their natural dew point by mixing water vapor with the combustion products. The formation of water condensate in the recuperative heat exchanger is thereby increased. Transition areas in the recuperative heat exchanger between dry and wet surface areas are eliminated. Such areas are particularly prone to corrosion since the acid concentration of condensate increases as condensate water evaporates. To reduce corrosion, heat transfer surface areas which dry out during the heating cycle are eliminated. Initially cool heat transfer surface areas which collect incipient condensate and then dry out later in the heating cycle are eliminated. Only non-heat transfer surfaces dry out during the heating cycle. Consequently, only these surfaces need be made from acid immune materials. The preferred heat exchanger operates at temperatures substantially reduced from conventional heat exchangers, and sufficient heat is transferred from the products of combustion to cool them below their natural dew point. In the preferred apparatus for heating air, the flow of combustion products is substantially opposite to the flow of air which is to be heated. A portion of the downward flowing condensate may preferably evaporate within the conduits thereby raising the dew point of the combustion products. The combustion product providing means may be a burner. The burner may be any suitable burner for oxidizing gaseous fuels. A burner suitable for oxidizing liquid fuels may also be used. The combustion product providing means may also be a nonrecuperative heat exchanger. Preferably, the dew point raising means may also provide means for removing a portion of gases from the combustion products which would otherwise form acids in the condensate. A portion of gases may be removed by directing combustion products against water wherein the water may absorb a portion of gases such as, for example, carbon dioxide, nitrogen dioxide and hydrogen chloride. The dew point raising means should raise the dew point of combustion products by at least 3°C (5°F.) Stated another way, the water vapor content of the combustion products should be increased by approximately 25%. It may be preferable that, where the means for providing combustion products is a burner enclosed, together with a water reservoir, in a firebox, means for spraying water particles into the firebox be provided. Water vapor would then be generated by evaporating the water particles in addition to, or in place of, evaporating water in the water reservoir. It may also be preferable for the furnace to include a blower for forcing a mixture of air and fuel into the burner. Preferably, the burner may be a radiant burner generating both combustion products and radiant energy to evaporate water in the water reservoir. The apparatus may preferably have an exhaust blower coupled to an exhaust vent for drawing combustion products from the heat exchanger. A pressure is thereby created within the heat exchanger that is less than the pressure outside the heat exchanger. In the unlikely event that a leak develops in the heat exchanger, combustion products will, therefore, not escape. The invention will now be described by way of example with reference to the accompanying drawings in which:- FIG. 1 is a block diagram of recuperative furnace 10; FIG. 2 is a partially broken away, right front perspective view of recuperative furnace 10 showing furnace chamber 14; FIG. 3 is a side elevation view of FIG. 2 with side panel 12 removed; FIG. 4 is a partially broken away, left front perspective view of recuperative furnace 10 showing components chamber 40 and a portion of furnace chamber 14; FIG. 5 is a partially broken away front elevation view of furnace chamber 14 showing firebox 22 and manifold 24; FIG. 6 is an illustrative electrical schematic for the controller of recuperative furnace 10; FIG. 7 is a partially broken away, left front perspective view of recuperative furnace 10 illustrating a liquid fuel burning embodiment; FIG. 8 is a partially broken away front elevation view of furnace chamber 14 showing manifold 24 and firebox 22 being adapted for a liquid fuel burning embodiment; and FIG. 9 is a right side elevation view of recuperative furnace 10 illustrating a two stage, dry and condensing, embodiment. Description of the Preferred Embodiment Referring first to FIG. 1, there is shown a simplified block diagram depicting operation of a recuperative furnace. Radiant burner 16 is shown firing on water reservoir 18. Recuperative heat exchanger 26, having an upwardly inclined combustion product flow path, is shown coupled to the output of reservoir 18. Those skilled in the art will recognize that the depicted operational parameters such as temperature, dew point, and BTU/hr., are for illustration and discussion only, and that the invention could be used to advantage with many other combinations of operational parameters. In the illustration of FIG. 1, radiant burner 16 oxidizes a mixture of air, water vapor, and fuel to generate 29.31 kilowatts (100,000 BTU/hr.) of heat of which approximately 14.66 kilowatts (50,000 BTU/hr.) are radiant heat. The remaining 14.66 kilowatts (50,000 BTU/hr.) of heat may be potentially recovered from both combustion product sensible heat and latent heat of vaporization. Assuming a 30% excess air/fuel mixture and input air having a 10% water vapor content, combustion products may typically be generated at a temperature of 1093°C (2,000°F) with an initial dew point of 53°C (128°F). Approximately 10.85 kilowatts (37,000 BTU/hr) of sensible heat and 3.8 kilowatts (13,000 BTU/hr.) of latent heat of vaporization may then potentially be recovered from the combustion products. Both radiant heat and combustion products from burner 16 are directed against water in water reservoir 18. As water is evaporated in reservoir 18, virtually all the 14.66 kilowatts (50,000 BTU/hr.) of radiant heat and approximately 1.47 kilowatts (5,000 BTU/hr.) of combustion product sensible heat are converted into latent heat of vaporization. Combustion products are thereby raised in dew point to approximately 78°C (172°F) and lowered in temperature to approximately 649°C (1200°F). About 19.9 kilowatts (68,000 BTU/hr.) of latent heat of vaporization and 9.38 kilowatts (32,000 BTU/hr.) of sensible heat may now potentially be recovered from the combustion products. The combustion products are directed from reservoir 18 into tubes 42 (FIG. 2) of recuperative heat exchanger 26. Heat exchanger 26 is of fin and tube construction wherein air from a home or other enclosure to be heated flows across the fins. Heat is transferred from the combustion products to the air thereby cooling the combustion products and heating the air. At the beginning of the heating cycle, a short transient phase occurs in heat exchanger 26, as will be explained in more detail later herein with particular reference to FIGS. 2-5. Briefly, combustion products are cooled as they flow through the upwardly inclined flow path of heat exchanger 26. A portion of the combustion products condense and flow downwards into warmer areas of heat exchanger 26 where they will re-evaporate. As condensate is evaporated, combustion product sensible heat is converted into latent heat. Combustion products will therefore be further lowered in temperature and further raised in dew point. Evaporation and condensation will then occur at successively earlier points within heat exchanger 26 until tubes 42 become totally covered by a film of condensate. Thus, heat exchanger 26 operates in what will be hereinafter referred to as the all wet mode. The condensation/evaporation cycle within heat exchanger 26 will also result in the conversion of another 8.65 kilowatts (29,500 BTU/hr.) of sensible heat into latent heat. The input to heat exchanger 26 will now see combustion products having approximately 28.6 kilowatts (97,500 BTU/hr.) of latent heat and 0.73 kilowatts (2,500 BTU/hr.) of sensible heat which may potentially be recovered. Heat exchanger 26 is designed to cool combustion products to approximately 38°C (100°F) before exhausting them. Therefore, approximately 27.3 kilowatts (93,000 BTU/hr.) of condensation heat and 0.59 kilowatts (2,000 BTU/hr.) of sensible heat are transferred to the forced air. The air forced across heat exchanger 26 will consequently be heated from a typical ambient temperature of approximately 19.5°C (67°F) to approximately 57°C (135°F). Stated another way, 97% of heat is transferred by heat of condensation while 95% efficiency is maintained. In sharp contrast, a conventional recuperative heat exchanger recovers approximately 3.5 kilowatts (12,000 BTU/hr.) of latent heat and 24.33 kilowatts (83,000 BTU/hr.) of sensible heat. It is therefore readily apparent that the increased recovery of latent heat has fully offset the smaller temperature differential between combustion products and air to be heated. Several anti-corrosion advantages are obtained by the configuration described above. One advantage is that by directing combustion products against water, a portion of potentially acidic forming gases are absorbed before entering the heat exchanger. Another advantage is that condensate will continuously drain into water reservoir 18 thereby flushing corrosive acids from heat exchanger 26. Still another advantage is obtained by operating totally in the wet mode along the entire length of tubes 42. That is, areas wherein condensate may dry during the heating cycle are eliminated. These areas would otherwise be particularly prone to corrosion since water evaporization would leave behind acids of maximum concentration. In prior recuperative furnaces, these areas were particularly prone to corrosion. A further advantage is obtained by reducing the operating temperature of heat exchanger 26. The chemical rate of any corrosive attack is markedly reduced in the event that acidic residue is not totally flushed. A more detailed description of the preferred embodiment is now made in reference to FIGS. 2 and 3. FIG. 2 illustrates a partially broken away, perspective view of recuperative furnace 10. For additional clarity, a side elevation view of furnace 10 with side panel 12 removed is shown in FIG. 3. Furnace 10 is broadly made up of furnace chamber 14, radiant burner 16, water reservoir 18, firebox 22, manifold 24, recuperative heat exchanger 26, exhaust plenum 32, blower assembly 34, return air port 36, hot air port 38 and components chamber 40. Recuperative heat exchanger 26 is of fin and tube construction which is well known in the art. Heat exchanger 26 is further defined by first and second heat exchanger segments 28 and 30, respectively, which are interconnected in series. Tubes 42 of heat exchanger 26 form a flow path through segments 28 and 30 for transferring combustion products between firebox 22 and exhaust plenum 32 as will be described later herein. The flow of fuel into firebox 22 is best seen in FIG. 4 which is a partially broken away left front perspective view of furnace 10 illustrating components compartment 40 and a portion of furnace chamber 14. Motor 46 is shown driving exhaust products blower 52. Fuel valve 56, input fuel line 58, fuel pipe 60 and burner 16 are connected together in series. Upon a call for heat and in accordance with the furnace controls as will be described later herein, with particular reference to FIG. 6, electric current flows through ceramic igniter tip 62 (FIGS. 3 and 5) heating it to approximately 2500°F. After a time delay from the call for heat, fuel valve 56 is opened and motor 46 activated. A gaseous fuel is then directed into burner 16. As will be described in more detail later herein, exhaust products blower 52 is also activated for drawing exhaust products from plenum 32 via exhaust pipe 66 and forcing the exhaust products out flue pipe 68. Referring now to FIG. 5 and continuing with FIGS. 2-4, the flow of combustion products is now described. Firebox 22 forms a chamber across the front of furnace chamber 14 partially enclosing burner 16 and water reservoir 18. Burner 16 is here shown as a conventional radiant burner defined by conduit 80 and metal screen 84. A baffle 70 extending downwards from firebox 22 towards water reservoir 18 provides an opening 74 above reservoir 18. Wall 72, having heat exchanger tubes 42 inserted therein, extends upwards from firebox 22 to furnace wall 76 defining manifold 24. Firebox 22, reservoir 18 and manifold 24 are shown without appropriate insulation or firewalls because a detailed description of them is not necessary for an understanding of the invention. Accordingly, the gaseous fuel enters conduit 80 and is deflected substantially downward through screen 84 at increased velocity and reduced pressure. Igniter 62 then ignites the fuel/air mixture generating combustion products across the surface area of screen 84. Radiant burner 16 therefore acts in a conventional manner as both a gas burner and infrared generator wherein a portion of combustion product heat is converted into infrared energy across screen 84. In this manner, the temperature of combustion products exiting burner 16 is less than 1093°C (2000°F) whereas a typical gas burner might generate combustion products at approximately 1538°C (2800°F). Although a conventional tubular burner could be used, an advantage is obtained by using a radiant burner since nitrogen oxides are not formed in significant quantities at temperatures less than 1093°C (2000°F). Thus, formation of nitric acid in the combustion product condensate is markedly reduced. Further, NO x flue emission standards of California will not be exceeded. See Rule 1111, issued by South Coast Air Quality Management District, 9150 Flair Drive, E1 Monte, California 91731. Burner 16 directs both combustion products and radiant energy against water reservoir 18 thereby evaporating a portion of the water contained therein. Consequently, combustion products become elevated indew point and lowered in temperature as a portion of combustion product sensible heat is converted into latent heat of vaporization. For the embodiment described herein, and assuming a 30% excess air fuel mixture, combustion products are elevated from a natural dew point of approximately 53°C (128°F) to a dew point of approximately 78°C (172°F) at an approximate temperature of 649°C (1200°F). Those skilled in the art will recognize that there are numerous other configurations for combustion product dew point elevation. For example, a water atomizer may be used in conjunction with reservoir 18, or in place of reservoir 18, to spray particles of water into the flow of combustion products. A slinger, such as the one illustrated in FIG. 8, may also be used to throw water from reservoir 18 throughout firebox 22. In this manner, the walls of firebox 22 would also be kept cooler. Continuing with FIGS. 2-5, dew point elevated combustion products are directed from firebox 22, through opening 74, through manifold 24, through tubes 42 of first heat exchanger segment 28, through tubes 42 of second heat exchanger segment 30, into plenum 32. Exhaust blower 52 draws combustion products from plenum 32 thereby creating a forced induction system for drawing combustion products through firebox 22 and heat exchanger 26. As air is drawn over heat exchanger 26, which will be explained later herein with particilar reference to FIG. 6, combustion products are cooled sufficiently by thermal transfer to condense partially within tubes 42 and, accordingly, transfer heat of condensation. The point at which condensation begins will vary during a brief transitory period at the beginning of the heating cycle. Condensation will first randomly occur on the initially cool surface areas of heat exchanger tubes 42. Additional condensate is formed throughout segment 30 when combustion products are cooled to their dew point. When steady state heat exchanger temperatures are approached, downward flowing condensate will re-evaporate in the warmer areas of segment 28. Combustion products will then become further elevated in dew point and further lowered in temperature. Condensation then occurs at successively earlier points within heat exchanger 26 until the entire inner surface areas of tubes 42 are substantially covered by a film of downward flowing condensate. The entire length of heat exchanger 26 then operates in the wet mode. The condensate drains by gravity from tubes 42, through manifold 24, into water reservoir 18. Since potentially acid forming combustion products are progressively absorbed by condensate as combustion products flow through heat exchanger 26, the condensate progressively becomes closer to a neutral pH. For example, the condensate in segment 30 will be at a higher pH than the condensate in segment 28. Consequently, heat exchanger tubes 42 are flushed by progressively purer condensate. Any residual acidic condensate which may be left in tubes 42 after the heating cycle is therefore minimized. Referring to FIG. 3, drain 90 is positioned in reservoir 18 and connected to the domestic sanitary drain system for disposal of excess condensate. Since combustion products are cooled to less than their natural dew point of 53°C (128°F), heat exchanger 26 produces more condensate than is evaporated by burner 16. Accordingly, excess condensate will flush reservoir 18 preventing the concentration of acids therein. In the event that the heating cycle begins with a dry reservoir 18, all wet operation will still be achieved. Heat exchanger 26 is designed to cool combustion products at 1093°C (2000°F) to less than 53°C (128°F). Condensation will therefore occur within tubes 42 when there is no water in reservoir 18. Condensate will then flow downward and re-evaporate in the manner previously described. Since furnace 10 is a net producer of water, the excess condensate will fill an initially dry reservoir 18. That is, furnace 10 is self priming. It is apparent from the above that heat exchanger 26 must be designed to achieve specific criteria. More specifically, the forced air flow across the heat exchanger fins, fin geometry, tube length, tube diameter and rate of flow of combustion products through the tubes should be designed to cool combustion products from firebox 22 to 32°C-49°C (90°F-120°F). The rate of combustion product flow should be designed for a high rate turbulent flow to maximize dry heat transfer coefficients, but the flow rate should not be large enough to force condensate into plenum 32. Further, in the event that reservoir 18 starts out dry, heat exchanger 26 should be designed to cool the hotter than normal combustion products to less than their natural dew point. Referring to FIG. 6, there is shown an electrical schematic which functions as the controller for recuperative furnace 10. Terminals 108 and 110 are connected to a standard household 115-volt AC source as shown. Accordingly, the AC voltage appears across the series of motor 46 and normally open fuel/air blower contacts 119, the series of air blower motor 114 and normally open air blower control contacts 116 of temperature activated switch 117, primary winding 113 of step down transformer 112, and terminals 1 and 5 of ignition control 120. The voltage across the secondary winding 122 of step down transformer 112 is reduced to approximately 24 volts. This secondary voltage appears across the series of water valve 94 and normally open water level contacts 100 of water level switch 98. Further, side 123 of secondary winding 122 is connected through a secondary limiter 124, which consists of manually reset normally closed temperature activated contacts 126, to one terminal of room thermostat 132. The opposite terminal of thermostat 132 is connected to power input terminal 8 of ignition control 120 and one terminal of relay 136. Side 125 of secondary winding 122 is connected through blower control resistant heater 140 to terminal 139 of pressure switch 138, to the opposite terminal of relay 136, through fuel valve 56 to terminal 141 of pressure switch 138, and to grounded terminal 9 of ignition control 120. Terminal 7 of ignition control 120 is connected to terminal 139 of pressure switch 138. Further, igniter 62, which was described earlier as a ceramic electronic igniter tip is connected across terminals 2 and 4 of igniter control 120. The components described herein with reference to FIG. 6 are conventional and well known in the art. For example, although the functions of ignition control 120 as described herein would be performed using a plurality of relays, it may be preferable to use a solid state device which is commercially available from Fenwal, Inc., Division of Kidde, Inc., of Ashland, Massachusetts, as Catalog Order No. 05-212225-107. Also, igniter 62 is commercially available as Model No. 201A from Norton Company of Milford, New Hampshire. Upon call for heat from room thermostat 132 as mentioned earlier herein, contacts internal thereto close allowing current to flow from secondary winding 122, through secondary limiter 124 and thermostat 132, thereby energizing power terminal 8 of ignition control 120. Current also flows from room thermostat 132 activating relay 136. The activation of relay 136 closes normally open contacts 119. Closing contacts 119 completes the line voltage circuit through motor 46 that drives exhaust products blower 52. Combustion products are drawn from plenum 32 and forced out flue pipe 68 by exhaust blower 52. Less than atmospheric pressure is therefore maintained in heat exchanger 26 which will prevent combustion products from leaking into furnace chamber 14 in the event of a leak in heat exchanger 26. The energizing of ignition control 120 causes igniter 62 to be energized by internally connecting pin 1 to pin 2 and pin 4 to pin 5 within ignition control 120. After a time delay in which igniter 62 heats to its specified ignition temperature, ignition control 120 opens fuel valve 56 and energizes air blower control resistance heater 140 by internally providing power from pin 8 to pin 7. Pin 7 is connected to fuel valve 56 through pressure switch 138. Activation of fuel valve 56 allows gas to enter burner 16 where it is ignited by igniter 62 as previously described. When air blower control resistance heater 140 has reached a predetermined temperature, air blower control contacts 116, which are in heat exchange relationship with heater 140, close to complete a line voltage circuit through air blower motor 114. Motor 114 turns blower assembly 34 to move return air through furnace 10 as shown in FIGS. 2 and 3. Cool air returning from the home or other enclosure to be heated is drawn into return air port 36 from a return air duct (not shown). It typically may be at a temperature of 19°C (67°F). The air is heated as it is drawn through segments 28 and 30 of tube and fin heat exchanger 26. The folded over segments 28 and 30 of heat exchanger 26 approximate forced air flow being substantially opposite to combustion product flow. More specifically, air is preheated as it is drawn across second heat exchanger segment 30 by transfer of both sensible heat and heat of condensation from combustion products. The temperature of the preheated air is typically elevated to approximately 27°C (81°F). Preheated air is then drawn across first heat exchanger segment 28 where it is heated further by transfer of both sensible heat and heat of condensation. Typically, the forced air will be heated to a temperature of approximately 57°C (135°F) before being expelled through hot air port 38 by blower assembly 34. From air port 38 the air may be forced through conventional duct work to the enclosure or home to be heated. Referring back to FIG. 6, when room thermostat 132 is satisfied, the secondary voltage circuit through it is opened thereby removing power to ignition control 120 and relay 136. Ignition control 120 then closes fuel valve 56. When power is removed from relay 136, contacts 119 open interrupting the line voltage circuit across motor 46. Drive is then removed from exhaust blower 52. Motor 114 however continues to drive blower assembly 34 to force heated air into the home or other enclosure to be heated until blower control contacts 116 open. Blower control contacts 116 are in heat exchange relationship with heat exchanger 26 and open when heat exchanger 26 is cooled to a predetermined temperature. Two safety interlocks are now described. The first interlock is provided by pressure activated switch 138 which is located on manifold wall 72 (not shown). Switch 138 will open, deenergizing fuel valve 56, in the unlikely event that a block of combustion products causes excessive back pressure within manifold 24. The second safety interlock is provided by water level switch 98. Inlet water pipe 92, water level valve 94, water reservoir conduit 96 and water reservoir 18 are interconnected in series (FIG. 4). Water level switch 98 is coupled to reservoir 18. Accordingly, if the water level of reservoir 18 becomes depleted due to evaporization during long standby periods, contacts 100 of switch 98 will close activating valve 94. Water reservoir 18 will then be refilled to a predetermined level. The water level back-up system however is not essential since heat exchanger 26 produces sufficient condensate to fill an initially dry reservoir 18. A second embodiment, for burning fuel oil or other liquid fuels, is now described with particular reference to Figs. 7 and 8. The operation of the liquid fuel embodiment is the same as the gaseous fuel embodiment, as previously described, with two general modifications. First, the flow of liquid fuel and combustion of liquid fuel are different. Second, the apparatus for mixing water vapor with combustion products in firebox 22 is modified. Except for the differences discussed below, all components of the liquid fuel embodiment are the same as those illustrated in Figs. 2-6. Radiant burner 16 is replaced by oil burner 150. Ignitor 62 is not shown since it is now an internal component of oil burner 150. Firebox 22 is shown having a larger volume to enable complete combustion of the liquid fuel. Slinger 154 is added to throw water from reservoir 18 into firebox 22. Gear driving mechanism 158 is added for turning slinger 154. The flow of liquid fuel into firebox 22 is first described in reference to Fig. 7 which is a partially broken away, left front perspective view of furnace 10 illustrating components compartment 40 and a portion of furnace chamber 14. Fig. 7 is the liquid fuel equivalent of Fig. 4 except for the differences noted above. A conventional liquid fuel burner 150 is shown having an electric motor 162. Motor 162 drives a fuel pump (not shown) and air blower (not shown) for generating a fuel/air mixture within burner chamber 166. Liquid fuel burner 150, valve 56, input fuel line 58 and fuel pipe 60 are connected together in series. Motor 162 is electrically connected in parallel (not shown) with exhaust products blower motor 46. After a time delay from a call for heat, as previously described with particular reference to FIG. 6, fuel valve 56 is opened and motor 162 activates. Liquid fuel is then forced through a conventional pump and nozzle assembly (not shown) into chamber 166. Air is also drawn in through apertures 164 thereby generating a fuel/air mixture in chamber 166. Referring now to FIG. 8, the flow of combustion products is described. FIG. 8 is a partially broken away front elevation view of furnace chamber 14 showing firebox 22 and manifold 24. FIG. 8 is the liquid fuel equivalent of FIG. 5 with the differences noted above. Chamber 166 having combustion product aperture 170 is positioned within firebox 22. Slinger 154 is a tubular screen which is positioned partially below water level 174 of water reservoir 18. A conventional gear driving mechanism 158 couples motor 162 to slinger 154. Other driving mechanisms may be used to turn slinger 154 such as, for example, a belt and pulleys. An electric motor may also be used to turn slinger 154. Accordingly, the fuel/air mixture in chamber 166 is ignited by an electrical ignitor (not shown) in accordance with the furnace controls previously described. Combustion products at approximately 1538°C (2800°F) are then forced through aperture 170 into firebox 22. Simultaneously, slinger 154 rotates thereby throwing particles of water throughout firebox 22. Combustion products will evaporate a portion of water in reservoir 18 and a portion of water particles in firebox 22. As combustion product sensible heat is converted into latent heat of vaporization, the combustion products will become elevated in dew point to approximately 77°C (170°F) and lowered in temperature to approximately 649°C (1200°F). Dew point elevated combustion products are then directed through manifold 24 into tubes 42 of heat exchanger 26. Furnace 10 will thereafter operate the same as the gaseous fuel embodiment described with reference to FIGS. 2-6. The same advantages obtained by the gaseous fuel embodiment will also apply to the liquid fuel embodiment. In addition, the downward flowing condensate will flush ash and carbon deposits from heat exchanger 26; when buning fuel oil, ash and carbon deposits would otherwise clog tubes 42. Also, a portion of combustion product particulates will be scrubbed away in firebox 22 by slinger 54. A portion of sulphur dioxide will also be absorbed by water in firebox 22 and condensate in tubes 42. Therefore, besides anticorrosion advantages, noxious exhaust emissions will also be reduced. A third alternate embodiment is now described wherein combustion products are first directed through a dry heat transfer process. The combustion products from the dry process are then elevated in dew point before being directed into a recuperative heat transfer process. This third embodiment illustrated in FIG. 9 which shows a side elevation view of furnace 10. A conventional tubular burner 216 is positioned in firebox 222. Tubes 242 of non-recuperative heat exchanger 228 are connected between firebox 222 and manifold 234. Manifold 234 encloses water reservoir 238. Tubes 252 of recuperative heat exchanger 230 are connected between manifold 234 and exhaust plenum 32. All other components of furnace 10 remain the same as described with particular reference to FIGS. 2-6. Burner 216 generates combustion products at approximately 1538°C (2800°F) with a dew point of approximately 53°C (128°F). The combustion products are drawn from firebox 222 through nonrecuperative heat exchanger 228 by the induced draft created by exhaust products blower 52, as described previously herein. Combustion products sensible heat is transferred to the air forced across heat exchanger 228 thereby cooling the combustion products to a temperature which is above their dew point. The combustion products are then drawn from heat exchanger 228, into manifold 234 and across water reservoir 238. A portion of remaining combustion product sensible heat is transferred into latent heat of vaporization as water is evaporated from reservoir 238. Dew point elevated combustion products are then drawn through recuperative heat exchanger 230 into exhaust plenum 32. As combustion products are cooled to approximately 38°C (100°F), both sensible heat and heat of condensation are transferred to the forced air flowing across recuperative heat exchanger 230. Condensate from combustion products flow downward through tubes 252 of heat exchanger 230 into reservoir 238. Since combustion products were elevated in dew point by reservoir 238, the flow of condensate is greater than the condensate flow in a conventional recuperative heat exchanger wherein only burner latent heat of vaporization is recovered. Consequently, the anticorrosion advantages of the preferred embodiment are applicable to this third alternate embodiment. Further, manifold 234 is operating at sufficiently low temperatures to permit the use of inexpensive, acid resistant materials such as, for example, polypropylene. Operating temperatures of manifold 234 are low for three reasons. First, entering combustion products have been reduced in temperature from heat transfer through nonrecuperative heat exchanger 228. Second, combustion products entering manifold 234 are further reduced in temperature by evaporating water in reservoir 238. Third, water reservoir 238 operates as a heat sink. A poor heat transfer material such as polypropylene may be used in the construction of manifold 234 since an adiabatic process occurs therein . That is, a portion of combustion product sensible heat is converted into latent heat of vaporization rather than being transferred to the forced air. In conclusion, the embodiments described provide numerous advantages. Many anticorrosion advantages are obtained, as follows. Acidic condensate is continuously flushed. Ash and carbon residues are also flushed. Residual condensate is minimized in acidity. Heat transfer areas which dry out during the heating cycle, thereby increasing acid concentration, are substantially eliminated. A portion of potentially acid forming gases are pre-absorbed before entering the recuperative heat exchanger. The most acidic condensate is confined to the reservoir and structures which enclose the reservoir. Only those areas need be constructed of acid immune material. The operating temperature of the recuperative heat exchanger is reduced thereby reducing the rate of corrosive attack. Noxious exhaust emissions are also reduced. A portion of noxious gases are absorbed in the water reservoir and recuperative heat exchanger. When a radiant burner is utilized, emission of nitrogen oxides are also minimized by lowering the temperature of burner output without sacrifice to efficiency. In addition, the embodiments described may burn gaseous or liquid hydrocarbon fuels.

Claims: An apparatus for heating air, comprising: means (16) for providing combustion products; a recuperative heat exchanger (26) having a plurality of conduits (42) for receiving said combustion products; and means (34) for moving air across said recuperative heat exchanger (26) wherein said air is heated by both sensible heat and heat of condensation from said combustion products, characterised in that said apparatus further comprises means (18) for raising the dew point of combustion products from said providing means (16), and said conduits (42) are upwardly inclined wherein condensate flows through the entire length of said conduits (42), the direction of flow of the condensate being opposite to that of the upward flow of said combustion products. An apparatus according to claim 1, characterised in that the recuperative heat exchanger (26) is positioned in an air duct (14) for receiving said combustion products from said dew point raising means (18), and the means for moving air comprises an air blower (34) communicating with said duct (14) for forcing air across said heat exchanger (26). Apparatus according to claim 1 or 2, characterised in that said providing means comprises a burner (16) for generating combustion products. Apparatus according to claim 1 or 2, characterised in that a nonrecuperative heat exchanger (228) connects said providing means (216) to said dew point raising means (238). Apparatus according to claim 1 or 2, characterised by a means (52) for exhausting said combustion products from said recuperative heat exchanger (26). Apparatus according to claim 1 or 2, characterised in that said dew point raising means (18) further comprises means for removing a portion of potentially acid forming gases from said combustion products. Apparatus according to claim 4, characterised in that said dew point raising means comprises a manifold (234) and water reservoir (238), said manifold (234) directing combustion products from said nonrecuperative heat exchanger (228) across said reservoir (238), said manifold (234) and said reservoir (238) being constructed from polypropylene. Apparatus according to claims 2 and 3, characterised in that said dew point raising means comprises a water reservoir (18) adjacently positioned to said burner (150) wherein combustion products from said burner (150) flow across said water reservoir (18) becoming elevated in dew point; a firebox (22) encloses said burner (150) and said water reservoir (18); the recuperative heat exchanger (26) has a plurality of upwardly inclined adjacent segments (28,30) positioned in said air duct (14), said segments (28,30) being interconnected in series thereby forming one continuous upwardly inclined heat exchanger; each of said segments (28,30) has an equal plurality of conduits (42) wherein respective conduits from said adjacent segments are interconnected in series thereby forming a plurality of continuous upwardly inclined conduits, said conduits (42) being connected to said firebox (22) for receiving said combustion products, said conduits (42) also being coupled to said reservoir (18) for draining combustion product condensate therein; and the air blower (34) is arranged to force air down said air duct (14) sequentially over said segments (28,30). Apparatus according to claim 8, characerised by means for spraying water particles into said firebox. Apparatus according to claim 8, characterised by means (162) for forcing a mixture of air and fuel into said burner (150). Apparatus according to claims 2 and 3, characterised in that said burner is positioned in a firebox (22); said dew point raising means comprises a water reservoir (18) positioned in said firebox (22) wherein combustion products from said burner (16) are directed over said water reservoir (18) thereby raising the dew point and lowering the temperature of said combustion products; the recuperative heat exchanger (26) has a serpentine shape, said heat exchanger being positioned in said air duct (14) at a continuous upward slope; said heat exchanger (26) having a plurality of tubes (42) surrounded by thermal transfer fins, each of said tubes (42) being connected to said firebox (22) for receiving said combustion products, said tubes (42) also being coupled to said water reservoir (18) for draining combustion product condensation therein; and said air blower (34) communicates with said air duct (14) for forcing air down said duct (14) and across said heat exchanger (26). Apparatus according to claim 11, characterised in that said burner (16) comprises a radiant burner for generating both combustion products and radiant energy. Apparatus according to claim 11, characterised by means for atomizing water within said firebox (22). Apparatus according to claims 2 and 3, characterised in that said dew point raising means comprises a water reservoir (18) positioned in a firebox (22), the burner (16) being positioned in said firebox (22) and directing combustion products against said water reservoir (18) thereby elevating the dew point and lowering the temperature of said combustion products; said air duct (14) is vertical; an exhaust vent (66) communicates with said air duct; the recuperative heat exchanger (26) has a first upwardly inclined segment (28) connected in series to a second upwardly inclined segment (30), said second segment (30) being folded back over said first segment (28), and said heat exchanger (26) having a plurality of substantially parallel upwardly inclined tubes (42) surrounded by metal fins in heat transfer relationship therewith, each of said tubes (42) having an inlet end coupled to said firebox (22) and an outlet end coupled to said exhaust vent (66) for transferring said combustion products therebetween, said inlet ends also being coupled to said water reservoir (18) for draining combustion product condensate therein; and said air blower (34) communicates with said air duct (14) for forcing air down said duct (14) across said heat exchanger (26), said air being preheated by thermal transfer from said second heat exchanger segment (30), said preheated air being further heated by thermal transfer from said first heat exchanger segment (28). Apparatus according to claim 14, characterised in that the dew point is elevated sufficiently for said combustion products to condense along the entire length of said heat exchanger tubes (42). Apparatus according to claim 14, characterised by an exhaust blower (52) coupled to said exhaust vent (66) for drawing said combustion products from said heat exchanger (26).

Inventors: CLAWSON, LAWRENCE G.
Applicants: RAYTHEON COMPANY
Application year: 1986
Main classification: F24H3
IPC codes: F24H1, F24H3, F24H8