Recuperative furnace

Abstract: Combustion products enter at an inlet (270) and flow across a water reservoir (274) thereby becoming elevated in dew point before flowing upwards through a heat exchanger (260) to a flue opening (272). Liquid to be heated is forced through the heat exchanger tubes (262) by a pump (264) and is heated both by transfer of sensible heat and heat of condensation from the combustion products. Condensation forms on the fins (276) of the exchanger (260) and flows down to the reservoir (274) thereby preventing corrosion.

Description: The invention relates to apparatus for heating a liquid. 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. Recuperative, or condensing, furnaces are furnaces which transfer both sensible heat and heat of condensation from combustion products. 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. 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 increase 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 exchanger 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 reoccur. 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. GB-A-2031572 describes an apparatus for exchanging heat between combustion products and a liquid 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. According to one aspect of the invention there is provided an apparatus for heating a liquid, comprising: means for providing hot combustion products; a water reservoir coupled to said providing means wherein said hot combustion products flow across said reservoir and thereby become elevated in dew point and lowered in temperature; and means positioned above said water reservoir for transferring sensible heat and heat of condensation from said dew point elevated combustion products to said liquid, the transferring means comprising a recuperative tube and fin heat exchanger, the said liquid being a liquid in the tube of the exchanger and means being provided for moving said liquid through the tube, the dew point elevated combustion products contacting the outside of the tube and fins of the exchanger, and the exchanger being arranged to allow condensate to flow back down the fins into the reservoir for recirculation by evaporation. According to another aspect of the invention there is provided a method of heating a liquid, comprising the steps of: (a) providing a recuperative heat exchanger having a tube surrounded by fins; (b) circulating said liquid through said tube; (c) locating a water reservoir below the recuperative heat exchanger; (d) contacting hot combustion gases with water in the water reservoir so as to elevate the dew point and lower the temperature of the combustion gases; and (e) directing the dew point elevated combustion gases upwardly across the recuperative heat exchanger, the dew point elevated combustion gases being provided at a given dew point and temperature and the liquid in the tube so cooling the combustion gases that substantially most of the water vapour in the combustion gases condenses within the heat exchanger and a steady-state cycle of condensation and evaporation is maintained in which condensate flows back down the fins into the reservoir at the same time as the hot combustion gases reevaporate water in the reservoir to be subsequently condensed in the heat exchanger. In a preferred embodiment of the invention, the heat exchanger has a plurality of conduits forming a flow path for the liquid to be heated, the liquid being heated by thermal transfer of sensible heat from the combustion products and from heat of condensation released by condensation forming on the heat exchanger, the condensation draining across the heat exchanger into the reservoir. When the combustion product providing means is a nonrecuperative heat exchanger, a polypropylene enclosure may be used for enclosing the heat exchanger and the water reservoir. Since combustion products have already been lowered in temperature through a nonrecuperative heat exchanger and water evaporation further lowered the combustion product temperature, an acid resistant plastic material such as polypropylene may be used. It may be preferable for the recuperative heat exchanger to be a coil wherein the liquid flows through the coil. Preferably, combustion product condensate drains across the heat exchanger in a direction opposite to the flow of the combustion products. The invention will now be described by way of example with reference to the accompanying drawings in which:- FIG. 1 is a partially broken away, right front perspective view of a recuperative heat exchanger embodiment for heating liquids; and FIG. 2 is a side elevation view of FIG. 1. In Figs. 1 and 2 a recuperative heat exchanger 260 embodying the invention is shown having tubes 262 connected between inlet water pump 264 and outlet pipe 266. Enclosure 268, having inlet coupling 270 and flue opening 272, is shown enclosing water reservoir 274 and heat exchanger 260. Water reservoir 274 is positioned at the bottom of enclosure 268 adjacent to inlet 270. Inlet 270 is coupled to the exhaust from a nonrecuperative heat exchange process (not shown) such as, for example, a nonrecuperative heat exchanger. Accordingly, combustion products from the nonrecuperative heat exchanger flow across reservoir 274 thereby becoming elevated in dew point. Combustion products then flow across fins 276 of heat exchanger 260 into flue opening 272. The liquid forced through heat exchanger tubes 262 is heated by thermal transfer of sensible heat from the combustion products and by heat of condensation released by condensation forming on fins 276. Condensate flows downwards along fins 276, opposite the flow of combustion products, into reservoir 274, giving anti-corrosion advantages. Further, the low operating temperature of this embodiment enables the use of inexpensive acid resistant materials such as, for example, polypropylene for enclosure 268. The liquid to be heated may be circulated through a secondary heat exchanger to heat air. The liquid may also be water for use as domestic hot water. Although this embodiment is described as being coupled to a nonrecuperative heat exchanger, those skilled in the art will recognize that it may also be coupled to the exhaust of any apparatus which burns hydrocarbon fuels. It may also be coupled directly to a burner. In conclusion, the embodiment described provides 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.

Claims: An apparatus for heating a liquid, comprising: means (270) for providing hot combustion products; a water reservoir (274) coupled to said providing means (270) wherein said hot combustion products flow across said reservoir (274) and thereby become elevated in dew point and lowered in temperature; and means (260) positioned above said water reservoir (274) for transferring sensible heat and heat of condensation from said dew point elevated combustion products to said liquid, the transferring means (260) comprising a recuperative tube and fin heat exchanger (262,276), the said liquid being a liquid in the tube (262) of the exchanger and means (264) being provided for moving said liquid through the tube (262), the dew point elevated combustion products contacting the outside of the tube (262) and fins (276) of the exchanger, and the exchanger (260) being arranged to allow condensate to flow back down the fins (276) into the reservoir (274) for recirculation by evaporation. Apparatus according to claim 1, characterised in that said combustion products providing means comprises a burner. Apparatus according to claim 1, characterised in that said combustion products providing means comprises a nonrecuperative heat exchanger. Apparatus according to claim 3, characterised by a polypropylene enclosure (268) for enclosing said water reservoir and said heat exchanger. A method of heating a liquid, comprising the steps of: (a) providing a recuperative heat exchanger (260) having a tube (262) surrounded by fins (276); (b) circulating said liquid through said tube (262); (c) locating a water reservoir (274) below the recuperative heat exchanger (260); (d) contacting hot combustion gases with water in the water reservoir (274) so as to elevate the dew point and lower the temperature of the combustion gases; and (e) directing the dew point elevated combustion gases upwardly across the recuperative heat exchanger (260), the dew point elevated combustion gases being provided at a given dew point and temperature and the liquid in the tube (262) so cooling the combustion gases that substantially most of the water vapour in the combustion gases condenses within the heat exchanger (260) and a steady-state cycle of condensation and evaporation is maintained in which condensate flows back down the fins (276) into the reservoir (274) at the same time as the hot combustion gases reevaporate water in the reservoir (274) to be subsequently condensed in the heat exchanger (260).

Inventors: CLAWSON LAWRENCE G, CLAWSON, LAWRENCE G.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1990
Main classification: F24H1
IPC codes: F24H1, F24H3, F24H8