Abstract:
A heat exchanger having a frame and a combustion tube mounted to the frame wherein the combustion tube is configured to contain a flame produced from a fuel/air mixture introduced therein, and the combustion tube is configured to exhaust combustion products of the fuel/air mixture, the improvement comprising an insert coupled to the frame and having a longitudinal axis extending along and within the combustion tube wherein the insert has a triangular cross-section normal the longitudinal axis and configured to intersect a periphery of the flame. A method of manufacturing a heat exchanger is also provided.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The invention is directed, in general, to heat exchangers and, more specifically, to an insert for a furnace heat exchanger having improved suppression characteristics of the production of nitrogen oxides. 
       BACKGROUND OF THE INVENTION 
       [0002]    Combustion heaters of conventional heating systems often employ tubular combustion chambers wherein air is mixed with a gaseous fuel, the mixture is burned, and the combustion products are directed to a flue and ultimately to an exhaust. Air to be conditioned is usually returned from a living/working space and passed over the tubular combustion chambers, taking on heat from the combustion chambers and then the air is routed back to the living/working space. As a result of the combustion process, combustion systems normally generate gaseous combustion products which include oxides of nitrogen (NO x ) which are vented to the atmosphere as flue gas. It is desirable to limit these NO x  emissions since NO x  is considered a pollutant and combustion systems sold in certain jurisdictions must meet strict NO x  emission standards. 
         [0003]    One technique for limiting NO x  emissions from a combustion system is to control peak combustion flame temperatures that contact the tubular combustion chambers as well as limiting the residence times at these peak combustion flame temperatures to minimize the formation of NO x . It has been known that peak combustion flame temperatures can be controlled by placing a flame holder inserted into the combustion tube to substantially contain the flame, thereby substantially preventing the flame from direct contact with the combustion tube. Tests have shown that such inserts having a substantially square cross-section will just meet the South Coast NO x  emissions requirements. Some prior art have gone so far as to state that theoretical work confirmed by experiments indicate that the precise shape of the flame holder is not critical; thereby implying that results will be substantially equivalent among all shapes. 
         [0004]    Accordingly, what is needed in the art is a device that provides a significant further reduction in furnace NO x  emissions over the conventional art. 
       SUMMARY OF THE INVENTION 
       [0005]    To address the above-discussed deficiencies of the prior art, one aspect of the invention provides a heat exchanger having a frame and a combustion tube mounted to the frame wherein the combustion tube is configured to contain a flame produced from a fuel/air mixture introduced therein and to exhaust combustion products of the fuel/air mixture. The improvement comprises an insert coupled to the frame and having a longitudinal axis extending along and within the combustion tube wherein the insert has a triangular cross-section normal the longitudinal axis and configured to intersect a periphery of the flame. A method of manufacturing a heat exchanger is also provided. 
         [0006]    The foregoing has outlined certain aspects and embodiments of the invention so that those skilled in the pertinent art may better understand the detailed description of the invention that follows. Additional aspects and embodiments will be described hereinafter that form the subject of the claims of the invention. Those skilled in the pertinent art should appreciate that they can readily use the disclosed aspects and embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the invention. Those skilled in the pertinent art should also realize that such equivalent constructions do not depart from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    For a more complete understanding of the invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0008]      FIG. 1A  illustrates a partially-exploded isometric view of a burner/heat exchanger constructed according to the principles of the present invention; 
           [0009]      FIG. 1B  illustrates an end view of the vestibule panel with the burner assembly of  FIG. 1A  removed and showing one insert; 
           [0010]      FIG. 2  illustrates a sectional view of the heat exchanger along plane A-A of  FIG. 1B  with the burner shown and an inset of the insert in relation to the combustion tube; 
           [0011]      FIG. 3  illustrates an isometric view of the insert of  FIG. 1 ; and 
           [0012]      FIG. 4  illustrates a graph of NO x  test results for a variety of inserts for comparison. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring initially to  FIG. 1A , illustrated is a partially exploded isometric view of a burner/heat exchanger  100  constructed according to the principles of the present invention. The burner/heat exchanger  100  comprises a frame  110 , at least one combustion tube  120 , at least one insert  130  and a burner assembly  140 . In a preferred embodiment, the burner/heat exchanger  100  comprises a plurality of combustion tubes  120   a - 120   g , each combustion tube  120   a - 120   g  having an insert  130  therein. In one embodiment, the frame  110  comprises a vestibule panel  111  coupled to the combustion tubes  120   a - 120   g  and a corbel plate  112  coupled to burners  113   a - 113   g . The plurality of combustion tubes  120   a - 120   g , inserts  130  (collectively), and vestibule panel  111  may be collectively referred to as a heat exchanger  150   
         [0014]    Referring now to  FIG. 1B , illustrated is an end view of the vestibule panel  111  with the burner assembly  140  of  FIG. 1A  removed and showing one insert  130 . Of course, it is understood that for maximum efficiency each combustion tube  120   a - 120   g  would have a similar insert  130 . In this view it can be seen that the insert  130  comprises three sides  131 - 133  and a tab  134 . The insert  130  is coupled to and supported by the tab  134  that is coupled to the frame  110  in any conventional manner. In one embodiment, the tab  134  is captured between the vestibule panel  111  and the corbel plate  112 . 
         [0015]    Referring now to  FIG. 2 , illustrated is a sectional view of the heat exchanger  150  along plane A-A of  FIG. 1B  with the burner  113   a  shown and an inset of the insert  130   a  in relation to the combustion tube  120   a  and primary combustion zone  231 . In this particular view the insert is specifically insert  130   a . In this sectional view, it can be seen that the tab  134  is captured between the vestibule panel  111  and the corbel plate  112  thereby supporting the insert sides  131 - 133  (side  132  not visible) above the inside bottom, and below the inside top, of the combustion tube  120   a . Therefore, only the tab  134  contacts any part of the frame  110  or the combustion tube  120   a . Note that a centerline  235  of the insert  130   a  is in line with a center line  114  of the burner  113   a . In the insert, it can more readily be seen that the insert sides  131 - 133  are prevented from contacting any portion of the combustion tube  120   a . A secondary combustion zone  232  occurs within the insert  130   a  wherein a periphery  233  of the secondary combustion zone  232  is intersected by the insert  130   a.    
         [0016]    Referring now to  FIG. 3 , illustrated is an isometric view of the insert  130   a  of  FIG. 1 . The insert  130   a  comprises first, second and third sides  131 - 133  forming a substantially equilateral triangular cross-section and a tab  134 . Note the substantially equilateral triangular cross-section normal to the centerline  235 . In one embodiment, the first, second and third sides  131 - 133  are porous, e.g., a wire mesh. In a preferred embodiment, the first, second and third sides  131 - 133  comprise a stainless steel, e.g., stainless steel alloy 310. Generally speaking, stainless steel is defined as an iron-carbon alloy with a minimum of 10.5% chromium content according to the American Iron and Steel Institute. More specifically, stainless steel alloy 310 alloy comprises between 24.0% and 26.0% chromium, between 19.0% and 22.0% nickel, and maximums of: 0.25% carbon, 2.0% manganese, 1.5% silicon, 0.045% phosphorus, and 0.030% sulfur. Stainless steel alloy 310 is especially suited for use in high temperature applications as it resists oxidation well at temperatures up to 1150° C. 
         [0017]    Referring now to  FIG. 4 , illustrated is a graph of NO x  test results for a variety of inserts for comparison. Conditions for testing were as follows. High fire and low fire refer to a two-stage burner/heat exchanger controlled by the gas valve. High fire condition is defined as 150 KBtu/hr; while low fire is defined as 105 KBtu/hr. References to percentages refer to the input of gas to the burner with 100% being the amount of gas for which the heat exchanger is designed and name plated as. References to 112% gas input refer to an American National Standards Institute (ANSI) American National Standard (ANS) 21.47, Section 2.8 requirement for over-fired heat exchangers. That is a maximum of 40 ng/J of NO x  emission with a carbon monoxide (CO) level not exceeding 400 ppm corrected air free. All cylindrical tubes were alloy 304 stainless steel (SS) with identical wall thickness of 0.045 in. The insert of the present invention comprised alloy 310 stainless steel mesh. 
         [0018]    As a standard against which the inserts can be judged, the first column shows that the NO x  emissions from a burner/heat exchanger without an insert were measured at 64 ng/J of NO x  at a high fire condition. Column two shows that with a cylindrical insert having a 1.25″ diameter and a 6″ length, the burner/heat exchanger produced 47 ng/J of NO x  at low fire conditions. Column three shows that for the same physical configuration as in column two, the burner/heat exchanger produced 45 ng/J of NO x  at 5% below a high fire condition. Therefore, a prior art insert of cylindrical design will significantly reduce NO x  emissions but not enough to meet a South Coast Air Quality Management District (SCQAMD) when applying the ANSI standard. By comparison, columns four and five show that with a cylindrical insert having a 1.25″ diameter and an 11″ length, the burner/heat exchanger produced 41 ng/J of NO x  at both low fire and high fire conditions—not quite meeting the SCAQMD standard. Therefore, it is clear that the length of the insert has a measurable effect on the effectiveness of the insert. 
         [0019]    Columns six and seven show that with a cylindrical insert having a 1.00″ diameter and a 6″ length, the burner/heat exchanger produced 41 ng/J of NO x  at low fire and 40 ng/J of NO x  at high fire. However, the 304 SS failed during this test, likely due to the smaller diameter causing impingement of a greater surface of the flame on the insert. Columns eight and nine show that for a cylindrical insert having a 0.75″ diameter and a 6″ length, the burner/heat exchanger produced 40 ng/J of NO x  at low fire and 42 ng/J of NO x  at high fire. Therefore, while a cylindrical insert does work to reduce emissions, the reduction is usually not quite enough to meet the SCAQMD Standard. 
         [0020]    Columns  10  and  11  show that for a rectangular/square insert having a 1.125″ side and a 10″ length, the burner/heat exchanger produced 39 ng/J of NO x  at low fire and 36 ng/J of NO x  at high fire, thereby meeting the SCAQMD Standard. This configuration approximates the prior art wherein it was stated that shape of the insert was not a governing factor in the performance of the insert. 
         [0021]    Columns  12  and  13  show that for a triangular insert having a 1.5″ side and a 10″ length, the burner/heat exchanger produced 35 ng/J of NO x  at low fire—a more than 10 percent improvement over the prior art at low fire. Additionally, with the triangular insert the burner/heat exchanger produced 30 ng/J of NO x  at high fire—an almost 17 percent improvement over the prior art at high fire. Thus, it has been shown that the cross sectional shape of the insert does have an effect on the performance in that rectangular/square inserts are better than round cross sections. Furthermore, triangular cross sectional inserts are significantly better performers compared to either rectangular/square or round cross sections. 
         [0022]    Thus, a triangular cross section heat exchanger insert has been described and shown to be significantly better at reducing NO x  emissions than any prior art despite statements in the prior art that cross sectional shape does not affect performance. The true benefit of using the triangular cross section heat exchanger insert is that furnaces now do not have to be de-rated in allowable gas flow in order to meet the SCAQMD Standard. 
         [0023]    Those skilled in the art to which the invention relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments without departing from the scope of the invention.