HEAT EXCHANGER WITH NOX-REDUCING TRIANGLE

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

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

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.

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.

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

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.

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

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:

FIG. 1A illustrates a partially-exploded isometric view of a burner/heat exchanger constructed according to the principles of the present invention;

FIG. 1B illustrates an end view of the vestibule panel with the burner assembly of FIG. 1A removed and showing one insert;

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;

FIG. 3 illustrates an isometric view of the insert of FIG. 1; and

FIG. 4 illustrates a graph of NO_x test results for a variety of inserts for comparison.

DETAILED DESCRIPTION

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 120a-120g, each combustion tube 120a-120g having an insert 130 therein. In one embodiment, the frame 110 comprises a vestibule panel 111 coupled to the combustion tubes 120a-120g and a corbel plate 112 coupled to burners 113a-113g. The plurality of combustion tubes 120a-120g, inserts 130 (collectively), and vestibule panel 111 may be collectively referred to as a heat exchanger 150

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 120a-120g 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.

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 113a shown and an inset of the insert 130a in relation to the combustion tube 120a and primary combustion zone 231. In this particular view the insert is specifically insert 130a. 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 120a. Therefore, only the tab 134 contacts any part of the frame 110 or the combustion tube 120a. Note that a centerline 235 of the insert 130a is in line with a center line 114 of the burner 113a. 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 120a. A secondary combustion zone 232 occurs within the insert 130a wherein a periphery 233 of the secondary combustion zone 232 is intersected by the insert 130a.

Referring now to FIG. 3, illustrated is an isometric view of the insert 130a of FIG. 1. The insert 130a 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.

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.

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.

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.

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.

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.

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.

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.

Claims

1. In a heat exchanger having a frame and a combustion tube mounted to said frame, said combustion tube configured to contain a flame produced from a fuel/air mixture introduced therein and to exhaust combustion products of said fuel/air mixture, the improvement comprising:

an insert coupled to said frame and having a longitudinal axis extending along and within said combustion tube, said insert having a triangular cross-section normal said longitudinal axis, said insert configured to intersect a periphery of said flame.

2. The heat exchanger as recited in claim 1 wherein said triangular cross section is substantially an equilateral triangular cross section.

3. The heat exchanger as recited in claim 1 wherein said insert is porous.

4. The heat exchanger as recited in claim 1 wherein said insert comprises a wire mesh.

5. The heat exchanger as recited in claim 1 wherein said insert comprises an alloy of stainless steel.

6. The heat exchanger as recited in claim 1 wherein said insert comprises stainless steel 310.

7. The heat exchanger as recited in claim 1 wherein said insert comprises an alloy of iron, chromium and nickel.

8. The heat exchanger as recited in claim 1 further comprising a mounting tab coupled to said insert, said mounting tab configured to support said insert within, but not in contact with, said combustion tube.

9. A method of manufacturing a heat exchanger having a frame and a combustion tube mounted to said frame, said combustion tube configured to contain a flame produced from a fuel/air mixture introduced therein and to exhaust combustion products of said fuel/air mixture, the improvement comprising:

coupling an insert to said frame, said insert having a longitudinal axis extending along and within said combustion tube, said insert having a triangular cross-section normal said longitudinal axis and configured to intersect a periphery of said flame.

10. The method as recited in claim 9 wherein coupling includes coupling an insert wherein said triangular cross section is substantially an equilateral triangular cross section.

11. The method as recited in claim 9 wherein coupling includes coupling an insert wherein said insert is porous.

12. The method as recited in claim 9 wherein coupling includes coupling an insert wherein said insert comprises a wire mesh.

13. The method as recited in claim 9 wherein coupling includes coupling an insert wherein said insert comprises an alloy of stainless steel.

14. The method as recited in claim 9 wherein coupling includes coupling an insert wherein said insert comprises stainless steel 310.

15. The method as recited in claim 9 wherein coupling includes coupling an insert wherein said insert comprises an alloy of iron, chromium and nickel.

16. The method as recited in claim 9 further comprising coupling a mounting tab to said insert, and configuring said mounting tab to support said insert within, but not in contact with, said combustion tube.