Recuperators For Incinerators

Abstract

A compact, low cost heat recuperator for incinerators and the like having one or more heat exchange tubes with internal and external fins, positioned within an outer tubular shell. Means at one end of the shell hold the heat exchange tubes in fixed position, and an expansion seal is provided at the other. Means are provided for conducting exhaust gases through the heat exchange tubes and process air around the heat exchange tubes for preheating. Connecting means are disclosed for attaching the recuperator assembly to an existing incinerator so that minimum modifications are required.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to the field of heat recuperators for incinerators, furnaces, and the like. More specifically, the present invention relates to a compact recuperator adapted for incorporation into an existing incinerator, requiring a minimum of modifications thereof.

Recuperators are widely used on furnaces in order to increase their efficiency and reduce fuel cost. Since the heat of the exhaust gases escaping out the chimney represents a loss, the recuperator is used to recover a good deal of this heat by using the exhaust gases to preheat the air being fed into the furnace. Less fuel is then required to heat the preheated air up to the operating temperature of the furnace. In the prior art, many different configurations have been used for recuperators. Many of these recuperators are very elaborate in design and are very expensive in construction, and this is especially true of recuperators designed for large furnaces, where the savings in fuel costs can be substantial. However, in medium sized and small sized furnaces or incinerators, recuperators are often not used because of the additional initial expense of the recuperator. Another stumbling block standing in the way of adding recuperators to existing incinerators is the fact that many prior art recuperators are integral with their furnaces, and would therefore require extensive modification to the incinerator for installation.

The increasing concern of the public over air pollution, and new, tighter laws governing factory emissions have placed new emphasis on the need for low cost and highly efficient recuperators for moderate sized incinerators. For example, certain industries such as paint and chemical industries are faced with the problem of disposal of noxious fumes or process air which are generated as a waste product, and which were formerly vented to the atmosphere. In many cases, buring of the fumes at high temperature is the only reasonable method of disposal. A low cost and highly efficient recuperator is necessary in order to keep costs down.

SUMMARY OF THE INVENTION

The present invention provides a recuperator for an incinerator which is both low in cost and very efficient in operation. Further, a recuperator according to the present invention can be added onto an existing incinerator with a minimum of modifications, or can be incorporated into new incinerators with a minimum of redesign.

According to the present invention there is provided a recuperator assembly for an incinerator, comprising an elongated generally cylindrical outer shell with one or more inner tubular members positioned within the outer shell and parallel therewith. The space between the inner tubular members and the outer shell defines a heating passage. The tubular member has a plurality of heat exchanging fins projecting both inside, and outside of the inner tube in the heating passage. The shell has an intake port at one end and an outlet port at the other. Means are provided for connecting the recuperator assembly to an incinerator. Means are provided for connecting the inner tubular member to receive exhaust gases from the incinerator, and further means are provided for connecting the outlet port to the air intake of the incinerator, so that process air may be preheated by the exhaust gases prior to combustion.

In one embodiment, a single heat exchange tube is used, and the relationship in size between the heat exchange tube and the shell is such that the shell fits snuggly around the external fins of the heat exchange tube so that process air in the heating passage is forced around the fins for efficient heat transfer. In another embodiment, a plurality of heat exchange tubes are used, and are positioned in fixed relationship within the shell by a tube sheet. One or more baffles are positioned inside the shell to direct process air flow transversely of the heat exchange tubes in this embodiment.

In one method of mounting, a flange at one end of the shell is used for vertical support and attachment of the recuperator assembly on the top of an incinerator. In a horizontal method of mounting the frame supports one end of the recuperator assembly, while a supporting duct connects the other end to the recuperator.

DESCRIPTION OF THE DRAWINGS

In the drawing,

FIG. 1 is a side elevation of an incinerator and recuperator according to the present invention, with portions thereof broken away to illustrate certain features thereof;

FIG. 2 is a perspective view of a portion of a heat exchanger tube showing the external fin arrangement;

FIG. 3 is an enlarged fragmentary detail showing the means for positioning the heat exchange tube within the outer shell;

FIG. 4 is an enlarged fragmentary detail showing the means for connecting the recuperator assembly to the incinerator;

FIG. 5 is a side elevation view showing a horizontal positioning of a recuperator assembly according to the present invention;

FIG. 6 shows a detail of an expansion joint used in the horizontal mounting embodiment of FIG. 5; and

FIG. 7 is a cross sectional view of a multi-tube recuperator according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, reference numeral 10 generally designates a recuperator according to the present invention. Recuperator 10 is shown mounted upon an incinerator generally designated 11. And exhaust stack (not shown) can be positioned on top of the recuperator.

The incinerator 11 comprises a housing 13 which defines an inner combustion chamber 14. Intake 15 introduces fumes or process air into the combustion chamber, and the burned exhaust gases pass upward from the combustion chamber through exhaust flu 16. Incinerator 11 also includes a burner assembly 12 for supplying fuel-air mixture to the combustion chamber, according to the particular design of the incinerator and the type of fuel which is intended to use. At the top of the housing 13 of incinerator 11, is a mounting flange 17.

Recuperator 10 comprises an outer shell 20 which forms the housing for the recuperator. Shell 20 is a generally elongated cylindrical steel tube. Positioned coaxially within shell 20 is an inner, heat exchange tube 21. Since heat exchange tube 21 is in direct contact with the hot exhaust gases, it is preferably made from chrome-moly steel. Heat exchange tube 20 has a plurality of heat exchange fins 22 welded to the inside thereof, and projecting inwardly. Tube 21 also has a plurality of heat exchange fins 23 welded to the outside thereof. The space between shell 20 and heat exchange tube 21 defines a heating passage, as indicated by flow arrow 24. In the single tube embodiment, shown in FIG. 1, heat exchange tube 21, fins 23, and shell 20 are sized so that shell 20 fits closely around fins 23.

The external fin structure of the heat exchange tube is shown better in FIG. 2. Fins 23 are made from a serrated strip which is spirally wound around and welded to the outer surface of tube 21. The serrations or notches divide the strip into individual fins. The notches on succeeding turns of the spiral tend to align so as to form a maze of spiral and zigzag passages. Since the outer shell 20 fits snuggly around fins 23, process air passing through the heating passage, in a generally longitudinal direction to the heat exchange tube, is forced into flow patterns which create maximum contact between the process air and fins 23, thereby helping a maximize efficiency of heat transfer. Since the external fins are usually not subjected to high temperatures, the normal material for their construction is carbon steel. In the preferred embodiment, the external fins are approximately one-eighth inch thick by one and a half inch high.

Internal fins 22 are made of lengths of bar stock, which are welded to the inner wall of tube 21. Fins 22 are welded at an angle and spaced in a spiral pattern so as to impart a spiral flow to the exhaust gases passing therethrough. This spiral flow ensures that maximum contact and heat transfer will be realized. Since the internal fins 22 are subjected to highest temperature and also to corrosion and errosion from the exhaust gases, they are preferably made of chrome-moly steel or stainless steel, depending upon the economics and life expectancy of the unit. In the preferred embodiment, internal fins 22 are approximately one-quarter inch by one inch by three inches.

Due to the plurality of internal fins 22, recuperators according to the present invention are not intended for use with incinerators which put out large amounts of dirt or soot, because of the likelihood of fouling or plugging. Rather, recuperators according to the present invention are better adapted for use on incinerators for burning off process air or toxic fumes.

Referring again to FIG. 1, an inlet port 35 is provided near the top of shell 20. Near the bottom of shell 20 is an outlet port 36. Air admitted to inlet port 35 travels down through the air passages as indicated by flow arrow 24 and exits at outlet port 36. An intake duct 38 connects from port 36 to the incinerator intake 15. An expansion joint is provided at 37 to allow for dimensional changes caused by heating. Shell 20 which is the outer housing of the recuperator is mounted to incinerator 11 by means of a flange 40, and is supported by a plurality of vertical support braces 41. The details of the mounting are shown more clearly in FIG. 4. The inside of incinerator housing 13 is lined with a refractory material 42 for protection from the heat generated within the combustion chamber 14. Annular flange 17 is welded to the top of housing 14. Another annular flange 40 is welded to the bottom of shell 20, at 43. Flanges 17 and 40 are held together by bolt and nut assembly 44 at a plurality of locations around the circumference thereof. Brace 41 helps give the vertical support needed for the structure.

FIG. 4 also shows the details of the expansion seal generally indicated by reference numeral 50. Member 51 in conjunction with the upper portion of incinerator housing 13 defines an annular channel around the top of the incinerator. The bottom of the heat exchange tube 21 extends loosely into this channel but is not attached to member 51, so that it can expand when heated. The seal is a accomplished by a sand packing 52 which fills the channel including the space between member 51 and the bottom of heat exchange tube 21.

As shown in FIGS. 1 and 3, a cap 30 is provided at the top of shell 20 for fixing the positioning of heat exchange tube 21 and shell 20. Cap 30 comprises a flange 54 and a plurality of braces 56. Flange 54 is welded to heat exchange tube 21, as shown at 47. A flange 45 is welded to the top of shell 20. A plurality of nut and bolt assemblies 55 hold flanges 54 and 45 together. Cap member 30 thus serves to fix the inner tube and outer shell in position with respect to each other. Since they are not fastened together at the bottom end, but rather are sealed by expansion seal 50, the heat exchange tube can be removed by undoing bolt and nut assemblies 55 and lifting tube 21 out the top.

A preferred installation of the heat recuperator assembly of FIG. 1 would one in which a portion of the stack of an existing incinerator would be removed. The existing incinerator would then be equipped with a sand seal and flange as shown in FIG. 4, for attaching the recuperator. If the incinerator is not strong enough it would of course be necessary to give additional support to the recuperator. A stack can then be attached to the top of heat exchange tube 21 which extends through the top of cap 30, by any convenient means.

If space would not permit a vertical installation, a horizontal installation such as shown in FIG. 5 could be used. In FIG. 5, a single tube recuperator assembly 60 is shown which is substantially identical to recuperator assembly 10 of FIG. 1. For purposes of clarity, like elements are given the same reference numerals as in FIG. 1. Horizontal mounting is accomplished through a frame 61 which supports one end of recuperator 60, and a supporting duct 62 which connects the other end of recuperator 20 to incinerator 11. Supporting duct 62 also functions to convey exhaust gases from the incinerator to the heat exchange tube 21. An exhaust duct 63 at the other end of recuperator 60 conveys exhaust gases to a stack (not shown). Because it is not feasible to use a sand expansion seal in the horizontal arrangement, an alternate expansion packing seal is used. In the embodiment of FIG. 5, heat exchange tube 21 and shell 20 are fixed together by an internal extension of flange 64, in a manner similar to that shown in FIG. 3. The expansion seal is used at the other end of the recuperator, as shown in FIG. 6, which is taken along lines 6--6 in FIG. 5.

In FIG. 6, heat exchange tube 21 has an annular bar which may be welded into place. A flange 71 is welded to the end of shell 20. Flange 71 has an annular groove 72 in which is placed the expansion packing, which may be asbestos-graphite material. A retainer ring 73 is fixed to flange 71 by nut and bolt assemblies 74. The seal is provided by annular bar 70 and the expansion packing, which also allows for expansion due to differential heating of the members.

In FIG. 7, reference numeral 110 generally designates a multiple tube recuperator assembly according to the present invention. Recuperator 110 comprises a shell member 120 which is similar to shell 20 of the embodiment of FIG. 1. A plurality of heat exchange tubes 121 are positioned within shell 120 and aligned generally parallel therewith. Heat exchange tubes 121 have internal and external fins and may be identical to heat exchange tube 21 of FIG. 1. Heat exchange tubes 121 are positioned into a bundle by upper tube sheet 122 and lower tube sheet 123. These tube sheets have a plurality of apertures for receiving and holding the ends of heat exchange tubes 121, which may be welded in place. Although only four heat exchange tubes are shown in FIG. 7, it will be understood that any number could be used and that any convenient arrangement such as a circular or rectangular pattern could be used. A pair of baffles 124 and 125 are positioned within shell 120 around heat exchange tubes 121, thereby forcing process air entering inlet port 135 to flow generally transversely of the heat exchange tubes and out outlet port 136. In use, ducts are provided for conveying the process air to port 135, and for conveying the heated process air from port 136 to the intake of the incinerator.

The entire assembly of heat exchange tubes 121, baffles 124 and 125 and upper and lower tube sheets 122 and 123 are fixed in position within shell 120 at the upper end. A flange 145 is welded around the top of shell 120. A cap member 130 has a matching flange 154. Tube sheet 122 extends between the two flanges and is bolted in place by a plurality of bolt assemblies. The tubes can therefore be removed by removal of cap 130 and lifting the tube assembly out through the top. A sand expansion seal similar to the one shown in FIG. 4 is provided at the lower end of the recuperator. A member 151 and the outer wall 13 of the incinerator cooperate to define a channel. A skirt 128 is welded around tube sheet 123, and extends downwardly into the channel. Sand packing 52 fills the channel to make the seal. A flange 140 is welded around the end of shell 120, to match with flange 17 at the top of incinerator 11. A plurality of bolt assemblies 144 connect the recuperator assembly to the incinerator. The bottom side of tube sheet 123 is covered with a refractory material 127, leaving holes aligned with heat exchange tubes 121 for passage of exhaust gases. The refractory material protects the tube sheet from the high temperatures developed in the combustion chamber.

The use of fins on both surfaces of the heat exchange tube, together with the compact configuration as illustrated in the foregoing paragraphs and drawings, results in a highly efficient and low cost recuperator.

Claims

I claim:

1. A recuperator assembly adapted for adding on to an existing incinerator between the main housing and exhaust stack thereof for preheating process air to be burned therein comprising:

a. a generally cylindrical elongated shell;

b. means attached to the bottom end of said shell for mounting said shell in vertical position on the main housing of the incinerator over the exhaust flu thereof;

c. a heat exchange tube having internal and external fins, said internal fins arranged and configured to impart a swirling flow to gases passing through said heat exchange tube;

d. means attached to the top end of said shell for mounting said heat exchange tube coaxially within said shell in alignment with the exhaust flue, the inside diameter of said shell being slightly larger than the outside diameter of said heat exchange tube plus fins, to define a heating passage therebetween;

e. means defining a sand expansion seal around the bottom of said shell and heat exchange tube;

f. a process air inlet port in said shell near the top end thereof;

g. a process air outlet port in said shell near the bottom end thereof; and

h. means for conducting the preheated process air from said outlet port to the intake of the incinerator.