US3566811A

US3566811A - Air damper

Info

Publication number: US3566811A
Application number: US812951A
Authority: US
Inventors: David J Tidd; Lawrence E Carroll
Original assignee: Alcan Research and Development Ltd; Loftus Engineering Corp
Current assignee: Alcan Research and Development Ltd; Loftus Engineering Corp
Priority date: 1969-03-12
Filing date: 1969-03-12
Publication date: 1971-03-02
Anticipated expiration: 1988-03-02

Abstract

There is disclosed a damper for use in furnace flues wherein opposed sheets or curtains of air are directed from ducts on opposite sides of a flue toward the center at a downward slope, and regulation thereof by controlling the damper by varying the air flow to the ducts.

Description

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[45] Patented Mar. 2, 1971 [7 3] Assignees said Carroll assignor to Alcan Research and 110/179 263/40 110/160X l 10/160X 805,295 11/1905 Hofmann..i........ 2,979,322 4/1961 Dai1ey................. Developmem 3,215,501 1 1/1965 Phillips 9 (311MB; 3,364,286 1/1968 Hanks said Tidd asslgnor to Lottus Engineering Corporation Primary Examiner-Edward G. Favors Attorney-Parmelee, Utzler & Welsh Pittsburgh, Pa.

ABSTRACT: There is disclosed a damper for use in furnace flues wherein opposed sheets or curtains of air are directed from ducts on opposite sides of a flue toward the center at a downward slope, and regulation thereof by controlling the damper by varying the air flow to the ducts.

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Attorneys.

PATENTED MAR 2197:

same or 3 v 3.566811 IMVENTQR. Dawd J. Tadd. BY Lawrence ECarrou. M W 7% Attorneys.

AER DAWER This invention relates to regulating the outflow of hot gases from a furnace or combustion chamber, and more specifically to the use of air in lieu of the usual mechanical devices heretofore commonly used. The invention is primarily intended for industrial uses, particularly where the flue gases are at a high temperature, such for example as the flue gases from metal meiting or metallurgical furnaces. While not limited to metallurgical furnaces, it will be herein described in connection therewith, but it will be understood that it may be used elsewhere.

It is desirable in many types of furnace operations to maintain a controlled pressure in the furnace. Melting operations, for example, may require a higher rate of combustion and a more free flow of spent combustion gases from the furnace than does a subsequent'step of holding the metal in a molten condition after the melting is completed and until the molten metal is to be used. Various types of dampers have been used to regulate the outflow of gasses from the furnace flue, and thereby control the pressure of the combustion gases in the furnace. These include the so-called butterfly type of damper, a sliding plate type, and a cap at the top of the furnace stack. Dampers of this type are customarily provided with internal water-cooling passages for the continuous circulation of cooling water therethrough where very hot gases are encountered. Water-cooling is attended with several disadvantages, not the least of which is the down-time that may result from leakage. The object of the present invention is to substitute an air curtain for the usual damper plate.

Consideration has been given to blowing air across the flue to provide a substitute for a metal element, but prior attempts have not heretofore proved to be successful. We have discovered that an effective controllable air curtain damper can be provided by correlating certain critical conditions that have heretofore not been recognized.

According to our invention, the flue passage is essentially four-sided, that is square or rectangular in section. Air ducts are provided on two opposing walls of the flue at about the same level. These ducts must be set back from the flue passage so that the ducts are out of the upflow of gases in the flue. Each has orifices arranged to project a sheet or curtain of air across the flue, and these curtains should be inclined down toward the center of the flue passage at an angle between about 45 and about 30, and they meet at about the midplane of the flue. Finally, each air curtain must sweep over a beveled or chamfered wall surface formed at the edge of the flue passage where the flue is offset to accommodate the air ducts, which, as above explained are set back from the flue passage.

Our invention may be understood by reference to the accompanying drawings showing a preferred embodiment thereof, and in which:

FIG. 1 is a vertical section through a flue with the damper applied thereto, the view being in the plane of line l-! of F 10.

FIG. 2 is a plan view of the damper of FIG. 1, the hood being omitted, the drawing being on a smaller scale;

E16. 3 is a fragmentary plan view on a larger scale;

FIG. 4 is a schematic view showing a typical arrangement for use in the aluminum industry where there is a melting furnace alongside a holding furnace; and

FIG. 5 is an explanatory table to assist one in adapting the invention to flue areas.

Referring to FIGS. 1 to 3 of the drawing, 2 designates the exterior shell of a rectangular flue with a refractory lining 3. Spaced above the refractory-lined flue is a hood 4 for allowing atmospheric air to enter and cool the gases flowing up the stack above the level of the refractory wall. The rectangular flue passage is designated 5.

There are two air ducts 6 anchored on the top of the refractory wall. Each is connected through a duct 7 to an air supply pipe 8. Air to the pipe 8 is supplied through a blower. Each of the two opposed air ducts 6 has closely'spaced air outlet openings 9 therealong for the full length of the long side of the flue passage. These openings are arranged to project air under pressure downwardly at an angle between about 30 and about 45 from the horizontal toward the longitudinal center plane of the flue. instead of individual holes, there may be provided a continuous slit for projecting the air flow laterally and downwardly but because of possible distortion of a slotted duct under repeated heating and cooling, spaced holes are preferred. I

The ducts 6 must be set back from the flue passage in order to be out of the direct upflow of combustion gases from the furnace, and the top of the refractory lining 3 is beveled or chamfered at 10 on a slope substantially parallel with the stream of air emerging from the holes 9 in the ducts 6. The air should leave the holes under sufficient pressure and in sufficient volume that the sheetlike curtains of air from the opposed ducts will meet at the midplane of the flue passage, and it is important that the air sweep smoothly over the chamfered surface of the refractory and that by the time the individual jets of air reach the inner edge of the chamfered surface, they shall have widened to a continuous curtain or stream across the full distance between the other two walls of the flue, and more or less retain that continuous curtain effected until the opposed streams intersect andcreate an area of turbulence at the midplane of the flue.

We have discovered that where there are opposed curtains of air meeting in this fashion, the furnace gases are deflected toward the two sidewalls of the flue along which the curtains of air are generated. If the air ducts are not recessed from the side of the flue, they are excessively heated especially when the air is shut off to provide a full open damper condition. If the air curtains are not projected downwardly they do not function satisfactorily and if the air ducts, though recessed are elevated above the top of the refractory to a height where chamfering of the refractory walls is not necessary, the rising currents of furnace gases will flow laterally under the curtain along each sidewall and defeat the purpose of the setting the ducts rearwardly from the flue. With this invention the inwardly-inclined chamfered surface is so positioned that there is a continuous curtain of air at the inner edge of the sloped refractory surface so that the rising flue gases cannot flow up the slope, but appear to be swept away from the sloping surface toward the center zone of turbulence and then escape to a level above the air curtain.

Referring to FlGS. 2, and 3 it will be seen how, in FIG. 3, the jets of air from the several openings widen out over the sloping surface, leaving free triangular open area between them. The base of the triangle so formed is the distance along the pipe between two adjacent holes and a line from the base to the apex in inches is the measure of open area between jets. This open area depends on several factors, such as the manifold pressure, the diameter of the hole 9, and how far the air must be projected from the duct to the midplane of the flue. The table shown in H6. 5 will enable one skilled in the art to select the proper hole size, blower, the dimension of the open space, the number of cubic feet of air per foot of duct length and the diameter of the duct. He should allow an adequate margin of safety. As an example, assume there is a flue section 4 ft. X6 in. A usual blower commonly provided for fuel burners for some furnaces develops 16 o.s.i. (ounces per square inch). Looking at table 2, he will see that with Vs-inch holes, this will give a projection of air for a distance of 17 inches from the opening which is more than adequate. He will see that /s.-inch holes spaced 1 inch from center-to-center forms an open space of 4 inches in length. Therefore he knows that the duct must be located a minimum of 4 inches on the slope back from the flue opening to have a continuous curtain of air at the plane of the nearest flue wall. Since the projection is 17 inches, he may locate the manifold further back than four inches and still have the air curtain extend to the opposite wall of the flue. Looking at table 8 he can determine that'with a manifold pressure of 16 o.s.i., with Via-inch holes on l-in'ch centers requires 17.5 c.f.m. per ft. of duct length. Following down the same column to table No. 4, it will be seen that with fi -inch holes on a 1-inch spacing, the duct 4 ft. in length should be a 2-inch diameter pipe.

In using the table, one generally figures a projection that will carry the air curtain across the full distance between walls if it were not intersected by the curtain from the opposing wall. In the case here assumed, the projection is 6 inches plus 4 inches, so that, as above stated, the ducts could be set further back and there would still be adequate coverage, that is, there is still a 7-inch excess. This allows effective dampering if the damper is to allow the outflow of flue gases at some intermediate level between full-off where no air curtain is provided to full-closed where the air curtain damper offers maximum but not complete restriction of outflow.

The table shown in FIG. is to facilitate one in adapting the invention to different flue areas and is not to be construed as limiting the invention to variations or exclude the use of elongated slits in place of holes, in which case the incomplete coverage factor might be entirely irrelevant. In the case of a continuous slit, there would be no open space and other factors would require adjustment, but these can be determined by experiment on the basis of the foregoing table as a guide to determine the procedure to follow.

Referring to FIG. 4, there is disclosed schematically a damper arrangement for use in the aluminum industry where a melting furnace is positioned close to a holding furnace. In this view, designates the flue of the melter, 21 the air curtain damper, and 22 the hood. The flue for the holder is designated 23, the air curtain damper is 24, and 25 is the hood. There is a single blower 26 with a duct 27 with two

branches

28 and 29. Duct 28 supplies air to the air curtain damper 21, and duct 29 supplies air to the air curtain damper 24.

There is a valve 30 in duct 28 and a similar valve 31 in duct 29. Each of these valves is operated by a separate control system to maintain the desired pressure in each furnace. Each system includes a probe 32 in the furnace responsive to the pressure of gases in the furnace, a pressure control recorder 33, a hydraulic relay or transducer 34 for converting signals from the probe to variations in hydraulic pressure, and an expansible chamber type of fluid pressure actuator 35, having a movable element that responds to pressure variations generated by the relay 2. The movable elements of the two actuators 35 are connected by a link to their

respective valves

30 and 31. This instrumentation per se is well known in the art, and has heretofore been used to control the furnace temperature where mechanical dampers have been employed, the actuators 35 in such cases being mechanically connected to a damper instead of an air valve.

Aside from the fact that an air curtain damper as herein described is not so directly exposed to hot furnace gases and therefore does not need to be frequently repaired, and continuously circulating water-cooling is unnecessary, with the attendant danger of leakage and possible down-time for repairs, the air curtain can be quickly replaced without necessarily shutting down the furnace. It will respond to variations in the air flow to the discharge duct to provide controlled response, and when the air flow is completely shut down, the ducts are out of direct flow of gases up the flue. Unlike a water-cooled damper in the closed position, it does not absorb heat, but it does desirably dilute the hot gases in the flue or hood above the plane of the damper.

The air damper may create a more nearly constant positive pressure within the furnace chamber in relation to the surrounding air pressure outside the furnace, as compared to a mechanical gate or butterfly which is much more responsive to variations of atmospheric conditions outside the furnace, and therefore less constant in its effect. It retards escaping flue gases longer to longer retain heat in the furnace, and thereby reduce fuel. It provides secondary air for the combustion of smoke resulting from unburned fuel escaping from the furnace, and by maintaining more uniform temperature, protects refractory furnace walls and roof to minimize thermal shock and improve the life of the refractory.

These and other ob ects and advantages result from the invention as herein disclosed.

We claim:

1. A flue damper comprising:

a. a flue having four substantially flat walls forming an enclosed flue passage with two walls at substantially right angles to the other two;

b. an air duct arranged along the full length of two opposed walls having openings therein arranged to discharge air therefrom at an angle that slopes downwardly so that air streams from the opposed ducts intersect at a level below the ducts and about midway between the two',

. said ducts being recessed from the vertical plane of the flue walls on which they are located;

d. the flue walls being sloped upwardly and outwardly from the flue passage toward the respective ducts at a level where the air discharge from the duct sweeps the said sloping surfaces whereby flue gases rising along said walls is confined against lateral flow toward the ducts;

e. means for supplying air under pressure to each of the ducts, the openings in the ducts being so arranged that there is a substantially continuous curtain of air projected crosswise into the flue from the inner edge of the sloping surface to at least the said midplane of the flue with the curtain being coextensive with the width of the flue wall.

2. The combination with a furnace flue having four walls forming a flue passage, of an air duct extending along each of two opposite walls recessed outwardly from the flue-forming faces of the walls and entirely clear of a straight line flow of gases up the flue, the walls of the flue in which the ducts are positioned being chamfered to form an upwardly and outwardly-sloping surface between the flue passage and said ducts, means for supplying air under pressure to the two ducts, the ducts having openings therein arranged to direct air down across the sloping chamfered surface and form a continuous curtain by the time the air reaches the lower edge of said surface to substantially preclude the up-flow of gases from the flue over said sloping surface, the curtain extending the full distance between the other two walls of the flue, said openings in the ducts comprising closely-spaced holes arranged to generate spaced jets of air with outwardly-narrowing open spaces between the jets, the ducts being recessed sufficiently from the flue passage so that the open spaces between the jets do not project beyond the inner edge of the sloping surface.

3. The combination defined in claim 2 wherein the means for supplying air to the ducts comprises a blower and a pipe for conveying air from the blower to both ducts, and valve means for variably controlling the flow of air to both ducts, said valve means being responsive to gas pressure in the furnace of which the flue is a part whereby the valve means are controlled according to pressure conditions in the furnace.

Claims

1. A flue damper comprising: a. a flue having four substantially flat walls forming an enclosed flue passage with two walls at substantially right angles to the other two; b. an air duct arranged along the full length of two opposed walls having openings therein arranged to discharge air therefrom at an angle that slopes downwardly so that air streams from the opposed ducts intersect at a level below the ducts and about midway between the two; c. said ducts being recessed from the vertical plane of the flue walls on which they are located; d. the flue walls being sloped upwardly and outwardly from the flue passage toward the respective ducts at a level where the air discharge from the duct sweeps the said sloping surfaces whereby flue gases rising along said walls is confined against lateral flow toward the ducts; e. means for supplying air under pressure to each of the ducts, the openings in the ducts being so arranged that there is a substantially continuous curtain of air projected crosswise into the flue from the inner edge of the sloping surface to at least the said midplane of the flue with the curtain being coextensive with the width of the flue wall.