US3779689A

US3779689A - Method and apparatus for non-polluting combustion of waste gases

Info

Publication number: US3779689A
Application number: US00216659A
Authority: US
Inventors: J Corble; H Koons; R Schwartz; H Glomm; R Reed; Zink J Smith
Original assignee: ZINC J CO
Current assignee: ZINC J CO; ZINC J CO US; KGI Inc
Priority date: 1972-01-10
Filing date: 1972-01-10
Publication date: 1973-12-18
Anticipated expiration: 1990-12-18

Abstract

A non-polluting waste-gas disposal system for processing plants or other operation subject to variable quantities of waste gas for disposal. The system includes a low-level burner normally adapted to handle the usual volumes of plant waste gas, required to be disposed, without visible flame, smoke or noise pollution. An elevated flare can be used in combination to consume gases in excess of the normal capacity of the low-level flare.

Description

United States Patent Reed et a1. Dec. 18, 1973 [5 METHOD AND APPARATUS FOR 3,273,627 9/1966 link et a1. 431/4 x NON-POLLUTING COMBUSTION 0F WASTE GASES 2,571,336 10/1951 Buckholdt 431/187 [75] Inventors: Robert D. Reed; John Smith Zink; 2,734,562 2/1956 Haberle et al. 431/5 X Robert E. Schwartz, all of Tulsa, Okla.; Horst Glomm, Frankfurt, Germany; John Corble SL Primary Examiner-William F. O Dea Albans, England; Harold F. Koons, Assistant Examiner-Peter D. Ferguson Tulsa, Attorney-Head & Johnson [73] Assignee: John Zinc Company, Tulsa, Okla.

[22] Filed: Jan. 10, 1972 57 CT [211 App]. No.: 216,659

A non-polluting waste-gas disposal system for processing plants or other operation subject to variable quan- [52] US. Cl 431/89, 431/5, 431/202, m f waste gas f di The system includes 3 431/350 low-level burner normally adapted to handle the usual [51] Int. Cl. F23n l/00 volumes f plant waste gas, required to be disposed, [58] Field of Search 431/4, 5, 6, 178, without visible flame, smoke or noise pollution An 1 431/179 188, 350 evated flare can be used in combination to consume gases in excess of the normal capacity of the low-level [56] References Cited flaw UNITED STATES PATENTS 1,807,977 6/l921 Frank 431/179 14 Claims, 9 Drawing Figures STEAM PATENTEU BEE 18 975 SHEET M !]F 4 WASTE GAS A ASTEAM .l FIG. '7

METHOD AND APPARATUS FOR NON-POLLUTING COMBUSTION OF WASTE GASES In many crowded areas of the world it is necessary, because of existing plant operation, to dispose of waste gases by burning or oxidizing, since the waste gases normally have no other utilitarian purpose. In order to maintain a plant in its existing community location, it is becoming increasingly necessary to provide such disposal without pollution of the atmosphere and without visible flame and noise and hence provide an acceptable disposal system to the surrounding community.

In addition, many plants, such as refineries, chemical processes and the like, form, at various stages of operation, variable quantities of gases which must be disposed. For example, in many processing operations the start-up or shut-down procedures typically will involve changes in the normal quantity of disposable gas until the efficient operation of the plant is reached. For one reason or another, many processes may be upset or overloaded, hence taxing the normal disposal system beyond its efficient operation. In the fuel burning art stable burning and complete combustion for safe operation can be expected when fuel flow ranges from two to 100 percent of design condition. When fuel flow falls below two percent of design flow there is great difficulty in securing safe and stable burning in a combustion space which is at high temperature level. At a temperature level of l50 F. or lower, safe, stable and complete burning in any typical burner or combustion system is difficult and virtually impossible. Because of the many operational variables found in many process plants it is vitally necessary that there be safe, stable and complete burning of any small or large waste gas volume in addition to stringent air-pollution regulations and safety. Because of widely varying quantities, the safe and efficient disposal is burdened. Some prior artisans have devised plural burner stages which operate sequentially upon reaching a given pressure but have the disadvantage of each stage starting at zero flow and these systems do not operate efficiently and economically SUMMARY It is the purpose and object of this invention to overcome the heretofore mentioned problems and provide a flare or disposal system for waste gas utilizing a lowlevel ground burning system which is substantially smokeless, noiseless, and non-polluting to the atmosphere. Such a low-level burner is designed to accept those quantities of gas for disposal under normal or non-normal operating conditions of the plant, and in some instances, may be used along or in combination with an elevated flare apparatus for consumption of excess quantities of gas beyond the rated capacities of the low-level burner.

The invention further provides and is directed to a flare stack which comprises one or a plurality of substantially low-level burner areas to receive and provide efficient combustion of variable quantities of waste gas, and an upper stack for the venting of the products of combustion. The inner diameter of the upper stack being substantially unrestrictive to the flow of the products of combustion, or in any event of design to create a maximum flow velocity immediately at the point of discharge.

To abate the noise of combustion flame, the invention further provides a screen either opposite each burner area, or completely encircling the lower level burner areas as a plenum chamber to abate the noise of combustion, visual display of the flame, and to prevent air pressure differentials caused by wind forces.

The invention further provides a waste gas flow staging method whereby a predetermined minimum pressure can be experienced by burner stages, subsequent to the first, without starting at zero flow. As additional flow of waste gas is experienced a condition sensing means allows flow to a subsequent stage to be combined with the previous stage or stages at a predetermined pressure and flow above zero.

The above purposes, objects and other objects of the invention may be more readily obtained and understood by reference to the following drawings and descriptive matter.

DRAWINGS DESCRIPTION OF THE VIEWS FIG. 1 is an elevational view, partly in section, depecting a typical low-level burner of this invention.

FIG. 2 is a front view of a typical burner nozzle taken along the line 2-2 of FIG. 1.

FIG. 2A is a partial sectional view of the burner nozzle taken along the line 2A2A of FIG. 2.

FIG. 3 is a partial sectional view of an additional embodiment.

FIG. 4 is a partial elevation of another embodiment.

FIG. 4A is a partial top sectional view of a burner opening utilizing the screen embodiment of FIG. 4.

FIGS. 5 and 6 are schematic descriptions generally depicting the gas disposal system of this invention.

FIG. 7 represents a schematic diagram of the flow staging for disposal of variable quantities of waste gases.

DESCRIPTION Before explaining the present invention in details, it is to be understood that this invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not limitation.

GROUND LEVEL FLARE STACK The embodiment shown in FIG. 1 includes a burner or flare stack 10 preferably of cylindrical shape fully opening at its upper end 12. The stack 10 may be fabricated to any desired height such that the effluent exhaust gases are at an acceptable elevation above the surrounding terrain or community. Typically such a stack is about 18 feet in diameter and may range from 40 to feet or more in height, which in the burner art is of a relatively low-level. Although the stack may be fabricated in a variety of ways typically, as shown, construction comprises an outer metallic shell 14 which is lined with a suitable refractory material 16. Although the flare stack 10 is described as being circular it is to be understood that this is not to be limiting as other cross-sectional configuration, e.g., round, square, rectangular or other geometrical configurations are to be inclusive of the invention. In any event in the preferred form of the apparatus the inner configuration of the stack is preferably the same throughout the vertical length then in any event not less than the internal crosssectional area at the location of the burners 20. The purposes for the maintaining of a substantially constant cross-sectional area throughout the length of the stack is based on the so-called natural-draft or chimneyeffect. That is, a structure as described generates a draft as the buoyant gases rise. Such an effect is useful in producing sufficient in-flow draft of combustion supporting air through openings 18 for burning the gases. Such a draft effect is a function of both the height of the stack and the temperature of the gases. Since the internal temperature level is usually fixed, the stack height becomes critical in the establishment of sufficient draft of incoming air to support smokeless combustion of the waste gas stream. Any reduction in crosssectional area in the vertical travel of the gases will produce an acceleration at the restriction but subtract from the draft-induced air because of the created pressure drop across the restriction. This may seriously reduce the air volume and the efficiency of the combustion process. However, additional factors enter into the design of the stack based largely upon present ecological requirements of government or other regulatory agencies. In addition, there are regulatory height restrictions and cost restrictions which must be considered. For example, it has been found that certain exit velocities from a stack must be maintained in order to have proper diffusion of pollutant products into the surrounding atmosphere to maintain a required partsper-million (ppm) condition.

A further factor is the makeup of the combustible waste gases and its burning temperature. For combustible gases there is chemically-fixed theoretical air demand according to the nature of the gases. Flame temperature, which may exist within the stack, is governed by the quantity of air in excess of the theoretical present as the burning progresses. For example, ethylene (C I-I utilizing theoretical air demand of 14.4 cubic feet/Cu.ft. of ethylene results in a temperature excess of 3,600 F. which far exceeds typical refractory endurance. A much more satisfactory temperature level is 2,000 F. which can be obtained by the admission or more than theoretical air for the burning. 250 percent excess air or 50.5 Cu.ft/cubic ft. of ethylene will result in temperature within which standard refractories can be utilized. The exit velocity of a typical stack based on 2,000 F. is in the order of 75 feet per second. In many instances this velocity is sufficient to diffuse pollutant products, such as S into the atmosphere within required parts/million. However, if the velocity of discharge required for proper diffusion is greater than 75 feet per second and where the volume of waste or disposal gases is fixed the required velocity increase must occur by other means where structures of minimal height are necessitated by regulation, cost, or other factors. Reducing the discharge cross-sectional area as in FIG. 3 is one such means. For example, if the velocity of a given quantity of combustion gas flow at the required temperature is 75 ft./second and 100 ft./second is required for proper diffusion of pollutants, there must be a ft./second acceleration. The height of a vertical furnace with substantially constant crosssectional area vertically for burning 30,000 lbs/hr C,H., is 62.60 feet which provides a flow velocity of 75 ft./second. The height necessary to achieve a discharge velocity of ft./second considering total pressure drop due to acceleration (0.53 inch Water Column) plus the pressure drop (0.35 inch WC) across the burners (0.88 inch WC total) divided by a net draft effect per foot of stack height (0.00905 inch WC) calls for a stack 97.24 foot high. If a flow restriction is placed within the stack, the pressure drop to be added is approximately:

( l 0.3 X l0O/75) O.3=0.233 inches ofwater(WC) At 2,000 F. the draft effect per foot of stack height is 0.0115 inch WC. Since the draft increase required is 0.233 inches WC the additional stack height necessary to provide for the added draft is:

0.233/0.00905 25.41 feet The total height required being:

62.68 feet 25.41 feet 88.09 feet This data, however, is based on flow efficiency at the restricted area of 100 percent which in most instances is not the case. For example, a thin edged orifice such as used in metering is approximately 61 percent efficient. When the ratio of the orifice length to diameter is 1.00 a coefficient becomes 0.85, but the point at which the preferred velocity occurs is down inside the orifice and is not effective as such in promoting diffusion. Accordingly, this invention teaches that maximum velocity discharge must occur immediately as the gases flow to the atmosphere for effective pollution control. This invention teaches a stack orifice contour which will provide exit flow coefficients ranging close to 1.00 which will provide a relatively low-stack height, proper diffusion of pollutants into the atmosphere and maximum velocity of diffusion yet providing cost savings. A typical example of such a stack discharge orifice cross-section is shown in FIG. 3. Preferably the restriction is positioned within the upper l0l5 percent of the stack height and comprises a converging wall 17 terminating with an arcuate contoured portion 19, being defined from a radius (R) centered anywhere along the terminal stack end 21.

BURNERS' Reference is made to FIGS. 1, 2 and 2A. Suitably arranged at one or a plurality of peripheral locations are burner areas defined by vertical openings 18 which are adapted to receive a plurality of vertically arranged burners 20, as shown. In one preferred embodiment the burner tip is angularly oriented slightly uwardly and inwardly, e.g., 20 radially into the interior of the stack 10. It is to be understood that other burner arrangements which will provide efficient mixing of the waste gas and air are inclusive of the invention. Although vertically arranged burner areas 18 are shown, the invention is adaptable to a horizontally arranged opening or plurality of horizontally oriented openings in the lower portion of the stack. Combustible waste gas is supplied from supply connection 23 to each burner nozzle 20 via a manifold 22 being divided to each burner nozzle via conduits 24. The manifold 22 may be appropriately supported by

cross brace numbers

26 and 28 to the stack side wall. Each of the burner nozzle designs as shown in this preferred embodiment include a diversion plate 30, surrounding the burner nozzle conduit 24 and which is provided with a plurality of circular or other shaped openings 32. The nozzle tip shown includes a central discharge orifice 25 and a plurality of angularly oriented openings 27. The latter openings are directed toward the upstream face of plate 30 to cause intimate mixing of the waste gas with the combusting supporting air draft flowing to and through plate 30 and perforations 32. It has been found that a burner nozzle design as shown is adaptable to provide acceptable time, temperature and turbulence to burn a wide variety of waste gas components of varying molecular weight without adjustments, without the formation of coke, carbon or other polymerized chemical deposits and with substantially complete combustion to eliminate atmospheric pollutants.

VISUAL, NOISE AND WIND SCREEN Circumferentially surrounding the bottom of stack is a screen generally designated by the numeral 36. In this embodiment the screen includes a plurality of slightly overlapping

panel members

38 and 40 which are alternately spaced vertically and horizontally of each other providing a gap for flow of combustion supporting air to the burners. In some instances a roof 42 is provided to prevent weather elements, snow, rain, etc., from interfering with the efficient operation of the burner and to define an air plenum surrounding the burner areas. The screen functions to (l) eliminate the sight of a visible flame; (2) reduce combustion noise; and (3) substantially eliminate air pressure differentials in the air plenum about the burner areas from the effect of wind.

Although in the above described embodiment a complete circumferential wind and noise screen or fence is shown it is to be understood that such a screen or fence may be segmented about each burner assembly, shown generally as 36A in the views of FIGS. 4 and 4A. That is, vertical wind and

noise screen louvers

50 and 52 extend upward from the ground opposite each burner MODIFICATIONS Referring now to FIGS. 5 and 6 the low-level flare or burner 10 is shown as used in combination with a high level flare 70. Such an elevated flare stack is well known in the art including at its upper end a combustion tip 72, as for example, shown in U. S. Pat. No. 3,539,285 and which generally includes devices to ensure ignition in areas where wind velocity may tend to extinguish the flame. It should be understood that the use of an elevated flare is but a safety addendum to the low-level stack of this invention, being useful as an emergency or secondary disposal means in the event the low-level stack is sought to operate beyond the rated design capacities. As shown in FIG. 5, waste gases to be disposed enter through conduit 80 which is in communication with conduits 90 and 82. Conduit 90 terminates below hydrostatic liquid level 92 while conduit 82 terminates below liquid level 86 which is of lesser level than 92. Hence waste gas initial flow occurs through 80-82 as long as the flow pressure is greater than liquid level 86 pressure but less than liquid level 92 pressure.

When the pressure in 80 rises to a point capable of displacing the liquid level 92 pressure flow of gases is initiated in 84-90 to be discharged to atmosphere through to 72-73 but this initiation of flow performs a useful service because of the Flow-Sensor (84) which at the initiation of flow creates a signal to the steam valve (94)-to cause opening of the steam valve in a substantially linear relationship to the volume of flow of gases from through 90. The flow sensor 84 is disclosed in U.S. Pat. No. 3,570,535 and creates a variable instrument signal, the magnitude of which is proportional to flow quantity, for transmission to 94.

Steam admitted by the valve 94 travels to the vicinity of 73 where the steam serves to suppress smoking as is well known in the arts.

Under normal operating conditions of flow the lowlevel stack 10 is maintained in constant operation to its design capacities with the auxiliary high level flare operating in the event of an upset in the plant operation involving larger quantities of gas which must be disposed or in which other intermediaries or products will be required to be removed to prevent explosions of other hazards.

FIG. 6 is another alternate embodiment of the integrated flare system with like parts having like numerals from FIG. 5. The primary difference is the elimination of the second liquid seal with waste gases passing directly through a flame or explosion arresting device 87 of a type well known to those skilled in the art, thence via conduit 21 to the particular burner area or areas. Excess gas flow through conduit 83 sufficient to overcome hydrostatic level 85 is then burned in the stack 70.

WASTE GAS STAGING FIG. 7 is incorporated as a part of this invention disclosure as a schematic diagram to depict the staging sequence for the burning of variable quantities of waste gas. The stack 10 depicts the c ircumferentially spaced burner areas designated by the numerals ll6 and are staged in the operation as required for the particular quantity of waste gas to be disposed. Before successive burners are staged in the operation, however, there must be a pre-purging of the burners with a gas of some nature typically steam via

conduits

60 and 62 to each burner area. The sequence in operation for the introduction of a gas stream and the steam are accomplished by pressure switches which are capable of emitting signals of controllable magnitude which in turn operate the sequencing valves as hereafter described. The pressure switches are shown in FIG. 7 as numbered squares (IL-1H. 2L-2H, 3L-3I-I, etc.). The L" after each number indicates a switch which emits its signal on a given small pressure rise of the waste gas while H indicates switches which emit signal on a rise greater in prssure than the L switch. The same is true for the operation of the steam control valve sequentially identified as steam-79, Ill, 13 and 15 which are used in conjunction with those valves identified as gas-8", l0, 12, 14 and 16 respectively. In the embodiment shown the odd numbered steam valves are set to respond to the smallest emitted signal while the even numbered gas valves respond only to the greatest emitted signal. Staging of the gas is accomplished in the following manner. Burner 1 is constantly in operation. It receives gas via conduit 21 which may be isolated by appropriate valves shown if required for any reason. Pressure switches 1L and l H are in communication with conduit 21 sensing the gas pressure therein. Controllable amounts of steam via

lines

60 and 62 maintain a constant injection or purge of burner 1 in the event there is no waste gas stream entering the conduit 21. When a small volume of gas is delivered to burner l the pressure in the conduit 21 is not capable of energizing pressure switch 1L. As the volume of gas increases the conduit pressure rises to a point where switch IL is energized, emitting a small signal adequate for opening steam-7 valve for admission of purge via conduit 60 and 62-7 into burners located in area 2. This pressure, however, is not adequate for the opening of gas-8 valve until a further rise in conduit 21 pressure causes switch 1H to emit a greater signal thus opening gas-8 valve and burner 2 is now placed into service, there being, of course, ignition means of those types well known in the art to ignite and maintain ignition of the flame. Further increases in presure then bring into

operation pressure switchs

2L and 2H to sequentially operate purge steam- 9 valve and gas-l valve to cause burner 3 to be placed into operation with the operation continuing to the other burners as the increase in waste gas flow and pressure increases. Of course as the waste gas flow decreases the sequencing procedure is reversed.

While the purpose of the purge is to prevent burner malfunction of impairment, any gas may be used for this purpose. Steam is preferred because of the favorable condition of smoke suppression, after purging, as it eneters a part of the reaction during the burning process.

What is claimed:

1. In a flare system for variable quantities of waste gas having,

a vertical flare stack of substantially unrestricted inner horizontal cross-sectional area along its effective upper height;

at least one burner area located about the lower portion of said stack the improvement in said burner area comprising:

A. at least one burner nozzle oriented to direct said waste gas into said stack, said nozzle comprising: a gas supply conduit; a coaxial air baffle plate positioned forwardly of said conduit; said plate having a plurality of perforations thereacross; said gas supply conduit having a converging portion at its forward end to a central opening; and a plurality of perforations across said converging portion such that a portion of said gas is diverted toward the back side of the said plate; B. an opening around said nozzle to provide passage of draft created combustion supporting air from the outside to the inside of said stack, the total cross sectional area of said opening for all burner areas not more than 90% of the said stack discharge area, C. means to supply said waste gas to said nozzle at any pressure greater than atmospheric pressure; D. means to cause sufficient mixing of said waste gas and air adjacent to and forwardly of said nozzle to cause complete and stable combustion when ignited under any waste gas flow quantity or combustion space temperature, and

E. means to ignite said gas-air mixture.

2. A flare system of claim 1 including, with each burner area, means to inject steam into and with said waste gas supply to said nozzle.

3. A flare system according to claim 1 including a plurality of burner areas wherein said openings are essentially vertical with a plurality of vertically spaced burner nozzles therein.

4. A flare system according to claim 1 including:

means to sense a pressure or flow condition of said waste gas;

means responsive to said condition to supply said waste gas to only that number of burners proportional to the quantity of said waste gas necessary to effect complete and stable combustion.

5. A flare system according to claim 4 including in combination:

a second flare system,

means to supply to said second flare system waste gas in excess of that capable of being efficiently and completely burned in said first ground level flare system.

6. A system according to claim 4 including in combination:

an elevated flare stack;

means to supply said elevated flare stack waste gas in excess of that capable of being efficiently and completely burned in said ground level flare system.

7. A flare system of claim 1 including:

a screen extending outwardly from each burner area and extending vertically at least the length of said area to abate noise and sight of said flame from a normal ground position away from said stack.

8. A system of claim 7 wherein said screen is a set of louvers.

9. A system of claim 8 wherein said louvers are adjustable.

10. A flare system of claim 1 including:

an air plenum encircling said lower periphery of said stack, said plenum comprising a screen to interrupt the wind or air force to said burner areas such that the air pressure within said plenum is substantially constant.

11. A system of claim 10 including:

means to close the top of said plenum between said screen and said stack.

12. A flare system of claim 10 wherein said screen comprises a plurality of overlapping panels alternately spaced vertically and horizontally from each other.

13. A flare system of claim 1 wherein said flare stack includes, at the top lO-l5 percent of said height, a restriction in said inner cross-sectional area to produce substantially maximum flow velocity at the point of discharge from said stack.

14. A flare system of claim 13 wherein said restriction terminates in a convex arcuate section the radius of which centers along said top of said stack.

I Inventor(s) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,779,55 2 Dated December 18, 1973 Robert 1). Reed et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the cover sheet, in the heading, item '53 John Zinc Company" should read John Zink Company Signed and sealed this 21st day oi May 19714,.

(SEAL) Atte st: r

C. MARSHALL DAIIII EDI'JARD ILFLEICIJL'I, JR I lommissioner of Patents Attesting Officer USCOMM-DC 60376-P69 v: us. GOVERNMEUT rnm'rma orrlcE I"! o-ut-su,

FORM PC4050 (10-69)

Claims

1. In a flare system for variable quantities of waste gas having, a vertical flare stack of substantially unrestricted inner horizontal cross-sectional area along its effective upper height; at least one burner area located about the lower portion of said stack the improvement in said burner area comprising: A. at least one burner nozzle oriented to direct said waste gas into said stack, said nozzle comprising: a gas supply conduit; a coaxial air baffle plate positioned forwardly of said conduit; said plate having a plurality of perforations thereacross; said gas supPly conduit having a converging portion at its forward end to a central opening; and a plurality of perforations across said converging portion such that a portion of said gas is diverted toward the back side of the said plate; B. an opening around said nozzle to provide passage of draft created combustion supporting air from the outside to the inside of said stack, the total cross sectional area of said opening for all burner areas not more than 90% of the said stack discharge area, C. means to supply said waste gas to said nozzle at any pressure greater than atmospheric pressure; D. means to cause sufficient mixing of said waste gas and air adjacent to and forwardly of said nozzle to cause complete and stable combustion when ignited under any waste gas flow quantity or combustion space temperature, and E. means to ignite said gas-air mixture.

4. A flare system according to claim 1 including: means to sense a pressure or flow condition of said waste gas; means responsive to said condition to supply said waste gas to only that number of burners proportional to the quantity of said waste gas necessary to effect complete and stable combustion.

5. A flare system according to claim 4 including in combination: a second flare system, means to supply to said second flare system waste gas in excess of that capable of being efficiently and completely burned in said first ground level flare system.

6. A system according to claim 4 including in combination: an elevated flare stack; means to supply said elevated flare stack waste gas in excess of that capable of being efficiently and completely burned in said ground level flare system.

7. A flare system of claim 1 including: a screen extending outwardly from each burner area and extending vertically at least the length of said area to abate noise and sight of said flame from a normal ground position away from said stack.

8. A system of claim 7 wherein said screen is a set of louvers.

9. A system of claim 8 wherein said louvers are adjustable.

10. A flare system of claim 1 including: an air plenum encircling said lower periphery of said stack, said plenum comprising a screen to interrupt the wind or air force to said burner areas such that the air pressure within said plenum is substantially constant.

11. A system of claim 10 including: means to close the top of said plenum between said screen and said stack.

13. A flare system of claim 1 wherein said flare stack includes, at the top 10-15 percent of said height, a restriction in said inner cross-sectional area to produce substantially maximum flow velocity at the point of discharge from said stack.