US3310036A - Gas off-take system - Google Patents

Description

March 1967 A. M. FRENDBERG ETAL 3,310,035

GAS OFF TAKE SYSTEM Filed Sept. 28, 1965 s Sheets-Sheet 1 FIG. 1

. INVENTORS Ar1hur M. Frendberg BY Richard Siegfried ATTORNEY March 21, 1967 A. M. FRENDBERG ETAL GAS OFF-TAKE SYSTEM 3 Sheets-Sheet 2 Filed Sept. 28, 1965 MaI'Ch 21, 1967 A. M. FRENDBERG ETAL 36 GAS OFF-TAKE SYSTEM Filed Sept. 28, 1965 j 3 Sheets-sheet s (LIONVECTJON 4.

4 I l PASS I 4 I I TO AIR HEATER United States Patent poration of New Jersey Filed Sept. 28, 1965, Ser. No. 490,813 8 Claims. (Cl. 1221) This invention relates generally to a gas off-take system, i.e. apparatus for withdrawing a portion of gas from a main gas stream, and more particularly to such a system as used in conjunction with a vapor generating unit employing the recirculation of cooled gaseous combustion products to the furnace for purposes of vapor temperature control.

In the design and operation of modern high capacity steam generating plants, the recirculation of gaseous combustion products is commonly used as a means of altering the heat absorption pattern Within the steam generator to effect substantially constant outlet steam temperature conditions over a wide load range. The gas to be recirculated is withdrawn from a relatively cool region (usually after the economizer) of the steam generator and is reintroduced to the furnace by means of a suitable gas recirculation system including a fan and connecting ductwork. Where coal or other high-ash fuel is burned in the furnace, the gaseous combustion products leaving the furnace contain significant quantities of particulate matter which, it carried over into the gas recirculation system, may cause serious erosion problems in the gas recirculation fan. Consequently, it is common practice to place a dust collector in the gas recirculating system upstream of the gas recirculation fan; however, such collectors are expensive and occupy a considerable amount of prime space in the immediate vicinity of the steam generator.

Accordingly, it is an object of the present invention to provide an improved gas off-take arrangement for withdrawing from the steam generator outlet the gas to be recirculated to the furnace. It is a still further object that this arrangement be effective for purposes of excluding from the gas recirculating system at least a substantial portion of the solid particulate matter carried by the main flue gas stream of the steam generator. Still further objects of the present invention are that the gas oil-take apparatus be arranged to afford structural rigidity to the duct from which the gas is being withdrawn, and to provide for the uniform withdrawal of gas from substantially the entire cross-sectional flow area of the main gas stream.

According to the present invention, these and other objects may be attained in a vapor generating unit having a furnace chamber wherein ash-bearing carbonaceous fuel is burned to produce solids-laden high temperature heating gases. An upright heating gas pass, communicating at its upper end with the furnace discharge, has arranged therein tubular heat exchange elements including an economizer. An air heater is conventionally arranged adjacent to and connects with the upright convection gas pass outlet for the flow of flue gas therefrom by a main duct which preferably includes a substantially right angle bend. A gas off-take system is arranged within the duct at the bend portion thereof, and includes a gas collecting and conveying duct interconnecting the main duct and the furnace chamber, and having interposed therein a gas recirculation fan. Within the main duct, and s aced at regular intervals thereacross, are a plurality of off-take channel elements, each of which is connected to opposite sides of the duct to afford structural rigidity thereto. Each channel element is in fluid flow communication with an intake manifold of the gas collecting duct, and

has formed therein inlet opening rneans disposed Within the main duct in a plane having a substantial horizontal component and being substantially perpendicular to the flow of the solids-laden gas in the main duct, whereby gas flowing from the main duct through the gas collecting duct is required to take a substantially turn to enter the channel members.

For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the following description which refers to the accompanying drawing in which:

FIG. 1 is a diagrammatic sectional side elevation of a vapor generator having a gas recirculation system wherein one embodiment of the gas off-take system of the present invention is employed; FIG. 2 is an enlarged plan view, having portions thereof broken away, taken along line 22 of FIG. 1; FIG. 3 is a diagrammatic sectional side elevation taken along line 33 of FIG. 4 of an alternate gas off-take system according to the present invention; and FIG. 4 is a partial sectional plan view taken along line 44 of FIG. 3.

Referring to FIG. 1 of the drawings, the main portions of the unit shown are an air heater 10, an upright furnace 12, a horizontal gas pass 14 and an upright convection gas pass 16. The boundary Walls of the furnace 12 and gas passes 14 and 16 are lined, in the conventional manner, with tubes through which fluid to be heated is passed for the absorption of heat by radiation and/or convection. Ambient temperature combustion air is supplied by a forced draft fan (not shown) via inlet duct 18 to the air heater 10, wherein the combustion air is passed in indirect heat exchange relation with combustion flue gas. The heated air passes via outlet duct 20 to the windbox 22 for distribution to the fuel burning equipment which includes a plurality of cyclone furnaces 24 wherein fuel (usually crushed coal) is thoroughly mixed with the air and burned to produce high temperature heating gases which are discharged into the furnace 12. The heating gasses pass upwardly through the furnace, then laterally through the horizontal gas pass 14 containing vapor heating sections 26, and then downwardly through theconvection gas pass 16 which has disposed therein additional vapor heating sections 28 and an economizer 30. A major portion of the heating gases then passes, in a U-shaped flow pattern, through the air heater 10 prior to being discharged to the atmosphere via duct 32.

For purposes of controlling steam temperature, a portion of the partially cooled heating gases may be withdrawn from the main gas stream at a location between the economizer 50 and the air heater 10, and recirculated to the furnace 12. Withdrawal of the gas to be recirculated is accomplished in a gas off-take system designated generally by the numeral 50 in FIG. 1, which system will be described in greater detail hereinafter. The gas withdrawn from the main stream is collected in the horizontally disposed cross duct or intake manifold 51 which connects at each of its ends (see FIG. 2) with a fan inlet duct 52 through which the recirculating gas is conveyed to a gas recirculating fan 55, or preferably to a pair of such fans operating in parallel. The fan (or fans) 55 provides the necessary energy to overcome the pressure differential between the point of withdrawal of the recirculated gases from the main gas stream and the furnace, including the pressure loss through the gas recirculating system. The recirculated gas passes from the fan 55 via fan outlet duct 53 to the lower recirculated gas plenum 56, from whence it is introduced into the fur nace 12 by way of recirculation gas ports 56A. Recirculated gas may also be delivered through the upwardly extending conduit 57 to the upper recirculated gas plenum 58 for introduction into the upper region of the furnace 12 through gas tempering ports 58A. It should be recognized that suitable dampers (not shown) may be pro vided in the gas recirculation system ducts to afford means for controlling the flow of recirculated gas and its apportionment to the upper and lower ports 58A and 56A.

Referring now particularly to the gas off-take system of FIGS. 1 and 2, the bottom of the convection pass 16 is formed with an ash collecting hopper 17, with suit able provisions for in-service clean out, and an upright convection pass outlet 16A. It should be noted that the air heater is significantly wider than the convection pass 16 (see FIG. 2) as is ofttimes the situation, depending on air heater arrangement and heating surface requirements. A sharply flaring, horizontally disposed transition duct section 61) interconnects the convection pass outlet 16A and the air heater inlet duct 19. Expansion joints 61A and 61B are provided in the transition duct section 61) to accommodate differential thermal expansion between the air heater 1t and the walls defining the convection gas pass 16.

Economy of space and cost dictate that the transition duct section 60 be as short as practicable. However, the resulting sharp divergence of the duct section 60 tends to cause an intolerable maldistribution of flue gas at the air heater inlet, with the gas flow being concentrated in the lateral center portion of the air heater. To correct this maldistribution, upright directional vanes 62, extending throughout the height of the transition duct section, are arranged at regularly spaced intervals thereacross, the vanes 62 being symmetrically oriented with respect to each other to provide equal angles of divergence between adjacent vanes 62.

Oriented in planes coextensive with the planes of the directional vanes 62 are a plurality of gas off-take boxes or channel elements 65 disposed directly above the first heating gas pass of the air heater, the boxes being arranged in the zone Where the flue gases are constrained to execute a right angle downward turn as indicated by the flow direction arrow 64. Each off-take box 65 includes a pair of spaced substantially parallel side plates 66 joined at their upstream edges by a vertical end C10- sure 67 which is preferably tapered to a relatively sharp edge to avoid excessive pressure drop and gas flow disturbance. The ends of the side plates 66 abut the plate which forms a part of the air heater inlet 19, so that the box ends opposite the end closure 67 are also closed. Each of the off-take boxes is open at its upper and lower ends only to afford a flow channel between the main gas stream and the intake manifold 51 in which the withdrawn gases are collected for conveyance to the gas recirculating fan 55. The spaces between adjacent boxes 65 are closed at the upper ends of the boxes by plates 68.

In operation, the solids-laden main gas stream passes downwardly between the off-take boxes 65 upon making the right angle turn from the horizontally disposed transition duct section 66 to the air heater inlet duct 19. Uniform distribution of the gases to the air heater inlet duct 19 is enhanced because of the above described orientation of the directional vanes 62 and the boxes 65. As the main gas stream passes the lower edge of the boxes 65, a portion of the gas makes a substantially 180 turn to fiow upwardly through the boxes 65 and into the intake manifold 51. Since the (bottom) inlet openings of the boxes 65 are horizontal, the normal separating eifect due to the 180 turn is further enhanced by the effect of gravity on the entrained solids.

It should be noted that the boxes 65 span, and therefore lend structural rigidity to, the air heater inlet duct 19 and the boundary of the convection pass outlet 16A. Moreover, since the inlet openings to the boxes 65 extend substantially wholly across the air heater inlet duct 19 and are evenly spaced across its width, representative portions of the main gas stream will be withdrawn for recirculation.

Referring to FIGS. 3 and 4, the alternate embodiment of the gas off-take system shown therein is arranged at the bottom of the upright convection pass 16 and forms the outlet therefrom. Connected with the boundary wall of the convection pass is a horizontally extending air heater inlet duct 79 in which the main gas stream is conveyed to an air heater (not shown). A plurality of gas off-take boxes or channel members 75 are substantially evenly spaced across the width of the unit at the junction of the convection pass 16 and the air heater inlet duct 79. Each gas off-take box 75 includes a pair of spaced, substantially parallel side plates 76 of generally triangular shape joined at their upper stream ends by a tapered end closure 77 to avoid excessive gas flow disturbance and pressure drop. The two remaining ends 75A and 75B of each box 75 are open to provide a flow channel therethrough for passage of the gas withdrawn from the main gas stream for recirculation. The box inlet ends 75A are disposed in a plane substantially perpendicular to the main gas flow stream. The outlet box ends 75B communicate with a laterally extending intake manifold or cross duct 71 in which the gases withdrawn from the main stream are collected. Each end of the intake manifold 71 connects with a recirculating fan inlet duct 72 through which the withdrawn gases are conveyed to a recirculating fan (not shown).

In operation, as the solids-laden flue gas flows downwardly through the convection pass 16 and executes the right angle bend into the air heater inlet duct 79, a portion of the gas is constrained to make a substantially turn to enter the off-take boxes 75 through their inlet ends 75A and flow therethrough into the intake manifold 71. The 180 turn of the gas entering the boxes 75 tends to separate entrained particulate matter, and a hopper 78 is provided immediately below the separation zone to collect the solids that settle out of the gas stream By arranging the inlet openings to the boxes 75 in a plane having a substantially horizontal component, the separation due to the 180 turn is enhanced by the effect of gravity acting on the entrained particles. It should be noted that the boxes 75 contribute significantly to the structural rigidity of the ductwork in which they are enclosed since they span the convection pass 16, the air heater inlet duct 79 and the intake manifold 71.

What is claimed is:

1. A gas off-take system comprising first duct means having a substantially rectangular cross-sectional flow area and forming a passageway confining the fiow of a stream of solids-laden gas, said passageway including a first portion, a second portion of greater width than said first portion, and a flaring portion interconnecting said first and second portions, second duct means disposed outside of said passageway transversely with respect to said first duct means for collecting and confining the flow of gas withdrawn from said gas stream, and means for evenly distributing gas across the width of said second portion of said first duct means including a plurality of gas off-take channel elements spaced within said flaring portion at regular intervals thereacross, said channel elements being oriented in diverging positions with respect to each other in the direction of gas flow, each of said channel elements being in communication with said second duct means and having formed therein inlet opening means disposed within said passageway in a plane having a substantially horizontal component and being substantially perpendicular to the flow of said solids-laden gas in said passageway, whereby gas flowing through said passageway and into said opening means is required to take a substantial 180 turn to enter said channel members.

2. A gas off-take system comprising first duct means having a substantially rectangular cross-sectional flow area and forming a passageway confining the flow of a stream of solids-laden gas, said passageway including a first portion, a second portion of greater width than said first portion, and a flaring portion interconnecting said first and second portions, second duct means disposed outside of said passageway transversely with respect to said first duct means for collecting and confining the flow of gas withdrawn from said gas stream, and means for evenly distributing gas across the width of said second portion of said first duct means including a plurality of gas off-take channel elements spaced within said flaring portion at regular intervals thereacross and connected to opposite sides of said first duct means to afford structural rigidity thereto, said channel elements being oriented in diverging positions with respect to each other in the direction of gas flow, each of said channel elements including a pair of spaced plates joined at their upstream ends by an end closure, said channel elements each having one end thereof in communication with said second duct means and having another end thereof formed with an inlet opening disposed Within said passageway in a plane having a substantial horizontal component and being substantially perpendicular to the flow of said solids-laden gas in said passageway, whereby gas flowing through said passageway and into said second duct means is required to take a substantially 180 turn to enter said channel members.

3. In a vapor generating unit, walls defining a furnace chamber having a heating gas outlet, means for burning ash-bearing carbonaceous fuel in said furnace whereby solids-laden heating gases are produced, walls forming an upright heating gas pass communicating at its upper end with said heating gas outlet, heat exchange means arranged in said heating gas pass, a combustion air heater, first duct means of substantially rectangular cross-section forming a passageway confining the flow of said solidsladen gas from the lower outlet end of said heating gas pass to said air heater, said passageway having at least one substantially right angle bend, and means for recirculating gas from said passageway to said furnace chamber including a gas off-take system arranged within said passageway at the bend thereof, said last named means including second duct means communicating between said gas off-take system and said furnace chamber, and a gas recirculating fan interposed in said second duct means, said gas off-take system comprising a plurality of fluid off-take channel elements spaced at regular intervals across the width of said passageway at and in the plane of the bend thereof and connected to opposite sides thereof to afford structural rigidity thereto, each of said channel elements being in fluid flow communication with said second duct means and having formed therein inlet opening means disposed within said passageway in a plane having a substantial horizontal component and being substantially perpendicular to the flow of said solids-laden gas in said passageway, whereby gas flowing through said passageway and into said second duct means is required to take a substantially 180 turn to enter said channel members.

4. In a vapor generating unit, walls defining a furnace chamber having a heating gas outlet, means for burning ash-bearing carbonaceous fuel in said furnace whereby solids-laden heating gases are produced, walls forming an upright heating gas pass communicating at its upper end with said heating gas outlet, heat exchange means arranged in said heating gas pass, a combustion air heater, first duct means of substantially rectangular cross section forming a passageway confining the flow of said solids-laden gas from the lower outlet end of said heating gas pass to said air heater, said passageway having at least one substantially right angle bend, and means for recirculating gas from said passageway to said furnace chamber including a gas off-take system arranged within said passageway at the bend thereof, said last named means including second duct means communicating between said gas off-take system and said furnace chamber, and a gas recirculating fan interposed in said second duct means, said gas off-take system comprising a plurality of fluid off-take channel elements spaced at regular intervals across the width of said passageway at and in the plane of the bend thereof and connected to opposite sides thereof to afford structural rigidity thereto, each of said channel elements being in fluid flow communication with said second duct means and having the downstream end thereof open for communication with said passageway, said downstream ends being disposed in a plane having a substantial horizontal component and being substantially perpendicular to the flow of said solids-laden gas in said passageway, whereby gas flowing through said passageway and into said second duct means is required to take a turn to enter said channel members.

5. In a vapor generating unit, walls defining a furnace chamber having a heating gas outlet, means for burning ash-bearing carbonaceous fuel in said furnace whereby solids-laden heating gases are produced, walls forming an upright heating gas pass communicating at its upper end with said heating gas outlet, heat exchange means arranged in said heating gas pass, a combustion air heater, first duct means of substantially rectangular cross section forming a passageway confining the flow of said solidsladen gas from the lower outlet end of said heating gas pass to said air heater, a portion of said passageway being substantially narrower than said air heater, said passageway having at least one bend portion and having an outwardly flaring portion disposed immediately upstream of said bend portion, and means for recirculating gas from said passageway to said furnace chamber including a gas Off-take system arranged within said passageway at the bend portion thereof, said last named means including second duet means communicating between said gas off-take system and said furnace chamber, and a gas recirculating fan interposed in said second duct means, said gas offtake system comprising a plurality of gas off-take channel elements spaced within said flaring portion at regular intervals thereacross and connected to opposite sides of said first duct means to afford structural rigidity thereto, said channel elements being oriented in diverging positions with respect to each other in the direction of gas flow to evenly distribute the gas leaving said flared portion, said channel elements each having one end thereof in communication with said second duct means and having another end thereof formed with an inlet opening disposed within said passageway in a plane having a substantial horizontal component and being substantial ly perpendicular to the flow of said solids-laden gas in said passageway, whereby gas flowing through said passageway and into said duct means is required to take a substantially 180 turn to enter said channel members.

6. In a vapor generating unit, walls defining a furnace chamber having a heating gas outlet, means for burning ash-bearing carbonaceous fuel in said furnace whereby solids-laden heating gases are produced, walls forming an upright heating gas pass communicating at its upper end with said heating gas outlet, a heat exchanger arranged in said gas pass, an air heater, wall structure forming a passageway confining the flow of said solids-laden gas from the lower outlet end of said heating gas pass to said air heater, said passageway having at least one substantially right angle bend, and means for recirculating gas from said passageway to said furnace chamber including a gas off-take system arranged within said passageway at the bend thereof, said gas recirculating means further including a duct between said gas off-take system and said furnace chamber, and a gas recirculating fan in said duct, said duct including an intake manifold disposed outside of said passageway transversely with respect to said passageway, said gas off-take system comprising a plurality of gas off-take channel elements spaced at regular intervals transversely across said passageway at the bend thereof and connected to the wall structure thereof to lend structural rigidity thereto, each of said channel elements being in fluid flow communication with the intake manifold of said duct and having formed therein inlet opening means disposed within said passageway in a plane having a substantial horizontal component and being substantially perpendicular to the flow of said solids-laden gas in said passageway, whereby that gas flowing through said passageway and withdrawn into 7 said intake manifold is required to make a substantially 180 turn to enter said channel elements.

7. The combination according to claim 6 wherein said passageway is elongated in the direction of gas flow, each of said channel elements including a pair of parallel plates which extend longitudinally and are spaced transversely relative to said passageway and which are joined at their upstream ends by an end closure, the regular spacing of said channel elements providing distributed flow areas between them in said passageway for said solids-laden gas and further providing fiow areas within them for the reverse flow therethrough of gas withdrawn from uniformly distributed locations across said passageway.

8. The combination according to claim 7 wherein said plates are of triangular shape, each with a first edge 8 disposed on the upstream side of said channel elements, a second edge on the downstream side of said channel elements within said passageway, and a third edge connected to said intake manifold.

References Cited by the Examiner UNITED STATES PATENTS KENNETH W. SPRAGUE, Primary Examiner.

Claims (1)

1. A GAS OFF-TAKE SYSTEM COMPRISING FIRST DUCT MEANS HAVING A SUBSTANTIALLY RECTANGULAR CROSS-SECTIONAL FLOW AREA AND FORMING A PASSAGEWAY CONFINING THE FLOW OF A STREAM OF SOLIDS-LADEN GAS, SAID PASSAGEWAY INCLUDING A FIRST PORTION, A SECOND PORTION OF GREATER WIDTH THAN SAID FIRST PORTION, AND A FLARING PORTION INTERCONNECTING SAID FIRST AND SECOND PORTIONS, SECOND DUCT MEANS DISPOSED OUTSIDE OF SAID PASSAGEWAY TRANSVERSELY WITH RESPECT TO SAID FIRST DUCT MEANS FOR COLLECTING AND CONFINING THE FLOW OF GAS WITHDRAWN FROM SAID GAS STREAM, AND MEANS FOR EVENLY DISTRIBUTING GAS ACROSS THE WIDTH OF SAID SECOND PORTION OF SAID FIRST DUCT MEANS INCLUDING A PLURALITY OF GAS OFF-TAKE CHANNEL ELEMENTS SPACED WITHIN SAID FLARING PORTION AT REGULAR INTERVALS THEREACROSS, SAID CHANNEL ELEMENTS BEING ORIENTED IN DIVERGING POSITIONS WITH RESPECT TO EACH OTHER IN THE DIRECTION OF GAS FLOW, EACH OF SAID CHANNEL ELEMENTS BEING IN COMMUNICATION WITH SAID SECOND DUCT MEANS AND HAVING FORMED THEREIN INLET OPENING MEANS DISPOSED WITHIN SAID PASSAGEWAY IN A PLANE HAVING A SUBSTANTIALLY HORIZONTAL COMPONENT AND BEING SUBSTANTIALLY PERPENDICULAR TO THE FLOW OF SAID SOLIDS-LADEN GAS IN SAID PASSAGEWAY, WHEREBY GAS FLOWING THROUGH SAID PASSAGEWAY AND INTO SAID OPENING MEANS IS REQUIRED TO TAKE A SUBSTANTIAL 180* TURN TO ENTER SAID CHANNEL MEMBERS.

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