GB2128725A - Device for heating air by flue gases - Google Patents

Abstract

One or more tube sections (2) mate with a supply flue (1). Each section (2) comprises heat-exchange tubes (3) attached to tube plates (4). A device for protection of the heat-exchange tubes (3) against abrasive wear incorporates at least one screen (5) formed by corrugated sheets facing each other and mating along convex portions, whereby cells (7) are defined, e.g. of bicuspid or hexagonal cross section. <IMAGE>

Description

SPECIFICATION Device for heating air byflue gases The present invention relates to devices for heating air by flue gases in steam boilers and furnaces.

The invention provides a device for heating air by flue gases in steam boilers and furnaces, comprising a supply flue, sections adjacent thereto, each made up of heat-exchange tubes attached to tube plates, and a device disposed in the supply flue and serving for protection ofthe heat-exchange tubes against abrasive wear, the device for protection ofthe heatexchangetubes against abrasive wear being in the form of at least one screen comprising corrugated sheets facing each other and mating along the lines of contact of convex portions whereby cells are formed.

The foregoing construction of the device prevents heavy wear of the heat-exchange tubes by virtue of straightening theflue gas flow in thesupplyflue, and maintaining even distribution of ash and other hard particles overthe entire cross-sectional area thereof.

Preferably, the height of each cell in the screen is two to five times the maximum dimension ofthe cross section thereof. This minimizes the probability of deflection ofthe flue gas flow in relation to the direction of the heat-exchangetubeaxes. In a screen where the height of each cell is less than two times the maximum dimension ofthe cross section thereof, abrasive wear ofthe heat-exchange tubes may rise more than threefold because some part of flue gases released from the cells is notably inclined to the axes of the heat-exchange tubes. In a screen where each cell has a height exceeding five times the maximum dimension of the cross section thereof, the efficiency of protection of the heat-exchange tubes against abrasive wear rises only insignificantly, becausethe possible deflection angle of some part of flowfrom the heat-exchangetube axes is small.Atthe same time, the metal consumption and the dimensions of the whole device increase.

To providefor better protection ofthe heatexchange tubes against abrasive wear, the screen disposed intheflue preferably adjoins a tube plate, with the axis of each cell thereof aligned with the axis ofthe respective heat-exchange tube, and with the distances between the cell axes equal to those between the heat-exchange tube axes. In a device of this construction, the velocity of flue gases sent to the screen cells is much less than the velocity of the gases entering the heat-exchange tubes, and, hence, the screen serving to straighten the gas flow undergoes little abrasive wear. As for the heat-exchange tubes, theabrasivewearthereofwill be precluded as the injection of the flue gas flow therein is stepped and approximately smooth.

Besides, minor displacement between the axes of the screen cells and heat-exchangetubes does not essentially affectthe efficiency of protection thereof against abrasive wear.

Asupplyflue incorporating a bent portion preferably contains two screens, with one screen invariably installed in the vertical portion oftheflueforfurther improvement of protection ofthe tubes against abrasive wear. In this case one screen located in the vertical portion of the supplyflue straightens the flue gasflowanddirectsitalongthe heat-exchangetube axes, and the other screen (which may be arranged on a tube plate) permits even inflow of flue gases into the heat-exchange tubes.

Each cell may be hexagonal in cross section for distributing the inlet flue gases more evenly over the entire perimeter of each heat-exchange tube, whereby wear of the tubes is reduced.

In orderto simplify the manufacture, assembly, and repair of the device, the screen preferably comprises separate blocks, each consisting of 50 to 150 cells. The screen blocks can be mounted so that the heatexchange tubes and screen cells are axially aligned to a higher degree of accuracy. If the number of cells in a blockwere less than fifty, the manufacture and assemblyofthe screen would be more labourconsuming, and if that number were above one hundred and fifty, the tolerances for linear dimensions of the blocks would increase, and, hence, the blocks might not be assembled as accurately.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 schematically represents a general longitudinal sectional view of a deviceforheating air by flue gases in steam boilers and furnaces; Figure 2 is a plan view of a screen in the device; Figure 3 is similar to Figure 1, but with the screen adjacent the tube plate; Figure 4 is a section on line IV-IV of Figure 3; Figure 5 is similar to Figure 1, but with two screens employed; Figure 6 shows an embodiment ofthe screen in which cells are hexagonal in cross section; and Figure 7 is a plan view of part of a screen comprising separate blocks.

Referring to Figure 1, the device for heating air by flue gases in steam boilers and furnaces comprises a supply flue 1 and adjacent sections 2 incorporating heat-exchange tubes 3 attached to tube plates 4. A device for protection of the heat-exchange tubes 3 against abrasive wear is arranged inside the supply flue 1 in the vertical partthereof. The protecting device incorporates at least one screen Sformed by corrugated sheets 6 (Figure 2) facing each other and mating along lines of contact of convex portions, whereby cells 7 are formed, e.g. of bicuspid cross section as shown. The corrugated sheets 6 can be manufactured from thin sheet metal or from heat-resistant plastics.

The single screen 5 shown in Figure 1 is spaced above the uppertube plate 4.

The height of each cell 7 in the screen 5 is two to five times the maximum dimension of its cross section.

Turning nowto Figure 3, here the screen 5 arranged in the vertical portion ofthe supply flue 1 is joined with the tube plate 4 ofthe section 2, the axis of each cell 7 (Figure 4) in the screen 5 being aligned with the axis of the respective heat-exchange tube 3, and the distances between the axes of the cells 7 being equal to distances between the axes ofthe heat-exchange tubes 3. The screen 5 is attached to the tube plate 4 by any convenient method, for example by welding.

The protecion device can incorporate two screens 5 (Figure 5), with one of them permanently arranged in the vertical portion ofthe supply flue 1. When the cells 7 (Figure 2) ofthe screen 5 are of elongate shape, it is expedient for the screen 5to be installed in such a mannerthatthe longer axes of the cross sectionsof the cells 7 are normal to the direction ofthe flue gas flowthrough the horizontal portion ofthe supply flue 1.

In orderfortheflue gas flow to be distributed more evenly atthe inlet, each cell 7 ofthe screen 5 (Figure 6) can be hexagonal in cross section. For simplicity of manufacture, assembly, and repair, the screen 5 can be formed of separate blocks 8 (Figure 7), each comprising from 50 to 150 cells 7. The dimensions of the blocks 8 and the tolerances thereof are selected so as to set up a clearance 9 between them to permit installing the blocks 8within tolerances and aligning the axes ofthe heat-exchange tubes 3 and cells 7.

The air heating device utilizing flue gases of steam boilers and furnaces operates as follows.

The flow offlue gases containing ash and other hard particles passes through the supply flue 1 (Figure 1 ) that can incorporate both a vertical and a horizontal portion, and a bend interconnecting them, and enters the heat-exchange tubes 3 attached to the tube plates 4 ofthe section or sections 2.

Airto be heated is supplied to the inter-tubular space ofthe section(s) 2. Ash and other hard particles carried byflue gases cause heavy abrasive wear ofthe heat-exchangetubes 3 wherein the walls are acted upon by the particles. The rate ofwear of the tubes 3 increases when the flue gas flow is deflected from the direction of the axes ofthe tubes 3 in the area where the flow is turned inside the supply flue 1.

For minimizing the rate of wear of the heat exchangetubes 3, the flue gas flow is straightened upstream ofthe tubes 3 by means of the device serving to protect the tubes 3 against abrasive wear and disposed inthevertical portion ofthesupplyflue 1. The device is in essence the screen 5 (Figure 1) comprising the corrugated sheets 6 (Figure 2) facing each other and mating along lines of contact of sheet convex portions, wherebythe cells 7 are formed whose height is two to five times the maximum dimension of the cross section thereof. The flue gas flow passing through the cells 7 of the screen 5 is sent to the heat-exchange tubes 3 in the direction approximately identical to that of the axes thereof.With the above relation between the dimensions ofthe cells in the screen, any deflection of the flue gas flow from the specified direction is minimal. Thus, the flue gas flow entering the heat-exchange tubes 3 is straightened, and this feature alone contributes to minimizing the rate of abrasive wear ofthe tubes 3. To achieve substantial reduction of abrasive wear of the tubes 3, the screen 5disposed inthevertical portion ofthe supply flue 1 should preferably directly adjoin the tube plate 4 (Figure 3), with the axes of the cells 7 set in line with the axes of the heat-exchange tubes 3, and with the distances between axes of the cells 7 made equal to the distances between the axes ofthe heatexchange tubes 3 (Figure 4).In this case the flue gas flow carried through the cel Is 7 of the screen 5 is separated into a number of jets, whereby each jet smoothly enters the respective heat-exchange tube 3 all over the cross section thereof without appreciable compression ofthe flow and, hence, without erosive action on the walls in the tubes3.

The maximum efficiency is provided when two screens 5 (Figure 5) are installed in theflue 1,with one screen invariably arranged inside the vertical portion thereof. One screen 5 can be mounted directly on the tube plate 4, and the other can be disposed at some distance above it. The first screen 5 in the way of flue gases installed in the vertical portion ofthe supply flue 1 takes up the major action of the gas flow, straightens it, and directs it along the axes of the heat-exchange tubes 3. However, the screen 5 proper does not undergo heavywearas it does not actually blockthe passage in the supply flue and, hence, the velocity of flue gases flowing through it is low. The other screen 5, disposed on the tube plate 4, breaks up the gasflow into separate shaped jets smoothly sent from each cell 7 ofthe screen Sto a respective heat-exchange tube 3.

In orderforthe flue gases entering each heat exchange tube to be more evenly distributed overthe whole perimeter thereof, the cells 7 in the screen 5 may be shaped so thattheir cross section is hexagonal (Figure 6).

To simplifythe manufacture and assembly ofthe screen 5, it is preferable thatthey are constructed of separate blocks, each block comprising 50 to 150 cells (Figure 7).

Claims (4)

1. A device for heating air by flue gases in steam boilers and furnaces, comprising a supply flue; at least on one tube section mating therewith, the section comprising heat-exchange tubes attached to tube plates; and meansfor protecting thetubes against abrasive wear, disposed in the flue, the said means comprising at least one screen formed by corrugated sheets facing each other and mating along their convex portions to define cells.

2. A device as claimed in claim 1, in which the height of each cell is two to five times its maximum cross-sectional dimension.

3. A device as claimed in claim 1 or 2, in which the screen is adjacent a tube plate, with the axis of each cell being aligned with the axis of a respective heat-exchangetube, and with the distances between the axes of the cells being equal to the distances between the axes ofthe heat-exchange tubes.

4. A device as claimed in any preceding claim, in which two screens are installed, one screen being arranged in a vertical portion of the flue.

4. A device as claimed in any preceding claim, in which two screens are installed.

5. A device as claimed in any preceding claim, in which each cell is hexagonal in cross section.

6. A device as claimed in any preceding claim, in which the or each screen is constructed of separate blocks, each block comprising 50 to 150 cells.

7. A device for heating air, substantially as de scribedwith reference to any embodiment illustrated in the accompanying drawings.

New claims or amendments to claims filed on 2216183.

Superseded claims 4.