EP0057102A2

EP0057102A2 - Flue terminal and method of promoting discharge of flue gases

Info

Publication number: EP0057102A2
Application number: EP82300389A
Authority: EP
Inventors: Eric James West
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-01-28
Filing date: 1982-01-26
Publication date: 1982-08-04
Also published as: DK36882A; EP0057102A3

Abstract

A flue terminal has an air inlet (14) defined between surfaces (15, 16) which establish laminar flow of air approaching the flue terminal at one side thereof upwardly adjacent to a peripheral wall 18 of the flue.

Description

This invention relates to a flue terminal, that is a structure which defines an upper part of a flue, from which part gases are discharged from the flue through an exit defined by the flue terminal. A flue is generally used to discharge products of combustion into the ambient atmosphere but the term flue is used herein to refer to any duct through which gases flow by convection from a lower position to a higher position at which the gases are discharged into the ambient atmosphere. The invention also relates to a method of promoting the discharge of flue gases from a flue.
In still air, flue gases at a temperature higher than the ambient temperature can be discharged from the flue by natural convection alone. Exposure of a flue terminal to wind often has an adverse effect on the flow of gases within the flue and sometimes results in the rate of upward flow through the flue decreasing to a rate which is less than the rate at which the flue gases are produced, or even results in downward flow of gases in the flue.
Some flue terminals which have been designed to avoid the adverse effect of wind on the flow of gases within a flue are non-rectilinear, that is to say, there is a cover extending over the flue at its upper end and the flue has one or more laterally facing exits so that flue gases rising through the flue terminal tend to impinge on the underside of the cover and are turned to flow laterally through the exit or exits.
It is also known to provide in a flue terminal an air inlet through which ambient air moving in a direction transverse to the axis of the flue terminal is admitted to the flue with the intenti_l that this additional air will promote upward flow within the flue. In practice, it is found that these known devices do not overcome the aforementioned problem.
According to a first aspect of the present invention, there is provided a method of promoting the discharge of flue gases from a flue terminal wherein ambient air flowing transversely of an axis of the flue terminal is admitted to the flue through an air inlet and flows upwardly within the flue to an exit at an upper end thereof, the flow of said ambient air within the flue terminal being substantially laminar flow. _-
The laminar flow of ambient air through the inlet and upwardly through the flue promotes discharge of flue gases from the outlet of the flue more effectively than does the admission of ambient air to known flues wherein turbulence is established.
The air admitted to the flue through the inlet preferably forms an annular barrier between the flue gases and the periphery of the flue in a region between the air inlet and the exit. This reduces the risk of deposition of water and other materials from the flue gases on the coolest part of the flue terminal.
According to a second aspect of the invention, I provide a flue terminal having at one end an exit and between its ends an air inlet defined by surfaces which promote laminar flow through the inlet into the flue of ambient air which approaches the inlet in a direction transverse to the axis of the flue terminal.
In a flue terminal in accordance with the invention, the air inlet is preferably spaced further from the exit end of the flue terminal than from an opposite end of the flue terminal; whereas in known flue terminals having lateral air inlets, the inlet or at least one of the inlets lies near to an exit end of the flue terminal. By spacing the inlet a substantial distance from the exit of the flue terminal, there can be established over a substantial part of the length of the flue a barrier of air between the peripheral boundary of the flue and the flue gases, thereby reducing the risk of deposition of water and other substances from the flue gases onto the coolest parts of the flue terminal.
That part of the flue which is defined by the flue terminal between the air inlet and the exit preferably tapers upwardly. The directions upwardly, downwardly and laterally, as applied herein and in the claims to the flue terminal, refer to the flue terminal when orientated for use, that is with its exit uppermost.
According to a third aspect of the invention, there is provided a flue terminal formed to define a part of a flue which, in use, extends downwardly from an exit of the flue terminal, said part converging from the exit to a throat and then diverging in a direction away from the exit and there being between the exit and the throat a first substantially frusto-conical surface which defines, in part, the periphery of the flue, said surface facing radially inwardly towards an axis of the flue terminal and axially outwardly towards the exit.
If the flue terminal is subjected, in use, to a wind acting in a direction having a downward component, the substantially frusto-conical surface assists with deflection of the wind in a region directly adjacent to the exit of the flue terminal in such a manner that entry of the wind into the flue terminal through the exit is reduced and passage of the wind over the flue terminal is promoted.
The length of the first substantially frusto-conical surface preferably does not exceed one quarter of the diameter of the flue at the exit of the flue terminal. By the length, we mean that dimension which is measured along the first substantially frusto-conical surface in a plane containing the axis of the flue terminal.
The preferred form of flue terminal embodies both the second and the third aspects of the invention. Furthermore, the preferred flue terminal is so formed that the exit and an entry for flue gases both face along the axis and that the flue between the entry and the exit is rectilinear, by which we mean that a cylinder having a length exceeding the length of the flue terminal can, whilst co-axial with the flue terminal, be passed through the flue terminal from the entry to the exit. With this arrangement, discharge of gases from the flue is not impeded by impingement of the gases on a cover, as occurs with certain known flue terminals. It is preferred that none of the surfaces which define the flue is perpendicular to the axis of the flue or is inclined to that axis at an angle exceeding 30%.
An example of a flue terminal embodying the second and third aspects of the invention and which is used in a method according to the first aspect of the invention, will now be described with reference to the accompanying drawing, wherein:-

FIGURE 1 shows a cross-section of the flue terminal in a vertical plane containing an axis of the terminal;
FIGURE 2 shows a plan view of the terminal.
FIGURE 3 shows diagrammatically a modification of the terminal of Figures 1 and 2.

The flue terminal illustrated in the accompanying drawing is intended for use to define an upper part of a flue, a lower part of which (not shown) is connected with an appliance in which a fuel is burned, for example a central heating boiler. The appliance and structure defining an adjacent part of the flue would normally be disposed within a building. At least the flue terminal is disposed outside the building and may stand on a chimney which defines the flue between the appliance and the flue terminal. The flue terminal is rectilinear, as hereinbefore defined, and is circular, as viewed in plan. A lower end portion 10 of the flue terminal is adapted to be joined to a chimney or other structure defining an adjacent part of the flue and itself defines an entry 11 for flue gases to the flue terminal. At its upper end, the flue terminal defines an exit 12.
The entry 11 and exit 12 both face along an axis 13 of the flue terminal, which axis is normally vertical when the flue terminal is in use.
Between the ends of the flue terminal, there is an annular air inlet 14 defined between a first surface 15 and a second surface 16. A radially outer end of the air inlet faces away from the axis 13 and the inlet extends from its outer end towards the axis and towards the exit 12 to a plane indicated by the reference numeral 17 where it joins with the flue.
The flue terminal comprises an upper wall 18 which presents the surface 15 and also presents a frusto-conical surface 19 which defines an upwardly tapering part of the flue immediately above the plane 17. The flue tapers upwardly from the plane 17 to a throat 20 and then diverges to the exit 12. The divergent part of the flue is defined by a further frusto-conical surface 21 of the wall 18. The throat 20 is preferably defined by a smoothly curved surface of the wall 18 which merges with the surfaces 19 and 21. Above the surface 21, the wall 18 presents an outwardly flared surface towards the exit 12. The cross-section shown in Figure 1 of that part of the wall 18 which presents the flared surface 22 may be an arc of a circle.
The apex angle defined by the surface 21 is within the range 20° to 60° and is preferably within the range 35° to 40°. The apex angle defined by the surface 19 is somewhat smaller, typically 10°. The length L of the surface 21 is not greater than one quarter the diameter of the flue at the exit 12 and is preferably not greater than one quarter the diameter of the flue at the throat 20. The surface 21 is spaced from the exit 12 by a distance within the range 5mm to 20mm. The axial distance from the throat 20 to the plane 17 is considerably greater than the length of the surface 21 and is selected to provide that the distance between the inlet 14 and the exit 12 is as large as is conveniently practicable, taking into consideration the requirement that, under various operation conditions, the introduction of kinetic energy to the flue by ambient air entering through the inlet 14 should not be entirely offset by increased drag within the flue above the inlet and that for reasons of cost and acceptable appearance, the overall length of the flue terminal should not be excessive.
The flue terminal comprises a second wall 25 which defines that part of the flue lying within the flue terminal below the plane 17. From the plane 17, the flue diverges downwardly to a cylindrical part of the flue defined by the lower end portion 10. This divergent part of the flue may be defined by a frusto-conical part of the wall 25 having an apex angle within the range 5° to 20°. Generally, the axial length of the wall 23 will be less than the distance between the plane 17 and the exit 12, although the length of the wall 23 may be varied as required to achieve a predetermined overall flue height. More preferably, the radially outer end of the air inlet 14 lies nearer to the entry 11 than to the exit 12. It will be noted that there is no further air inlet or exit between the air inlet 14 and the exit 12.
As shown in the cross-section of Figure 1, in the vicinity of the plane 17, the surface 15 merges with the surface 19. From that plane, the surface 15 curves smoothly downwardly and radially outwardly to an outer marginal portion 23 which is spaced further from the axis 13 than is any part of the flue defined by the flue terminal. The marginal portion 23 is horizontal, that is perpendicular to the axis 13. The radius of curvature of the surface 15, as viewed in the cross-section of Figure 1, may vary gradually between the plane 17 and the marginal portion 23, having a minimum value adjacent to the marginal portion. As viewed in the cross-section of Figure 1, the surface 16 may have at all positions a somewhat larger radius of curvature than the minimum radius of curvature of the surface 15. Adjacent to the plane 17, the surface 16 is of substantially frusto-conical form and defines an apex angle equal to that defined by the adjacent peripheral boundary of the flue. A radially outer marginal portion of the surface 16 is inclined downwardly and outwardly to diverge from the marginal portion 15. As can be seen from Figure 1, the surface 16 also is smoothly curved.
The area of the air inlet 14 in a horizontal plane just below the plane 17 is considerably smaller than the area of the flue in the same plane, preferably within the range 10% to 25% of the area of the flue in that plane. The area of the air inlet 14 just below the plane 17 is approximately equal to the difference between the area of the flue at the throat 20 and the area of the flue at the exit 12.
Preferably, the area of the inlet 14 just below the plane 17 is approximately 15% of the area of the flue in the same plane.
When the flue terminal is in use, flue gases rise through a lower part of the flue, enter the flue terminal at the entry 11 and are discharged from the flue terminal at the exit 12. In the absence of any wind, air from the ambient atmosphere will be drawn through the air inlet 14 to enter the flue at the plane 17 and to rise with the flue gases to the exit 12. The outward flaring of the flue adjacent to the exit 12 promotes turbulence in the gases discharged from the flue so that mixing with the ambient atmosphere is promoted. When the flue terminal is subjected, during use, to a wind, the direction of flow of air which approaches the inlet 14 in any direction will be changed gradually by the action of the surfaces 15 and 16 so that the air is maintained in laminar flow and is directed into the flue at the plane 17 along a path substantially parallel to the surface 19. Air which approaches the inlet 14 in a direction having, in addition to a lateral component, an upward component or a downward component will tend to behave in the same way and enter the flue flowing in a laminar condition. In a case where the wind blows onto the flue terminal in a direction having a downward component, air which is incident on the surface of the wall 18 which is opposite to the surface 15 will be deflected upwardly and outwardly away from the air inlet. Any air which enters the flue terminal through the exit 12 whilst the terminal is subjected to wind blowing in a direction having a downward component, will tend to be deflected by the surface 21 upwardly and outwardly away from the flue so that discharge of flue gases from the flue terminal is not unduly inhibited.
Air which enters the flue from the inlet 14 in laminar flow forms a substantially complete annular barrier between the flue gases and the wall 18 so that there is little risk of water or other substances being deposited on the wall 18.from the flue gases. It will be understood that the upper end portion of a structure defining a flue is normally the coolest part of that structure and therefore the part on which deposition of solids and liquids from the flue gases is most likely to occur. The deposition of substances from the flue gases onto the flue terminal tends to cause corrosion of the flue terminal and to obstruct, at least partly, the flue.
The flue terminal illustrated in the accompanying drawing enables air which is admitted through the inlet 14 to flow in a laminar condition almost to the exit 12 where turbulence is desired to promote mixing of the flue gases with the ambient atmosphere.
The particular example of flue terminal illustrated in the accompanying drawing may be fabricated from sheet metal or formed from metal tube. Suitable materials are stainless steel and aluminium. Alternatively, the flue terminal may be formed by casting metal or by moulding a ceramic or like material. In this case, the walls of the flue terminal would generally be somewhat thicker than those illustrated and the vacant space between the inlet 14 and the flue may be smaller or eliminated entirely.
Struts 24 are provided for connecting the wall 18 with the wall 25. The particular form of struts used depends upon the manner of manufacture of the flue terminal. The struts may be formed integrally with the walls 18 and 25. Alternatively, sheet metal struts may be secured by riveting or welding to the walls 18 and 25.
If required, the walls 18 and 25 may be connected together by means which provides for relative adjustment of these walls along the axis 13. Furthermore, a separate wall portion which presents the surface 16 could be connected with the wall 25 by means providing for adjustment of the surface 16 towards and away from the surface 15 independently of the wall 25.
In the modified flue terminal shown in figure 3, there is provided an electrically energisable impellor 30 for causing ambient air to flow into the flue when there is no wind or insufficient wind to assist upward flow of the flue gases above the plane 17. The impellor is housed in a cylindrical housing 31 which is spaced from and extends around the wall 25 at a level below the surface 16. The housing has an air inlet in a plate 32 which partly closes the lower end of the housing. In use, the impellor causes air to be d seharged from the housing into the inlet 14 through gaps formed in the wall presenting the surface 16. Alternatively, the impellor may discharge air into the flue through gaps 33 provided as shown between the wall 25 and the wall presenting the surface 16.

Claims

1. A method of promoting the discharge of flue gases from a flue terminal wherein ambient air flowing transversely of an axis of the flue terminal is admitted to the flue terminal through an air inlet (14) and flows upwardly within the flue terminal to an exit (12) at an upper end thereof, the flow of said ambient air within the flue terminal being substantially laminar flow.

2. A method according to Claim 1 wherein the ambient air forms an annular or part annular barrier between the flue gases and the periphery of the flue in a region between the air inlet (14) and the exit (12).

3. A flue terminal having at one end an exit (12) and between its ends an air inlet (14) defined by surfaces (15, 16) which promote laminar flow through the inlet into the flue terminal of ambient air which approaches the inlet in a direction transverse to the axis (13) of the flue terminal.

4. A flue terminal according to Claim 3 wherein said surfaces (15,16)are adapted to promote laminar flow into the flue terminal of ambient air which approaches the inlet in a direction perpendicular to the axis (13) or approximately perpendicular thereto.

5. A flue terminal according to Claim 3 or Claim 4 wherein the air inlet (14) is spaced further from the exit end (12) of the flue terminal than from an opposite end (11) of the flue terminal.

6. A flue terminal according to any one of Claims 3, 4 and 5 wherein there is between tne air inlet (14) and the exit (12) a substantial length of the flue terminal which is devoid of any substantial air inlet or exit

7. A flue terminal according to any one of claims 1 to 6 wherein a second (16) of the surfaces which define the inlet (14) is spaced from said first surface (15) and diverges from the first surface in a direction from the flue towards said marginal portion of the first surface.

8. A flue terminal formed to define a part of a flue which, in use, extends downwardly from an exit (12) of the flue terminal, said part converging from the exit to a throat (20) and then diverging in a direction away from the exit and there being between the exit and the throat a first substantially frusto-conical surface which defines, in part, the periphery of the flue, said substantially frusto-conical surface (21) facing radially inwardly towards an axis (13) of the flue terminal and axially outwardly towards the exit.

9. A flue terminal according to Claim 8 comprising a further substantially frusto-conical surface (19) below the throat (20).

10. A flue terminal according to Claim 9 wherein the apex angle defined by said first substantially frusto-conical surface (21) exceeds the apex angle defined by said further substantially frusto-conical surface (19).