GB2289424A - Device for diluting boiler flue gas with air - Google Patents

Abstract

Flue gas from a boiler is diluted with air in a device comprising a chamber 8 which communicates with a flue gas inlet 4, an air inlet 2 and a mixed gas outlet 6. A fan 10 is situated within an inner cylinder 12 arranged coaxially inside chamber 8. Weir gaps 15, 17 are provided to enhance the mixing of input gases. The weir gaps are formed by rings 14, 16 having an arcuate radial cross-section. In use air from inlet 2 is drawn through the inner cylinder 12 by the fan 10. Flue gas from inlet 4 is fed into chamber 8 where an upstream portion is drawn into weir gap 15 and mixed with air in cylinder 12, the downstream portion of the flue gas passing into weir gap 17 for mixing with the flue gas/air mixture. <IMAGE>

Description

MIXING DEVICE The present invention relates to a mixing device suitable for installation in a gas flow path. More particularly, but not exclusively, the invention relates to a dilution device for installation in place of a chimney or flue in the flue gas outlet of a boiler, although it is to be understood that the invention can be employed in a wide range of other systems where intimate mixing of two or more gas streams is required.

Conventional mixing systems, such as dilution systems, dilute the products of combustion from a boiler with air drawn from outside the boiler room, for example.

The mixed air and combustion products are discharged outside the boiler room. This enables the discharge to be carried out at a low level and obviates the need for a chimney which would, otherwise, carry the products up to a high level where they would be discharged and diluted by the surrounding wind turbulence.

In order to comply with emission regulations and building regulations, the discharge from a dilution system must typically not contain more than 1% carbon dioxide. If the discharge is not diluted correctly, the discharge mixture can have an excessive carbon dioxide level due to stratification.

To achieve correct dilution, it is known to induce flue gases into a low pressure region created by a fan in a dilution device. To date, however, the necessary equipment has been bulky and expensive to install on site.

The present invention sets out to provide a dilution device which is as small as possible and which can be installed in a "cassette" form (i.e. as a single, modular component).

According to the invention there is provided a mixing device suitable for installation in a gas flow path, comprising a chamber, an inner wall dividing the chamber into first and second flow paths, suction means provided in the first flow path for propelling gas through the first flow path, a first inlet feeding directly into both ofthe flow paths, an outlet feeding directly from both ofthe flow paths, and a second inlet which feeds directly into the second flow path and is arranged to feed gas into the first flow path by means of a weir gap at least partially defined by the said inner wall.

Preferably the chamber has a circular cross section and the inner wall is cylindrical and situated coaxially within the chamber, thereby giving the first flow path a circular cross section and the second flow path a coaxial annular cross section, with the first flow path being surrounded by the second flow path. The first inlet and the outlet may be aligned to feed gas into and accept gas from the flow paths in a direction substantially parallel to the common axis of the flow paths. The second gas inlet may be arranged to feed radially into the second flow path via a wall of the chamber. The weir gap may be defined by a gap between an upstream edge ofthe cylindrical inner wall and an edge of a ring fitted upstream of and coaxially with the cylindrical inner wall inside the chamber.

A second weir gap may be provided downstream of the inner wall and may be partially defined by a downstream edge ofthe inner wall. In the preferred arrangement described above, the second weir gap may be further defined by a second ring fitted downstream ofthe downstream edge ofthe cylindrical inner wall.

Each of the rings may have a cross section which has a generally radial upstream portion and a generally axial downstream portion with an intermediate, contiguous arcuate portion.

The upstream diameter of each ring may be substantially the same as the inner diameter of the wall of the chamber and the downstream diameter of each ring may be substantially the same as the diameter of the cylindrical inner wall.

The suction means may be a fan and preferably an axial flow fan arranged coaxially within the cylindrical inner wall.

The device may be particularly adapted for installation as a dilution device in a flue gas outlet.

A mixing device according to the invention, therefore, enables intimate and complete mixing to take place between gas flows such as flue gas and dilution air. Use of such a device can therefore serve to ensure that the carbon dioxide level ofthe discharged mixture from a flue meets the required levels.

An embodiment of the invention will now be described, by way of example, with reference to the attached drawing which shows a dilution device in accordance with the present invention installed in a flue gas outlet.

A dilution device in accordance with the present invention comprises a square chamber 8 which has its axis aligned vertically and communicates with a radially fed flue gas inlet 4. The chamber 8 communicates axially with an air inlet 2 and a mixed gas outlet 6. The air inlet 2 opens into the bottom of the chamber 8 and feeds air initially from a horizontal direction, subsequently turning it through 900 to supply it to the chamber in a generally vertical direction. The outlet 6 extends generally vertically from the top of the chamber 8 and directs mixed outlet gas initially vertically, turns it through 90" and subsequently directs it horizontally away from the chamber 8.

An axial flow fan 10 is situated coaxially within an upper portion of an inner cylinder 12, which is situated coaxially inside the chamber 8. The fan 10 is arranged to draw gas upwards. The cylinder 12 has a slightly shorter axial length than the chamber 8.

The suction face of the fan is situated approximately in the centre of chamber 8.

Two rings 14 and 16 are each situated in a respective axial end region ofthe chamber 8. Ring 14 is situated at the lower edge ofthe chamber 8 and ring 16 is situated somewhat lower than the upper edge of the chamber 8. These rings are also coaxial with the chamber 8. Each ring 14, 16 defines a respective weir gap 15, 17 between itself and the nearest axial edge of the inner cylinder 12.

Each ring 14,16 has an arcuate radial cross section, which extends between a generally axially aligned portion and a generally radially aligned portion. The two rings are aligned in the same direction, that is to say with their axially aligned cross sectional portion downstream ofthe radially aligned cross sectional portion. This means that the diameter of the aperture of each ring decreases with flow direction. The greatest diameter of each ring 14,16 is roughly equal to that of the chamber 8, the smallest diameter of each ring is roughly equal to that ofthe inner cylinder 12.

An annular flow path is defined between the radially outer surface ofthe inner cylinder 12 and a radially inner surface ofthe chamber 8. The flue gas inlet 4 feeds directly into this radially. The air inlet 2 and mixed gas outlet 6 communicate with this annular flow path (as well as the inside ofthe inner cylinder 12) via the lower ring 14 and the upper ring 16 respectively.

In use, the fan 10 is caused to rotate, which generates a suction region immediately upstream ofthe fan 10. This region is bounded by the inner walls ofthe inner cylinder 12.

A lower weir gap 15 is defined between ring 14 and the upstream axial edge of the inner ring 12. An upper weir gap 17 is defined between ring 16 and the downstream axial edge of the inner ring 12.

Air flows through the air inlet 2, continues largely uninterrupted through the ring 14 and is drawn to the suction region by the fan 10. From there it is directed axially upwardly across the weir gap 17, and subsequently through the ring 16 towards the mixed gas outlet 6. Flue gas travels through the flue gas inlet 4 and is fed radially into the chamber 8. Flue gas from an upstream region of the flue gas inlet is drawn initially radially into the chamber 8 until it meets the outer surface of the wall of the inner cylinder 12 where it is pulled downwards towards the weir gap 15 where it turns upwards as it passes through the weir gap 15 and is drawn over the upstream edge of the inner cylinder 12 under the influence of the fan.As the flue gas is drawn over the upstream edge ofthe inner cylinder 12 it is distributed broadly across the axial flow path through the inner cylinder 12. Flue gas from a downstream region of the flue gas inlet 4 is drawn initially radially into the chamber 8 and is directed by the outer surface ofthe image cylinder 12 and inner surface ofthe chamber 8 towards ring 16, where it passes in to the weir gap 17 and on out through ring 16 under the influence of a Venturi effect caused by the fan.

The flue gas, therefore, mixes with the inlet air at weir gap 15, as it passes over the upstream edge of the cylinder 12, and at weir gap 17, as it is drawn into the weir gap 17 to mix with the mixed gases propelled by the fan into ring 16.

From the foregoing it can be seen that cylinder 12 effectively acts as a flow straightener. Furthermore, the combination of the inner cylinder 12 and ring 16 ensures good mixing by preventing laminar differentiation across the flow cross section.

The radial cross sectional shape of each of the rings 14 and 16 provides a nozzle effect, thereby enhancing flow direction.

The size and positioning of the weir gaps is important to achieve effective distribution and hence mixing in the correct regions.

The downstream weir gap 17 could be omitted, because the upstream weir gap provides a very effective mixing ofthe flue gas and the inlet air. However, the mixing would generally not be as complete if the downstream weir gap were omitted.

This embodiment ofthe arrangement enables both power levels and noise levels to be lower than in known dilution devices. Furthermore, the distribution system provided by the weirs enables smaller fans to be used as distribution is considerably more even across the discharge duct than in known systems. Because all ofthe components of the device are essentially enclosed by the chamber 8, the device can be provided as a unit, which can easily be installed in an air outlet.

An embodiment ofthe invention has successfully diluted a flue gas containing 9% C02, to an extent that the mixed outlet gas contains only 1% C02 Although the above embodiment employs an axial flow fan, any other appropriate suction device and particularly any other fan could be used.

The invention is not limited for use in a flue gas outlet, but can be used wherever multi-gas streams require intimate mixing.

Claims (15)

1. A mixing device suitable for installation in a gas flow path, comprising a chamber, an inner wall dividing the chamber into first and second flow paths, suction means provided in the first flow path for propelling gas through the first flow path, a first inlet feeding directly into both ofthe flow paths, an outlet feeding directly from both of the flow paths, and a second inlet which feeds directly into the second flow path and is arranged to feed gas into the first flow path by means of a weir gap at least partially defined by the said inner wall.

2. A mixing device according to Claim 1, wherein the chamber has a circular cross section and the inner wall is cylindrical and situated coaxially within the chamber, thereby giving the first flow path a circular cross section and the second flow path a coaxial annular cross section, with the first flow path being surrounded by the second flow path.

3. A mixing device according to Claim 2 wherein the first inlet and the outlet are aligned to feed gas into and accept gas from the flow paths in a direction substantially parallel to the common axis of the flow paths.

4. A mixing device according to Claim 2 or 3, wherein the second gas inlet is arranged to feed radially into the second flow path via a wall of the chamber.

5. A mixing device according to any one of Claims 2 to 4, wherein the weir gap is defined by a gap between an upstream edge of the cylindrial inner wall and an edge of a ring fitted upstream of and coaxially with the cylindrical inner wall inside the chamber.

6. A mixing device according to Claim 5, wherein the ring has a cross section which has a generally radial upstream portion and a generally axial downstream portion with an intermediate, contiguous arcuate portion.

7. A mixing device according to any preceding claim, wherein a second weir gap is provided downstream of the inner wall.

8. A mixing device according to Claim 7, wherein the second weir gap is defined partially by a downstream edge ofthe inner wall.

9. A mixing device according to Claim 7 or 8 when dependent on Claim 2, wherein the second weir gap is partially defined by a second ring fitted downstream of the downstream edge ofthe cylindrical inner wall.

10. A mixing device according to Claim 9, wherein the second ring has a cross section which has a generally radial upstream portion and a generally axial downstream portion with an intermediate, contiguous arcuate portion.

11. A mixing device according to Claim 5 and/or Claim 9 or any claim dependent on each or either of Claim 5 and Claim 9, wherein the upstream diameter of the or each ring is substantially the same as the inner diameter ofthe wall ofthe chamber and the downstream diameter ofthe or each ring is substantially the same as the diameter of the cylindrical inner wall.

12. A mixing device according to any preceding claim, wherein the suction means is a fan.

13. A mixing device according to Claim 12, when dependent on Claim 2, wherein the fan is an axial flow fan arranged coaxially within the cylindrical inner wall.

14. A mixing device according to any preceding claim and particularly adapted for installation as a dilution device in a flue gas outlet.

15. A mixing device substantially as hereinbefore described with reference to the accompanying drawing.