GB2334901A - Mixing device for diluting boiler flue gas with air - Google Patents

Abstract

Flue gas 10 from a boiler is diluted with air 12 in region 24 of a mixing chamber 4 before being discharged through an outlet duct 14. The air enters the chamber through an annular inlet 8 formed between the gas outlet duct and the chamber whilst the flue gas enters the chamber in the opposite direction through an inlet 6. Within the body of the outlet duct is a suction fan 18 that creates a low pressure in region 24 drawing the flue gas and air into the chamber and causing the mixed gases to exit the chamber via the outlet duct. A cone shaped baffle 20 deflects the flue gas entering the mixing chamber and this together with the disruption caused by reversing the direction of air flow assists in mixing the gases. The mixed gases following flow path 16 in the outlet duct are throttled by cone 28.

Description

Mixing Device The present invention relates to a mixing device which has particular, although not exclusive, relevance as use as a dilution device for installation in place of a chimney or flue in the flue gas outlet of a boiler, or the like. The invention has further application, however, in that it has utility wherever there is a need for intimate mixing of gases.

It is known that, in order to comply with emission and building regulations, the discharge from a dilution system must typically not contain more than 1% of CO2. If the discharge is not diluted correctly, then a discharge mixture can have excessive levels of CO2. This can occur, for example, because the flue gas (typically having a concentration of CO2 in the region of 9%) must be diluted with air (having a concentration of CO2 of 0%) and mixed thoroughly in order that the final outlet of mixed gas have a concentration of CO2 of no more than 1%.

Furthermore. even if the correct concentration of gases for dilution is achieved, if the mixing is inefficient, this can lead to stratification in the discharge mixture.

Stratification is a well known effect and results in localised pockets of relatively high or low - concentration of certain gases. Depending upon the position in which a sensor is placed for detecting the concentration of CO2 in the discharge mixture, then stratification in the outlet may result in localised areas of the gas having a concentration of CO2 upto that of the undiluted flue gas, namely 9%.

It is known to provide a dilution devices having a mixed gas outlet of no more than 1% CO2. Even in such devices, it is still possible for stratification to occur. This is due to the fact that flue gas entering a mixing chamber is not necessarily efficiently or completely constrained within the mixing chamber before being passed to an outlet of the device. Hence localised areas of stratification can occur.

It is an object of the present invention, therefore, to provide a mixing device which at least alleviates the aforementioned short coming. It is a further object of the present invention to provide a mixing device which is compact in size and can therefore be constructed as a modular unit and installed in a cassette form, or the like. It is further object of the present invention that a mixing device be adapted for installation as a dilution device in a flue gas output.

According to one aspect of the present invention there is provided a mixing device suitable for installation in a gas flow path comprising: a chamber having a plurality of gas inlets and a gas outlet; a suction device for causing gas to flow into the chamber via the gas inlets of the plurality and exit the chamber via the gas outlet; and a baffle positioned within the chamber to disrupt the flow of gas from at least one of the plurality of gas inlets, the baffle and the suction device being spaced apart thereby to define a mixing region for mixing together the gases from the plurality of gas inlets.

The baffle serves to disrupt the flow of inlet gas thereby causing mixing of the inlet gases in the mixing region defined by a space between the baffle and the suction device. By having a clearly defined mixing region and the baffle disrupting the inlet air in this way, a more thorough mixing of the inlet gases is achievable than has been the case in the prior art.

Preferably the mixing region is upstream of the suction device. Furthermore gases from the plurality of gas inlets, having been mixed in the mixing region, may pass via the suction device into the gas outlet, which gas outlet is downstream of the suction device.

Advantageously the chamber defines a central axis and the baffle, the mixing region and the suction device are all coaxially aligned therewith. By virtue of such coaxial alignment, a compact device is able to be constructed.

Preferably a duct is arranged within the chamber such that gas from another of the plurality of gas inlets flows between the chamber and the duct. This enables a more compact design to be constructed by making use of otherwise redundant space in a device. Advantageously the duct comprises the gas outlet.

In a preferred embodiment the at least one of the plurality of gas inlets is coaxially aligned with the central axis of the chamber such that the gas flowing into the chamber therethrough is coaxial with the central axis, and on impinging upon the baffle is deflected in a direction away from the central axis. This enables the disrupted gas flow to be more readily mixed with inlet gas from another part of the device and helps avoid the problem of stratification. Advantageously the direction of deflection is substantially normal to the central axis.

Preferably gas from the another of the plurality of gas inlets flows toward the mixing region in a direction opposite to that of the outlet gas flowing via the duct. This also helps to reduce the overall size of the mixing device.

Preferably the suction device comprises a fan and the fan may be position concentrically within the duct. Furthermore the device may comprise a gas throttle disposed within the gas outlet.

According to another aspect of the present invention, there is provided a mixing device suitable for installation in a gas flow path comprising: a chamber having a plurality of gas inlets and a gas outlet; a suction device for causing gas to flow into the chamber via the gas inlets of the plurality and exit the chamber via the gas outlet, and wherein at least one of the plurality of gas inlets surrounds the gas outlet and wherein the direction of flow of gas through the at least one plurality of gas inlets and the gas outlet are opposite to each other.

The present invention will now be described, by way of example only, and with reference to the accompanying drawings of which: Figure 1 shows a schematic sectional view of a mixing device in accordance with the present invention: Figure 2 shows an end view from one side of the device, and: Figure 3 shows an end view from the other side of the device.

Referring now to the drawings Figure 1 shows a mixing device 2 in accordance with the present invention which, in this example, communicates with the flue gas outlet of a domestic gas boiler system (not shown). The device 2 comprises a chamber 4 of generally square cross-section having a plurality of gas inlets. The gas inlets of the chamber 4 comprise a flue gas inlet 6 and an air inlet 8. The flue gas passing through the inlet 6 comprises typically 9% CO2, whilst the air inlet 8 comprises typically 0% CO2. Each of the inlets 6, 8 defines a flow path for the gas passing therethrough.

These can be seen by the enlarged arrows in the figure and the flow path for the flue gas inlet 6 is shown as 10, whilst the flow path for the air inlet is shown as 12.

Housed concentrically within the chamber 4 is a cylindrical duct having a circular cross-section, here an outlet duct 14. The purpose of the outlet duct 14 is to provide an outlet passage for gas leaving the mixing device 2. The flow path of the outlet gas passing through the outlet duct 14 is shown by the arrows marked 16.

Because the outlet duct 14 is arranged concentrically within the chamber 4, the air inlet 8 which defines the flow path for air 12 is generally annular. In this way, therefore, air entering the mixing device 2 via the air inlet 8 will follow the flow path 12 into the device whilst generally annularly surrounding the outlet duct 14 which provides the flow path 16 for outlet gas.

The mixing device 2 includes a suction device, in this example a fan 18. The fan 18 is arranged coaxially with the chamber 4 and is also housed within the body of the outlet duct 14.

Also housed within the chamber 4, and in the example of Figure 1 coaxially aligned therewith, is a baffle, here mixing cone 20. The purpose of the mixing cone 20 is to disrupt the flow path for flue gas 10 entering via the flue gas inlet 6. The mixing cone 20 is held in position by way of cone supports 22.

The fan 18 is arranged to create a suction region, by way of low pressure, upstream thereof and thereby draw gas into the chamber 4 via the flue gas inlet 6 and air inlet 8. The flue gas flow path 10 shows the passage of flue gas entering the chamber 4 having passed through the flue gas inlet 6. The flow path 10 is disrupted on meeting the mixing cone 20 and follows generally the path defined by the arrows 10.

Also air following the air flow path 12 having entered via the generally annular air inlet 8 initially passes along the region defined by the inside of the cylindrical chamber 4 but outside the outlet duct 14 before meeting with the flue gas flow path 10 in a mixing region 24, upstream of the fan 18 and downstream of the mixing cone 20. In the mixing region 24, intimate mixing between the flue gas and the air following their respective paths 10 and 12 occurs, as will be described in detail below. It will be seen from figure 1 that the air flow path 12, on entering the mixing region 24 is turned through 180 before entering into the outlet duct 14 having been mixed with the flue gas following flow path 10, following which both gases (having been mixed) follow the outlet flow path 16 and are ejected from the mixing device 2.

It will be seen from figure 1 that the chamber 4 defines a central axis 26 with which are coaxially aligned the mixing cone 20, the fan 18 and the outlet duct 14. Such coaxial alignment provides a significant advantage over the prior art in that it allows a compact and efficient design of mixing device 2 having a high degree of symmetry about this central axis 26.

In use of the mixing device 2, which is generally oriented substantially horizontally, flue gas containing around 9% CO2 enters via the flue gas inlet 6 under the influence of the fan 18, as has been described above. Because the fan creates a low pressure upstream thereof; the suction caused by this low pressure causes air to enter the device 2 via the air inlet 8 concomitantly with flue gas entering via the flue gas inlet 6.

Because the flue gas inlet 6 is circular, as can be seen most easily from figure 2 and because the air inlet 8 is generally annular, as can be seen most easily from figure 3, then their respective flow paths 10, 12 are initially likely to be laminar.

However, due to the fact that the mixing cone 20 disrupts the flow path 10 of the flue gas by causing a deflection of the path 10 in a direction substantially normal to that of this central axis 26 then the laminar flow is disrupted. Also the mixing region 24 is at a lower pressure than any other region of the device 2 due to being immediately upstream of the fan 18. This causes the deflected flue gas air flow path 10 to be attracted into the mixing region 24. Additionally, this low pressure attracts the air flow path 12 thereby causing its direction to change by firstly 90 so that it travels perpendicularly toward the central axis 26. Finally the air flow path turns through a further 90 to reverse its initial direction. This too will disrupt any laminar flow of the air following the air flow path 12.

When both the flue and air gases 10, 12 enter the mixing region 24, because their flow has been disrupted, they are likely to mix together. Additionally, because both of these flow paths 10, 12 have to pass through the rotating fan 18, this will cause an intimate mixing of the two gases.

Because of the disruption to their flow paths and also because they have passed via the rotating fan 18, the discrete flow paths 10, 12 no longer exist downstream of the fan 18 and the flue and air gases have been intimately mixed and exit the device 2 via the outlet duct 14 by following the flow path 16.

The intimately mixed gas following the flow path 16 has been found to have a concentration of CO2 less than or equal to 1%. This meets with the requirements of emission and building regulations.

There exist other regulations, however, which can require that the velocity of gas being discharged from a dilution system such as the mixing device 2 reach a minimum value. For this reason, it is advantageous that disposed within the outlet duct 14 is a throttle, here a distribution cone 28. As can be seen from the figures, the distribution cone 28 is arranged to throttle the flow of gas following outlet path 16 there passed such that the gas forms an annular flow path therearound and, because the distribution cone 28 is shaped so that its upstream diameter is less than its downstream diameter, this causes an effective reduction in radial width of the annular flow path followed by the outlet gas 16 thereby increasing its velocity, in known manner.

The distribution cone 28, therefore enables the minimum velocity associated with any building regulation to be met.

The mixing device 2 includes within the outlet duct 14 an airflow proving device 30. The proving device 30 is connected to a differential pressure switch 32 situated outside of the chamber 4 and the two devices 30, 32 act as safety devices to provide an interlock back to the gas boiler (not shown) and coupled to the path followed by the flue gas entering via the inlet 6. These devices are well known to those skilled in the art and so will not be described in any detail. Should there be no, or an insufficient, flow of flue gas via the inlet 6, then the arrangement of devices 30 ,32 will prevent the boiler (not shown) from operating.

One aspect of the present invention is particularly suitable for the avoidance of stratification of CO2 within the outlet gas flow path 16 due to the effect of the disruption to the flow paths 10, 12 caused by the mixing cone 20. Also the rotation through 180 flow path 12 between inlet 8 and outlet duct 14 further disrupts the flow paths and so help avoid stratifications. If however further disruption of the flow paths 10, 12 is required, this may be achieved, for example, by altering the size or axial position of the mixing cone 20. Because the mixing cone 20 is positioned via cone supports 22, these may be altered to move the axial position of the cone 20 nearer to or further from the flue gas inlet 6 in order to provide increased or decreased disruption of the flue gas flow path 10 therethrough.

Additionally, or alternatively, it may be possible to provide baffles in the air flow path 12 to provide further disruption thereto. Furthermore there could be provided in the mixing region 24 upstream of the fan 18 and downstream of the mixing cone 20, a further baffle 4 enhancing the intimate mixing between the two air flow paths 10, 12.

Whilst in the above description the chamber 4 has been described as having a generally square cross section, it may take any cross-sectional shape such as circular or rectangular. Additionally the outlet duct 14 may take any suitable shape and is not necessarily of circular cross-section.

Advantages may be gained, however, from using generally circular cylindrical shapes for the chamber 4 and outlet duct 14. This will allow axial alignment of the inlet paths, outlet paths and mixing cone 20 and fan 18. Such axial alignment enables a Whilst an axial flow fan 18 has been disclosed in the above embodiment, any suitable device for creating a region of suction upstream thereof by low pressure and a region of excess pressure downstream thereof will suffice. Once again, due to the axial alignment of the components within the device 2, a smaller fan than his hitherto been used is able to operate efficaciously in the present embodiment. Because smaller fans are able to be used, this reduces the power consumption and noise level generated by the device 2 and enables the unit to be arranged in a compact nature which can be easily installed with a gas boiler or the like.

The present invention is not limited for use in an air flue gas outlet but may be used wherever multi-gas streams require intimate mixing.

Claims (13)

1. CLAIMS: 1. A mixing device suitable for installation in a gas flow path comprising: a chamber having a plurality of gas inlets and a gas outlet; a suction device for causing gas to flow into the chamber via the gas inlets of the plurality and exit the chamber via the gas outlet; and a baffle positioned within the chamber to disrupt the flow of gas from at least one of the plurality of gas inlets, the baffle and the suction device being spaced apart thereby to define a mixing region for mixing together the gases from the plurality of gas inlets.
2. 2. A mixing device according to claim 1 wherein the mixing region is upstream of the suction device.
3. 3. A mixing device according to claim 2 wherein gases from the plurality of gas inlets, having been mixed in the mixing region, pass via the suction device into the gas outlet, which gas outlet is downstream of the suction device.
4. 4. A mixing device according to any one of the preceding claims wherein the chamber defines a central axis and the baffle, the mixing region and the suction device are all co-axially-axial aligned therewith.
5. 5. A mixing device according to claim 4 further comprising a duct arranged within the chamber such that gas from another of the plurality of gas inlets flows between the chamber and duct.
6. 6. A mixing device according to claim 5 wherein the duct comprises the gas outlet.
7. 7. A mixing device according to claim 6 wherein the at least one of the plurality of gas inlets is co-axially aligned with the central axis of the chamber such that gas flowing into the chamber therethrough is co-axial with the central axis, and on impinging upon the baffle is deflected in a direction away from the central axis.
8. 8. A mixing device according to claim 8 wherein the direction of deflection is substantially normal to the central axis.
9. 9. A mixing device according to claim 6 wherein gas from the another of the plurality of gas inlets flows toward the mixing region in a direction opposite to that of the outlet gas flowing via the duct.
10. 10. A mixing device according to any one of the preceding claims wherein the suction device comprises a fan.
11. 11. A mixing device according to claim 10 when appendant to claim 5 wherein the fan is positioned concentrically within the duct.
12. 12. A mixing device according to any one of the preceding claims further comprising a gas throttle disposed within the gas outlet.
13. 13. A mixing device suitable for installation in a gas flow path comprising: a chamber having a plurality of gas inlets and a gas outlet; a suction device for causing gas to flow into the chamber via the gas inlets of the plurality and exit the chamber via the gas outlet, and wherein at least one of the plurality of gas inlets surrounds the gas outlet and wherein the direction of flow of gas through the at least one plurality of gas inlets and the gas outlet are opposite to each other.