WO2008010242A1

WO2008010242A1 - Device for pollution reduction and relative method for fume treatment

Info

Publication number: WO2008010242A1
Application number: PCT/IT2006/000544
Authority: WO
Inventors: Valerio Abate; Alberto Conti; Michela Bianchi
Original assignee: Easy International S.R.L.
Priority date: 2006-07-18
Filing date: 2006-07-18
Publication date: 2008-01-24
Also published as: EP2097674A1

Abstract

The invention relates to a fume treatment device (10) for use in a chimney flue (3) of a boiler (2), said flue having a minimum project section (A). The device comprises a branch-off (32) that divides the flue (3) into a main duct (30) and a by-pass pipe (31 ), and a purifier assembly (11 ) positioned along said by-pass pipe (31 ). The branch-off (32) is static and the main duct (30) is completely open for passage therethrough during normal boiler operation.

Description

DESCRIPTION

"Device for pollution reduction and relative method for fume treatment"

[0001] The present invention relates to a device for pollution reduction, in particular, for exhaust fumes discharged from boilers for central heating and/or domestic hot water systems.

[0002] According to a further aspect, the present invention relates to a method for treating exhaust fumes from boilers. [0003] According to another further aspect, the present invention relates to a system for measuring the flow rate in chimney flues. [0004] It has always been a necessary priority to improve the quality of the air in urban centres. The most common causes of air pollution include internal combustion transport means and boilers for central heating in civil buildings.

[0005] As far as limiting the pollution caused by internal combustion self- driven motors is concerned, research has produced considerable results in recent years, for example by improving combustion quality, and above all through the treatment of exhaust fumes using suitable catalysts and filters, before the fumes are released into the atmosphere.

[0006] On the contrary, in order to reduce pollution caused by boilers, some device or method is still required to treat exhaust fumes to drastically reduce the amount of pollutants before they are released into the atmosphere. [0007] Unfortunately, it is not possible to transfer a direct application of the systems used on vehicles onto civil buildings. In fact, appropriate safety and systems used on vehicles onto civil buildings. In fact, appropriate safety and fire prevention standards severely forbid the reduction or blocking of the necessary open section required in exhaust flues for boiler fumes. [0008] Therefore these standards prevent the use of filters and/or catalysts inside boiler exhaust flues for safety reasons.

[0009] The aim of the present invention is therefore to provide a device and a method for treating boiler exhaust fumes, and for reducing the pollutants contained in the fumes, in order to at least partially resolve the problems described in relation to prior art. [0010] This aim is achieved by means of a device according to claim 1 , and by means of a method according to claim 22.

[0011] For a better understanding of the invention and in order to appreciate its advantages, certain embodiments are described below as examples which are by no means limiting, with reference to the appended drawings wherein:

[0012] Figure 1 is a schematic view of a civil building equipped with a boiler and the related fume exhaust flue comprising a device according to the invention;

[0013] Figure 2 is a schematic view of the detail indicated by numeral Il in figure 1 , comprising a device according to the invention;

[0014] Figure 3 is a schematic view of the detail indicated by numeral III in figure 2;

[0015] Figure 4 is a schematic view of a flow rate measuring device according to the invention; [0016] Figure 5 is a schematic view of another flow rate measuring device according to the invention;

[0017] Figure 6 is a schematic view of the detail shown in figure 3 in a first flow configuration;

[0018] Figure 7 is a schematic view of the detail shown in figure 3 in a second flow configuration.

[0019] With reference to figure 1 , the building 1 comprises a boiler 2 and its specific chimney flue 3, which is terminated with a chimney pot 4. A fume treatment device according to the invention is positioned along the chimney flue 3, said device being identified in its whole with the numeral 10. [0020] Hereafter, the term "upstream" will refer to a position along the course of the chimney flue 3 relatively near the boiler 2, while the term "downstream" will refer to a position along the course of the chimney flue 3 relatively near the chimney pot 4. [0021] The chimney flue 3 is dimensioned in a manner known in the art in order to guarantee combustion quality inside the boiler, ensure correct fume dispersion, and guarantee the absorption of the pressure wave front typical of boiler operation such as during start-up. In this manner, a minimum project section A is defined according to proven project design techniques. [0022] The device 10 comprises a branch-off 32, which divides the chimney flue 3 into a main flue duct 30 and a by-pass pipe 31. Preferably, the device will also comprise a header 33 that will reconnect the by-pass pipe 31 to the main flue duct 30 once more.

[0023] The internal section of the main flue duct 30 is larger than or equal to the minimum project section A along its total length from the branch-off 32 as far as the header 33. In this manner the main flue duct 30 will guarantee that the whole chimney flue 3, up or downstream, or from the boiler 2 to the chimney pot 4, will continue to maintain the required size without any reduction in its section under the size of the minimum project section A. [0024] Furthermore, the branch-off section 32 is static since it does not include any separators, gate valves or similar elements to direct the flow into the by-pass pipe 31. In other words, the main flue duct 30 is completely open for passage through it during normal boiler 2 operations. At this point, and further on, the chimney flue 3 and the main flue duct 30 are considered as being completely open for passage through them when they possess an open operating section greater than or equal to the minimum project section A.

[0025] Any conditions where the main flue duct 30 is not perfectly free, are considered as serious faults, which can cause a potential danger for the operation of boiler 2. Such faults may occur, for example, in the case where some foreign material provokes the modification of the internal section of the chimney flue 3.

[0026] The characteristics of the main flue duct 30 thus described, ensure that the chimney flue 3 comprising device 10 according to the invention, comply with the strictest safety and fire prevention standards. [0027] The device 10 also includes a purifier assembly 11 on the by-pass pipe 31. This purifier assembly 11 comprises means, known per se in the prior art, for separating the polluting substances from the current that is expelled and dispersed in the atmosphere. [0028] For example, the purifier assembly 11 can comprise static filters, cyclone dust separators, electrostatic devices or similar equipment 111 conceived to separate dust and particles from the main flow composed of gas and/or vapour/steam.

[0029] This purifier assembly 11 can also comprise catalysts 112 able to let the gaseous pollutants present in the fumes react so as to reduce the concentration before their dispersion into the atmosphere.

[0030] In accordance with a preferred embodiment, the device 10 also includes two flow rate measuring devices 12 and 13, a motor driven fan 14, and a control unit 15. [0031] Said control unit 15 can be electrically connected to both the flowo rate measuring devices 12 and 13, as well as to the motor driven fan 14. Furthermore, said control unit 15 is preferably of the electronic type and comprises, for example, a micro-controller or a microprocessor. [0032] The first flow rate measuring device 12 is able to measure the flow rate inside the chimney flue 3, upstream the branch-off 32. The second flow5 rate measuring device 13 is able to measure the flow rate downstream the branch-off 32. Preferably the second flpw rate measuring device 13 is able to measure the flow rate in the by-pass pipe 31 downstream the branch-off 32. [0033] The motor driven fan 14 is preferably positioned along the by-pass o pipe 31 downstream the purifier assembly 11.

[0034] The motor driven fan 14 is conceived to generate in the by-pass pipe a suction pressure equal to or higher than the resistance flow or pressure drop caused by the purifier assembly 11. Said motor driven fan 14 is activated from the control unit 15 according to the data measured by the two5 flow rate measuring devices 12 and 13. In particular this data can be composed of indicative digital signals of the flow rate measured by aforesaid measuring devices 12 and 13 for transmission to the control unit 15.

[0035] A general embodiment of the fume treatment method according to the invention comprises the following steps: [0036] providing a device 10 for fume treatment according to the invention;

[0037] measuring the fume flow rate in the chimney flue 3 upstream the branch-off 32;

[0038] measuring the fume flow rate in the chimney flue 3 downstream the branch-off 32; [0039] transmitting the flow rate measurement results to the control unit 15 in order to establish the difference in flow rate between the chimney flue 3 upstream the branch-off 32, and the by-pass pipe 31 downstream the branch-off 32;

[0040] actuating the motor driven fan 14 in order to create a suction pressure in the by-pass pipe 31 , which is sufficient to eliminate the difference in flow rate between the chimney flue 3 upstream the branch-off

32, and the by-pass pipe 31 downstream the branch-off 32;

[0041] making the fumes pass through the purifier assembly 11 ; and

[0042] releasing the residue fumes into the atmosphere. [0043] Those skilled in the art will easily understand from the aforesaid description that the flow rate measuring devices 12 and 13 are intended for measuring the flow rate of a flow composed of a fluid mixture with suspended solid particles.

[0044] Many categories of common speed meters, for measuring flow rate (venture-meters, anemometers, etc) have been revealed as unsuitable for use in the device 10 according to the invention. The environmental conditions in which these instruments must operate would compromise the operating action in a relatively short time.

[0045] For this reason, yet a further aspect of the present invention concerns a flow rate Measuring device for chimney flues.

[0046] Further on, with particular reference to figure 4, is the detailed description of flow rate measuring device 12 according to the invention, arranged so that it measures the flow rate in chimney flue 3 upstream the branch-off 32. This description is naturally equally valid for the flow rate measuring device 13 arranged to measure the flow rate in the by-pass pipe 31 downstream the branch-off 32, and which can be produced in a similar manner.

[0047] The mass flow rate Q in the chimney flue 3 must be calculated from the measurements of the volume flow rate and the density of the fumes. The volume flow rate Q_v is calculated thanks to the known value of the chimney flue 3 section, and to the speed V of the flow. In turn, the speed V can be calculated using the Bernoulli theorem in relation to non-ideal piping in which pressure drop R occurs. [0048] In figure 4 , a section 'a' upstream, and a section 'b¹ downstream can be seen; the following equation can be composed for general application:

V₂ V_a ² + gz_a + Pa/p = ¹/₂ V_b ² + gz_b + P_b/p + R_ab (V) where V_a and V_b are the flow speeds of section a and section b respectively; g is the gravity acceleration; Z₃ and Z_b are the flow altitude of section a and section b respectively; P_a and P_b are the flow pressures of section a and section b respectively; p is the fume density; R_ab (V) is the pressure drop that has occurred between sections a and b, pressure drop which is function, inter alia, of the flow speed V.

[0049] The equation provided above can be simplified in the specific case. For example if the two sections of the flue 3, a and b, are considered as having the same area, from the flow rate continuity equation- it will result that the speeds V₃ and V_b are equal to each other. It is also possible to consider that the potential terms of gz_a and gz_b will be the same. [0050] The result is that a link is obtained between the difference in pressure ΔP = P_a - P_b and the pressure drop function of the speed R_ab (V). When the function that links the pressure drop R with the speed V is known, it is possible to determine the speed V from ΔP.

[0051] Still using figure 4 as reference^ the value of ΔP can be obtained by means of a differential pressure gauge 120. The differential pressure gauge 120 comprises a first pressure sensor 121 and a second pressure sensor 122. The pressure sensors 121 and 122 are positioned in order to read the flow pressure of section a and section b respectively. [0052] A first logic unit 123 obtains the ΔP value from the differential pressure gauge and based on this it can provide the value of the flow speed V. Finally a second logic unit 124 obtains the value of V and based on this it can provide the value of the mass flow rate Q.

[0053] It can be seen that said first and second logic units 123 and 124, can be implemented using electronic modules (hardware type) and can be included for example, in a digital signal processor or DSP. In addition, said logic units 123 and 124 can also be implemented using software modules, in other words- a sequence of program instructions and controlled by a suitable microprocessor.

[0054] Advantageously, other accessory sensors can be used in order to make these calculations even more accurate. One or two thermometers 125 and one or two hygrometers 126 can provide useful information for constant updating of fume density p values, a value that is necessary for the second logic unit 124 in order to calculate the mass flow rate Q. Alternatively, the p value can be obtained through appropriate tables, for example, those relative to operating stages of the boiler or the like. [0055] Those skilled in the art will easily understand from the aforesaid description, that the heart of the problem in calculating the flow mass rate Q lies in the link between the pressure drop R and the flow speed V in the chimney flue 3.

[0056] One case where said link has been widely researched in technical writings is the case of the diaphragm. The diaphragm is a separator in the shape of a circular crown, including a central free passage, arranged along a pipe set at right angle to the axis of the pipe in question. The diaphragm introduces a concentrated pressure drop into the pipe. The link of this loss with the speed of the fluid has been researched extensively and is known on an empiric basis. In particular: R (V) = V₂ β V²

Where β is a constant known in technical writings, and is called "accidentality factor".

[0057] An example of a known diaphragm is that described in the UNI EN ISO 5167 standards issued in 2004, to which reference should be made for further specifications. [0058] A plain diaphragm of the common well known type cannot be used in the chimney flue 3 because of the reduction this will provoke in the flue section. As was seen previously, safety and fire prevention standards prevent any kind of restriction, impediment, or reduction that modifies the section of a flue to a size less than the minimum project section A.

[0059] The embodiment of the flow rate measuring device 12 according to the invention shown in figure 5, overcomes this problem. In fact the flow rate measuring device12 comprises a divergent 127 conceived to increase the section of the chimney flue 3. [0060] In accordance with one embodiment, the flow rate measuring device 12 also comprises a convergent 129 conceived to reduce the flue 3 section once more. The maximum possible reduction provided by the convergent 129 is obviously that which returns the flue section 3 to the size which it possessed upstream the convergent 127, which should normally be the minimum project section A.

[0061] In accordance with another possible embodiment, the flow rate measuring device 12 also comprises a portion with an increased section 128, set between the divergent 127 and the convergent 129. [0062] Along this portion of increased section, for example, along the length of the constant increased section 128, a diaphragm 130 of a commonly known type, is installed. The diaphragm 130 is chosen in order to ensure that the central passage 130' has an area equal to or larger than that of the minimum project section A of the flue 3. [0063] Those skilled in the art will understand from the description that along the total length of the flow rate measuring device 12, the internal section of the flue 3 is greater than or equal to the section upstream the divergent 127. In other words, the internal section of the flue 3 is greater than or equal to the minimum project section A. A flow pipe with this section is indicated by dotted lines in figure 5. [0064] As described previously, and once again with reference to figure 5, the flow rate measuring device according to the invention also comprises a differential pressure gauge 120, which in turn includes a pressure sensor 121 upstream the diaphragm 130, and. a pressure sensor 122 downstream the diaphragm. [0065] The differential pressure gauge 120 provides the first logic unit 123 with the ΔP value in order to calculate the value of the flow speed V. Said first logic unit 123 provides the value of the speed V to the following second logic unit 124 which calculates the value of the mass flow rate Q. [0066] Other accessory sensors can also be used to advantage in this embodiment as well to make calculations even more accurate. A thermometer 125 and/or a hygrometer 126 can provide useful information for constant updating of fume density p values, necessary for the second logic unit 124 in order to calculate the mass flow rate Q. For the sake of simplicity the connections between* the thermometer 125 and the hygrometer 126 and the said second logic unit 124 have been omitted in figure 5.

[0067] The diaphragm 130 of the flow rate measuring device 12 according to the invention will not generally operate in exactly the same manner as an identical diaphragm of a commonly known type positioned in a pipe with a constant section. In any case, even if the accidentality factor β cannot be obtained from technical writings, it can be determined on an empiric basis during an initial taring stage.

[0068] In accordance with the embodiment illustrated in figure 3, the flow rate measuring device 12 and the branch-off 32 are combined together in order to occupy as short a length as possible along the chimney flue 3.

[0069] As can be seen easily in figure 3, the branch-off 32 is inserted downstream the diaphragm 130, for example on the convergent 129. The measurement of the mass flow rate Q must be carried out upstream the branch-off 32 in any case. [0070] Because of the close vicinity of the diaphragm 130 and the branch- off 32, it is advantageous to move the central passage 130' laterally in order to facilitate the flow when it is deviated towards the by-pass pipe 31. In fact fluid dynamics studies on the present case, and practical experimental tests showed that the flow continuity gains an advantage from having the central passage 130' moved in the opposite direction to that where the by-pass pipe 31 branches off. This off-centered position is shown clearly in figure 3. [0071] In accordance with the embodiment of the invention shown in figure 3, the flow rate measuring device13 also comprises a diaphragm with an off- centre passage. This particular geometry permits easier flow through from the elbow joint of the branch-off 32.

[0072] Figure 7 shows a field simulation of a flow speed, which moves into the by-pass pipe 31 at the branch-off point 32. This simulation shows the advantage gained from the off-centered position of the centre passages of the two diaphragms. [0073] From the aforesaid description, those skilled in the art will easily understand the advantages of static branch-off such as that described. The branch-off 32 of the main flue pipe 30 to the branch of bypass pipe 31 is referred to as "static" because it contains no form of separator, gate valve or mobile parts. [0074] The correct operation of the static branch-off described is guaranteed by the control unit installed. When it has received the data indicating the mass flow rate Q calculated by the flow rate measuring devices 12 and 13, the control unit 15 activates the motor driven fan 14 in order to perform the elimination of the difference between the two flow rates. [0075] The architecture of this system has proved to be particularly robust because during normal working operations, both flow rate Q values are finite and far different from zero. For this reason oscillation and shifting caused by systematic error and/or background noise picked up by the instruments can be avoided. [0076] In order to correctly achieve the working conditions described above, the system must have control over the transient which starts when the boiler 2 is started up. Below is a brief description of the transient control. [0077] Forced ventilation boilers currently available on the market are equipped with a safety system which activates the fan to evacuate fumes from the combustion chamber at each start-up, before the flame is actually lit up. This system is necessary to evacuate any undesirable amounts of inflammable gas. Combustible gas could be present in the combustion chamber (methane, GPL or others) because the valve on the supply pipe is not perfectly hermetic. However it is more probable that carbon monoxide (CO) is present having been produced at the previous switch-off. These inflammable gases can cause a deflagration in the combustion chamber at start-up.

[0078] Such anticipated switching on of the fan to evacuate fumes results, prior to lighting the flame, in pressure oscillation in the chimney flue 3. [0079] The fume treatment method according to the invention provides the advantageous stage of activating the motor driven fan 14 when pressure oscillation occurs in the chimney flue 3. In this manner, before the flame is lit, a suction pressure is created in the by-pass pipe 31 which will attract the deviated flow shown in figure 7. [0080] From the aforesaid description it is obvious how the device 10 according to the invention provides excellent fume treatment without reducing the section of the flue 3 to a size less than the minimum project section A. [0081] Furthermore, those skilled in the art will be well aware that thanks to the active control of the flow rate in outflow, the device 10 according to the invention ensures excellent boiler operation. In particular it permits ideal combustion even when pressure oscillation is present, perhaps caused by specific wind conditions on chimney pot 4, or particular atmospheric pressure conditions. [0082] Naturally, those skilled in the art are able to apply further modifications and variants to the fume treatment device according to the present invention in order to satisfy specific and current needs, while still remaining within the protective context of the invention as defined in the appended claims.

Claims

1. Fume treatment device (10) for use in a chimney flue (3) of a boiler (2), said flue having a minimum project section (A), said device comprising:

- a branch-off (32) that divides said flue (3) into a main duct (30) and a by-pass pipe (31 ); and

- a purifier assembly (11 ) positioned along said by-pass pipe (31 ); characterised in that said branch-off (32) is static and that said main duct (30) is completely open for passage therethrough during normal boiler (2) operation.

2. Device (10) according to claim 1 wherein said main duct (30) constantly has an operating section larger than or equal to said minimum project section (A).

3. Device (10) according to claim 1 or 2, wherein said purifier assembly (11) comprises devices (111) able to separate dust, flour and particles from the fume flow.

4. Device (10) according to any previous claim wherein said purifier assembly (11) comprises static filters, cyclone dust separators, electrostatic devices and the like.

5. Device (10) according to any previous claim wherein said purifier assembly (11 ) comprises devices (112) able to let the with the gaseous pollutant present in fumes react so as to reduce their concentration before they are dispersed in the atmosphere.

6. Device (10) according to any previous claim wherein said purifier assembly (11) comprises catalysts.

7. Device (10) according to any previous claim which also includes a header (33) conceived to reconnect said by-pass pipe (31) and said main duct (30) in said flue (3).

8. Device (10) according to claim 1 , which also comprises:

- a first flow rate measuring device (12) suitable for measuring the flow rate in the flue (3) upstream the braήch-off (32);

- a second flow rate measuring device (13) suitable for measuring the flow rate in the by-pass pipe (31 ) downstream the branch-off (32);

- a motor driven fan (14) positioned along the by-pass pipe (31 );

- a control unit (15) conceived to activate said motor driven fan (14) according to the difference in flow rate measured by said first and second flow rate measuring devices (12,13).

9. Device (10) according to claim 8 wherein said motor driven fan (14) is conceived to generate in the by-pass pipe (31) a suction pressure that is greater than or equal to the pressure drop introduced into the flow by the purifier assembly (11 ).

10. Device (10) according to claim 8 wherein each of said flow rate measuring devices (12) comprise a differential pressure gauge (120), a first logic unit (123), and a second logic unit (124), said differential pressure gauge (120) being able to read the difference in pressure (ΔP) between two sections (a, b,) of the flue (3) and to transmit the value of said pressure difference (ΔP) to said first logic unit (123), said first logic unit (123) being able to calculate the value of the flow speed (V) and to transmit the value of the said speed (V) to said second logic unit (124), said second logic unit (124) being able to calculate the value of the mass flow rate (Q), and to transmit the value of said mass flow rate (Q) to said control unit.

11. Device (10) according to claim 8 wherein each of said flow rate measuring devices (12, 13) also comprise at least one thermometer (125) able to measure the value of the flow temperature and to transmit the value of said temperature to said second logic unit (124).

12. Device (10) according to claim 10 or 11 , wherein each of said flow rate measuring devices (12, 13) also comprise at least one hygrometer (126) able to measure the value of the humidity of the flow and to transmit said value to said second logic unit (124).

13. Device (10) according to claim 8 wherein the control unit (15) is of the electronic type and comprises a micro-controller or a microprocessor.

14. Measuring device (12) for the flow in a chimney flue (3), said flue being defined with a minimum project section (A), said measuring device comprising:

- a divergent (127) adapted to increase the section of the flue(3); - a diaphragm (130) positioned in a portion of the flue (3) having an enlarged section and having a central passage opening (130') equal to or larger than the minimum project section (A) of the flue (3); and

- a differential pressure gauge (120); wherein said differential pressure gauge (120) is able to read a difference in pressure (ΔP) between one section (a) of the flue (3) just upstream the diaphragm (130) and a section (b) of the flue (3) just downstream the diaphragm (130).

15. Measuring device (12) according to claim 14, further comprising a first logic unit (123) and a second logic unit (124), wherein said differential pressure gauge (120) is able to transmit the value of said pressure difference (ΔP) to said first logic unit (123), said first logic unit (123) being able to calculate the value of the flow speed (V) and to transmit the value of said speed (V) to said second logic unit (124), said second logic unit (124) being able to calculate the value of the mass flow rate (Q) and to transmit the value of said mass flow rate (Q).

16. Measuring device (12) according to; claim 14 or 15, further comprising a convergent (129) adapted to reduce the section of the flue (3) once more, said reduction being limited to measuring no less than the minimum project section (A).

17. Measuring device (12) according to any claim from 14 to 16, further comprising a branch-off (32), said branch-off being static and being located downstream the diaphragm (130).

18. Measuring device (12) according to claim 17, wherein the central passage (130') of said diaphragm (130) is moved laterally in the direction opposite to that in which the branch-off (32) extends.

19. Measuring device (12) according to claim 14, wherein said first and second logic unit (123, 124) are implemented by means of electronic modules of the hardware type, included in a digital signal processor or DSP.

20. Measuring device (12) according to claim 14, wherein said first and second logic unit (123, 124) are implemented by means of software modules, in other words, sequences of program instructions, and controlled by a suitable microprocessor.

21. Device (10) according to claim 8 wherein at least one of said flow rate measuring devices (12, 13) is in accordance with any of the claims from 14 to 20.

22. Method for the treatment of exhaust fumes from a boiler (2) comprising the following steps of:

- providing along the flue (3) of said boiler (2) a fume treatment device (10) according to claim 8; - measuring the fume flow rate in the flue (3) upstream the branch-off (32);

- measuring the fume flow downstream the branch-off (32);

- transmitting of the flow rate results to the control unit (14) in order to calculate the difference in the flow rate between the flue (3) upstream the branch-off, and the flue (3) downstream the branch-off ;

- actuating the motor driven fan (14) in order to create a suction in the bypass pipe (31) that is sufficient to eliminate the flow rate difference between the flue (3) upstream the branch-off (32) and the by-pass pipe (31) downstream the branch-off (32); - directing the fumes through the purifier assembly (11 ); and

- dispersing the residual fumes into the atmosphere.

23. Method according to claim 22, wherein the stage of measuring the fume flow rate downstream the branch-off (32) comprises the stage of measuring the fume flow rate in the by-pass pipe (31) downstream the branch-off (32).

24. Method according to claim 22 or 23, wherein each measuring stage of the flow rate (Q) comprises the steps of:

- measuring the difference in pressure (ΔP) between two successive flow sections (a, b);

- calculating the flow speed (V) by means of said pressure difference (ΔP); - calculating the mass flow rate (Q) by means of said speed (V).

25. Method according to claim 24, wherein the stage of calculating the mass flow rate (Q) comprises the stage of measuring the flow temperature.

26. Method according to claim 24 or 25, wherein the stage of calculating the mass flow rate (Q) comprises the stage of measuring the flow humidity.

27. Method according to any claim from 22 to 26, also comprising the stage of re-directing the fumes into the flue (3) through a header (33) downstream the purifier assembly (11 ).