GB2610624A

GB2610624A - Pollution control system

Info

Publication number: GB2610624A
Application number: GB2113011.7A
Authority: GB
Inventors: Purnell Mark
Original assignee: Individual
Current assignee: Individual
Priority date: 2021-09-13
Filing date: 2021-09-13
Publication date: 2023-03-15
Anticipated expiration: 2041-09-13
Also published as: GB2610624B; GB202113011D0

Abstract

The system comprises one or more inlets 12, an air cleaning unit 30, a duct 14 connected to the inlet and air cleaning unit and an outlet 40 to vent the clean air back out the system. The system also comprises a mechanism for forcing air into the inlet, through the ducting, through the air cleaning unit and out of the outlet. Wherein the mechanism can be reversible to force the air through the ducting and out the inlets. The mechanism may include a fan or blower 32 and the air cleaning unit may contain a number of filters e.g. a pre-filter, HEPA filter and/or an activated carbon filter. The fan or blower may be arranged to operate in one direction with a high-pressure side and low pressure side. The system may comprise a control unit to control the mechanism and may also comprise a series of sensors detecting levels of air pressure, pollution level and air flow within the duct.

Description

POLLUTION CONTROL SYSTEM

The present invention relates to a pollution control system, particularly to a pollution control system for controlling pollution from vehicles in queueing traffic.

BACKGROUND TO THE INVENTION

Pollution from vehicles is known to cause serious health problems which can lead to the development of respiratory diseases, worsening pre-existing respiratory diseases as well as being carcinogenic and affecting cardiac health. Pollutants include ultrafine particles, nitrogen oxides, carbon monoxide, unburned hydrocarbons, and other harmful substances.

Emissions from vehicle exhausts can reach particularly high concentrations where vehicles are queuing on a road, for example at a set of traffic signals, a busy junction, or a pedestrian crossing. Vehicles idling or in stop-start driving conditions can be particularly polluting.

The places where emissions concentrations are particularly high are often also the places where there are large numbers of people likely to be harmed. The occupants of the vehicles in the queue will themselves be exposed to high pollution levels, but these traffic conditions often occur in town centres, outside schools, and in other places where there are many people being dropped off and picked up, or just present in the area.

Staff working in drive-through takeaways and other drive-through booths, e.g. ticket booths, are also potentially exposed to high and unhealthy levels of pollution from vehicles queuing to be served at the booth It is known to use fans / compressors to move polluted air from inlets at the side of the road, through ducting, to a filtration / purification system which removes the harmful pollution. However, there is a problem of how to deal with water ingress where inlets are provided on a road surface.

Another problem is that debris will inevitably build up on the road surface and can cover intake holes, reducing the effectiveness of the system over time. As well as reducing the amount of pollution which is removed, blocked intakes can result in increased power draw from fans, and could result in some cases in overloading and equipment failure.

It is an object of the present invention to reduce or substantially obviate the aforementioned problems.

STATEMENT OF INVENTION

According to the present invention there is provided a self-cleaning pollution control system for reducing harmful vehicle pollution near an area of queuing traffic, the system comprising: one or more inlets; an air cleaning unit; a duct connected to the inlets and connected to the air cleaning unit; an outlet for venting cleaned air back into the atmosphere; means for forcing air into the inlet, through the ducting, through the air cleaning unit and out of the outlet, characterised in that the means for forcing air is reversible to force air through the ducting and out of the inlets, for clearing debris from the ducting and inlets.

The means for forcing air may be for example a fan, blower or compressor. In most embodiments a fan or blower is specified to move a high volume of air. A modest pressure rise may be required to force air through filters in the air cleaning unit.

In some embodiments, the fan or blower could simply be reversed, for example by causing suitable fan blades to turn in the other direction, in order to effect the reversible flow. However, it is preferable to arrange the fan or blower with a low pressure side and a high pressure side, and use an arrangement of valves to control the flow direction, as described in further detail below.

The fan or blower is used to suck in polluted air at the inlets, force it through the ducts, into and through the air cleaning unit before it is vented back into the atmosphere after being cleaned. When the means for forcing air is reversed flow within the system is forced in the opposite direction, through the ducts and out of the inlets. This helps to clear any debris blocking the inlets. For example, leaves, dirt, litter, etc may accumulate over the inlets, and if not cleaned then over time this will reduce the effectiveness of the system. Reversing the flow allows the system to be self-cleaning to clear this debris.

Pressure sensors may be provided to measure the air pressure within the duct. As debris accumulates, blocking the inlets, the air pressure in the duct will drop. The pressure sensors can detect this, and this can be used as a trigger to reverse the flow and clear the inlets. As an alternative to siting the pressure sensors in the duct, they may be positioned anywhere convenient within the flow path of air, where a blockage is likely to result in a detectable change in pressure. For example, the pressure sensor may be positioned in part of the air cleaning unit, for example at an inlet stage of the air cleaning unit. Preferably, the pressure sensor is positioned somewhere in the flow path between the inlet and the fan or blower.

Alternatively or in addition, air flow volume may be measured. With some kinds of fan or blower, it may also be possible to detect blockages indirectly by measuring current draw. However blockages might be detected, this can be used to trigger a debris-clearing process by reversal of air flow.

A control unit may be provided. The control unit may be made from analogue or digital electronics, including microprocessors, and may in some embodiments include mechanical components. Pressure sensors, air flow sensors and/or current sensors, as described above, and/or any other types of sensor which may be useful to indicate a blockage, may form inputs to the control unit.

Mien a blockage is detected, e.g. because the air pressure in the duct drops below a threshold value, the flow of air may be automatically reversed to clear the blockage.

In other embodiments, instead of attempting to detect blockages, the flow or air may be simply reversed on a timed basis. The reverse flow, to clear blockages, may be timed to take place at times when there is likely to be little or no traffic and few pedestrians, for example in the night-time. In this way the pollution control system continues to work effectively at busy times during the day.

Another alternative is to use sensors to measure the level of pollution in the duct and to reverse the flow to clear blockages only when the amount of harmful pollution being drawn away from the road is at a low level. Again, this ensures that the system operates when it is needed to extract high levels of pollution, but reverses at less busy times to keep the inlet vents clear.

The control unit in some embodiments may include inputs from sensors designed to detect blockages (e.g. pressure sensors), and also inputs from sensors designed to measure pollution levels, and the control unit may also include a timer. In embodiments, the control unit may take into account any one, two, or all three of detected blockages, pollution levels, and time in order to trigger a cleaning process by reversing the air flow. For example, the control unit may trigger cleaning when a blockage is detected, but may delay the cleaning cycle if the pollution level is high or if the current time is within a pre-set period in which it is not desirable to reverse the flow, for example near the start or the end of a school day in a system installed outside a school.

In some embodiments, the control unit may be capable of varying the speed of the fan. In particular more power may be delivered to the fan in order to mitigate some level of blockage, especially at a time where it is undesirable to activate the reverse flow for cleaning (e.g. at the start or the end of a school day). In reverse flow mode, the speed of the fan may also be varied according to detected blockage levels as increasing the rate of air flow through the ducts can help remove heavier debris and also remove particulates that may have become stuck within the duct.

An arrangement of valves is preferably provided for controlling the direction of air flow within the system. The valves may be a shutters, screens, flaps or any similar devices that can open and close to allow or prevent a flow of air. In a preferred embodiment, valves in the form of movable louvred flaps are provided. When the flaps are tilted at one orientation, air can pass between the flaps, and when the flaps are titled to another orientation, the passage of air is substantially blocked. Note that this type of valve may not necessarily form a complete seal when closed. However, it is sufficient to control the bulk of the air flow.

In particular, there may be provided a cleaning unit inlet valve and a cleaning unit outlet valve. The cleaning unit inlet valve is provided on an inlet to the cleaning unit and the cleaning unit outlet valve is provided on an outlet to the cleaning unit. When the cleaning unit inlet valve and cleaning unit outlet valve are both open, air can flow into and through the cleaning unit, and cleaned air can flow out of the air cleaning unit and be vented out into the atmosphere. The cleaning unit inlet valve is between the low pressure side of the fan, and the ducts which in turn connect to the inlets. When the cleaning unit inlet valve is open, polluted air is drawn through the valve, from the low pressure side of the fan to the high pressure side of the fan. The cleaning unit outlet valve is between the high pressure side of the fan and the outlet vent to the atmosphere.

There may also be provide an air reversing inlet valve and an air reversing outlet valve. The air reversing inlet valve is provided to open or close an auxiliary air inlet. The auxiliary air inlet is open to the atmosphere and the air reversing inlet valve is between the auxiliary air inlet and the low pressure side of the fan. The air reversing outlet valve is between the high pressure side of the fan and the ducting, which in turn is connected to the inlets (i.e. the pollution inlets which may be in a road). When the air reversing inlet valve and air reversing outlet valves are both open, air is drawn into the auxiliary air inlet, through the fan, into the ducts, and out of the inlet. The is used to effect the reversal of flow to blow out any debris that are blocking the inlets. In addition, the reversal of air flow can also blow out some small particulates that may have settled within the duct out through the inlets.

The air cleaning unit may include one or more filters through which air is forced in order to filter out pollutants. Preferably, multiple filters may be provided, for example a prefilter, NEPA filter and a carbon filter. This allows the air cleaning unit to remove large particulates (e.g., dust, skin, insects and soot) by the pre-filter, ultrafine particles (e.g., pollen, moisture, some aerosols and some viruses and bacteria) by the HEPA filter and toxic exhaust gases by the carbon filter.

The cleaning unit inlet valve is preferably positioned within the air cleaning unit close to the connection of the air cleaning unit to the duct and next to the air cleaning filter(s).

This means that the air cleaning filter(s) are between the duct and the low pressure side of the fan. When the cleaning unit inlet valve and cleaning unit outlet valve are both open, polluted air will flow through the filter(s), then through the fan, then out of the vent outlet.

The auxiliary inlet may be positioned at the back of the unit, facing away the road where the exhaust gases from the queuing traffic or pollutants are less likely to be sucked in.

An auxiliary filter may be provided on the auxiliary inlet. The auxiliary filter keeps any large particles from the atmosphere from entering the system, in particular protecting the fan. The auxiliary filter may also have some pollution filtering effect although in some embodiments the auxiliary filter is a different type of filter from the air cleaning filter, since expensive filters designed to effectively remove pollution may not be required. However, using equally good filters both for cleaning the polluted air from the inlets, and for cleaning the reverse-flow air from the auxiliary inlets, is of course possible and may be desirable.

In one example, of the pre-filter, HEPA filter and carbon filter which may be used for the air cleaning filter, only the first two may be used on the auxiliary inlet. The pre-filter can filter out large particulates such as dust, skin, insects and soot. The HEPA filter can filter out ultra-fine particulates including pollen, moisture and some aerosols allowing a slightly cleaner air to enter the system. The fan is thus protected from some build-up of fine material than may block it. Furthermore, the pre-filter and HEPA filter may also be used together to make sure that larger and smaller particulates can be filtered out. An advantage of having the auxiliary inlet on a side of the unit facing away from queuing traffic is that the auxiliary filter does not necessarily have to be a high-grade filter designed to get rid of the toxic exhaust gases. The air blowing back out of the inlets during a reversal of flow can be safely breathed in by people within the vicinity.

The auxiliary inlet and vent outlet may both be located in similar positions, and in some embodiments may in fact share some ducting to a shared auxiliary inlet! vent outlet location, preferably facing away from the road.

Alternatively, the vent outlet and the auxiliary inlet can be positioned anywhere that will allow the air to be released into the atmosphere or sucked in to be used as reverse flow to clear debris.

The control unit may be used for controlling the opening and closing of the valves. In particular, when the control unit determines from whatever inputs is has and from whatever rules it has been programmed with, that flow reversal should take place, the air cleaning inlet and air cleaning outlet valves can be closed, and the air reversing inlet valve and air reversing outlet valve may be opened. The valves can be operated by motors, solenoids, or other actuators capable of being activated by the control unit.

The arrangement of valves described means that the fan only needs to be operated in one direction. It also means that during flow reversal, there does not need to be a reverse air flow through the air cleaning filter. This would be undesirable because of the risk that previously captured particulates would be dislodged from the filter and forced back out of the inlets in the road.

The air cleaning unit may be provided in a roadside cabinet. The outlet for venting the air to the atmosphere in most embodiments is simply a vent from the air cleaning unit. It may be preferable in some installations to site the air cleaning unit set back from a pavement and/or to duct air from the air cleaning unit to an outlet spaced from the air cleaning unit. However, in principle the air cleaning unit should sufficiently clean the air that it can be safely breathed by pedestrians and other road users.

Where space is at a premium, in principle the air cleaning unit could be provided in an underground chamber.

The inlets may be in a road, for example substantially horizontal inlets forming part of the surface of the road, over which vehicles may drive.

The duct may be formed as a trench dug into the road. The trench may be substantially elongate, for example around 70mm wide and running along the length of the road where the traffic is likely to queue and pollution control is required. The trench may run for example for a few tens of metres along a stretch of road in front of a school, running up to a pedestrian crossing, in the queue for a drive-through food service booth, or in other areas.

The inlets may be formed as a mesh or grille forming a cover over at least part of the trench. In one example, the mesh or grille covers only a portion across the width of the trench. E.g. for a 70mm wide trench, a mesh cover or grille may be 20mm wide. In an embodiment, 20mm mesh covers are provided on either side of a 70mm trench, leaving a 30mm wide section between the mesh covers which is covered by a solid cover.

The mesh or grille is designed to allow polluted air to be sucked through the inlet into the duct. The mesh or grille should prevent large debris from entering the system while not materially obstructing the passage of air.

A layer of porous material, for example pea gravel, may be provided at the bottom of the trench to facilitate drainage.

A perforated cover may be disposed above the porous material to allow water to flow out of the duct, through the layer of porous material, and drain into the ground. The perforated cover helps to separate the porous material from the duct, and limits the amount of debris which can fall into and clog the porous material.

Water that enters the duct through the inlets can be removed through the drainage system set up at the bottom of the duct. In embodiments where the inlets are in the form of a horizontal mesh or grille, in the surface of the road and forming a cover of the duct, rainwater will inevitably enter through the mesh or grille. Where the mesh forming the inlet is only a part of the total width of the cover over the trench, this may help to limit the amount of water which enters the trench, since the surface area through which water can enter is smaller than the surface area at the bottom of the trench, through which water can drain away.

Water that enters the duct through the inlets can be removed through the drainage system set up at the bottom of the duct. The air that reaches the filtration system can pass through a set of filters that remove particulates and toxic gases.

The ducts guide polluted air to an air cleaning unit. Ducting in addition to the above-described trenches may be provided to pass polluted air from the trenches under the road surface to the air cleaning unit.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which: Figure 1 shows a plan view of self-cleaning pollution control system according to the invention installed in a road in an area with queuing traffic; Figure 2 shows a cross-section through a duct, forming part of the system of Figure 1; Figure 3 shows a plan view of mesh inlets on a road, forming part of the system of Figure 1; and Figure 4 shows a schematic of an arrangement of valves, a fan and filters, forming part of the system of Figure 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring firstly to Figure 1, a plan view of a self-cleaning pollution control system 10 for areas with queueing traffic is shown. The pollution control system 10 includes inlets 12 in the surface of a road 100. The inlets 12 are in the form of mesh covers over a long and continuous trench that is dug into the road, and runs along the road in an area with a lot of queuing traffic. For example, the system may be installed in front of a school 102. Typically, the length of the trench can be chosen based on the likely length of queue which may build up, and taking into account the area that needs to be protected from pollution.

The trench, which runs under the inlets 12 (mesh covers), forms a duct 14 which in use carries air from the inlets. Secondary ducting 16, which may be for example in the form of pipes buried in the road, then carries the air to a roadside cabinet 18. The roadside cabinet contains components such as a fan, a valve arrangement, and an air cleaning unit in the form of filters, as will be described in more detail below.

Figure 2 is a cross section of the duct 14. From this view, two mesh covers 12 are located, one on either side of the duct 14, each with a width of 20 mm. The mesh covers 12 allow the air to get into the system while preventing debris from entering the duct 14. The mesh covers 12 form inlets into the system, through which polluted air from vehicles on the road may be drawn into the system. These inlets 12 are separated by a solid cover section 20 across the centre of the duct 14. The solid cover section in this embodiment is 30 mm in width. The duct is 85 mm deep below the road surface level. A layer of porous material, in this embodiment, fine pea gravel 22 is laid at the bottom of the duct to allow the drainage of water that enters the inlets covering the duct. A perforated cover 24 is provided above the porous material to prevent smaller debris that gets into the duct from causing a blockage in the drainage.

Underneath the mesh and solid trench covers 12, 20 and above the perforated cover 24, a substantially unrestricted space is provided. This is the duct which carries air from the inlets 12 to other parts of the system, as will be explained.

Referring to Figure 3, a plan view shows how the inlets 12 are arranged on either side of the duct at road surface level. Each inlet (mesh cover) is rectangular in shape and the inlets on either side of the duct are positioned parallel to each other and side-byside. The mesh covers 12 may be substantially flush with the road surface. Many inlets are provided on both sides of the duct along the length of the duct. These inlets along the duct are spaced evenly along the length of the duct. Between the inlets, a solid cover 26 about a quarter of the length of the each mesh cover 12 separates the inlets.

Figure 4 shows an arrangement of valves, fans and filters. The arrangement of Figure 4 may be provided for example in the roadside cabinet (18, Figure 1). It incorporates a fan 32 for forcing air, a series of valves 36, 38, 42, 44, an air cleaning unit in the form of a filter 30, an auxiliary filter 48, a connection 28 to the duct (14) and an auxiliary inlet 46.

Cleaning filters 30 are provided between the connection 28 to the duct (14) and the fan 32. These are used to remove the large and small particulates and toxic exhaust gases from the polluted air.

The fan 32 is used to force air through the system. The fan 32 has a high pressure side 32a and a low pressure side 32b. The fan in this embodiment always runs in the same direction, and so the high pressure side 32a and low pressure side 32b are always as shown in the drawings. The flow of air within the system is caused to go in one direction or the other, i.e. it can be reversed using an arrangement of valves 36, 38, 42, 44. Specifically, an air cleaning inlet valve 36 and an air cleaning outlet valve 38 are opened to allow air to flow from the duct (14), through the air cleaning filter 30, and to allow cleaned air to be released back into the atmosphere through outlet vent 40. The air cleaning inlet valve 36 is provided between the duct (14) and the low pressure side 32b of the fan 32, and the air cleaning outlet valve 38 is provided between the high pressure side of the fan 32a and the outlet vent 40 that allows air to be released back into the atmosphere.

On the other hand, an air reversing inlet valve 42 and an air reversing outlet valve 44 are opened when the flow of air is reversed within the system. To reverse the flow of air, the air cleaning inlet valve 36 and air cleaning outlet valve 38 are both closed, and the air reversing inlet valve 42 and air reversing outlet valve 44 are both opened. With the air reversing valves 42, 44 open air entering an auxiliary inlet 46 is forced into the duct (14) and out of the inlets (12) to remove any debris. The air reversing inlet valve 42 is provided between the auxiliary air inlet 46 and the low pressure side of the fan 32b and the air reversing outlet valve 44 is provided between the high pressure side of the fan 32a and the duct (14).

When the flow of air within the system is reversed, an auxiliary filter 48 allows the air entering the auxiliary inlet 46 to be cleaned before entering the system. This filter is positioned between the auxiliary inlet 46 and the air reversing inlet valve 42. The auxiliary filter 48 in some embodiments is not the same as the air cleaning filter 30. For example, the auxiliary filter 48 may be a relatively inexpensive filter designed only to capture larger particles. Such a filter may also have a smaller pressure drop across it, which is advantageous when trying to force air through a system which is partially blocked.

The pollution control system uses mesh inlets in the surface of the road, and is highly effective to capture exhaust emissions from vehicles, especially in a queue or in slow moving traffic. This can significantly improve the air quality, for example at the approach to traffic lights, in front of a school, at a drive-through service booth, etc. Inevitably debris will clog the inlets, but the system can detect this and clean itself by reversing the flow of air. Minimal manual maintenance is therefore required, providing a robust solution which will last a long time.

The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

CLAIMS1. A self-cleaning pollution control system for reducing harmful vehicle pollution near an area of queuing traffic, the system comprising: one or more inlets; an air cleaning unit; a duct connected to the inlets and connected to the air cleaning unit; an outlet for venting cleaned air back into the atmosphere; means for forcing air into the inlet, through the ducting, through the air cleaning unit and out of the outlet, characterised in that the means for forcing air is reversible to force air through the ducting and out of the inlets, for clearing debris from the ducting and inlets.
2. A self-cleaning pollution control system as claimed in claim 1, in which the means of forcing air includes a fan or blower.
3. A self-cleaning pollution control system as claimed in claim 2, in which the fan or blower is arranged to operate in a single direction, having a high pressure side and a low pressure side, and in which the means of forcing air is reversible by use of an arrangement of valves.
4. A self-cleaning pollution control system as claimed in claim 3, in which an air cleaning inlet valve is provided between the duct and the low pressure side of the fan, the air cleaning inlet valve allowing polluted air to flow into the air cleaning unit from the duct, and in which an air cleaning outlet valve is provided between the high pressure side of the fan and the outlet vent to the atmosphere, the air cleaning outlet valve allowing cleaned air to flow out of the system into the atmosphere.
5. A self-cleaning pollution control system as claimed in claim 3 or claim 4, in which an air reversing inlet valve is provided between an auxiliary air inlet and the low pressure side of the fan, the air reversing inlet valve allowing air to enter the ducts from the auxiliary air inlet, and in which an air reversing outlet valve is provided between the high pressure side of the fan and the duct, the air reversing outlet valve allowing air to flow in reverse, through the duct and out of the inlets connected to the duct.
6. A self-cleaning pollution control system as claimed in any of claims 3 to 5, in which at least one of the valves is in the form of one or more movable shutters that can open or close to allow or prevent a flow of air.
7. A self-cleaning pollution control system as claimed in any of the preceding claims, in which the air cleaning unit includes one or more filters.
8. A self-cleaning pollution control system as claimed in claim 7, in which the filters of the air cleaning unit include at least one of a pre-filter, NEPA filter and carbon filter.
9. A self-cleaning pollution control system as claimed in claim 7 or claim 8, when dependent on claim 2, in which the filter(s) are provided between the duct and the fan, for cleaning air as it passes from the duct to the fan.
10. A self-cleaning pollution control system as claimed in any of the preceding claims, in which an auxiliary inlet is provided for allowing air to enter the system from the atmosphere when the means for forcing air is reversed, and in which an auxiliary filter is provided for filtering air as it enters the auxiliary inlet.
11. A self-cleaning pollution control system as claimed in any preceding claim, in which the inlets are meshes or grilles.
12. A self-cleaning pollution control system as claimed in claim 11, in which the meshes or grilles form part of the surface of a road over which vehicles may drive.
13. A self-cleaning pollution control system as claimed in claim 12, in which the duct is a trench dug into the road.
14. A self-cleaning pollution control system as claimed in claim 13, in which a porous material is provided at the bottom of the duct, for allowing water to drain from the duct.
15. A self-cleaning pollution control system as claimed in claim 14, in which a perforated cover is provided over the porous material.
16. A self-cleaning pollution control system as claimed in any of the preceding claims, in which a control unit is provided for controlling the means for forcing air, including controlling when the means for forcing air is operated in reverse.
17. A self-cleaning pollution control system as claimed in claim 16, in which sensor(s) are provided for detecting a condition in the system, and in which the control unit controls operation in reverse flow according to input(s) from the sensor(s).
18. A self-cleaning pollution control system as claimed in claim 17, in which the sensor(s) include at least one pressure sensor and in which the condition sensed is pressure.
19. A self-cleaning pollution control system as claimed in claim 18, in which the pressure sensor(s) are positioned within the duct, for sensing the air pressure in the duct.
20. A self-cleaning pollution control system as claimed in any of claims 17 to 19, in which the sensor(s) include at least one pollution level sensor for sensing the amount of pollution in air in the duct.
21. A self-cleaning pollution control system as claimed in any of claims 17 to 20, in which the sensor(s) include at least one air flow sensor.
22. A self-cleaning pollution control system as claimed in any of claims 17 to 21, in which at least one of the sensors can detect a blockage and the control unit controls the means for forcing air to cause a reverse flow based on whether a blockage is detected.
23. A self-cleaning pollution control system as claimed in any of claims 17 to 22, in which at least one of the sensors can detect a pollution level and in which the control unit controls the means for forcing air to cause a reverse flow based on the detected pollution level.
24. A self-cleaning pollution control system as claimed in any of claims 17 to 23, in which the control unit has a timer and the control unit controls the means for forcing air to cause a reverse flow depending on the time.