TECHNICAL FIELD

The present invention relates to a filtration system for separating gas borne impurities from a polluted gas flow. The gas borne impurities may e.g. be metal working residues mixed with a metal working fluid such as oil, or commixtures at least partially comprising oil or other metal working fluid. The filtration system may be used within e.g. the manufacturing industry.

BACKGROUND

In traditional manufacturing within the automotive industry the cooling and lubrication provided by metal working fluids has been essential when speeding up production in order to increase productivity. A natural consequence has been that the concentration of fluid such as oil or oil emulsion mist, which is used to cool down and lubricate the metal working processes, in the exhaust air from such metal working processes has increased and that the size distribution of liquid particles at the same time has decreased. The continuous decrease of particle sizes has made that particles below 0,1 micron in size are now very common. The biggest problem is not to collect the liquid particles, it is to drain the fluid droplets that are formed when the metal working fluids agglomerate away from the used filters fast enough in order to minimize the risk of increased pressure drop due to clogged filters.

This development has increased the demand for filtration systems capable of managing fluid mist comprising small metal working particles. The penetration of smaller particles is higher than for larger particles which affect both the filtration efficiency and the draining properties for a given filter. Higher concentration of particles also affects the drainage properties due to the need for larger amounts of metal working fluids drained from the filters. The increased cost to handle the increasing amounts of metal working fluids has forced the development towards processes with lower usage of metal working fluids, but still without jeopardizing productivity. A common solution to this is Minimal Quantity Lubrication.

In Minimal Quantity Lubrication cooling with compressed air is used wherein the friction is reduced by lubricant spray carried to the work piece by the compressed air stream. Obvious benefits are reduction of coolant consumption, cost reduction in cleaning of the used coolant and less coolant on the metal chips and turnings removed by the process. A downside has previously been limitations in machining speed and wear of the tooling, but recent development have addressed these problems and the Minimal Quantity Lubrication technology is rapidly growing. However, applying Minimal Quantity Lubrication adds new problems in regards to the air borne emissions. The casing of the machines using Minimal Quantity Lubrication needs to be held with a negative pressure to prevent air leakage containing hazardous particles into the working environment. Traditionally, there are two distinctive areas of emission control in metal working, oil mist collectors for fluid particles created by wet processes using large amounts of metal working fluids, as has been described above, and dust collectors for dry solid particles created in dry processes. Minimal Quantity Lubrication is located somewhere in the boundary between the dry and wet application. Thus, in order to achieve and maintain good air quality not affecting the operations further development is needed.

SUMMARY

An object of the present invention is to provide a filtration system which at least partially overcomes or alleviates the problems associated with previously known filtration systems, particularly by improving the flexibility of the filtration system. The object is achieved by a filtration system according to claim 1. Further exemplifying advantageous aspects of the present invention are disclosed in the dependent claims. The present invention further relates to an arrangement comprising a number of filtration systems and methods of operating said filtration system.

The exhaust gas from the metal working processes using e.g. Minimal Quantity Lubrication technology can be classified according to;

Semi-dry, wherein Semi-dry is defined by that the exhaust gas comprises a certain amount of wet content together with very sticky, semi-fluid and solid non-self-drainable particles, Medium wet, wherein Medium wet is defined by a combination of drainable fluid droplets together with non-self-drainable particles, and

Wet, wherein exhaust gas classified as wet contain essentially 100% of fluidly self- drainable particles. The conventional filters used for cleaning the polluted exhaust gas from metal working processes are configured for continuously draining themselves as the metal working fluids are agglomerated, forming fluid droplets which are drained away from the filters by the influence of gravity. However, the continuously ongoing streamlining of metal working processes has led to that the amounts of metal working fluids that are used at metal working processes continuously is decreasing, together with that the particle sizes of the metal working residues decreases. Exhaust gas classified as Wet can be managed with conventional filtration technology whereas for exhaust gas classified as Medium-wet and Semi-dry conventional filtration technology is not always that convenient. The smaller amounts of used metal working fluids affects the drainage properties of the filters wherein if the formed agglomerating droplets of fluid and metal working residues formed on the filters are not drained fast enough from the used filters the filters may be clogged. This in turn increases the risk of pressure drop due to clogged filters which ultimately may affect the filtration properties. The above disclosed problem is at least partially solved by the present invention by providing a filtration system with functionality to adjust the actual fluid content of the polluted gas flow to a predetermined content of fluid by adding additional fluid to the polluted gas flow and thereby adjusting the fluid content of the polluted gas flow such that the filtration is efficient.

Thus, according to the present invention a filtration system for separating gas borne impurities from a polluted gas flow comprises:

a gas flow inlet, wherein the gas flow inlet is configured to receive an incoming polluted gas flow,

a filter unit, wherein the filter unit forms a gas passage and wherein the gas flow inlet is in fluid communication with the filter unit, and

- a gas flow outlet, wherein the gas flow outlet is in fluid communication with the filter unit and the gas flow inlet.

The received polluted gas flow enters the filtration system at the gas flow inlet. At the gas flow inlet the polluted gas is subsequently guided through the filter unit where the polluted gas flow pass by at least one, but preferably more than one, gas filters cleaning the received polluted gas flow. After passing the filter unit the now cleaned gas flow subsequently passes the gas flow outlet. The number of gas filters, being at least one, and the properties of the filters, are preferably selected based on the condition of the received polluted gas and the required quality of the cleaned gas flow being discharged from the filtration system. It is also possible to complement said at least one filter with additional means that may be used to clean a polluted gas flow such as e.g. magnets or forms of electrostatic precipitators. The filtration system of the present invention is further characterized in that the filtration system further comprises: a fluid content control system, wherein the fluid content control system is configured to control such that an actual content of fluid in the received polluted gas flow is adjusted to be a predetermined content of fluid by adding additional fluid to the polluted gas flow before the polluted gas flow passes through the filter unit.

Filter units as the one referred to above are generally most effective when being exposed to a polluted gas with a predetermined content of fluid or with a polluted gas with a fluid content within a predetermined interval. At what predetermined fluid content filters are effective may also be influenced by the particle size distribution of the particles comprised in the polluted gas. By controlling that the fluid content of the received polluted gas flow is essentially equal to such predetermined content of fluid, or at least within the predetermined interval, it can be secured that the fluid drainage properties are sufficient considering the present filter unit and that the filtration efficiency is adequate.

Further, and which is important to point out, it should be noted that when herein referring to gas what in most cases is intended is air. Air is the preferred gas for all aspects of the present invention disclosed herein. However, it is also possible to apply any aspect of the present invention for other gases that air. Thus, in order not to discriminate any valid applications of the present invention gas will be referred to throughout the description. Thus, according to an aspect of the present invention, which applies for all aspects of the present invention disclosed herein, the gas is air.

According to an aspect of the present invention the fluid of the received polluted gas flow consists of a commixture comprising essentially of, or at least partially of, oil or oil emulsion. Today oil is the most commonly used fluid for obtaining the desired cooling and lubrication effect, but it is also possible that other fluids with similar properties are used as metal working fluid. The present invention, and all aspects of the present invention disclosed herein, is applicable for oil as well as for all other today, and in the future, known fluids with corresponding properties.

According to another aspect of the present invention the filter unit comprises at least one gas filter arranged inside of the filter unit, wherein said gas filter is arranged such that a received polluted gas flow passes through said gas filter. The filter unit may be defined as a housing forming a gas passage through which the received polluted gas can flow and wherein at least one gas filter is arranged inside of the housing in a way such that a gas passing through the filter unit has to flow trough said gas filter.

According to yet another aspect of the present invention the gas flow outlet provides suction facilitating that the received polluted gas flow enters the gas flow inlet, passes the filter unit and is discharged from the filter unit through the gas flow outlet. The suction is preferably provided by that a fan or like is provided at the gas flow outlet and by that said fan is configured to create an underpressure in the outlet. By forming an underpressure at the gas flow outlet a suction force is provided whereby the received polluted gas entering the gas flow inlet will be guided through the filter unit comprising at least one gas filter and to the gas flow outlet. After passing the gas flow outlet the now cleaned received gas flow may be discharge into the free air or o a fluidly connected ventilation pipe or like.

According to still one aspect of the present invention the filter unit comprises a progressive filter stack. The progressive filter stack comprises said at least one gas filter and said gas filter is selected considering the predetermined content of fluid and/or particle size distribution of the received polluted gas flow.

A progressive filter stack is generally arranged to comprise a number of filters together providing the desired filtration capability. The first filter of the progressive filter stack may be preceded by a pre-filter. The pre-filter, which according to aspects of the present invention may be in form of a folded filter material arranged in a frame, is exposed to the received polluted gas flow first and is preferably in form of a sacrificial filter which easily, and to a lesser expense, can be replaced relatively frequently. Hence, sacrificial filters are preferably cheap and easy to replace. According to one aspect of the present invention the pre-filter comprises a filter made of a non-woven polyester fabric. The subsequent filters of the progressive filter stack may be of higher quality, which often entails that they are more expensive, and may be configured to handle one specific form of pollutants and/or one specific size of particles. The last filter of the progressive filter stack may be a filter of HEPA, High Efficiency Particulate Arrestor, type. HEPA filters are highly effective particulate filters which may be able to collect as much as 99,95% of MPPS (Most Penetrating Particle Size). HEPA filters are generally relatively expensive, and consequently it is desirable that the HEPA filter is not that frequently replaced. According to one aspect of the present invention the progressive filter stack comprises four filters, including a first pre-filter. The pre-filter may be configured to handle the largest metal working residues and unexpected pollutants, the subsequent first, second and third gas filters may be of higher quality and may be configured to sequentially filter out smaller and smaller gas borne particles. The configuration and combination of suitable filters in regards to polluted gas flow to be cleaned is considered to be part of common general knowledge.

In order to determine the amount of additional fluid that needs to be added to the actual content of fluid, such that the fluid content can be adjusted to the predetermined content of fluid, the actual content of fluid in the received polluted gas flow needs to be determined. According to an aspect of the present invention the actual content of the fluid in the polluted gas flow is provided by measurements performed before the filtration system is put in operation, such as when the filtration system is being assembled. Such measurements may e.g. be performed by measurement equipment configured for performing: particle size measurements, particle concentration measurements, particle composition/content measurements and/or relative humidity. By determining the actual content of fluid, the particle size distribution etc., before the filtration system is put in operation no additional measurement equipment is required making the aspect of the present invention cheap and less complex. Also, since no additional measurement equipment is needed no regular recalibration of the measurement equipment is needed and there is no additional measurement equipment that can break or interrupt operation. Thus, this aspect of the present invention provides a very robust filtration system.

According to another aspect of the present invention the actual content of fluid in the polluted gas flow is provided by measurements performed by measurement means such as sensors, detectors or like arranged such that the sensors or like continuously can determine the actual fluid content of the polluted gas flow. Such sensor may e.g. be arranged at the gas flow inlet of the filtration system or at a metal working process exhaust outlet guiding the polluted gas flow from a metal working machine to the filtration system. Such sensors may further be used to continuously measure e.g. the actual fluid content and/or particle size distribution of the polluted gas flow, such that the amount of fluid, in order to adjust the fluid content of the polluted gas flow to be the predetermined fluid content, can be determined.

Thus, it is within the scope of the present invention that the actual fluid content of the polluted gas flow can be measured continuously or sequentially with certain time intervals, such that the fluid content can be continuously adjusted to the predetermined content of fluid.

Depending on the application of the filtration system, and the characteristics and possible fluctuations of the received polluted gas flow, the measurements may be performed relatively frequently, such as once every 10th second or once every minute, or more seldom, such as once every 24 hours or once a week.

Applying continuous in-flow measurements has the advantage that possible changes, temporarily or permanent, of the actual fluid content of the polluted gas flow are identified and may be compensated for. Thus, continuous in-flow measurements improves the possibility to continuously add suitable amount of fluid to the polluted gas flow such that the actual fluid content is adjusted to be the predetermined fluid content.

It is also possible to apply sensors or like at a position downstream of, thus after, the fluid content of the polluted gas flow has been adjusted to the predetermined fluid content. Such sensors can be used to control that the fluid content control system and the functionality of adding additional fluid is correctly configured. Applying such sensors also offer the possibility to implement a feedback system wherein possible deviations of the actual fluid content from the predetermined fluid content, after the fluid content has been adjusted, can be compensated for immediately.

The measurements referred to may comprise particle size measurements, particle concentration measurements, particle composition/content measurements and/or relative humidity. As is stated above, except for the actual fluid content also the particle size distribution may be measured. The particle size distribution may be an important aspect to consider when configuring the filter unit, which preferably comprises at least one gas filter or even more preferably a progressive filter stack. Thus, according to all aspects of the present invention, in addition to the actual fluid content is also the particle size distribution measured.

According to another aspect of the present invention the fluid content control system comprises a pump box and at least one filter spraying nozzle. The at least one filter spraying nozzle is arranged such that the polluted gas flow flows past the filter spraying nozzle and the pump box is configured to provide fluid to the at least one filter spraying nozzle. The filter spraying nozzle is further configured to spray fluid into the polluted gas flow such that additional fluid is provided to the polluted gas flow. The filter spraying nozzle may be part of a filter spraying system through which the filter spraying nozzle is provided with the fluid to be added to the polluted gas flow or be supplied with fluid via a filter spraying system.

The filter spraying nozzle and the filter spraying system used to provide additional fluid to the polluted gas flow, in order to obtain the predetermined fluid content of the polluted gas flow, may also be used in embodiments of the present invention where no pump box is used. For such embodiments the fluid such as oil may be provided to the filter spraying nozzle by means of other arrangements, such as e.g. by means of a pump connected to a tank.

As previously disclosed, the fluid and particles of the received polluted gas flow accumulates at the lower arranged filters, such as the pre-filter, if one is present, and the first and possibly second gas filters of the filter unit. The accumulated fluid agglomerates form droplets which ultimately are drained from the surface of respective filter when they become sufficiently large by the influence of gravity. The droplets are ultimately collected at a drainage box of the filtration system and from the drainage box the agglomerated droplets are drained via a drain pipe to the fluid container of the pump box. The drainage box may also be equipped with a dustbin for collecting solid particles. Inside the dustbin there may be a non-woven dust bag for receiving solid particles either separated from the discharged fluid droplets or directly from the polluted gas flow. The dustbin may be removed and replaced upon service of the filtration system or when cleaning of the filtration system. According to yet one aspect of the present invention the pump box additionally comprises a fluid recirculation arrangement, wherein the fluid recirculation arrangement is configured to collect discharged fluid, which is provided from the drainage box to the pump box via the drain pipe, and clean the discharged fluid such that the cleaned fluid may be reused in the pump box. The fluid recirculation arrangement may be in form of a non-woven polyester fabric through which the discharged fluid is filtered such that metal working residues, solid agglomerates etc. are separated from the fluid. The pump box may comprise a pump for pumping fluid from a fluid container of the pump box to the filter spraying system which in turn provides such that the filter spraying nozzle can spray fluid into the polluted gas flow. The pump box may also comprise additional components such as e.g. pressure equalization means and means for indicating the fluid level of the fluid container.

According to yet an aspect of the present invention the gas flow inlet comprises a redirecting portion arranged to be able to redirect the received polluted gas flow in desirable direction. According to one aspect of the present invention the redirecting portion redirects the received polluted gas flow in a down-flow direction, pointing essentially downwards. The redirecting portion is provided upstream of where additional fluid is added to the polluted gas flow, thus before additional fluid is added to the polluted gas flow, and consequently the received gas flow at the redirecting portion is, even if not completely dry, at least dryer than after additional fluid has been added by the fluid content control system. Redirecting the polluted gas flow in a downward direction is e.g. advantageous since it enables solid state particles such as turnings etc. to be collected in a controlled manner before additional fluid is added to the received polluted gas flow and before the polluted gas flow reaches the filter unit. Removing solid state particles before additional fluid is added and before the polluted gas flow reaches the filter unit is preferable e.g. since collected waste may be easier to take care of and it may prolong the life time of the filter unit. Thus, according to one aspect pf the present invention the redirecting portion further comprises a replaceable filter tray, wherein the replaceable filter tray is arranged such that a received polluted gas flow flows past the replaceable filter tray. Since the received polluted gas flow pass the redirecting portion before entering the filter unit the filter tray, and the filter of such filter tray, is the first filter that the received polluted gas flow pass through. According to one aspect of the present invention the replaceable filter tray comprises a filter at least partially made of wire netting. Filter trays comprising e.g. filters of wire netting has the advantage that the filters may be reused after being cleaned. Depending on current filtration system design it is also possible to use the redirecting portion to redirect the received polluted gas flow in another direction than downwards. The redirecting portion may e.g. be in form of a tubular portion into which the polluted gas is guided, wherein the tubular portion may have an opening pointing in a down-wards direction. This will be disclosed more in detail in the forthcoming detailed description.

The present invention also relates to a filtration system arrangement comprising a plurality of filtration systems, wherein the plurality of filtration systems are arranged in parallel such that respective filtration system is configured to receive a polluted gas flow. For such filtration system arrangement at least two of the filtration systems arranged in parallel may share one common pump box. This filtration system arrangement has the advantage that even though all filtration systems by definition comprise a pump box all filtration systems do not have to comprise an own pump box. Some filtration systems may share pump box which will reduce the overall number of pump boxes, thus reduce cost.

One aspect of the present invention relates to a method for operating a filtration system of the present invention. As previously disclosed the filtration system comprises a gas flow inlet, wherein the gas flow inlet is configured to receive an incoming polluted gas flow, a filter unit, wherein the filter unit forms a gas passage, and a gas flow outlet, wherein the gas flow inlet is in fluid communication with the gas flow outlet via the gas passage formed by the filter unit. The filtration system further comprises a fluid content control system. The method of operating the filtration system comprises the steps of:

· receiving an incoming polluted gas flow at the gas flow inlet of the filtration system,

• adding fluid to the polluted gas flow such that a fluid content of the polluted gas flow is adjusted to be a predetermined fluid content, and

• guiding the received polluted gas flow with adjusted fluid content through the filter unit to the gas flow outlet of the filtration system. Operating a filtration system according to any aspect of filtration systems disclosed herein, in accordance with the aspect of the disclosed method, provides the advantages previously disclosed with such filtration system.

According to another aspect of a method of operating a filtration system of the present invention the method further comprises the method step of, before fluid is added to the polluted gas flow:

• determining an actual fluid content of the received incoming polluted gas flow.

The determination of the actual fluid content, as well as other aspects such as for example particle size distribution, the dispersion of the fluid etc. is provided by measurements performed by utilizing measurement means provided in the gas flow. The measurements are preferably, but not restricted to, performed before additional fluid is added to the polluted gas flow.

As previously disclosed, the determination of the actual fluid content may be performed by measurements performed before the filtration system is taken into operation or according to any aspect of continuous measurements during operation of the filtration system. The advantages with respective aspect of how the actual fluid content can be determined have been disclosed previously. It is considered that related methods for controlling respective aspect of continuous measurements during operation are within the scope of the present invention.

The following description of example embodiments of the invention provided is presented only for purposes of illustration and should not be seen as limiting. The description is not intended to be exhaustive and modifications and variations are possible in light of the above teachings or may be acquired from practice of various alternatives to the provided embodiments. The examples discussed herein were chosen and described in order to explain the principles and the nature of various example embodiments and its practical application to enable one skilled in the art to utilize the example embodiments in various manners and with various modifications as are suited to the particular use contemplated. It should be appreciated that the aspects presented herein separately may be practiced in any combination with each other. BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a more detailed description of embodiments of the invention cited as examples. Fig. 1 shows a schematic view of an example of a filtration system according to the present invention,

Fig. 2 shows a fist schematic view of an example of a gas flow inlet of a filtration system according to the present invention, where examples of a fluid content control system and a pump box are disclosed, and

Fig. 3 shows a second schematic view of an example of a gas flow inlet of a filtration system according to the present invention, where examples of a fluid content control system and a pump box are disclosed.

DETAILED DESCRIPTION OF EXAM PLE EM BODIMENTS OF THE INVENTION

Example embodiments of the present invention are hereinafter disclosed with reference to the accompanying drawings. Like reference signs refer to like elements throughout the detailed description of fig. 1-3.

Fig. 1 shows a schematic view of an example of a filtration system 1 according to the present invention. The filtration system 1 comprises a gas flow inlet 2, a filter unit 3 and a gas flow outlet 4, wherein the gas flow inlet 2 is arranged to be in fluid communication with the gas flow outlet 4 via the filter unit 3. A metal working process exhaust outlet 13 is fluidly connected to the gas flow inlet 2 such that polluted exhaust gas generated during metal working is guided to the filtration system 1. The guiding of polluted exhaust gas to the gas flow inlet 2, via the filter unit 3 and to the gas flow outlet 4 is further facilitated by that the filtration system 1 comprises a means for providing suction 6 capable of creating an underpressure in the gas flow outlet 4 or a suction force facilitating the flow of polluted exhaust gas through the filtration system 1. The means for providing suction 6 may be in form of a fan or like. Once processed and cleaned in the filtration system 1 the cleaned gas flow may be discharged out into the free air or, as is exemplified in the embodiment of fig. 1, be guided to a ventilation pipe 17 or like. The embodiment of the filtration system 1 disclosed in fig. 1 shows a filter unit 3 comprising a first, a second and a third gas filter 18a, 18b, 18c. The first, second and third gas filters 18a, 18b, 18c together form an example of a progressive filter stack 24. According to a preferred aspect of the present invention the filter unit 3 comprising the first, second and third gas filters 18a, 18b, 18c may be preceded by a (not shown) pre-filter.

It is also within the scope of the present invention to use a filter unit 3 with less than three, or more than three, gas filters. The number of gas filters, the possible use of pre-filter and their properties are e.g. dependent on the degree of pollution and characteristics of the received polluted gas flow and the properties of the present gas filters.

The filtration system 1 further comprises a drainage box 7 with a dustbin 8. The drainage box 7 is provided in order to collect formed fluid droplets, in the embodiment shown in fig. 1 in the form of agglomerated oil droplets, which are drained from the filter unit 3. The dustbin 8 is provided to collect solid material that are separated from the polluted gas flow and/or that are discharged from the filter unit 3.

The arrows in fig. 1 indicate the route of the polluted gas flow, starting at the metal working process exhaust outlet 13, passing the gas flow inlet 2, the filter unit 3, the gas flow outlet 4 and being discharged at the ventilation pipe 17 or like.

In later developments of new metal working processes compressed air together with a lubricating fluid is used in order to reduce friction and cool the metal working piece and the tools processing the working piece. This has the advantage that less fluid is used which reduces the amount of waste fluid that needs to be taken care of. Also, the new metal working processing methods has led to that the residue particle size distribution is changed. The gas filters used in a filter unit as of a filtration system as of the present invention are generally configured to be used for cleaning polluted gas with a predetermined fluid content and are most effective at a certain fluid content or within a certain fluid content interval. Thus, from an exhaust gas cleaning aspect the dryer exhaust gas occurring in later developments of metal working machines is problematic since the risk of clogging the filters that are used to clean the exhaust gas is increased. According to the present invention the filtration system 1 further comprises a pump box 9 and a fluid content control system 5. The fluid content control system 5 will be discussed more in detail in relation to fig. 2 and 3. A drain pipe 10 fluidly connects the drainage box 7 with the pump box 9 such that discharged oil collected in the drainage box 7 can be guided to the pump box 9. The pump box 9 comprises at least a fluid container 11 configured to retain fluid provided by the drain pipe 10, a fluid recirculation arrangement 16 configured to clean and regenerate the fluid drained from the drainage box 7, such that the used fluid can be recirculated and reused, and a pump 12 configured for pumping fluid, such as e.g. regenerated fluid such as regenerated oil, from a fluid container 11 to the fluid content control system 5 via a fluid supply line 19. The fluid container 11 and the fluid recirculation arrangement 16 will also be disclosed more in detail in relation to fig. 2 and 3. The pump box 9 may also comprise e.g. a pressure equalization means 14, configured to control the pressure in the pump box 9 and/or fluid container 11, and means for indicating the fluid level of the fluid container 11. The fluid content control system 5 of the present invention, which may be supplied with a fluid such as oil from a pump box 9, is configured to add additional fluid, preferably by means of spraying, to the polluted gas flow provided via the metal working process exhaust outlet 13. By controlling the fluid content of the polluted gas flow to be the predetermined fluid content at which the filter unit 3 comprising the progressive filter stack 24 is most efficient, in terms of e.g. filtration efficiency and filter drainage, the filtration system 1 of the present invention can be operated efficiently and without problems associated with clogged filters independently of the actual fluid content of the polluted gas flow.

Fig. 2 shows a fist schematic view from the side of an example of a gas flow inlet 2 of a filtration system according to the present invention, where examples of a fluid content control system 5 and a pump box 9 are disclosed. Fig. 2 shows an exemplary arrangement of the fluid container 11 and the fluid recirculation arrangement 16 of the pump box 9.

According to the embodiment of the present invention disclosed in fig. 2 the fluid content control system 5 comprises a filter spraying system 22 to which fluid is provided from the pump box 9 by means of the pump 12 and the fluid supply line 19. The filter spraying system 22 further comprises two filter spraying nozzles 23. In the preferred embodiment shown in fig. 2 the filter spraying system 22 comprises two nozzles 23 directed in an upwards direction. However, it is also within the scope of the present invention to use one or more than two nozzles possibly directed in another direction than upwards. The nozzles 23 are configured to spray additional fluid into the passing polluted gas flow and thereby adjust the actual fluid content of the polluted gas flow to the predetermined fluid content. The nozzles 23 and the fluid supply line 19 are fluidly connected by pipes of the filter spraying system 22.

According to other aspects of the present invention, not shown, the filter spraying system may be provided with fluid from another fluid source than a pump box such as an external container provided with a pump and a control system controlling the operation of such supply system.

Fig. 2 also discloses a circular redirecting portion 20. Metal working process exhaust outlets 13 (of which only one is disclosed in fig. 1) may be fluidly connected to at least one side of the redirecting portion 20. The other side of the redirecting portion 20 (if a metal working process exhaust outlet 13 is only connected to one side of the redirecting portion 20) may be sealed or be connected to another filtration system. The redirecting portion 20 has an opening portion 21 directed in a downwards direction such that a received polluted gas flow entering the redirecting portion 20 will be redirected in a down-flow direction before being redirected once again and flowing upwards to the filter unit. In fig. 2 is also the means for indicating the fluid level 15 of the pump box 9 shown.

Referring now to fig. 3, showing a second schematic view from above of an example of a gas flow inlet 2 of a filtration system according to the present invention, where examples of a fluid content control system 5 and a pump box 9 are disclosed. Fig. 3 is intended to further clarify how respective, previously disclosed, components may be arranged according to the present invention.

Obvious modifications of the filtration system, and the components thereof, of the present invention, which e.g. in regards to present degree of pollution and characteristics of the polluted gas flow, filtration system design restrictions etc. are considered to be within the scope of the present invention. The invention should not be seen as limited, e.g. in regards to how the filter spraying system 22 is configured, by the embodiments of the filtration system disclosed in any of fig. 1 to 3. It should be noted that the word "comprising" does not necessarily exclude the presence of other elements or steps than those listed and the words "a" or "an" preceding an element do not exclude the presence of a plurality of such elements. It should further be noted that any reference signs do not limit the scope of the claims, that the example embodiments may be implemented at least in part and that several "means", "units" or "devices" may be represented by the same item of hardware. It is further to be understood that the present invention is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.