[0001] The invention relates to a method and an arrangement for recirculating air in a drying process. BACKGROUND OF THE INVENTION

[0002] Drying is an energy intensive operation due to the high latent heat of vaporization and the inefficiency of using hot air as the drying medium. In a typical industrial the drying process moisture is removed from a product into an air flow creating a moisture laden exhaust air flow.

[0003] It is known to recover heat from the moisture laden exhaust air and use the recovered heat in the plant comprising the drying process. The cooled moisture laden exhaust air flow is then exhausted to the atmosphere possibly causing emissions. In some areas even exhausted plumes of a clean water vapour are unacceptable. The visible water vapour plumes can be of concern to the population living close to industrial plants. Further, plume induced fogging and icing rise accident risks and may be dangerous for crops and equipment.

[0004] If the moisture laden exhaust air itself is recirculated in the plant it requires dehumidification and cooling of the exhaust air together with the removal of unwanted particles before it can be reused. However, the moisture removal in the dehumidification process is often inadequate and fresh dry air has to be mixed with the dehumidified exhaust air to obtain an acceptable moisture content level for reuse. Further, the cost of the dehumidification and cooling installation makes the recirculation of the moisture laden exhaust air easily unprofitable. BRIEF DESCRIPTION OF THE INVENTION

[0005] An object of the present invention is to solve the above mentioned drawbacks and to provide an efficient and cost effective method and an arrangement for implementing the method for recirculating air in a drying process.

[0006] This object is achieved with a method for recirculating air in a drying process according to independent claim 1 and with an arrangement for recirculating air in a drying process according to independent claim 19. The preferred embodiments of the invention are disclosed in the dependent claims.

[0007] In the method for recirculating air in a drying process within a plant in the drying process moisture is removed from a product into an air flow creating an exhaust air flow. The exhaust air flow is guided to a device wherein the moisture content of the exhaust air flow is reduced. The exhaust air flow exiting the device is divided at least into an air flow recirculated to the drying process and into an air flow to at least one room related to the drying process.

[0008] An arrangement for recirculating air in a drying process within a plant comprises an exhaust air flow from a drying process. The arrangement comprises a device for reducing the moisture content of the exhaust air flow. The arrangement comprises means for dividing the exhaust air flow exiting the device at least into an air flow recirculated to the drying process and into an air flow to at least one room related to the drying process.

[0009] An advantage of the method and arrangement of the invention is that the amount of the exhaust air flow exhausted to the atmosphere and the formation of visible water vapour plumes can be reduced to minimum. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the following the invention will be described in greater detail by means of preferred embodiments with reference to the accompanying drawings, in which

Figure 1 shows an arrangement for recirculating air in a drying pro- cess;

Figure 2 shows an arrangement for recirculating air in a drying process with heat recovery exchangers;

Figure 3 shows an arrangement for recirculating air in a drying process comprising a cleaning unit;

Figure 4 shows an arrangement for recirculating air in a drying process comprising a cleaning unit;

Figure 5 shows an arrangement for recirculating air in a drying process comprising a cleaning unit, a supplementary air flow and a venturi scrubber.

DETAILED DESCRIPTION OF THE INVENTION

[0011] Figure 1 shows an arrangement for recirculating air in a drying process. In the drying process 1 moisture is removed from a product into a hot air flow creating a moisture laden exhaust air flow 2a. The drying process 1 is preferably continuous energy intensive drying process where the moisture laden exhaust air flow 2a may comprise 130-160 grams of water vapour per kg of air, for instance. Examples of continuous energy intensive drying processes are drying of paper, pulp, wood, lumber or waste sludge.

[0012] A device 3 reduces the moisture content of the exhaust air flow 2a. The device 3 cools the exhaust air flow 2a to remove moisture, or absorbs moisture from the exhaust air flow 2a, or adsorbs moisture from the exhaust air flow 2a, or performs any combination of these three, for instance. The reduction of moisture content of the exhaust air 2a reduces the visible water vapour in the exhaust air flow 2a when it is exhausted from the device 3. The water vapour becomes visible as a result of water vapour condensing and forming water droplets. Clouds or fog are visible forms of atmospheric water vapour.

[0013] At least part of the exhaust air flow 2b-c exiting the device 3 is recirculated to the drying process 1. The reduction of the water vapour in the exhaust air flow 2b-c and the recirculation of the exhaust air 2b-c reduces the amount of the exhaust air flow exhausted to the atmosphere and the formation of visible water vapour plumes.

[0014] Plumes of the moisture-laden exhaust air 2a have high moisture content, and exhausting them to atmosphere under certain meteorological conditions without adequate moisture removal would result in visible plume formation. The visible water vapour plume formation depends on meteorological conditions such as wind speed, wind direction, atmospheric stability class, ambi- ent temperature, mixing heights, and relative humidity of the ambient air. By removing water vapour from the exhaust air 2a with a device 3 the occurrence of the visible water vapour plume and the length and the height of the plume is reduced.

[0015] The exhaust air flow 2b-c exiting the device 3 is divided at least into a recirculated air flow to the drying process 2b and into an air flow to at least one room related to the drying process 2c. The air flow divided to the drying process 2b comprises 65-85% and the air flow divided to at least one room related to the drying process 2c comprises 35-15% of the exiting exhaust air flow 2b-c, for instance. The space surrounding the drying process 1 is not leak-proof and typi- cally the amount of recirculated dry air 2b brought to the space is less than the amount of moisture laden exhaust air 2a drawn from the space.

[0016] Typical examples of rooms related to the drying process 1 are finishing treatment rooms, service spaces, control and electrical rooms, machine room, finishing area, special rooms such as motor control center and rack rooms. The at least one room related to the drying process may comprise a production area of a plant where the drying process is part of the production process. The air flow 2c divided from the dried and cooled exhaust air flow 2b-c exiting the device 3 may be used for heating, cooling or ventilating the room or rooms related to the drying process 1.

[0017] During the winter time the moisture content and the tempera- ture of the ambient air is low at many geographic locations. With these weather conditions the probability of occurrence of the visible water vapour plumes becomes high. Additionally, the inlet air to the rooms related to the drying process 1 taken from outdoor needs to be heated and preferably humidified for maintaining ideal working conditions for the personnel and plant machinery. The recircula- tion of the exhaust air 2b also to the rooms related to the drying process 1 is advantageous as it improves their inlet air quality and at the same time reduces the amount of the exhaust air flow exhausted to the atmosphere and the formation of visible water vapour plumes. The method and the arrangement for recirculating air in a drying process allows a full recirculation of exhaust air on a stable produc- tion flow of the plant particularly during a cold and dry season of the year.

[0018] In an embodiment of the method and the arrangement for recirculating air in a drying process a wet scrubber 28 is utilized for reducing the temperature and the moisture content of a moisture laden exhaust air 2a from a drying process 1, i.e. the moisture laden exhaust air 2a is dried in a wet scrubber 28. The scrubber 28 is advantageous in drying the moisture laden exhaust air as it offers large volume for the heat and mass transfer. The wet scrubber 28 is cost effective as the required heat and mass transfer surface is considerably lower compared to conventional plate heat exchanger systems or a tube-in-a-shell heat exchanger systems. Further, the scrubber 28 also removes unwanted substances from the moisture laden exhaust air 2a. Examples of removable substances are dust, dirt, odorous vapours and water soluble chemicals.

[0019] In the Figure 1 the moisture laden exhaust air flow 2a from a drying process 1 is guided to a wet scrubber 28 for reducing the moisture content and the temperature of the exhaust air flow 2a. Part of the exhaust air 2b is recir- culated to the drying process 1 and part of the exhaust air 2c is led to at least one room related to the drying process 1.

[0020] The scrubber 28 comprises a scrubber tower which has a circular base surface 4 and a cylindrical shell 5. Another base shape may be rectangular, for instance. The moisture laden exhaust air flow 2a is guided horizontally to the bottom part 6 of the wet scrubber tower through an inlet opening 7. The flow guiding may comprise pressurizing the moisture laden exhaust air flow with one or more fans 8.

[0021] In the scrubber 28 the exhaust air flow 2a flows upwards and droplets 9 of a cooling water 10 are injected into the exhaust air flow from a plurality of nozzles 11. The cooling water droplets 9 are flowing down-wards in counterflow to the exhaust air flow 2a. The moisture laden exhaust air 2a collides with the cooling water 10 creating a turbulent zone where the air/water interface is continuously and rapidly renewed. Scrubbing is effectively accomplished by the thorough mixing of the exhaust air 2a and scrubbing cooling water 10 in the scrubber 28.

[0022] Air is saturated when it has reached its water vapour capacity and contains all the water vapour that it can hold at a particular temperature. In saturated air, the water vapour content equals its capacity. At the dew point temperature the air is saturated, and the dew point is the temperature at which the relative humidity of the air is 100%. In the scrubber 28 the dew temperature of the cooling water 10 is lower than the dew temperature of the exhaust air flow 2a.

[0023] The dried and cooled exhaust air 2b-c is discharged from the scrubber 28 through one or more exhaust air outlets 12 at the top part 13 of the scrubber shell 5. The scrubber 28 may comprise an outlet 12 to atmosphere as shown in Figure 4 which is used during the start-up or halting of the scrubber 28, for instance.

[0024] In another embodiment of the method and arrangement the state of exiting exhaust air 2b-c may be controlled. The arrangement comprises means for measuring the temperature and the moisture content of the exhaust air flow 2b-c exiting the scrubber 28. The temperature and moisture content of the exhaust air 2b-c exiting from the scrubber 28 may be measured by a wet and dry bulb thermometers, for instance, or other online humidity measuring devices may be used.

[0025] The arrangement comprises means for comparing 15 the measured temperature and the moisture content values to set values for the exit- ing exhaust air flow 2b-c. The comparison may be made with a control algorithm provided in a control device or a computer, for instance.

[0026] In the method the temperature and moisture content of the exhaust air flow lb-c exiting the scrubber 28 are measured, the measured temperature and the moisture content values are compared to set values for the exit- ing exhaust air flow 2b-c and based on the comparison result the cooling water 10 flow to the plurality of nozzles 11 and the pressure at the plurality nozzles 11 is controlled.

[0027] The set values for the exiting exhaust air 2b-c from the scrubber 28 depend on the drying process 1 into which the exiting exhaust air 2b-c is recirculated. However, the moisture content of the exiting exhaust air 2b-c has to be low enough to have free water vapour capacity. The set value range maximum for from the scrubber 28 exiting exhaust air 2b-c temperature preferably comprises 35°C, and the set value range maximum for the moisture content of from the scrubber 28 the exiting exhaust air 2b-c preferably comprises 35 grams of water vapour per kg of air.

[0028] Further, a suitable set value range for the from the scrubber 28 exiting exhaust air 2b-c temperature could comprise 25-35°C, and the set value range for the moisture content of from the scrubber 28 exiting exhaust air 2b-c could comprise 20-35 grams of water vapour per kg of air, preferably 25-30 grams of water vapour per kg of air. The arrangement further comprises means for controlling 16 the cooling water 10 flow to the plurality of nozzles 11 and the pressure at the plurality of nozzles 11 based on the comparison result. The controlling means 16 is able to reduce or increase the cooling water 10 flow to the plurality of nozzles 11 and to reduce or increase the pressure at the plurality of nozzles 11. The controlling of the cooling water 10 flow to the plurality of nozzles 11 and the pressure at the plurality of nozzles 11 may be performed with pumps

17 and valves, or with adjustable nozzles, for instance.

[0029] In the Figure 1 the scrubber 28 comprises three successive modules 18a-c arranged on top of each other in the vertical direction y. A module

18 comprises a row of a plurality of spraying nozzles 11 for injecting droplets 9 of a cooling water 10, a mass transfer area 19a-c where the exhaust air 2a collides with droplets 9 of the cooling water 10, a droplet separator 20a-c for separating droplets from the exhaust air 2 and a water collecting tray 21a-c for collecting the separated droplets from the droplet separator 20a-c. The mass transfer area 19a- c is created below each row of a plurality of nozzles 11. The droplet separator 20a-c and a water collecting tray 21a-c are arranged above the plurality of nozzles 11 as the exhaust air flow 2a moves upwards in vertical direction y. The number of the modules is preferably from 2 to 5 in one scrubber. In some cases it is possible to use only one module 18a-c if the required reduction in the moisture content and the temperature of the moisture laden exhaust air 2a is small.

[0030] In the method the droplets 9 of the cooling water 10 are injected to the mass transfer area 19a-c created below the row of a plurality of nozzles 11, and the droplets moving upwards with the exhaust air flow 2a are separated in the droplet separator 20a-c and the droplet separator 20a-c is drained to the water collecting tray 21a-c. The droplet separator 20a-c is arranged above the water collecting tray 21a-c and may further comprise a shape for creating a falling film from the separated droplets.

[0031] The arrangement may further comprise means for controlling the size of the droplets 22 of the cooling water 10 in each module 18a-c. Droplets 9 are usually described by their perceived size, i.e., diameter. The controlling of the size of the droplets may be performed with pumps 17 and tanks 27, or with adjustable nozzles, for instance.

[0032] In the method the size of the droplets 9 of the cooling water 10 may be controlled. The size of the droplets 9 of the cooling water 10 is be controlled by controlling the pressure at the row of the plurality of nozzles 11, and the controlling of the size of the droplets 9 is performed separately for each mod- ule 18a-c. Preferably the size of the droplets 9 is the smallest at the plurality of nozzles 11 located in the bottom part 6 of the shell 5 and increases towards the vertically y higher located nozzles being the greatest at the plurality of nozzles 11 in the top part 13 of the shell 5.

[0033] In the Figure 1 the cooling water 10 is first fed to the plurality of nozzles 11 in the top part 13 of the shell 5, i.e. to the vertically highest located module 18c. Then the subsequent modules 18a-b in a vertically descending order use the cooling water 10 collected from the previous modules 18c. This kind of layout minimizes the use of cooling water 10 in the scrubber 28. However, other layouts are also applicable. For instance, if the cooling water 10 temperature is high compared to the required moisture content and temperature values for the exhaust air flow 2b-c exiting the scrubber 28 then it may be necessary to provide the first two highest located modules 18b-c with the cooling water 10 at its original temperature.

[0034] In order to enhance the moisture removal from the exhaust air 2a the scrubber 28 may comprise at least two droplet separators 20a-c and the vertically y upper mounted droplet separator 20c is connected to a flowing cooling media for providing a cooled separator surface. The method may then comprise the step of cooling the vertically upper droplet separator 20c with a flowing cooling media for providing a cooled separator surface.

[0035] The arrangement comprises means for recirculating 29 at least part of the dried and cooled exhaust air flow 2b-c exiting the scrubber 28 to the drying process 1 where in the drying process 1 moisture is removed from the product into the dried and cooled exhaust air flow 2b creating a moisture laden exhaust air flow 2a which is guided to the scrubber 28. The means for recirculating 29 may comprise a piping and a fan, for instance.

[0036] The cooling water 10 can comprise raw water. In the scrubber

28 used cooling water may then be utilized in a production process of the product which is dried in the drying process 1. This arrangement reduces the energy consumption of the plant comprising the drying process as from the reduction of the water vapour from the exhaust air flow obtained heat is transferred to the cooling water 10.

[0037] The temperature of raw water is low during the winter time at many geographic locations. Low cooling water 10 temperature intensifies the moisture removal of the exhaust air 2a in the scrubber 28. This is advantageous as at the same time the occurrence of the visible water vapour plumes becomes high if the exhaust air 2a is exhausted to the atmosphere.

[0038] The cooling water 10 is preferably filtered to reduce the amount of possible solid particles in it for avoiding choking of the plurality of nozzles 11. The cooling water 10 in the scrubber 28 can comprise water soluble substance and water solution, e.g. liquid caustic soda. Further, it may comprise chemicals for reducing the surface tension of the water. The method for recirculating air may comprise the steps of filtering the cooling water 10 and adding chemicals to the cooling water 10 for reducing the surface tension of the cooling water 10.

[0039] The temperature of the cooling water 10 rises as the cooling water 10 flows downwards in the scrubber 28. The temperature of the cooling water 10 exiting from the scrubber bottom 6 is preferably 2-3°C lower than the temperature of the moisture laden exhaust air flow 2a at the inlet 7 on the bottom 6 of the scrubber 28. The bottom 6 of the scrubber 28 comprises an outlet 23 for discharging the cooling water 10 from the scrubber 28.

[0040] Figure 2 shows an arrangement for recirculating air in a drying process 1 with a multi-stage heat recovery system 24. The arrangement of Figure 1 is connected to a three stage heat recovery system 24 comprising three heat recovery heat exchangers 25a-c arranged in series. The heat recovery heat exchangers 25a-c are equipped with washers 26a-c for batch washing.

[0041] In the first stage heat recovery heat exchanger 25a the moisture laden exhaust air 2a transfers heat to inlet air of the drying process, i.e. to the dried and cooled exhaust air 2b flowing from the device 3. In Figure 2 the device 3 comprises a scrubber 28. In the second 25b and third 25c heat recovery heat exchangers the moisture laden exhaust air 2a transfers heat to other flowing media. From the multi-stage heat recovery system 24 the moisture laden exhaust air 2a is guided to the device 3, e.g. the scrubber 28.

[0042] In Figure 2 the dried and cooled exhaust air 2b which exits device 3, e.g. the scrubber 28, flows into a heat recovery exchanger 25a for being heated before it is recirculated to the drying process 1.

[0043] The arrangement may comprise two or more devices 3 ar- ranged in parallel for increasing air recirculating capacity for the drying process 1. When several devices 3 are arranged in parallel the set values for the exiting exhaust air 2b-c from each device 3 are preferably substantially the same. The moisture laden exhaust air flow 2a from the drying process 1 is then guided to two or more devices 3 arranged in parallel.

[0044] In the arrangement comprising the scrubber the scrubber 28 dimensions depend on the drying process 1 from and into which the exiting exhaust air 2a-c is recirculated and on the available cooling water temperature 10 for the drying and cooling of the moisture laden exhaust air 2a. The dimensioning method comprises the steps of: determination of the scrubber tower diameter based on the volumetric flow rate of the moisture laden exhaust air 2a and the moisture laden exhaust air 2a velocity through the scrubber 28; determination of the number and the height of modules 18a-c required for dehumidification and cooling based on mole fractions in the exhaust air 2a at the inlet and outlet and on the required exhaust air 2a residence time in the droplet 9 flow; calculating a total volume of spray section based on the number of modules 18a-c, the overall volumetric gas phase mass transfer coefficient and the flow rate of the gas phase.

[0045] Figure 3 shows an embodiment of an arrangement for recirculating air in a drying process where the arrangement comprises a cleaning unit 30. In the Figure 3 the moisture laden exhaust air flow 2a from a drying process 1 is guided to a cleaning unit 30 for cleaning the exhaust air 2a before it enters to the device 3 which device 3 reduces the moisture content of the exhaust air flow 2a. The cleaning unit 30 is arranged in front of the device 3 in the flow direction of the exhaust air flow 2a. The cleaning unit 30 cleans the exhaust air flow 2a by reducing the contaminant content of the exhaust air flow 2a. The cleaning unit 30 cleans the exhaust air flow 2a by removing at least part of the unwanted substances from the moisture laden exhaust air 2a. Examples of removable substanc- es are dust, dirt, odorous vapours and water soluble chemicals.

[0046] The cleaning unit 30 comprises a scrubber. In the scrubber 30 the exhaust air flow 2a flows upwards and droplets of a washing water 31a are injected into the exhaust air flow from a plurality of nozzles 11. The exhaust air 2a is scrubbed with the washing water 31a. The washing water 31a is recirculated with preferably adding some fresh water 31b. Cleaning chemicals 32, e.g. NaOH, may also be added. Water overflow 31c, the excess amount of water overflows to water treatment.

[0047] The washing water 31a circulation is relatively closed where the amount of added fresh water 31b comprises 18-25% of the circulated washing water 31a.

[0048] From the top of the cleaning unit 30 the cleaned exhaust air flow 2a is guided into the bottom part 6 of the device 3 through an inlet opening 7. As shown in Figure 3 the cleaning unit 30 and the device 3 are two separate equipment connected with a flow channel 33. The cleaning unit 30 is not contained in the shell 5 of the device 3.

[0049] The cleaning unit 30 reduces the contaminant content of the exhaust air flow 2a entering the device 3. Therefore the contaminant content of the cooling water 10 used in the device 3, e.g. in the scrubber 28, remains on a lower level. Thus the cooling water 10 having a low contaminant level may then be reused more efficiently in the device 3 requiring less fresh water. For instance, the washing water 31a consumption in the cleaning unit 30 comprises 15-25% of the cooling water 10 consumption of the device 3.

[0050] Figure 4 shows an embodiment of an arrangement for recircu- lating air in a drying process where the arrangement comprises a cleaning unit 30. In the Figure 4 the moisture laden exhaust air flow 2a from a drying process 1 is guided to a cleaning unit 30 for cleaning the exhaust air 2a before it enters to the device 3 which device 3 reduces the moisture content of the exhaust air flow 2a. The cleaning unit 30 is arranged inside the shell 5 of the device 3. The clean- ing unit 30 is arranged to the bottom part 6 of the shell 5 surrounding the device 3. The cleaning unit 30 is in front of the device 3 wherein the moisture content of the exhaust air flow 2a is reduced in the flow direction of the exhaust air flow 2a. The cleaning unit 30 cleans the exhaust air flow 2a by reducing the contaminant content of the exhaust air flow 2a. The cleaning unit 30 cleans the exhaust air flow 2a by removing at least part of the unwanted substances from the moisture laden exhaust air 2a. Examples of removable substances are dust, dirt, odorous vapours and water soluble chemicals.

[0051] The cleaning unit 30 comprises a scrubber. In the scrubber 30 the exhaust air flow 2a flows upwards and droplets of a washing water 31a are injected into the exhaust air flow from a plurality of nozzles 11. The exhaust air 2a is scrubbed with the washing water 31a. The washing water 31a is recirculated with preferably adding some fresh water 31b. Cleaning chemicals 32, e.g. NaOH, may also be added. Water overflow 31c, the excess amount of water overflows to water treatment.

[0052] From the top of the cleaning unit 30 the cleaned exhaust air flow 2a flows upwards. In Figure 4, the device 3 comprises a scrubber 28 comprising two successive modules 18d-e arranged on top of each other in the vertical direction y. The cleaned exhaust air flow 2a flows upwards from the cleaning unit 30, i.e. from the scrubber comprising washing water 31, to a vertically lowest module 18c of the scrubber 28.

[0053] The vertically lowest module 18c of the scrubber 28 comprises an outlet 23 for discharging the cooling water 10 from the scrubber 28.

[0054] As shown in Figure 4 the cleaning unit 30 and the device 3 are two separate equipment contained in the same housing, in the shell 5 of the device 3. The scrubber comprising washing water 31 cleaning the exhaust air 2a and the scrubber 28 reducing the moisture content of the exhaust air flow 2a have separate scrubbing water circulations, i.e. the washing water 31a circulation is separate from the cooling water 10 circulation.

[0055] The cleaning unit 30 reduces the contaminant content of the exhaust air flow 2a entering the device 3. Therefore the contaminant content of the cooling water 10 used in the device 3, e.g. in the scrubber 28, remains on a lower level. Thus the cooling water 10 having a low contaminant level may then be reused more efficiently in the device 3 requiring less fresh water. For instance, the washing water 31a consumption in the cleaning unit 30 comprises 15-25% of the cooling water 10 consumption of the device 3.

[0056] The operation of the scrubber 28 is explained in the description of Figure 1.

[0057] The embodiment shown in Figure 4 where the cleaning unit 30 is contained in the same housing with the device 3 provides saving in space.

[0058] Figure 5 shows an embodiment of an arrangement for recircu- lating air in a drying process comprising a cleaning unit, a supplementary air flow and a venturi scrubber. Figure 5 shows the arrangement of Figure 3 provided with a supplementary air flow and a venturi scrubber between the cleaning unit 30 and the device 3. In Figure 5 shown cleaning unit 30 for cleaning the exhaust air 2a is similar to the cleaning unit 30 shown in Figure 3 and its operation is explained in the description of Figure 3.

[0059] The exhaust air 2a is cleaned in the cleaning unit 30 and the cleaned exhaust air 2a is led to one or more heat exchangers 34. The exhaust air flow 2a is cooled with a supplementary air flow 35 in one or more heat exchangers 34. The heat exchanger 34 is an air-to-air heat exchanger. The type is a cross flow heat exchanger, for instance. The supplementary air flow 35 comprises pref- erably outside air.

[0060] In the heat exchanger 34 cooled exhaust air flow 2a is mixed with supplementary air flow 35 in a mixing chamber 36. The mixture comprises 30-60% of exhaust air flow 2a, preferably 45-55%. The relative humidity of the air flow comprising the cooled exhaust air flow 2a mixed with the supplementary air flow 35 is significantly less than 100%. The aim of the mixing is to reduce the relative humidity of the air flow comprising the exhaust air flow 2a to allow a phase change in the venturi scrubber 37.

[0061] Additional fans 8 can be used to balance the exhaust air flow 2a and the supplementary air flow 35.

[0062] The exhaust air flow 2a mixed with supplementary air flow 35 is guided to a venturi scrubber 37. The exhaust air flow 2a mixed with supplementary air flow 35 is cooled in a venturi scrubber 37. The venturi scrubber 37 comprises a quench section providing evaporative cooling. The venturi scrubber 37 comprises also a high pressure pump 38 to produce small droplets, i.e. mist, with a high water pressure for the venturi scrubber 37. As shown in Figure 5 the air flow comprising the exhaust air flow 2a mixed with supplementary air flow 35 flows downwards in vertical direction in the venturi scrubber 37.

[0063] As shown in Figure 5 the one or more heat exchangers 34, the mixing chamber 36 and the venturi scrubber 37 are installed between the clean- ing unit 30 and the device 3 in the direction of the exhaust air flow 2a.

[0064] The effect of using a supplementary air flow 35 and a venturi scrubber 37 after cleaning the exhaust air flow 2a is that the exhaust air flow 2a mixed with supplementary air flow 35 entering the device 3 is cooler and drier than the moisture laden exhaust air 2a arriving from the drying process 1.

[0065] In a further embodiment of an arrangement for recirculating air in a drying process the arrangement comprises the components as shown and described with Figure 5 except the heat exchangers 34 for cooling the exhaust air 2a with the supplementary air flow 35. After the cleaning of the exhaust air 2a in the cleaning unit 30 the exhaust air 2a is guided directly to the mixing chamber 36. The supplementary air flow 35 is also guided to the mixing chamber 36 where the exhaust air flow 2a is mixed with supplementary air flow 35.

[0066] The use of the supplementary air flow 35 increases the air flow volume from the device 3. Then the air flow divided to the drying process 2b comprises 32-18% and the air flow divided to at least one room related to the drying process 2c comprises 68-82% of the exiting exhaust air flow 2b-c, for in- stance.

[0067] In Figures 3 and 5 shown device 3 for reducing the moisture content of the exhaust air flow 2a is similar to the device 3 shown in Figure 1 and its operation is explained in the description of Figure 1.

[0068] The embodiments of an arrangement for recirculating air in a drying process shown in Figures 3, 4 and 5 may be applied also to the arrangement shown in Figure 2 comprising a multi-stage heat recovery system 24. From the multi-stage heat recovery system 24 of Figure 2 the moisture laden exhaust air 2a is guided to the cleaning unit 30. Further, in Figures 3, 4 and 5 shown exhaust air 2b which exits device 3 flows then into a heat recovery exchanger 25a as shown in Figure 2 for being heated before it is recirculated to the drying process 1.

[0069] With the arrangement and the method for recirculating the moisture laden exhaust air can be recirculated within the plant comprising the drying process without discharging it to the atmosphere. Major part of the recir- culated exhaust air can be reused in the drying process. This increases the efficiency of the system and saves energy. The emission to the atmosphere and the formation of the plumes of a water vapour can be avoided.

[0070] It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.

[0071] Part list: 1 a drying process, 2a-c an exhaust air flow, 3 a device, 4 a base surface, 5 a shell, 6 a bottom part, 7 an inlet opening, 8 a fan, 9 droplets, 10 cooling water, 11 a plurality of nozzles, 12 an outlet, 13 a top part, 14 means for measuring a temperature and a moisture content of an air flow, 15 comparing means, 16 controlling means, 17 a pump, 18a-c a module, 19a-c a mass transfer area, 20a-c a droplet separator, 21a-c a water collecting tray, 22 a droplet size controlling means, 23 a discharging outlet, 24 a heat recovery system, 25a-c a heat recovery exchanger, 26a-c a washer, 27 a tank, 28 a scrubber, 29 recirculating means, 30 a cleaning unit, 31a-c washing water, 32 cleaning chemicals, 33 a flow channel, 34 a heat exchanger, 35 a supplementary air flow, 36 a mixing chamber, 37 a venturi scrubber, 38 a venturi scrubber pump, y vertical direction.