FIELD OF THE INVENTION

The present invention relates to an air re-circulation system and a method for a dryer section of a board or paper machine, and more particularly to a dryer section where water from a moving web-like material is evaporating to air creating moist exhaust air during a run of the board or paper machine.

BACKGROUND OF THE INVENTION

Drying of a web-like material in a dryer section or sections of a board or paper machine is an energy intensive operation due to the high latent heat of vaporization. The water from a moving web-like material is evaporating to a surrounding hot air creating a moisture laden exhaust air.

It is known to recover heat from the moisture laden exhaust air. For instance, a fresh supply air can be pre-heated or heated with the moisture laden exhaust air. The cooled moisture laden exhaust air is then exhausted to atmosphere. As the cooled moisture laden exhaust air flow is exhausted to the atmosphere it causes 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.

Thus, there is a need for an efficient and energy saving system and a method for re-circulating air in a dryer section of a board or paper machine which also reduces the amount of the exhaust air flow exhausted to the atmosphere.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is to provide an air re-circulation system and a method for a dryer section of a board or paper machine so as to solve or at least alleviate the prior art disadvantages. The objects of the invention are achieved by an air re-circulation system which is characterized by what is stated in claim 1. The objects of the present invention are also achieved by an air re circulation method which is characterized by what is stated in claim 11.

The preferred embodiments of the invention are disclosed in the dependent claims. The invention is based on the idea of an air re-circulation system for a dryer section of a board or paper machine. In the dryer section water from a moving web-like material is evaporating to air creating moist exhaust air during a run of the board or paper machine. The system comprises at least three air re circulation parts connected to a dryer section hood one after the other in a run direction of a web-like material. Each air re-circulation part comprises one or more heat recovery units. Each heat recovery unit comprises a circulation air fan for drawing moist exhaust air from the dryer section hood through the heat recovery unit and for supplying at least part of the conditioned moist exhaust air to the dryer section hood, and an air-to-fluid heat exchanger for conditioning the moist exhaust air by cooling and reducing a moisture content of the moist exhaust air. The air-to- fluid heat exchanger comprises a fluid circuit connected to a fluid cooling unit, and a spraying unit in connection with the air-to-fluid heat exchanger for cleaning the moist exhaust air. A first air re-circulation part is connected to a beginning of the dryer section hood, a second air re-circulation part is connected to a middle of the dryer section hood and a third air re-circulation part is connected to an end of the dryer section hood.

During a run of the board or paper machine, the dryer section hood comprises several zones in a run direction of the web-like material where the moisture contents of the exhaust air are at different levels. In the system and method, the dryer section hood comprises at least three zones. In the beginning of the dryer section hood is a zone where the moisture content of the exhaust air is low as the web-like material in warming up. In the beginning of the dryer section hood the evaporating rate is typically 0-20 kg/m ² h. In a middle of the dryer section hood is a zone where the moisture content of the exhaust air is high as the web-like material has a high evaporating rate. In the middle of the dryer section hood the evaporating rate is typically 20-40 kg/m ² h. In the end of the dryer section hood is a zone where the moisture content of the exhaust air is low again due to a decreased drying rate of the web-like material. In the end of the dryer section hood the evaporating rate is typically 20-0 kg/m ² h. Thus, the beginning and the end of the dryer section hood comprise zones where the moisture content of the exhaust air is lower than in the zone in the middle of the dryer section hood. The zones are preferably defined by an evaporating rate and may be separated by light weight walls, e.g. flaps. However, the zones need not to be separated from each other with separation walls constructed to limit the movement of the moist exhaust air from one zone to another. In the system and the method, the moist exhaust air which removes the water vapor evaporated from the web-like material out of the hood of the dryer section in the dryer section is conditioned by cooling and drying and cleaning, and at least part of the conditioned moist exhaust air is re-circulated to the dryer section hood as a supply air.

With the system and the method, the zones having different moisture content levels inside the dryer section hood creating moist exhaust air at different moisture content levels can be handled separately. Each zone may be provided with a supply air, conditioned moist exhaust air, having a temperature and moisture content independent of another zone. In the middle of the dryer section hood the temperature and humidity of the supplied air, the conditioned moist exhaust air, is preferably higher than in other zones.

As the air re-circulation in the heat recovery unit enables also a fully closed or largely closed air re-circulation system, only one circulation air fan needs to be provided for each heat recovery unit.

A dryer section hood is an enclosure surrounding the dryers in the drying section. The locations in the dryer section hood, e.g. in the beginning, in the middle and in the end of the dryer section hood, are defined in the length direction of the dryer section hood in the run direction of the web-like material. The web like material comprises a paper web, a board web or a fibre web, for instance. In the fluid circuits of the air-to-fluid heat exchangers and the second air-to-fluid heat exchangers flowing cooling fluid comprises a water-glycol mixture, for instance, or a liquid having a low freezing point, e.g. below -15°C.

The fluid cooling unit may comprise a cooling tower and/or a thermally driven heat pump. The thermally driven heat pump utilizes the moist exhaust air having a higher temperature as a heat source and further, it may provide chilling to the moist exhaust air to have a lower temperature.

The fluid cooling unit may comprise an absorption chiller. The fluid circuit of the air-to-fluid heat exchanger of the second air re-circulation part which is connected to a middle of the dryer section hood is connected to one or more generators of the absorption chiller for driving the absorption chiller. The fluid circuit of the air-to-fluid heat exchanger of the first air re-circulation part and/or the third air re-circulation part is connected to one or more evaporators of the absorption chiller.

The absorption chiller utilizes the moist exhaust air having a higher temperature as a heat source and provides chilling to the moist exhaust air to have a lower temperature. In the beginning and in the end of the dryer section hood the moisture content of the exhaust air is lower than in the zone in the middle of the dryer section hood and by providing chilling to condition the moist exhaust air the moisture content of the conditioned moist air is further reduced. As the dryer section hood requires an amount of 55%...80% of the moist exhaust air flow supplied as a dry heated supply air part of the conditioned moist exhaust air is exhausted to atmosphere. The main part of the exhausted conditioned moist exhaust air is preferably exhausted from the beginning and from the end of the dryer section hood. Therefore, the reduced moisture content of the conditioned moist air decreases the risk of visible water vapour plumes.

The fluid cooling unit may comprise a thermally driven heat pump, and the fluid circuit of the air-to-fluid heat exchanger of the second air re-circulation part is connected to one or more generators of thermally driven heat pump for driving the thermally driven heat pump. The one or more evaporators of the thermally driven heat pump produces cooling power during the operation of the thermally driven heat pump. Further, the fluid circuit of the air-to-fluid heat exchanger of the first air re-circulation part and/or the third air re-circulation part may be connected to one or more evaporators of the thermally driven heat pump. In this case the thermally drive heat pump produces cooling power to the air re circulation system. Furthermore, the one or more evaporators of the thermally driven heat pump may comprise an inlet connection and an outlet connection to a chilled water network of the paper or pulp production mill for providing chilled water for process and building cooling purposes. In this case the thermally driven heat pump produces cooling power to the paper or pulp production mill comprising the air re-circulation system.

The fluid cooling unit may comprise a thermally driven heat pump and a cooling tower. The fluid circuit of the air-to-fluid heat exchanger of the heat recovery unit of the second air re-circulation part is connected to the thermally driven heat pump for driving the thermally driven heat pump. The thermally driven heat pump is connected to a cooling fluid circuit of the cooling tower for releasing heat from the thermally driven heat pump to the cooling fluid. The heat gained from a hot and moist exhaust air drawn from the middle of the dryer section hood where the web-like material has a high evaporating rate is utilized to drive a thermally driven heat pump.

The heat recovery unit may further comprise an air-to-air heat exchanger for heating from the air-to-fluid heat exchanger flowing conditioned moist exhaust air with the moist exhaust air and a spraying unit in connection with the air-to-air heat exchanger for cleaning the moist exhaust air, a heater for heating the conditioned moist exhaust air flowing from the air-to-air heat exchanger prior to supplying at least part of the conditioned moist exhaust air to the dryer section hood.

The heat recovery unit may further comprise a heater for heating the conditioned moist exhaust air prior to supplying at least part of the conditioned moist exhaust air to the dryer section hood and a second air-to-fluid heat exchanger comprising a second fluid circuit connected to a cooling fluid circuit of the thermally driven heat pump where the cooling fluid enters the thermally driven heat pump at a lower temperature than entering to the second air-to-fluid heat exchanger.

The second air re-circulation part connected to the middle of the dryer section hood may comprise two heat recovery units. The heat recovery unit further comprises an air-to-air heat exchanger for heating from the air-to-fluid heat exchanger flowing conditioned moist exhaust air with the moist exhaust air and a spraying unit in connection with the air-to-air heat exchanger for cleaning the moist exhaust air, and a heater for heating the conditioned moist exhaust air flowing from the air-to-air heat exchanger prior to supplying at least part of the conditioned moist exhaust air to the dryer section hood. The fluid cooling unit may comprise a thermally driven heat pump, and the fluid circuits of the air-to-fluid heat exchangers of the two heat recovery units are connected to one or more generators of the thermally driven heat pump for driving the thermally driven heat pump. Further, the fluid circuits of the air-to-fluid heat exchangers of the two heat recovery units may be connected in parallel to the fluid cooling unit.

During the run of the board or paper machine the dryer section hood may comprise at least three zones comprising exhaust air at different moisture content levels inside the dryer section hood one after the other in a run direction of a web-like material.

The at least three zones are located to the beginning, to the middle and to the end of the dryer section hood. In the system the first air re-circulation part is arranged to re-circulate moist exhaust air from the zone located to the beginning, the second air re-circulation part is arranged to re-circulate moist exhaust air from the zone located to the middle, and the third air re-circulation part is arranged to re-circulate moist exhaust air from the zone located to the end.

The invention is based on the idea of an air re-circulation method for a dryer section of a board or paper machine and in the dryer section water from a moving web-like material is evaporating to air creating moist exhaust air during a run of the board or paper machine. The method comprises at least three air re circulation parts connected to a dryer section hood one after the other in a run direction of a web-like material. Each air re-circulation part comprises one or more heat recovery units. Each heat recovery unit comprises a circulation air fan and an air-to-fluid heat exchanger. The circulation air fan is drawing moist exhaust air from the dryer section hood through the heat recovery unit and is supplying at least part of the conditioned moist exhaust air to the dryer section hood. The air-to-fluid heat exchanger is conditioning the moist exhaust air by cooling and reducing a moisture content of the moist exhaust air. A fluid circuit of the air-to-fluid heat exchanger is connected to a fluid cooling unit, and the fluid in the fluid circuit of the air-to-fluid heat exchanger is cooled in the fluid cooling unit. A spraying unit in connection with the air-to-fluid heat exchanger is cleaning the moist exhaust air. A first air re-circulation part is connected to a beginning of the dryer section hood and a first circulation air fan is drawing the moist exhaust air and is supplying at least part of the conditioned moist exhaust air to the beginning of the dryer section hood. A second air re-circulation part is connected to a middle of the dryer section hood and a second circulation air fan is drawing the moist exhaust air and is supplying at least part of the conditioned moist exhaust air to the middle of the dryer section hood. A third air re-circulation part is connected to an end of the dryer section hood and a third circulation air fan is drawing the moist exhaust air and is supplying at least part of the conditioned moist exhaust air to the end of the dryer section hood.

During the run of the board or paper machine the amount of water from a web-like material evaporating to air may be varied in a run direction of a web like material in the dryer section hood. At least three zones creating exhaust air at different moisture content levels is provided inside the dryer section hood one after the other in a run direction of a web-like material. The at least three zones are located to a beginning of the dryer section hood, to a middle of the dryer section hood and to an end of the dryer section hood. In the method the first air re circulation part is re-circulating moist exhaust air from the zone located to the beginning, the second air re-circulation part is re-circulating moist exhaust air from the zone located to the middle and the third air re-circulation part is re circulating moist exhaust air from the zone located to the end of the dryer section hood. The fluid cooling unit may comprise a cooling tower. The fluid circuit of the air-to-fluid heat exchanger is connected to the cooling tower, and the fluid in the fluid circuit is cooled in the cooling tower.

The fluid cooling unit may comprise a thermally driven heat pump. The fluid circuit of the air-to-fluid heat exchanger of the second air re-circulation part is connected to one or more generators of thermally driven heat pump, and the fluid in the fluid circuit of the air-to-fluid heat exchanger of the second air re circulation part is cooled in the thermally driven heat pump by driving the thermally driven heat pump.

The fluid cooling unit may comprise a thermally driven heat pump and a cooling tower. The fluid in the fluid circuit of the air-to-fluid heat exchanger of the heat recovery unit of the second air re-circulation part is flowing to the thermally driven heat pump for driving the thermally driven heat pump. The cooling fluid in the cooling fluid circuit of the cooling tower is flowing to the thermally driven heat pump and the thermally driven heat pump is releasing heat to the cooling fluid.

Further, the heat recovery unit may further comprise a heater and a second air-to-fluid heat exchanger comprising a second fluid circuit connected to a cooling fluid circuit of the thermally driven heat pump. The heater is heating the conditioned moist exhaust air prior at least part of it is supplied to the dryer section hood. The fluid in the second fluid circuit of a second air-to-fluid heat exchanger is cooling the conditioned moist exhaust air flowing from the air-to-fluid heat exchanger in the second air-to-fluid heat exchanger, and the fluid in the second fluid circuit of a second air-to-fluid heat exchanger is entering the thermally driven heat pump at a lower temperature than entering to the second air-to-fluid heat exchanger.

The fluid cooling unit may comprise an absorption chiller. The fluid circuit of the air-to-fluid heat exchanger of the second air re-circulation part is connected to one or more generators of the absorption chiller. The fluid in the fluid circuit of the air-to-fluid heat exchanger of the second air re-circulation part is cooled in one or more generators of the absorption chiller by driving the absorption chiller. The fluid circuit of the air-to-fluid heat exchanger of the first air re-circulation part and/or the third air re-circulation part is connected to one or more evaporators of the absorption chiller. The fluid in the fluid circuit of the air- to-fluid heat exchanger of the first air re-circulation part and/or the third air re circulation part is chilled in one or more evaporators of the absorption chiller. The heat recovery unit may further comprise an air-to-air heat exchanger, a spraying unit in connection with the air-to-air heat exchanger and a heater. The moist exhaust air drawn from the dryer section hood is heating the conditioned moist exhaust air flowing from the air-to-fluid heat exchanger in the air-to-air heat exchanger, and the spraying unit is cleaning the moist exhaust air by spraying washing liquid, and the heater is heating the conditioned moist exhaust air flowing from the air-to-air heat exchanger prior at least part of it is supplied to the dryer section hood.

The fluid cooling unit may comprise a thermally driven heat pump, and the fluid circuit of the air-to-fluid heat exchanger of the second air re-circulation part is connected to one or more generators of the thermally driven heat pump. The fluid in the fluid circuit of the air-to-fluid heat exchanger of the second air re circulation part is cooled in one or more generators of the thermally driven heat pump by driving the thermally driven heat pump. The fluid circuit of the air-to- fluid heat exchanger of the first air re-circulation part and/or the third air re circulation part is connected to one or more evaporators of the thermally driven heat pump. The fluid in the fluid circuit of the air-to-fluid heat exchanger of the first air re-circulation part and/or the third air re-circulation part is chilled in one or more evaporators of the thermally driven heat pump.

The second air re-circulation part connected to the middle of the dryer section hood may comprise two heat recovery units. The heat recovery unit further comprises an air-to-air heat exchanger, a spraying unit in connection with the air- to-air heat exchanger and a heater, and the moist exhaust air drawn from the dryer section hood is heating the conditioned moist exhaust air flowing from the air-to- fluid heat exchanger in the air-to-air heat exchanger, and the spraying unit is cleaning the moist exhaust air by spraying washing liquid, and the heater is heating the conditioned moist exhaust air flowing from the air-to-air heat exchanger prior at least part of it is supplied to the dryer section hood.

Further, the fluid cooling unit may comprise a thermally driven heat pump and the fluids in the fluid circuits of the air-to-fluid heat exchangers of the two heat recovery units are flowing to one or more generators of the thermally driven heat pump for driving the thermally driven heat pump.

The air re-circulation system and a method for a dryer section of a board or paper machine of the present invention provide an efficient and energy saving system and method and it reduces the amount of the exhaust air flow exhausted to the atmosphere. BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail by means of specific embodiments with reference to the enclosed drawings, in which

Figure 1 shows an air re-circulation system for a dryer section of a board or paper machine;

Figure 2 shows an air re-circulation system for a dryer section of a board or paper machine;

Figure 3 shows an air re-circulation system for a dryer section of a board or paper machine.

DETAILED DESCRIPTION OF THE INVENTION

Figures 1-3 show an air re-circulation system 1 for a dryer section of a board or paper machine 2. During a run of the board or paper machine water from a moving web-like material 3 is evaporating to air creating moist exhaust air 4a-c in the dryer section. The shown system comprises three air re-circulation parts 5a- c connected to a dryer section hood 6 one after the other in a run direction r of a web-like material 3. The system may comprise four or more air re-circulation parts 5a-c. The shown air re-circulation part 5a-c comprise a heat recovery unit 7a-c. The air re-circulation part 5a-c may also comprise two or more heat recovery units 7a- c arranged one after the other in a run direction r of a web-like material 3.

The heat recovery unit 7a-c comprises a circulation air fan 8a-c for drawing moist exhaust air 4a-c from the dryer section hood 6 into the heat recovery unit 7a-c. The circulation air fan 8a-c circulates the moist exhaust air 4a- c through the heat recovery unit 7a-c and supplies at least part of the conditioned moist exhaust air 9a-c to the dryer section hood 6 as a supply air.

The heat recovery unit 7a-c further comprises an air-to-air heat exchanger lOa-c and an air-to-fluid heat exchanger lla-c. The circulation air fan 8a-c moves the moist exhaust air 4a-c through the air-to-air heat exchanger lOa-c where it is cooled as it transfers heat to the conditioned moist exhaust air flowing from the air-to-fluid heat exchanger lla-c. The heat recovery unit 7a-c comprises also a heater 12a-c for heating the conditioned moist exhaust air flowing from the air-to-air heat exchanger lOa-c prior to supplying at least part of the conditioned moist exhaust air 9a-c to the dryer section hood 6 as a supply air.

In Figures 1-3 the circulation air fan 8a-c is located to precede the air- to-air heat exchanger lOa-c in the flow direction of the moist exhaust air 4a-c in the heat recovery unit 7a-c. The circulation air fan 8a-c may also be located after the air-to-air heat exchanger lOa-c in the flow direction of the moist exhaust air 4a-c in the heat recovery unit 7a-c. Further, the circulation air fan 8a-c may also be located after the heater lOa-c in the flow direction of the moist exhaust air 4a-c in the heat recovery unit 7a-c.

The moist exhaust air 4a-c is conditioned in the air-to-fluid heat exchanger lla-c by cooling and reducing the moisture content of the moist exhaust air 4a-c. The air-to-fluid heat exchanger lla-c comprises a fluid circuit 13a-c connected to a fluid cooling unit 14. The fluid in the fluid circuit 13a-c of the air- to-fluid heat exchanger is cooled in the fluid cooling unit 14.

In the embodiment of Figure 1 the fluid cooling unit 14 comprises a cooling tower or an air cooled air-to-fluid heat exchanger, e.g. a lamella heat exchanger. The cooling tower may be a closed circuit evaporative cooling tower, for instance. A closed circuit, where the fluid in the fluid circuit 13a-c circulates within the flow channels inside the cooling tower, allows the fluid to maintain its chemical and physical characteristics and prevents the pollution of the fluid by external particles. The flow channel is continuously wetted with a spray system wherein part of the spray water evaporates.

During a run of the board or paper machine, the dryer section hood 6 comprises several zones 15a-c one after the other in a run direction r of the web like material 3 where the moisture contents of the exhaust air 4a-c are at different levels. In Figures 1-3 the dryer section hood comprises three zones 15a-c. In the beginning of the dryer section hood 6 is a zone 15a where the moisture content of the exhaust air is low as the web-like material in warming up. In a middle of the dryer section hood 6 is a zone 15b where the moisture content of the exhaust air is high as the web-like material has a high evaporating rate. In the end of the dryer section hood 6 is a zone 15c where the moisture content of the exhaust air is low again due to a decreased evaporating rate of the web-like material. Thus, the beginning and the end of the dryer section hood comprise zones 15a,c where the moisture content of the exhaust air is lower than in the zone 15b in the middle of the dryer section hood.

The moisture content of the moist exhaust air 4a-c is not constant within a zone 15a-c inside the dryer section hood 6. In the beginning of the dryer section hood 6 the moisture content increases in the run direction of the web-like material r and in the end of the dryer section hood 6 the moisture content decreases in the run direction of the web-like material r. In the Figures the zones 15a-c are represented with dashed lines.

Each air re-circulation part 5a-c is located to be in connection with one zone 15a-c. A first air re-circulation part 5a is connected to a beginning of the dryer section hood 6, a second air re-circulation part 5b is connected to a middle of the dryer section hood 6 and a third air re-circulation part 5c is connected to an end of the dryer section hood 6.

The first air re-circulation part 5a is re-circulating moist exhaust air 4a from the zone 15a located to the beginning, the second air re-circulation part 5b is re-circulating moist exhaust air 4b from the zone 15b located to the middle and the third air re-circulation part 5c is re-circulating moist exhaust air 4c from the zone 15c located to the end of the dryer section hood 6.

In the embodiments shown in Figures 2 and 3 the fluid cooling unit 14 comprises a cooling tower and a thermally driven heat pump 16. Thermally driven heat pumps 16 work at several temperature levels. Driving heat is supplied at a high temperature level to one or more generators of the thermal heat pump 16. Useful cold in a cooling operation is supplied at a low temperature level from one or more evaporators of the thermally driven heat pump 16. The supplied heat is released at a medium temperature level to a cooling fluid 17 flow of the thermally driven heat pump 16. Thermally driven heat pumps 16 enable utilization of low- grade heat. Examples of thermally driven heat pumps 16 are absorption chillers and adsorption chillers. In the embodiments shown in Figures 2 and 3, the heat gained from a hot and moist exhaust air 4b drawn from the middle of the dryer section hood 6 where the web-like material 3 has a high evaporating rate is utilized to drive a thermally driven heat pump 16.

In the embodiments shown in Figures 2 and 3 comprising the thermally driven heat pump 16 the air-to-air heat exchanger 10b in the heat recovery unit 7b of the second air re-circulation part 5b may be omitted. In this case the moist exhaust air 4b can release more heat in the air-to-fluid heat exchanger lib for driving the heat pump 16.

In the embodiments shown in Figures 2 and 3 the thermally driven heat pump 16 comprises an absorption chiller, for instance. Then the fluid circuit 13b of the air-to-fluid heat exchanger lib of the second air re-circulation part 5b is connected to one or more generators of the absorption chiller. The fluid in the fluid circuit 13b of the air-to-fluid heat exchanger lib of the second air re-circulation part 5b is cooled in one or more generators of the absorption chiller by heating the work medium of the absorption chiller. As an example, the temperature of the fluid in the fluid circuit 13b of the air-to-fluid heat exchanger lib of the second air re circulation part 5b entering the thermally driven heat pump comprises 55-75°C.

The fluid circuit 13a of the air-to-fluid heat exchanger 11a of the first air re-circulation part 5a and/or the third air re-circulation part 5c is connected to one or more evaporators of the thermally driven heat pump 16. The fluid in the fluid circuit 13a, c of the air-to-fluid heat exchanger 13a, c of the first air re-circulation part 5a and the third air re-circulation part 5c is chilled in one or more evaporators of the thermally driven heat pump 16. As the one or more evaporators of the absorption chiller operate at a lower temperature that the one or more generators of the absorption chiller the temperature of the fluid entering to the air-to-fluid heat exchangers lla,c in the beginning and in the end of the dryer section hood 6 is lower than the temperature of the fluid entering to the air-to-fluid heat exchanger lib in the middle of the dryer section hood 6. This enables the reduction of the moisture content of the conditioned moist exhaust air 9a-c to a lower level in the beginning and in the end of the dryer section hood 6 than in the middle of the dryer section hood 6. As an example, the temperature of the fluid in the fluid circuit 13a, c of the air-to-fluid heat exchanger 11a, c of the first and third air re circulation parts 5a, c entering the air-to-fluid heat exchanger 11a, c comprises 13- 18°C.

In the embodiment shown in Figure 2 the cooling fluid 17 of the thermally driven heat pump 16 is cooled by means of a cooling tower. The cooling fluid 17 flow flowing from the cooling tower enters the thermally driven heat pump 16, e.g. the absorption chiller, and to the thermally driven heat pump 16 supplied heat is released to the cooling fluid 17 flow. In the thermally driven heat pump 16 heated cooling fluid flow returns to the cooling tower. As an example, the temperature of the cooling fluid 17 entering the thermally driven heat pump 16 comprises 27-33°C.

In the embodiment shown in Figure 3 the heat recovery units 7a-c in the re-circulation parts 5a-c further comprise a second air-to-fluid heat exchanger 18a-c for conditioning the moist exhaust air 4a-c. The moist exhaust air 4a-c is conditioned in the second air-to-fluid heat exchanger 18a-c comprising a second fluid circuit 19a-c connected to a cooling fluid 17 circuit of the thermally driven heat pump 16. The cooling fluid 17 of the thermally driven heat pump 16 is cooled by means of a cooling tower. The cooling fluid 17 flow flowing from the cooling tower enters the thermally driven heat pump 16, e.g. the absorption chiller, and to the thermally driven heat pump 16 supplied heat is released to the cooling fluid 17 flow.

In the thermally driven heat pump 16 heated cooling fluid 17 flow is further divided into flow parts which flow to the second air-to-fluid heat exchangers 18a, c connected to the beginning and to the end of the dryer section hood 6. The moist exhaust air 4a, c releases heat to the cooling fluid 17 flow parts in the second air-to-fluid heat exchangers 18a, c. In the second air-to-fluid heat exchangers 18a, c heated cooling fluid 17 flow parts flow to the second air-to-fluid heat exchanger 18b connected to the middle of the dryer section hood 6. The cooling fluid 17 flowing from the second air-to-fluid heat exchanger 18b connected to the middle of the dryer section hood 6 enters the cooling tower.

In the embodiment shown in Figure 3, the cooling fluid 17 flowing from the cooling tower enters the thermally driven heat pump 16 at a lower temperature than it enters to the second air-to-fluid heat exchangers 18a-c. In the flow direction of the cooling fluid 17 the second air-to-fluid heat exchangers located to the beginning and to the end of the drying section hood 6 precede the second air-to- fluid heat exchanger located to the middle of the drying section hood. Thus, the temperature of the cooling fluid 17 when it enters the second air-to-fluid heat exchanger 18b located to the middle of the drying section hood 6 comprises a higher temperature than when it enters the second air-to-fluid heat exchangers 18a, c located to the beginning and to the end of the drying section hood 6. As an example, the temperature of the cooling fluid 17 entering the thermally driven heat pump 16 comprises 27-33°C and the temperature of the cooling fluid 17 entering the second air-to-fluid heat exchangers 18b connected to the beginning and to the end of the dryer section hood 6 comprises 33-39°C.

In the re-circulation part 5b connected to the middle of the dryer section hood 6, the air-to-fluid lib and the second air-to-fluid heat exchangers 18b are arranged such that the air-to-fluid heat exchanger lib precedes the second air- to-fluid heat exchanger 18b in the flow direction of the moist exhaust air in the heat recovery unit 7b.

In the re-circulation parts 5a, c connected to the beginning and to the end of the dryer section hood 6, the air-to-fluid 11a, c and the second air-to-fluid 18a, c heat exchangers are arranged such that the second air-to-fluid heat exchanger 18a, c precedes the air-to-fluid heat exchanger 11a, c in the flow direction of the moist exhaust air 4a, c in the heat recovery unit 7a, c.

In the embodiment shown in Figure 3 the fluid cooling unit 14 arranged to provide fluid cooling to the air-to-fluid lla-c and to the second air-to-fluid heat exchangers 18a-c comprises a thermally driven heat pump 16 and a cooling tower. The thermally driven heat pump 16 provides cooling to the air-to-fluid heat exchangers lla-c and the cooling tower provides cooling to the second air-to-fluid heat exchangers 18a-c. The cooling tower provides cooling also to the thermally driven heat pump 16.

At certain times of the year the outdoor air temperature can be high enough allowing the conditioned moist exhaust air 9a-c from the beginning and from the end of the dryer section hood 6 to be exhausted to the atmosphere without a risk of creating visible plumes. Then the system 1 shown in Figure 3 comprises one or more valves closing the fluid circuit 13a, c of the air-to-fluid heat exchanger 13a, c of the first air re-circulation part 5a and the third air re-circulation part 5c. The system 1 also comprises by-pass channels to by-pass the air-to-fluid heat exchanger 13a, c of the first air re-circulation part 5a and the third air re-circulation part 5c. Additionally, the system 1 comprises an inlet connection to and an outlet connection from the thermally driven heat pump which can be used to provide chilled water to a chilled water network of the paper or pulp production mill for cooling purposes.

In all embodiments each air recirculation part 5a-c comprises at least one washing system 20 which is a continuous washing system comprising a washing liquid tank and a washing liquid pump. The air-to-air heat exchanger 10a- c, the air-to-fluid heat exchanger lla-c and the second air-to-fluid heat exchanger 18a-c comprise a spraying unit 21a-c for cleaning the moist exhaust air 4a-c. In the Figures the washing liquid inlets to the spraying units 21a-c and the washing liquid outlets from the heat exchangers lOa-c, lla-c, 18a-c are shown by arrows and the connection to the washing system 20 is shown only for one heat exchanger 10b in one heat recovery unit 5b. The reference numbers for the spraying units 21a-c are marked only to Figure 1 for a sake of clarity. The continuous washing of the moist exhaust air 4a-c is preferably made in one of the heat exchangers lOa-c, lla-c, 18a- c in each recovery unit 7a-c and a periodical washing of the moist exhaust air 4a-c is made in the other heat exchangers lOa-c, lla-c, 18a-c. The washing of the moist exhaust air 4a-c in the re-circulation part 5a-c is necessary in order to meet the quality requirements set to the supply air.

The embodiments shown in Figures 1-3 may comprise two heat recovery units 7a-c in the second air re-circulation part 5b connected to the middle of the dryer section hood 6 and arranged one after each other in the run direction r of the web-like material 3. The fluid circuits of the air-to-fluid heat exchangers of the two heat recovery units are then connected in parallel to the fluid cooling unit.

Usually modern dryer section hoods 6 require an amount of 55%...80% of the moist exhaust air 4a-c flow supplied as a dry heated replacement air. The remainder is supplied by infiltration around the hood seals and doors to prevent hot moist air from escaping into the machine room. In an embodiment, the first circulation air fan 8a connected to a beginning of the dryer section hood 6 is supplying at least 60% of the volume of the conditioned moist exhaust air 9a, and the second circulation air fan 8b connected to the middle of the dryer section hood 6 is supplying at least 90% of the volume of the conditioned moist exhaust air 9b and the third circulation air fan 8c connected to the end of the dryer section hood 6 is supplying at least 60% of the volume of the conditioned moist exhaust air 9c to the dryer section hood 6 as a supply air.

The invention has been described above with reference to the examples shown in the figures. However, the invention is in no way restricted to the above examples but may vary within the scope of the claims.

Part list: 1 an air re-circulation system; 2 a dryer section of a board or paper machine; 3 a web-like material; 4a-c moist exhaust air; 5a-c an air re circulation part; 6 a dryer section hood; 7a-c a heat recovery unit; 8a-c a circulation air fan; 9a-c conditioned moist exhaust air; lOa-c an air-to-air heat exchanger; 11a- c an air-to-fluid heat exchanger; 12a-c a heater; 13a-c a fluid circuit; 14 a fluid cooling unit; 15a-c a zone; 16 a heat pump; 17 a cooling fluid; 18 a-c a second air- to-fluid heat exchanger; 19a-c a second fluid circuit; 20 a washing system; 21a-c a spraying unit; r run direction.