Abstract:
The invention relates to a machinery arrangement, which comprises a combus-tion unit ( 3 ) arranged in an engine room ( 11 ), and an exhaust gas duct assem-bly ( 4 ) connected to the combustion unit ( 3 ) for receiving an exhaust gas flow and for leading the exhaust gas flow to the atmosphere through an exhaust gas cleaning system ( 5 ) provided with an exhaust gas pipe ( 52 ). In order to condi-tion the exhaust plume, the exhaust gas pipe ( 52 ) is arranged in an enclosure ( 114 ), whereby the enclosure ( 114 ) is arranged in flow connection with the en-gine room ( 11 ) in order to provide heat for the exhaust gas pipe ( 52 ) by means of a heated air flow from the engine room.

Description:
TECHNICAL FIELD 
       [0001]    The invention relates to a machinery arrangement for a marine vessel, which machinery arrangement comprises a combustion unit arranged in an engine room, and an exhaust gas duct assembly connected to the combustion unit for receiving an exhaust gas flow and for leading the exhaust gas flow to the atmosphere through an exhaust gas cleaning system provided with an exhaust gas pipe, according to the preamble of claim  1 . The present invention also relates to a method for operating a machinery arrangement of a marine vessel, according to the preamble of claim  12 . 
       BACKGROUND ART 
       [0002]    Machinery arrangements for marine vessels conventionally comprise one or more combustion units, such as diesel engines, oil-fired boilers, incinerators, etc. The combustion units usually have independent exhaust gas duct assemblies, whereby also the cleaning of exhaust gases takes place separately for each combustion unit. 
         [0003]    Gases coming from oil-fired boilers are sometimes referred to as flue gases. Gases from diesel engines are mostly called exhaust gases. In the following the general term exhaust gas will be used with regard to gases from all relevant types of combustion units. 
         [0004]    Reduction of exhaust gas emissions can be obtained by improving the performance of the combustion units, by using cleaner fuel, or by cleaning the exhaust gases. 
         [0005]    In previously known solutions, with regard to cleaning the exhaust gases, machinery arrangements have been provided with exhaust gas cleaning systems for nitric oxides (NO x ). Presently, however, due to increasing demands to reduce exhaust gas emissions, especially in port conditions and near the coast, there will be a need to clean sulphuric oxides (SO x ) emanating from sulphur rich fuels, such as conventional heavy fuel oil. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the invention is to avoid the above mentioned problems and to provide a machinery arrangement with an effective exhaust gas emission control. This object is attained by a machinery arrangement according to claim  1  and a method according to claim  12 . 
         [0007]    The basic idea of the invention is to regulate the final discharge of exhaust gas into the atmosphere by exhaust gas plume conditioning. The exhaust gas pipe is arranged in an enclosure, which is arranged in flow connection with the engine room in order to heat the exhaust gas pipe by means of a heated air flow from the engine room. Further, the heated ventilation air from the engine room can mix with the wet exhaust gas discharged into the atmosphere from the exhaust gas pipe. Further, as the exhaust gas pipe is heated, undesired condensation of the wet exhaust gas in the exhaust gas pipe can be avoided. 
         [0008]    The engine room is advantageously provided with an engine room ventilation outlet, which is in flow connection with the enclosure. 
         [0009]    Appropriate end heat treatment of the exhaust gas pipe discharge is advantageously carried out by providing the enclosure with a jacket portion surrounding at least part of the exhaust gas pipe. 
         [0010]    The exhaust gas cleaning system advantageously comprises a cleaning device for sulphuric oxides (SO x ), i.e. preferably a scrubber unit provided with the exhaust gas pipe. 
         [0011]    The scrubber unit is advantageously arranged outside the enclosure so that the treatment of exhaust gases in the scrubber unit can be carried out in a controlled manner. 
         [0012]    The combustion unit is advantageously connected by means of a second section of the exhaust gas duct assembly and a first section of the exhaust gas duct assembly to the exhaust gas cleaning system. 
         [0013]    In such an arrangement, the first section of the exhaust gas duct assembly, which is connected to the exhaust gas cleaning system, is advantageously arranged in the enclosure. In this case the first section is advantageously un-insulated in the enclosure. 
         [0014]    The heated ventilation air from the engine room can thus advantageously be provided with additional heat from the first section of the exhaust gas duct assembly. 
         [0015]    In a machinery arrangement, which comprises one or more combustion units, the first section of the exhaust gas duct assembly advantageously comprises a first branch pipe and the second section of the exhaust gas duct assembly advantageously comprises a second branch pipe, whereby the first branch pipe is arranged in the enclosure. In this case the first branch pipe is advantageously un-insulated in the enclosure. 
         [0016]    The heated ventilation air from the engine room can thus advantageously be provided with additional heat from the first branch pipe of the exhaust gas duct assembly. 
         [0017]    In a machinery arrangement, which comprises a number of combustion unit, the exhaust gas duct assembly advantageously comprises corresponding first branch pipes and second branch pipes, whereby the first branch pipes are arranged in the enclosure. 
         [0018]    Further advantageous features of the machinery arrangement according to the present invention are given in claims  2 - 11  and of the method according to the present invention in claims  13 - 15 . 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]    In the following the invention will be described, by way of example only, with reference to the accompanying schematic drawings, in which 
           [0020]      FIG. 1  illustrates a first embodiment of the present invention, 
           [0021]      FIG. 2  illustrates a second embodiment of the present invention, 
           [0022]      FIG. 3  illustrates a third embodiment of the present invention, and 
           [0023]      FIG. 4  illustrates a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    In  FIG. 1  a machinery arrangement for a marine vessel  1  is generally indicated by reference sign  2 . The machinery arrangement comprises a combustion unit  3 . The combustion unit may be a diesel engine, oil-fired boiler, incinerator, etc. 
         [0025]    The combustion unit  3  is arranged in an engine room generally indicated by reference sign  11  (discussed more in detail below). 
         [0026]    The machinery arrangement  2  comprises an exhaust duct assembly  4  for receiving an exhaust gas flow from the combustion unit  3  and for leading the exhaust gas flow to the atmosphere. The exhaust gases are treated by an exhaust gas cleaning system  5  arranged in the flow direction (main intended flow direction of the exhaust gas is indicated by a block arrow) of the exhaust gas. The exhaust gas cleaning system  5  includes a cleaning device for sulphuric oxides (SO x ), e.g. a scrubber unit  51 , which is provided with an exhaust gas pipe  52 . 
         [0027]    The exhaust gas duct assembly  4  includes a first section  41  connected to and thus leading to the exhaust gas cleaning system  5 , i.e. the scrubber unit  51 , and a second section  42  connected to and extending form the combustion unit  3 . The first section  41  is located downstream of the second section  42  in the flow direction of the exhaust gas, whereby the first section and the second section connect the combustion unit  3  to the scrubber unit  51 . 
         [0028]    Further, in the present invention, the engine room  11  comprises an engine room casing  111  with an engine room ventilation outlet  112  provided with a fire damper  113  that can be closed, e.g. in case of fire. The engine room  11  is in flow connection with the an enclosure  114 , or more particularly, the ventilation outlet  112  is in flow connection with, in other words leads to the enclosure  114 , which encloses the exhaust pipe  52  of the scrubber unit  51 . The enclosure  114  is advantageously located outside the fire damper  113  of the engine room casing  111 . The enclosure is open-ended and is provided by a jacket portion  115  surrounding at least part of the exhaust gas pipe  52  of the scrubber unit  51 . 
         [0029]    The scrubber unit  51  is located outside the enclosure  114 . The second section  42  of the exhaust gas duct assembly  4  is located within the engine room casing  111  and the first section  41  of the exhaust gas duct assembly  4  is at least partly located outside the engine room casing  111 . 
         [0030]    In the following the operation of the machinery arrangement according to the invention shall shortly be described. 
         [0031]    As the combustion unit  3  is driven, exhaust gas is generated and is led into the exhaust gas duct assembly  4 , firstly through the second section  42  connected to the combustion unit  3  and then forward through the first section  41  connected to the scrubber unit  51 . 
         [0032]    In operation, a heated air flow is provided by ventilation air from the engine room  11  through the ventilation outlet  112  into the enclosure  114 . The heated air flow flowing through the enclosure  114  towards the jacket portion  115  heats the exhaust gas pipe  52  of the scrubber unit  51 . The heated air flow is thus led into contact with and is mixed with the wet exhaust gas from the exhaust gas pipe  52  which is discharged into the atmosphere. Consequently, the resulting mixture has reduced relative humidity, reduced risk for condensed droplets, reduced opacity and increased buoyancy. The heated air flow also prevents cooling inside the exhaust gas pipe  52 , thus avoiding undesired condensation. 
         [0033]    Depending on the desired effect of the heated air flow from the engine room  11 , the jacket portion  115  surrounds at least part of the exhaust gas pipe  52 , i.e. advantageously ends somewhat below, at the same level, or somewhat above the outlet of the exhaust gas pipe  52 . The jacket portion  115  may also be constructed as a lateral support of the exhaust gas pipe  52 . 
         [0034]    In  FIG. 2  a machinery arrangement for a marine vessel  1  is generally indicated by reference sign  2 . The machinery arrangement comprises a combustion unit  3 . The combustion unit may be a diesel engine, oil-fired boiler, incinerator, etc. 
         [0035]    The combustion unit  3  is arranged in an engine room generally indicated by reference sign  11  (discussed more in detail below). 
         [0036]    The machinery arrangement  2  comprises an exhaust duct assembly  4  for receiving an exhaust gas flow from the combustion unit  3  and for leading the exhaust gas flow to the atmosphere. The exhaust gases are treated by an exhaust gas cleaning system  5  arranged in the flow direction (main intended flow direction of the exhaust gas is indicated by a block arrow) of the exhaust gas. The exhaust gas cleaning system  5  includes a cleaning device for sulphuric oxides (SO x ), e.g. a scrubber unit  51 , which is provided with an exhaust gas pipe  52 . 
         [0037]    The exhaust gas duct assembly  4  includes a first section  41  connected to and thus leading to the exhaust gas cleaning system  5 , i.e. the scrubber unit  51 , and a second section  42  connected to and extending form the combustion unit  3 . The first section  41  is located downstream of the second section  42  in the flow direction of the exhaust gas, whereby the first section and the second section connect the combustion unit  3  to the scrubber unit  51 . 
         [0038]    Further, in the present invention, the engine room  11  comprises an engine room casing  111  with an engine room ventilation outlet  112  provided with a fire damper  113  that can be closed, e.g. in case of fire. The engine room  11  is in flow connection with the an enclosure  114 , or more particularly, the engine room ventilation outlet  112  is in flow connection with, in other words leads to an enclosure  114 , which encloses the first section  41  of the exhaust gas duct assembly  4  as well as the exhaust pipe  52  of the scrubber unit  51 . The enclosure is open-ended and is provided by a jacket portion  115  surrounding at least part of the exhaust gas pipe  52  of the scrubber unit  51 . The scrubber unit  51  is located outside the enclosure  114 . The second section  42  of the exhaust gas assembly  4  is located within the engine room casing  111 . 
         [0039]    The enclosure  114  has a cross sectional area larger than the engine room ventilation outlet  112  leading to the enclosure  114  from engine room  11 . This minimizes flow resistance and impact on engine room ventilation. The enclosure  114  is advantageously located outside the fire damper  113  of the engine room casing  111 . 
         [0040]    In the following the operation of the machinery arrangement according to the invention shall shortly be described. 
         [0041]    As the combustion unit  3  is driven, exhaust gas is generated and is led into the exhaust gas duct assembly  4 , firstly through the second section  42  connected to the combustion unit  3  and then forward through the first section  41  connected to the scrubber unit  51 . 
         [0042]    In operation, the advantageously un-insulated first section  41  of the exhaust gas duct assembly  4  is ventilated by and provides additional heat to a heated air flow provided by ventilation air from the engine room  11  through the ventilation outlet  112 . The heated air flow flowing towards the jacket portion  115  also heats the exhaust gas pipe  52  of the scrubber unit  51 . The heated air flow is further led into contact with and is thus mixed with the wet exhaust gas from the exhaust gas pipe  52  which is discharged into the atmosphere. Consequently, the resulting mixture has reduced relative humidity, reduced risk for condensed droplets, reduced opacity and increased buoyancy. The heated air flow also prevents cooling inside the exhaust gas pipe  52 , thus avoiding undesired condensation. 
         [0043]    Depending on the desired effect of the heated air flow from the engine room  11 , the jacket portion  115  surrounds at least part of the exhaust gas pipe  52 , i.e. advantageously ends somewhat below, at the same level, or somewhat above the outlet of the exhaust gas pipe  52 . The jacket portion  115  may also be constructed as a lateral support of the exhaust gas pipe  52 . 
         [0044]    In  FIG. 3  a machinery arrangement for a marine vessel  1  is generally indicated by reference sign  2 . The machinery arrangement comprises a combustion unit  3 , in this embodiment a number of combustion units. The combustion units may include diesel engines, oil-fired boilers, incinerators, etc. 
         [0045]    The combustion units are arranged in an engine room generally indicated by reference sign  11  (discussed more in detail below). 
         [0046]    The machinery arrangement  2  comprises an exhaust duct assembly  4  for receiving an exhaust gas flow from the combustion units and for leading the exhaust gas flow to the atmosphere. The exhaust gases are treated by an exhaust gas cleaning system  5  arranged in the flow direction (main intended flow direction of the exhaust gas is indicated by a block arrow) of the exhaust gas. The exhaust gas cleaning system includes a cleaning device for sulphuric oxides (SO x ), e.g. a scrubber unit  51 , which is provided with an exhaust gas pipe  52 . 
         [0047]    The exhaust gas duct assembly  4  includes a first section  41  comprising a first branch pipe  411  connected to and thus leading to the exhaust gas cleaning system  5 , i.e. to the scrubber unit  51 , and a second section  42  comprising a second branch pipe  421  connected to and extending from the combustion unit  3 , also leading to the atmosphere. The first section, comprising the first branch pipe, and the second section, comprising the second branch pipe, connect the combustion unit  3  to the scrubber unit  51 , whereby the second section  42 , comprising the second branch pipe  421 , also forms a by-pass leading to the atmosphere through a funnel section  116  of an engine room casing  111  of the engine room  11  (discussed more in detail below). In this embodiment, which includes a number of combustion units, each combustion unit  3  is connected to a first branch pipe  411  and a second branch pipe  421  respectively. A fan unit  6  is provided in flow connection with each first branch pipe  411 . In this embodiment the fan unit  6  is arranged between the first branch pipes  411  and the exhaust gas cleaning system  5 , in a common collecting branch  43 , which connects the various first branch pipes and the scrubber unit  51 . 
         [0048]    Each of the first branch pipes  411  is provided with a first valve means  7 , which advantageously is arranged to function as a shut-off valve and flow control valve. 
         [0049]    In the following the operation of the machinery arrangement according to the invention shall shortly be described. 
         [0050]    In operation, as one or more combustion units  3  are driven, exhaust gas is generated and is led into the exhaust gas duct assembly  4 . The purpose is to draw substantially all of the exhaust gas by the fan unit  6  through the first branch pipes  411  and to push the exhaust gas via the common collecting branch  43  into the scrubber unit  51  in order to clean the same. The second branch pipes  421  basically function as stand-by exhaust pipe for the respective combustion units  3 , preferably so that a minimum of fresh air is provided to the scrubber unit  51  in order to prevent escape of un-cleaned exhaust gases. 
         [0051]    The fan unit  6  is advantageously equipped with a frequency converter and/or guide vanes for flexible control of its appropriate function in relation to the drive mode of the respective combustion units  3 . Exhaust gas flow-back from the common collecting branch  43  to other combustion units is controlled by the first valve means  7  provided in each first branch pipe  411 . The first valve means  7  are arranged to balance the flow in the active first branch pipes  411 , i.e. the first branch pipes connected to a driven combustion unit  3 . On the other hand, the first valve means  7  are also arranged to shut-off the flow in the inactive first branch pipes  411 , i.e. the first branch pipes connected to a non-operational combustion unit  3 . 
         [0052]    This arrangement also ensures that the exhaust gas back pressure, with regard to each combustion unit  3 , remains about the same or slightly lower than in a conventional machinery installation. 
         [0053]    In case e.g. of malfunction of the scrubber unit  51 , the first valve means  7  can be used to prevent the flow of hot exhaust gases to the scrubber unit  51 . In a situation like this, the exhaust gases can be led directly through the second branch pipes  421  to the atmosphere, whereby exhaust gas emission control may be achieved by using fuel with lower sulphur content. 
         [0054]    Each of the second branch pipes  421  are advantageously provided with a second valve means (not shown), a temperature sensor (not shown) and a flow sensor (not shown). 
         [0055]    The temperature sensors in the second branch pipes  421  can thus be used to control that there is no inadvertent flow of hot, un-cleaned exhaust gases into the atmosphere. The flow sensors in the second branch pipes  421  can correspondingly be used to control the flow of fresh air, i.e. said minimum of fresh air discussed above, provided to the scrubber unit  51 . The second valve means may regulate the flow as such in the second branch pipes. 
         [0056]    For optimizing the overall operation of the exhaust gas cleaning system the fan unit  6 , the first valve means  7 , and also as appropriate the second valve means, the temperature sensor and the flow sensor, are connected to a central control unit (not shown) for monitoring the exhaust gas flow. 
         [0057]    In conclusion, such a central control unit provides the means to ensure that a minimum of fresh air continuously can be sucked backwards from the active second branch pipes, that the air flow to and therefore the exhaust gas load (combined air and exhaust gas flow) to the scrubber unit is minimised, and that power consumption can be optimised. Furthermore, by minimizing the flow of cold air from the second branch pipes, any risk for condensation of sulphuric acid at the point of mixing of said cold air and hot exhaust gases can be avoided. The deployment of the central control unit remains within the knowledge of a person skilled in the art, and is therefore not discussed in more detail in this connection. 
         [0058]    Further, in the present invention, the engine room  11  comprises an engine room casing  111  with an engine room ventilation outlet  112  provided with a fire damper  113  that can be closed, e.g. in case of fire. The engine room  11  is in flow connection with the an enclosure  114 , or more particularly, the engine room ventilation outlet  112  is in flow connection, in other words leads to an enclosure  114 , which encloses exhaust pipe  52  of the scrubber unit  51 . The enclosure  114  is advantageously located outside the fire damper of the engine room casing  111 . The enclosure is open-ended and is provided by a jacket portion  115  surrounding at least part of the exhaust gas pipe  52  of the scrubber unit  51 . 
         [0059]    The scrubber unit  51  is located outside the enclosure  114 . The second branch pipes  421  are located in the funnel portion  116  of the engine room casing  111  and the first branch pipes  411  are at least partly located outside the engine room casing  111 . 
         [0060]    In operation, a heated air flow is provided by ventilation air from the engine room  11  through the ventilation outlet  112  into the enclosure  114 . The heated air flow flowing through the enclosure  114  towards the jacket portion  115  heats the exhaust gas pipe  52  of the scrubber unit  51 . The heated air flow is thus led into contact with and is mixed with the wet exhaust gas from the exhaust gas pipe  52  which is discharged into the atmosphere. Consequently, the resulting mixture has reduced relative humidity, reduced risk for condensed droplets, reduced opacity and increased buoyancy. The heated air flow also prevents cooling inside the exhaust gas pipe  52 , thus avoiding undesired condensation. 
         [0061]    Depending on the desired effect of the heated air flow from the engine room  11 , the jacket portion  115  surrounds at least part of the exhaust gas pipe  52 , i.e. advantageously ends somewhat below, at the same level, or somewhat above the outlet of the exhaust gas pipe  52 . The jacket portion  115  may also be constructed as a lateral support of the exhaust gas pipe 
         [0062]    It is clear that the number and type of combustion units can vary, whereby they can be connected to the same exhaust gas cleaning system. The combustion units may also be provided with an exhaust gas boiler. In view of the location of the combustion units onboard a marine vessel, it is also possible to have more than one exhaust gas cleaning system provided with a corresponding enclosure. 
         [0063]    The fan unit may alternatively be arranged after the scrubber unit  51 , in the exhaust gas pipe  52  of the scrubber unit. Another possibility is to arrange a fan unit in each of the first branch pipes  411 , upstream of the valve means  7  with regard to the flow direction (block arrow) of the exhaust gas. 
         [0064]    In  FIG. 4  a machinery arrangement for a marine vessel  1  is generally indicated by reference sign  2 . The machinery arrangement comprises a combustion unit  3 , in this embodiment a number of combustion units. The combustion units may include diesel engines, oil-fired boilers, incinerators, etc. 
         [0065]    The combustion units are arranged in an engine room generally indicated by reference sign  11  (discussed more in detail below). 
         [0066]    The machinery arrangement  2  comprises an exhaust duct assembly  4  for receiving an exhaust gas flow from the combustion units and for leading the exhaust gas flow to the atmosphere. The exhaust gases are treated by an exhaust gas cleaning system  5  arranged in the flow direction (main intended flow direction of the exhaust gas is indicated by a block arrow) of the exhaust gas. The exhaust gas cleaning system includes a cleaning device for sulphuric oxides (SO x ), e.g. a scrubber unit  51 , which is provided with an exhaust gas pipe  52 . 
         [0067]    The exhaust gas duct assembly  4  includes a first section  41  comprising a first branch pipe  411  connected to and thus leading to the exhaust gas cleaning system  5 , i.e. to the scrubber unit  51 , and a second section  42  comprising a second branch pipe  421  connected to and extending from the combustion unit  3 , also leading to the atmosphere. The first section, comprising the first branch pipe, and the second section, comprising the second branch pipe, connect the combustion unit  3  to the scrubber unit  51 , whereby the second section  42 , comprising the second branch pipe  421 , also forms a by-pass leading to the atmosphere through a funnel section  116  of an engine room casing  111  of the engine room (discussed more in detail below). In this embodiment, which includes a number of combustion units, each combustion unit  3  is connected to a first branch pipe  411  and a second branch pipe  421  respectively. A fan unit  6  is provided in flow connection with each first branch pipe  411 . In this embodiment the fan unit  6  is arranged between the first branch pipes  411  and the exhaust gas cleaning system  5 , in a common collecting branch  43 , which connects the various first branch pipes and the scrubber unit  51 . 
         [0068]    Each of the first branch pipes  411  is provided with a first valve means  7 , which advantageously is arranged to function as a shut-off valve and flow control valve. 
         [0069]    In the following the operation of the machinery arrangement according to the invention shall shortly be described. 
         [0070]    In operation, as one or more combustion units  3  are driven, exhaust gas is generated and is led into the exhaust gas duct assembly  4 . The purpose is to draw substantially all of the exhaust gas by the fan unit  6  through the first branch pipes  411  and to push the exhaust gas via the common collecting branch  43  into the scrubber unit  51  in order to clean the same. The second branch pipes  421  basically function as stand-by exhaust pipe for the respective combustion units  3 , preferably so that a minimum of fresh air is provided to the scrubber unit  51  in order to prevent escape of un-cleaned exhaust gases. 
         [0071]    The fan unit  6  is advantageously equipped with a frequency converter and/or guide vanes for flexible control of its appropriate function in relation to the drive mode of the respective combustion units  3 . Exhaust gas flow-back from the common collecting branch  43  to other combustion units is controlled by the first valve means  7  provided in each first branch pipe  411 . The first valve means  7  are arranged to balance the flow in the active first branch pipes  411 , i.e. the first branch pipes connected to a driven combustion unit  3 . On the other hand, the first valve means  7  are also arranged to shut-off the flow in the inactive first branch pipes  411 , i.e. the first branch pipes connected to a non-operational combustion unit  3 . 
         [0072]    This arrangement also ensures that the exhaust gas back pressure, with regard to each combustion unit  3 , remains about the same or slightly lower than in a conventional machinery installation. 
         [0073]    In case e.g. of malfunction of the scrubber unit  51 , the first valve means  7  can be used to prevent the flow of hot exhaust gases to the scrubber unit  51 . In a situation like this, the exhaust gases can be led directly through the second branch pipes  421  to the atmosphere, whereby exhaust gas emission control may be achieved by using fuel with lower sulphur content. 
         [0074]    Each of the second branch pipes  421  are advantageously provided with a second valve means (not shown), a temperature sensor (not shown) and a flow sensor (not shown). 
         [0075]    The temperature sensors in the second branch pipes  421  can thus be used to control that there is no inadvertent flow of hot, un-cleaned exhaust gases into the atmosphere. The flow sensors in the second branch pipes  421  can correspondingly be used to control the flow of fresh air, i.e. said minimum of fresh air discussed above, provided to the scrubber unit  51 . The second valve means may regulate the flow as such in the second branch pipes. 
         [0076]    For optimizing the overall operation of the exhaust gas cleaning system the fan unit  6 , the first valve means  7 , and also as appropriate the second valve means, the temperature sensor and the flow sensor, are connected to a central control unit (not shown) for monitoring the exhaust gas flow. 
         [0077]    In conclusion, such a central control unit provides the means to ensure that a minimum of fresh air continuously can be sucked backwards from the active second branch pipes, that the air flow to and therefore the exhaust gas load (combined air and exhaust gas flow) to the scrubber unit is minimised, and that power consumption can be optimised. Furthermore, by minimizing the flow of cold air from the second branch pipes, any risk for condensation of sulphuric acid at the point of mixing of said cold air and hot exhaust gases can be avoided. The deployment of the central control unit remains within the knowledge of a person skilled in the art, and is therefore not discussed in more detail in this connection. 
         [0078]    Further, in the present invention, the engine room  11  comprises an engine room casing  111  with an engine room ventilation outlet  112  provided with a fire damper  113  that can be closed, e.g. in case of fire. The engine room  11  is in flow connection with the an enclosure  114 , or more particularly, the engine room ventilation outlet  112  is in flow connection, in other words leads to an enclosure  114 , which surrounds the first branch pipes  411  as well as the exhaust pipe  52  of the scrubber unit  51 . The enclosure is open-ended and is provided by a jacket portion  115  surrounding at least part of the exhaust gas pipe  52  of the scrubber unit  51 . In this embodiment, the fan unit  6  is also arranged within the enclosure  114 . The scrubber unit  51  is located outside the enclosure  114 . The second branch pipes  421  are located in the funnel portion  116  of the engine room casing  111 . 
         [0079]    The enclosure  114  has a cross sectional area larger than the engine room ventilation outlet  112  leading to the enclosure  114  from engine room  11 . This minimizes flow resistance and impact on engine room ventilation. The enclosure  114  is advantageously located outside the fire damper of the engine room casing  111 . 
         [0080]    In operation, the advantageously un-insulated first branch pipes  411  are ventilated by and provide additional heat to a heated air flow provided by ventilation air from the engine room  11  through the ventilation outlet  112 . The heated air flow flowing towards the jacket portion  115  also heats the exhaust gas pipe  52  of the scrubber unit  51 . The heated air flow is further led into contact with and is mixed with the wet exhaust gas from the exhaust gas pipe  52  which is discharged into the atmosphere. Consequently, the resulting mixture has reduced relative humidity, reduced risk for condensed droplets, reduced opacity and increased buoyancy. The heated air flow also prevents cooling inside the exhaust gas pipe  52 , thus avoiding undesired condensation. 
         [0081]    Depending on the desired effect of the heated air flow from the engine room  11 , the jacket portion  115  surrounds at least part of the exhaust gas pipe  52 , i.e. advantageously ends somewhat below, at the same level, or somewhat above the outlet of the exhaust gas pipe  52 . The jacket portion  115  may also be constructed as a lateral support of the exhaust gas pipe 
         [0082]    It is clear that the number and type of combustion units can vary, whereby they can be connected to the same exhaust gas cleaning system. The combustion units may also be provided with an exhaust gas boiler. In view of the location of the combustion units onboard a marine vessel, it is also possible to have more than one exhaust gas cleaning system provided with a corresponding enclosure. 
         [0083]    The fan unit may alternatively be arranged after the scrubber unit  51 , in the exhaust gas pipe  52  of the scrubber unit. Another possibility is to arrange a fan unit in each of the first branch pipes  411 , upstream of the valve means  7  with regard to the flow direction (block arrow) of the exhaust gas. 
         [0084]    In view of all the embodiments discussed above, it may be noted that an exhaust gas cleaning system can use a scrubber unit that is run with sea water, fresh water, or a combination of both. The scrubber unit material can be corrosion resistant metal or glass-fibre reinforced plastic (GRP), whereby the latter has advantages in view of its lightness, as it is located high up in the marine vessel. 
         [0085]    The description and thereto related drawings are only intended to clarify the basic idea of the present invention. The invention may vary in detail within the scope of the ensuing claims.