Patent Publication Number: US-2022235773-A1

Title: Compressed air station

Description:
The invention relates to a compressed air station comprising at least two compressed air components that yield waste heat, wherein each compressed air component is designed either as a compressor, in particular as a screw compressor, or as a refrigeration dryer, and at least one exhaust air duct for discharging waste heat from a room. 
     A compressed air refrigeration heat exchanger is provided within the refrigeration dryer, in which the compressed air is cooled by way of a refrigerant conducted in a refrigerant circuit, wherein the refrigerant circuit comprises a refrigerant compressor, a condenser, an expansion valve and the compressed air refrigeration heat exchanger. The waste heat produced by the compressor(s) is traditionally already discharged via one or more exhaust air ducts from a plant room in which the compressed air components are installed. In the case of refrigeration dryers, which usually produce a small amount of waste heat compared to the compressors present within a compressed air station, the waste heat has traditionally been either introduced into the plant room or conducted away via extraction hoods arranged above the refrigeration dryers. However, particularly when discharging the waste heat from the refrigeration dryer(s) into the plant room or even when this is conducted away only incompletely via an extraction hood arranged above the refrigeration dryers, the problem arises that the ambient air in the plant room heats up and the efficiency both of the compressors, in particular the screw compressors, and of the refrigeration dryers decreases. 
     In contrast, the object of the present invention is to propose a compressed air station and a corresponding method in which an improved discharging of waste heat from a plant room is made possible, including the waste heat from a refrigeration dryer present in the compressed air station. 
     This object is achieved in terms of the device by a compressed air station having the features of claim  1  and in terms of the method by a method for actuating the fan motor of a fan of a refrigeration dryer according to the features of claim  12 . Advantageous further developments are specified in the dependent claims. 
     In terms of the device, the compressed air station is characterized in that it further comprises a dryer exhaust air duct, which is provided for discharging a cooling air flow that is conducted through the refrigeration dryer, and which connects a cooling air outlet of the refrigeration dryer to a refrigeration dryer connection on the exhaust air duct, wherein the refrigeration dryer has a fan with a speed-adjustable fan motor, and the fan is designed to convey the cooling air flow even against a backpressure currently prevailing in the exhaust air duct, wherein the refrigeration dryer has a flow sensor for detecting a respective current value for the cooling air volume flow V act , and wherein the refrigeration dryer has a controller or interacts with a controller, which is configured and designed to record and process the data from the flow sensor and to actuate the fan motor of the fan in such a way that, regardless of the current backpressure in the exhaust air duct, the respective current cooling air volume flow V act  follows a setpoint for the cooling air volume flow V soll . 
     In terms of the method, it is proposed that the fan of a refrigeration dryer within a compressed air station is actuated in order to compensate for even a fluctuating backpressure within an exhaust air duct, which is also fed by at least one further compressed air component, the method being designed as follows: 
     A method for actuating the fan motor of a fan of a refrigeration dryer within a compressed air station, wherein the compressed air station comprises at least two compressed air components that yield waste heat, wherein each compressed air component is designed either as a compressor, in particular as a screw compressor, or as a refrigeration dryer, and at least one exhaust air duct for discharging waste heat from a room, wherein at least one of the compressed air components, namely a refrigeration dryer, is connected to the exhaust air duct, and wherein a further compressed air component is connected to the same exhaust air duct, 
     wherein a compressed air refrigeration heat exchanger is provided within the refrigeration dryer, in which the compressed air is cooled by way of a refrigerant conducted in a refrigerant circuit, wherein the refrigerant circuit comprises a refrigerant compressor, a condenser, an expansion valve and the compressed air refrigeration heat exchanger, 
     wherein the compressed air station further comprises a dryer exhaust air duct, which is provided for discharging a cooling air flow that is conducted through the refrigeration dryer, and which connects a cooling air outlet of the refrigeration dryer to a refrigeration dryer connection on the exhaust air duct, 
     wherein the refrigeration dryer has a fan with a speed-adjustable fan motor, and the fan is designed to convey the cooling air flow even against a backpressure currently prevailing in the exhaust air duct, wherein the method comprises the following steps:
         specifying a setpoint V soll  for the cooling air volume flow,   detecting a respective current value for the cooling air volume flow V act , and   actuating the fan motor ( 21 ) of the fan ( 20 ) in such a way that the respective current cooling air volume flow V act  follows the respectively specified setpoint for the cooling air volume flow V soll .       

     One core consideration of the present invention is that of controlling the fan motor of the fan in the refrigeration dryer in such a way that a residual pressure of the exhaust air flow from the refrigeration dryer is adjusted to the pressure level of an exhaust air duct system used jointly with at least one screw compressor. The speed of the fan motor of the fan is therefore varied such that the amount of exhaust air from the refrigeration dryer per unit of time, i.e. the cooling air volume flow of the refrigeration dryer, remains unchanged regardless of the current backpressure in the exhaust air duct. 
     The condensation pressure and the cooling capacity of the refrigeration dryer can thus be kept at the same level regardless of the current backpressure, wherein it remains possible for the condensation pressure or the cooling capacity to vary according to other criteria, but the current backpressure in the exhaust air duct or in the exhaust air duct system has no influence on the condensation pressure or the cooling capacity of the refrigeration dryer. A current residual pressure will be understood here to mean the current pressure reserve of the fan for overcoming additional flow resistances, such as that of an exhaust air duct for example. The maximum achievable residual pressure of the fan used in the refrigeration dryer is to be regarded as sufficient if the backpressure, as results from the interaction with other components, such as a connected screw compressor, and the additional flow resistances, as occur inter alia in an exhaust air duct of customary length, can be overcome. 
     Although, with regard to the refrigerant circuit, mention is made of a condenser on the one hand and an expansion valve on the other hand, it is clarified that the refrigerant circuit need not necessarily be operated with a phase transition of gas-liquid liquid-gas, but rather in the case of certain refrigerants is also operated via transcritical processes, such as for example in the case of CO 2  (R-744). There is then no liquefaction, but rather heat is output on the high-pressure side of the refrigerant circuit and the gas phase is retained. The gas cooler outlet temperature is then to be seen as a value equivalent to the condensation temperature. 
     A speed-adjustable fan motor in the sense of the invention will be understood to be a fan motor that can be adjusted with regard to its speed, in particular by frequency reversal or by phase control. 
     In the present application, the cooling air flow conducted through the refrigeration dryer is referred to as the cooling air flow per se. Where the cooling air volume flow is mentioned, this refers to the quantitative value as a volume flow, for example expressed in the unit m 3 /s, i.e. the volume of cooling air transported by the cooling air flow per period of time. 
     In a preferred embodiment of the present invention, the flow sensor is designed as a differential pressure sensor. 
     In a preferred embodiment of the compressed air station proposed here, the fan of the refrigeration dryer is thus also designed as a radial fan. Admittedly, traditionally used axial fans have a lower power consumption at the nominal point and thus initially appear more favourable from an energy point of view. However, this applies particularly in connection with traditionally used refrigeration dryers, which blow out their waste heat directly into a plant room in which they are installed. Radial fans appear more suitable for direct connection to an exhaust air duct or to an exhaust air duct system since they are able to ensure a higher residual pressure. 
     In a further preferred embodiment, it is provided that a screw compressor and the refrigeration dryer are connected to one another via a compressed air line, and the compressed air line is designed to transfer the compressed air output by the screw compressor to the refrigeration dryer for drying purposes, wherein a cooling air outlet of the screw compressor is connected via a compressor connection to the exhaust air duct, to which the refrigeration dryer is also connected. In this possible embodiment, the screw compressor and the refrigeration dryer convey their waste heat via corresponding cooling air outlets to the same exhaust air duct. At the same time, the compressed air output by the screw compressor is transferred to the refrigeration dryer for drying purposes. However, it is also conceivable that, although the compressed air generated by the screw compressor is transferred to the refrigeration dryer, the screw compressor and the refrigeration dryer output their waste heat to different exhaust air ducts. 
     In the compressed air station proposed here, one or more oil-injected screw compressors and/or one or more oil-free compressors may be used. 
     In a preferred embodiment of the compressed air station proposed here, the controller interacts with an ambient air sensor, preferably a temperature sensor, for detecting a value representative of the state of the supply air, in particular a value representative of the supply air temperature, and transmitting it to the controller. 
     In a possible embodiment of the present invention, two or more screw compressors and/or two or more refrigeration dryers are connected to a common exhaust air duct. Particularly large pressure fluctuations or pressure differences may occur in a common exhaust air duct used in this way, and therefore an adjustment of the fan motor of the fan of the refrigeration dryer, as proposed in the context of the present invention, is particularly important. 
     In a preferred embodiment, the setpoint for the cooling air volume flow V soll  results from a design value for the cooling air volume flow V nenn , which is specific to the given refrigerant circuit with the given refrigerant compressor, according to the following formula: 
         V   soll   =V   nenn   *F   Hub/KMK   *F   T , 
     where the factors F Hub/KMK  and F T  are correction factors, each of which can alternatively be set to 1 or to a value between 0 and 1. 
     In a preferred embodiment, an exhaust air damper is arranged in the dryer exhaust air duct, which damper is designed and configured to close the dryer exhaust air duct when the refrigerant compressor is idle. This prevents the exhaust air from flowing back into a plant room in which the refrigeration dryer is installed. 
     In this case, in a preferred embodiment, the exhaust air damper may be designed as a gravity-operated exhaust air damper, which opens whenever the fan is conveying the exhaust air through the dryer exhaust air duct and closes whenever the fan is idle. 
     In an alternatively possible embodiment, the exhaust air damper may interact with a drive motor, wherein the drive motor is actuated by the controller in such a way that the exhaust air damper is opened or closed depending on the operating state of the fan. In particular, the exhaust air damper is or will be opened when the fan is operating. The exhaust air damper is or will be closed in particular when the fan is idle. 
     In a specific further development of the proposed method, the respective current value for the cooling air volume flow V act  is detected by way of a differential pressure measurement. A differential pressure measurement is a relatively simple and at the same time reliable measurement method for determining with sufficient certainty a current value for the cooling air volume flow V act . 
     In a preferred embodiment, the preferred differential pressure measurement is performed immediately before the cooling air flow flows into the fan. In other words, the pressure is detected in the region immediately before the air flow flows into the fan and is compared with a reference pressure. Such a reference pressure may be provided for example between the condenser and the fan. As already mentioned above, this is a particularly favourable position within the course of the cooling air flow for performing a differential pressure measurement. 
     In a possible embodiment, the setpoint for the cooling air volume flow may be adapted as a function of the load state of the refrigerant compressor and/or as a function of the ambient temperature. If the cooling air volume flow is adapted as a function of the load state of the refrigerant compressor, in the case of just one refrigerant compressor for example the usage state of one, possibly frequency-controlled, refrigerant compressor is to be taken into account. If a plurality of refrigerant circuits are operated in parallel with one another in a plant, the load state may also include for example the extent to which each refrigerant compressor is loaded. 
     In a preferred embodiment of the present method, the setpoint for the cooling air volume flow V soll  results from a design value for the cooling air volume flow V nenn , which is specified for the refrigerant circuit and the refrigerant compressors installed therein. 
     In a further preferred exemplary embodiment, the setpoint for the cooling air volume flow V soll  results from a design value for the cooling air volume flow V nenn , which is specified for the refrigerant circuit and the refrigerant compressors installed therein, taking into account one or more correction factors F. 
     In a specifically preferred embodiment of the present method, the setpoint for the cooling air volume flow V soll  results from the design-based nominal value for the cooling air volume flow V nenn , according to the following formula: V soll =V nenn *F Hub/KMK *F T , where F Hub/KMK  is a correction factor for taking into account the respective current stroke volume in the refrigerant circuit, and F T  is a correction factor for taking into account temperature fluctuations in the supply air of the cooling air flow, where: 0≤F Hub/KMK ≤1 and 0≤F T ≤1. Therefore, V soll  results directly from V nenn , with one or more correction factors being used in certain operating situations and/or under certain operating conditions, which correction factors ensure that V soll  is reduced in comparison to V nenn . V soll  can thus be a certain percentage between 0 and 100% of V nenn . 
     In a further preferred embodiment, a correction factor F T  takes into account a current value of the supply air temperature, wherein the value F T  is set to 1 in the case of supply air temperatures above a limit temperature T 0,amb , and 0≤F T &lt;1 applies only in the case of cooling air inlet temperature values T&lt;T 0,amb . In a very specifically preferred embodiment, the correction factor F T  for the temperature T of the supply air (cooling air inlet temperature) in ranges below the limit temperature T 0,amb  can be calculated as follows: 
     
       
         
           
             
               
                 T 
                 
                   0 
                   , 
                   amb 
                 
               
               = 
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   T 
                 
                 
                   
                     T 
                     
                       0 
                       , 
                       amb 
                     
                   
                   - 
                   
                     T 
                     
                       amb 
                       , 
                       act 
                     
                   
                 
               
             
             , 
           
         
       
     
     where Δ T  denotes a dryer-specific supplement in ° C., T 0,amb  denotes a fixed limit temperature, and T amb,act  denotes the current supply air temperature. 
     In a further preferred embodiment, the correction factor F Hub/KMK  can be formed from a ratio of the current stroke volume and of the maximum stroke volume in the refrigerant circuit, according to the following formula 
     
       
         
           
             
               F 
               
                 Hub 
                 / 
                 KMK 
               
             
             = 
             
               
                 current 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 stroke 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 volume 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 KMK 
               
               
                 maximum 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 stroke 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 volume 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 KMK 
               
             
           
         
       
     
     By way of the correction factor F HUb/KMK , therefore, it is possible to take into account operating situations in which the current stroke volume KMK is reduced in comparison to a maximum stroke volume KMK for which the refrigerant circuit is designed. This may be achieved for example in that, if using multiple refrigerant compressors, one or more fluid compressors are switched off or, if using one refrigerant compressor with an adjustable output, the output of the refrigerant compressor is currently reduced. 
     In a specifically preferred embodiment of the method according to the invention, a PID controller, a PI controller, a deadband controller or a three-point controller may be used to actuate the fan motor of the fan in order to bring the current cooling air flow V act  towards the setpoint for the cooling air volume flow V soll , wherein a control deviation e results from V soll −V act , and wherein V soll  denotes the setpoint for the cooling air volume flow and V act  denotes the current cooling air volume flow. 
    
    
     
       The invention will be explained in greater detail below, including with regard to further features and advantages, based on the description of exemplary embodiments and with reference to the following drawings, in which: 
         FIG. 1  shows an exemplary embodiment of a compressed air station according to the invention, in a schematic illustration; 
         FIG. 2  shows an embodiment of a compressed air station according to the invention, modified in comparison to the embodiment shown in  FIG. 1 , in a schematic illustration; 
         FIG. 3  shows an embodiment of a compressed air station, once again modified in comparison to  FIGS. 1 and 2 , in a schematic illustration; 
         FIG. 4  shows an embodiment of a compressed air station, once again modified in comparison to  FIGS. 1 to 3 , in a schematic illustration; 
         FIG. 5  shows an embodiment of a compressed air station, once again modified in comparison to  FIGS. 1 to 4 , in a schematic illustration; 
         FIG. 6  shows a schematic illustration to explain the mode of operation of a refrigeration dryer according to the present invention; 
         FIG. 7  shows a flowchart to explain an embodiment of the method according to the invention; and 
         FIG. 8  shows an embodiment of a possible curve of the correction factor F T  as a function of the supply air of the cooling air flow T amb . 
     
    
    
       FIG. 1  shows an exemplary embodiment of a compressed air station according to the invention, in which, as compressed air components, a screw compressor  11  and a refrigeration dryer  12  are connected to a common exhaust air duct  13 . The screw compressor  11  supplies compressed air via a compressed air line  14  to the refrigeration dryer  12 . Via a further compressed air line  43 , the compressed air dried in the refrigeration dryer  12  is supplied to a consumer  44 . Instead of one consumer, a plurality, in particular a large number, of consumers may also be supplied with compressed air through the further compressed air line  43  via a compressed air network. 
     The screw compressor  11  and the refrigeration dryer  12  produce waste heat, which according to the invention is discharged via the common exhaust air duct  13  by way of corresponding cooling air flows. A screw compressor exhaust air duct  45  forms the first section of a common exhaust air duct  13 , to which the refrigeration dryer  12  is also connected further downstream via a dryer exhaust air duct  15 . Specifically, the screw compressor  11  has a cooling air outlet  18 , to which the screw compressor exhaust air duct  45  is directly connected. The refrigeration dryer  12  likewise has a cooling air outlet  19 , to which the dryer exhaust air duct  15  is directly connected, namely preferably in such a way that only the cooling air or the waste heat of the refrigeration dryer  12  is discharged and no mixing takes place with ambient air not conducted via the refrigeration dryer  12 . 
     The dryer exhaust air duct  15  is connected to the common exhaust air duct  13  at the aforementioned refrigeration dryer connection  16 . In addition, an exhaust air damper  29  is arranged between the refrigeration dryer  12  and the refrigeration dryer connection  16  on the exhaust air duct  13 , in particular within the dryer exhaust air duct  15 , by means of which damper the dryer exhaust air duct  15  can be closed. 
     A further exhaust air damper  46  may also be arranged between the screw compressor  11 , i.e. between the cooling air outlet  18  of the screw compressor  11  and the refrigeration dryer connection  16 , in particular within the screw compressor exhaust air duct  45 . As passive backflow dampers, the exhaust air damper  29  of the refrigeration dryer and/or the exhaust air damper  46  of the screw compressor can either be opened using a sufficient flow in the conveying direction or closed under the effect of gravity. However, it is also possible (see illustration in  FIG. 6 ) to open and close the exhaust air damper  29  by means of a drive motor  39 . Of course, the exhaust air damper  46  of the screw compressor can also be opened and closed by means of a drive motor (not shown). 
       FIG. 2  shows an embodiment of a compressed air station according to the invention, modified in comparison to the embodiment shown in  FIG. 1 , in which, as compressed air components, a screw compressor  11  and a first refrigeration dryer  12  and also a further refrigeration dryer  12 ′ are connected to a common exhaust air duct  13 . In this embodiment, the compressed air transferred from the screw compressor to the compressed air line  14  is split at a branching point  47  into a first partial line  48  and a second partial line  49 . 
     The first refrigeration dryer  12  is arranged in the first partial line  48 , and the second refrigeration dryer  12 ′ is arranged in the second partial line  49 . The dried compressed air leaves the first refrigeration dryer  12  via a third partial line  50 . The dried compressed air that flows via the second partial line  49  into the second refrigeration dryer  12 ′ and is dried therein leaves the refrigeration dryer  12 ′ via a fourth partial line  51 . The third partial line  50  and the fourth partial line  51  are brought together at a joining point  52  and merge into the compressed air line  43 , which conducts the compressed air to at least one consumer  44 . 
     The screw compressor  11 , the refrigeration dryer  12  and the refrigeration dryer  12 ′ each direct their cooling air flows into a common exhaust air duct  13 . To this end, the screw compressor  11  is connected to the exhaust air duct  13  in the manner already described with reference to the embodiment shown in  FIG. 1 . The first refrigeration dryer  12  is also connected to the common exhaust air duct  13 , in the manner already described with reference to the embodiment shown in  FIG. 1 , in order to transfer the cooling air or waste heat output from the cooling air outlet  19 . Downstream of the refrigeration dryer connection  16 , which is assigned to the first refrigeration dryer  12 , a second refrigeration dryer connection  53  is provided on the exhaust air duct  13 , at which the cooling air of the second refrigeration dryer  12 ′ is also introduced downstream into the exhaust air duct  13 . To this end, a cooling air outlet  54  of the second refrigeration dryer is connected to a dryer exhaust air duct  55 , which connects the cooling air outlet  54  of the second refrigeration dryer  12 ′ to the refrigeration dryer connection  53  on the exhaust air duct  13 , so that the cooling air also of the second refrigeration dryer is discharged without any ambient air not conducted via the second refrigeration dryer  12 ′. Also in this dryer exhaust air duct assigned to the second refrigeration dryer  12 ′, an exhaust air damper  64  is provided so as to be able to close the exhaust air duct  55 , in particular when the second refrigeration dryer  12 ′ is idle. 
       FIG. 3  shows an embodiment of a compressed air station, once again modified in comparison to the embodiments shown in  FIG. 1  and  FIG. 2 , in which, as compressed air components, a first screw compressor  11 , a second screw compressor  11 ′, a first refrigeration dryer  12  and a second refrigeration dryer  12 ′ are connected to a common exhaust air duct  13 . In the embodiment of a compressed air station illustrated in  FIG. 3 , two screw compressors are provided, namely the first screw compressor  11  and the second screw compressor  11 ′, which operate in parallel with one another with regard to compressed air generation, i.e. the screw compressor  11  outputs compressed air on a first output line  56  and the screw compressor  11 ′ outputs compressed air on a second output line  57 . At a joining point  58 , the first output line  56  and the second output line  57  join to form a common compressed air line  14 . From the compressed air line  14 , the compressed air is conducted at a branching point  47  to a first partial line  48  and to a second partial line  49 , in which a respective refrigeration dryer  12  or  12 ′ is connected. The compressed air is thus dried by the two refrigeration dryers  12 ,  12 ′ in parallel, so that the arrangement of the two refrigeration dryers corresponds exactly to the arrangement of the two refrigeration dryers according to the embodiment shown in  FIG. 2 . The discharging of the cooling air or waste heat via dryer exhaust air duct  15  or dryer exhaust air duct  55  also takes place exactly as in the arrangement shown in  FIG. 2 . 
     In manner differing from the arrangement shown in  FIG. 2 , however, in the embodiment shown in  FIG. 3  not only are two refrigeration dryers  12 ,  12 ′ connected to the common exhaust air duct  13 , but also the aforementioned two screw compressors  11 ,  11 ′. The first screw compressor  11  is connected to the common exhaust air duct  13  in the manner already described with reference to  FIGS. 1 and 2  and represents the component yielding waste heat that is arranged furthest upstream in relation to the flow direction of the exhaust air duct  13 . The screw compressor  11  is therefore the waste heat supplier placed most upstream within the common exhaust air duct  13 . 
     The aforementioned second screw compressor  11 ′ has a cooling air outlet  59 , by which it is connected to a screw compressor exhaust air duct  60 . The screw compressor exhaust air duct  60  connects the cooling air outlet  59  of the second screw compressor  11 ′ to a compressor connection  61 , at which the screw compressor exhaust air duct  60  is connected to the common exhaust air duct  13 , namely at a section between the compressor connection  17  of the first screw compressor  11  and the refrigeration dryer connection  16  of the first refrigeration dryer  12  or the second refrigeration dryer connection  53  of the refrigeration dryer  12 ′. 
       FIG. 4  illustrates an embodiment that has once again been modified, in which, as compressed air components, a first refrigeration dryer  12  and a second refrigeration dryer  12 ′ are connected to a common exhaust air duct  13 . In this case, therefore, a common exhaust air duct  13  is not fed by one refrigeration dryer  12  and one screw compressor  11 , but rather by two refrigeration dryers  12 ,  12 ′. The connection of the refrigeration dryers  12 ,  12 ′ corresponds to the connection of the refrigeration dryers  12 ,  12 ′ in the arrangement of the embodiment shown in  FIG. 3 , with the sole exception that no cooling air or waste heat from a screw compressor  11 ,  11 ′ is introduced upstream of the connection points where the refrigeration dryers  12 ,  12 ′ are connected to the common exhaust air duct  13 . 
       FIG. 5  illustrates an embodiment that has once again been modified, in which, as compressed air components, a first screw compressor  11  discharges waste heat into a first exhaust air duct  13  via a screw compressor exhaust air duct  45 . The screw compressor  11  supplies compressed air via a compressed air line  14  to a first refrigeration dryer  12 . The compressed air dried in the refrigeration dryer  12  is conducted via a first output line  66  to a joining point  68 , to which compressed air dried in a second refrigeration dryer  12 ′ is also conducted via a second output line  67 . From the joining point  68 , the combined dried compressed air is fed to a consumer  44 . 
     A second screw compressor  11 ′ generates compressed air and transfers this compressed air via the compressed air line  14 ′ to the aforementioned second refrigeration dryer  12 ′. Waste heat from the second screw compressor  11 ′ is supplied via a screw compressor exhaust air duct  60  to a second exhaust air duct  13 ′, which is separate from the exhaust air duct  13 . Waste heat from the first refrigeration dryer  12  is also discharged into this second exhaust air duct  13 ′ via a dryer exhaust air duct  15 . The second refrigeration dryer  12 ′, which in terms of compressed air is connected to the second screw compressor  11 ′, in contrast feeds its exhaust air via a dryer exhaust air duct  55  to the exhaust air duct  13 , which is also fed by the first screw compressor  11 . Here, therefore, the pairs of screw compressors and refrigeration dryers connected one behind the other are connected to two different exhaust air ducts  13 ,  13 ′ in a crossed fashion. 
     The basic structure and the basic mode of operation of a refrigeration dryer  12 ,  12 ′ according to the present invention will be explained in greater detail below with reference to  FIG. 6 . The refrigeration dryer first has a compressed air inlet  62  and a compressed air outlet  63 . The compressed air flowing into the refrigeration dryer  12 ,  12 ′ via the compressed air inlet  62  is cooled by a refrigerant at a compressed air refrigeration heat exchanger  23  and leaves the refrigeration dryer through the compressed air outlet  63 . As is well known to a person skilled in the art, prior to flowing into the compressed air refrigeration heat exchanger  23 , the compressed air is usually pre-cooled in a pre-heat exchanger, namely by the compressed air that has already flowed through the compressed air refrigeration heat exchanger  23 , which is thus heated again before flowing out of the compressed air outlet  63 . However, the pre-heat exchanger described above is not shown in the present case. 
     To provide the cooling capacity required at the compressed air refrigeration heat exchanger  23 , the latter is part of a refrigerant circuit  24 , which forms a compression refrigeration machine, known per se, and thus comprises, as seen in the direction of flow of the refrigerant from the compressed air refrigerant heat exchanger  23  a refrigerant compressor  25 , a condenser  26  connected thereto and an expansion valve  27  downstream thereof. The gas expanded after flowing through the expansion valve  27  is cooled by the expansion process and in the compressed air refrigerant heat exchanger  23  transfers cold to the compressed air. The refrigerant compressor  25  compresses the refrigerant. The heat produced during this is output in the condenser  26  to the supply air. For this purpose, a cooling air flow  65  is conducted through the refrigeration dryer  12 ,  12 ′, the supply air usually being ambient air and being sucked into the refrigeration dryer  12 ,  12 ′ at a supply air inlet  41 , namely under the effect of a fan  20  driven by a fan motor  21 . 
     The cooling air flow  65  absorbs heat in the condenser  26 , so that the refrigerant in the condenser  26  is cooled. According to the invention, the resulting waste heat is to be introduced via a dryer exhaust air duct  15  to an exhaust air duct  13 , into which waste heat from other compressed air-generating or compressed air-processing components is also conducted. For this purpose, the dryer exhaust air duct  15  is connected directly to the cooling air outlet  19  of the refrigeration dryer  12 ,  12 ′. An exhaust air damper  29 , which can be moved by way of a drive motor  39 , in particular from an open position to a closed position or vice versa, interacts with a controller  22 , which actuates the exhaust air damper  29  and closes or opens it as required. 
     However, the controller  22  also controls the fan motor  21  of the fan  20 . 
     The fan motor  21  is speed-adjustable. A current value for the cooling air volume flow V act  is detected within the cooling air flow  65  by a flow sensor  30 . This preferably takes place in that the flow sensor  30  is designed as a differential pressure sensor and detects on the one hand the pressure within the cooling air flow  65  in the region of an inflow nozzle  31 , which is arranged immediately in front of the fan  20 , and on the other hand in an upstream region of the cooling air flow way before it flows into the inflow nozzle  31 . From these two pressure values and the resulting pressure difference, it is possible to calculate the flow rate of the cooling air flow and thus, based on the given flow cross-section, the current volume flow. The current value of the cooling air volume flow V act  detected by the flow sensor  30  is transmitted to the controller  22 , which then actuates the fan motor  21  in such a way that, regardless of the current backpressure in the exhaust air duct  13 , a respectively specified value for the cooling air volume flow V soll  is adhered to as precisely as possible, that is to say the fan  20  is adjusted in such a way that the current cooling air volume flow V act  follows the respectively pre-set setpoint for the cooling air volume flow V soll . 
     Although the differential pressure measurement proposed here is a particularly simple, inexpensive and reliable method for determining the current cooling air volume flow V act , the invention is of course not limited to this specific way of detecting the value of the cooling air volume flow. Instead, various other measurement techniques and methods are conceivable for detecting the current value of the cooling air volume flow. 
     A preferred adjustment, taking into account the respective current value for the cooling air volume flow V act , will be explained in greater detail below with reference to the flowchart shown in  FIG. 7 : 
     In a step  100 , the refrigeration dryer  12 ,  12 ′ is first started. In a step  101 , the cooling capacity requirement is determined and the refrigerant circuit  24  is started by setting the refrigerant compressor  25  in operation. In step  102 , the fan  20  is started at a specified minimum speed. 
     Then, in step  103 , a current differential pressure Δp act  is determined by the flow sensor  30 , and in a step  104  a current cooling air volume flow V act  is calculated from the current differential pressure Δp art . In parallel with this, a setpoint for the cooling air volume flow V soll  is defined. In the present embodiment, this takes place by first reading in a signal for the supply air temperature T amb  in a step  105 . In a step  106 , a setpoint for the cooling air volume flow V soll  is calculated, taking into account the temperature of the supply air T amb  and optionally also a current stroke volume. Based on a design value for the cooling air volume flow V nenn , which is specific to the given refrigerant circuit  24  with the given refrigerant compressor  25 , a setpoint for the cooling air volume flow is determined according to the following formula, using two correction factors: 
         V   soll   =V   nenn   *F   Hub/KMK   *F   T , 
     where the factors F Hub/KMK  and F T  are correction factors. The correction factor F T  is used only in the case where the supply air temperature is below a limit temperature T 0,amb , and is otherwise set equal to 1, cf. in this regard  FIG. 8 , which shows an example of the curve of the correction factor F T , here expressed in %, with the limit temperature T 0,amb  having been set here to 15° C. If the temperature of the supply air is below the limit temperature T 0,amb , the correction factor can be defined for example as follows: 
     
       
         
           
             
               F 
               T 
             
             = 
             
               
                 
                   Δ 
                   T 
                 
                 
                   
                     T 
                     
                       0 
                       , 
                       amb 
                     
                   
                   - 
                   
                     T 
                     
                       amb 
                       , 
                       act 
                     
                   
                 
               
               . 
             
           
         
       
     
     The correction factor for the stroke volume can be calculated according to the following formula: 
     
       
         
           
             
               F 
               
                 Hub 
                 / 
                 KMK 
               
             
             = 
             
               
                 current 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 stroke 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 volume 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 KMK 
               
               
                 maximum 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 stroke 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 volume 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 KMK 
               
             
           
         
       
     
     In a step  107 , the current cooling air volume flow V act  is compared with the setpoint for the cooling air volume flow defined in step  106 , and the difference between V soll  and V act  is formed, this difference defining a control deviation e. The fan  20  is then actuated in such a way that the cooling air volume flow V act  follows the specified setpoint for the cooling air volume flow V soll , namely in that, in step  108 , a current speed signal is transmitted to the fan motor  21  for the fan  20 . The method then begins again at step  103  or  105 , at a predefined sampling rate of, for example, 100 ms. 
     REFERENCE SIGNS 
     
         
           11 ,  11 ′ screw compressor 
           12 ,  12 ′ refrigeration dryer 
           13 ,  13 ′ exhaust air duct 
           14 ,  14 ′ compressed air line 
           15  dryer exhaust air duct 
           16  refrigeration dryer connection 
           17  compressor connection 
           18  cooling air outlet (screw compressor) 
           19  cooling air outlet (refrigeration dryer) 
           20  fan 
           21  fan motor 
           22  controller 
           23  compressed air refrigeration heat exchanger 
           24  refrigerant circuit 
           25  refrigerant compressor 
           26  condenser 
           27  expansion valve 
           28  pressure sensor 
           29  exhaust air damper 
           30  flow sensor 
           31  inflow nozzle 
           39  drive motor 
           41  supply air inlet 
           42  ambient air sensor/temperature sensor 
           43  further compressed air line 
           44  consumer 
           45  screw compressor exhaust air duct 
           46  exhaust air damper (screw compressor) 
           47  branching point 
           48  first partial line 
           49  second partial line 
           50  third partial line 
           51  fourth partial line 
           52  joining point 
           53  further refrigeration dryer connection 
           54  cooling air outlet 
           55  dryer exhaust air duct 
           56  first output line 
           57  second output line 
           58  joining point 
           59  cooling air outlet 
           60  screw compressor exhaust air duct 
           61  compressor connection 
           62  compressed air inlet 
           63  compressed air outlet 
           64  exhaust air damper 
           65  cooling air flow 
           66  first output line 
           67  second output line 
           68  joining point 
           69  exhaust air damper