Patent Publication Number: US-2005123399-A1

Title: Compact diagonal fan

Description:
BACKGROUND OF THE INVENTION  
      The invention relates to a compact diagonal fan according to the preamble of patent claim  1 .  
     PRIOR ART  
      At first glance there seems to be very little difference between diagonal fans and the familiar axial fans, which is why they are also frequently referred to as “semi-axial” blowers. Like an axial fan, a diagonal fan according to  FIG. 3  essentially comprises a housing  100 , an electric motor  101  supported within the housing  100  whose hub  102 , together with a plurality of blades  103  formed on the hub  102 , go to make up the impeller  104  of the fan. The impeller  104  is rotatably supported about a rotational axis  105  and is driven by the electric motor  101 . The electric motor  101  is held within the housing  100  by means of bridges  106  which essentially extend in a radially outwards direction.  
      For both diagonal and axial compact fans, the motor  101 , the commutation electronics (if used), the impeller  104  and the housing  100  are all integrated into a single unit. The motor is an outer rotor motor where the rotor rotates about the internally positioned stator. This goes to create a very compact design since the impeller can be directly fixed to the outer rotor. The motor itself is usually either a split-pole motor or a capacitor motor in the case of an AC power network (the latter only for high-power) or a commutator motor or a brushless DC motor for a DC power supply.  
      In the case of a diagonal fan, the air is sucked in axially but flows out diagonally. By making the hub conical in shape and specifying the air conduction in the outer housing, an outflow angle of between 0 and 90 degrees with respect to the rotational axis can be achieved. The peripheral velocity at the hub, which is necessary for building up pressure, is increased in particular by the diameter of the hub widening in the direction of flow. As a consequence, a diagonal fan having the same overall dimensions and the same rotational speed as an axial fan can generate a greater rise in pressure than the axial fan. This makes diagonal fans very attractive for users and they find application in telecommunication electronics in particular, since flow resistance in switch cabinets is becoming ever greater in line with the growing integration in this area, making powerful fans necessary. To date, small ventilators are hardly ever found with a diagonal design, which can perhaps be primarily attributed to the complicated geometry of the impeller.  
      As mentioned above, the stator of the outer rotor motor is generally fixed to the fan housing by means of bridges. In the case of an axial fan, it is known to position these bridges either at the air intake opening or at the air exit opening. This has hardly any effect on the impeller itself or on the operating noise of the fan, since the cross-section of the flow channel does not change—as opposed to a diagonal fan. For various reasons, however, axial compact fans are almost always designed to blow over the bridges. Conventional diagonal fans are likewise designed to blow over the bridges, i.e. the bridges are located at the air exit opening.  
     SUMMARY OF THE INVENTION  
      The object of the invention is to provide a diagonal fan which has considerably lower operating noise compared to a conventional diagonal fan with the same airflow rate and rise in pressure.  
      This object has been achieved in accordance with the invention by the characteristics outlined in claim  1 .  
      Beneficial embodiments and further developments of the invention can be derived from the subordinate patent claims.  
      The invention is characterized by the fact that the bridges used to secure the motor to the housing are arranged in the region of the air intake opening of the flow channel. This makes it possible to fit a larger fan wheel with the overall dimensions of the fan remaining unchanged. A larger fan wheel has a greater air flow so that the fan according to the invention can be operated at a relatively lower rotational speed to achieve the same air flow as a conventional fan. Operating the fan at a lower speed, however, means that the operating noise is also lowered, which was the actual objective of the invention.  
      Arranging the bridges in the region of the air intake opening makes it possible to fix the blades of the impeller close to the air exit opening of the flow channel and thus in the region where the diameter of the hub is at its largest so that the overall diameter of the fan wheel is increased.  
      The cross-section of the flow channel runs at a sharp angle radially outwards with respect to the rotational axis of the impeller, so that the air flowing through the fan is expelled diagonal to the rotational axis. One way of improving the rise in pressure compared to an axial fan is by decreasing the cross-section of the flow channel in the direction of the air exit opening.  
      To reduce the noise of the fan even further, provision is made for the profiled surface of the air conduction sleeve to be rounded off at the air intake opening in a radially outwards direction.  
      In a preferred embodiment of the invention, the electric motor is an outer rotor motor whose stationary part is held to the housing by the bridges and whose rotor forms the hub with the impeller.  
      The invention is explained in more detail below on the basis of an embodiment schematically illustrated in the drawings. 
    
    
     SHORT DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows an axial section through a diagonal fan according to the invention;  
       FIG. 2  shows a perspective view of the fan according to  FIG. 1 ;  
       FIG. 3  shows an axial section through a diagonal fan according to the prior art. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION  
      The fan illustrated in  FIGS. 1 and 2  essentially comprises a housing  10 , an electric motor  11  supported within the housing  10  whose hub  12 , together with a plurality of blades  13  formed on the hub  12 , go to make up the impeller  14  of the fan. The impeller  14  is rotatably supported about a rotational axis  15  and is driven by the electric motor  11 . The electric motor  11  is held within the housing  10  by means of bridges  16  which essentially extend in a radially outwards direction.  
      The housing  10  comprises an air conduction sleeve  17  adapted to the diameter of the impeller  14 . Together with the air conduction sleeve  17  surrounding the impeller, the hub encloses an essentially annular flow channel  18  which has an air intake opening  19  and an air exit opening  20 . The air is sucked into the air intake opening  19 , flows through the fan in the direction of flow  22  and is expelled again at the air exit opening  20 .  
      The hub  12  essentially takes the form of a truncated cone widening in the direction of the air exit opening  20 . The air conduction sleeve  17  is likewise essentially profiled, its diameter expanding towards the air exit opening  20 . The intake opening  19  of the air conduction sleeve  17  restrains the air intake opening of the flow channel  18  and is rounded towards the outside over an intake radius  21  to prevent turbulence being generated on the intake side.  
      The surface of the air conduction sleeves  17  preferably has a more acute angle with respect to the rotational axis  15  in the direction of the air exit opening  20  than the surface of the hub  12  so that the diameter of the annular flow channel  18  decreases particularly in the region of the impeller  14  in the direction of the air exit opening  20 .  
      According to the present invention it was established that in the case of a diagonal fan—unlike an axial fan—it is more advantageous for flow purposes to position the bridges  16  used to hold the electric motor  11  on the air intake side  19 .  
      This is explained using  FIGS. 1 and 3  as a basis.  
       FIG. 3  shows a conventional diagonal compact fan whose bridges  106  are arranged on the air exit side. The stationary part of the motor  101  held by the bridges  106  is thus arranged on the air exit side while the fan wheel  104  together with the hub  102  is moved in the direction of the air intake opening. The blades  103  are fixed on the side of the hub  102  having the smallest diameter.  
       FIG. 1  shows the diagonal fan according to the invention in which the bridges  16  are arranged on the air intake side  19 . It is clear that compared to the fan according to  FIG. 3 , this results in an impeller  14  with a larger diameter on the air exit side  20  due to the sloping walls of the hub  12  and of the air conduction sleeve  17  since the blades  13  of the impeller  14  are now fixed to the side of the hub  12  having the largest diameter. In the illustrated case, the diameter of the impeller  14  on the air exit side according to  FIG. 1  is about 10% larger than the diameter of the impeller  104  according to  FIG. 3 .  
      Enlarging the diameter of the impeller  14  in this way has several effects on the operation of the fan.  
      The airflow rate, i.e. the volume flow, of a fan depends among other factors on the rotational speed and the diameter D of the impeller  14 . As the diameter of the impeller increases so does the airflow rate, increasing by a power of five. This means, for example, that an impeller having a 10% larger diameter (factor 1.10) achieves a 61% greater airflow rate at the same rotational speed, since 
 
1.10 5 =1.61 
 
      The airflow rate also depends on the rotational speed of the impeller and changes with the cube of the rotational speed. This means that from the above example, the rotational speed of a fan whose impeller diameter is 10% larger can be reduced by 15% in order to produce the same airflow rate as a fan having a 100% impeller diameter, since (1/1.61) 1/3 =0.85=1.0−0.15  
      Assuming the other operating conditions remain the same, a reduction in rotational speed also means a reduction in operating noise. In practice, the following empirical equations can be used for calculations: 
 
 L   W   =A  log ( N   1   /N   2 ), 
 
 where 
          L W =Sound level in dB     A=50 to 55 (empirically determined value)     N 1 =Nominal speed     N 2 =Reduced speed        

      This means that a reduction in rotational speed of 15% makes it possible to reduce the noise of the fan by 
 
 L   W =50 (or 55) log (1.0/0.85)=3.5 dB (or 3.9 dB) 
 
      A reduction of 3.5 to 3.9 dB means that the original sound level is reduced by more than half. It is consequently very advantageous if the same airflow rate can be achieved at a lower rotational speed using the diagonal fan according to the invention  
      However, many other factors play a part in producing the sound level, including the diameter D of the impeller itself. Nevertheless, in general terms it is possible to say that with respect to the sound level, it is more advantageous to enlarge the diameter D of the impeller  14  and as a consequence to reduce the rotational speed. This can be explained by the fact that the tangential speed of the impeller is linearly proportional to both its radius as well as to the rotational speed.  
      Calculating for the above example (110% diameter, 85% rotational speed), the maximum tangential speed of the impeller  14  is 6.5% lower compared to the original impeller  104  (100% diameter, 100% rotational speed): 
 
1.10×0.85=0.935=1−0.065 
 
      In summary it can be said that the design of a diagonal fan according to the invention as presented in  FIGS. 1 and 2 , having the same overall dimensions and the same flow operating point (air volume and rise in pressure) as a conventional diagonal fan, can be operated at a lower rotational speed and thus with less operating noise.  
     Identification Reference List  
     
         
           10  Housing  
           11  Electric motor  
           12  Hub  
           13  Blades  
           14  Impeller  
           15  Rotational axis  
           16  Bridges  
           17  Air conduction sleeve  
           18  Flow channel  
           19  Air intake opening  
           20  Air exit opening  
           21  Intake radius  
           22  Direction of flow  
           100  Housing  
           101  Electric motor  
           102  Hub  
           103  Blades  
           104  Impeller  
           105  Rotational axis  
           106  Bridges