US20050123399A1 - Compact diagonal fan - Google Patents

Abstract

The invention relates to a diagonal fan having a housing, an impeller driven by an electric motor whose hub together with an air conduction sleeve surrounding the impeller defines an essentially annular flow channel having an air intake opening and an air exit opening. The hub takes the form of a truncated cone that widens towards the air exit opening, the air conduction sleeve being essentially profiled, the electric motor being held in the housing by means of bridges that essentially extend in a radially outwards direction. The invention is characterized by the fact that the bridges to secure the motor are arranged in the region of the air intake opening of the flow channel.

Description

BACKGROUND OF THE INVENTION
• The invention relates to a compact diagonal fan according to the preamble of patent claim1.
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 toFIG. 3 essentially comprises ahousing100, anelectric motor101 supported within thehousing100 whosehub102, together with a plurality ofblades103 formed on thehub102, go to make up theimpeller104 of the fan. Theimpeller104 is rotatably supported about arotational axis105 and is driven by theelectric motor101. Theelectric motor101 is held within thehousing100 by means ofbridges106 which essentially extend in a radially outwards direction.
• For both diagonal and axial compact fans, themotor101, the commutation electronics (if used), theimpeller104 and thehousing100 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 claim1.
• 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 toFIG. 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 inFIGS. 1 and 2 essentially comprises ahousing10, anelectric motor11 supported within thehousing10 whosehub12, together with a plurality ofblades13 formed on thehub12, go to make up theimpeller14 of the fan. Theimpeller14 is rotatably supported about arotational axis15 and is driven by theelectric motor11. Theelectric motor11 is held within thehousing10 by means ofbridges16 which essentially extend in a radially outwards direction.
• Thehousing10 comprises anair conduction sleeve17 adapted to the diameter of theimpeller14. Together with theair conduction sleeve17 surrounding the impeller, the hub encloses an essentiallyannular flow channel18 which has an air intake opening19 and anair exit opening20. The air is sucked into the air intake opening19, flows through the fan in the direction offlow22 and is expelled again at the air exit opening20.
• Thehub12 essentially takes the form of a truncated cone widening in the direction of the air exit opening20. Theair conduction sleeve17 is likewise essentially profiled, its diameter expanding towards theair exit opening20. The intake opening19 of theair conduction sleeve17 restrains the air intake opening of theflow channel18 and is rounded towards the outside over anintake radius21 to prevent turbulence being generated on the intake side.
• The surface of theair conduction sleeves17 preferably has a more acute angle with respect to therotational axis15 in the direction of the air exit opening20 than the surface of thehub12 so that the diameter of theannular flow channel18 decreases particularly in the region of theimpeller14 in the direction of the air exit opening20.
• 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 thebridges16 used to hold theelectric motor11 on theair intake side19.
• This is explained usingFIGS. 1 and 3 as a basis.
• FIG. 3 shows a conventional diagonal compact fan whosebridges106 are arranged on the air exit side. The stationary part of themotor101 held by thebridges106 is thus arranged on the air exit side while thefan wheel104 together with thehub102 is moved in the direction of the air intake opening. Theblades103 are fixed on the side of thehub102 having the smallest diameter.
• FIG. 1 shows the diagonal fan according to the invention in which thebridges16 are arranged on theair intake side19. It is clear that compared to the fan according toFIG. 3, this results in animpeller14 with a larger diameter on theair exit side20 due to the sloping walls of thehub12 and of theair conduction sleeve17 since theblades13 of theimpeller14 are now fixed to the side of thehub12 having the largest diameter. In the illustrated case, the diameter of theimpeller14 on the air exit side according toFIG. 1 is about 10% larger than the diameter of theimpeller104 according toFIG. 3.
• Enlarging the diameter of theimpeller14 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 theimpeller14. 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
  1.10⁵=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=Alog (N₁/N₂),
  where
• • L_W=Sound level in dB
  • A=50 to 55 (empirically determined value)
  • N₁=Nominal speed
  • N₂=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 theimpeller14 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 theimpeller14 is 6.5% lower compared to the original impeller104 (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 inFIGS. 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

Claims (8)

1. A diagonal fan having a housing (10), an impeller (14) driven by an electric motor (11) whose hub (12) together with an air conduction sleeve (17) surrounding the impeller defines an essentially annular flow channel (18) having an air intake opening (19) and an air exit opening (20), the hub (12) taking the form of a truncated cone that widens towards the air exit opening (20), and the air conduction sleeve (17) being essentially profiled, the electric motor (11) being held in the housing (10) by means of bridges (16) that essentially extend in a radially outwards direction, characterized in that,

the bridges (16) are arranged in the region of the air intake opening (19) of the flow channel (18).

2. A diagonal fan according toclaim 1, characterized in that the blades (13) of the impeller (14) are fixed in the region where the diameter of the hub (12) is at its largest.

3. A diagonal fan according toclaim 1, characterized in that the impeller (14) is arranged in the region of the air exit opening (20) of the flow channel (18).

4. A diagonal fan according toclaim 1, characterized in that the cross-section of the flow channel (18) with respect to the rotational axis (15) of the impeller (14) extends radially outwards at an acute angle.

5. A diagonal fan according toclaim 1, characterized in that the cross-section of the flow channel (18) decreases in the direction of the air exit opening (20).

6. A diagonal fan according toclaim 1, characterized in that the diameter of the air conduction sleeve (17) widens towards the air exit opening (20).

7. A diagonal fan according toclaim 1, characterized in that the profiled surface of the air conduction sleeve (17) has an outwardly rounded intake radius (21) at the air intake opening (19).

8. A diagonal fan according toclaim 1, characterized in that the electric motor (11) is an outer rotor motor whose stationary part is held to the housing (10) and whose rotor forms the hub (12) with the impeller (14).

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