JP2010117062A - Duct fan - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a duct fan capable of adjusting a total air quantity to a low level by reducing an air quantity of part of centrifugal air blowers, in the duct fan which includes the plurality of built-in centrifugal air blowers and cannot largely suppress the total air quantity since a rotational frequency of a motor cannot be largely reduced due to heat generation and deterioration in the efficiency of the motor when the air quantity is desired to temporarily be reduced as much as possible. <P>SOLUTION: The plurality of centrifugal air blowers 10-1, 10-2 are provided in a box-shaped body 3 including a body suction port 1 and a body blowout port 2 opposing each other, and the air blowers 10-1, 10-2 are arranged forward and backward toward an inspection opening 18 and forward and backward toward the body suction port 1. By adjusting the total air quantity to a lower level by reducing the air quantity of the centrifugal air blower 10-1 with respect to the total air quantity in a state of the normal operation of all of the centrifugal air blowers 10-1, 10-2, the duct fan can largely suppress the total air quantity when the air quantity is desired to temporarily be reduced as much as possible. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a structure of a duct fan that is installed in a space such as a ceiling of a building, has a plurality of centrifugal fans built therein, and ventilates indoor air through a duct.

  Conventionally, as this type of duct fan, if you want to temporarily reduce the air volume as much as possible, the motor speed cannot be greatly reduced due to the heat generation of the motor or the efficiency reduction, so the total air volume can not be greatly suppressed, A damper type air volume adjusting member for reducing the air volume is provided separately from the fan main body, and the air volume is reduced by reducing the air volume (for example, see Patent Document 1).

  Hereinafter, the duct fan will be described with reference to FIG.

  As shown in FIG. 7, the airframe suction port 101 and the airframe outlet 102 are provided on opposite side surfaces of the box-shaped airframe 103, and the airframe 103 includes a suction adapter 105 that connects the air intake side duct 104 to the airframe suction port 101, In addition, a blow-out adapter 107 that connects the blow-out side duct 106 is disposed at the machine blow-out port 102. A damper type air volume adjusting member 108 is disposed between the airframe outlet 102 and the outlet adapter 107, and the damper type air volume adjusting member 108 includes a slide plate 109 and a guide part 110.

When the motor 111 is operated, the single suction centrifugal blower 113 is driven via the pulley belt 112, and the suction air flow 114 inside the machine body 103 passes through the scroll casing 115 and flows out from the discharge port 116 as the discharge air flow 117. The air volume is reduced by colliding with the slide plate 109.
JP-A-11-270885

  In such a conventional duct fan, when adjusting the air volume, it is necessary to directly operate the slide plate 109, and there is a problem that it takes time and effort to reduce the air volume.

  In addition, there is a problem that the amount of reduction in noise and power consumption is not proportional to the amount of airflow reduction due to the air loss due to the collision of the discharged airflow 117 with the slide plate 109.

  Moreover, since it is necessary to equip the airframe 103 with the damper type air volume adjusting member 108 for reducing the air volume separately from the fan body, there is a problem that the structure becomes complicated.

  The present invention solves such a conventional problem, and temporarily reduces the air volume of some centrifugal fans with respect to the total air volume in a state where all the centrifugal fans in the airframe are normally operated. An object of the present invention is to provide a duct fan capable of greatly reducing the total air volume when it is desired to reduce the air volume as much as possible.

  In order to achieve the above object, a duct fan according to the present invention includes a plurality of centrifugal fans inside a box-shaped body provided with a body suction port and a body air outlet facing each other, and the centrifugal fan includes a discharge port and a casing. A scroll casing having a suction port is provided, and the impeller is driven by a rotating shaft of a motor fixed in the vicinity of the casing suction port, so that inspection is performed on one surface of the body other than the surface having the body suction port and the body air outlet. An opening is provided, a partition plate is provided in the airframe to partition the airframe inlet side and the airframe outlet side, and the plurality of centrifugal fans are arranged to be front and rear toward the inspection opening; and In the duct fan, which is arranged so as to be front and back toward the airframe suction port, and provided with an opening in the partition plate and connected to the discharge port of the centrifugal blower, all centrifugal fans are operated as standard. The total air volume condition, a portion of adjusting low total air volume by reducing the air volume of the centrifugal blower, and the centrifugal blower to reduce the air volume and configuration arranged such that the body inlet side.

  Another means is that the centrifugal blower for reducing the air volume is configured to reduce the operation of the motor.

  As another means, the centrifugal blower for reducing the air volume is configured to include a backflow prevention shutter that stops operation and closes backflow in the scroll casing.

  In addition, the other means is configured such that a backflow prevention shutter is arranged at the discharge port of the centrifugal blower.

  As another means, the centrifugal blower for reducing the air volume is configured to include an orifice-shaped backflow prevention nozzle that stops operation and suppresses backflow in the scroll casing.

  In addition, the other means is configured such that the backflow prevention nozzle is arranged at the discharge port of the centrifugal blower.

  According to the present invention, a plurality of centrifugal fans are provided inside a box-like body provided with an air inlet and an air outlet facing each other, and the centrifugal fan includes a scroll casing provided with an outlet and a casing inlet. The impeller is driven by a rotating shaft of a motor fixed in the vicinity of the casing suction port, and an inspection opening is provided on one surface of the airframe other than the surface having the airframe suction port and the airframe outlet, A partition plate is provided for partitioning the airframe suction port side and the airframe air outlet side, and the plurality of centrifugal fans are arranged to be front and rear toward the inspection opening, and front and rear toward the airframe suction port. In a duct fan in which an opening is provided in the partition plate and the discharge ports of the centrifugal blower are connected to each other, the total number of centrifugal blowers in a standard operation state is partially By reducing the air flow of the centrifugal blower and adjusting the total air flow low, and the centrifugal blower that reduces the air flow is placed on the air inlet side, the air flow can be reduced and noise and power consumption can be suppressed. In addition, a duct fan having an effect that the suction airflow from the airframe suction port can be cooled by hitting a low-rotation motor, and further, the backflow of the scroll casing of the centrifugal blower in which the airflow from the airframe suction port reduces the air volume can be suppressed. Can be provided.

  Moreover, in the centrifugal blower that reduces the air volume, by adopting a configuration in which the motor is operated at a low speed, it is possible to provide a duct fan that can reduce the air volume and suppress noise and power consumption.

  Moreover, in the centrifugal blower that reduces the air volume, the operation is stopped and the backflow prevention shutter that closes the backflow in the scroll casing is provided, so that the air volume can be reduced and noise and power consumption can be suppressed. A duct fan can be provided.

  In addition, by adopting a configuration in which the backflow prevention shutter is arranged at the discharge port of the centrifugal blower, it is possible to provide a duct fan having an effect that the backflow prevention shutter can have a simple structure.

  Moreover, in the centrifugal blower that reduces the air volume, the operation is stopped and the configuration including the orifice-shaped backflow prevention nozzle that suppresses the backflow in the scroll casing can reduce the air volume and suppress noise and power consumption. An effective duct fan can be provided.

  Moreover, the duct fan which has the effect that a backflow prevention nozzle can be made into a simple structure can be provided by having set it as the structure which distribute | arranges a backflow prevention nozzle to the discharge port of a centrifugal blower.

  According to a first aspect of the present invention, a duct fan includes a plurality of centrifugal fans inside a box-shaped body provided with an air inlet and an air outlet facing each other, and the centrifugal fan includes an outlet and a casing inlet. Provided with a scroll casing provided, driven by a rotating shaft of a motor fixed in the vicinity of the casing suction port, and provided with an inspection opening on one surface of the body other than the surface having the body suction port and the body air outlet. A partition plate that divides the airframe inlet side and the airframe outlet side is provided in the airframe, and the plurality of centrifugal fans are arranged to be front and rear toward the inspection opening, and the airframe In the duct fan, which is arranged so as to be front and back toward the suction port, and provided with an opening in the partition plate and connected to the discharge port of the centrifugal fan, the total of the state in which all the centrifugal fans are in standard operation The total air volume is adjusted to be lower by reducing the air volume of some centrifugal fans, and the centrifugal fan that reduces the air volume is arranged to be on the airframe inlet side. The airflow from the air intake when the airflow of some centrifugal fans is reduced by reducing the airframe width from the inspection opening side to the anti-inspection opening side. The distribution of air flow can be reduced and turbulence can be suppressed. By reducing the air volume of some centrifugal fans as compared to the total air volume when all centrifugal fans are in standard operation, the air volume is temporarily reduced as much as possible. When it wants, it has the effect | action that a total air volume can be restrained largely. Furthermore, the airflow from the air inlet can be cooled by hitting a low-rotation motor, and the motor can be cooled to generate heat with low cooling due to a decrease in the airflow at the casing air inlet near the motor. Has the effect that the motor can be cooled by hitting the low-rotation motor. Further, the suction airflow from the airframe suction port can suppress the backflow of the scroll casing of the centrifugal blower that reduces the air volume, and the discharge backflow airflow of the centrifugal blower that operates as a standard operation is generated by flowing in the scroll casing of the centrifugal blower that is stopped. With respect to the reverse flow in the airframe, the suction airflow from the airframe suction port collides to suppress the reverse flow, and the operation of some centrifugal blowers can be stopped to reduce the air volume.

  Moreover, the centrifugal blower that reduces the air volume has a configuration in which the operation of the motor is set to low rotation, and has an effect that the air volume can be reduced by setting the operation of some motors to low rotation.

  In addition, the centrifugal blower that reduces the air volume is configured to include a backflow prevention shutter that stops operation and closes backflow in the scroll casing, and the centrifugal blower that discharges the backflow of the centrifugal blower that performs standard operation stops. By blocking the reverse flow generated in the scroll casing by the reverse flow prevention shutter, the operation of some centrifugal fans can be stopped and the air volume can be reduced.

  In addition, the backflow prevention shutter is arranged at the outlet of the centrifugal blower, and the backflow prevention shutter is arranged at the outlet of the centrifugal blower so that the size of the backflow prevention shutter is reduced to match the size of the outlet. In addition, when the centrifugal blower is in operation, there is an effect that a simple backflow prevention shutter structure in which the backflow prevention shutter is opened by the wind pressure of the discharged backflow airflow can be achieved.

  In addition, the centrifugal blower for reducing the air volume is configured to include an orifice-shaped backflow prevention nozzle that stops operation and suppresses backflow in the scroll casing, and the discharge backflow airflow of the centrifugal blower that performs standard operation is stopped. By suppressing the backflow generated in the scroll casing of the centrifugal blower from flowing in the body by the backflow prevention nozzle, the operation of some of the centrifugal blowers can be stopped to reduce the air volume.

  In addition, the backflow prevention nozzle is arranged at the discharge port of the centrifugal blower, and the backflow prevention nozzle is arranged at the discharge port of the centrifugal blower, so that the size of the backflow prevention nozzle is reduced to the size of the discharge port. It has the effect | action that it can be made and can be made a simple backflow prevention nozzle structure.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
As shown in FIG. 1, an air inlet 1 and an air outlet 2 are provided on opposite sides of a box-shaped airframe 3, and the airframe 3 has a suction adapter 5 that connects a suction side duct 4 to the airframe inlet 1, The air outlet 2 is provided with an air outlet adapter 7 for connecting the air outlet side duct 6. Inside the airframe 3, there is an air passage 9 through which the suction air flow 8 from the airframe inlet 1 to the air outlet 2 flows, and two centrifugal fans 10-1 and 10-2 are arranged in the airway 9. Yes. Centrifugal blowers 10-1 and 10-2 include an impeller 13 fixed to the motors 11-1 and 11-2 via the rotary shaft 12, a discharge port 14 facing the fuselage outlet 2, and a scroll casing 15. have. On the side of the scroll casing 15, a motor-side orifice 17 having a casing suction port 16 opened on the motor side is disposed.

  An inspection opening 18 is provided on one surface of the airframe 3 other than the surface having the airframe inlet 1 and the airframe outlet 2, and a partition that divides the airframe inlet 1 side and the airframe outlet 2 side in the airframe 3. A plate 19 is provided. The plurality of centrifugal blowers 10-1 and 10-2 are arranged so as to be front and rear toward the inspection opening 18, and an opening 20 is provided in the partition plate 19 to provide each of the centrifugal blowers 10-1 and 10-2. The discharge port 14 is connected. The rotary shafts 12 of the plurality of centrifugal blowers 10-1 and 10-2 are arranged to face the inspection opening 18. A detachable inspection panel 21 is fixed to the inspection opening 18. The distance from the surface on the inspection opening 18 side of the body 3 to the facing surface is defined as the body width dimension 22.

  Centrifugal blowers 10-1 and 10-2 reduce the air volume of centrifugal blower 10-1 on the air inlet 1 side with respect to the total air volume in a state where all the units (centrifugal fans 10-1 and 10-2) are operated as standard. Then, the total air volume is adjusted low, and the air volume is reduced as a whole. Centrifugal blower 10-1 which reduces an air volume shall reduce an air volume by making the rotation speed of motor 11-1 low. In general, as a method of reducing the operation of the motor 11-1, the motor 11-1 itself has a plurality of speed adjustment functions, or a power supply frequency is changed by an inverter.

  As shown in FIG. 2, in the actual use state, the airframe 3 is constructed by connecting the suction side duct 4 to the airframe inlet 1 and the air outlet duct 6 to the airframe outlet 2 in the ceiling 24. When the centrifugal blowers 10-1 and 10-2 are operated, the air in the room 25 passes through the blowout duct 6 via the suction duct 4 and the centrifugal blowers 10-1 and 10-2, and is exhausted to the outdoor 26 for use. To do. The ceiling 24 and the room 25 are partitioned by a ceiling material 27, and a ceiling inspection port 28 is provided at the lower part of the body 3 of the ceiling material 27.

  Moreover, as shown in FIG. 3, the centrifugal blower 10-1 for reducing the air volume is arranged so as to be on the airframe suction port 1 side, and the centrifugal air blower 10-1 in which the suction airflow 8 from the airframe suction port 1 reduces the air volume. The backflow 29-1 of the scroll casing 15 is suppressed.

  With the above configuration, when the air flow of the centrifugal blower 10-1 on the airframe inlet 1 side is reduced and the centrifugal blower 10-2 on the airframe outlet 2 side is set as a standard operation, the centrifugal blower 10-1 is placed on the airframe inlet 1 side. Therefore, it is easy to inhale air, and the air volume can be ensured even when operating at a low speed. Conversely, when the centrifugal blower 10-2 on the airframe outlet 2 side is operated at a low rotation, the centrifugal blower 10-1 on the airframe suction port 1 side has a strong suction force and is far from the airframe suction port 1. -2 cannot be secured, and the desired air volume cannot be obtained as a whole. Further, since the plurality of centrifugal blowers 10-1 and 10-2 are arranged to be shifted back and forth with respect to the airframe inlet 1 (aircraft outlet 2), the airframe from the surface on the inspection opening 18 side of the airframe 3 to the opposite surface. The width dimension 22 is reduced. Also by reducing the airframe width dimension 22, when the air volume of the centrifugal blower 10-1 is reduced, the uneven distribution of the suction airflow 8 from the airframe suction port 1 can be reduced and turbulence can be suppressed. On the contrary, when the airframe width dimension 22 is large, the suction airflow 8 is biased toward the centrifugal blower 10-2 on the side where the air volume is large, so that the airflow distribution inside the airframe 3 becomes uneven and turbulence is likely to occur. .

  In addition, the cooling of the motor 11-1 can be cooled by the suction airflow 8 from the airframe suction port 1 hitting the low-rotation motor 11-1 in addition to the casing suction airflow 23 being cooled by the motor 11-1. At that time, since the motor 11-1 is rotating at a low speed, the motor 11-1 generating less heat due to a decrease in the airflow of the casing suction port airflow 23 in the vicinity of the motor 11-1, and the suction airflow 8 from the airframe suction port 1 is rotating at a low speed. The motor 11-1 can be cooled by hitting the motor 11-1. This is also effective for heat generation caused by a reduction in motor efficiency, which is generally generated when the rotational speed of the motor is greatly reduced.

  In addition, a reverse flow 29-1 in the airframe 3 generated by flowing through the scroll casing 15 of the centrifugal blower 10-1 in which the discharge backflow airflow 29-2 of the centrifugal blower 10-2 operating as a standard is stopped is supplied to the airframe inlet 1 When the suction airflow 8 from the air collides with the backflow 29-1, the backflow 29-1 can be suppressed and the operation of the centrifugal blower 10-1 can be stopped to reduce the air volume. At this time, by suppressing the backflow 29-1, generation of noise due to turbulent flow and increase in power consumption are also suppressed.

  Thus, according to the duct fan of Embodiment 1 of the present invention, the air volume can be reduced, noise and power consumption can be suppressed, and the motor can be cooled.

(Embodiment 2)
As shown in FIG. 4, the duct fan according to the second embodiment is provided with a centrifugal fan 10-1 that reduces the air volume, when the operation of the motor 11-1 is stopped, and the reverse flow 29-in the scroll casing 15. 1 is provided with a backflow prevention shutter 30 that blocks the discharge port 14 to prevent it. The backflow prevention shutter 30 is configured to freely rotate about the shutter rotation shaft 31. The backflow prevention shutter 30 is provided at the discharge port 14 of the centrifugal blower 10-1.

  With the above configuration, the backflow air flow 29-2 discharged from the centrifugal fan 10-2 that operates as a standard flows through the scroll casing 15 of the stopped centrifugal fan 10-1, thereby generating the backflow 29-1 in the body 3. By closing this backflow 29-1 with the backflow prevention shutter 30, the operation of the centrifugal blower 10-1 is stopped to reduce the air volume. At this time, the backflow 29-1 is blocked, so that generation of noise due to turbulent flow and increase in power consumption are suppressed.

  Further, by arranging the backflow prevention shutter 30 at the discharge port 14 of the centrifugal blower 10-1, the size of the backflow prevention shutter 30 can be reduced according to the size of the discharge port 14. Moreover, it can be made a simple structure in which the backflow prevention shutter 30 is opened by the wind pressure of the discharged backflow airflow 29-2 during the operation of the centrifugal blower 10-1.

  Thus, according to the duct fan of Embodiment 2 of the present invention, the air volume can be reduced, noise and power consumption can be suppressed, and the backflow prevention shutter can be made in a simple structure.

(Embodiment 3)
As shown in FIG. 5, the duct fan according to the third embodiment is provided with a centrifugal fan 10-1 that reduces the air volume when the operation of the motor 11-1 is stopped. 1 is provided with an orifice-like backflow prevention nozzle 32 that suppresses 1. The backflow prevention nozzle 32 is provided at the discharge port 14 of the centrifugal blower 10-1.

  With the above configuration, the backflow air flow 29-2 discharged from the centrifugal fan 10-2 that operates as a standard flows through the scroll casing 15 of the stopped centrifugal fan 10-1, thereby generating the backflow 29-1 in the body 3. By suppressing this backflow 29-1 by the backflow prevention nozzle 32, the operation of the centrifugal blower 10-1 is stopped and the air volume is reduced. At this time, by suppressing the backflow 29-1, generation of noise due to turbulent flow and increase in power consumption are suppressed.

  Further, by arranging the backflow prevention nozzle 32 at the discharge port 14 of the centrifugal blower 10-1, the size of the backflow prevention nozzle 32 can be reduced according to the size of the discharge port 14, and a simple backflow prevention nozzle structure can be achieved.

  As described above, according to the duct fan of Embodiment 3 of the present invention, the air volume can be reduced, noise and power consumption can be suppressed, and the backflow prevention nozzle can be made in a simple structure.

(Embodiment 4)
As shown in FIG. 6, the duct fan of the fourth embodiment is provided with an airframe inlet 1 and an air outlet 2 on opposite side surfaces of a box-shaped airframe 3. A suction adapter 5 that connects the suction side duct 4 and a blower adapter 7 that connects the blowout side duct 6 are arranged in the air blower outlet 2. Inside the airframe 3, there is an air passage 9 through which an intake air flow 8 from the airframe inlet 1 to the air outlet 2 flows, and the air passage 9 includes three centrifugal fans 10-1, 10-2 and 10-3. Is arranged. Centrifugal blowers 10-1, 10-2, and 10-3 include impellers 13 fixed to motors 11-1, 11-2, and 11-3 through a rotating shaft 12, and discharges that face the air outlet 2. An outlet 14 and a scroll casing 15 are provided. A motor-side orifice 17 having a motor-side casing suction port 16 opened is disposed on the side surface of the scroll casing 15.

  An inspection opening 18 is provided on one surface of the airframe 3 other than the surface having the airframe inlet 1 and the airframe outlet 2, and a partition that divides the airframe inlet 1 side and the airframe outlet 2 side in the airframe 3. A plate 19 is provided. The three centrifugal fans 10-1, 10-2, and 10-3 are arranged so as to be front and rear toward the inspection opening 18, and an opening 20 is provided in the partition plate 19 so that the centrifugal fans 10-1, 10 are provided. -2 and 10-3 are connected. The rotary shafts 12 of the three centrifugal blowers 10-1, 10-2, and 10-3 are arranged to face the inspection opening 18, and a detachable inspection panel 21 is fixed to the inspection opening 18. . The distance from the surface on the inspection opening 18 side of the body 3 to the facing surface is defined as the body width dimension 22.

  The total air volume is adjusted to be low by reducing the air volume of the two centrifugal fans 10-1 and 10-2 with respect to the total air volume when all the centrifugal fans 10-1, 10-2, and 10-3 are in the standard operation. The configuration.

  With the above-described configuration, the plurality of centrifugal fans 10-1, 10-2, and 10-3 are arranged so as to be shifted back and forth with respect to the airframe suction port 1 (aircraft air outlet 2). By reducing the airframe width dimension 22 to the inspection opening side, the distribution of the suction airflow 8 from the airframe inlet 1 when the airflow of the centrifugal blowers 10-1 and 10-2 is reduced is reduced, and the turbulent flow By reducing the air volume of the centrifugal fans 10-1 and 10-2 with respect to the total air volume in the state where all the centrifugal fans 10-1, 10-2, and 10-3 are standardly operated, Therefore, when it is desired to reduce the air volume as much as possible, the total air volume can be greatly suppressed. In addition, when the airframe width dimension 22 is large, since the suction airflow 8 is biased to the centrifugal blower 10-3 on the side where the air volume is large, the airflow distribution inside the airframe 3 becomes non-uniform and turbulence tends to occur.

  Thus, according to the duct fan of Embodiment 4 of this invention, an air volume can be reduced and a noise and power consumption can be suppressed.

  In addition to the purpose of air conveyance such as ventilation and blower equipment, air conditioners, dehumidifiers, humidifiers, and air purifiers, the equipment can be cooled using the air blown from the fuselage outlet, reducing the air volume as necessary. It can be applied to the use of blasting equipment that can be used.

Plan view of duct fan of embodiment 1 of the present invention Side view showing the installation state of the duct fan Plane configuration diagram showing the reverse flow of the duct fan Plane | planar block diagram of the duct fan of Embodiment 2 of this invention Plane | planar block diagram of the duct fan of Embodiment 3 of this invention Plane | planar block diagram of the duct fan of Embodiment 4 of this invention Plan view of a conventional duct fan

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Airframe inlet 2 Airframe blowout outlet 3 Airframe 8 Suction air flow 10-1 Centrifugal blower 10-2 Centrifugal blower 10-3 Centrifugal blower 11-1 Motor 11-2 Motor 11-3 Motor 12 Rotating shaft 13 Impeller 14 Discharge port 15 Scroll casing 16 Casing inlet 18 Inspection opening 19 Partition plate 20 Opening 30 Backflow prevention shutter 32 Backflow prevention nozzle

Claims (6)

A box-shaped machine body provided with a machine body suction port and a machine body outlet facing each other is provided with a plurality of centrifugal blowers, the centrifugal blower is provided with a scroll casing provided with a discharge port and a casing suction port, and the impeller is connected to the casing. Driven by a rotating shaft of a motor fixed in the vicinity of the air inlet, an inspection opening is provided on one surface of the airframe other than the surface having the airframe air inlet and the airframe outlet, and the airframe inlet side and the airframe are provided in the airframe. A partition plate is provided to divide the machine air outlet side, and the plurality of centrifugal fans are arranged so as to be front and back toward the inspection opening, and are arranged so as to be front and back toward the machine air inlet. In a duct fan in which an opening is provided in the partition plate and the discharge ports of the centrifugal blower are connected, the airflow of some centrifugal fans is lower than the total airflow in a state where all the centrifugal fans are in standard operation. To adjust the total air volume of the low duct fan, characterized in that the centrifugal fan to reduce the air flow has a structure arranged such that the body inlet side. The duct fan according to claim 1, wherein the centrifugal blower for reducing the air volume is configured such that the motor is operated at a low speed. 2. The duct fan according to claim 1, wherein the centrifugal blower for reducing the air volume includes a backflow prevention shutter that stops operation and closes backflow in the scroll casing. 4. The duct fan according to claim 3, wherein the backflow prevention shutter is arranged at a discharge port of a centrifugal blower. 2. The duct fan according to claim 1, wherein the centrifugal blower for reducing the air volume is configured to include an orifice-shaped backflow prevention nozzle that stops operation and suppresses backflow in the scroll casing. The duct fan according to claim 5, wherein the backflow prevention nozzle is arranged at a discharge port of a centrifugal blower.

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