Centrifugal fan and fan with sound-muffling box having the centrifugal fan built-in

1. A centrifugal fan in which an impeller is disposed inside a fan casing,

the fan casing including a casing side plate with a casing inlet for taking in a gas, a spiral scroll, a tongue, and a casing outlet, and

the impeller including a main plate fixed to a rotating shaft that transmits rotation of a motor, a retaining ring disposed facing the main plate, and a plurality of blades arranged between the main plate and the retaining ring,

wherein

the centrifugal fan takes in the gas from the casing inlet, passes the gas through an air-intake space surrounded by the blades and between the blades, and discharges the gas from the casing outlet by rotating the impeller,

the centrifugal fan further includes an airflow accelerator in the air-intake space for increasing an airflow speed of the gas toward the blades,

the airflow accelerator is a thin plate including a first side whose length is shorter than a distance from the main plate to the retaining ring, a second side whose length is shorter than a distance from a circumferential plane extending from a radially outermost periphery of the motor to the blades, and a third side perpendicular to the first side and the second side,

a line aligned with the second side of the airflow accelerator and projecting from an end of the airflow accelerator intersects a face formed by the casing outlet at a substantially orthogonal angle, and

the face is a plane connecting outlet ends of the tongue and the scroll at the casing outlet.

2. The centrifugal fan of claim 1 ,

wherein:

the first side is perpendicular to the main plate; and

an end of the second side at a blade side is disposed ahead of an end of the second side at a motor side relative to a rotating direction of the impeller.

3. The centrifugal fan of claim 2 ,

wherein

the airflow accelerator is disposed such that the second side is positioned on a straight line connecting an airflow inlet end of one of the blades at an inner periphery of the impeller and an airflow outlet end of said one of the blades at an outer periphery of the impeller.

4. The centrifugal fan of claim 2 ,

wherein

the airflow accelerator is disposed between a line connecting a tip of the tongue and a rotation center of the rotating shaft and a line extended from the rotation center parallel to a face formed by the casing outlet when the impeller rotates toward the tongue as the rotating direction.

5. The centrifugal fan of claim 1 ,

wherein the centrifugal fan is a sirocco fan, and

an end of the airflow accelerator at a side of the casing side plate is fixed to the casing side plate.

6. A fan housing with a sound-muffling box in which a centrifugal fan is built in a box housing provided with a housing inlet and a housing outlet,

in the centrifugal fan, an impeller is disposed inside a fan casing,

the fan casing including a casing side plate with a casing inlet for taking in a gas, a spiral scroll, a tongue, and a casing outlet, and

the impeller including a main plate fixed to a rotating shaft that transmits rotation of a motor, a retaining ring disposed facing the main plate, and a plurality of blades arranged between the main plate and the retaining ring,

wherein

the centrifugal fan takes in the gas from the casing inlet, passes the gas through an air-intake space surrounded by the blades and between the blades, and discharges the gas from the casing outlet by rotating the impeller,

the centrifugal fan further includes an airflow accelerator in the air-intake space for increasing an airflow speed of the gas toward the blades,

the airflow accelerator is a thin plate including a first side whose length is shorter than a distance from the main plate to the retaining ring, a second side whose length is shorter than a distance from a circumferential plane extending from a radially outermost periphery of the motor to the blades, and a third side perpendicular to the first side and the second side,

a line aligned with the second side of the airflow accelerator and projecting from an end of the airflow accelerator intersects a face formed by the casing outlet at a substantially orthogonal angle, and

the face is a plane connecting outlet ends of the tongue and the scroll at the casing outlet.

7. The fan housing of claim 6 , wherein the length of the first side of the airflow accelerator is set such that during fan operation accelerated deflecting airflow is applied to an entire blade of the plurality of blades, corresponding to a distance of the entire blade from the main plate to the retaining ring.

8. The fan of claim 1 , wherein the length of the first side of the airflow accelerator is set such that during fan operation accelerated deflecting airflow is applied to an entire blade of the plurality of blades, corresponding to a distance of the entire blade from the main plate to the retaining ring.