Centrifugal fan

Abstract

A centrifugal fan includes an impeller arranged to rotate about a rotation axis, a motor arranged to rotate the impeller, and a case arranged to accommodate the impeller and the motor. The case includes an upper case, a lower case, and a joint portion at which the upper and lower cases are fixed to each other. An interior of the case includes a first annular space positioned radially outward of the motor. The first annular space includes a second annular space defined by a portion of a space defined by an annularly continuous collection of imaginary circles each of whose diameter is the shortest distance between an inner circumferential edge of the joint portion and a radially outermost circumferential portion of a motor accommodating portion and each of which touches the inner circumferential edge of the joint portion.

What is claimed is: 
     
         1 . A centrifugal fan comprising:
 an impeller arranged to rotate about a rotation axis;   a motor arranged to rotate the impeller; and   a case arranged to accommodate the impeller and the motor; wherein   the case includes:
 an upper case; 
 a lower case including a motor accommodating portion arranged to cover an outer circumference of the motor; and 
 a joint portion at which the upper and lower cases are fixed to each other; 
   an interior of the case includes a first annular space positioned radially outward of the motor;   the first annular space includes a second annular space defined by a portion of a space defined by an annularly continuous collection of imaginary circles each of whose diameter is a shortest distance between an inner circumferential edge of the joint portion and a radially outermost circumferential portion of the motor accommodating portion and each of which touches the inner circumferential edge of the joint portion, the portion of the space being defined by a portion of an inner wall of the upper case and a portion of an inner wall of the lower case; and   a portion of the impeller is arranged in the second annular space.   
     
     
         2 . The centrifugal fan according to  claim 1 , wherein the first annular space further includes a space positioned axially below the second annular space. 
     
     
         3 . The centrifugal fan according to  claim 1 , wherein
 the impeller includes:
 a hub having a substantially circular plate shape, and joined to a shaft arranged to rotate about the rotation axis; 
 a plurality of first blades arranged axially above the hub; and 
 a plurality of second blades arranged axially below the hub; and 
   the second blades are arranged in the second annular space.   
     
     
         4 . The centrifugal fan according to  claim 1 , wherein
 the impeller includes:
 a hub having a substantially circular plate shape, and joined to a shaft arranged to rotate about the rotation axis; 
 a plurality of first blades arranged axially above the hub; and 
 a plurality of second blades each of which is defined by a cut away portion of an outer circumferential edge of the hub radially inward; and 
   the second blades are arranged in the second annular space.   
     
     
         5 . The centrifugal fan according to  claim 3 , wherein a peripheral portion of the hub is arranged to project radially outward relative to a peripheral portion of each first blade. 
     
     
         6 . The centrifugal fan according to  claim 3 , wherein
 each first blade is arranged to extend axially downward and radially outward away from the rotation axis; and   a portion of an inner wall of the case which covers each first blade includes an inner wall surface arranged to extend along an axially upper end portion of each first blade.   
     
     
         7 . The centrifugal fan according to  claim 1 , wherein a portion of an inner wall of the case which covers the second annular space includes a curved surface in a vertical section. 
     
     
         8 . The centrifugal fan according to  claim 3 , wherein the second blades are arranged radially inward within the second annular space. 
     
     
         9 . The centrifugal fan according to  claim 1 , wherein
 the impeller includes:
 a hub joined to a shaft arranged to rotate about the rotation axis, and arranged to obliquely extend axially downward and radially outward from the shaft; and 
 a plurality of first blades arranged axially above the hub; and 
   a portion of each first blade is arranged in the second annular space.   
     
     
         10 . The centrifugal fan according to  claim 9 , further comprising an annular shroud arranged on an axially upper end portion of each first blade, wherein the shroud is positioned axially over the second annular space.