Multiblade fan

1. A multi-blade fan comprising:

a spiral shaped housing including a bell-mouth orifice at one side, a sucking inlet and an exhausting outlet;

an impeller disposed in the housing and including a plurality of blades supported by a main plate and a lateral plate at both axial ends of each one of the blades, said lateral plate located closer to the bell-mouth orifice than said main plate; and

a motor for driving the impeller,

wherein a cross section, cut along vertically with respect to a rotary shaft within a first given length, of each one of the blades has a given shape which allows a main air stream to flow along a back face of each one of the blades from a leading edge to a trailing edge of each one of the blades,

wherein an axial length of each of the blades comprises a thin-walled section of a second given length and a thick-walled section of a third given length, said thick-walled section extending from said thin-walled section at a transition section so as to reach the trailing edge, the second given length being smaller than the third given length, a thickness of the thin-walled section being smaller than a thickness of the thick-walled section, said transition section oriented substantially perpendicular to the axial length of the blade.

2. The multi-blade fan of claim 1 , wherein the first given length is an entire axial length of each one of the blades.

3. The multi-blade fan of claim 1 , wherein the given shape includes a change such that the thin-walled section of the back face changes over to the thick-walled section from the leading edge to the trailing edge of each one of the blades.

4. The multi-blade fan of claim 3 , wherein the second given length of the thin-walled section is not shorter than 1/20 and not greater than ⅓ of a chord length.

5. The multi-blade fan of claim 3 , wherein a the thickness of the thin-walled section is not smaller than 1/10 and not greater than ½ of the thickness of the thick-walled section.

6. The multi-blade fan of claim 3 , wherein a junction between the thin-walled section and the thick-walled section shapes like an arc, of which length is not shorter than 1/20 and not longer than 1/10 of a chord length.

7. The multi-blade fan of claim 3 , wherein a thickness of the thick-walled section becomes thinner gradually toward the trailing edge.

8. The multi-blade fan of claim 3 , wherein the main plate has a circumference smaller than a circumference of the trailing edge of the plurality of blades of the impeller.

9. The multi-blade fan of claim 3 , wherein the change comprises a junction between the thin-walled section and the thick-walled section, said junction being substantially orthogonal to the transition section.

10. The multi-blade fan of claim 1 , wherein the first given length is not shorter than ⅓ and not greater than ⅔ of the axial length of each one of the blades extending from the main plate.

11. The multi-blade fan of claim 1 , wherein the first given length is not shorter than ⅓ and not greater than ⅔ of the axial length of each one of the blades extending from the lateral plate.

12. The multi-blade fan of claim 1 , wherein the orifice is formed of a first orifice nearer to the impeller and a second orifice, and the two orifices are disposed with a given interval in between for cutting off sound.

13. The multi-blade fan of claim 12 , wherein the first orifice has an inner diameter not shorter than 8/10 and not greater than 1/1 of an inner diameter of the impeller.

14. The multi-blade fan of claim 12 , wherein the given interval is not shorter than 1/10 and not longer than ½ of an inner diameter of the first orifice.

15. The multi-blade fan of claim 12 , wherein the second orifice has a contour identical to a contour of the first orifice.

16. The multi-blade fan of claim 12 , wherein the second orifice has an inner diameter not smaller than ⅔ and not greater than 1/1 of an inner diameter of the first orifice.