It is the usual practice to strengthen with angle iron or flat bars, the large areas of metal forming the sides of centrifugal fan casings. This prevents the “drumming” of the relatively thin sheet. The areas of sheet metal or plate so formed may be treated as rectangles of sides a and b subjected to a uniform pressure and supported around its perimeter. Then the maximum bending stress f will be:
= thickness (m)
= pressure (Pa)
A & b = dimensions (m)
F = stress (Pa)
The circumferential surfaces and also the casings of axial flow and other in-line fans may be considered as thin cylinders.
The direct stress will be due to the force p-2 r I across the resisting section of area 2■ t ■ I. I being the length of the casing. Thus the direct stress will be:
P • 2 • r • I _ pr
2 M “7
Where an electric motor must be supported in an axial flow fan casing, this will often be suspended by tie rods or brackets, for which additional load the casing must be designed.
Posted in Fans Ventilation A Practical Guide