In a common design of pneumatic conveying systems the materials being conveyed and the conveying gas together pass through a materials handling fan located in a pneumatic conveying line between the material pick-up or feed point and the material discharge point.
Fans which only handle gases normally have impeller blades attached to a circular front and rear shroudplate. In such a fan gas passes from the inlet opening at the center of the impeller radially outwards in the constricted paths defined by the blades and the front and rear shroudplates to the periphery of the impeller where it leaves the fan casing via an exit opening.
In designing a fan handling only gases or in a materials handling fan the principal design objective is to get a specific throughput. That throughput is a function of pressure difference across the fan and volume of gas passing through the fan. These parameters, in turn, are a function of fan blade tip speed and blade width, respectively. Blade tip speed is, of course, a function of the blades' radius and rotational speed. Thus, for a given rotational speed the pressure difference across the fan can be increased by increasing the fan blade length or impeller radius. Similarly, to increase volume through the fan the width of the fan blade can be increased. Of course, as these dimensions increase the stresses on the impeller increase dramatically for the reasons described in more detail below.
The double shroud design of the previously described gas handling fan is not suitable for use as a materials handling fan because the conveyed material cannot readily pass through the constricted path defined by the blades and the front and rear shroudplates.
The problems associated with pneumatically conveying materials through a fan with a double shroudplate impeller have led to the design of materials handling fans with impellers having radial blades shroudplate. These open impellers are not as efficient as the double shrouded gas handling impellers because of the gas turbulence that takes places between the stationary fan casing and the unshrouded side or sides of the radial impeller blades. The materials handling fan impellers with one shroudplate are substantially more efficient than the impellers with no shroudplate and the single shroud design is therefore most favored and is almost always used in the larger diameter, higher rotational speed fans used in systems requiring a substantial generation of operating gas pressure.
This invention deals only with the more efficient materials handling fan impeller using a single shroudplate or backplate.
The single shroudplate radially bladed materials handling impeller design is subject to unusually high stresses near the hub on the open or materials feeding side of the impeller because of its asymmetric design. During rotation the centrifugal force acting on the impeller blades and the shroudplate causes stresses and strains in both these components. At the same rotational speed the strain on the impeller blade alone will be substantially greater than the strain on the shroudplate of equal diameter because of the constraint of the circumferential ligaments in the circular shroudplate. As a specific example the radius of a 20" radius .times. 3/8 thick steel disk used as a shroudplate rotated about its central axis at 3600 rpm will increase approximately 0.005 inches, whereas the radius of a 20" long .times. 3/8 thick radial blade rotated about one end at 3600 rpm will increase approximately .009 inches.
When a materials handling fan impeller, made up of a multiplicity of radial blades welded or otherwise affixed to a single circular shroudplate, is rotated about its central axis, the greater strain or radial growth of the radial blades as compared to the strain or radial growth of the shroudplate will cause the impeller to distort so that the impeller, when viewed from the radial blade side, will be convex with the highest stresses being present on the non-shroud side of the impeller closest to the impeller hub.
These high stress levels, compared to the stress levels in a similar sized double shroud impeller operated at the same speed, have dramatically limited the diameter and operating speed of commercial materials handling fan rotors with a single shroudplate and hence their performance capabilities.
Sophisticated designs of materials handling fan impellers of large diameters for high rotational speeds which incorporated tapered blades and back plates, large specially shaped hubs, etc., have been developed which have the materials of construction strategically located so that the large centrifugal forces present during rotation generate acceptably low and safe levels of stress. These impellers require either extensive and expensive machining in order to distribute the materials satisfactorily or, alternatively, large and expensive castings.
It would therefore be desirable if an impeller could be constructed which is basically adaptable for use with fans of varying diameters. Absent careful design of such impellers, however, high failure rates for fans of larger diameters are likely because of the inability of the impeller to withstand stress at the impeller hub.
A background patent of general interest in this area is U.S. Pat. No. 4,285,635 which discloses a centrifugal-blower impeller -- not a materials handling fan -- having a center part and a cicumferential outer part of lesser thickness than the center part. The outer part consists of ring segments welded to each other and the periphery of the center part to create compressive forces in the center part.