10-10-2012, 05:55 PM
BASIC GUIDELINES FOR PLASTIC CONVERSION OF METAL AXIAL FLOW FANS
BASIC GUIDELINES FOR PLASTIC CONVERSION.pdf (Size: 935.68 KB / Downloads: 369)
INTRODUCTION
This guideline outlines in brief the basic steps recommended for the development of a plastic conversion of
a metal fan. It is limited with respect to axial flow type fans, and does not necessarily address a single
classification within that family. The field of fan design is quite extensive and complex, it is therefore
impossible to address all aspects of axial fan design within the scope of this paper. It is suggested that these
rules be utilized in general sense as a starting point in the development process especially when initial
geometric data is lacking. It is also essential to integrate a testing program throughout the different
development stages, to evaluate the performance of the various basic design changes and their impact on
achieving the desired outcome.
DEVELOPMENT GUIDELINES
Axial Flow Fans
Axial flow fans, while incapable of developing high pressures, they are well suitable for handling large
volumes of air at relatively low pressures. In general, they are low in cost and possess good efficiency,
most have a large hug and can have blades of airfoil shape. The blades are usually not close together, they
can be made in many forms, but the most effective have airfoil sections. Angle change and twist are given
to the blade at various positions outward from the hub to tip. Usually inlet guide vanes are used to align
and direct the air into the fan blades, which in turn impart energy to the incoming air.
Axial flow fans show good efficiencies, and can operate at high static pressures if such operation is
necessary. The fan can be so designed that the horsepower is flat and non-overloading. The swirl imparted
to the air by the fan blades can be eliminated by the guide vanes on the inlet side and, in some designs, on
the outlet side as well.
Concepts of Fan Pressures
The flow of air between two zones is due to a pressure difference between the two zones. This pressure
difference forces the air to flow from the high-pressure zone to the low-pressure zone. The flow of air
through a system requires energy to overcome any static pressure at the entry or outlet of the system. The
fan provides this energy by the increase in total pressure from the fan’s inlet to the fan’s outlet.
Blade Angle or Twist, and Velocity Distribution
Air flows through an axial-flow fan in “an approximately axial direction”. On the inlet side, the direction
of the flow is axial, i.e., parallel to the axis of rotation. The fan blade then deflects the airflow. Past the
blades as shown in figure 4, therefore, the pattern of the deflected air flow is of helical shape, and the air
velocity can be resolved into two components: an axial velocity and a tangential or circumferential
velocity. The axial velocity is a useful components, the tangential or circumferential velocity component
can be a partial or total energy loss.
For good efficiency, the airflow of an axial-flow fan should be evenly distributed over the working face of
the fan wheel, which means that the axial air velocity should be the same from hub to tip. The velocity of
the rotating blade, on the other hand, is far from evenly distributed: it is low near the center and increases
toward the tip. This gradient should be compensated by a twist in the blade, resulting in larger blade angles
near the center and smaller blade angles toward the tip. At high static pressures, the blade twist is
important, because without it, the inner portion of the blade will stall and permit reversed airflow, which, of
course, will seriously affect the fan efficiency.