Page 109 - Root Cause Failure Analysis

100 Root Cause Failure Analysis
The limitations of backward-curved blades are

Weighs more and occupies considerably more space than other designs of
equal volume and pressure.
Large wheel width.
Not to be used in dusty environments or where sticky or stringy materials
are used because residues adhering to the blade surface cause imbalance
and eventual bearing failure.

Forward-Curved Blades This design is commonly referred to as a squirrel-cage
fun. The unit has a wheel with a large number of wide, shallow blades; a very large
intake area relative to the wheel diameter; and a relatively slow operational speed. The
advantages of forward-curved blades include

Excellent for any volume at low to moderate static pressure using clean
air.
Occupies approximately same space as backward-curved blade fan.
More efficient and much quieter during operation than propeller fans for
static pressures above approximately 1 in. of water (gauge).

The limitations of forward-curved blades include

Not as efficient as backward-curved blade fans.
Should not be used in dusty environments or handle sticky or stringy mate-
rials that could adhere to the blade surface.
BHP increases as this fan approaches maximum volume as opposed to
backward-curved blade centrifugal fans, which experience a decrease in
BHP as they approach maximum volume.

Radial Blades Industrial exhaust fans fall into this category. The design is rugged
and may be belt driven or driven directly by a motor. The blade shape varies consider-
ably from flat to various bent configurations to increase efficiency slightly or to suit
particular applications. The advantages of radial-blade fans include

Best suited for severe duty, especially when fitted with flat radial blades.
Simple construction that lends itself to easy field maintenance.
Highly versatile industrial fan that can be used in extremely dusty environ-
ments as well as clean air.
Appropriate for high-temperature service.
Handles corrosive or abrasive materials.

The limitations of radial-blade fans include
Lowest efficiency in centrifugal-fan group.
Highest sound level in centrifugal-fan group.
BHP increases as fan approaches maximum volume.