Page 175 - Root Cause Failure Analysis

Dust Collectors 163
Collection Ejiciency

Since cyclones rely on centrifugal force to separate particulates from the air or gas
stream, particle mass is the dominant factor that controls efficiency. For particulates
with high densities (e.g., ferrous oxides), cyclones can achieve 99 percent or better
removal efficiencies, regardless of particle size. Lighter particles (e.g., tow or flake)
dramatically reduce cyclone efficiency.

These devices generally are designed to meet specific pressure-drop limitations. For
ordinary installations operating at approximately atmospheric pressure, fan limita-
tions dictate a maximum allowable pressure drop corresponding to a cyclone inlet
velocity in the range of 20-70 ft per second. Consequently, cyclones usually are
designed for an inlet velocity of 50 ft per second.

Varying operating conditions change dust-collection efficiency by only a small
amount. The primary design factor that controls collection efficiency is cyclone diam-
eter. A small-diameter unit operating at a fixed pressure drop has a higher efficiency
than a large-diameter unit. Reducing the gas-outlet duct diameter also increases the
collection efficiency.

Installation

As in any other pneumatic-conveyor system, special attention must be given to the
piping or ductwork used to convey the dust-laden air or gas. The inside surfaces must
be smooth and free of protrusions that affect the flow pattern. All bends should be
gradual and provide a laminar-flow path for the gas. See the appropriate section in
Chapter 9 for specific installation information on pneumatic conveyors.

Cyclones are designed for continuous operation and must be protected from plugging.
In intermittent applications, the operating practices must include specific steps to
purge the entire system of particulates prior to shutdown.

Pressure drop is the only factor that can be effectively controlled by an operator.
Using the fan dampers, the operator can increase or decrease the cyclone’s load by
varying the velocity of the entering dirty air.