Page 174 - Root Cause Failure Analysis

162 Root Cause Failure Analysis

Flow Pattern
The path the gas takes in a cyclone is through a double vortex that spirals the gas
downward at the outside and upward at the inside. When the gas enters the cyclone,
the tangential component of its velocity, V,,, increases with the decreasing radius as
expressed by

In this equation, r is the cyclone radius and n is dependent on the coefficient of fric-
tion. Theoretically, in the absence of wall friction, n should equal 1.0. Actual mea-
surements, however, indicate that n ranges from 0.5 to 0.7 over a large portion of the
cyclone radius. The spiral velocity in a cyclone may reach a value several times the
average inlet-gas velocity.

Pressure Drop
The pressure drop and the friction loss through a cyclone are most conveniently
expressed in terms of the velocity head based on the immediate inlet area. The inlet
velocity head, h,,, which is expressed in inches of water, is related to the average
inlet-gas velocity and density by

h,, = 0.0030r V,'
where

h,,, = inlet-velocity head (in. of water);
r = gas density (Ib/ft');
V, = average inlet-gas velocity (ft/sec).

The cyclone friction loss, F,,, is a direct measure of the static pressure and power that
a fan must develop. It is related to the pressure drop by

where

F,,, = friction loss (inlet-velocity heads);
Ap,, = pressure drop through the cyclone (inlet-velocity heads);
A, = area of the cyclone (ft2);
D, = diameter of the gas exit (ft).

The friction loss through cyclones may range from 1 to 20 inlet-velocity heads,
depending on its geometric proportions. For a cyclone of specific geometric propor-
tions, F,, and Ap,,? essentially are constant and independent of the actual cyclone size.