Aspect Ratio

The aspect ratio, which relates the length of an ESP to its height, is an important factor in
reducing rapping loss (dust reentrainment).When particles are rapped from the electrodes,
the gas flow carries the collected dust forward through the ESP until the dust reaches the
hopper. Although the amount of time it takes for rapped particles to settle in the hoppers is
short (a matter of seconds), a large amount of "collected dust" can be reentrained in the gas
flow and carried out of the ESP if the total effective length of the plates in the ESP is small
compared to their effective height. For example, the time required for dust to fall from the
top of a 9.1-m plate (30-ft plate) is several seconds. Effective plate lengths must be at least
10.7 to 12.2 m (35 to 40 ft) to prevent a large amount of "collected dust" from being carried
out of the ESP before reaching the hopper.
The aspect ratio is the ratio of the effective length to the effective height of the collector
surface. The aspect ratio can be calculated using Equation

The effective length of the collection surface is the sum of the plate lengths in each consecutive
field and the effective height is the height of the plates. For example, if an ESP
has four fields, each containing plates that are 10 feet long, the effective length is 40 feet.
If the height of each plate is 30 feet, the aspect ratio is 1.33 as shown below:

Aspect ratios for ESPs range from 0.5 to 2.0. However, for high-efficiency ESPs (those
having collection efficiencies of > 99%), the aspect ratio should be greater than 1.0 (usually
1.0 to 1.5) and in some installations may approach 2.0.