Gas Distribution Criteria for Electrostatic Precipitators (ESP)

  1. Gas distribution tests are performed after the first (A) field of the ESP. For specific cases the gas distribution can be measured after the ESP’s last field.
  2. Gas distribution is typically measured from every second gas passage starting from the very first passage next to the casing wall. If there are even number of gas passages in the field, then also the last passage next to the casing wall must be measured.
  3. The measurement points are in the middle of each gas passage at the rear edge of the collecting plate rows.
  4. The highest and lowest measurement points are 500mm from the top and bottom of the collecting plate vertical ends.
  5. The number of vertical measurement points depend on the ESP height, but the distance between each point is typically 10% of the effective height.
  6. Before starting the measurements colored smoke is used to determine any possible gas sneakage areas especially on the top part of the field as well as below the field. Smoke test will also show whether the gas flows in correct direction as it is intended to.
  7. If the calculated average gas volume deviates from the design value more than +/- 10% the measurement should be rejected and performed again with correct flow rate.
  8. As a result of the measurements a Coefficient of Variation (CV factor) is calculated. If the CV factor remain below 20 the distribution can usually be accepted, however the aim being for 15.
  9. Bigger individual deviations can be accepted on case-by-case basis especially if there are larger internal casing columns right behind a measurement point.
  10. Any individual measured point with deviation of more than +/- 25% of the average will lead to rejecting the measurement and to corrective actions in the gas distribution screens.
  11. If the gas distribution is measured after the last field of the ESP the flow pattern is preferred to have higher velocities in the middle and top part of the field, and considerably lower velocities at the bottom part of the field in order to prevent fly-ash re-entrainment