MECH6019 Chap.7 Particulate Collection Equipment
Particulate Collection Equipment
Gravity settlers, cyclones and electrostatic precipitators capture particles through different physical mechanisms. A gravity chamber slows gas and provides time for particles to fall. A cyclone creates rotating flow so particle inertia drives material toward the wall. An electrostatic precipitator charges particles and moves them across the gas toward collection surfaces.
Mechanism determines which particle properties and operating variables control performance.
A gravity settler favours coarse particles with useful settling velocity and offers low pressure drop, but it needs space and does little for fine material. A cyclone strengthens inertial separation and can handle robust dust duties, but pressure loss rises with the velocities used to improve capture.
Electrostatic precipitation can reach finer particles with low gas-side pressure loss, yet charging, electrical resistivity, collection area and surface condition become important.
Grade efficiency reports capture for a defined particle-size band. Overall efficiency weights the grade curve by the inlet distribution. Calculate inlet mass in each band, multiply by that band's efficiency, then add captured masses.
A simple average of percentages is valid only when the corresponding mass weights are equal. The residual distribution is usually finer than the inlet even when overall mass removal is high.
Collectors in series are handled stage by stage. The second device receives the mass and size distribution leaving the first, not the original inlet.
Penetration, one minus efficiency, can simplify repeated stages for a single size band: combined penetration is the product of stage penetrations when bypass is absent. Captured mass must also be assigned to the correct hopper or waste stream.
Selection goes beyond efficiency.
Pressure drop affects fan energy, footprint affects installation, abrasive or sticky dust affects wear and fouling, and high loading affects cleaning frequency. Temperature, moisture and electrical properties can rule out an otherwise attractive mechanism.
A useful comparison states the target size range, inlet loading, required outlet and the operating constraint most likely to control design.
Close every calculation with inlet equals captured plus emitted. Then interpret both overall mass efficiency and the remaining fine fraction. A high mass percentage can coexist with poor number capture if the uncaptured particles are small.
Report the weighting basis so performance is not detached from the health, visibility or process objective that made capture necessary.
What this chapter covers
- 01
Gravity settling chambers
- 02
Cyclone separators
- 03
Electrostatic precipitators
- 04
Dividing collectors
- 05
Grade efficiency
- 06
Overall mass efficiency
Worked example · free
Combine two grade efficiencies
- 1Capture 38 kg/h from the first band.
- 1Capture 30 kg/h from the second.
- 1Add to 68 kg/h.
- 1Divide by 100 kg/h for 68%.
Key terms
- Gravity settler
- A chamber that allows particles to fall from slow-moving gas.
- Cyclone
- An inertial collector using rotating gas motion.
- Electrostatic precipitator
- A collector using particle charging and electrical migration.
- Grade efficiency
- Capture fraction for a defined particle-size interval.
Particulate Collection Equipment FAQ
Why is one efficiency number insufficient?
The same device captures different particle sizes at different rates. Overall performance changes when the inlet size distribution changes. Grade efficiency must be weighted by inlet mass in each band, and the residual distribution should be carried forward when another collector follows.
What makes a cyclone useful?
Centrifugal inertia enhances separation compared with gravity alone, making cyclones robust for suitable dust sizes and loadings, though pressure drop matters. Gas velocity, cyclone geometry, particle density and the required fine-particle outlet decide whether that trade-off is useful for the actual stream.
How should a collector calculation begin?
Divide the inlet into size bands, multiply each band mass by its grade efficiency, then sum captured and residual mass separately. Check that the two totals close to the inlet and carry the residual distribution forward if another collection stage follows.
Exam move
Build a device-mechanism table, then practise weighted size-band balances. Always check that captured plus emitted mass equals inlet mass. Give each device one mechanism row, one favourable condition and one limiting condition. For gravity settlers, connect residence time with terminal velocity and footprint. For cyclones, connect inertial separation with gas velocity and pressure drop.
For electrostatic precipitators, connect charging and migration with resistivity, collection area and electrical operation. Then work a grade-efficiency table by calculating captured mass in each size band before adding totals. Never average percentages until the weights are shown. Carry the residual distribution forward and explain why it becomes finer after a coarse precollector.
Practise two devices in series using stage-by-stage mass balances and, for a single size band, penetration multiplication. Assign captured mass to each hopper and include any duct deposition or bypass in the boundary. Compare designs with the same inlet distribution and outlet requirement rather than isolated catalogue efficiencies. Add fan energy, maintenance, loading and temperature to the qualitative choice.
In final review, sketch a particle-size axis and place each mechanism across the range without drawing hard universal cut-offs. Finish every selection answer with the measurement plan: inlet distribution, grade efficiency or outlet distribution, flow and pressure drop. This shows whether performance is merely expected or actually verified.
