Topic 5 of 5
Discharge hazards and particle collection
Accumulated charge can lead to an unwanted spark. Controlled charging can also give suspended particles an electric force that removes them from a gas stream.
Electrons carry negative charge. A conducting Earth connection can accept or supply electrons, and an electric-field arrow gives force direction for a positive test charge.
Explain how the hazard develops
Contact and separation between materials can separate charge. If the charge cannot flow away, it can accumulate and produce a strong electric field across an air gap. If the air becomes conducting, a rapid discharge through it can produce a spark.
- Flammable vapour during liquid handling
- Flow and contact between liquid and surfaces can separate charge. A discharge spark can ignite a suitable flammable vapour-air mixture. The dangerous combination is an ignition source and an ignitable mixture, not merely the existence of any charged object.
- Combustible dust in air
- Particle movement and contact can lead to charge accumulation. A spark can ignite a suspended cloud when the combustible dust and air form an ignitable mixture.
A suitable conducting path to Earth allows excess charge to flow away and can reduce accumulation. Connecting conductors together, called bonding, allows charge to redistribute between them and reduces the conditions for a spark between them. These connections must actually conduct; an insulating coating or broken connection can leave the intended path unavailable.
This reasoning is different from induction's deliberate isolation step. Induction disconnects a path at a chosen stage to retain charge; reducing a static hazard often requires a path that prevents charge from building up.
Collect particles with an electrostatic precipitator
A dry electrostatic precipitator removes suspended dust or smoke particles from a gas stream. The model here uses negative discharge wires between earthed collecting plates.
A negative dust particle is forced towards a collecting plate
This is a cross-sectional force model of a dry wire-and-plate precipitator. A representative negative discharge wire lies between two earthed collecting surfaces.
Charged gas particles transfer negative charge to suspended dust. The electric force drives it towards a collecting surface while gas continues through. A dust grain does not have to touch a wire to become charged.
Later, mechanical shaking removes the deposited dust into a hopper. This is particle removal; it does not remove every gaseous pollutant.
- Charge the suspended particles. The strong field near the discharge wires ionises gas, producing charged particles. Electrons can attach to gas molecules, forming negative ions that transfer negative charge to dust as they collide with it. Dust grains need not touch a wire to become charged.
- Exert a sideways electric force. In the space between the negative wires and earthed collectors, negatively charged dust experiences force towards the collecting plates, opposite the local field direction.
- Deposit the particles. Dust collects on the plates while gas continues through the device with fewer suspended particles.
- Remove the deposit. Periodic mechanical shaking dislodges the collected material into a hopper, allowing collection to continue.
Earthing a plate does not make it electrically inactive. The connection allows charge to redistribute between the conducting collector and Earth. Its facing surface can attract negative dust in the field produced by the negative wires. This model does not require a separate positive supply attached to the collecting plates.
The collected particles can move across the gas flow because an electric force acts on them. Removing particles does not mean that every gaseous pollutant has been removed, or that every particle is captured.
Use the same reasoning in a different separator
Application to a supplied design
Particles enter upwards through a leftward field
For the supplied field in the separator example, positive particles have force left and negative particles force right. Side collectors can therefore receive different charge signs.
The design reasoning is: identify the charge sign, read the field direction, predict the force, then consider how it changes the existing motion.
Explain charging and collection separately. The gas first transfers charge to the dust; the field then exerts a force on that charged dust. Neither step is explained by saying only that the particles are attracted to a plate.