Question

() (ii) Explain how both a cyclone and an electrostatic precipitator work, and then, giving your reasons, rationalise why, out of the two technologies, you would recommend using electrostatic precipitation to be used for extracting dust during laser cutting of plastics (6 marks) The efficiency (E) of an electrostatic precipitator with plate collectors is given by: E = 1 - exp(-AW/O) where: A is the area of the plates (in m?) Q is the gas flow rate (in m/s) W is the particle drift velocity in an electric field (in m/s) which can be estimated as being numerically equal to one- twentieth of the average particle diameter (in microns). If the laser cutting of a plastic produces PM2s particles with a concentration of 12 g/m in an air flow rate from the production line of 8 m'is estimate both the plate area required to achieve 90% efficiency in removing the particles from the air stream, and the daily particulate mass removed by the electrostatic precipitator. (4 marks) END OF ASSESSMENT o

Answer 1

Working of cyclone separator

Cyclone separator is used for separation of solid from solid gas mixture using centrifugal force and interia. In this technique the mixture is fed to a conical veritical cyliner with high velocity such tha it forms a vortex inside and due to greator inertia of solid particles this centrifugal force throws particles towards chamber periphery and gas comes out from top and hence mixture is spearated

Working of electrostatic precipitator

In electrostatic precipitator electric field is used to separate fine particles from gas mixture by application of electric field. In this device the gas is passed between two electricllay charged plates maintained at high voltage such that when particles come inside this filed they get polarised hence attract and deposit on plates based on charge they aquire thus get separated from mixture

In laser cutting of plastic very fine powder dust is generated which cannot be separated by cyclone due to same inertia of mixture but can be separated by electrifying in ESP as charge can be induced easily to particles of plastic without ionising air.

For Q=8 m3/s

n=0.9

w=1/20*2.5*10^-6m= 1.25*10^-7m

using in given equation

exp(-AW/Q)=0.1

AW/Q=2.3

A=(2.3*8)/(1.25*10^-7=1.472*10⁸ m²