The particulate removal mechanism in the wet scrubbers Essay

The power required for ID fan can be computed by using the Equation no. 6.4.
Power Calculation of ID fan
Power input at fan shaft = power drawn by motor motor efficiency ..(6.4)
Fan Static efficiency .(6.5)
i.e. Combining (6.4) and (6.5)
Fan static efficiency
Pmotor = Power drawn by motor, Kilowatt, KW
Q = Flow rate, m3/sec.
Pressure drop across bag house, mm WC
Motor efficiency (8090%)
Efficiency of fan (7080%)
6.12.7. Wet Scrubbers
The particulate removal mechanism in the wet scrubbers is the extension of the natural process of atmosphere cleansing by precipitation  (i.e. rain). The basic requirement of wet scrubbers is the establishment of contact between the particulate to be removed and the scrubbing liquid, which usually is water. All particulate wet scrubber designs utilize particle and/ or droplet inertia as the fundamental force to transfer particles from the gas stream to the liquid stream. Within the scrubber, the particleladen gas is forced to contact the liquid droplets, sheets of liquid on a packing media, or jets of liquid depending on the type of scrubber used. The particles with too much inertia impact on the water droplets (as in a spray chamber), water sheet (as in a packedbed scrubber) or water jet (as in a venturi scrubber). Irrespective of the particle contact system adopted, the basic collection mechanisms are the same as in fabric filters (as discussed in the preceding sections) i.e. (i) inertial impaction, (ii) interception and (iii) diffusion.
Generally, inertial impaction and interception are the predominant mechanisms for particles of diameter above 0.3 m and for particles below 0.3 m, diffusion begins to prevail. Other mechanisms which can also influence the particulate removal are diffusiophoretic, electro static charge induced on to the droplet during atomization and agglomeration. However, their influence is practically insignificant as compared to impaction and interception.
The ability of wet scrubbers to remove particles depends on two or more of the following variables
a. The size (aerodynamic diameter) of the particle
b. The velocity of the particle  and
c. The velocity of the droplet, sheet or jet.
Collection efficiencies for wet scrubbers vary with the particle size distribution of the waste gas stream. In general, collection efficiency decreases as the PM size decreases.
Performance of Wet scrubbers
The parameters affecting the overall performance of a wet scrubber are
(i) Particle size distribution and loading
(ii) Waste gas flow rate, temperature and humidity
(iii) Gas velocity and pressure drop
(iv) Liquidtogas [L/G] ratios
(v) Droplet size and
(vi) Residence time.
The detailed discussion of each parameter is beyond the scope of this book.
6.12.7.1. Types of Wetscrubbers
There are various types of scrubbers depending on the method of establishing the contact between the particles and the scrubbing liquid. Various types of scrubbers, which are used widely, are shown in Figure 6.23 and are listed here under
(a) Spray towers or Spray chambers
(i) Countercurrent flow
(ii) Crossflow
(b) Centrifugal or Cyclone spray chambers
(c) Packed beds and Plate columns