Capturing Fugitive Dust by Using Bag Filter

In various industries such as cement plants, ore / stone crushing units, mining industries, dust is generated and thrown to general atmosphere. To control the fugitive dust generated, a common arrangement bag filter (fabric)  type dust collector is used. A dust bearing gas is intercepted by a permeable fabric in such a manner that the gas passes through the fabric whilst the dust particles are restrained on the fabric. The separation of particles happens on the external surface of filters.

 

The bag filter system, often called bag house, has three sections.

  • Plenum/ Clean air chamber
  • Housing
  • Hopper

Other related components, for operating a bag filter type dust collector, are

  • Fan
  • Compressed air (for jet pulse)
  • Rotary air lock

Figure 1: Basic components of a bag filter

Working principle of bag house

Fugitive dusts from various locations are drawn into bag house through duct lines. The chamber where the dust bearing common duct enters is called dusty chamber and the (dust free) clean air chamber is called as clean chamber. The clean air is pulled out by draft generating fan (induced draft fan). The dust retained in the outer surface of filter bag is pulsed by compressed air (Jet Pulse Type Bag Filter) from inside and the disposed dust material is passed through rotary air lock system where dust can pass and entry of air is restricted. The efficacy of the system is largely governed by suction capacity, dimension and layout of ducts and engineering thereof, type of filter cloths, cleaning mechanism, and location of the installation which may vary from plant to plant. Some common operational control factors are as follows –

a)      Suction pressure at de-dusting point is controlled by using damper in the de-dusting line. If the suction pressure is high, it may collect process materials along with fugitive dust which results material loss. If the suction pressure is less, it will not capture fugitive dust properly resulting dust emission nuisance at workplace.

b)      The jet pulse pressure should be well maintained. If the compressed air pressure is high enough, it may cause filter bag bursting, on the other hand if the pressure is low, it will not dislodge the dust from filter bag surface properly which can cause clogging of filter bag soon.

c)      Creating sufficient draft by induced fan (the requirement is estimated by air to cloth ratio)

Figure 2: Cross Section of Woven and Felted Filter Fabric

Bag filter system sizing
Air to cloth ration

Air to cloth ration, or filtration velocity, is the value of the gas flow rate divided by the area of filter cloth surface through which the gas passes.

The surface area of a single filter bag = a, m2

Total no. of filter bag = n

Total filtration area = (n x a), m2

Air volume rate drawn by fan = b, m3/min

Air to cloth ratio = b/ (n x a), m/min [m3/min/m2]

Selection of Fabric

Fibers used in fabric manufacture for filtration are almost exclusively synthetic and they are either woven or needle felted (Figure 2). Woven fabrics are smoother, easier to clean than the needle felts and at low load condition hardly any cleaning device is needed due to the self-cleaning property of the fabric. Woven fabrics are not suitable for vigorous cleaning as this would break down the entire dust cake and force the dust between the fibers so that the dust emission would be high. Needle felts are less permeable than woven fabrics, but they can be operated at considerably higher filtration velocities. The pores in needle felts are very small compared to the woven fabrics, so dust penetration is low.

Generally, the filter elements, whether of woven or felted fabric, are cylindrical, but some manufacturers have adopted flat panel, or envelope elements.

Troubleshooting

Any sort of problems with bag filter ultimately leads to visible dust emission either through stack of the bag house or at fugitive dust collecting point itself. However, the common problems with bag filters can be identified as broken filter bags, blockage of filter bags, leakage or gap in the area where filter bag hanged, etc. High pressure of jet pulse air, sharp edge of metallic cage can cause breaking of filter bag. Once the filter bag is burst, the manometer reading (differential pressure between clean chamber and dusty chamber) will show lower value. On the other hand, blockage of filter fabric shows high differential value. Hopper full with disposed dust blocking the common inlet duct can cause practically no suction in the de-dusting lines to collect fugitive dust from collecting points, hence allowing fugitive dust emission from areas of collecting points. So, time setting of jet pulse and its pressure should be optimized, the ribs of metallic cages should be smooth to avoid bursting of filter bags. Hopper level should be monitored periodically to avoid blocking the common inlet de-dusting line.
 

 

To cite this article, please use following information:

(use the given format or any standard citation format)

Mohiuddin, M., Capturing Fugitive Dust Using Bag Filter, ChE Thoughts 2 (1), 13-15, 2011.

 

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