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Systems for Dust Producing Applications
Arunava Das, Chairman, Comtech Synthesis Pvt Ltd & Managing Director, Synthesis EPC Projects Ltd Increase in pollution caused by rapid urbanisation and industrialisation has drawn the attention of environmental activists globally, which has compelled and environment regulatory bodies to lay stringent policies to monitor the emissions to curb the greenhouse effect in order to address the issue of global warming. Arunava Das answers some of the questions regarding the efficiency of baghouse filters and use of cyclone separators to separate particulate matter from air, gas or liquid streams.

Particulate Management Systems, generally referred to as dust collectors are used in industrial and commercial processes to collect dust and other impurities from air or gas. These are designed to handle heavy dust loads. A typical dust collector system consists of a blower, dust filter, a filter cleaning system and a dust receptacle or dust removal system. It can be distinguished from air cleaners which use disposable filters to remove dust. Five principal types of dust collectors are inertial separators, fabric filters, wet scrubbers, electrostatic precipitators and unit collectors. Synthesis specializes in cyclone separators (inertial), fabric filters and wet scrubbers. The selection criteria of the Air Pollution Control (APC) equipment is based on various parameters which include product or dust characteristics like the particulate size, particle density, static charge and hygroscopic nature, etc. Fabric filters commonly known as bag house filters are designed around the use of engineered fabric, filter tubes, envelopes or cartridges in the dust capturing, separation or filtering process. The fact that the system can be engineered for almost any dust producing application under almost any set of circumstances puts them into a totally different class. Because of the wide range of available fabric filter media, the baghouse has proven that it will remain a prime player in the worldwide quest for cleaner air.

Baghouses can be found in virtually all sorts of industries from foundry and steel operations to pharmaceuticals, food and chemical producers in nearly every country in the world. The range is staggering. One of the major merits of baghouse filters is that they can come in any size to suit an available location or process and are able to work in almost any reasonably dry dusty atmosphere. This gives the system a lot of flexibility. Another remarkable quality is that there are many retrofits available for almost any make or model and their robustness allows them to withstand high physical abuse and operate within design specifications. Apart from this, a large array of filter media available to suit every type of process also gives convenience to the design. The only setbacks are the operating temperatures and the chemical as well as weather conditions. Overhaul costs is relatively optimised if the machine is serviced on a regular basis. Although there are many apprehensions related to design and operation of bagfilters, the interpretations depend on everything from availability of the space to the dust drop required from the chamber which in turn is a subset of the particle size.

Breaking the Myth
It is a myth that baghouses cannot render high dust removal efficiency. The simple reason that this misconception still prevails is that most baghouses actually do not give high performance efficiency. This may sound paradoxical, but there are ample of reasons behind this notion which prevails, most important is the mismanagement of dust cake. The dust accumulation on the surface of the bag is usually the most dynamic part of the system.

One can manage the baghouse performance through proper management of dust cake. The dust particles, gas stream and media in a system will produce a characteristic dust cake “profile” for a particular process. Large dust particles create a barrier that can capture the incoming fine particles; however, it is very difficult to deal with the fine particles of size 1 micron in diameter or lesser. Most “conventional” fabrics have difficulty trapping these “fines”. The dust cake accumulation on the bag will many times reflect the issues or problems within the system. However, a managed dust cake, proper surface treatment on the filter media, intended to act as a barrier and a dust cake builder (filter aid) which can be introduced into the system gas stream will take care of the given problem. Unfortunately, the assistance of the said aids are either incorrectly used or altogether ignored. Synthesis is among those few that implements and puts these assistances to good use. Apart from this, the style of bag cleaning system will reflect how the cake is managed. The down time heating of the unit also plays an important role in maintaining the efficiency of baghouses, especially, during the monsoon.

Fabric Filters – A Problem Area
Not all problems in a baghouse are related dust cake media. Another aspect that governs the performance of baghouses is the fabric filters. There are two basic approaches to utilize a fabric fitter. One is the engineered media approach and the other being off the shelf method, that is the price approach. Synthesis seeks specific fabrics for specific applications (engineered media approach) like for surface filtration, depth filtration, high cloth compact cartridge filtration and so on so forth. Also the larger the dust drop out area for dust particles before the filter media, the less stress there usually is on the filtering process.

Separating Particulates from Air, Gas or Liquid Streams
Cyclone separator is another widely used technique to remove particulates from an air, gas or liquid stream which utilises vortex separation instead of using filters. Rotational effects and gravity are used to separate mixtures of solids and fluids. The method can also be used to separate fine droplets of liquid from gaseous stream. A high speed rotating (air) flow is established within a cylindrical or conical container called a cyclone. Air flows in a spiral pattern, beginning at the top (wide end) of the cyclone and ending at the bottom (narrow) end before exiting the cyclone in a straight stream through the center of the cyclone and out the top. Larger (denser) particles in the rotating stream have too much inertia to follow the tight curve of the stream, and strike the outside wall, then falling to the bottom of the cyclone, from where they can be removed. In a conical system, as the rotating flow moves towards the narrow end of the cyclone, the rotational radius of the stream is reduced, thus separating smaller and smaller particles. The cyclone geometry, together with flow rate, defines the cut point of the cyclone. This is the size of particle that will be removed from the stream with 50 per cent efficiency. Particles larger than the cut point will be removed with a greater efficiency and smaller particles with a lower efficiency.

The Baghouse filters and Cyclone separators needn’t necessarily be used only for pollution control. They are very well and widely used for process and production systems as well to control product loss. For instance, most of the drying plants and systems are installed with cyclone separators followed by baghouses. These systems can be designed as individual units or in combination with each other, depending on the pollutant, emission norms, product type and process requirement among many other criteria. Proper design and regular maintenance is reflected by these systems efficient and stable performance.