Selecting the Proper
Filter Media for Dust Collection System
Previously, "The Basic" course covered four basic questions to ask when selecting the proper filter media for a dust collecting system. After determining the continuous gas stream temperature, the design of the collector, the possible presence of water and the potential for any chemical reaction, media selection can be narrowed down to correct fiber and correct fabric design for a dust collector bag. The Advanced Course in Media Selection - understanding and analysis - is required to optimize performance of the selected filter media. Since the predominant collector design today is the pulsejet collector, Course 2 will focus on needled felts as the primary filter medium since there are more variables, options and design opportunities.
Understanding and anticipating dust characteristics is the heart of this next level of analysis. Also requiring consideration are the effects of moisture or hydrocarbons on the dust, the potential for static buildup and the potential for sparks or live particles.
Basic dust characteristics, once known, will influence the felt fiber size and distribution as well as cake side surface design, scrim type and specifications and, finally, chemical finishes and surface treatments. Major dust characteristic questions are:
Question #1: Is the dust free flowing or does it tend to agglomerate?
Question #2: What is the particle size, shape and distribution?
Question #3: Is the dust abrasive?
Question #4: Is the dust fibrous?
These four questions, parts of which can be scientifically determined and parts of which are more subjective, gives "the rest of the story" for choosing the optimum media. The single most important issue above is whether the dust is free flowing or whether it agglomerates because that determines whether the media must be efficient enough to separate a small particle, which does not normally form a cohesive filter cake or whether the focus has to be on dust cake release because the dust will be difficult to pulse off the collecting surface.
There are multiple options available to deal with fine dust and free flowing particles. Depending on the severity or complexity of the problem, felt media options include:
Note: High efficiency felts (Dual Density, micro-denier and composite felts) normally have a singed cake side surface to allow optimum clean-ability of a free flowing dust when pulsing.
These many options provide the OEM, the bag converter and the end user with many suitable options for dealing with difficult dust and meeting current PM and emission standards. Particle size analysis (PSA) and dust evolution will indicate to the experienced technician, which medium provides the best combination of efficiency, pressure drop, bag life and long term cost.
On the other side of the spectrum is dust, which readily agglomerates or dust with moisture or hydrocarbons in the gas stream. In this situation, filter media efficiency is generally not an issue, regardless of particle size. Here, the dust literally collects itself and long bag life depends on the ability of the medium to release the filter cake effectively and thoroughly. One must also avoid sticky solids penetrating the depth of the felt and becoming lodged in the media ultimately causing the felt to blind prematurely.
In the case of agglomerating or sticky dusts, the media options are:
The mechanical treatment option is very basic and adds relatively little cost to the medium or finished bag. Typical mechanical treatments are singeing of the cake side surface fibers in non-thermoplastic materials such as acrylic, aramid and polyimide fibers and glazing (melting/smoothing process) of thermoplastic fibers such as polypropylene, polyester and PPS. In both cases, the objective is to provide a sticky dust with limited opportunity to attach itself to the fibers, which are present on the surface of an unfinished felt medium.
However, mechanical treatments are generally not enough to handle aggressively agglomerating solids and, in that situation, one turns first to oil and water repelling chemicals such as fluorocarbon bath treatments. These treatments encapsulate each fiber and will actually create a filter felt, which will bead water and oil. The combination of mechanical surface and durable chemical bath treatment will significantly extend the life of a dust collector bag when moisture, fats or oils are present in the gas stream or product itself.
PTFE bath treatments, although not repellent, do increase the anti-adhesion characteristic of a fiber and are used in many applications, especially in utility boiler baghouses. Lastly, membrane laminates work well with releasing wet dust, but caution must be taken to avoid using membranes when hydrocarbons or natural fats/oils are present.
Regarding abrasive dust, the most common solution is to treat the media - whether a synthetic fiber or a woven fabric - with a heavy silicone solution. The key is to deposit enough silicone so that, after the media is dried (or cured), each fiber is fully coated. For effective abrasion resistance, the minimally acceptable amount of silicone for effective performance (measured after drying) is 4-5 percent of the media's basis weight. When the silicone coating is adequate, it tends to deflect dust particles' direct abrasive contact on the media surface. As particles penetrate the media, the silicone acts as a lubricant to protect the fibers inside the media from abrasive wear.
Fibrous dusts present a unique situation in a baghouse because that problem may create significant downtime if not addressed early in the media selection process. The fibrous dust cake formed on the filter bags can eventually bridge between bags and between the bags and the collector wall, packing these areas so densely that the only way to clean out the baghouse is to go inside and remove the fibrous dust cake by hand. Typical fibrous dusts include dust from coarsely sawn wood, fiberglass, diaper components, and polypropylene-polyethylene streamers. A few filter media that can mitigate fibrous dust problems include:
In all cases above, the recommended media presents very small openings on the cake side surface, which limit the ability of fibrous dust to attach its tendrils to the media surface. This, in effect, stops the problem before it starts.
Static problems are being addressed today in numerous seminars related to plant maintenance and housekeeping. From a filter media perspective, there are products readily available today, which make highly resistive synthetic felts conductive and allow the end user to know that a very high percentage of static, which may build up on a filter bag, can be conducted to ground.
Bag and collector design are also important here. The past and more current technologies for manufacturing static dissipating filter media are:
Of these three, item (3) above is the current technology and employs a woven scrim in the middle of the felt with alternating synthetic and stainless steel yarns, in which the stainless steel yarns are spaced approximately 1/2" - 5/8"/13 - 16 mm apart. Current resistivity standards for a typical static dissipating media require that it not exceed surface resistivity of 106ohm using the DIN 54345/1 - 5 test methods. In actual testing, most static dissipating felts do not exceed 104.
The majority of static dissipating felts are polyester but there are applications for almost every generic fiber group. In conjunction with other related dust control procedures, a static dissipating media will effectively contribute to a plant's overall reduction of potential explosion hazards.
Lastly, there may be a situation where sparks or live particles may be present or potentially present. Hot or burning particles can create pinholes in the media. In the worst case, a hot or burning particle or spark can create a baghouse fire that destroys the filters and damages the baghouse's metal structure and related components.
A common way to prevent these problems is to install a mechanical or chemical spark- and fire-prevention system inside the baghouse. Selecting a particular specialty media or media treatment is another approach. These options include:
If no mechanical or process related solution for sparks or live particles is possible, the above alternates may improve the situation.
To the first time buyer of bags or a dust collector system, the "Basic" and "Advanced" steps may appear to be a daunting task. The key to properly selecting the correct filter media for a system is to know and understand three major parts of the project:
1) Know the process, how it works and what it produces in gas stream and particulate.
2) Know the dust, what it is and how it performs.
3) Know the filter media options and solutions or best features, which match the process and the dust.
Carefully considering all of thesefactors and making an informed decision will result in a safe, efficient and effectively performing filter bag and dust collector system.