BAGHOUSE/FABRIC FILTERS
FUNDAMENTALS OF BAG FILTERS
Gas phase filtration using bag filters is actually an adaptation of an intermittent or discrete process, i.e., filtration cycle sequences over a certain period of time, usually dictated by limits on the allowable pressure drop, and subsequent cleaning stages.
In order to achieve continuity of service, it has been decided to operate or design traditionally in one of two ways:
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add additional capacity/compartment to that required in order to guarantee continuity while offline sections are being restored;
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Perform cleaning in extremely short intervals, e.g., between 0.03 and 0.1 seconds, increasing the frequency by alternating filtering and cleaning operations, e.g., according to a manometrically determined schedule.
The cleaning method employed somehow leads to the traditional classification of bag filters , namely:
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mechanically cleaned bag filters (periodic shaking)
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cleaning bag filters by reversing the air or gas flow
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bag filters cleaned by short air pulses
While the classification may seem trivial, the particle collection mechanisms involved are fundamentally different . Whereas in mechanically cleaned bag filters and cycle reversal bag filters, filtration is achieved through the formation of a layer of the particulate material itself (the mesh or fabric is merely a substrate or support for the formation of this layer, or, if you will, a " pre-coat "), in short-pulse cleaned bag filter alternatives, the mesh or felt itself is responsible for the retention or capture of dust/particles.
In the case of mechanically cleaned bag filters, particulate removal is achieved through relatively vigorous shaking , which reduces the lifespan of the bags. In reverse-flow bag filters, the flow direction is cyclically reversed, resulting in a gentler operation. Short-pulse cleaning of bag filters is performed using abrupt bursts or shocks of air that impart a sinusoidal wave to the bag, not only causing particulate removal but also creating a new residual structure/layer.
In general, the operating speeds of pulse-cleaned filters are approximately twice those of periodic shake bag filters and reverse-cycle bag filters. While this naturally results in initial savings in terms of capital investment, the operating/energy costs associated with the compressed air (e.g., 0.03 to 0.1 seconds of air at 60 to 100 psig) required for cleaning can equal or exceed the cost of the main blower that drives or induces the gas phase to be filtered.
Both periodic shaking bag filters and reverse-cycle bag filters tend to primarily use filter fabrics or textiles/yarns , resulting in a clearly ordered texture. In the case of short-pulse cleaning bag filters, felts are usually employed ; that is, pressed fibrils forming a mat supported by an open backing fabric, generally creating more robust "sandwich" structures. This is another property that allows operation at the characteristically higher speed ranges of this type of filter. A wide variety of construction materials are available: polypropylene, polyester, polyamide, Ryton, Nomex, PTFE, Teflon, PET, and fiberglass.
As for the advantages of bag filter technology, it can be said that they constitute an alternative to gas phase filters with excellent capture/efficiency levels , are relatively insensitive to fluctuations in feed or load conditions, do not usually present corrosion problems (taking care to operate above the dew point), the operation is simple and high voltages are not required.
Bag filters may not be advisable in environments with extremely sticky (e.g., galvanizing plants), oily, or viscous particulate matter. In some cases, dust/particulate concentrations, e.g., close to 50 g/m³, can pose a risk of explosion or fire.
Although the sizing of bag filters is practically automated, for illustrative purposes it is possible to manually obtain an estimate of scale or order of magnitude.
In the case of a pulse bag filter, we can get an idea of the nominal speed V to be used using the following generic formula:
V = 2.878 * A * B * T^-0.2335 * L^-0.06021 * (0.7471+0.0853 Ln D)
being
A = a factor between 15 (tobacco, flour, grain, sawdust) and 6 (activated carbon, carbon black, detergents, powdered milk, soaps)
B = a factor associated with the service that must be fulfilled, e.g., conveying, venting, process, between 1 and 0.8
L = particulate load, eg 4 gr/cu.ft., (= 9153 mg/m3)
T = temperature, °F
D = average particle diameter, e.g. 7 µ
Typically the pressure drop ranges from 4 to 10 inches of water column , approximately between 4” and 8” for reverse cycle and between 3” and 6” for pulse cleaning bag filters.
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