by Robert P. Donovan
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A competitive contamination control journal recently carried the following headline: “Litigation Creates $15 Billion Fabric Filter Market.” This pronouncement headed a short note that predicts explosive growth in the market for this type of filter as a result of settlements being reached between industry and the Environmental Protection Agency (EPA).
Does this headline refer to the filters we've been discussing over the past several months? Is fabric filter a synonym for fibrous filter or membrane filter? Is this development just more evidence of the growing material and support demands of the semiconductor manufacturing industry? No, No and No.
While fabric filters are indeed filters and are usually made of fiberglass, like cleanroom HEPA filters, or from various polymers, like many membrane filters, they are distinctly different from both and have little use or impact at present in cleanrooms.
A fabric filter features filtration media that, when new, consists of a textile cloththat is, a woven or knitted assembly of fiber-based yarns or threads formed into a cloth having stable dimensions without the need for supporting screens or housings. The qualifier, “when new,” is important and reflects the fact that the textile cloth is primarily a support scrim on which a dust cake, formed from the particles collected from the air being filtered, builds up to create the primary filtration media of the fabric filter.
This dust cake is a dynamic layer composed of the very particles being separated from the feed gas stream. It increases in thickness and weight in time.
This filtration action of a fabric filter and the applications in which fabric filters best match differ significantly from those of fibrous or membrane filters for the following reasons:
1. Fabric filters are used to remove aerosol particles from heavily contaminated industrial stack emissions such as flyash from coal-fired boilers. Reducing the aerosol particle concentration in these gases exiting a stack to levels typical of clean, outdoor ambient air (say, ±108 particles/ft3) represents ideal performance for a fabric filter. For fibrous HEPA filters, on the other hand, this concentration represents the particle concentration in the feed gas to the HEPA filters or their prefilters. Ideal filter performance here means reducing particle concentration in the filter discharge to the noise levels of contemporary particle counters (say, ±1 particle/ft3).
2. Filter packing density (the volume fraction of a filter occupied by the fibers) is typically 0.2-0.3 for a fabric filter. The packing density of a typical fibrous filter is in the 0.01-0.1 range.
3. Gas face velocity (the gas flow rate upstream of the filter divided by the area of the filter face) through a fabric filter is typically 1-3 cm/s. Gas face velocity through a cleanroom HEPA filter is often 50 cm/s. While in most other fibrous filter applications, such as the filtration of high-pressure gases, face velocities are much higher.
4. Fabric filters must be regenerated (cleaned) in situ by various actions such as shaking, temporarily reversing gas flow through the filter or pulsing with short, high-pressure bursts of cleaned gas in the reverse flow direction. Fibrous filters are not generally regenerated in situ or at allthey're simply replaced.
5. Fabric filters are typically huge in fabric area and have huge housings. Many industrial fabric filters are called baghouses because the individual filter elements have been sewn into 30-foot-long, cylindrical bags, a foot or so in diameter, and are hung in large numbers in an enclosure into which the dusty air to be cleaned is fedthe baghouse. Incoming air must pass from either inside or outside the cloth bag to the other side before discharge.
Fabric filters are quite different from fibrous or membrane filters. Cleanroom people should not get too excited over headlines promising explosive growth in the fabric filtration marketplace. These headlines may be good news for society in general but will not directly impact life in the cleanroom.
Robert P. Donovan is a process engineer assigned to the Sandia National Laboratories as a contract employee by L & M Technologies Inc., Albuquerque, NM.