**7. Required efficiency**

Pressure drop in the entrance equation:h<sup>f</sup> = K ∙ VP (6)

Dusts, toxic or corrosive gases, and fumes should not be discharged to the atmosphere. Each exhaust system handling such materials should be provided with an adequate air cleaner. Air cleaning devices remove contaminants from an air or gas stream after ventilated from indoor spaces and before exhausted to the atmosphere. They are available in a wide range of designs to meet variations in air cleaning requirements. Quantity and characteristics of the contaminant to be removed, conditions of the air or gas stream, and degree of removal required will all have a bearing on the device selected for any given application. In addition, fire safety and explosion control must be considered in all selections. For particulate contaminants, air cleaning devices are divided into two groups: dust

Air filters are designed to remove low dust concentrations of the magnitude found in atmospheric air. This kind of air cleaning device is typically used in air-conditioning, ventilation, and heating systems where dust concentrations seldom exceed 1.0 grains per thousand cubic feet of air and are usually well below 0.1 grains per thousand cubic feet of air. Where the air or gas to be cleaned originates in local exhaust systems or process stack gas effluents, usually duct collectors are designed for the much heavier loads from industrial processes. For each cubic foot of air or gas, contaminant concentrations will vary from less than 0.1 to 100 grains or more. Therefore, dust collectors are, and must be, capable of handling concentrations 100–20,000 times greater than those for which air filters are designed. Small, inexpensive versions of all categories of air cleaning devices are available. The principles of selection, application, and operation are the same as for larger equipment. However, much of the available equipment is of light duty design and construction due to the structure of the market that focuses on small, quickly available, and inexpensive equipment. One of the major economies of unit collectors implies recirculation, for which such equipment may or may not be suitable. Application engineering is just as essential for unit collectors as it is for major systems for adequate prevention of health hazards, fires,

WG), K is the entrance friction factor ("

WG), and VP is the

where h<sup>f</sup>

velocity pressure ("

**Table 3.** Entrance friction factor

**6. Air cleaning devices**

filters and air cleaners.

and explosions.

is the loss of entrance ("

 **degrees Friction factor**

152 Air Pollution - Monitoring, Quantification and Removal of Gases and Particles

30 0.18 45 0.28

WG).

The required degree of collection can depend on plant location, nature of contaminant, and the regulations of governmental agencies when the cleaned air is to be discharged outdoors. Damage to farms or contribution to air pollution problems of distant cities can affect the need for and importance of effective collection equipment in remote locations. Many industries, originally located away from residential areas, have failed to anticipate the residential building construction which frequently develops around a plant. Such lack of foresight has required installation of air cleaning equipment at greater expense. Nowadays, the remotely located plant must comply with the same regulations as the plant located in an urban area in most cases. Management can continue to expect criticism for excessive emissions of air contaminants whether located in a heavy industry section of a city or in an area closer to residential zones with the present emphasis on public nuisance, public health, and preservation and improvement of community air quality. Also, the mass rate of emission will affect selection of equipment. For a given concentration, the larger the exhaust volumetric flow rate, the greater the need for better equipment. While a smaller industrial pulverized fuel boiler might be able to use slightly less efficient collectors, large central steam-generating stations might select high efficiency electrostatic precipitators or fabric collectors for their pulverized coal boiler stacks.

impacting, or bonding. The fabric represents a porous mass through which the gas is passed in directional such that dust particles are retained on the dirty side and the cleaned gas passes on through. The ability of the fabric to pass air is called "permeability." It is defined as the cubic feet of air passed through one square foot of fabric each minute at a pressure

40 cfm. A highly efficient fabric that cannot be cleaned represents an excessive resistance to air flow and is not an economical engineering solution. Final fabric selection is generally a compromise between efficiency and permeability. The efficiency of the fabric as a filter is meaningful when new fabric is first put into service. Even after cleaning, a residual and/or redeposited dust cake provides higher collection efficiency and additional filtration surface than obtainable with new fabric*.* Fabric collectors are not 100% efficient. But well-designed, adequately sized, and properly operated fabric collectors can be expected to operate at efficiencies in excess of 99% and often as high as 99.9% or more on a mass basis*.* The fabric collector should be leak tested for mechanical leaks where extremely high collection efficiency is essential. The combination of fabric and collected dust becomes increasingly efficient as the dust cake accumulates on the fabric surface*.* Fabric collectors are suitable for service on relatively heavy dust concentrations. The amount of dust collected on a well-designed and single square yard of fabric may exceed 5 pounds per hour. Commercially available fabric collectors employ fabric configured as bags or tubes, envelopes (flat bags), rigid elements, or pleated cartridges. Most of the available fabrics are employed in either bag or envelope configuration. The variable design features of the many available fabric collectors are as

**1.** Housing configuration (single compartment and multiple compartment)

Due to many variables and their range of variation, fabric collector sizing is judged based on experience. Also, a combination of shaking and reverse air flow has been utilized. It is possible that reverse-jet, continuous-duty fabric collectors use envelopes or tubes of nonwoven fabric, pleated cartridges of non-woven mat (paper-like) in cylindrical or panel configuration, or rigid elements such as sintered polyethylene. Based on our experience, when tubes have 6–11 inches diameter and can be as long as 10 feet, permeability 10–25, reverse-jet 6–8 atmosphere (in high load of pollution each 1 minute, 1 second pulse jet; and in low load of pollution each 2–3 minute, 1 second pulse jet), and air velocity inside the chamber selected 300 fpm, the bag filter becomes optimal and economic in removal efficiency. Solenoid valves which control the pulses of compressed air may be open for a tenth of a second or less. An EPA-sponsored research has shown that superior performance results from downward flow of the dirty air stream. This downward air flow reduces redeposition since it aids gravity in moving dust particles toward the hopper. **Figure 5** shows

**2.** Type of reconditioning (shaker, reverse air, pulse-jet)

**4.** Type of fabric (woven or non-woven)

**5.** Intermittent or continuous service

the fabric collector [1, 2].

**3.** Fabric configuration (bags or tubes, cartridges, envelopes)

wg. Usual permeability amounts for commonly used fabrics range from 25 to

Industrial Air Pollution Control

155

http://dx.doi.org/10.5772/intechopen.80678

drop of 0.5 "

follows:

A safe recommendation in selecting equipment is to select the collector that will allow the least possible amount of contaminant to escape and also while meeting all prevailing air pollution regulations is reasonable in first cost and maintenance. It must be remembered that visibility of an effluent will be a function of the light reflecting surface area of the escaping material. Surface area per pound increases inversely as the square of particle size. In other words, the removal of 80% or more of the dust on a weight basis may remove only the coarse particles without altering the stack appearance.

The contaminant characteristics will also affect equipment selection. Emitted chemicals may attack collector elements or corrode wet type collectors. Sticky materials can adhere to collector elements, plugging collector passages. Linty materials adhere to certain types of collector surfaces or elements. Abrasive materials in moderate to heavy concentrations will cause rapid wear on dry metal surfaces. Particle size, density, and shape will rule out certain designs. The combustible nature of many finely divided materials requires specific collector designs to assure safe operation. The characteristics of the carrier gas stream can have a marked bearing on selecting equipment. It is possible that temperature of the gas stream limit the material choices in fabric collectors. Condensation of water vapor will cause packing and plugging of air or dust passages in dry collectors. We can reach optimum and high removal efficiency with optimization and more study about design parameters in each type of collectors.
