Workplace

Air

Keep it Clean

Staff Report

As an environmentally concerned manufacturer, you face some of the air quality problems that exist. This guide, from United Air Specialists in Cincinnati, Ohio, is designed to help you make informed decisions in selecting the proper air cleaning system and solution for the workplace.

  Airborne contaminants are part of our lives. They pose a constant threat to our environment and health. Around most large cities and in heavy industrial areas, literally billions of particulate pollutions can be found per cubic foot of outside air.

  Airborne pollutants further abound within industrial plants. Although these pollutants never can be fully eliminated, they can be reduced and controlled to within legal limits.

  In addressing the removal of particulates from the air, it is necessary to take a look at the nature of airborne particulates. Particles are measured in terms of microns. One micron is 1/25,400 of an inch. To understand the nature of airborne particles, they can be defined as bodies with the following:

  •Definite physical boundaries in all directions.

  •Diameters ranging from 0.001 microns to 100 microns.

  •Liquid or solid characteristics.

  Airborne particles can be in the form of smoke, mist, fumes, duct and aerosols. To remove airborne particles we rely on air filtration. This involves the separation of particles from airstreams.

  As particles become very small, they are visible only because they are present in dense concentrations. it is difficult to tell if such small particles are suspended in air (particles) or diffused throughout it (gas or vapor). The bottom boundary where particles act as true particles is about 0.01 microns. The normal theory of separation does not apply to particles below 0.01 microns and removing them from air requires techniques used for gaseous materials. Particles above 0.01 microns are considered to be filterable or separable and can be addressed with air cleaners.

  Visible particles (greater than about 50 microns) generally comprise less than 10 percent of the total airborne particles in an industrial environment and tend to settle on horizontal surfaces of equipment, machinery, finished inventory, floors, etc., where they can be removed by dusting or vacuuming. Visible particles are usually filtered out by the body’s natural mechanisms and are easily collected by filtering equipment and systems. Their greatest harm is in the house- keeping problems they create.

  Sub-micronic or invisible particles (less than 50 microns) can collect on vertical as well as horizontal surfaces. In industry, this contributes to the soil and grime that is on walls, windows, machinery and clothing. These singly invisible particles (less than 10 microns) are the most harmful to human health.

The respirable fraction

  Although the human body has astounding filtering capabilities, it is vulnerable to heavy concentrations of small particles less than 10 microns in size, which are called or termed the respirable fraction.

  These particles are visible only under a microscope. This size range is typical of oil mist, smoke and thermally generated hydro-carbons, welding smoke and metal oxides. Because of their extremely small size, these particles can by-pass the body’s natural filtration mechanisms, enter the lung and bloodstream, and stay permanently lodged within the body, making them the most dangerous to our health. Potential chemical reactions, surface reactions and immunological effects make these particles a serious health threat.

  The most practical way to control particulate in the inhalable or respirable range (0.01 to 10 microns) is with two-stage electrostatic precipitator air cleaning systems or reverse pulse cartridge dust collector systems.

  The preferred choice-to filter and capture industrial dusts 10 microns and larger is with mechanical filtration devices.

  Nearly every industrial operation generates airborne contaminants which pose potential health problems. In most industrial settings, there are likely to be multiple manufacturing processes under one roof and multiple pollution sources, each creating its own problem. This can result in the need for a combination of filtration technologies and air control measures.

  Particulate air cleaning methods can basically be divided into two categories:

  •Two-stage Electrostatic Precipitators (ESP)-electronic air cleaners.

  •Mechanical Filtration Devices.

  The first step to deciding which type of system will best solve your air quality problem is a thorough evaluation of your facilities and manufacturing operations. This evaluation should include a review of:

  •The process(es) to be controlled.

  •The problem(s) created by the pollutants, such as breathing problems, electronic control failures, employee exposure, damage to production equipment, potential downtime, annoying odors, HVAC coil fouling, visual smoke, soiling, etc.

  •The best method for capture of the pollutants, be it source capture or general cleaning of the ambient workspace.

  •The consistency and concentration of pollutants to be filtered.

  •The disposal of pollutants collected.

  •The degree of cleanliness desired and/or legally mandated.

  A clean air professional can help you with your evaluation- making time-saving suggestions.

  To control air pollutants from most processes in the range of 10 microns or smaller, electrostatic precipitators (electronic air cleaners) are your best solution. They provide high collection efficiency at a low expenditure of energy.

Theory of electrostatic precipitation

  To understand the theory of electrostatic particle attraction, think of such common electrical phenomena as a comb attracting small bits of paper or static cling in clothing. Both illustrate the attractive force employed in electronic air cleaners. Under normal conditions, particles in the air tend not to have visible attraction to one another because they are electrically neutral, carrying little, if any, charge. In the example of the comb or clothing, the rubbing of these items causes them to change the electrical balance, causing them to attract. In the same manner, an electronic air cleaner alters the electrical balance of particles in the air by imparting a high positive charge to those particles.

How electronic air cleaners work

  Two-stage electrostatic precipitators, invented by Dr. Gaylord Penney, are designed to filter and collect particles down to 0.01 micron, such as those associated with industrial oil smoke and metal oxides from welding.

  A two-stage precipitator is composed of two sections - a charging section and a collecting section. The charging section contains a series of wires suspended between metal plates. The collecting section is a series of parallel, flat, metal plates spaced about 1/4' apart, with alternate plates charged and grounded. Particles are first given an electrical charge, then collected on the ground plates where they are driven by a repelling force from the charged plates toward the ground plates where they are collected.

  Electronic air cleaners also typically contain prefilters and afterfilters. These heavy-duty, reusable filters serve to aid in air distribution across the face of the units and to mechanically remove large particles.

  Mechanical filter systems are the preferred choice to filter and capture a wide variety of industrial dusts. Mechanical filters are designed primarily for dry industrial dusts, but some tolerate a minimal amount of liquid contaminants. Excess fluids of any type, however, will lead to the failure of mechanical filters due to the plugging of the filter media. Mechanical filter systems capture dust at the closest possible point to its generation (source capture) or by filtering dust in an open air space (ambient). These filtration systems take several forms.

Continuous cleaning cartridge filter units

  These dust collectors use pleated paper or polyester cartridge filters to clean the air. The cartridge filter elements are cleaned on-line during the dust collection process. This feature maintains a relatively constant pressure drop across the filter media, allowing a constant air volume for particulate capture. Cartridge units offer high filtration efficiency and are capable of trapping up to 99 percent of sub-micronic materials and virtually 100 percent of larger dust particles. Selecting the right size unit is done using a simple air-to-media ratio based on the type of contaminant. Your clean air expert can determine this ratio for each of your applications.

Cyclones

  Cyclones are the simplest and oldest type of dust collection device. They operate by spinning the collected material within the device using centrifugal force to direct the dust to the outside wall of the separator. Gravity and mechanical internal deflectors direct the dust-laden air in a downward spiral and discharge the dust out of the cyclone cone bottom. The airstream turns and spirals up the center of the cyclone and out the top. Newer, high-efficiency cyclones are even more effective than their predecessors.

Bag House

  This type of dust collector is a tubular bag device that can be cleaned on-line while the dust collection process is being performed. An advantage of the jet bag collector is that it maintains consistent pressure drop during the cleaning process, resulting in a relatively constant airflow and energy saving benefits. These devices are highly efficient in the collection of fibrous and other large size process particulate at relatively high concentration levels.

Self-contained media fan/filter units

  A media filter unit is the simplest form of industrial filtration device. it removes dust particles 10 microns or larger, typically referred to as nuisance dust. It works by passing contaminated air through a pre-filter where larger particles are collected, then through a primary filter where smaller particles can be captured. The cleaned air is then returned to the workspace.

Source capture systems

  Source capture is a technique by which contaminants generated by processes are collected directly at the point of generation. Source capture is more effective than unducted operation and is recommended because it draws off contaminants before they pass through the worker’s breathing zone or disperse in the facility’s air. This technique can be critical in applications where the contaminant represents a respiratory hazard to the employee.

  A source capture system consists of three distinct components, each with a definite function.

Capture velocity

  Many types of source capture devices (hoods, enclosures, etc.) are available with one thing in common-they rely upon the movement of air past the smoke generation source at a velocity sufficient to draw the contaminant into the source capture device. The capture velocity is the basis for all source capture design.

Capture velocity range criteria

  Lower end:

  •Room air currents minimal or favorable to capture.

  •Contaminants of low toxicity or of nuisance value only.

  •Contaminants release direction complements capture velocity direction.

  •Intermittent, low production.

  • Large hood, large air mass in rotation.

  Higher end:

  •Disturbing room air currents.

  •Contaminants of high toxicity.

  •Contaminant release direction does not complement capture velocity    direction.

  •High production, heavy use.

  •Small hood, local control only.

Enclosing and flanging

  If the contaminant generation area is well enclosed, the contaminant has little area through which to escape. Applying the recommended capture velocity to a small, open area results in a small capture volume, which means an economical installation. An effective hood design method is to enclose the operation completely and then provide access and work openings as required.

  Free standing hoods should have flanged perimeters whenever possible. Flanges eliminate airflow from ineffective zones where no contaminants exist, concentrating the capture capability of the hood. Flanges can increase hood effectiveness by 25 percent and significantly reduce hood entry static pressure loss.

Overhead canopy hoods

  Canopy hoods are effective for many machining and other operations where thermally-generated contaminants rise rapidly. They should not be used when workers must work directly over the active process because the flow of air must then pass through a worker’s breathing zone.

Side draft hoods

  Side draft hoods offer significant benefits. They can be located so that access and working space requirements are not hindered. They also draw the contaminants away from the worker’s breathing zone. The primary disadvantage is that the capture velocity direction must overcome the predominantly upward release direction of most contaminants. Thus, they require more total air volume than do other types of hoods. As is the case with other hood types, enclosing the generation area to any extent will allow for a decrease in air volume.

  Many types of hoods, especially side draft, are equipped with baffles over the face of the hood. The purpose of the baffles is to evenly distribute the capture volume across the face of large or narrow hoods; otherwise, the capture velocity would be concentrated near the ductwork opening. Baffles do not result in less air volume. They merely provide better air distribution.

Unducted air cleaning systems

  Unducted air cleaning systems consist of one or more air cleaning units positioned in the overhead plant space to create a planned air circulation pattern. This method effectively cleans the overall ambient plant air. Rather than eliminating the contaminant at the point of generation, as is done in source capture systems, unducted systems rely upon constant cleaning and mixing of the entire room volume to remove contaminants. Unducted systems never remove 100 percent of the contaminants. The objective of this method is to achieve a substantial reduction in steady-state contaminant concentrations.

Why an unducted system?

  Source capture systems are always the recommended method of air cleaning because they capture the contaminant before it can escape to the ambient air. There are many factors, though, which can make a source capture system impractical. An unducted system can be the best available approach in applications where:

  •Work is done on large parts and the worker has no fixed operating position, making source capture difficult to impossible.

  •Workers object to hooded systems. Some source capture systems may require physical positioning by the worker. If it is unlikely the worker will perform that positioning, the system will be rendered ineffective.

  •There are a large number of small smoke producers in a confined area. Hooding and sheet metal costs can escalate to the point that the source capture effectiveness benefit is nullified.

  •Overhead cranes make ductwork installation impossible. Unducted systems, designed properly, can keep the air cleaning units out of the craneways and still achieve effective results.

  Floor layout revisions are anticipated and could result in expensive ductwork modifications. Unducted systems are more flexible.

  Ductwork is simply not desired, for either aesthetic or other reasons.

Unducted principle of operation

  As contaminants are generated, they rise to where they are entrained in the collection unit’s air circulation pattern and are routed to an air cleaning unit inlet. The most important component of the air pattern is the outlet airstream of each unit. Each unit’s outlet throw pushes the contaminants toward another unit’s inlet where it can be drawn in.

  Self-contained unit mounting heights should be from 9' to 15' above the floor, regardless of room ceiling height. An air pattern created at that height aspirates air from both upper and lower spaces, effectively treating air in the work zone as well as reducing temperature stratification.

  Minor amounts of contaminants can be expected to rise past the air pattern. By consistently removing contaminants from the room, the unducted system prevents build-up and subsequent contaminant stacking down to the floor level.