DOWN FLOW FILTERING

Properly engineered air collection and filtration systems not only improve plant air quality, they can help welders become more productive and reduce expensive welding mistakes and rework. Here is a filtering technology that improves air quality at a low filter replacement cost for welding operations.

Focused on keeping their workers healthy, manufacturers have a key interest in improving indoor air quality in environments where human operators come in contact with airborne particulates such as smoke, oil mist, dusts, and other contaminants emitted during production processes.

Fine and ultra fine particulates are generated during processes such as welding, grinding, plasma and laser cutting, buffing, polishing, sanding, mixing operations, blasting and many more applications. To address these particulates in welding, for example, certain types of collectors are used at many weld stations to capture welding smoke and grinding dust and remove it from the environment. Not only must these stations prevent contaminants from entering the weld puddle, they must prevent contaminants from entering the air which the workers breathe.

Collecting these particulates is only successful when re-entrainment and blinding problems are eliminated by using down flow filtering devices. Down-flow filtering technology, combined with its special smoke capturing concept, reduces smoke to improve air quality at a low filter replacement cost for applications.

Let’s examine some collection issues and compare other solutions to down flow filtering devices.

RE-ENTRAINMENT
Re-entrainment of particulates means that once dust is pulsed off of the cartridge filter, it does not go back to the filter again. A technology called Smart-Flow™ has been created that defines that gases will be introduced above the filters so that fan energy, gravity and cleaning energy will work together in the same direction, i.e., down towards the hopper. Cleaning moves particles outward into a downward moving gas flow. Thus, any rising flow is compromising to favor re-entrainment.

Airflow velocity is a consideration in most air filtration systems because dust storms and velocity spikes will destroy materials and cause unrestrained re-entrainment. Blinding occurs when dust re-entrainment is advanced to the point where airflow through a filter is insufficient for the task. Staggered blinding is where filters in a group are unevenly loaded with dust. The collection design of Smart-Flow filtering systems eliminates dust re-entrainment, removing the dust completely.

LAMINAR AIR FLOW
Laminar air flow systems require excellent deflector design and lots of headroom, which can be costly and limited. With poor deflector design, it is easy to overcome the established airflow velocity and introduce reverse currents that can re-introduce contaminants into the work area.

With laminar airflow, you must have perfectly balanced air velocity downward across the horizontal area of the housing. With unbalanced flow, staggered blinding of the filters occurs. What can be done? Be certain that the collector throughout the housing zone below the inlet deflectors is without deflectors, sheet metal containments, or anything, which is purported to control airflow. Why?

If velocity exiting the inlet deflectors is low (900 fpm to 1800 fpm), then any remaining imbalance of velocity will best be adjusted by large open areas, which exist from the deflectors down to the filters and down between the filters. Also, deflectors or screens located in the filter zone space should be negligible.

VERTICAL VS. HORIZONTAL FILTERS
There is zero (cleaning) re-entrainment if each particle removed from a filter goes to the hopper and not back on a filter. From a design standpoint, it is best to position filters vertically in down flowing dirty air. If filters are mounted horizontally, the top third of each filter becomes a 2 in deep dust trap. Dust cleaned from upper filters falls onto lower filters, causing re-entrainment.

DIRECT TO THE HOPPER
Dust, as it passes the inlet deflectors, has an aerial view of the filters and it sees only the top of filters and the open space between . . . direct to the hopper. In the Smart-Flow™ design, these two areas are typically equal. Given laminar flow, the gas velocity (at A/C = 1.5:1) entering the open space between the filters is 412 fpm. Another example: if A/C = 2:1, the gas velocity downward entering the open space between the filters is 549 fpm. This velocity, regardless of A/C, becomes 0 fpm at the base of each 52 in filter stack.