When you think of criminals, the first thing that comes to mind probably isn’t humidity or hazardous mists and vapors. But these so-called ‘criminals’ — indoor air quality criminals, that is — can escape from open surface tanks causing irritation or harm to operators, as well as corroding the surrounding building structure and supporting utility systems. Properly ventilating exhaust and supply air of open surface tanks is key to thwarting these fugitives before they can inflict damage — and can ultimately save a company a whole lot of trouble.
Air quality outlaws often leave behind clues of their escape in the form of corrosion on or around the tank area. The following evidence may point to ineffective capture by the ventilation system:
- Corrosion to the exterior of metallic ventilation hoods and ducts
- Discoloration, bubbling or other change in coatings or non-metallic materials above or to the side of the tanks
- Corrosion or discoloration of building structural components and wall components in the tank room or area.
- Repeated malfunction of system components (valves, lights, sprinkler heads, etc.) in the tank room or area
- Precipitation on the walls, ceilings or floors
- Odors or haziness in the air around the tank area
- Corrosion on the exterior of the building adjacent to roll-up or personnel doors
- Employee sickness or complaints
Air quality offenders often evade capture due to a lack of or ineffective use of ventilation. With the help of accomplices like cross drafts and poorly-placed high velocity supply-air diffusers, vapors can be dispersed from the tank surface and away from exhaust inlets. Inadequate exhaust velocities may fail to capture vapors and mists.
There are two types of ventilation systems typically used to capture open surface tank emissions: pull-pull ventilation and push-pull ventilation.
Pull-pull ventilation hoods exhaust air from two or more sides of the tank at the tank lip, while push-pull ventilation involves one exhaust hood and one push-air pipe located on opposite sides of the tank at the tank lip. The push-air directs emissions from the tank surface to the exhaust hood and often requires a fraction of the air required for pull-pull ventilation systems for the same effectiveness. Many factors influence the design of the exhaust hoods, push-air pipes and ductwork including:
- Tank size and configuration
- Process chemistry and temperature
- Process agitation and rectification
The Special Agents
Experienced industrial ventilation designers know that open surface tanks warrant special attention and ventilation techniques to effectively capture humidity and vapors. The American Conference of Governmental Industrial Hygienists (ACGIH) publishes the book Industrial Ventilation: A Manual of Recommended Practice for Design, containing recommendations for ventilation in various scenarios including open surface tanks.
Section 13.70 Open Surface Tanks is geared toward the metal finishing industry and provides data specific to anodizing, plating, pickling, etching, and alkaline cleaning (among other processes). Many engineers and designers use this publication when designing new or retrofitted ventilation systems. As always, understanding the chemical process is the key to designing an effective ventilation system.
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