Arresting Indoor Air Quality Criminals
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 indoor air quality “criminals” can escape from open surface tanks used in metal finishing, causing irritation or harm to operators, as well as corroding the surrounding building structure and supporting utility systems.
Properly ventilating exhaust from and supply air to these open surface tanks is key to thwarting these fugitives before they can inflict damage, and doing so ultimately can save a company a whole lot of trouble.
Air quality outlaws often leave behind clues to 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 and discoloration 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 the building’s structural components and wall components in the tank room or area; and corrosion on the exterior of the building adjacent to roll-up or personnel doors.
- Repeated malfunction of system components (valves, lights, sprinkler heads, etc.) in the tank room or area.
- Precipitation on walls, ceilings or floors.
- Odors or haziness in the air around the tank area.
- Compromised work environment, meaning ineffective and/or insufficient ventilation can lead to an unpleasant work environment, employee illnesses or even death.
The term “unpleasant work environment” can mean different things to different people, but most would agree that an environment filled with irritating vapors, and uncontrolled heat and humidity would constitute an unpleasant work environment. Uncomfortable employees are likely to be less efficient than comfortable employees. In fact, cost estimators will factor in the work environment when they tabulate the potential cost of labor for construction.
Proprietary chemical suppliers often can provide safer alternatives for metal finishing that still meet industry-recognized specifications. For instance, citric acid sometimes is an acceptable alternative to nitric acid in some passivation processes, offering advantages for both safety and the environment. Whether or not a safer chemical alternative is available, proper ventilation for the processing tank is still necessary and should be evaluated by a professional. Local tank exhaust should keep all vapors and mists away from the operators’ breathing zones to protect them from irritating, corrosive and toxic materials.
Air quality offenders often evade capture due to a lack or ineffective use of ventilation. Accomplices like cross-drafts and poorly placed, high-velocity supply air can disperse vapors from the tank surface and away from exhaust inlets. Inadequate exhaust velocities also may fail to capture vapors and mists.
Two types of ventilation systems typically are used to capture open-surface tank emissions: lateral-exhaust and push-pull systems.
Lateral-exhaust ventilation uses one or more slot hoods that pull air across the surface of the liquid in the open tank. One hood is usually sufficient for a small tank in which the width (the dimension in the direction of airflow) of the liquid surface is no more than 36 inches. For widths greater than 36 inches, two slot hoods placed opposite each other on the tank would typically be recommended. (Under special circumstances, one slot hood may be sufficient to capture emissions on a tank with a width as large as 48 inches. In these cases, cross-drafts, as mentioned above, are absent.) Lateral exhaust using one hood is often called “pull” ventilation, while lateral exhaust using two or more hoods is called “pull-pull” ventilation.
In order for the exhaust slot hoods to effectively capture emissions, exhaust air must be moving at the proper capture velocity across the liquid surface. The required lateral exhaust airflow can be calculated using several parameters that are described in detail later in “The Special Agents” section of this article. In short, important parameters include:
- The tank dimensions.
- The hazard potential of the liquid chemistry, which is determined using threshold limit values (TLVs) as adopted by the American Conference of Governmental Industrial Hygienists (ACGIH), and the liquid flash point. Established TLVs are the airborne concentrations of chemicals (in gas or vapor form) that have been determined to be safe for repeated exposure to personnel during normal operating time periods.
- The liquid temperature relative to its boiling point.
- The amount of gassing or misting created in the process, rated from zero to high. The rate of gassing and mist evolution is effected by the type and aggressiveness of liquid agitation, the current density in the electrochemical processes, the surface area of the work being acted upon, and the chemical reaction occurring in the tank.
Push-pull ventilation involves the use of an exhaust hood and a push-air pipe located on opposite sides of the tank at the tank lip. The push-air pipe is often made of polyvinyl chloride (PVC) or chlorinated polyvinyl chloride (CPVC) with a linear set of holes that runs the length of the tank. Push-air blowers provide clean, pressurized air for the “pushing” process. In the push-pull scenario, the push-air pipe blows the push-air jet across the liquid surface. The push-air jet carries the emissions to the exhaust hood, which then captures and removes the air.
Exhaust airflow required for push-pull ventilation systems is calculated differently than that required for lateral exhaust and also may differ based on the designer’s preference. ACGIH literature states that rates of 75 cubic feet per minute per square foot (cfm/ft2) of tank area to 200 cfm/ft2 have been used. Low airflow rates have been shown to be successful, but process tanks using agitation air, high current densities and vigorous reactions should use higher exhaust airflow rates. A good rule of thumb is 100 cfm/ft2 for most metal-finishing processes. Push airflow is calculated directly from this literature (see The Special Agents) using push nozzle design data. Parameters for calculation include tank dimensions, nozzle (hole or slot) sizes and nozzle diameter or width.
Push-pull ventilation often requires only a fraction of the air necessary for lateral ventilation systems (using exhaust alone) to get the same removal effectiveness. In plating and metal-finishing applications in which this is the case, it is often economically justifiable, then, to install a push-pull ventilation system. Lower ventilation rates translate to smaller exhaust ducts, and smaller exhaust fans and scrubbers. Any air exhausted from a space needs to be replaced and conditioned. Push-pull systems normally require smaller makeup-air units and less heating for the makeup air. These smaller exhaust-air and makeup-air requirements can reduce both capital expenditures and operational expenditures for a business.
Whether the system used to capture the criminals is lateral-exhaust or push-pull, it will be most effective when the exhaust hood slots are located near the liquid surface and the tank is designed so that the exhaust air stream is directed across the surface rather than in the area surrounding the tank. Removable tank covers assist with containing emissions within the tank until they can be captured by the slot hood, while baffles (vertical plates installed along the tank lip) direct exhaust airflow across the tank width.
Although locating the exhaust slot right above the liquid surface seems desirable, it can lead to disaster if not done properly. There should be ample vertical distance between the liquid surface and the bottom of the slot to accommodate a change in liquid depth and eliminate overflow into the hood. The actual vertical distance should be determined based on the liquid displacement caused by the largest-volume load placed in the tank. For instance, immersion of a metal sheet will cause less initial liquid displacement than immersion of a jet engine part or barrel of the same length. An additional distance of 6 to 8 inches is often recommended to accommodate splashing and liquid surface disturbance. That being said, it is always prudent to include drains and cleanouts in low points of these exhaust systems as well.
After witnessing overflow into a slot hood or other hood-related mishaps, well-meaning maintenance personnel sometimes will relocate the exhaust point much too high above the tank surface. Unfortunately, there also may be a misunderstanding that having an exhaust point located vertically inline or above the operator’s breathing zone will protect the operator, but this actually has the opposite effect. Raising the exhaust point in this manner draws hazardous vapors closer to the operator and increases the airflow required to capture emissions. Most vapors and mists originating from metal-finishing tanks will be heavier than air and will tend to migrate toward the floor without an efficient capture velocity.
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 ACGIH publishes the book “Industrial Ventilation: A Manual of Recommended Practice for Design,” which contains recommendations for ventilation in various scenarios including open-surface tanks. Section 13.70, Open Surface Tanks (in the 27th edition of the publication), 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, and understanding the chemical process is the key to designing an effective system.
In addition to open-tank ventilation guidelines, the ACGIH manual provides ventilation recommendations for many metal-product manufacturing applications, including:
- Sanding and use of lathes
- Milling, abrasive blasting and grinding
- Metal shearing
- Use of melting pots, furnaces and die casting
- Operation of paint booths, drying ovens and paint storage rooms
- Welding and torch-cutting
- Metal spraying
When hiring engineers to design a new or retrofitted ventilation system, it is beneficial for manufacturers to recognize the standards that these engineers will use as their basis of design. The ACGIH industrial ventilation manual is a good baseline standard that can be referenced in the scope of work provided to the engineer to guarantee that the design meets recognized industrial practice.
Angela Vawter is with Burns & McDonnell and has a degree in chemical engineering. She provides engineering services to the manufacturing industry with an emphasis on chemical metal finishing. Visit burnsmcd.com for information. Thanks to Jessup Engineering for the photos used in this article.