Open surface tanks used for cleaning, etching, plating, and other finishing tasks all require ventilation to satisfy OSHA, protect workers, protect property, and prevent smelly or corrosive fumes from contaminating areas or nearby processes.

Exhaust ventilation can be an expensive proposition, with energy costs for temperature and humidity control among the costliest contributors. However, thoughtful design can not only reduce ongoing utility and “housekeeping” expenses, but also minimize investment costs. And although the best time to consider ventilation design is before treatment tanks are installed, the reasons to consider retrofitting are compelling.

This is especially true if chemistries have changed (and now include chromium, nickel or cobalt), if steps have been added, if Permissible Exposure Limits (PELs) have lower thresholds than in the past, or if parts handling equipment obstructs the airflow that was once considered sufficient. Ventilation also warrants a fresh look if process variables such as temperature, concentration or agitation have changed, if there has been a change in plant make-up air or control of the air pollution – or if there is a problem.

Are there visible fumes emanating from treatment tanks?

Strong smells? Corrosion on equipment?

All of these require further investigation. Eliminating these symptoms through better tank ventilation will solve these problems while improving what is likely a hidden worker exposure problem, reducing the need for maintenance and repair, and extending the life of sensitive components such as instrumentation and controls.

Open surface tank venting design falls into four broad categories: side exhaust, push-pull, cowl covers and enclosure.

Ventilation configuration on a manual HCl strip line.

Side-draft exhaust uses a slotted hood installed at a right angle and adjacent to the work area to pull emissions onto the tank surface. Tank width, hazard class, and the presence (or absence) of cross drafts are factors that determine the quantity, height, and configuration of exhaust hoods. In cases where the draw distance exceeds 4 feet, side exhaust alone is usually insufficient.

push pull employs an air jet above the surface of the solution and a side exhaust hood, on opposite sides of the tank. A curtain of air sweeps across the surface of the tank, forming a thin layer on either side of the jet. This slows down the fluid in the jet and speeds up the surrounding fluid. With a push-pull system, the walls and contents of the reservoir prevent the entrainment of fluid from the low side of the jet, deflecting it towards the surface.

Push-pull ventilation requires significantly lower exhaust volumes compared to side hoods, which operate using the exhaust alone. The lower ventilation rates of a push-pull system directly translate to smaller exhaust ducts, exhaust fans, and scrubbers.

Exhaust air must be replaced and conditioned, and push-pull systems have the advantage that they can also be combined with smaller make-up air units. Make-up air creates positive or negative pressure, depending on whether the make-up air volume is greater or less than the exhaust air. ANSI Z9.1 specifies that makeup air should be 90 to 110 percent exhaust air. If the exhausts are hazardous, the make-up air cannot exceed 100% of the exhaust.

Metal finishers are advised to maintain a slight negative air pressure, so that when an exterior door is opened, the pressure within resists the strain. Otherwise, an adjustment of the top-up rate is necessary. Since make-up air is usually heated in the winter, keeping it at a minimum allowable level can result in significant energy savings.

Depending on the chemistry, unheated outside air can sometimes be directed into the tank to compensate for exhaust air. When an anodizing solution requires cooling, unheated supply air also reduces cooling requirements.

The push-pull is most often used with large tanks, in applications where high capture efficiency is essential and where access or ergonomic considerations preclude the use of an overhead canopy.

Canopy hoods can be freestanding and open on each side, or open on only three sides. They control airflow directly in applications with sufficient control velocity, but are not suitable where highly toxic chemicals are used, or where thermal currents from hot processes, spot cooling, or machinery movement create inevitable cross currents.

Canopy hoods are rarely used to vent open surface tanks for several reasons: 1) hoods placed directly above process tanks can interfere with cranes/hoists and other process equipment and 2) vertically directed fumes from the surface of the solution to an exhaust hood above are likely to pass through an operator’s breathing zone.

Covers closed partially or completely surround the tank. They consist of a side cover and either a panel or panels at each end. This venting option reduces cross drafts by directing most of the hood airflow above the tank surface. Enclosed hoods become an impractical option when operators need access to the tank.

Lower side hood (BSL) with air manifold pushed over the anodizing process for architectural extrusions.

Exhaust equipment for trim tanks is typically made of a thermoplastic material, provided solution temperatures are below the threshold where an alloy option is required. The materials most often specified are PP (co-poly or homo-poly), PVC, PVDF and CPVC. Of these, PP and PVC are the most common.

“Polyro” is stiff and stiff, with strong resistance to impact, stress fatigue and temperature extremes. It is highly resistant to corrosion and chemical leaching, making it a worthy companion to solvents, bases and acids. It is a good choice for installations prone to incidents with cranes, forklifts and other motorized hazards, as it is impact resistant and can be easily welded/repaired in the field. PP also holds a charge well at higher temperatures.

When ethylene is combined with propylene during the polymerization process, the result is a polypropylene copolymer. This material has increased flexibility and improved optical properties, so it is suitable for applications where transparency and good aesthetics are an advantage.

Illustration showing a pair of tanks evacuated through a single lower side hood with a double inlet (BSL-D).

PVDF has excellent piezoelectric properties, thermal stability and mechanical strength. Hoods made from this material have excellent resistance to acids, bases, organic solvents, oils and greases, and exceptional insulating properties. Its continuous use temperature limit is 302°F, so it is advantageous for finishing lines where one or more steps are maintained at elevated temperatures.

PVC has a high density for a plastic material, so it is extremely strong, light and economical. PVC’s peak operating temperature is 140°F; The CPVC can handle up to 200^ohF. Notably, their fittings and bonding agents are not interchangeable due to differences in ASTM specifications.

Major manufacturers also offer special flame retardant PVC, polypropylene and PVDF. PVC, for example, only burns when held in the flame. When it gets hot, it collapses and is less able to conduct air, which can fuel or spread a fire.

Ventilation is a critical engineering control that directly affects everyone—and everything—in your shop. And whether you’re planning a new line, expanding an existing line, or simply considering a new chemistry, getting input from a company that offers a wide range of standard and custom tank venting solutions can only make you smarter – and probably save you a lot of money. , short and long term.

Tri-Mer Corp. (Owosso, MI) manufactures metal finishing systems, ventilation systems, wet and dry scrubbers, odor control systems and dust collectors. The company provides turnkey project delivery and project management worldwide. Go to tri-mer.com.