What Makes a Good Chrome Scrubber? A Spec Guide for Plating Line Exhaust

Say you're laying out exhaust for a plating line: a hard chrome tank at one end, with nickel and nitric strip tanks down the same wall on the other end. The tidy solution is one header and one wet scrubber for the situation.

The trouble is, however, those tanks aren't putting out the same thing. Before you can say what a good chrome scrubber looks like, you have to decide which tanks it's serving.

If you split the line first, the chrome spec gets a lot easier to write.

Key Takeaways

• Split the line by chromium content. Permit records include plating plants that send chrome tanks and chromium-bearing etches to mesh pads and tanks without chromium to packed beds.
• Mesh pads suit chrome because the mist is fine. Our engineering data rates composite mesh pads at 99% removal of 3-micron mist, against 99% at 20 microns for horizontal blades.
• Washdown belongs in the design. Nozzles sized to the pad area keep chrome from plugging the pad, and removable headers keep the nozzles serviceable.
• Ask how a pad behaves when loaded. In our field experience, our polypropylene pads hold together even when almost completely filled with chrome.
• Chromic acid ratings fall as concentration rises. On Viron's chart, PVC goes from 140°F at 10% chromic acid to not recommended at 50%.

A Plating Line Is Two Exhaust Problems

You should sort the tanks on a plating line by one question: Do they carry chromium? That answer decides the exhaust layout before you pick any equipment.

The two differ in what they emit and how they're regulated. Chromic acid leaves a chrome bath as a fine mist, and most of a chrome tank's emissions are in that mist, so the control device's job is to catch it. Chrome plating and chromic anodizing tanks also fall under the federal chrome rule, 40 CFR Part 63 Subpart N, with its emission limits and ongoing monitoring.

Tanks without chromium, such as nickel, acid copper, and nitric strip, sit outside the chrome rule, though their permits still set limits. Their acid fumes are the kind of load a packed bed scrubber absorbs in recirculating liquid.

Permit records show plants often draw the line this way.

For instance, in two Michigan inspection reports (in July 2021 and November 2019) for plating-on-plastics plants, chrome plating tanks and chromium-bearing etch tanks run to composite mesh pads, with fume suppressant in the baths. Nickel, acid copper, and nitric strip tanks run to packed bed systems. In one of those plants, the etch line carries its own chromium emission limit and a closed-loop chrome recovery system.

The split decides which tanks share a system, not which device the chrome side has to use. The federal rule accepts composite mesh pads, packed bed scrubbers followed by a mist eliminator, fiber-bed mist eliminators, and fume suppressants as chrome controls. And one of the same Michigan plants runs a group of chromium tanks through a packed-bed scrubber.

What this argues against is one header for the whole line. That asks a single scrubber to do both jobs, and it mixes other acids into washdown liquor you might otherwise return to the chrome bath.

For the non-chrome side, our horizontal scrubbers and blower scrubbers list plating, anodizing, and steel pickling fumes among their standard applications.

What Makes a Good Chrome Scrubber

A good chrome scrubber is mostly a mist eliminator—including enough mesh-pad stages for your efficiency target, and a dependable way to keep them clean.

Stage the pads to the efficiency target

Mist eliminator ratings show why mesh pads get the chrome job. In our scrubber engineering data, composite mesh pads remove 99% of 3-micron mist, while horizontal and vertical blade eliminators reach 99% at 20 and 32 microns.

Our Viro-Chrome 9000 uses a three-stage knitted polypropylene pad about 6 inches deep in the direction of the airflow, with 1 to 3 pads depending on the removal efficiency the job calls for. Give the vendor your efficiency target and let the pad count follow.

Put a differential pressure gauge in the spec, too.

Where a tank complies through an add-on control device, the federal rule requires that mesh-pad pressure drop be monitored at least daily and kept within 2 inches of the water column of the performance-test value. Magnehelic and Photohelic gauges are options on our chrome scrubbers.

Washdown, and what happens when it's missed

Chrome that builds up on a pad plugs it. We wash the pads with PVC spray nozzles sized for 0.25 to 1 GPM per square foot of pad area, fed by Schedule 80 PVC headers with true unions, so each header comes out for nozzle maintenance. That detail matters the first time a nozzle plugs.

Schedules still slip, and sooner or later someone finds the washdown timer switched off.

In our field experience, our polypropylene pads can be almost completely filled with chrome and still hold together, and they're built to withstand chromic acid at or above hard-chrome-bath strength. We recommend scheduled washdown anyway, and the federal rule requires specific operation and maintenance practices.

But ask every vendor what happens to their pad when it's loaded; the answer to that question tells you how it was built.

Capture Chrome at the Tank and Getting It Back

Chrome control starts at the hood. Our chrome hood carries its own removable polypropylene mesh pad over a drip pan and a sloped bottom that drains captured solution back into the plating tank. It comes in PVC or FRP. For tanks that need frequent access, our pivoted fume hood swings away from the tank for service.

Where several hoods share a duct, our VHM inline duct mist eliminators sit in the duct run, in standard sizes from 500 to 14,000 CFM. At the scrubber, the Viro-Chrome is designed to reclaim re-entrained chrome, return that solution to the bath, and cut waste treatment costs.

That's one more reason to keep chrome on its own system: you don't want nitric or other acids riding back into a chrome bath.

Housing Material for Chromic Acid

PVC and FRP both work as chrome scrubber housings; the deciding factors are the concentration and temperature the housing will see.

On Viron's chemical resistance chart, PVC is rated to 140°F at 10% chromic acid, 100°F at 20% and 70°F at 30%, and isn't recommended at 50%. The chart's FRP column is vinylester, which holds 180°F at 10% and 140°F at 20%, then drops out at 30%.

Our FRP chrome housings specify Hetron 197 or equal, a resin the chart doesn't rate, over a 10 to 20 mil veil and an 80 mil chopped corrosion liner; the PVC version uses Trovidur 150 or equal (ASTM D1784, 12454-B). We won't tell you one material wins every time. Check the resin against your actual liquor concentration and temperature, not the chart's vinylester numbers.

The duct is a separate call. SSTeelcoat, our default duct recommendation, is on the table: its ECTFE coating is rated for 10% to 50% chromic acid on our SSTeelcoat chart. PVC duct is the lower-cost route, and metal plating is one of its listed applications, as long as the air stays within PVC's temperature range.

Design for Where the Rules Are Going

Both EPA's 2012 revision and California's 2023 amendments tightened chrome limits.

We'd build margin into today's design. Under 40 CFR Part 63 Subpart N, tanks that don't comply through a fume suppressant must meet total chromium limits. New tanks (built or reconstructed after February 8, 2012) are held to 0.006 mg/dscm; existing tanks range from 0.007 to 0.015 mg/dscm by type and size.

California's amendments, in effect since January 1, 2024, set a hexavalent chromium limit of 0.00075 mg/amp-hr for chrome plating tanks. They also phase hex chrome out of decorative plating by 2027 (2030 on an alternative pathway) and out of functional plating by 2039.

The units differ, per amp-hour versus per dry standard cubic meter, so the numbers don't compare directly. If your air district follows California's lead, or might, size the chrome side for a tighter number than today's permit. A third pad stage is easier to settle at the design table than after a failed source test.

One System From Tank Hood to Stack

We build the pieces of a plating exhaust system that often come from separate vendors, from tank hoods and PVC or FRP duct through mist eliminators, chrome and packed-bed scrubbers, fans, dampers, control panels and stacks. One engineering team sizes hood capture and scrubbers together, and you have one call to make if something isn't right.

At one plating facility, we designed, built, and installed a ventilation system over a single weekend, with wall-type hoods exhausting more than 50,000 CFM of acid-copper fumes. What arrives on site has already been plumbed and water-tested for leaks, and a PVC scrubber headed somewhere very hot or cold may ride in a heated or refrigerated truck so it doesn't warp or turn brittle.

Before You Write the Spec

If you're writing the exhaust spec for a plating line, send us the tank list with each bath's chemistry, plus what our scrubber data sheet asks for: airflow, chemical removed, loading rate, temperature and required removal efficiency. Our engineers will walk through how we'd split the streams and what the chrome side needs to hit your target.

Viron has manufactured corrosive air handling systems for over 50 years across semiconductor fabs, municipal wastewater facilities, metal finishing operations, and industrial chemical applications. We can help you determine which scrubber is right for your situation. Request a quote today.

Frequently Asked Questions About Scrubbers and Plating Line Exhaust