Acid Mist Scrubber Packing for Metal Finishing: PP Pall Ring vs Tellerette Ring

Metal finishing, electroplating, pickling, anodizing, and acid-cleaning processes can generate corrosive exhaust containing acid vapor, reactive gases, and entrained acidic droplets.

Typical exhaust sources may include:

• hydrochloric acid pickling tanks
• sulfuric acid process tanks
• plating baths
• acid cleaning lines
• anodizing tanks
• chemical surface-treatment equipment

Packed wet scrubbers are often used to treat these ventilation streams.

Inside the scrubber, contaminated air contacts a circulating liquid over a bed of random packing.

Two plastic packing options that may be considered are:

• PP Pall Ring
• Plastic Tellerette Ring

Both provide gas-liquid contact, but their geometry creates different balances between:

• pressure drop
• gas capacity
• liquid distribution
• fouling tolerance
• mass-transfer area
• long-term maintenance

The practical engineering question is:

When is PP Pall Ring the better general-purpose packing, and when does the more open Tellerette Ring geometry make sense in a metal-finishing acid mist scrubber?

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Why Metal-Finishing Exhaust Is Different From a Clean Chemical Gas Stream

Acid-process ventilation air is often not perfectly clean.

Depending on the operation, the exhaust may contain:

• acid vapor
• fine acidic droplets
• metal salts
• dust
• process aerosols
• water droplets
• treatment-chemical residues

The circulating scrubber liquid can also accumulate contaminants over time.

This means packing must remain effective not only when new, but after extended exposure to:

• corrosive chemistry
• deposits
• salts
• high ventilation airflow
• continuous wet operation

For this reason, hydraulic openness and maintainability can be just as important as nominal surface area.

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1. What the Packing Actually Does

Random packing provides a large contact zone between:

contaminated exhaust gasandscrubbing liquid.

The packing helps:

• spread liquid over many surfaces
• break liquid into films and droplets
• repeatedly redirect the gas
• create gas-liquid contact
• maintain open gas passages

The packing does not replace the chemistry.

Actual contaminant removal also depends on:

• acid or contaminant type
• gas concentration
• scrubbing-liquid chemistry
• pH
• liquid circulation rate
• gas flow
• bed height
• mist-elimination system

A good packing cannot compensate for exhausted or chemically unsuitable scrubbing liquor.

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2. Acid Vapor and Acid Mist Are Not the Same Thing

This distinction is important.

A packed bed is highly effective for gas-liquid mass transfer.

But acid mist can consist of physical liquid droplets suspended in the gas.

Very fine droplets may require a dedicated mist eliminator.

Therefore, a metal-finishing scrubber may need:

packed bed → chemical absorption / gas contact

followed by:

mist eliminator → droplet removal

If visible acid carryover remains at the outlet, simply increasing packing height may not solve the problem.

The demisting section must also be evaluated.

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3. Why PP Pall Ring Is a Common Starting Point

PP Pall Ring is one of the most common plastic random packings used in wet scrubbers.

Its open cylindrical geometry provides:

• wall windows
• internal contact surfaces
• substantial void space
• multiple gas-flow paths
• repeated liquid redistribution

PP construction also offers practical advantages:

• low weight
• economical cost
• easy loading
• low support load
• good resistance to many wet-scrubber environments

For many ordinary acid-exhaust systems, PP Pall Ring is therefore a logical general-purpose starting point.

However, material compatibility must always be checked against the actual acid concentration and temperature.

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4. Why Tellerette Ring Is Different

Tellerette Ring uses a very open lattice or looped geometry.

Compared with a conventional ring wall, the structure contains large interconnected voids.

This can provide:

• lower physical obstruction to airflow
• large gas passages
• good liquid drainage
• multiple wettable edges and curved surfaces
• greater tolerance to partial deposits

Its main attraction is not simply “more advanced geometry.”

It is greater hydraulic openness.

This can matter in metal-finishing ventilation systems where large volumes of low-pressure air must move through the scrubber.

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5. Ventilation Airflow Can Be Very Large

Metal-finishing lines often use local exhaust systems to collect fumes from multiple tanks.

The contaminant concentration may be modest, but the total air volume can be substantial.

The fan must overcome pressure losses across:

• hoods
• ductwork
• scrubber inlet
• packing support
• packed bed
• mist eliminator
• outlet stack

If the packed bed creates excessive resistance:

• fan power increases
• capture airflow may decrease
• process fumes may escape from the source
• energy consumption rises

This makes low packed-bed pressure drop especially valuable in ventilation-driven scrubber systems.

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6. When Pall Ring Is Usually Enough

PP Pall Ring is often sufficient when:

• airflow is within a normal range
• packing remains reasonably clean
• pressure drop is acceptable
• contaminant absorption is effective
• standardization matters
• replacement availability is important

If an existing Pall Ring scrubber already performs reliably, changing packing geometry simply because Tellerette Ring is more open may not produce a meaningful benefit.

A geometry change should solve a documented problem.

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7. When Tellerette Ring Becomes More Attractive

Tellerette Ring becomes more interesting when the scrubber experiences:

High Ventilation Airflow

Large gas volume makes low hydraulic resistance valuable.

Limited Fan Pressure

The fan cannot tolerate a large packed-bed pressure drop.

Frequent Packing Fouling

Deposits reduce the open area of a tighter packing bed.

Metal Salt Accumulation

Reaction products or aerosols gradually contaminate the packing.

Increasing Differential Pressure

The bed becomes hydraulically restrictive before cleaning intervals are reached.

In these situations, a more open packing can provide a practical maintenance advantage.

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8. Fouling Is Often a Major Long-Term Issue

Metal-finishing scrubbers may accumulate material such as:

• metal salts
• crystalline deposits
• sludge
• scale
• dust
• chemical reaction products

As deposits build up:

• gas passages become smaller
• liquid distribution worsens
• pressure drop rises
• gas channels through cleaner regions
• effective contact decreases

This is why the clean-bed packing data are only part of the selection.

Long-term operating behavior matters.

A packing that performs slightly better when clean but plugs rapidly may be worse over the full maintenance cycle.

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9. Open Packing Does Not Eliminate Fouling

Tellerette Ring can provide greater tolerance to deposits, but it is not self-cleaning.

Severe fouling can still block the bed.

A good maintenance strategy may include:

• periodic water flushing
• chemical cleaning where appropriate
• recirculation blowdown
• solids removal
• distributor inspection
• differential-pressure monitoring

If the scrubber liquor is allowed to become heavily saturated or contaminated, no packing geometry will remain open indefinitely.

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10. Packing Size Still Matters

Pall Ring and Tellerette Ring are both available in different sizes depending on supplier.

Packing size changes the trade-off.

Smaller Packing

Generally provides:

• greater surface area
• more frequent liquid redistribution
• potentially stronger mass-transfer performance

But also:

• smaller flow passages
• greater pressure drop
• higher fouling sensitivity

Larger Packing

Generally provides:

• greater open area
• lower resistance
• better deposit tolerance
• higher gas capacity

But lower surface area per unit packed volume.

For dirty metal-finishing exhaust, selecting the smallest available packing may therefore be a poor long-term decision.

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11. Material Compatibility Must Be Checked Acid by Acid

“Acid scrubber” is not a complete material specification.

A plating or pickling line may contain:

• HCl
• H₂SO₄
• HNO₃
• mixed acids
• alkaline cleaners
• oxidizing chemicals

PP may perform very well in one duty but have a different service margin in another.

Material selection should consider:

• exact chemical
• concentration
• operating temperature
• maximum temperature
• oxidizing conditions
• cleaning chemicals

Do not select PP merely because another acid scrubber at the same factory uses PP packing.

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12. Nitric and Oxidizing Service Need Extra Caution

Strongly oxidizing environments deserve special material review.

If nitric acid or other oxidizing chemicals are present, the actual polymer compatibility should be confirmed carefully.

This applies not only to the random packing but also to:

• distributor
• support
• tower lining
• piping
• mist eliminator

The most chemically resistant packing in the tower will not help if another internal component fails first.

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13. Liquid Distribution Is Critical

The scrubbing liquid must reach the entire packed-bed cross-section.

Poor distribution can create:

• dry zones
• gas channeling
• localized flooding
• uneven contaminant removal
• uneven fouling

Distributor openings may themselves become blocked by salts or solids.

For dirty metal-finishing service, the distributor should be inspected regularly.

Sometimes declining scrubber performance is caused primarily by a partially blocked distributor rather than by the packing.

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14. Wall Flow Reduces Effective Contact

Liquid that runs directly down the vessel wall bypasses much of the random packing.

This reduces useful gas-liquid contact.

Wall flow can be caused by:

• poor distributor coverage
• packing that is too large relative to tower diameter
• improper packing loading

Therefore, packing size should always be reviewed relative to the scrubber diameter.

A very large Tellerette Ring may offer excellent gas capacity but perform poorly if the tower itself is too small for that packing size.

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15. Pressure Drop Is a Useful Maintenance Indicator

One of the simplest ways to track packed-bed condition is differential pressure.

If gas and liquid rates stay similar but pressure drop gradually rises, likely causes include:

• packing fouling
• salt deposition
• support blockage
• excessive liquid holdup
• mist eliminator fouling

Trending differential pressure can help identify problems before airflow becomes severely restricted.

This is especially valuable in production lines where ventilation reliability affects worker exposure and process safety.

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16. The Packing Support May Be the Real Restriction

The packing support below the bed must provide:

• adequate mechanical strength
• sufficient open area
• chemical resistance
• reliable drainage

In dirty scrubber service, deposits can collect on the support openings.

This can cause:

• liquid backup
• localized flooding
• increasing pressure drop

During shutdown, the support should be inspected together with the packing.

Replacing the random packing while leaving a blocked support may provide little improvement.

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17. Mist Eliminator Condition Must Also Be Checked

Metal-finishing scrubbers commonly use mist eliminators after the packed section.

If the demister becomes fouled:

• total tower pressure drop increases
• droplet carryover can rise
• fan airflow can decrease

Therefore, if the scrubber suddenly develops high differential pressure, do not automatically blame the packing.

Check:

packing + support + mist eliminator

separately where possible.

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18. Pall Ring vs Tellerette Ring: Practical Comparison

There is no universal winner.

The correct choice depends on the actual operating problem.

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19. When PP Pall Ring Is the Better Choice

PP Pall Ring is usually the stronger starting point when:

• the scrubber is relatively clean
• pressure drop is acceptable
• the existing design is proven
• standardization matters
• product availability matters
• replacement simplicity matters
• there is no serious fouling problem

For many plating and pickling exhaust systems, this remains a practical and economical solution.

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20. When Tellerette Ring Is Worth the Change

Tellerette Ring becomes more attractive when:

• the existing packing plugs frequently
• fan pressure is limited
• ventilation airflow is high
• pressure drop increases rapidly during service
• deposits are unavoidable
• longer cleaning intervals are important

In these cases, its open geometry can solve a specific hydraulic or maintenance problem.

That is a much stronger reason to change packing than simply selecting a different shape.

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21. Retrofit Projects Should Identify the Real Failure Mode

Before replacing the packing, determine what is actually wrong.

Poor Contaminant Removal + Normal Pressure Drop

Check:

• scrubbing chemistry
• pH
• liquid circulation
• distributor condition
• packed height

High Pressure Drop + Good Chemistry

Check:

• packing fouling
• support blockage
• mist eliminator fouling

Insufficient Exhaust Airflow

Check:

• fan capacity
• duct resistance
• packing pressure drop
• mist eliminator pressure drop

Only then should a new packing geometry be selected.

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22. Data Needed for an Acid Mist Scrubber Packing RFQ

Useful information includes:

• tower internal diameter
• gas flow rate
• maximum ventilation flow
• acid type
• acid concentration
• contaminant concentration
• scrubbing-liquid composition
• operating pH
• liquid circulation rate
• operating temperature
• maximum temperature
• current packing type
• current packing size
• packed-bed height
• current pressure drop
• fouling or salt-deposit history
• distributor type
• support design
• mist eliminator type
• tower material

For retrofit projects, photographs of removed packing and deposits are especially useful.

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Final Selection Principle

PP Pall Ring and Tellerette Ring can both be effective random packings in metal-finishing acid scrubbers.

PP Pall Ring remains a strong general-purpose choice where:

reliable mass transfer + availability + economical replacement + proven operation

are the main priorities.

Tellerette Ring becomes more attractive where:

high ventilation airflow + low fan-pressure margin + frequent fouling + deposit tolerance

are the dominant operating concerns.

The correct engineering question is therefore:

Is the scrubber limited mainly by gas-liquid mass transfer, or by long-term hydraulic restriction and fouling?

If mass transfer and standardization dominate, PP Pall Ring is often sufficient.

If airflow and fouling repeatedly limit performance, Tellerette Ring is worth serious evaluation.