25 mm vs 38 mm Pall Ring: Which Size Should Be Selected?

25 mm and 38 mm Pall Rings can both be suitable for packed towers, but they do not provide the same balance of mass-transfer area, pressure drop, hydraulic capacity and fouling tolerance. In general, 25 mm Pall Rings are considered when higher packing surface area and stronger contacting performance are important, while 38 mm Pall Rings are often considered when lower hydraulic resistance, larger flow passages or greater tolerance to fouling are more important.

The correct choice is not simply:

smaller packing = better efficiency

or:

larger packing = better capacity.

The selection should consider the complete application, including:

• tower diameter;
• gas flow;
• liquid load;
• pressure-drop limitation;
• fouling tendency;
• required mass-transfer performance;
• existing tower internals;
• replacement constraints.

The key product-selection question is:

When does the additional surface area of 25 mm Pall Ring justify its higher hydraulic resistance, and when is the more open 38 mm Pall Ring the better choice?

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1. What Is the Main Difference Between 25 mm and 38 mm Pall Rings?

The most obvious difference is nominal packing size.

But changing size also changes the structure of the complete packed bed.

A smaller Pall Ring generally creates:

• more packing elements per unit bed volume;
• greater geometric surface area;
• smaller gas/liquid flow passages;
• greater hydraulic resistance.

A larger Pall Ring generally creates:

• fewer packing elements per unit volume;
• larger open flow paths;
• lower resistance to gas and liquid movement;
• lower specific surface area.

Therefore, the real comparison is:

More Contacting Area

versus

More Hydraulic Openness

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2. 25 mm Pall Ring: Why Would Engineers Consider It?

25 mm Pall Rings may be attractive when the process places relatively high value on:

• mass-transfer area;
• efficient gas-liquid contacting;
• compact packed height;
• smaller tower geometry.

Smaller packing creates more individual elements within the same packed volume.

This generally increases the available geometric surface area.

That can be useful in applications where:

• separation duty is demanding;
• absorber performance is important;
• packed height is constrained.

However, higher surface area is not free.

The hydraulic trade-off must also be considered.

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3. 38 mm Pall Ring: Why Would Engineers Consider It?

38 mm Pall Rings provide a more open bed structure than smaller 25 mm packing.

This can be attractive when the application prioritizes:

• lower pressure-drop tendency;
• higher hydraulic capacity;
• easier liquid drainage;
• greater tolerance to solids or deposits.

This often makes 38 mm packing worth considering in towers where:

• gas throughput is relatively high;
• fouling risk is significant;
• pressure-drop margin is limited.

Again, this does not mean 38 mm is universally superior.

The larger size sacrifices some specific surface area compared with smaller packing.

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4. Specific Surface Area: 25 mm Usually Has the Advantage

For the same Pall Ring family and material, smaller packing generally provides more geometric surface area per unit packed volume.

This is one reason 25 mm Pall Ring can be attractive for mass-transfer duty.

More surface area can provide more opportunity for:

• liquid spreading;
• gas-liquid contact;
• mass transfer.

But actual tower performance also depends on:

• liquid distribution;
• gas distribution;
• wetting;
• physical properties;
• operating loads.

Therefore:

Higher geometric surface area does not automatically guarantee higher real tower efficiency.

It creates potential for stronger contacting, but the rest of the tower must use that area effectively.

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5. Pressure Drop: 38 mm Usually Has the Hydraulic Advantage

A larger packing size generally creates:

• larger flow passages;
• less obstruction to gas flow.

Therefore, 38 mm Pall Ring typically has a lower pressure-drop tendency than 25 mm Pall Ring under comparable service.

This can be important when:

• blower capacity is limited;
• tower pressure drop must be minimized;
• gas flow is high;
• the tower operates under low-pressure or vacuum-sensitive conditions.

Pressure drop should still be evaluated using project-specific hydraulic data.

A simple statement such as:

“38 mm has lower pressure drop”

is useful for preliminary selection, but it is not a substitute for actual tower evaluation.

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6. Hydraulic Capacity: 38 mm Often Provides More Margin

Larger flow passages can provide additional hydraulic capacity.

This may allow a packed bed to tolerate greater:

• gas flow;
• liquid flow

before approaching hydraulic limitations.

Therefore, if two candidate sizes both satisfy the mass-transfer requirement, 38 mm may provide more operating margin.

This can be valuable in plants that expect:

• future throughput increases;
• operating-rate variation;
• occasional high-load conditions.

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7. Smaller Packing Is Not Automatically Better for Every Absorber

Absorption performance depends on more than packing surface area.

A 25 mm Pall Ring may provide stronger contact potential, but if the tower operates too close to hydraulic limits, higher resistance can offset the benefit.

For an absorber, selection should balance:

• mass-transfer requirement;
• gas velocity;
• liquid circulation;
• packed height;
• pressure drop.

The best packing size is the one that satisfies both:

Process Performance + Hydraulic Stability

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8. Fouling Tolerance: 38 mm Often Has an Advantage

Fouling can involve:

• suspended solids;
• salts;
• scale;
• precipitates;
• biological growth;
• process deposits.

Larger packing generally provides larger openings.

This can make 38 mm Pall Ring more tolerant of fouling than 25 mm packing.

Smaller passages are more easily restricted when deposits accumulate.

Therefore, for dirty service:

The most efficient clean packing is not always the most reliable operating packing.

A slightly more open bed may provide longer operating intervals between maintenance shutdowns.

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9. Why 25 mm Packing May Be More Sensitive to Solids

Smaller random packing creates:

• smaller void spaces;
• more contact points;
• more narrow liquid pathways.

If the process contains solids, these areas can become sites for:

• deposition;
• bridging;
• blockage.

As fouling develops, pressure drop may rise faster.

This does not mean 25 mm Pall Ring cannot be used in dirty service.

It means fouling tendency should be given more weight in the size decision.

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10. Why 38 mm Packing Can Be Attractive for Scrubbers

Many scrubber applications involve:

• relatively high gas throughput;
• circulating liquid;
• potential solids;
• reaction products;
• salt deposition.

In these services, hydraulic openness can be especially valuable.

38 mm Pall Ring may therefore be attractive when:

• pressure drop matters;
• contamination is present;
• maintenance interval is important.

But if the scrubber requires very high removal performance within limited packed height, 25 mm may still deserve evaluation.

The process requirement must decide.

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11. Why 25 mm Packing Can Be Attractive for Cleaner Absorption Service

If the process fluid is relatively clean and the tower is not strongly hydraulically constrained, the additional surface area of 25 mm Pall Ring may be useful.

Examples may include applications where:

• gas-liquid contacting is the main priority;
• fouling risk is low;
• packing height is limited.

The final decision still depends on actual:

• gas load;
• liquid load;
• tower diameter.

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12. Distillation Applications Require More Than a Size Comparison

For distillation, packing size can affect:

• pressure drop;
• effective mass-transfer performance;
• capacity.

Smaller packing may be attractive when efficiency is important.

Larger packing may be attractive when capacity or low pressure drop matters more.

However, demanding distillation service may also involve comparison with:

• structured packing;
• trays.

Therefore, choosing between 25 mm and 38 mm Pall Rings should be part of the broader separation decision rather than treated as an isolated rule.

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13. Tower Diameter Matters

The packing should be reasonably matched to tower diameter.

In small-diameter columns, very large packing can create a relatively small number of packing elements across the vessel cross-section.

This can make the bed less representative of an ideal random packing structure and may increase wall effects.

Smaller packing can therefore be more attractive in smaller towers.

However:

There is no single universal tower-diameter rule that should be applied blindly to every Pall Ring design.

Actual suitability depends on:

• packing geometry;
• column diameter;
• process duty.

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14. Why Wall Effects Matter

Random packing near the tower wall does not behave exactly like packing in the center of the bed.

If packing size becomes large relative to tower diameter, wall effects can become more significant.

Possible consequences include:

• uneven void structure;
• preferential liquid flow near the wall;
• less uniform bed behavior.

This is another reason very large random packing is not automatically the best choice for a small column.

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15. 25 mm Pall Ring May Be Better Suited to Some Small Columns

Where tower diameter is limited, 25 mm Pall Ring provides more individual packing pieces across the tower cross-section than 38 mm packing.

This can help create a more representative random bed.

However, column diameter is only one selection factor.

Engineers should still review:

• pressure drop;
• gas velocity;
• liquid load;
• required separation.

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16. 38 mm Pall Ring May Be Attractive as Tower Diameter and Throughput Increase

As towers become larger and gas throughput increases, hydraulic capacity often becomes increasingly important.

In these situations, 38 mm Pall Ring can offer a useful balance between:

• sufficient mass-transfer area;
• larger open passages;
• manageable pressure drop.

This is one reason mid-size random packing is widely considered in industrial towers.

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17. Material Choice Does Not Eliminate the Size Decision

25 mm and 38 mm Pall Rings may be manufactured in:

• plastic;
• metal;
• ceramic.

Selecting the material does not automatically determine the size.

For example:

PP Pall Ring

still requires a separate decision between:

• 25 mm;
• 38 mm;
• 50 mm;
• other available sizes.

Material answers:

What should the packing be made from?

Size answers:

What hydraulic and mass-transfer balance should the packed bed provide?

These are related but separate decisions.

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18. Plastic 25 mm vs 38 mm Pall Ring

For plastic Pall Rings, size selection may be influenced by:

• lightweight construction;
• corrosion resistance;
• scrubber service;
• fouling tendency.

A clean absorber may justify the additional area of 25 mm.

A fouling-prone scrubber may favor 38 mm or a more open alternative.

Chemical compatibility must still be confirmed separately.

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19. Metal 25 mm vs 38 mm Pall Ring

For metal Pall Rings, size affects both:

• bed geometry;
• total packing weight.

Wall thickness and bulk density also matter.

Two suppliers' 25 mm Pall Rings may not have exactly the same:

• geometry;
• bulk density;
• surface area.

Therefore, when comparing quotations, use actual product datasheets rather than nominal size alone.

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20. Ceramic 25 mm vs 38 mm Pall Ring

Ceramic packing introduces additional considerations such as:

• brittle handling;
• higher bed weight;
• transport breakage.

Size still affects:

• surface area;
• hydraulic resistance;
• fouling tolerance.

For corrosive or higher-temperature service, ceramic may be considered when polymer materials are unsuitable, but packing size still requires its own engineering decision.

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21. Does 25 mm Pall Ring Always Give Better Efficiency?

No.

Smaller packing generally offers more surface area, but real tower efficiency depends on:

• distribution;
• wetting;
• gas/liquid loading;
• process properties;
• bed installation.

If liquid distribution is poor, additional packing surface may not be fully utilized.

Therefore, it is safer to say:

25 mm Pall Ring can provide greater mass-transfer opportunity, but actual efficiency is application-specific.

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22. Does 38 mm Pall Ring Always Give Lower Pressure Drop?

Typically it tends toward lower hydraulic resistance than a smaller equivalent packing, but actual pressure drop depends on:

• gas rate;
• liquid rate;
• material;
• packing geometry;
• fluid properties.

Supplier hydraulic data or engineering evaluation should be used for final selection.

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23. Which Size Is Better for Fouling Service?

If fouling tolerance is a major design priority, 38 mm Pall Ring will often deserve stronger consideration because of its more open flow passages.

However, the decision should consider the severity and type of fouling.

For very severe fouling, even 38 mm may not be sufficiently open.

Engineers may need to evaluate:

• larger packing;
• alternative geometry;
• upstream solids control.

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24. Which Size Is Better for High Gas Flow?

Where gas throughput is high and pressure-drop margin is limited, 38 mm Pall Ring may provide greater hydraulic flexibility.

But tower capacity is not determined by packing size alone.

Also review:

• tower diameter;
• gas density;
• liquid load;
• process pressure.

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25. Which Size Is Better When Packed Height Is Limited?

If the tower has limited available packed height, stronger mass-transfer performance per unit height may become important.

This can make smaller packing such as 25 mm more attractive.

But it is not valid to assume that changing from 38 mm to 25 mm will always produce the required process performance.

The actual separation duty still requires evaluation.

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26. Which Size Is Better for Low Pressure Drop?

If low pressure drop is a major requirement, 38 mm Pall Ring generally has the stronger preliminary position.

This can be relevant in:

• gas scrubbers;
• blower-limited systems;
• low-pressure processes.

For highly pressure-sensitive separation service, structured packing may also deserve comparison rather than automatically selecting a larger random packing.

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27. What If the Tower Already Uses 25 mm Pall Rings?

A common replacement question is:

Can we replace 25 mm Pall Rings with 38 mm Pall Rings?

Possibly, but the size change should not be treated as a simple one-for-one substitution.

The change may affect:

• specific surface area;
• pressure drop;
• packed-bed performance;
• support-grid compatibility;
• required packed height.

The original process duty should be reviewed first.

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28. Why Operators Sometimes Want to Change from 25 mm to 38 mm

Typical reasons include:

• high pressure drop;
• frequent fouling;
• limited hydraulic capacity;
• planned throughput increase.

In these cases, 38 mm may reduce hydraulic restriction.

But the engineering question becomes:

Will the tower still provide the required mass-transfer performance after reducing packing surface area?

That must be checked before replacement.

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29. Why Operators Sometimes Want to Change from 38 mm to 25 mm

The opposite change may be considered when:

• separation efficiency is insufficient;
• packed height cannot easily be increased;
• operating loads remain hydraulically moderate.

But 25 mm may increase:

• pressure drop;
• fouling sensitivity.

Therefore, improved contacting cannot be evaluated without hydraulic consequences.

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30. Support Grid Compatibility Must Be Checked During Size Changes

A packing support designed for 38 mm Pall Rings may have openings that are inappropriate for smaller 25 mm packing.

Possible consequences include:

• packing falling through;
• packing becoming trapped;
• need for an additional retaining layer.

Therefore, retrofit projects should inspect:

• support-grid opening;
• beam arrangement;
• packing retention.

Size selection affects tower internals as well as packing itself.

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31. Hold-Down Requirements May Also Change

Different packing sizes can have different:

• bulk density;
• bed movement behavior.

For lightweight plastic Pall Rings, engineers should review whether the existing:

• hold-down grid;
• bed limiter

remains appropriate after the size change.

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32. Do Not Mix 25 mm and 38 mm Pall Rings Randomly to “Average” the Performance

Using both sizes in one uncontrolled mixed bed does not automatically create an ideal compromise.

Smaller packing may occupy voids between larger packing and create:

• uneven bed density;
• unpredictable pressure drop;
• maldistribution.

If different packing sizes are intentionally used, they should normally be treated as defined engineered sections rather than casually mixed.

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33. Packing Quantity Changes with Size and Bulk Density

The required packed volume is primarily defined by:

Tower Area × Packed Height

But the shipping weight can change with packing size because bulk density differs.

Therefore, switching from 25 mm to 38 mm may affect:

• net weight;
• freight;
• support load.

When comparing supplier quotations, use the same packed-volume basis.

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34. Do Not Compare 25 mm and 38 mm by Price per Kilogram Alone

A smaller packing may have a different:

• bulk density;
• piece count;
• manufacturing cost.

The meaningful commercial comparison should consider:

• USD/m³;
• required packed volume;
• freight;
• expected process performance.

Price per kilogram alone can distort the comparison.

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35. Fouling Cost Can Be More Important Than Packing Price

Suppose 25 mm packing provides slightly stronger clean-service mass-transfer potential but requires:

• frequent cleaning;
• shorter operating campaigns.

A 38 mm packing with greater fouling tolerance might provide better lifecycle economics.

The best product decision should consider:

Initial Cost + Operating Reliability + Maintenance Frequency

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36. Pressure-Drop Cost Can Also Matter

Higher pressure drop may increase:

• blower duty;
• compressor energy;
• operating cost.

Therefore, a lower-cost 25 mm packing may not necessarily provide lower lifecycle cost if its additional pressure drop creates meaningful energy penalties.

The significance depends on the process.

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37. Efficiency Loss Can Make Oversizing Packing Expensive

The opposite is also possible.

Choosing 38 mm solely for lower pressure drop may reduce mass-transfer performance enough that the tower requires:

• more packed height;
• larger equipment;
• lower throughput.

Therefore, larger packing is not automatically the lowest-cost option.

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38. 25 mm vs 38 mm Pall Ring Decision Table

This table is for preliminary product comparison only.

Final selection depends on project-specific operating conditions.

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39. Common Mistake 1: “Smaller Packing Is Always More Efficient”

Smaller packing generally provides more surface area.

But efficiency also depends on:

• distribution;
• wetting;
• hydraulics.

Higher surface area does not guarantee better operating performance.

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40. Common Mistake 2: “Larger Packing Is Always Better Because Pressure Drop Is Lower”

Lower resistance is valuable, but the packing must still provide sufficient mass-transfer performance.

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41. Common Mistake 3: Ignoring Fouling

Clean-service performance data may be misleading for a tower that handles:

• solids;
• salts;
• polymerizing material.

Fouling tolerance can dominate the size decision.

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42. Common Mistake 4: Ignoring Tower Diameter

A packing size appropriate for a large industrial tower may not be the best choice for a small column.

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43. Common Mistake 5: Changing Size During Replacement Without Checking the Support

A smaller packing may not be safely retained by the existing support grid.

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44. Common Mistake 6: Comparing Only Supplier Price

Different sizes affect:

• packed-bed performance;
• shipping weight;
• energy cost;
• maintenance frequency.

Compare total engineering value.

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45. What Data Should Be Confirmed Before Choosing 25 mm or 38 mm Pall Ring?

Tower Information

• tower internal diameter;
• available packed height;
• number of packed beds.

Gas Conditions

• gas flow;
• pressure;
• temperature;
• composition.

Liquid Conditions

• liquid flow;
• density;
• viscosity where relevant;
• solids or fouling tendency.

Process Duty

• absorption;
• scrubbing;
• stripping;
• distillation;
• other gas-liquid contacting duty.

Operating Priority

Determine whether the project prioritizes:

• efficiency;
• low pressure drop;
• high capacity;
• fouling tolerance.

Existing Tower Information

For retrofit projects:

• current packing size;
• current pressure drop;
• current performance;
• support-grid details.

For broader hydraulic evaluation, project data can also be screened with the DAIER Tower Packing Engineering Assistant:

https://www.pxdaier.com/tower-packing-engineering-assistant.html

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Frequently Asked Questions

Is 25 mm Pall Ring more efficient than 38 mm Pall Ring?

25 mm packing generally provides more specific surface area, which can support stronger mass-transfer performance. Actual tower efficiency still depends on operating conditions and distribution.

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Does 38 mm Pall Ring have lower pressure drop?

It generally has a lower pressure-drop tendency because of its larger open flow passages, but actual pressure drop depends on gas and liquid loading and product geometry.

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Which Pall Ring size is better for a scrubber?

For clean service with demanding mass-transfer duty, 25 mm may deserve consideration. For higher gas load or fouling-prone scrubber service, 38 mm may provide greater hydraulic tolerance.

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Which Pall Ring size is better for fouling service?

38 mm is generally the stronger preliminary candidate because of larger flow passages, but severe fouling may require an even more open packing design.

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Which Pall Ring size is better for a small tower?

25 mm may be more appropriate in some smaller towers because more packing elements fit across the column diameter. Exact suitability depends on tower geometry and process duty.

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Can I replace 25 mm Pall Rings with 38 mm Pall Rings?

Possibly, but the change affects hydraulic resistance, mass-transfer area and possibly support-grid compatibility. The existing tower duty should be reviewed first.

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Can 25 mm and 38 mm Pall Rings be mixed in one bed?

Uncontrolled mixing is generally not recommended because it creates uncertain bed structure and hydraulic behavior.

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Is 38 mm Pall Ring always cheaper than 25 mm?

Not necessarily.

Price depends on:

• material;
• manufacturing;
• bulk density;
• supplier;
• quantity.

Compare cost per required packed volume and total delivered project cost.

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Selection Takeaway

The choice between 25 mm and 38 mm Pall Ring is fundamentally a trade-off between greater contacting area and greater hydraulic openness.

A useful preliminary decision logic is:

Consider 25 mm Pall Ring when:

• mass-transfer intensity is a major priority;
• packed height is constrained;
• fouling risk is relatively low;
• tower diameter favors smaller packing;
• hydraulic margin is adequate.

Consider 38 mm Pall Ring when:

• lower pressure drop is important;
• gas throughput is high;
• fouling tolerance matters;
• larger flow passages are desirable;
• greater hydraulic capacity is required.

The correct question is not:

“Which size is better?”

It is:

“Which size provides the better balance of mass-transfer performance, pressure drop, capacity and fouling tolerance for this specific tower?”