Vacuum Deaeration Tower Packing: Plastic vs Stainless Steel Pall Ring

Vacuum deaeration towers remove dissolved gases from water by reducing the operating pressure and increasing gas-liquid interfacial contact inside a packed column.

Typical dissolved gases may include:

• oxygen
• carbon dioxide
• air
• other non-condensable gases

Vacuum deaeration can be used in boiler-water treatment, process-water systems, district heating, chemical plants, cooling-water preparation, and other applications where dissolved gases contribute to corrosion or interfere with downstream processing.

Random packing is often installed inside the vacuum tower to spread the water into thin films and droplets, creating more surface area for dissolved gases to escape.

Pall Ring is one packing family that may be considered for this service.

But the next decision is important:

Should the vacuum deaerator use Plastic Pall Ring or Stainless Steel Pall Ring?

The answer depends on more than corrosion resistance.

Vacuum level, temperature, pressure drop, water chemistry, packing size, mechanical requirements, and long-term operating conditions all influence the correct selection.

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How Vacuum Deaeration Works

The amount of gas that remains dissolved in water depends partly on pressure and temperature.

When water enters a lower-pressure environment, the equilibrium changes and dissolved gases become easier to release.

A vacuum deaeration system therefore combines:

reduced pressure + gas-liquid surface renewal

to remove dissolved gases.

Inside a packed tower:

1. water enters near the top
2. the distributor spreads it across the packing
3. water flows downward over the packing surfaces
4. reduced pressure promotes gas release
5. liberated gases move toward the vacuum outlet
6. deaerated water leaves the bottom

The packing does not chemically remove oxygen or CO₂.

Its role is to create the contact and surface renewal required for degassing.

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1. Why Packing Is Important in a Vacuum Deaerator

If water simply falls through an empty vessel, only a limited surface is available for gas release.

Random packing breaks the liquid into:

• films
• rivulets
• droplets
• repeatedly redistributed streams

This increases the effective interfacial area.

A suitable packing should therefore provide:

• good water spreading
• adequate wetting
• low gas-flow resistance
• sufficient void space
• stable drainage
• mechanical reliability

The best packing is not necessarily the one with the maximum nominal surface area.

Under vacuum, pressure drop becomes particularly important.

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2. Why Pressure Drop Matters More Under Vacuum

Suppose a tower operates near atmospheric pressure.

A small packed-bed pressure loss may represent only a small fraction of the total absolute pressure.

Under vacuum, the situation changes.

If the absolute pressure is already low, the same pressure loss can represent a much larger percentage of the operating pressure.

Excessive packed-bed resistance can:

• reduce the effective vacuum in the lower part of the tower
• increase required vacuum-pump duty
• reduce degassing performance
• restrict gas release
• limit hydraulic capacity

Therefore, vacuum deaerator packing should provide enough surface interaction without consuming unnecessary pressure differential.

This is one reason open random packing geometries such as Pall Ring are attractive.

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3. Why Pall Ring Is Used Instead of a Simple Raschig Ring

Pall Ring was developed from the older Raschig Ring concept.

A Raschig Ring is essentially a short cylindrical element.

Pall Ring opens the cylindrical wall and adds internal structural elements.

This provides:

• more open flow area
• better access to internal surfaces
• repeated liquid redistribution
• relatively high void fraction
• reduced obstruction to released gases

For vacuum deaeration, these characteristics are useful because the gases released from the water must leave the packed bed with minimal resistance.

The packing should assist degassing—not create a hydraulic barrier to it.

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4. Plastic Pall Ring: Why It Can Be Attractive

Plastic Pall Ring is widely used in water-treatment and gas-liquid contacting systems.

Its main advantages include:

• very low weight
• corrosion resistance
• economical cost
• easy installation
• low support loading
• multiple size options
• good hydraulic openness

For vacuum deaeration systems operating at moderate temperatures, a suitable plastic material can provide a very practical solution.

PP is one commonly evaluated polymer in industrial water systems, although final suitability depends on the actual operating environment.

Plastic becomes especially attractive where:

• tower weight should be minimized
• water chemistry is corrosive to ordinary carbon steel
• temperature remains within polymer limits
• economical large-volume packing is required

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5. Stainless Steel Pall Ring: Why It May Be Preferred

Stainless steel Pall Ring becomes more attractive when the process places greater demands on:

• temperature resistance
• mechanical strength
• dimensional stability
• long-term structural durability

Possible grades may include:

• SS304
• SS316
• SS316L

depending on the water chemistry and equipment specification.

Stainless steel packing is particularly useful when:

• feedwater temperature is relatively high
• thermal cycling occurs
• polymer creep or deformation is a concern
• mechanical robustness is important
• plant metallurgy requires metallic internals

However, stainless steel is heavier and more expensive than PP.

Therefore, it should not automatically be specified when plastic can safely meet the service conditions.

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6. Plastic vs Stainless Steel Is Not Mainly a Mass-Transfer Decision

If two Pall Rings have comparable geometry and size, the difference between plastic and stainless steel is primarily related to:

material operating envelope + mechanical behavior + surface properties

rather than an entirely different separation mechanism.

A simplified comparison is:

The actual choice should follow the process conditions.

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7. Packing Size Strongly Influences Deaeration Performance

Material selection is only one part of the decision.

Packing size can be equally important.

Smaller Pall Ring

Generally provides:

• more elements per cubic meter
• greater surface area
• more frequent water redistribution
• potentially better mass transfer per unit packed height

But also:

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

Larger Pall Ring

Generally provides:

• lower resistance
• larger gas-release passages
• greater hydraulic capacity
• better fouling tolerance

But lower surface area per cubic meter.

For vacuum deaeration, the correct size therefore balances:

surface renewal + pressure drop + tower diameter + water flow.

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8. Very Small Packing Can Be Counterproductive

Because deaeration requires mass transfer, it may seem logical to choose the smallest packing available.

But under vacuum, extremely small packing can create unnecessary hydraulic resistance.

This can reduce the benefit of the low-pressure operating environment.

The tower may then gain surface area but lose vacuum efficiency.

Therefore, the engineering objective is not:

maximum surface area

but:

sufficient surface area at minimum practical hydraulic resistance.

This is a different optimization from some high-efficiency absorption duties.

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9. Tower Diameter Must Be Compatible With Packing Size

Packing size should also be selected relative to vessel diameter.

If the packing is too large compared with the column diameter, wall effects become significant.

This can create:

• non-uniform packing arrangement
• preferential water paths
• reduced surface utilization
• poor gas removal

This is particularly important in:

• small packaged vacuum deaerators
• pilot systems
• laboratory water-treatment columns

A packing suitable for a 2-meter industrial tower may be completely inappropriate for a 200-mm column.

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10. Water Distribution Is Critical

A vacuum pump cannot compensate for poor liquid distribution.

If water enters the bed in a few concentrated streams:

• much of the packing remains underused
• effective interfacial area decreases
• dissolved gases remain in poorly contacted water
• local hydraulic loading develops

The water distributor should provide reasonably uniform coverage across the full tower area.

Important considerations include:

• number of distribution points
• distributor levelness
• hole or nozzle blockage
• water-flow range
• wall flow

Packing and distributor must be designed as one system.

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11. Wall Flow Is Especially Wasteful in Degassing

Water flowing directly down the vessel wall may bypass much of the random packing.

That reduces the surface renewal intended by the packed bed.

Wall flow may become worse when:

• packing size is too large
• distributor coverage is poor
• tower internals encourage edge flow

For vacuum deaeration, good liquid redistribution is essential because the process depends on repeatedly exposing water to the reduced-pressure environment.

A tower containing large amounts of bypassing liquid may fail to reach the expected dissolved-gas level even when the vacuum is adequate.

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12. Vacuum Level and Packed Height Must Be Considered Together

Deeper vacuum generally increases the thermodynamic driving force for gas release.

More packed height generally increases the opportunity for water-surface renewal.

But neither variable should be increased blindly.

A deeper vacuum requires:

• larger or more capable vacuum equipment
• tighter sealing
• greater energy consumption
• more attention to leakage

A taller packing bed creates:

• more pressure drop
• more packing cost
• greater vessel height

The optimum deaerator therefore balances:

vacuum level + packing efficiency + bed height + operating cost.

Packing selection is part of that optimization.

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13. Air Leakage Can Destroy Vacuum Performance

A poorly sealed vacuum system may perform badly even with excellent packing.

Possible air-inleakage points include:

• flanges
• manways
• instrument connections
• pump seals
• piping joints
• maintenance openings

Air leakage increases the non-condensable gas load handled by the vacuum system.

This can:

• reduce attainable vacuum
• increase pump duty
• reduce deaeration performance

Therefore, if dissolved oxygen suddenly increases, replacing the packing should not be the first response.

Vacuum integrity should also be checked.

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14. Poor Deaeration Is Not Always a Packing Problem

If an existing tower fails to achieve the required dissolved oxygen or CO₂ level, possible causes include:

• insufficient vacuum
• air leakage
• poor water distribution
• excessive water flow
• high water temperature variation
• blocked packing
• incorrect packed height
• vacuum pump deterioration

Changing the Pall Rings without identifying the real problem may produce little improvement.

A retrofit should begin with operating data.

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15. Fouling Can Increase Pressure Drop

Even relatively clean industrial water can contain contaminants such as:

• iron oxides
• suspended solids
• scale
• treatment-chemical residue
• biological matter

Deposits can accumulate inside the packing.

Over time this may cause:

• reduced void area
• higher pressure drop
• uneven liquid distribution
• poorer degassing

This is particularly important under vacuum because additional pressure loss directly reduces the effective low-pressure environment.

Where fouling is expected, a somewhat larger packing may provide more reliable long-term operation.

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16. Plastic Packing Deformation Must Be Avoided

Plastic Pall Rings provide major weight and cost advantages, but they should operate within their mechanical and thermal limits.

Potential problems from unsuitable service conditions include:

• creep
• distortion
• flattening
• reduced void space
• bed compaction

If packing deforms, packed-bed pressure drop may increase significantly.

Therefore, material selection should consider:

• continuous operating temperature
• maximum temperature
• packing depth
• wet-bed load
• long-term polymer strength

The lowest-cost polymer is not automatically the lowest lifecycle-cost solution.

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17. Stainless Steel Packing Requires Corrosion Review

Stainless steel solves many thermal and mechanical concerns, but it is not universally corrosion-proof.

Water chemistry can contain:

• chlorides
• dissolved oxygen
• treatment chemicals
• acidic contaminants
• alkalinity

These conditions influence alloy selection.

SS304 may be adequate for one system while SS316L may be preferred in another.

The correct grade should follow the plant's water chemistry and corrosion requirements.

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18. Packing Support Can Become the Hidden Pressure-Drop Source

Vacuum deaeration requires a support grid that provides high open area.

The support must:

• retain the packing
• carry the wet-bed weight
• resist corrosion
• allow water drainage
• allow released gases to move freely

A restrictive support plate can consume significant pressure differential.

If a high-capacity Pall Ring is installed above a low-open-area support, the packing upgrade may produce little benefit.

This should be checked carefully during retrofit projects.

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19. Plastic vs Stainless Steel for Retrofit Projects

For an existing vacuum deaerator, the simplest decision may be to retain the existing material if it has already demonstrated acceptable service life.

Keep Plastic Pall Ring When:

• the existing polymer remains chemically stable
• temperature remains safely within limits
• deformation is not occurring
• pressure drop is acceptable
• lightweight construction is valuable

Consider Stainless Steel When:

• plastic deformation has occurred
• temperature has increased
• mechanical durability is inadequate
• plant specifications require metallic internals
• longer structural life justifies additional cost

The objective should be to solve a documented problem rather than upgrading material unnecessarily.

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20. When Plastic Pall Ring Is the More Logical Choice

Plastic Pall Ring can be particularly attractive when:

• water temperature is moderate
• polymer compatibility is confirmed
• low packed-bed weight matters
• large packing volume is required
• corrosion resistance is important
• cost efficiency is important
• tower support capacity is limited

For many ordinary industrial vacuum-degassing systems, these advantages can make plastic a strong starting point.

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21. When Stainless Steel Pall Ring Is the More Logical Choice

Stainless steel may be preferable when:

• operating temperature is elevated
• mechanical strength is critical
• long-term dimensional stability matters
• thermal cycling occurs
• polymer service margin is insufficient
• plant metallurgy standards favor stainless steel

The additional material cost should be evaluated against operating risk and expected service life.

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22. Data Needed for a Vacuum Deaerator Packing RFQ

Useful information includes:

• tower internal diameter
• operating absolute pressure
• target vacuum level
• water flow rate
• inlet water temperature
• outlet water temperature
• inlet dissolved oxygen
• required outlet dissolved oxygen
• CO₂ removal requirement if applicable
• current packing type
• current packing material
• packing size
• packed-bed height
• allowable pressure drop
• water chemistry
• chloride concentration where relevant
• fouling history
• distributor design
• packing support information
• vacuum equipment capacity

For retrofit projects, current tower differential pressure and dissolved-gas performance are especially useful.

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Final Selection Principle

Pall Ring can be an effective random packing for vacuum deaeration because its open geometry provides:

water surface renewal + gas-release paths + relatively low hydraulic resistance.

The choice between plastic and stainless steel should then be made according to the actual service conditions.

Plastic Pall Ring is attractive where:

low weight + corrosion resistance + economical cost

are important and temperature remains within the polymer's safe operating range.

Stainless Steel Pall Ring becomes more attractive where:

temperature + mechanical durability + dimensional stability

justify metallic construction.

For vacuum systems, however, the most important principle remains:

Do not maximize packing surface area at the expense of vacuum pressure drop.

The correct packing should provide enough liquid-surface renewal to achieve the required deaeration while preserving the low-pressure environment that makes vacuum degassing effective.