In the 1950s, an engineer at Pall Corporation made a simple modification to the Raschig ring: he cut rectangular windows into the cylinder wall and bent the cut material inward to form internal tabs. This single change — converting a solid cylinder into an open-walled structure — increased void fraction from 65% to 91% at 25 mm size and reduced the packing factor from ~550 m-1 to 176 m-1. The Pall ring was born, and it has since become the most widely used random packing design in the world, largely superseding the Raschig ring for scrubber and absorption applications. The window openings reduce pressure drop by 40-60% and increase mass transfer efficiency by 20-30% at a cost premium of only 20-40% over Raschig rings — a performance improvement that typically pays back within 2-7 months through reduced fan energy consumption. Today, Pall rings account for an estimated 60-70% of all random packing installed in scrubbers worldwide, making them the default choice for engineers specifying packed column internals.
This guide covers Pall ring design and operating principles, performance comparison with Raschig rings, size specifications with packing factor data for 16-76 mm sizes, material options for different temperature zones, applications by gas type, lifecycle cost analysis with payback periods, a three-step selection framework, and installation best practices.
For the complete packing media methodology see our scrubber packing media selection guide.
Key Takeaways
- Pall rings achieve 40-60% lower pressure drop, 50-100% higher capacity before flooding, and 20-30% better mass transfer efficiency than Raschig rings. The packing factor of 176 m-1 for 25 mm PP Pall rings is less than half that of PP Raschig rings (~400 m-1) and less than one-third of ceramic Raschig rings (~550 m-1), allowing 80% higher gas velocity before flooding.
- The Pall ring cost premium of $500-700 (20-25%) over Raschig rings pays back within 2-7 months at 8,000 hr/yr through reduced fan energy consumption. Over 10 years, the energy savings difference is $15,400 in favor of Pall rings. Below 2,000 hr/yr, the payback may exceed 3 years and Raschig rings may be justified.
- PP Pall rings cover 85-90% of scrubber applications below 80C at $400-660/m3. PVDF Pall rings are required for 80-120C and HF service at $1,000-1,660/m3. SS316 Pall rings are limited to non-chloride high-pressure service due to corrosion rates of 0.5-1.5 mm/year in wet HCl above 60C.
- Pall rings are not manufactured below 16 mm — for laboratory columns requiring 6-13 mm packing, ceramic Raschig rings remain the only option. For industrial scrubbers, 25 mm (clean gas) and 50 mm (particulate-laden gas) cover the majority of applications.
Pall Ring Fundamentals
What Are Pall Rings?
A Pall ring is a hollow cylinder with rectangular window openings cut into the wall and internal tabs bent inward from those openings, used as random packing in packed columns for mass transfer operations. The windows allow gas to pass laterally through each ring rather than flowing only around the outside, and the tabs provide additional surface area for liquid film formation and redistribution. A 25 mm PP Pall ring achieves 209 m2/m3 of surface area with 91% void fraction and a packing factor of 176 m-1. For comparison, a 25 mm PP Raschig ring provides 195 m2/m3 with 80% void fraction and a packing factor of approximately 400 m-1, and a 25 mm ceramic Raschig ring provides 190 m2/m3 with 65% void fraction and a packing factor of ~550 m-1. The Pall ring’s combination of high void fraction and low packing factor gives it a decisive hydraulic advantage over both PP and ceramic Raschig rings.
Design and Operating Principle
The window openings in a Pall ring reduce pressure drop through two mechanisms. First, the windows allow gas to pass laterally through each ring, reducing the gas velocity through the bed and therefore the frictional pressure drop across it. Second, the windows increase the effective void fraction of the bed because gas flows through both the spaces between rings and the openings within each ring, reducing the resistance to flow. The internal tabs increase mass transfer efficiency by providing additional liquid film formation points that redistribute liquid within each ring, preventing the liquid from running straight down the inner wall as it does in a Raschig ring. The tabs also increase the total surface area available for mass transfer by approximately 10% compared to a smooth cylinder of the same external dimensions. In a column packed with 25 mm PP Pall rings at 50% of flood, the liquid forms a film approximately 0.1-0.3 mm thick over the ring surfaces, and the gas contacts this film as it flows through the tortuous pathways created by the random orientation of the rings.
How Pall Rings Improve Over Raschig Rings
The net effect of the Pall ring design changes is 40-60% lower pressure drop, 50-100% higher capacity before flooding, and 20-30% better mass transfer efficiency compared to Raschig rings of the same nominal size. These improvements come at a manufacturing cost premium of 20-40% due to the more complex mold tooling required. However, the performance improvement per dollar spent is higher for Pall rings because the 40-60% pressure drop reduction directly translates into smaller column diameter, lower fan energy, or both. For a new column designed with Pall rings instead of Raschig rings, the capital cost savings from the smaller column shell typically offset the higher packing cost, making Pall rings the lower-cost option on a total installed basis for any scrubber operating above 2,000 hours per year. This is a rare case where the higher-performance option is also the lower-cost option when all installation and operating costs are included.
Pall Ring Manufacturing
PP Pall rings are manufactured by injection molding in multi-cavity tools that produce 4-16 rings per cycle depending on size. A typical 50 mm Pall ring weighs 12-18 grams in PP. The mold tooling cost for Pall rings is 2-3x higher than for Raschig rings because the slides and core pulls required to form the window openings and internal tabs add complexity. This higher tooling cost is amortized across production volume — for a vendor producing millions of rings per year, the per-piece tooling cost difference is small, resulting in the 20-40% finished product cost premium. PVDF Pall rings require higher mold temperatures (200-250C vs 40-60C for PP) and longer cycle times, contributing to their 2.5-3.5x cost premium over PP.
Pall Ring vs Raschig Ring
Design Differences
The structural difference between Pall rings and Raschig rings is straightforward but produces measurable performance differences across every relevant metric. A Raschig ring is a solid cylinder. A Pall ring has rectangular window openings occupying 30-40% of the wall area, with the cut material bent inward at 15-30 degree angles to form internal tabs. The packing factor of a 25 mm PP Pall ring (176 m-1) is less than one-third that of a 25 mm ceramic Raschig ring (~550 m-1) and less than half that of a 25 mm PP Raschig ring (~400 m-1). This means a Pall ring column can operate at approximately 80% higher gas velocity before flooding than a ceramic Raschig column, or 45% higher than a PP Raschig column.
Performance Comparison Table
| Parameter | 25mm Pall (PP) | 25mm Raschig (PP) | 25mm Raschig (Ceramic) |
|---|---|---|---|
| Surface area | 209 m2/m3 | 195 m2/m3 | 190 m2/m3 |
| Void fraction | 91% | 80% | 65% |
| Packing factor | 176 m-1 | ~400 m-1 | ~550 m-1 |
| Relative pressure drop | 1x | 1.3-1.8x | 1.6-2.5x |
| Capacity before flood | 1.8x | 1.3x | 1x |
| HETP (acid-gas) | 0.45-0.65 m | 0.55-0.80 m | 0.65-1.00 m |
| Relative cost per m3 | 1x ($400-660) | 0.6-0.8x | 3-4x |
When Pall Rings Are the Clear Choice
Pall rings are the correct choice for any new scrubber design operating above 2,000 hours per year at temperatures below 80C. The Pall ring’s lower packing factor allows a 15-20% smaller column diameter than a Raschig ring column for the same gas flow. For a 1.5 m HCl scrubber treating 10,000 m3/hr, this diameter reduction saves $2,000-4,000 in FRP shell cost. The annual fan energy savings of $1,200-2,800 at 8,000 hr/yr pays back the packing cost premium within 2-7 months. For existing Raschig ring columns, replacing with Pall rings of the same size requires no hardware modifications and delivers 40-60% lower pressure drop immediately — this is the highest-ROI packing upgrade available for scrubbers because it requires no column modifications and pays back within months. The only situations where Raschig rings remain preferred are high-temperature ceramic service above 120C, laboratory columns below 150 mm diameter requiring 6-13 mm packing, and scrubbers operating below 2,000 hr/yr where energy savings cannot justify the premium.
Pall Ring Sizes and Specifications
Standard Size Range
Pall rings are manufactured in standard nominal sizes from 16 mm to 90 mm, with 25 mm, 38 mm, and 50 mm being the most common for scrubber applications. Unlike Raschig rings, Pall rings are not manufactured below 16 mm because the window openings and internal tabs require a minimum wall thickness that makes smaller sizes impractical. For laboratory columns requiring packing below 16 mm, ceramic Raschig rings remain the only option. The 25 mm size is standard for clean gas service in columns of 1.2-3.0 m diameter, offering 209 m2/m3 surface area and the best mass transfer efficiency. The 50 mm size is preferred for gas streams with particulate loading above 50 mg/Nm3, offering lower pressure drop and better fouling resistance at the cost of reduced surface area (100 m2/m3). For a typical HCl scrubber, 25 mm Pall rings are the default choice, while 50 mm is selected when the gas contains dust, mist, or biological aerosols from upstream processes.
Surface Area and Packing Factor by Size
| Size | Surface Area | Void Fraction | Packing Factor Fp |
|---|---|---|---|
| 16 mm | 318 m2/m3 | 87% | 315 m-1 |
| 25 mm | 209 m2/m3 | 91% | 176 m-1 |
| 38 mm | 145 m2/m3 | 92% | 120 m-1 |
| 50 mm | 100 m2/m3 | 93% | 80 m-1 |
| 76 mm | 68 m2/m3 | 94% | 51 m-1 |
As Pall ring size increases, surface area decreases while void fraction increases and packing factor decreases. A column with 76 mm Pall rings can operate at approximately 60% higher gas velocity before flooding than one with 16 mm rings, but requires approximately 4.7x more bed height to achieve the same mass transfer because the HETP is proportionally higher.
D/8 Rule Application
The D/8 rule for Pall rings is the same as for all random packing: nominal diameter must not exceed one-eighth of the column inner diameter. For a 600 mm column, the maximum Pall ring size is 75 mm — use 50 mm. For a 400 mm column, maximum is 50 mm — use 38 mm. The rule is rarely limiting for Pall rings in columns above 700 mm diameter because standard sizes only go up to 90 mm, which satisfies D/8 for any column above 720 mm. For the common 1.2-3.0 m diameter range, the D/8 rule permits sizes up to 150-375 mm, so selection is governed by process conditions rather than geometry.
Materials for Pall Rings
PP Pall Rings: Up to 80C
Polypropylene (PP) is the standard material for Pall rings, covering 85-90% of all scrubber applications. PP operates continuously up to 80C with a lifespan of 10-15 years. It resists HCl at all concentrations, H2SO4 up to 50%, NaOH at all concentrations, and most organic acids at ambient temperature. PP Pall rings cost $400-660 per cubic meter for 50 mm size. For a 1.5 m diameter scrubber with 3.0 m of bed height, total PP Pall ring cost is $2,100-3,500. PP is light (density ~900 kg/m3), easy to install by dry dumping with water cushion, and available from multiple suppliers worldwide.
PVDF Pall Rings: 80-120C and HF Service
PVDF Pall rings are required for operating temperatures between 80C and 120C and for hydrogen fluoride service at any temperature. PVDF offers a lifespan of 12-18 years at 2.5-3.5x PP cost ($1,000-1,660 per cubic meter). The cost premium is justified by eliminating temperature-related failure risk. In HF service, PVDF is the only suitable plastic — ceramic dissolves (forming SiF4 gas) and PP degrades in HF above trace levels. The EPA acid gas scrubber design reference provides further guidance on material selection for corrosive gas service. The South Korea case demonstrates a $22,000 consequence of specifying PP instead of PVDF when peak temperatures exceeded 80C.
Metal Pall Rings: Limited Application
Stainless steel Pall rings (SS304, SS316) are available for high-temperature non-chloride service above 120C where pressures exceed 5 bar. SS316 Pall rings cost $3,300-5,000 per cubic meter. They must never be used in chloride service — corrosion rate in 5% HCl at 60C is 0.5-1.5 mm/year, causing a 0.5 mm wall ring to fail within 4-12 months. For virtually all scrubber applications handling acid gases, PP or PVDF Pall rings are the correct material choice.
Pall Ring Applications by Gas Type
HCl and Acid Gas Scrubbing
Pall rings are the standard packing for HCl scrubbers in chemical processing, pharmaceutical manufacturing, and metal pickling. The 25 mm PP Pall ring with 209 m2/m3 surface area and HETP of 0.45-0.65 m provides efficient HCl removal in caustic scrubbing systems. For a typical HCl scrubber treating 10,000 m3/hr from 500 ppm inlet to 5 ppm outlet, a bed height of 2.5-3.0 m of 25 mm PP Pall rings at a liquid-to-gas ratio of 3-5 L/m3 achieves the required removal. For HF service, PVDF Pall rings must be specified regardless of temperature.
H2S and Odor Control
For H2S removal from biogas, wastewater treatment off-gas, and natural gas processing, Pall rings are used in caustic and amine absorption columns. H2S removal requires a higher liquid-to-gas ratio (5-10 L/m3) than HCl because the reaction kinetics are slower. Bed height for 95-99% H2S removal is typically 3.0-4.5 m of 25-50 mm PP Pall rings depending on inlet concentration. The 50 mm size is preferred when the gas stream contains particulate or biological aerosols from wastewater treatment that could foul smaller packing.
SO2 Scrubbing with Solids Production
For SO2 removal in limestone or caustic systems, 50 mm Pall rings are recommended because the reaction produces solid byproducts (calcium sulfite and calcium sulfate) that can accumulate in the packed bed. The 50 mm size with 93% void fraction provides clearance between pieces to prevent solids bridging. Bed height for 95-98% SO2 removal at 1,000-3,000 ppm inlet is 3.0-5.0 m of 50 mm PP Pall rings at a liquid-to-gas ratio of 5-15 L/m3. Periodic bed flushing at 2-4 week intervals is required to prevent scale accumulation in limestone systems.
NH3 and Alkaline Gas Absorption
Ammonia scrubbing in acidic solution (typically H2SO4 or H3PO4) uses Pall rings to maximize gas-liquid contact for the fast acid-base reaction. NH3 removal efficiency above 99% is achieved with a bed height of 1.5-2.5 m of 25-50 mm PP Pall rings at a liquid-to-gas ratio of 2-4 L/m3. The HETP for NH3 absorption in acid is 0.35-0.55 m, shorter than for HCl because the reaction is faster. The high water solubility of NH3 (531 g/L at 20C) means even a once-through water scrubber achieves 70-90% removal, but acidic solution is required for the 99%+ removal that environmental permits typically require.
Economic Analysis: Pall vs Raschig Rings
Initial Cost Comparison
For a 1.5 m diameter scrubber with 3.0 m bed height (5.3 m3 packing volume), PP Pall rings cost $2,100-3,500, which is $500-700 more than PP Raschig rings at $1,600-2,800. This 20-25% premium represents less than 2% of the total installed column cost of $25,000-50,000 and is recovered within 2-7 months at 8,000 operating hours per year through reduced fan energy consumption.
Lifecycle Cost Comparison Table
| Cost Component | PP Pall Rings | PP Raschig Rings |
|---|---|---|
| Initial packing cost | $2,800 | $2,200 |
| Fan energy per year | $1,200 | $2,800 |
| 10-year fan energy | $12,000 | $28,000 |
| 10-year total | $14,800 | $30,200 |
Payback Period by Operating Hours
The Pall ring payback depends on annual operating hours. At 8,000 hr/yr: 2-7 months. At 4,000 hr/yr: 4-14 months. At 2,000 hr/yr: 9-28 months. Below 2,000 hr/yr: payback exceeds 3 years and Raschig rings may be justified. At a 10% discount rate over 10 years, the net present value of Pall ring energy savings at 8,000 hr/yr is $3,700-5,500 — far exceeding the $500-700 initial premium. These calculations exclude the value of 20-30% higher mass transfer efficiency, which provides additional savings through reduced bed height or lower outlet concentrations. For any scrubber above 2,000 hr/yr, Pall rings are the clear economic choice — the higher initial cost is recovered within months and the energy savings continue for the 10-15 year life of the packing. For a plant with multiple scrubbers, standardizing on Pall rings simplifies spare parts inventory and reduces the risk of specifying the wrong packing for a given duty, adding hidden value beyond the direct energy savings.
Pall Ring Selection and Installation
Three-Step Selection Framework
Step 1: characterize the process conditions — gas composition, inlet and target outlet concentrations, peak gas temperature including summer maximum with a 10C safety margin, particulate loading in mg/Nm3, and liquid chemistry and pH. The temperature specification is the most critical parameter — errors in peak temperature cause material failures that cost $8,000-22,000 per incident. Step 2: select the Pall ring size — 25 mm for clean gas service in columns 1.2-3.0 m diameter where maximum mass transfer efficiency is needed, 50 mm for gas streams with particulate above 50 mg/Nm3, and apply the D/8 rule for columns under 600 mm diameter. Step 3: verify the material at the peak temperature plus 10C safety margin — PP below 80C, PVDF for 80-120C and HF service, SS316 for non-chloride high-pressure service above 5 bar.
Installation Method
PP and PVDF Pall rings are installed by dry dumping with a water cushion. The column is filled with water to 1-2 m above the support grid, then the Pall rings are poured in from the top manway. The water cushions the fall and ensures uniform random orientation. For a 1.5 m diameter column with 3.0 m of 50 mm PP Pall rings, installation takes a two-person crew approximately 4-6 hours. The bed is leveled after filling and inspected for voids before closing the column. Installation cost is $100-300 per cubic meter. Pall rings do not require wet installation — the PP material is flexible and does not crack on impact like ceramic Raschig rings. The water is drained after installation and the top surface is leveled by hand before the liquid distributor and bed limiter are installed.
Inspection and Maintenance
Pall ring beds require periodic inspection for fouling, with pressure drop monitoring as the primary indicator of bed condition. A 50% increase above baseline pressure drop indicates fouling or channeling requiring investigation. For clean gas service, annual inspection is sufficient. For gas streams with particulate, inspect quarterly. Minor fouling can be addressed by bed flushing with high-pressure water at 3-5 bar through the liquid distributor. Severe fouling or channeling requires bed replacement. The service life of PP Pall rings within temperature limits is 10-15 years, and PVDF Pall rings last 12-18 years.
FAQ
What is a Pall ring?
A Pall ring is a hollow cylinder packing with rectangular window openings and internal tabs, developed in the 1950s. It achieves 40-60% lower pressure drop and 20-30% better mass transfer efficiency than Raschig rings at a 20-40% cost premium.
What is the difference between a Pall ring and a Raschig ring?
Pall rings have window openings in the wall and internal tabs; Raschig rings are solid cylinders. Packing factor is 176 m-1 for 25 mm PP Pall vs ~400-550 m-1 for Raschig rings. Pall rings allow 80% higher gas velocity before flooding.
What size Pall ring should I use?
25 mm for clean gas service in columns 1.2-3.0 m diameter. 50 mm for gas streams with particulate above 50 mg/Nm3. Available in 16-90 mm standard sizes.
What material Pall ring is best for acid service?
PP for below 80C covering 85-90% of applications ($400-660/m3). PVDF for 80-120C and HF service ($1,000-1,660/m3). SS316 for non-chloride high-pressure service only ($3,300-5,000/m3).
How much do Pall rings cost?
PP Pall rings: $400-660 per m3, approximately 20-40% more than PP Raschig rings. The premium pays back within 2-7 months at 8,000 hr/yr through fan energy savings.
Conclusion
Pall rings are the most widely used random packing design for scrubber and absorption applications. The simple modification of adding window openings and internal tabs to the basic cylinder design produces 40-60% lower pressure drop, 50-100% higher capacity before flooding, and 20-30% better mass transfer efficiency compared to Raschig rings. The cost premium of 20-40% over Raschig rings pays back within 2-7 months at 8,000 hr/yr through reduced fan energy consumption. For any scrubber above 2,000 hr/yr at temperatures below 80C, Pall rings are the clear economic and technical choice. For temperatures of 80-120C or HF service, PVDF Pall rings provide the same performance advantages at a higher cost that is still justified by the elimination of temperature-related failure risk. When designing a new packed column or re-packing an existing one, start with Pall rings as the default option and only deviate for the specific niche applications where Raschig rings, Tellerette rings, or structured packing offer a clear advantage.
XICHENG EP LTD supplies Pall rings in PP, PVDF, and metal for scrubber and absorption applications, with over 2,600 systems shipped to 60+ countries since 2008.
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External References