Every wet scrubber produces a gas stream saturated with liquid droplets after the gas-liquid contact section. Without mist elimination, these droplets carry dissolved pollutants, particulate matter, and scrubbing chemicals out of the stack — causing visible emissions, violating permit limits, and damaging downstream equipment such as fans, ductwork, and stacks. A mist eliminator is the final barrier that removes these droplets before the gas exits the column. The selection among wire mesh pads, chevron vanes, and high-efficiency mist eliminators depends on the droplet size distribution, gas velocity, allowable pressure drop, and fouling tendency of each application. Selecting the wrong type leads to either inadequate removal (permit violations and fines) or excessive pressure drop (wasted fan energy and higher operating costs). This guide covers mist eliminator fundamentals, types with quantified performance data including cut points and pressure drop ranges, selection by application across scrubbers, process columns, and environmental compliance, design and sizing methodology using the Souders-Brown equation, and installation and cost considerations for each type.
For the complete scrubber design methodology see our scrubber packing media selection guide.
Key Takeaways
- Mist eliminators remove liquid droplets from scrubber exhaust using three mechanisms: inertial impaction (>5 microns), direct interception (1-5 microns), and Brownian diffusion (<1 micron). Standard wire mesh pads achieve 99% removal at 3-5 microns with 0.5-2.0 in wc pressure drop. Chevron vanes achieve 99% at 8-10 microns with 0.3-0.8 in wc. High-efficiency types achieve sub-micron removal at higher cost and pressure drop.
- The Souders-Brown equation governs mist eliminator sizing: Vmax = k × √((ρl − ρg)/ρg). For wire mesh pads, k = 0.35-0.45 ft/s for clean service. For chevron vanes, k = 0.40-0.60 ft/s. Design velocity is set at 50-75% of Vmax. For a 10,000 m3/hr HCl scrubber, the required diameter is 1.5-1.8 m.
- Selection depends on droplet size, gas velocity, fouling tendency, and required removal efficiency. Wire mesh is best for fine droplets in clean gas. Chevron vanes are best for high velocity and fouling service. Fiber-bed is required for sub-micron acid mist. Double-pocket vanes bridge the gap between mesh and standard chevron for fouling service requiring fine droplet capture.
- Mist eliminator cost is 5-15% of total scrubber cost but the wrong selection can cause permit violations or excessive fan energy. For a 1.5 m scrubber: PP mesh pad $500-1,500, SS316 chevron $2,000-5,000, high-efficiency vane $3,000-8,000. Mesh pad replacement is 5-10 years, chevron vane replacement 10-15 years.
Mist Eliminator Fundamentals
What Is a Mist Eliminator?
A mist eliminator is a device installed in the upper section of a scrubber or process column to remove entrained liquid droplets from a gas stream before it exits the vessel. The droplets range from 1 to 1000 microns in diameter, with most scrubber-generated mist falling in the 5-200 micron range. Without a mist eliminator, a scrubber operating at 99% gas removal efficiency can still produce visible stack emissions due to droplet carryover, because the liquid droplets contain the dissolved pollutants at the scrubber’s outlet concentration. Mist eliminators use one or more of three physical mechanisms — inertial impaction, direct interception, and Brownian diffusion — to capture droplets from the gas stream and return the captured liquid to the scrubber sump.
How Mist Eliminators Work
The three droplet capture mechanisms operate in different droplet size ranges. Inertial impaction dominates for droplets above 5 microns — the gas stream flows around the mist eliminator fibers or vanes, but the heavier droplets cannot follow the gas streamlines and impact onto the capture surface. Direct interception captures droplets in the 1-5 micron range when the droplet touches a fiber as it passes within one droplet radius of the fiber surface. Brownian diffusion captures droplets below 1 micron by random molecular motion that causes them to collide with capture surfaces. The combination of these three mechanisms determines the overall efficiency of a mist eliminator as a function of droplet size. The minimum droplet size at which a mist eliminator achieves 99%+ efficiency — known as the cut point — is the primary performance specification for mist eliminator selection.
Key Performance Parameters
Four parameters define mist eliminator performance. Removal efficiency (%) as a function of droplet size — typically specified as the cut point (99% removal at 3-5 microns for mesh, 8-10 microns for vane). Pressure drop (in wc) across the mist eliminator at design gas velocity — mesh pads typically run at 0.5-2.0 in wc, chevron vanes at 0.3-0.8 in wc. Maximum gas velocity (ft/s or m/s) — exceeding the design velocity causes re-entrainment of captured liquid, drastically reducing efficiency. Fouling resistance — the tendency of the mist eliminator to plug with solids, scale, or sticky deposits. These four parameters interact: increasing removal efficiency typically increases pressure drop, and the optimal mist eliminator balances both against the specific requirements of each application. The Souders-Brown equation ties these parameters together by defining the maximum gas velocity as a function of the liquid and gas densities and a design constant k that varies by mist eliminator type.
Droplet Size Distribution in Scrubbers
Understanding the droplet size distribution produced by the scrubber is essential for mist eliminator selection. Spray nozzles produce droplets in the 100-1000 micron range — easily captured by any mist eliminator. Packed beds produce droplets in the 10-200 micron range, with most between 20-100 microns — well within the capture range of both mesh pads and chevron vanes. High-energy scrubbers like venturi scrubbers produce droplets in the 1-20 micron range, with significant mass below 5 microns — requiring mesh pads or high-efficiency mist eliminators for adequate capture. Condensation droplets formed when saturated gas cools after leaving the scrubber are typically 0.5-5 microns — the most difficult to capture. A mist eliminator selection must account for the full droplet size distribution, not just the mean droplet size, to ensure the required overall removal efficiency.
Types of Mist Eliminators
Wire Mesh Pads
Wire mesh mist eliminators consist of layers of knitted wire mesh, typically 100-150 mm thick, compressed to a specific density and held between support grids. The knitted wire creates a tortuous path of interconnected fibers (0.1-0.5 mm wire diameter) that capture droplets through inertial impaction and interception. Standard mesh pads achieve 99% removal of droplets above 3-5 microns at pressure drops of 0.5-2.0 in wc. The Kimre technical guide provides detailed wire type and selection data. Wire mesh pads are the most common mist eliminator type for scrubbers, offering the best efficiency for fine droplets at moderate pressure drop. They are available in SS304, SS316, PP, PVDF, and other materials. Key limitations: fouling in particulate-laden gas streams, limited liquid handling capacity, and risk of flooding at high gas velocities.
Chevron/Vane Mist Eliminators
Chevron or vane mist eliminators consist of a series of corrugated plates with directional changes that force the gas stream to follow a zigzag path. Droplets impact on the vane surfaces by inertial impaction and drain downward through gravity to collection channels. Chevron vanes achieve 99% removal of droplets above 8-10 microns at pressure drops of 0.3-0.8 in wc — lower pressure drop than mesh pads but larger cut point. The MACH Engineering comparison guide provides application guidance. Chevron vanes are preferred for high gas velocities, particulate-laden gas streams (better fouling resistance than mesh), and applications requiring low pressure drop. They are available in PP, PVDF, SS304, SS316, and FRP. The vane spacing (12-25 mm) determines the cut point — closer spacing captures smaller droplets but increases pressure drop and fouling risk.
High-Efficiency Mist Eliminators
High-efficiency mist eliminators combine multiple capture mechanisms to achieve sub-micron droplet removal. These include double-pocket vanes (two-stage chevron with drainage channels), fiber-bed mist eliminators (packed fiber layers for 0.1-1 micron removal), and electrostatic mist eliminators (charged particle collection). Double-pocket vanes are the most common high-efficiency type for scrubbers, achieving 99% removal of droplets above 3-5 microns at 0.6-1.5 in wc — mesh pad efficiency with improved fouling resistance. Fiber-bed mist eliminators are used for sub-micron acid mist and oil mist applications (H2SO4 plants, PVC production) but require low gas velocities and clean gas streams. Electrostatic mist eliminators are specialized for fine particulate and mist below 1 micron at high collection efficiency but high capital cost.
Performance Comparison Table
| Type | Cut Point (99%) | Pressure Drop | Max Velocity | Fouling | Cost |
|---|---|---|---|---|---|
| Wire mesh pad (std) | 3-5 microns | 0.5-2.0 in wc | 8-12 ft/s | Moderate | $ |
| Chevron vanes | 8-10 microns | 0.3-0.8 in wc | 12-20 ft/s | Good | $$ |
| Double-pocket vane | 3-5 microns | 0.6-1.5 in wc | 10-16 ft/s | Good | $$$ |
| Fiber-bed | 0.1-1 micron | 2.0-6.0 in wc | 2-5 ft/s | Poor | $$$$ |
Selection by Application
Scrubber Mist Elimination
For packed bed and spray tower scrubbers, the standard mist eliminator selection is a wire mesh pad mounted 300-600 mm above the highest liquid distributor or spray nozzle. The gas leaving the packing or spray section carries droplets of scrubbing liquid containing the removed pollutants at the outlet concentration. For most acid-gas scrubbers (HCl, H2S, NH3) where the target outlet concentration is 1-5 ppmv, a standard wire mesh pad with 99% removal at 5 microns provides adequate mist elimination. If the scrubber handles particulate-laden gas that could foul the mesh, chevron vanes are preferred at the cost of reduced fine-droplet capture. For scrubbers requiring visible emission elimination (opacity-based permits), high-efficiency double-pocket vanes or a mesh pad followed by a chevron vane in series is typically specified. The series arrangement captures fine droplets on the mesh pad and prevents re-entrainment with the chevron section, achieving 99.9%+ removal at the cost of double the pressure drop.
Process Column Applications
In distillation and absorption columns, mist eliminators prevent product loss and protect downstream equipment. For distillation columns handling high liquid-to-gas ratios, chevron vanes are preferred because they handle higher liquid drainage loads without flooding. For amine absorption columns where amine carryover causes downstream corrosion and fouling, wire mesh pads with 99% removal at 3-5 microns are standard, typically specified in SS316 or SS304 to resist amine stress corrosion cracking. For sulfuric acid plants where acid mist below 1 micron must be captured for environmental compliance, fiber-bed mist eliminators are required despite their higher pressure drop and cost. For evaporators and crystallizers where salt or scale formation is likely, chevron vanes with wide spacing (20-25 mm) provide the best balance of capture efficiency and fouling resistance.
Environmental Compliance
Environmental permits increasingly specify mass emission limits for liquid droplets (mg/Nm3 of total liquid carryover) or opacity limits that require mist elimination. The US EPA’s MACT standards for chemical plants, pharmaceutical manufacturing, and hazardous waste combustors include specific requirements for mist elimination efficiency. A typical permit limit of 0.1 gr/dscf (0.23 g/m3) of total carryover requires 99%+ removal efficiency for droplets above 5 microns, achievable with a standard wire mesh pad. More stringent limits below 0.01 gr/dscf require high-efficiency mist eliminators with cut points below 3 microns. The OSHA permissible exposure limits for the scrubbed pollutant also affect the required mist eliminator efficiency, as dissolved pollutant in carryover droplets contributes to the workplace exposure concentration.
Design and Sizing
Souders-Brown Equation
The maximum allowable gas velocity through a mist eliminator is calculated using the Souders-Brown equation: Vmax = k × √((ρl – ρg)/ρg), where Vmax is the maximum gas velocity in ft/s, ρl is the liquid density, ρg is the gas density, and k is a constant that depends on mist eliminator type and operating conditions. For wire mesh pads, k typically ranges from 0.35-0.45 ft/s for clean service and 0.25-0.35 ft/s for fouling service. For chevron vanes, k ranges from 0.40-0.60 ft/s. For high-efficiency double-pocket vanes, k is 0.30-0.45 ft/s. For fiber-bed mist eliminators, k is 0.15-0.25 ft/s. The required cross-sectional area of the mist eliminator is the gas volumetric flow rate divided by the design velocity, which is typically set at 50-75% of Vmax to provide a safety margin for process upsets and future capacity increases. For a typical HCl scrubber treating 10,000 m3/hr at 50C with a wire mesh pad (k=0.40), Vmax is approximately 12 ft/s, and the design velocity at 60% of Vmax is 7.2 ft/s, requiring a mist eliminator area of approximately 2.0-2.5 m2 — equivalent to a 1.6-1.8 m diameter vessel.
Pressure Drop Calculation
Mist eliminator pressure drop affects the fan static pressure requirement and operating cost. For wire mesh pads, pressure drop increases with gas velocity, pad thickness, and mesh density. A standard 100 mm thick mesh pad at 50% of flood produces 0.3-0.8 in wc pressure drop, increasing to 1.0-2.0 in wc at 75% of flood. For chevron vanes, pressure drop is approximately 0.3-0.8 in wc at design velocity, with the vane spacing being the primary variable — 12 mm spacing produces higher pressure drop but smaller cut point than 25 mm spacing. For fiber-bed mist eliminators, pressure drop is 2.0-6.0 in wc, significantly increasing the fan operating cost. The annual fan energy cost for a mist eliminator at $0.08/kWh and 8,000 hr/yr is approximately $200-600 for mesh pads, $150-500 for chevron vanes, and $1,000-3,000 for fiber-bed units for a 10,000 m3/hr scrubber.
Material Selection
Mist eliminator material selection follows similar principles to packing media selection. PP is the standard for corrosive scrubber service up to 80C, available in wire mesh (knitted PP monofilament) and chevron vanes (molded PP). PVDF is required for 80-120C and HF service. SS304 and SS316 are used for high-temperature service above 120C and for amine absorption applications where plastic materials are not chemically compatible. SS316L is preferred for chloride service to minimize pitting corrosion risk at the welded wire junctions in mesh pads. For chevron vanes, FRP (fiberglass reinforced plastic) is also available for large-diameter columns where the structural strength of FRP provides advantages over plastic at a lower cost than stainless steel.
Installation and Cost
Installation Methods
Wire mesh pads are installed as prefabricated sections cut to fit the column cross-section, with each section laid on a support grid and secured with a hold-down grid. The mesh sections must overlap by at least 50 mm at the seams to prevent gas bypass. For columns above 1.5 m diameter, multiple mesh sections are required, and the seams should be staggered between layers if a multi-layer pad is specified. Chevron vanes are installed as prefabricated vane packs that are bolted or clamped in place on a support frame. The vane packs must be oriented perpendicular to the gas flow direction. Both types require access via column manways for installation and removal. Installation cost is typically $200-500 per square meter for mesh pads and $300-600 per square meter for chevron vanes.
Cost Comparison
For a 1.5 m diameter column (1.77 m2 cross-section), the mist eliminator purchase cost ranges from $500-1,500 for a PP wire mesh pad assembly to $2,000-5,000 for a SS316 chevron vane assembly. High-efficiency double-pocket vanes cost $3,000-8,000 for the same size. Fiber-bed mist eliminators cost $5,000-15,000 plus $2,000-5,000 for the support housing and drainage system. The mist eliminator cost typically represents 5-15% of the total scrubber system cost, making it a small fraction of the total investment. However, selecting the wrong type can cause permit violations (fines, shutdowns) or excessive fan energy costs that far exceed the initial cost difference between types.
Maintenance and Replacement
Wire mesh pads require periodic inspection and cleaning to prevent fouling and maintain efficiency. For clean gas service, annual inspection is sufficient. For particulate-laden gas, quarterly inspection is recommended. Cleaning is performed by water washing or chemical cleaning in place. Mesh pads typically last 5-10 years in scrubber service before wire breakage or fouling requires replacement, at a cost of $500-1,500 for a 1.5 m column. Chevron vanes have a longer service life of 10-15 years due to their more robust construction and better fouling resistance, with replacement cost of $2,000-5,000. The annual maintenance cost, including inspection labor and cleaning, is approximately $200-500 for mesh pads and $100-300 for chevron vanes.
FAQ
What is a mist eliminator?
A device installed in the upper section of a scrubber or column to remove entrained liquid droplets from the gas stream before it exits. Types include wire mesh pads, chevron vanes, and fiber-bed mist eliminators.
What is the difference between a mesh pad and a chevron vane?
Wire mesh pads achieve 99% removal at 3-5 microns with 0.5-2.0 in wc pressure drop. Chevron vanes achieve 99% at 8-10 microns with 0.3-0.8 in wc. Mesh pads are better for fine mist; chevron vanes have better fouling resistance.
How do I size a mist eliminator?
Use the Souders-Brown equation: Vmax = k × √((ρl−ρg)/ρg). For mesh pads, k = 0.35-0.45. For chevron vanes, k = 0.40-0.60. Set design velocity at 50-75% of Vmax.
What mist eliminator is best for scrubbers?
Wire mesh pads are standard for most scrubbers due to their fine droplet capture at moderate cost. Chevron vanes are preferred when fouling is a concern. Fiber-bed is required for sub-micron acid mist compliance.
How much does a mist eliminator cost?
For a 1.5 m column: PP wire mesh $500-1,500, SS316 chevron $2,000-5,000, high-efficiency vane $3,000-8,000, fiber-bed $5,000-15,000.
Conclusion
Mist eliminator selection is governed by the droplet size distribution, gas velocity, allowable pressure drop, and fouling tendency of each application. Wire mesh pads offer the best fine-droplet capture for clean gas streams. Chevron vanes provide better fouling resistance and lower pressure drop at the cost of a larger cut point. High-efficiency types — double-pocket vanes and fiber-bed units — serve applications requiring sub-micron removal for environmental compliance or product recovery. The Souders-Brown equation provides the sizing methodology, with the k factor selected based on mist eliminator type and operating conditions. For most scrubber applications, a standard wire mesh pad provides adequate mist elimination at moderate cost, with chevron vanes specified when fouling resistance or low pressure drop is the primary requirement.
XICHENG EP LTD supplies mist eliminators in wire mesh, chevron vane, and high-efficiency configurations for scrubber and process column applications.
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External References