Every mist eliminator — whether a wire mesh pad, a vane pack, or a fiber-bed coalescer — is sized using the same governing equation: the Souders-Brown relationship that balances droplet gravitational settling against upward gas drag. The differences between types reduce to three variables: the K-factor (capacity factor) that captures the geometry-specific efficiency, the allowable pressure drop, and the cut point defining the smallest droplet captured at 99% efficiency. Getting the mist eliminator design right depends on selecting the correct K-factor for the type and service, calculating the required cross-sectional area from the gas flow rate, verifying that the resulting pressure drop is within the fan or compressor budget, and confirming that the vessel geometry provides adequate disengagement height above and below the mist eliminator. This guide covers the complete mist eliminator design calculation methodology: the Souders-Brown equation with K-factor selection tables for both wire mesh and vane types, pressure drop correlations with worked numerical examples, collection efficiency prediction using Stokes number theory, derating factors for non-ideal conditions including vacuum, high pressure, and viscous liquids, and vessel design requirements for mist eliminator installation.
Key Takeaways
- Every mist eliminator type — mesh, vane, or fiber-bed — is sized using the Souders-Brown equation with type-specific K-factors ranging from 0.061 m/s (vacuum mesh) to 0.259 m/s (pocketed vanes, horizontal); using the wrong K-factor for your type guarantees either flooding or wasted vessel capacity.
- The design velocity should never exceed 80% of Vmax under normal conditions; operating at 90% or above puts the mist eliminator inside the re-entrainment zone where captured liquid is torn off and re-enters the gas stream regardless of how well the unit captures incoming droplets.
- Stokes number theory predicts collection efficiency from first principles: Stk > 1 means high capture probability, Stk < 0.1 means droplets follow gas streamlines and miss the capture surface entirely — this determines whether a mesh pad achieves 99.9% or 50% efficiency at your target droplet size.
- Multiple non-ideal conditions multiply their effects: a vane pack at 80 bar with viscous oil (8 cP) in fouling service needs a combined K-factor derating of 0.58 — not three separate partial deratings — reducing design velocity by 42% from the base case.
- GPSA requires a minimum of 600 mm or one vessel diameter (whichever is larger) between the mist eliminator top and the outlet nozzle centerline; violating this disengagement height causes liquid carryover even with a correctly sized mist eliminator.
Mist Eliminator Design Fundamentals
Every mist eliminator design procedure follows a consistent sequence: determine the gas and liquid physical properties at operating conditions, select the mist eliminator type based on droplet size and fouling tendency, choose the appropriate K-factor for that type and service, calculate the maximum allowable velocity using the Souders-Brown equation, set the design velocity at 70-80% of maximum, compute the required cross-sectional area, verify pressure drop, and confirm that the vessel geometry provides adequate disengagement height. Each step requires specific data inputs and engineering judgment. This section covers the three most critical elements: the Souders-Brown equation, K-factor selection, and droplet size distribution analysis.
The Souders-Brown Equation
The Souders-Brown equation is the universal starting point for mist eliminator sizing. It defines the maximum superficial gas velocity at which a liquid droplet of a given size can settle against the upward gas flow: Vmax = K × √[(ρl — ρg) / ρg], where Vmax is the maximum allowable velocity (m/s), K is the capacity factor (m/s) specific to the mist eliminator type and geometry, ρl is the liquid density (kg/m³), and ρg is the gas density (kg/m³). The equation derives from balancing the gravitational settling force on a droplet against the drag force from the upward gas stream. At Vmax, the smallest droplet that can settle is approximately equal to the cut point of the mist eliminator. Operating above Vmax causes liquid to be carried upward through the eliminator and out of the vessel — a condition called flooding in mesh pads and re-entrainment in vane packs. The standard design practice is to operate at 70-80% of Vmax to provide a safety margin against process upsets and fouling accumulation.
K-Factor Selection for Different Types
The K-factor is the single most important variable in the Souders-Brown equation and varies by mist eliminator type, geometry, and flow configuration. The values below represent starting points for air-water systems at ambient conditions. Apply derating factors for non-ideal conditions as described in the derating section.
| Mist Eliminator Type | Configuration | K (m/s) | Typical Design Velocity (m/s) |
|---|---|---|---|
| Wire mesh — standard | 100 mm thick, 150 kg/m³ | 0.107 | 2.5-3.5 |
| Wire mesh — high-efficiency | 150 mm thick, 182 kg/m³ | 0.076-0.090 | 2.0-3.0 |
| Wire mesh — high-capacity | 100 mm thick, 98 kg/m³ | 0.120-0.150 | 3.5-4.5 |
| Vane — standard, no pocket, vertical | 20-30 mm spacing | 0.152 | 3.5-5.0 |
| Vane — standard, no pocket, horizontal | 20-30 mm spacing | 0.198 | 4.5-6.0 |
| Vane — pocketed, vertical | 15-25 mm spacing | 0.244 | 5.0-6.5 |
| Vane — pocketed, horizontal | 15-25 mm spacing | 0.259 | 5.5-7.0 |
| Wire mesh — vacuum service | Standard mesh | 0.061-0.076 | 1.5-2.5 |
The K-factors above assume clean service with liquid viscosity below 1 cP, liquid density near that of water, and droplet loading below 2% by volume. Operating conditions that deviate from these assumptions require derating as covered in the derating factors section.
Droplet Formation and Size Distributions
Entrained liquid in a gas stream does not consist of uniform droplets but a broad range of sizes characterized by a log-normal distribution around a mean value. The droplet generation mechanism determines the distribution shape and the mean size, which in turn dictates the cut point required for the mist eliminator. Droplet classifications follow three categories based on the AMACS engineering manual: fogs (sub-micron to 3 microns, formed by condensation or gas-phase chemical reactions), mists (3-20 microns, formed by surface evaporation from packing or trays), and sprays (20+ microns, formed by mechanical atomization from nozzles).
Each droplet source produces a characteristic distribution. Packed scrubber outlets typically produce mist with Sauter mean diameters (SMD) of 20-50 microns and a narrow distribution. Spray nozzles produce spray with droplets from 50 to 1,000 microns — coarse from hydraulic atomization and fine from secondary breakup against vessel walls. Distillation trays produce the widest range: sieve trays generate 10-30 micron droplets from vapor jetting through perforations, while valve trays produce 20-50 micron droplets. Condensation fogs, such as the “blue smoke” from metal stamping lubricants or SO₃ reaction mist in flue gas, require fiber-bed coalescers below 3 microns. When site-specific droplet data is unavailable, assume 90% of the liquid mass is above 10 microns in mist service or above 50 microns in spray service for conservative design. Specifying a mist eliminator cut point without understanding the source distribution risks either oversizing (wasted capital) or undersizing (carryover).
Wire Mesh Demister Sizing Procedure
Wire mesh demisters are the most common mist eliminator type for clean services. The sizing procedure below uses standard design practice from KLM Technology Group and GPSA Engineering Data Book guidelines. The same procedure applies to all mesh types — standard, high-efficiency, and high-capacity — with the appropriate K-factor selected from the table.
K-Factor Table for Wire Mesh
Wire mesh K-factors depend on mesh density, pad thickness, and operating conditions. Standard mesh at 100 mm thickness with 150 kg/m³ density provides K = 0.107 m/s. High-efficiency mesh (182 kg/m³, 150 mm) reduces K to 0.076-0.090 m/s due to denser packing that restricts gas flow and increases the probability of re-entrainment. High-capacity mesh (98 kg/m³, 100 mm) increases K to 0.120-0.150 m/s by providing more open area. For vacuum service where gas density drops below 1.0 kg/m³, use K = 0.061-0.076 m/s to compensate for the reduced drag force on droplets in low-density gas.
Pressure Drop Calculation for Mesh
The pressure drop across a wire mesh demister follows the modified Darcy equation for porous media: ΔP = CD × ρg × U² × L / (2 × ε² × Df), where CD is the mesh drag coefficient (0.8-1.2 for standard mesh), U is the face velocity (m/s), L is the pad thickness (m), ε is the mesh porosity (0.97-0.98), and Df is the wire diameter (m). For a standard 100 mm mesh pad with ε = 0.98, Df = 0.23 mm, at U = 2.5 m/s with air at 20°C (ρ = 1.2 kg/m³), the pressure drop is approximately 100-150 Pa (0.4-0.6 in wc). Doubling velocity increases ΔP by roughly 4x, making velocity the primary lever for controlling pressure drop. A ΔP exceeding 500 Pa in a clean mesh pad indicates either flooding or severe fouling. The KLM Technology Group design guidelines provide detailed pressure drop correction factors for non-standard mesh densities and pad thicknesses, and should be consulted for final design verification. For high-efficiency mesh (32 layers per 100 mm, 182 kg/m³), multiply the standard ΔP by 1.3-1.5 at the same velocity because the increased wire surface area creates more flow resistance. For high-capacity mesh (20 layers per 100 mm, 98 kg/m³), multiply by 0.7-0.8.
Worked Example: Sizing a Mesh Demister for a 2m Scrubber
Given: A scrubber handling 18,000 m³/hr of air at 60°C (ρg = 1.05 kg/m³), water droplets (ρl = 985 kg/m³). Vessel ID = 2.0 m. Target removal: 99% at 8 microns. Allowable ΔP: 250 Pa.
Step 1: Select standard mesh with K = 0.107 m/s.
Step 2: Calculate Vmax: 0.107 × √[(985 — 1.05) / 1.05] = 0.107 × 30.62 = 3.28 m/s.
Step 3: Set Vdesign = 3.28 × 0.75 = 2.46 m/s.
Step 4: Required area: Q = 18,000 / 3,600 = 5.0 m³/s; A = 5.0 / 2.46 = 2.03 m².
Step 5: Actual velocity in 2.0 m vessel: A_actual = 3.14 m²; U_actual = 5.0 / 3.14 = 1.59 m/s — 48% of Vmax. Adequate safety margin. ΔP ≈ 50-80 Pa, well within 250 Pa limit.
Anti-Fouling Two-Stage Mesh Design
For gas streams with moderate fouling potential where vane packs are not viable (sub-5-micron cut point required), use a two-stage wire mesh demister with a coarser bottom layer and a finer top layer. The KLM Technology Group design guidelines recommend a lower stage of high-capacity mesh (98 kg/m³, 100 mm thick) that captures bulk droplets and larger fouling particles without flooding, followed by an upper stage of standard or high-efficiency mesh (150 kg/m³, 100 mm thick) that polishes the gas to the target cut point. The total thickness increases to 200 mm and the combined K-factor drops to approximately 0.085-0.095 m/s due to the denser total packing. The staged approach extends time between cleaning by 2-3× compared to a single high-efficiency pad because the coarse bottom layer prevents foulants from reaching the fine top layer. Apply the two-stage design in amine contactors, glycol dehydrators, and scrubbers handling partially fouled recycle gas where mesh is preferred for fine mist capture but standard single-layer pads blind within weeks.
Vane Mist Eliminator Sizing Procedure
Vane packs use the same Souders-Brown methodology but with significantly higher K-factors than mesh due to their open channel geometry. The design procedure is identical to mesh sizing, but the K-factor selection must account for flow direction (vertical vs horizontal) and vane type (standard vs pocketed).
K-Factor Table for Vanes
Vane K-factors vary primarily with flow direction and pocket design. Standard (non-pocketed) vanes in vertical flow use K = 0.152 m/s; horizontal flow increases K to 0.198 m/s because gravity assists liquid drainage perpendicular to the gas flow. Pocketed vanes shield collected liquid from re-entrainment, allowing K up to 0.259 m/s in horizontal flow. For fouling services where partial plugging is expected, apply a 0.8-0.85 derating to the listed K-factors.
Pressure Drop Calculation for Vanes
Vane pressure drop is lower than mesh for the same gas flow rate because vanes have greater open area (>99% vs 97-98%). Use the simplified correlation: ΔP = N × Cv × ρg × U², where N is the number of bends (typically 4-6), Cv is the loss coefficient per bend (0.1-0.3 depending on angle), and U is the face velocity. For a 5-bend vane at 3.5 m/s with air at 20°C: ΔP = 5 × 0.15 × 1.2 × 12.25 ≈ 110 Pa. This low pressure drop makes vanes particularly attractive for fan-limited systems.
Worked Example: Sizing a Vane Pack for a 2.5m FGD Scrubber
Given: FGD scrubber handling 40,000 m³/hr of flue gas at 60°C (ρg = 1.05 kg/m³), limestone slurry (ρl = 1,100 kg/m³). Vessel ID = 2.5 m. Moderate fouling expected. Target: 99% at 15 microns.
Step 1: Select standard vanes (fouling resistance), vertical flow, K = 0.152 m/s.
Step 2: Apply fouling derating 0.85: K_design = 0.129 m/s.
Step 3: Vmax = 0.129 × √[(1,100 — 1.05) / 1.05] = 0.129 × 32.36 = 4.18 m/s.
Step 4: Vdesign = 4.18 × 0.75 = 3.14 m/s.
Step 5: Required area: Q = 40,000 / 3,600 = 11.11 m³/s; A = 11.11 / 3.14 = 3.54 m².
Step 6: Actual velocity in 2.5 m vessel: A_actual = 4.91 m²; U_actual = 11.11 / 4.91 = 2.26 m/s — 54% of Vmax. ΔP ≈ 45-90 Pa. Suitable for FGD service with weekly water wash.
Advanced Mist Eliminator Designs
Standard mesh and vane designs cover the majority of industrial mist elimination needs, but certain applications benefit from combined or advanced configurations that leverage the strengths of multiple technologies in a single assembly. Three designs deserve attention: combined vane-mesh assemblies, two-stage mesh with graded density, and inlet diffuser placement that protects the mist eliminator from bulk liquid carryover.
Combined Vane-Mesh Assemblies
A combined vane-mesh assembly places a vane pack as the lower stage followed by a wire mesh pad as the upper stage in the same vessel section. The vane stage removes bulk droplets above 10-15 microns at high capacity and resists fouling from solids in the gas stream. The wire mesh stage above polishes the gas to capture fine mist down to 3-5 microns. The combined assembly handles liquid loads up to 10% by volume and extends the cleaning interval by 3-5× compared to mesh alone, while achieving cut points below 5 microns that neither technology achieves individually in fouling service. The combined K-factor is approximately 0.080-0.095 m/s — lower than either type alone because the total assembly creates more restriction — and the total height is typically 300-400 mm (150 mm vanes + 100-150 mm mesh plus 50-100 mm separation gap). Specify combined assemblies in amine contactors, glycol dehydrators, and refinery gas processing where both high liquid load and fine mist capture are required.
Inlet Diffuser Design
The inlet arrangement below the mist eliminator determines whether the gas velocity is uniformly distributed across the full cross-section of the mist eliminator. A maldistributed inlet causes localized high-velocity zones that exceed Vmax in one area while other areas receive little flow, dramatically reducing overall efficiency. The KLM guidelines and GPSA recommend one of four inlet diffuser types depending on the inlet nozzle momentum (ρV²/gc). For low momentum below 21 lbf/ft², a simple half-pipe diverter plate is adequate. For moderate momentum of 21-104 lbf/ft², use an inlet deflector or perforated baffle plate. For high momentum above 104 lbf/ft², install a cyclonic inlet or Schoepentoeter device that removes bulk liquid by centrifugal force before the gas reaches the mist eliminator. The inlet diffuser must be located at least 300 mm below the mist eliminator support ring, with a straight vessel section of minimum 1 D between the inlet nozzle and the support ring for proper velocity profile development. A well-designed inlet device reduces the required K-factor derating from 0.75 to 0.95 by preventing liquid carryover from disrupting the mist eliminator operation.
Collection Efficiency Calculation
Predicting mist eliminator collection efficiency requires understanding the Stokes number — a dimensionless parameter describing how closely a droplet follows the gas flow streamlines. The Stokes number determines the probability of a single droplet colliding with a wire or vane surface, and the total efficiency is the cumulative result across all layers or bends in the mist eliminator.
Stokes Number and Single-Wire Efficiency
The Stokes number is defined as Stk = ρl × dp² × U / (18 × μg × Df), where dp is the droplet diameter (m), μg is the gas viscosity (Pa·s), and Df is the wire diameter or the characteristic vane channel width (m). When Stk >> 1, droplets continue in a straight line when the gas turns and impact the surface — high capture probability. When Stk << 1, droplets follow the gas streamlines and flow around the surface without contact — low capture probability. The critical Stokes number for 50% capture efficiency is approximately Stk50 = 0.25 for wire mesh and Stk50 = 0.5-1.0 for vanes depending on the bend angle. For a standard wire mesh demister with Df = 0.23 mm, gas velocity of 2.5 m/s, and μg = 1.8 × 10⁻⁵ Pa·s (air at 20°C), Stk for a 10-micron water droplet is Stk = 995 × (10⁻⁵)² × 2.5 / (18 × 1.8 × 10⁻⁵ × 2.3 × 10⁻⁴) ≈ 2.9 — well above the critical value, giving high single-wire capture efficiency.
Multi-Layer Efficiency
Single-wire efficiency ηw describes the fraction of droplets of a given size that collide with a single wire. For Stk > 1, ηw approaches 1. The total efficiency of a multi-layer mesh pad is ηtotal = 1 — (1 — ηw)n, where n is the number of effective layers in the pad. For a standard 100 mm mesh pad with 25 layers, the total efficiency at 10 microns is ηtotal = 1 — (1 — 0.85)25 ≈ 99.999+%. For a 5-micron droplet at the same velocity with ηw ≈ 0.30, ηtotal = 1 — (1 — 0.30)25 ≈ 99.99%. These theoretical efficiencies are rarely achieved in practice because of non-uniform gas distribution, liquid re-entrainment, and bypass at the vessel wall. A well-designed mist eliminator achieves approximately 90% of the theoretical multi-layer efficiency. For typical design work, use the published vendor efficiency curves rather than calculating from first principles, but understand the Stokes number relationship to evaluate how efficiency changes when operating conditions deviate from the vendor’s reference case.
Design Derating Factors
The K-factors in the previous tables apply to air-water systems at ambient conditions with clean gas and low liquid loading. Real operating conditions almost always require derating — multiplying the base K-factor by a factor less than 1 to maintain safe operation. The table below provides conservative derating factors for common non-ideal conditions.
| Condition | Derating Factor | Applicable To |
|---|---|---|
| Liquid viscosity > 5 cP | 0.70-0.85 | Both mesh and vanes |
| Liquid viscosity > 20 cP | 0.50-0.70 | Both mesh and vanes |
| Inlet liquid loading > 5% by volume | 0.75-0.85 | Both mesh and vanes |
| Operating pressure > 50 bar | 0.80-0.90 | Both mesh and vanes |
| Vacuum service (P < 1 bar abs) | 0.60-0.75 | Mesh only (vanes less affected) |
| Fouling service (partial plugging expected) | 0.80-0.85 | Vanes only (mesh should not be used) |
| Horizontal vessel installation | 1.05-1.10 | Both (more area available) |
| Polymerizing or sticky compounds | 0.60-0.70 | Both (conservative — frequent cleaning needed) |
When multiple non-ideal conditions apply simultaneously, multiply the individual derating factors. For example, a vane pack handling viscous oil (μ = 8 cP, factor 0.80) at high pressure (80 bar, factor 0.85) in fouling service (factor 0.85) would use a combined derating of 0.80 × 0.85 × 0.85 = 0.58 applied to the base K-factor. Always verify derating factors with the mist eliminator vendor before finalizing the design, as specific geometries may have different sensitivities to each condition.
Galvanic Corrosion Warning
A frequently overlooked design issue is galvanic corrosion between the mist eliminator and the vessel. When a stainless steel wire mesh or vane pack is installed in a carbon steel vessel handling polar liquids — water, acids, caustics, or any aqueous solution — the dissimilar metals in contact with an electrolyte create a galvanic cell. The carbon steel vessel becomes the anode and corrodes preferentially, thinning the vessel wall at the support ring contact points. KLM Technology Group identifies this as one of the top five demister design flaws. The fix: use vessel material for the support ring and demister frame, or isolate the stainless steel demister from the carbon steel vessel with a non-metallic gasket (PTFE or reinforced rubber) at every contact point. In highly corrosive environments, consider an all-plastic demister assembly (PP or PTFE) to eliminate galvanic potential entirely. This issue is especially critical in FGD scrubbers and amine contactors where the electrolyte conductivity is high and corrosion rates can reach 1-2 mm/year at unprotected contact points.
Vessel Design for Mist Eliminators
The vessel containing the mist eliminator must provide adequate space above and below the unit for proper gas distribution and liquid disengagement. The design of this space is governed by guidelines from GPSA and ASME Section VIII. Below the mist eliminator, provide a minimum of 300 mm of straight vessel height for gas inlet distribution when the inlet nozzle is below the demister, and 150 mm when inlet baffles or distributors are installed. Above the mist eliminator, provide a minimum vertical distance from the top of the pad to the outlet nozzle centerline equal to the vessel diameter D but not less than 600 mm. This disengagement height allows any liquid re-entrained from the top of the mist eliminator to settle back down before reaching the outlet nozzle. The support ring for the mist eliminator should be designed for a minimum of 2.5 kPa uniform load (equivalent to approximately 250 kg/m²) to account for the wet weight of the demister plus any liquid hold-up during flooding conditions. Access shall be provided via a manway or handhole located either at the mist eliminator elevation for sectional units that pass through the opening, or above the unit for upload-type mesh pads that are installed through the top opening. For vessels larger than 2.5 m diameter, provide two access openings at opposite sides of the vessel to allow installers to reach the full cross-section without stretching across unsupported mesh or vane sections. All internal support structures — support rings, beams, and grating — should be fabricated from the same material as the vessel or a compatible alloy to prevent galvanic corrosion at the contact points with the mist eliminator.
Mist Eliminator Selection by Operating Conditions
The final design step is verifying that the selected mist eliminator type is appropriate for the full range of operating conditions — not just the design point but also turndown, startup, and upset scenarios. The decision matrix below summarizes which type to select for each combination of operating conditions.
| Operating Condition | Wire Mesh | Standard Vane | Pocketed Vane | Fiber-Bed |
|---|---|---|---|---|
| Cut point below 5 μm | Yes (hi-eff) | No | No | Yes |
| Cut point 5-10 μm | Yes | No | Yes (tight spacing) | Over-specified |
| Cut point above 10 μm | Yes | Yes | Yes | Over-specified |
| High velocity (> 4 m/s) | No | Yes | Yes | No |
| Fouling / solids present | No | Yes | Yes | No |
| High liquid load (> 5% vol) | No | Marginal | Yes | No |
| Very low ΔP required | Yes | Yes | Marginal | No |
| Vacuum service | Yes (derated) | Yes (derated) | Yes (derated) | No |
| Online cleaning needed | No | Yes | Yes | No |
| Sub-micron removal | No | No | No | Yes |
For detailed design guidance on each specific type, see the mist eliminator selection guide, the wire mesh demister pad design guide, and the vane mist eliminator design guide.
Common Design Mistakes to Avoid
KLM Technology Group’s engineering guidelines identify several recurring specification errors. The most common: awarding the demister to the lowest bidder without performance guarantees, specifying a mesh pad in fouling service where vanes would extend runtime 3-5x, installing SS316L in a carbon steel vessel handling aqueous streams without galvanic isolation, and failing to verify disengagement height — the most frequent cause of carryover in field installations with a correctly sized mist eliminator. Every specification should include three vendor guarantees: hydraulic capacity at design velocity, separation efficiency at the target droplet size, and pressure drop at the design point — all verified at end-of-run conditions, not start-of-run.
FAQ
What is the Souders-Brown equation for mist eliminator design?
The Souders-Brown equation calculates the maximum allowable gas velocity through a mist eliminator: Vmax = K × √[(ρl — ρg) / ρg]. The K-factor depends on the mist eliminator type — 0.107 m/s for standard wire mesh, up to 0.259 m/s for pocketed vanes in horizontal flow.
How do I select the right K-factor for my mist eliminator?
Start with the base K-factor for your type (wire mesh standard = 0.107, vane standard vertical = 0.152, etc.), then apply derating factors for viscosity, pressure, fouling, and liquid loading. Multiply all applicable derating factors and apply to the base K before calculating Vmax.
What is the typical pressure drop across a mist eliminator?
Wire mesh demisters: 50-250 Pa (0.2-1.0 in wc) at design velocity. Vane packs: 50-200 Pa (0.2-0.8 in wc). Fiber-bed coalescers: 500-2,500 Pa (2-10 in wc). The pressure drop increases with gas velocity squared, so small increases in flow rate produce significant ΔP increases.
How do I calculate the required vessel diameter for a mist eliminator?
The required diameter is determined by the Souders-Brown equation. Calculate the required cross-sectional area A = Q / Vdesign, then D = √(4A/π). If the calculated diameter is larger than the vessel diameter, either increase vessel size, switch to a higher-capacity mist eliminator type (e.g., pocketed vanes instead of mesh), or accept a lower safety margin.
What derating factor should I use for vacuum service?
For vacuum service below 1 bar absolute, use K-factor derating of 0.60-0.75 for wire mesh demisters. Vane packs are less affected by low gas density because their separation mechanism relies more on droplet inertia in directional changes than on gravitational settling, but a 0.80-0.85 derating is still recommended below 0.5 bar.
What is the minimum height required above a mist eliminator?
GPSA guidelines recommend a minimum vertical distance from the top of the mist eliminator to the outlet nozzle centerline equal to the vessel diameter D but not less than 600 mm. Below the mist eliminator, provide at least 300 mm for gas distribution or 150 mm if inlet baffles are installed.
Can I use the same K-factor for vertical and horizontal vessels?
No. Vanes in horizontal flow can operate at higher K-factors (up to 0.259 m/s) than vertical flow (up to 0.244 m/s) because gravity assists liquid drainage perpendicular to the gas flow. For wire mesh, horizontal installation is not recommended — the pad orientation prevents proper drainage and leads to flooding at lower velocities.
Conclusion
Mist eliminator design follows a single governing equation — the Souders-Brown relationship — applied with type-specific K-factor selection and appropriate derating for non-ideal conditions. The correct design procedure ensures that the mist eliminator achieves its target separation efficiency at the allowable pressure drop, with adequate safety margins for process upsets and fouling accumulation. The selection between wire mesh, standard vanes, pocketed vanes, and fiber-bed coalescers depends on the droplet size distribution, gas velocity, fouling potential, and liquid load — each type has a specific operating window where it outperforms the alternatives.
XICHENG EP LTD provides complete mist eliminator design services, including type selection, K-factor determination, vessel sizing, and pressure drop verification. Contact our applications engineering team with your process conditions — gas composition, flow rate, temperature, pressure, and target separation efficiency — for a design recommendation and equipment quote.