A sizing run turns a duty sheet into a vessel diameter, and every answer it returns is only as good as the inputs behind it. At its core, mist eliminator design is arithmetic over a short list of variables, and the failure most worth catching is an input that is wrong by an order of magnitude.
Key Takeaways
- Six inputs decide the vessel, and the K-factor is the one most often wrong. Actual gas flow, gas density, liquid density, K-factor, target droplet size and liquid load; a size computed from a blank line is a number someone else reverses at start-up.
- The same K reads as two constants in two unit systems. A mesh pad at 0.35 ft/s and one at 0.107 m/s are the same pad; multiplying by 0.3048 is the whole conversion, and it moves the diameter directly.
- 0.35 is an average, not a constant. Service multipliers and pressure derating come before the number is usable for a specific duty.
- Capacity and capture are two separate checks. Souders-Brown gives the velocity ceiling where a vessel stops re-entraining; it says nothing about which droplet sizes are captured.
- An existing tower’s usable gas flow depends on the gas flow, not on the tower. The same 1.2 m vessel runs at 60% of its limit at 8,000 m³/h and 135% at 18,000 m³/h.
- Pressure-drop energy is real but small at moderate scale. Fifty to 150 Pa spans roughly three times the annual fan cost, which is still hundreds of dollars a year at 18,000 m³/h.
The Design Inputs a Sizing Run Needs
The Six Inputs, and What to Do When One Is Missing
Every sizing run starts from six inputs, and each one carries a unit that must be checked before arithmetic begins. The capture mechanisms behind the type you pick sit in the mist eliminator overview; this page starts after that choice is made.
- Gas flow rate at actual operating conditions, in actual volume per time. A flow stated at standard conditions must be corrected for temperature and pressure first.
- Gas density, from temperature, pressure, and molecular weight.
- Liquid density, the other side of the Souders-Brown density term.
- K-factor, selected from the mist eliminator type, orientation, and service using the manufacturer’s table for that geometry (N-01, N-04).
- Target droplet size the eliminator must capture. Gravity settling captures down to roughly 50 µm, and below 5 µm the service needs an additional mechanism (N-01).
- Liquid load, the droplet mass carried per volume of gas, which drives the re-entrainment check.
When an input is missing, record it as open and name the owner. A diameter computed from a blank line is a number someone else will reverse at start-up.
Which Input Actually Moves the Answer
The diameter hides inside the answer, but the inputs push it at different rates. The K-factor is linear in velocity: the Souders-Brown limit is Vt = K·√((ρl−ρg)/ρg) (N-01 Eq. 11), so doubling K doubles the allowable velocity. Flow rate enters through area, so doubling gas volume grows the diameter by only √2, about 1.41 times. The density ratio also sits under a square root, so a given percentage error in density moves the answer less than the same error in K or flow. Target droplet size works on a separate axis: it sets the efficiency ceiling while the diameter follows flow, K, and the density ratio.
The Short Answer Before Any Arithmetic
Before running the numbers, the answer already has a shape. Gas volume, fluid properties, and target droplet size decide how large the vessel must be; the equipment model does not. A duty sheet that lists only a model number and a flow rate omits the densities that actually enter the Souders-Brown limit. Get the flow and the densities right first, select a K-factor honestly, and the diameter follows. Every later refinement — the design velocity, the re-entrainment line, the pressure drop — adjusts that first number without changing which variables drive it.
The Souders-Brown Equation in Both Unit Systems
The Equation and Where It Comes From
Souders-Brown is an empirical correlation built from decades of separator field data, set out in the Laurance Reid gas-conditioning fundamentals text with its K and C tables (N-01, University of Oklahoma). It sets the maximum superficial gas velocity a vertical separator can run before it re-entrains liquid: Vt = K·√((ρl−ρg)/ρg), where ρl and ρg are the liquid and gas densities and K is an empirically determined capacity factor (N-01 Eq. 11). The form shows the physics plainly: the density difference drives separation, and the whole term shrinks when the gas is dense or the liquid is light. A light gas and a heavy liquid therefore permit a higher velocity, which is why a vacuum or low-pressure duty behaves differently from a high-pressure one.
A horizontal separator longer than 10 ft gains extra allowance, so the same limit picks up a length correction: Vt = K·√((ρl−ρg)/ρg)·(L/10)^0.56 (N-01 Eq. 13). A mass-flow form also exists, Gm = C·√(ρg(ρl−ρg)), for rating a vessel by throughput (N-01 Eq. 12). All three forms share one property: K, like C, is empirical and must be determined by experiment for the actual mesh type, liquid load, and liquid properties (N-01). In a sizing run the equation is the single step that turns the density ratio and the chosen K into a velocity ceiling, which then divides the gas flow to yield the required area and, from it, the diameter. That is why the same mist eliminator design arithmetic can carry different K tables from one vendor manual to the next.
The Same K in Feet and in Metres
The most confusing number in mist eliminator design is the K-factor, because one physical limit appears in two unit systems and reads as two different constants. The AMACS engineering handbook gives a pad a capacity factor of 0.35 ft/s in its standard coefficient table (N-04, The Engineered Mist Eliminator). Converted, 0.35 ft/s × 0.3048 = 0.1067 m/s. A separate compilation lists a standard 100 mm, 150 kg/m³ mesh at K = 0.107 m/s (N-03). The two independent sources land within about 0.0003 m/s of each other — the same number written once in feet and once in metres. An engineer who mixes the two units gets a velocity limit off by the full conversion factor, and the error lands directly in the diameter.
That agreement turns a source of confusion into a check: any K stated in ft/s multiplies by 0.3048 to give m/s, and a mesh K near 0.107 m/s is the same pad a US reference calls 0.35 ft/s. The value itself is not a universal constant, though. The primary reference is explicit: “The familiar 0.35 factor is merely an average value” (N-01). A mesh pad in clean, low-load service justifies a higher number; a loaded or foaming service does not. The unit conversion fixes the arithmetic, and the service fixes the K.
Where the Equation Stops Being Valid
Souders-Brown corrects only the gas and liquid densities. It carries no term for liquid load, viscosity, flow maldistribution, or foaming, so any condition that affects drainage or re-entrainment must be handled by scaling the capacity factor in proportion, exactly as the manufacturer states for its own factor (N-04). A mesh that drains poorly at high liquid load will flood at a velocity the equation still calls safe, because the equation never saw the liquid load.
The density ratio is also where the equation overreaches in vacuum service. When ρg is very small, the term √((ρl−ρg)/ρg) is amplified, and the velocity limit climbs to a value no physical pad will hold. The reference drops K to 0.20 ft/s for vacuum service (N-01), a correction the equation itself does not supply. Relying on the raw equation in vacuum would size a pad too small for the duty.
Droplet size is absent from the equation entirely. Souders-Brown gives only the ceiling where a vessel stops re-entraining liquid; it says nothing about which droplet sizes are actually captured. That ceiling and the capture target are two separate questions, and only the first one is answered here.
Selecting a K-Factor for Your Service
K-Factors by Type and Orientation
K begins as a property of the eliminator element, not of the vessel around it. A mesh pad and a vane pack hold different velocities on the same gas, and the flow direction through the pack shifts the number again. The base figures below come from a manufacturer’s standard table (N-04 Table 3); the metric column is the ×0.3048 conversion, cross-checked against an independent compilation (N-03). Which of the two element types suits a given duty is a separate comparison; here both are sized by the same arithmetic.
| Type and flow direction | K (ft/s) | K (m/s) |
|---|---|---|
| Mesh pad | 0.35 | 0.107 |
| MisterMesh | 0.42 | 0.128 |
| Vane pack, horizontal flow | 0.65 | 0.198 |
| Vane pack, vertical flow | 0.50 | 0.152 |
The two vane rows carry a vocabulary trap worth flagging once. The manufacturer names the pack by plate orientation and the compilation by flow direction, so the same two numbers — 0.152 and 0.198 m/s — reappear under swapped labels in the two sources. Read by flow, horizontal flow holds the higher K (0.65 ft/s) and vertical flow the lower (0.50 ft/s), an ordering the authority reference confirms (N-01 Table 3). Mist eliminator design arithmetic fails most often here, when an engineer drops a horizontal-flow K into a vertical-flow vessel.
The ordering itself is physical. In horizontal flow, gravity pulls the collected liquid down while the gas moves sideways, so the gas carries fewer droplets back into the airstream; in vertical flow, the gas must push against the draining liquid, which caps the velocity lower. That is why the horizontal rows sit above the vertical rows in every table that separates the two.
Service Corrections That Move the Number
The base K is a clean-service starting point. Real duties knock it down with multipliers the reference states plainly (N-01): no demister at all ×0.5, glycol or amine systems ×0.6–0.8, and compressor suction ×0.7–0.8. Each multiplier encodes a service condition the base table never sees, so applying them in sequence folds those conditions back into the number.
Service also changes the picture directly. The reference lists wet steam at 0.25 ft/s, vacuum at 0.20 ft/s, and salt or alkali evaporators at 0.15 ft/s (N-01 Table 3), each well below the clean mesh pad’s 0.35. A corrosive or fouling duty lowers the K, and the correct value comes from the table row for that service rather than from the clean baseline.
Pressure Corrections Above Atmospheric
Raising the operating pressure lowers the allowable design velocity, because denser gas carries liquid more readily. The reference expresses this as a percentage of the atmospheric design velocity (N-01): 100% at atmospheric, 90% at 150 psig, 85% at 300 psig, 80% at 600 psig, and 75% at 1150 psig. The correction is a multiplier applied after the type, orientation, and service steps, so a pad sized at 300 psig runs 15% slower than the same pad at atmospheric pressure. Denser gas carries more momentum per unit volume, so a droplet that would fall out at atmospheric pressure gets dragged along. The percentage correction folds that extra momentum into the velocity limit before the area is computed.
Why “0.35” Is Only an Average
0.35 ft/s is the number most engineers reach for, and it is only an average. The authority reference calls 0.35 an average value (N-01), one that shifts with mesh type, liquid load, liquid properties, and flow maldistribution. Two pads of the same nominal type can hold different K values because their drainage differs.
The order is what makes 0.35 safe or unsafe: type first, then orientation, then the service corrections, then the pressure correction. Each step moves the number down from the clean baseline. Skipping a step leaves the K too high, and a K that is too high becomes a vessel that floods at a velocity the drawing still calls acceptable.
Worked on a compressor-suction mesh pad, the sequence runs: 0.35 ft/s for the type, the vertical orientation, ×0.7 for compressor suction, then the pressure step for the suction pressure. Each step lowers the number, and the final K is the value that actually goes into the velocity equation.
Design Velocity and the Re-Entrainment Line
From Maximum Velocity to a Design Velocity
The Souders-Brown number is a ceiling, and a vessel is never run at its ceiling. Normal operation stays at or below 80% of the maximum velocity, and 90% or higher pushes into the re-entrainment zone (N-03, a secondary compilation; the same margin is implied by the manufacturer’s instruction to scale the capacity factor for any condition that affects drainage or re-entrainment, N-04). The 20% gap is the operating margin: it absorbs a flow swing, a liquid-load spike, or a patch of poor distribution without crossing the line.
The cost of crowding the line is pressure drop, which rises with the square of velocity (N-04 Eq. 3; N-03 states that doubling velocity roughly quadruples pressure drop). A pad run at 90% of Vmax pays about 27% more pressure drop than the same pad at 80%, for a margin it no longer holds. The design velocity is the number the sizing arithmetic actually uses, and it sits deliberately short of the ceiling so that a real, unsteady duty stays clear of the line. Mist eliminator design therefore treats the design velocity as a decision made inside the safe band.
What Re-Entrainment Looks Like on a Running Unit
Re-entrainment rarely announces itself as an overspeed alarm, because there is no velocity gauge reading the eliminator’s cross-section. It shows up as three consequences that have to be read backwards, and by the time they show up the pad has been running past its limit long enough to affect whatever sits downstream.
First, pressure drop climbs while the gas flow has not changed. Liquid that was once held in the pad is now being carried back into the gas and passing through again, so the pad behaves as if it were handling more liquid than it is.
Second, the droplet size at the outlet grows. The larger droplets that the pad was sized to catch begin reappearing downstream, because the gas is now fast enough to strip them off the drainage surfaces and carry them through.
Third, the drainage volume rises with no change in liquid load. Liquid is re-entrained, carried forward, and captured again on the next pass, so the same droplets are drained several times over.
The common thread is that every one of these signals is a consequence, and none is a direct reading of velocity. That is why re-entrainment is usually found only after it has already been running for a while.
Horizontal and Vertical Are Not the Same Duty
The same eliminator element gives different numbers depending on which way the gas runs through it, and the reason is drainage. In horizontal flow, gravity pulls the collected liquid down while the gas moves sideways, so the two directions are perpendicular and the liquid takes the shortest path out of the airstream. In vertical flow the gas pushes upward against a film that is trying to drain downward; the two directions fight each other, and the velocity ceiling drops accordingly.
The authority reference makes the gap concrete: at a 10 ft height a vertical element runs 0.18–0.35 ft/s while a horizontal element runs 0.40–0.50 ft/s (N-01 Table 3), two ranges that barely overlap. That is why reorienting a unit changes its allowable throughput even though the pad inside is unchanged. A vessel sized as a horizontal-flow unit will re-entrain if it is reinstalled as a vertical-flow unit and fed the same gas volume, because the ceiling it was designed against has moved down. The direction of flow is therefore part of the duty specification, and a K chosen without it is a K chosen against the wrong velocity ceiling.
Capacity Limits and Collection Efficiency
Two Different Questions, Two Different Calculations
The task of mist eliminator design splits into two questions that are easy to collapse into one, and the collapse is where designs go wrong. The capacity question is whether the vessel will flood: the Souders-Brown limit and the chosen K give a maximum velocity, and staying under it keeps the pad from re-entraining liquid. The collection question is whether the pad will catch the droplet size the duty actually requires, and that is answered by inertial impaction and the Stokes number, not by K.
The two checks have different failure modes, which is why they cannot share one number. A pad can sit comfortably inside its capacity limit and still miss the target droplet size, because a low velocity may be safe yet too slow to throw the small droplets onto the wires. A pad can also hit its target size and still flood, because the velocity needed for capture has already crossed the capacity ceiling. Capacity failure shows up as carryover downstream; collection failure shows up as the wrong droplet getting through. Each must be checked on its own, with its own calculation.
The Inertial Parameter and What It Predicts
Collection on a wire or fiber is a collision problem, and the variable that predicts it is the inertial parameter: K = (ρl−ρg)·Vd² / (9·µ·D), where Vd is the gas velocity, µ the gas viscosity, and D the wire or fiber diameter (N-04 Eq. 2). The parameter is dimensionless and reads directly: a droplet with enough inertia will not follow the streamline around a wire, so it collides instead.
The formula shows the two levers. K rises with the square of velocity and falls as the wire gets thicker or the gas more viscous, so a thin wire and a fast gas both push the collision probability up. The same velocity, though, is the one that carries the pad toward its capacity ceiling. Raising gas speed to catch smaller droplets and raising it far enough to flood the pad are the same motion, which is why efficiency and capacity pull against each other on this single variable. The Stokes-number rule of thumb states the same cut in another form: a droplet well above unity tends to be captured, and one well below about 0.1 follows the streamlines and misses the surface (N-03, a secondary compilation).
The manufacturer then converts that collision fraction into an overall efficiency through the pad’s surface area and thickness: Efficiency% = 100 − 100/e^(0.213·E·SO), where SO folds in the specific surface area, the thickness, and a 0.67 factor for the wires that are not perpendicular to the flow (N-04). K predicts a collision; the pad’s geometry then decides how many chances a droplet gets.
Droplet Size Sets the Ceiling
Collection efficiency is never a single number. It is always efficiency at a given droplet size, and a pad that removes nearly all of a 100 µm droplet may remove almost none of a 10 µm one. Any efficiency figure quoted without the droplet size it applies to is incomplete; the correct question is always the size, never a bare percentage.
The size boundaries come from the settling physics. Gravity settling is effective down to about 50 µm; below that, gravity alone loses hold and the droplet needs mechanical assistance, and below about 5 µm it needs coalescing, filtration, or an electrostatic means to be caught at all (N-01). A duty that must remove 2 µm droplets is asking for a different mechanism than one that must remove 50 µm droplets, and the efficiency target has to name that size before any number can be assigned.
Sizing a New Unit From Start to Finish
A Full Sizing Run on One Duty
The full mist eliminator design calculation runs end to end in seven steps. The duty is an air stream at 18,000 m³/h and 60 °C carrying water droplets, with a gas density of 1.05 kg/m³, a liquid density of 985 kg/m³, and a requirement to catch 8 µm droplets at 99% within an allowable pressure drop of 250 Pa. The 99% capture is a collection requirement, and it is settled before the capacity arithmetic: a mesh is chosen to meet the droplet size, and the velocity calculation below then settles how large the vessel must be. The duty is a warm, near-atmospheric exhaust, and its low gas density pushes the velocity ceiling higher than a denser, pressurised gas would allow.
A standard mesh is the starting element, with K = 0.107 m/s (N-03, a 100 mm, 150 kg/m³ mesh; the same value as N-04’s 0.35 ft/s). The maximum velocity follows from the Souders-Brown limit: Vmax = K·√((ρl−ρg)/ρg) = 0.107 × √((985−1.05)/1.05) = 0.107 × 30.61 = 3.28 m/s. That ceiling is the flood line for this gas and liquid pair, and every step after it divides against it.
A 75% design line is chosen here to leave margin for liquid-load swings, the fluctuation the 70–80% band exists to absorb: Vdesign = 3.28 × 0.75 = 2.46 m/s. The flow is 18,000/3,600 = 5.00 m³/s, so the required area is A = 5.00/2.46 = 2.04 m², and the diameter follows: D = √(4A/π) = √(4×2.04/3.1416) = 1.61 m.
A round vessel is specified at the next standard size up, 2.0 m, whose area is π×1.0² = 3.14 m². The actual velocity then drops to 5.00/3.14 = 1.59 m/s, which is 48.6% of Vmax and a comfortable margin. The jump from the required 1.61 m to the standard 2.0 m is what buys that margin. At that velocity the pressure drop is roughly 50–80 Pa (N-03, a secondary compilation), well under the 250 Pa allowance. The arithmetic first produces a 1.61 m requirement, and the delivered answer is the 2.0 m vessel that carries a margin; keeping those two figures distinct is the point of the checks that follow.
Checking the Answer Three Ways
A single number is easy to trust, so the diameter is checked three independent ways before it is accepted. The three checks use three different inputs — velocity, area, and pressure drop — so a mistake in any one of them is caught by the other two.
The velocity check confirms the operating point sits far from the re-entrainment line: 1.59 m/s is 48.6% of Vmax, well clear of the 80% ceiling, so the pad is nowhere near flooding at the design flow.
The throughput check works the area in reverse. At the 80% line of 2.62 m/s, the 3.14 m² vessel passes 2.62 × 3.14 × 3,600 ≈ 29,600 m³/h, about 1.6 times the design flow, so the tower absorbs a future throughput increase without crossing the line.
The pressure-drop check confirms the fan budget holds: 50–80 Pa against a 250 Pa allowance leaves the fan with margin to spare, so the pad adds no need for a larger fan. Only when all three checks pass is the size accepted.
What Changes When the Duty Changes
Mist eliminator design keeps its linearity up to a point, so the effect of a changed duty is predictable. Doubling the gas flow doubles the required area and grows the diameter by only √2, about 1.41 times, because diameter scales with the square root of area. The required 1.61 m would become about 2.28 m at twice the flow, and the same relationship runs in reverse: halving the flow shrinks the diameter to about 0.71 times.
Temperature changes the density side of the equation. A higher temperature lowers ρg, which enlarges the density-ratio term √((ρl−ρg)/ρg) and raises Vmax, because a lighter gas permits a higher velocity. That gain is real but partial: a higher temperature also evaporates or condenses droplets and shifts the droplet size distribution, and that effect is not in the Souders-Brown equation. A temperature-driven sizing change must therefore be checked against the droplet-size requirement, which the velocity equation alone does not answer.
Reverse-Checking an Existing Tower Diameter
What the Existing Diameter Already Delivers
The reverse question starts from a tower that already stands: 1.2 m in diameter, and the job is to find how much gas it can carry before it crosses the line. The forward run asked how large a new vessel should be; the reverse run asks how large the flow may be on a vessel whose diameter can no longer change. The duty is the same as the forward case — ρl = 985 kg/m³, ρg = 1.05 kg/m³, and a standard mesh at K = 0.107 m/s. The cross-section is fixed at A = π × (1.2/2)² = 1.131 m².
The ceiling does not move with the tower. Vmax = 0.107 × √((985−1.05)/1.05) = 3.28 m/s, the 80% design line sits at 2.62 m/s, and the 90% re-entrainment line sits at 2.95 m/s (the 80% and 90% thresholds come from a secondary compilation, N-03). With the area fixed, each flow rate becomes a velocity, and each velocity becomes a fraction of Vmax:
| Flow (m³/h) | Actual velocity (m/s) | % of Vmax | Verdict |
|---|---|---|---|
| 8,000 | 1.97 | 60% | Comfortable |
| 10,000 | 2.46 | 75% | Comfortable |
| 10,669 | 2.62 | 80% | Tight (on the line) |
| 12,003 | 2.95 | 90% | Over the line |
| 14,000 | 3.44 | 105% | Over the line |
| 18,000 | 4.42 | 135% | Over the line (far) |
Three Zones: Comfortable, Tight, Over the Line
The table falls into three zones, and the zone a tower occupies depends on the flow, not on the tower. Comfortable means the velocity sits below the 80% design line, so the pad runs with its full safety margin intact. Tight means the velocity has reached 80%, and the design margin is spent: the band from 80% up to 90% still runs, but any flow swing or liquid-load spike now pushes the pad toward the re-entrainment line. Over the line means 90% and beyond, where the pad has entered the re-entrainment zone and is shedding liquid downstream.
The same 1.2 m tower demonstrates the point on its own. At 8,000 m³/h it runs at 60% of Vmax and is safe; at 18,000 m³/h it runs at 135% and is deep in re-entrainment. Whether the device can do the job is decided entirely by the gas volume against the fixed cross-section, which is why the reverse check starts from the flow and works toward the zone. The zone is read off the flow, not off a nameplate, because the diameter is fixed while the flow is the variable the owner actually sets.
Raising Throughput Without Replacing the Tower
The 80% line is the product of K and the cross-section, so there are only three levers that raise throughput without changing the vessel, and each one carries a cost.
Swapping to a higher-K element, such as a vane, raises Vmax and lets the same area pass more gas. The cost is collection: a higher-K geometry also raises the smallest droplet it can reliably catch, and a duty that must still hit the 8 µm target may fall short.
Enlarging the effective flow area — rearranging internals or clearing blocked or blanked-off sections — raises the cross-section term. The cost is a structural modification and the downtime to carry it out.
Reducing the gas volume or splitting it, with part of the stream routed through a bypass pretreatment stage, keeps the tower under its line. The cost is a second treatment train, with its own capital and operating load.
Each lever moves the same two terms, K and area, and moving either one shifts the 80% line to a new value; the choice among them belongs to type selection and is made against the droplet target. For mist eliminator design on an existing tower, the working rule is that the ceiling follows the flow, and the flow is the only number the owner fully controls.
Pressure Drop and What It Costs
Dry Pressure Drop Versus Wet
Pressure drop has two states, and the second one is what a running unit actually sees. Dry pressure drop is the resistance of the clean geometry, set by the pad’s structure and the square of the gas velocity (N-04 Eq. 3). Wet pressure drop adds the resistance of the liquid the pad is holding and draining: once the bottom pool of captured liquid reaches about 2 inches deep, that head must be added on top of the dry figure (N-04). Wet therefore runs higher than dry, and the gap widens as liquid load rises, because a wetter pad holds more liquid and drains it against the gas. A pad sized on its dry figure alone under-predicts the running drop once liquid is present, so the wet figure is the number a fan budget must absorb.
Turning that drop into a fan and motor selection is its own sizing problem, and the wet figure is what enters it.
Annual Fan Energy at a Stated Power Price
The cost of that pressure drop is fan power, and the clean way to compute it is first principles rather than a vendor coefficient: P(kW) = Q(m³/s) × ΔP(Pa) / η / 1000, where η is the fan efficiency. The equation is the definition of power as flow times pressure, so every figure below is reproducible by hand.
The worked duty is the M06 case, Q = 5.0 m³/s, run continuously for 8,760 hours, with a fan efficiency of 0.65 and electricity at $0.08 or $0.12/kWh; all three are stated assumptions, to be replaced with the site’s own values. A unit that runs only 8,000 hours scales every energy and dollar figure by 8,000/8,760.
| ΔP (Pa) | Power (kW) | Annual energy (kWh) | @ $0.08/kWh | @ $0.12/kWh |
|---|---|---|---|---|
| 50 | 0.385 | 3,369 | $270 | $404 |
| 80 | 0.615 | 5,391 | $431 | $647 |
| 150 | 1.154 | 10,108 | $809 | $1,213 |
Raising the pressure drop from 50 Pa to 150 Pa, a difference of only 100 Pa, triples the annual bill from $270 to $809 at the lower price. The more useful number is the magnitude: at 18,000 m³/h a mesh element’s pressure-drop energy runs a few hundred dollars a year, small against the vessel and fan capital, so pressure drop rarely decides the selection at this scale.
When Pressure Drop Becomes the Binding Constraint
Pressure drop enters the decision in three situations. In vacuum service the gas density is tiny, so the same mass flow means a much larger volumetric flow, and every pascal costs proportionally more fan work. On a large-diameter, high-flow tower the absolute flow is high enough that the annual bill stops being a rounding error. And on a retrofit where the fan is already selected and fixed, any added pressure drop eats directly into the remaining fan margin, so a pad that fits the diameter can still be disqualified by the pressure it adds.
In ordinary near-atmospheric, mid-size duty the magnitude is small enough that other criteria carry the decision; only vacuum, very large towers, or a fixed fan turn a small annual figure into a binding one. For mist eliminator design, the useful habit is to price the pressure drop once, at the site’s own power rate and efficiency, and then let the number either bind or drop out.
Frequently Asked Questions
What K-Factor Should I Start With?
Start from 0.107 m/s for a standard mesh in clean, low-load service, the metric form of 0.35 ft/s. That figure is a starting point, not an answer: fold in the service corrections — a demister-less service, an amine or glycol system, or a compressor suction all derate it — and then the pressure correction. The result of that four-step sequence is the K for the duty. A K used without the derating is too high by exactly the margin those steps exist to remove.
Can I Use the Same K for Horizontal and Vertical?
No. The same vane geometry takes a different K by flow direction: about 0.198 m/s (0.65 ft/s) in horizontal flow and 0.152 m/s (0.50 ft/s) in vertical flow. Horizontal flow drains perpendicular to the gas and holds the higher figure; vertical flow pushes the gas against a downward-draining film and holds the lower one. Using the horizontal K on a vertical vessel overestimates the allowable gas volume, and the same element reoriented from horizontal to vertical loses throughput without any change to the pad itself.
How Do I Know My Mist Eliminator Is Flooding?
Watch for three consequences: pressure drop rising while the gas flow has not, larger droplets appearing downstream, and drainage volume climbing with no change in liquid load. There is no gauge that reads the cross-section velocity directly, so flooding is inferred from those consequences and is usually caught after it has begun. When any one appears, check the actual flow against the 80% of Vmax line first, then check the liquid load and whether the drain path is clear, because a plugged drain can flood a pad that the velocity arithmetic still calls safe.
Does a Bigger Pad Always Capture More?
No. A thicker pad gives a droplet more chances to collide, which helps efficiency, but it also raises pressure drop and liquid holdup and lowers the allowable velocity, which hurts capacity. The two effects pull against each other, and beyond some thickness the pressure-drop penalty arrives before the efficiency gain. The right thickness is set by the target droplet size and the allowable pressure drop together, and a duty with a 2 µm target needs a different mechanism, not simply more of the same pad.
