Your Scrubber Lost Efficiency Because You Picked the Wrong Cone: Full vs Hollow

A scrubber or absorption tower rarely fails all at once. More often it drifts: outlet readings creep up, the treatment chemical bill creeps with them, and nobody can see why, because the nozzles are still spraying. Nine times out of ten the nozzles are spraying the wrong pattern. The pump is fine, the liquid is fine, but the cone shape does not match the job, and a spiral spray nozzle that fills the center is quietly doing the opposite of what the tower needs. This guide breaks down full cone versus hollow cone, shows which limiting step each pattern serves, and gives you the selection table that stops the guessing.

The spiral nozzle spray pattern, thrown by what most purchase orders call spiral jet spray nozzles, is where that decision is made. The cone choice is the cheapest performance decision in a tower. It costs nothing to change at the specification stage, and it costs weeks of drifting outlet readings to discover in service. Read this before you write the nozzle line of the BOM. The cone is decided by the duty, and the duty is decided by where the gas actually flows.

What “Cone” Actually Means at the Tower

A full-cone nozzle lays liquid across the whole circle of the spray, including the center. Stand under it and you get a filled disc of droplets from the axis out to the edge. A hollow-cone nozzle lays liquid in a ring and leaves the center empty: a donut of spray with a dry hole in the middle. Same flow rate, same angle, completely different distribution of where the water goes.

That difference is the whole game in a tower, because gas and liquid only meet where the liquid actually is. Water aimed at the center of the tower, where there may be little or no gas to treat, is water spent on nothing. Water aimed at the annular region where the gas stream actually flows is water doing the work.

The distribution difference also shows up in droplet size and velocity. A hollow cone spins the liquid harder to throw it outward, which produces finer droplets moving faster: more surface area per unit volume of liquid, exactly what gas absorption wants. A full cone delivers a wider range of droplet sizes with more liquid mass near the center, which is what a quenching or surface-wetting duty wants: liquid that lands and stays. The cone is not a cosmetic choice between two circles; it is a choice between two different mechanisms of contacting.

Full Cone vs Hollow Cone at a Glance

Property	Full cone	Hollow cone
Liquid distribution	Filled disc, axis to edge	Ring, center dry
Where liquid concentrates	Center-weighted	Periphery
Droplet character	Broad size range, more mass	Finer, faster droplets
Surface area per volume	Lower	Higher
Best limiting step	Wetting, quenching, cooling	Gas-to-liquid mass transfer
Overlap tolerance	Forgiving: discs blend	Needs deliberate spacing
Typical failures	Center over-wet, sump load	Dry wedges between rings

Keep this table in front of you when you spec. Every row of it is a different way of saying the same thing: full cone puts liquid where the target is, hollow cone puts liquid where the gas is.

Full Cone: Wetting, Quenching, Cooling a Target

Use a full-cone pattern when the limiting step is uniform wetting of a surface or a volume, not gas contact per se:

Gas quenching: dropping the temperature of a hot gas stream fast, where you want the whole cross-section cooled, not just a ring. The center of the stream carries enthalpy too, and it has to meet liquid.
Cooling a solid or a bed: any duty where the target occupies the center and you need it wetted evenly, from a packed bed to a hot plate to a flue-gas duct wall.
Dust knocking-down in a filled chamber: where droplets need to intersect particles across the whole area, not just at the periphery.
Tank and vessel filling: where the duty is adding liquid or wetting a vessel interior rather than contacting a gas stream.
Fire-suppression deluge: where the whole footprint must be wetted, including the center, the moment the system opens.

A full cone is also forgiving on overlap. Because it fills the disc, two adjacent full cones blend into even coverage with less precise spacing. For a spiral spray nozzle used in these duties, the filled pattern means you can space nozzles on a grid and trust the whole plane is wetted.

Hollow Cone: Maximizing Gas-to-Liquid Contact

Use a hollow-cone pattern when the limiting step is mass transfer between gas and liquid, absorption, scrubbing, reaction at the interface:

Gas absorption and scrubbing: the gas flows through the annulus, and the ring of droplets gives a large surface area for the pollutant to dissolve into.
Counter-current contact: where you want the liquid film presented to the moving gas rather than dumped into the middle of it.
Evaporative cooling in a duct or tower: where the liquid must evaporate into the gas stream, and fine peripheral droplets evaporate faster than heavy center droplets.
Odor and VOC treatment: where the duty is contact area per liter, and a hollow cone delivers more of it at the same flow.
Spray drying of fine products: where finer, more uniform peripheral droplets give a narrower product size band.

The hollow cone wins here because per unit of liquid it presents more peripheral interface to the gas. The center of the tower, where a full cone wastes liquid, often carries little gas in a well-designed tower, so leaving it dry is correct. A spiral spray nozzle set to a hollow-cone pattern puts the droplets exactly where the gas is, and because the spiral makes that ring with an open ramp instead of a fine vane chamber, it keeps the pattern in dirty recirculated water where a vane hollow cone would start bridging.

Matching the Cone to the Duty

Duty	Limiting step	Cone	Why
Quench hot gas to a temperature target	Cooling across the whole cross-section	Full	Center carries enthalpy; it must meet liquid
Absorb SO₂, HCl or NH₃	Gas-to-liquid mass transfer	Hollow	Peripheral surface area per liter
Cool a packed bed or solid target	Wetting the target	Full	Target occupies the center
Knock down dust in a chamber	Droplet–particle collision everywhere	Full	Particles are everywhere
Evaporative cooling / spray drying	Droplet surface area	Hollow	Finer peripheral droplets evaporate faster
Odor / VOC contact scrubber	Interface area	Hollow	More area per liter at same flow
Tower with dead center (duct enters at wall)	Where the gas actually is	Hollow	No gas in the middle; do not wet it

Print this table and attach it to the spec. It is the answer to “which cone?” for ninety percent of tower duties. The remaining ten percent are systems where the gas distribution itself is wrong, and no cone fixes that.

Why the Wrong Choice Steals Efficiency

Pick a full cone for an absorption duty and you pour liquid into the center where there is no gas to meet. The liquid that should be maximizing contact at the periphery is instead dripping through the middle doing nothing, so you either raise the dose to compensate (more chemical, more pump, more drift to the sump) or you accept a higher outlet concentration. Either way the tower’s effective efficiency drops for no fault of the pump.

The waste is measurable. In a tower where the gas flows in the outer 60% of the cross-section, a full cone directing a third of its liquid into the dead center is spending roughly a third of the absorbed-duty liquid on zero contact, and the chemical bill, pump energy and sump disposal all carry that dead weight. The same liquid redirected into the annulus is actual removal.

Pick a hollow cone for a quenching or surface-cooling duty and you leave the center of the target dry. The hot gas or the hot surface in the middle never meets liquid, so cooling stalls and the treated stream leaves hotter than spec. Same nozzle family, opposite failure, same root cause: pattern not matched to the limiting step.

Spiral Nozzles Make Both, and That Is the Trap

Here is the part that catches specifiers. A spiral spray nozzle is not inherently one cone or the other. The number of turns on the spiral ramp and the body geometry set whether the discharge is full cone or hollow cone. A single-turn design tends toward hollow cone; a multi-turn design fills the pattern toward full cone. So you cannot specify “a spiral nozzle” and assume the pattern. You have to specify the cone you need, then confirm the body delivers it.

Our reference line shows both patterns in the same family:

Model ref.	Pattern	Flow @ 1.5 bar	Free passage	Connection	Material
SP-1/8	Full cone	0.2–0.8 L/min	2.5 mm	1/8“	316L / PP
SP-1/4	Full cone	0.6–2.5 L/min	4.0 mm	1/4“	316L / PP / PVDF
SP-3/8	Full cone	1.5–6 L/min	6.0 mm	3/8“	316L / PP
SP-1/2	Full cone	4–18 L/min	9.0 mm	1/2“	316L / PP
SP-3/4	Hollow cone	8–40 L/min	12 mm	3/4“	316L
SP-1	Hollow cone	20–90 L/min	18 mm	1“	316L

Reference values at 1.5 bar with water. Confirm the actual cone shape at your operating pressure before ordering in quantity.

This is also why spiral nozzles suit towers better than many vane nozzles in dirty service: they keep the open path that resists clogging (the narrowest point is the exit, not an internal swirl chamber) while still giving you the cone choice. Just do not let the cone choice be an afterthought. Spiral spray nozzles and spiral nozzles across a tower should be picked cone-first, then laid out by coverage.

How the Pattern Behaves as Pressure Moves

Cone shape is not a fixed property stamped into the metal; it is a function of pressure. Every pressure-atomicizing nozzle, spiral, vane or tangential, shifts its pattern as the pressure moves off the rating point:

Above the rating pressure, the cone angle typically widens and the liquid concentrates toward the periphery. A full cone starts to hollow out at the center; a hollow cone’s ring thins and throws farther.
Below the rating pressure, the cone narrows and the center fills. A hollow cone that should be a clean ring degrades into a ragged, center-weighted spray; a full cone shrinks toward a stream.
The minimum pattern pressure is the number to know: below it, neither cone exists, just a dribbling jet.

That is why the selection table above quotes flow at 1.5 bar and why you should confirm the cone at your actual operating pressure, not the catalog rating. If your tower holds 1 bar and the body is rated at 3, you will get a pattern that looks nothing like the spec sheet. The same rule applies on a recirculation loop where pressure drifts with sump level and strainer condition. The cone drifts with it.

A Five-Question Check Before You Spec

What is the limiting step? Wetting/cooling a target → full cone. Gas-to-liquid mass transfer → hollow cone. If you cannot name the limiting step, you are not ready to name a cone.
Where does the gas actually flow? If the center is dead space, a hollow cone is usually right. Confirm with a velocity traverse instead of assuming uniform flow, towers rarely deliver what the drawing promises.
What is the water quality? Dirty recirculated water favors the open path of a spiral design over a vane nozzle either way; the cone choice stays the same but the family changes.
What pressure do you have? Spiral nozzles sit in a low band roughly 0.7–3 bar; confirm the cone holds at your operating pressure, because pattern shape shifts as pressure moves: a full cone hollows out above rating, a hollow cone fills in below it.
How do the patterns overlap? Hollow cones need tighter, more deliberate spacing than full cones to avoid dry wedges between rings. Lay the grid out on paper before the manifold is welded.

Spacing Hollow Cones Without Dry Wedges

Hollow cones are less forgiving on layout than full cones. Two hollow cones that are spaced too far apart leave a dry wedge between their rings; the gas slips through that wedge untreated. Lay them on a spacing where the rings overlap by a margin, and stagger rows so a gap in one row is covered by the next.

A quick geometry check saves most of the losses. A hollow cone at angle θ spreads to a ring of diameter roughly 2·d·tan(θ/2) at distance d: the same coverage formula as a full cone, but the liquid is in the ring, not the disc. Two nozzles spaced on a pitch greater than the ring diameter leave the center of the pitch uncovered. Rule of thumb for a first layout: pitch at 60–75% of the ring diameter, stagger alternate rows, and sketch the overlap before you order. The spiral nozzle product range lists angle and flow per cone type so you can lay out overlap instead of guessing, and a quick coverage sketch before install stops most of these losses.

When the duty is a focused wash on a wall or a screen rather than area coverage, a spiral jet variant trades the ring for a directed, higher-impact stream, still open-path, still clog-resistant, just aimed instead of fanned. The spiral jet guide covers when that trade is the right one.

When the Pattern Has Already Drifted

If your tower is already underperforming, do not assume the cone was wrong from day one. It may have drifted. Clogging narrows any nozzle toward a weaker, distorted pattern, and a hollow cone that is partially blocked stops presenting its ring and starts dribbling. Pull one nozzle, check free passage against your water solids, and compare the actual pattern to the spec. Often the fix is restoring the open path, not re-coning the whole tower.

The fastest field test is to pull a nozzle and run it into a bucket at operating pressure: a healthy hollow cone throws a clean ring, a clogged one throws a lopsided stream. Match what you see to the spec sheet before you order a different cone. And if the pattern is right but the tower still underperforms, the problem is gas distribution, packing, or liquid loading. The cone was never the issue.

Frequently Asked Questions

How do I know if my tower needs full cone or hollow cone? Name the limiting step: wetting or cooling a target is full cone; gas-to-liquid mass transfer is hollow cone. If the gas flows in the annulus and the center is dead space, hollow cone is usually right.

Does a spiral spray nozzle make both patterns? Yes. The number of turns on the ramp and the body geometry set the pattern: single-turn designs tend hollow cone, multi-turn designs tend full cone. Specify the cone explicitly; do not assume from the word “spiral.”

Why is my hollow cone not forming a clean ring? Pressure below the minimum pattern pressure, partial clogging, or a body whose rating does not match the header. Test at operating pressure into a bucket and compare to spec.

Which cone gives better absorption? Hollow cone. It spins the liquid harder, producing finer, faster droplets with more surface area per liter, concentrated where the gas actually flows.

Can I switch a tower from full to hollow cone without changing the manifold? Usually yes if the bodies share the same connection, but the flow, angle and pressure rating all change. Re-check the layout for dry wedges: hollow cones need tighter spacing than the full cones they replace.

What pressure do spiral spray nozzles need for a stable cone? Roughly 0.7–3 bar, with the minimum pattern pressure depending on the body. Confirm the cone at your operating pressure. Pattern shape shifts as pressure moves.

Why is my full cone spraying like a stream? Pressure has dropped below the minimum pattern pressure, or the body is clogged. Check the header pressure at the manifold and inspect the nozzle.

Which cone should a dirty-water scrubber use? The same answer as a clean-water scrubber, the cone follows the duty, but the spiral family wins on both cones because the open ramp resists the clogging a vane chamber would suffer.

Selection Checklist

Limiting step named before the cone is chosen
Gas flow distribution confirmed, not assumed
Cone matched to duty (full: wet/cool; hollow: contact)
Pattern confirmed at actual operating pressure
Hollow-cone pitch at 60–75% of ring diameter, rows staggered
Free passage checked against water solids
Bucket test scheduled at commissioning
Cone type recorded on the BOM

For a new tower or a retrofit, send the gas flow, the duty (absorb, quench or cool) and your operating pressure. The engineering desk will set the cone type and the overlap spacing so the spray does the work instead of the sump. For the broader spray-pattern picture, when a flat fan beats a cone, and when a jet beats both, the spray patterns guide covers the full family, and the spiral nozzle overview explains the clog-resistance that makes these bodies the dirty-water default.