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, and how to stop picking the wrong one.
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.
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.
- Cooling a solid or a bed — any duty where the target occupies the center and you need it wetted evenly.
- Dust knocking-down in a filled chamber — where droplets need to intersect particles across the whole area, not just at the periphery.
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.
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.
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.
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.
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.
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.
- Where does the gas actually flow? If the center is dead space, a hollow cone is usually right.
- What is the water quality? Dirty recirculated water favors the open path of a spiral design over a vane nozzle either way.
- 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.
- How do the patterns overlap? Hollow cones need tighter, more deliberate spacing than full cones to avoid dry wedges between rings.
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. 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.
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.
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.