Process Technology · Wet Scrubbing
Low pressure drop by design — and where the saving goes when the duty gets harder.
An open spray scrubber is a hollow column: spray levels, a mist eliminator, a sump, and nothing in between. Removing the packing removes the component that fouls, and it gives the lowest gas-side pressure drop of any wet scrubber — 125 to 750 Pa against 2,490 to 19,920 Pa for a venturi scrubber. What that saving is worth, and what it costs elsewhere, is the subject of this article.
01 The column with nothing in it
In an open spray scrubber the gas enters the column containing neither packing nor trays. An array of nozzles atomises the absorption liquid into a dense droplet field; as the gas rises, soluble components transfer into the liquid phase, and particulate matter is captured on the droplet surfaces. The treated gas passes a mist eliminator and leaves at the top, while the spent liquid drains to the sump for recirculation or blowdown.
Everything that distinguishes the design follows from that empty volume. There is no bed to blind, no distributor to foul, no packing to replace. Gas-side pressure drop is limited to the column, the absorption stage and the mist eliminator, which is why the open scrubber sits at the bottom of every published pressure-drop table for wet gas scrubbers.
The corresponding dependency is equally direct: with no internals to distribute the liquid, mass transfer rests entirely on the spray. Droplet size distribution, spray pattern, droplet velocity, liquid flow rate and the position of each nozzle in the absorption stage together determine how complete the gas–liquid contact is.
02 What the pressure drop is worth
The EPA Air Pollution Control Cost Manual puts a counter-current spray column at 0.5 to 3 inches of water column, or 125 to 750 Pa, for the complete scrubber. A venturi in the same duty runs from 2,490 to 19,920 Pa, and at extreme throat velocities to 37,350 Pa. A random-packed bed is quoted per metre of packing, at 409 to 817 Pa, so a working column of two to four metres lands far above the open column — the EPA's own worked example for an acid gas absorber totals 2,129 Pa over 3.14 m of bed.
One qualification belongs here rather than in a footnote. The 125 to 750 Pa excludes the mist eliminator, which adds, depending on the type of droplet separator, a further 125 to 350 Pa and is not optional: at the gas velocities that make an open column compact, droplet entrainment has to be avoided. European sources also quote higher figures — VDI 3679 Part 2 gives 600 to 1,000 Pa, and the Dutch IPLO fact sheet 2,000 to 5,000 Pa, almost certainly for a complete installation including ductwork. The ranking between technologies is unaffected; the absolute number depends on where the system boundary is drawn.
Pressure drop only becomes meaningful once it is converted into power. Fan shaft power follows P = Q × Δp / η, with the EPA quoting combined fan and motor efficiencies of 0.4 to 0.7. At η = 0.65, every 1,000 m³ of gas costs 0.32 kWh at 750 Pa and 4.27 kWh at 10,000 Pa.
For a plant treating 50,000 m³/h over 8,000 operating hours, the difference between an open column at 750 Pa and a venturi at 10,000 Pa is roughly 1.6 GWh of fan energy per year — on the order of a quarter of a million euros at current Dutch industrial tariffs. That is the implication for the empty column, stated in the only unit that settles arguments.
03 Where the saving goes when the duty gets harder
A low pressure drop is not the same thing as low energy consumption, and any honest comparison has to follow the second half of the balance. With no packing to hold liquid, the open column creates its contact area entirely from droplets, which means more absorption liquid per unit of gas. The EPA gives 0.07 to 2.7 l/m³ for particulate matter duty, notes that fine particulate requires more than 2.7 l/m³ and lists 5.3 to 13.4 l/m³ for limestone flue-gas desulphurisation. As a comparison a packed column performs acid gas absorption within 0.27 to 2.7 l/m³.
The absorption liquid has to be pumped and atomised, and the same EPA method applies P = Q × Δp / η. Placing fan and pump energy side by side gives the picture that matters for electrical operating cost.
For straightforward particulate duty, the open spray column consumes 0.06–0.55 kWh per 1,000 m³, against approximately 1.05 kWh for the packed column in the EPA's worked example and 1.07–8.5 kWh for a venturi scrubber. Push the same column towards fine particulate or chemical absorption, however, and the liquid-to-gas ratio rises until the pump accounts for two thirds of total power draw. The efficiency advantage does not disappear — overall energy consumption remains below that of the alternatives — but the margin narrows and shifts.
This is a design variable rather than a defect. The engineering question is not how to eliminate the energy but where to spend it: in a fan working against packing, in a fan working against a venturi throat, or in a pump feeding a droplet field that can be sized, positioned and staged for the duty at hand.
04 The particle size where it stops
Particle capture in the open spray tower proceeds by inertial impaction on falling droplets. As particle size decreases, particles increasingly follow gas streamlines around the droplet surface, rendering impaction ineffective. Published collection efficiency data reflects this mechanism directly.
For particles above 5 µm, mass collection efficiency exceeds 90%; in the 3–5 µm range, efficiency falls to 60–80%; below 3 µm, collection drops below 50%. In unfavourable service, a simple spray column achieves 40–70% overall efficiency. The cut diameter lies in the range 2–8 µm. For the majority of industrial dust applications, and for all soluble gas duties, this performance is entirely adequate; for submicron particulate, it is not — and increasing the liquid-to-gas ratio does not alter this fundamental constraint.
Framed as an engineering trade-off rather than a limitation, the comparison is unambiguous: energy expenditure purchases collection fineness. A venturi scrubber converts fan power into high relative velocity at the throat, achieving a cut diameter of approximately 0.2 µm; the open spray tower operates at a fraction of that specific energy input and collects particles an order of magnitude coarser. Neither configuration is inherently superior. The particle size distribution of the duty stream determines the appropriate technology.
05 Fouling: what the empty column actually buys
The strongest practical argument for the open spray tower is not collection efficiency but operational availability. A randomly packed bed carries a documented inlet dust loading limit — the EPA cites 450 mg/m³, while European references apply considerably stricter criteria — and the packing itself is a consumable, with service lives of one to five years in aggressive duty and liquid redistributors required at intervals of 2.4–6.1 m of bed depth. Where the process gas carries sticky, crystallising, or scaling components, bed fouling is the principal cause of unplanned plant shutdowns.
An open column eliminates these constraints, but fouling is not avoided — it is redistributed to components that remain accessible during operation.
- Spray nozzles require clean recirculation liquid of appropriate quality. Blockage and abrasive wear at high solids loading are well-documented failure modes; a blocked nozzle reduces collection efficiency immediately and proportionally.
- The mist eliminator is subject to fouling and requires periodic washing. Wire mesh demisters blind rapidly in dust-laden service, which is why chevron-type designs are standard in this application.
- Blowdown maintains suspended solids within approximately 20–30% by weight and generates a waste stream that typically requires neutralisation prior to disposal.
The distinction is categorical rather than one of degree. Repacking a bed requires a planned shutdown; cleaning a nozzle lance or washing a chevron pack is routine maintenance. For any plant in which operational availability carries greater economic weight than fan power, this distinction is generally sufficient to determine technology selection.
06 Designing to the duty, not to a catalogue
Because the column internals are absent, virtually every performance parameter is established at the design stage rather than specified through component selection. This is the practical meaning of a custom-engineered open spray tower — and it is also where its limitations must be clearly understood. The height of a transfer unit for a spray column ranges from 1.5 to 6.0 m, against 0.3 m for a packed column under comparable conditions: ten to twenty times the bed height for equivalent separation. The EPA states this directly: of the absorber configurations it evaluates, the spray column exhibits the lowest mass transfer capability per unit height.
The design consequences follow directly:
- Height and staging are set by the actual number of transfer units required, not by a standard model. Demanding absorption duty is met with additional spray levels or a second stage, not with a taller empty vessel.
- Spray nozzle selection governs droplet size and spray pattern, and droplet size governs both impaction efficiency and the surface area available for absorption. Finer droplets improve contact and increase carry-over; the mist eliminator has to be designed for the spray that was chosen.
- Liquid-to-gas ratio is a cost decision, visible directly in the pump bar of the second figure.
- Materials and gas velocity follow from the chemistry and the load profile rather than from a catalogue envelope.
The open spray tower is not, in other words, a lower-cost alternative to the packed column. It is a distinct piece of equipment, appropriate where the duty involves soluble gases, coarse to medium particulate, or a gas stream that would foul any device with internal structure — and one that must be sized rigorously against the number of transfer units the process genuinely requires.
Sources. Pressure drops, liquid-to-gas ratios, cut diameters and removal efficiencies from the US EPA Air Pollution Control Cost Manual (Section 5.2 Chapter 1 and Section 6 Chapters 1 and 2) and the EPA CATC fact sheets. Height per transfer unit from Flagiello et al. (2018). Cost indications from the EPA fact sheets (2002 price level) and IPLO/InfoMil (approximately 2021 price level); the two sets are not interchangeable. Energy figures are calculated from the EPA fan and pump equations at a combined efficiency of 0.65, with a spray pressure of 2 to 3 bar assumed.