Spiral Jet Nozzle Guide: Structure, Flow and Selection

The pump is running, the pressure gauge looks normal, and the spray still does not do the job. This is the classic failure pattern in scrubbers, cooling towers and dust suppression systems: everything upstream checks out, but the nozzle was never the right one for the duty. The spiral jet nozzle is one of the most mis-specified nozzles in industry, because it looks simple and behaves differently from the conical nozzles most engineers learned on. It gives you high flow at low pressure, a huge free passage, and a cone that is easy to get wrong. The spiral nozzle spray pattern it throws is the same cone you size from the capacity curves below, whatever the catalogue calls the part. This guide covers how the spiral jet is built, what the numbers mean, and how to select it without guessing.

What a Spiral Jet Nozzle Actually Is

A spiral jet nozzle (often written as spiral nozzles when specifying a range, and on purchase orders as spiral jet spray nozzles) is a one-piece body with a helical groove cut into the core of the discharge end. Liquid enters the wide inlet, flows down the body, and is thrown outward by the spiral ramp. The ramp splits the stream into one or more concentric rings of droplets that fan out as a cone. There are no moving parts, no internal vanes, and no swirl chamber. The spiral groove is the entire mechanism.

The geometry is the product of a single machining operation. The groove is cut progressively deeper and wider toward the outlet, so the flow area opens up as the liquid travels. That open design is what separates the spiral jet from a conventional conical nozzle. A standard full-cone nozzle forms its pattern inside a swirl chamber with tangential slots. A spiral jet forms its pattern in the open, on the ramp itself.

Three physical facts follow directly from this construction:

• The narrowest cross-section of the whole flow path is the outlet. There is no internal restriction smaller than the exit.
• The spray angle is set by the groove pitch and the ramp angle, and it is fixed at manufacture. You cannot tune it in the field.
• The droplet cloud is wide and mixed in size. The spiral jet is a coverage tool, not a precision atomizer.

Materials matter as much as geometry. Standard spiral jets are made of brass, 303 or 316 stainless steel, and plastics such as PP, PVDF and PTFE. For flue gas desulfurization (FGD) service, 316L or higher alloys are common because the slurry is acidic and abrasive. For washdown and dust suppression, PP is cheap and adequate. Material choice should follow the fluid, not the budget.

How the Spiral Groove Makes a Cone

The spiral ramp does two jobs at once. It accelerates the liquid tangentially, and it peels the stream into separate layers. The number of turns on the groove decides which cone you get.

A single-turn spiral throws most of the liquid to the outer edge of the pattern. The center of the spray stays thin, and the result is close to a hollow cone: a ring of droplets with a light or empty middle. A multi-turn spiral spreads the liquid more evenly across the radius, filling the center and producing a full cone.

The practical difference is large. A hollow cone concentrates the liquid in the annulus, which is where gas actually flows in a packed tower. A full cone wets the entire cross-section, which is what you want for quenching, surface cooling and knock-down of dust. Same family of nozzles, opposite spray distribution. So the first selection question is never “spiral or not”. It is “full cone or hollow cone”.

This is also why you cannot judge a spiral jet nozzle from a photo. Two units with the same thread size and the same rated flow can deliver different cone shapes because the groove pitch differs. If the pattern matters, confirm the cone type on the datasheet before you buy.

Orifice Size, Flow Rate and Spray Angle

Spiral jet nozzles are specified by three numbers: thread size, capacity, and spray angle. The thread size (1/8 inch up to 4 inches NPT or BSPT) sets the inlet. The capacity number describes the flow the nozzle passes at a reference pressure. The spray angle (60, 90, 120 or 170 degrees are the common options) sets the coverage width at a given height.

The table below gives typical values for a mid-range spiral jet range. These are representative figures from public industry data, not a brand datasheet. Always verify the exact curve for the specific model before you commit a design.

Thread size	Free passage (approx.)	Flow at 2 bar	Spray angle options	Typical service
1/8“	2.0 mm	4 to 8 L/min	60°, 90°	Rinsing, small washdown
1/4“	3.0 mm	8 to 20 L/min	60°, 90°, 120°	Cooling, dust control
3/8“	4.0 mm	15 to 40 L/min	90°, 120°	Scrubber banks, quenching
1/2“	5.0 mm	25 to 70 L/min	90°, 120°, 170°	Cooling towers, FGD scrubbers
1“	8.0 mm	80 to 200 L/min	120°, 170°	FGD absorber headers, big cooling duty
2“	12.0 mm	250 to 600 L/min	120°, 170°	Desulfurization towers, large ducts

Two rules of thumb follow from the table. First, flow scales with the square root of pressure. Double the pressure and you get roughly 1.41 times the flow, not twice. Second, spray angle and coverage interact with distance: a 120-degree cone covers about 3.5 times its height as diameter, so a nozzle 1 meter above a surface wets a circle about 3.5 meters across. A 60-degree cone covers about 1.2 times its height.

Angle choice is a coverage math problem, not a preference. For a given height, the 120 degree pattern spreads the liquid over more area, so the droplet density per square meter drops. If you need heavy local wetting, pick a narrower angle or more nozzles. If you need to blanket a large plane cheaply, wide angle wins.

Flow Capacity and the K-Factor

The capacity number printed on a spiral jet datasheet is a K-factor. The relationship is Q = K × √P, where Q is flow in liters per minute and P is pressure in bar. K is fixed at manufacture by the groove cross-section. Raising the pressure changes Q, not K.

The table below converts the earlier flow bands into K values at the 2 bar reference point. Ranges are approximate and typical of public industry data, not a brand curve.

Thread size	K-factor range (L/min at 1 bar)
1/8“	3 to 6
1/4“	6 to 14
3/8“	11 to 28
1/2“	18 to 50
1“	57 to 141
2“	177 to 424

Worked example: take a 1/2 inch spiral jet with K = 18. At 1 bar it passes 18 L/min. At 2.5 bar it passes 18 × √2.5 ≈ 28 L/min. That single number lets you check a pump curve in seconds: pick the K whose mid-band flow sits at the pump’s comfortable point.

K is a capacity number, never a pattern number. Two nozzles with the same K can produce different cones, because the number of groove turns decides the distribution. Always pair the K-factor with a stated cone type. For the full range structure, the spiral nozzles overview guide walks through how the family is organized.

High Flow at Low Pressure

The defining operating trait of a spiral spray nozzle is that it does a lot of work at low pressure. Typical operating bands sit around 0.7 to 3 bar. That is roughly 10 to 45 psi. In that window a spiral jet passes substantially more water than a conventional full-cone nozzle of the same thread size, because the open ramp does not choke the flow the way a swirl chamber does.

Why does this matter in practice? Because most industrial loops are volume-limited, not pressure-rich. A scrubber recirculation pump is sized for cubic meters per hour, and the piping is sized for the flow. When you replace a vane-type nozzle with a spiral jet, you usually get the same or better coverage at a lower discharge pressure, which cuts pump energy and reduces wear on the seal and impeller.

The low-pressure band also changes how you treat the system curve. A conventional nozzle operating at 5 to 7 bar leaves little margin on a pump that is already near its duty point. A spiral jet at 1 to 2 bar sits in the comfortable middle of most pump curves, so flow stays stable as the line fouls or the filter loads. The nozzle holds its pattern at pressures where a swirl-chamber nozzle would already be distorting.

There is a limit to this logic. Below about 0.5 bar the spiral ramp loses the energy it needs to peel the stream, and the pattern collapses into a dribble. Above about 4 bar you gain little extra coverage, and you accelerate wear on the groove edge. Stay inside the 0.7 to 3 bar band unless the supplier’s curve says otherwise.

The Clogging Trade-Off in Sewage and Dirty Water

The spiral jet is the standard answer for dirty water, and there is a real reason behind the reputation. Because the narrowest point in the flow path is the outlet itself, any solid that enters the body can pass through it. A piece of debris larger than the exit is rejected at the face, where it is visible and easy to clear. Nothing gets trapped inside, because there is no inside to trap it.

Compare that with a conventional conical nozzle in the same service. Its swirl chamber and tangential slots are smaller than the outlet, so solids lodge upstream of the orifice. The clog is invisible, the pattern collapses, and the failure is found only when the process numbers drift. This is the classic failure in sewage and recirculated-water service, and it is exactly the failure a spiral jet avoids by geometry.

Sewage and process water push the design to its limits in two ways. First, fibrous material: rags, hair, paper pulp. A fiber can wrap around the spiral ramp and gradually choke the groove even though it never blocks the outlet. For heavy fiber loads, ask for a larger free passage or a dedicated non-clog design. Second, scaling. In hard or alkaline water, scale deposits on the ramp and narrows the effective flow area over weeks. The open geometry slows this, but it does not stop it.

The honest trade-off is droplet quality. The spiral jet trades a tight, uniform droplet band for open flow. If your process needs a precise droplet size, such as a coating or a fogging step, a spiral jet is the wrong tool. If your process needs reliable coverage in water that carries solids, it is the right one. That is the selection fork most specifiers miss.

Free Passage Rules for Dirty-Water Service

The anti-clog argument comes down to one number: free passage, the narrowest point a solid must travel through. In a spiral jet the free passage equals the outlet, because the groove opens up toward discharge. In a vane-type full cone the passage sits inside the swirl chamber. Typical catalog designs run it at roughly 50 to 70 percent of the outlet size.

Internal geometry	Free passage vs outlet	Failure mode
Spiral open ramp	about 100 percent	Blocks at the face, visible and clearable
Vane full cone	about 50 to 70 percent	Lodges upstream of the orifice, invisible
Swirl chamber	about 40 to 60 percent	Internal jam, needs disassembly

The sizing rule follows from the geometry. As a working guideline, keep the largest solid at or below about 75 to 90 percent of the free passage. From the earlier table that means roughly 2 mm for a 1/8 inch unit and 12 mm for a 2 inch unit. Solids larger than the passage will not enter the body. They sit at the face where they are easy to see and clear.

Fiber is the exception to every passage rule. Rags and hair pass the opening easily, then wrap around the ramp. For fibrous loads, plan on periodic inspection rather than trusting the passage number.

Strainers change the equation. A strainer mesh opening should be about half to two-thirds of the smallest free passage in the system. Mesh that fine adds pressure drop, so size the pump with the strainer in the curve. Spiral jets let you lean further toward passing solids than any slot-based design.

Where Spiral Jet Nozzles Earn Their Keep

Four applications dominate spiral jet use, and each one stresses a different strength of the design.

Flue gas desulfurization (FGD). In a wet scrubber, spiral jets hang in banks across the absorber, spraying limestone slurry into the rising flue gas. The duty demands high flow, a wide cone, and resistance to abrasive, acidic slurry. Spiral jets are the workhorse here precisely because no other nozzle family combines that flow with that free passage. Material is the main selection battle: slurry loops call for 316L or better, and hard chrome or ceramic coatings extend life in abrasive service.

Cooling towers. Open recirculating circuits carry algae, scale and grit. A spiral jet at low pressure keeps the fill wetted with minimal pumping head, and its open path shrugs off the solids that a fine-orifice nozzle would accumulate. The wide 120 or 170 degree cone covers a large fill area from a single header point.

Dust suppression. Bulk handling, crushing and conveyor transfer points need heavy, low-pressure wetting that stays on during dirty conditions. Feed water is often settled process water, not a filtered supply. A spiral jet delivers the volume and survives the water. The common mistake here is undersizing: dust systems are usually upgraded from fine mist nozzles, and the fix is almost always more flow at lower pressure, which is the spiral jet’s home turf.

Firefighting and deluge. Fixed deluge systems use open nozzles because they must never clog and must flood a large area instantly. Spiral jets suit monitor and deluge duty where high flow at moderate pressure is the spec. The wide cone covers a large floor area from a single point, which reduces header count and piping cost.

Washdown and tank cleaning fall in the same family. For open-tank rinsing, floor washing and flume cleaning, the spiral jet’s coverage and clog resistance beat a fan nozzle in anything but perfectly clean water.

Spiral Jets in Flue Gas Desulfurization

Wet FGD absorbers are the largest single market for big spiral jets, and the service has published numbers worth knowing. Public limestone scrubbing references report a liquid-to-gas ratio of about 5 to 15 liters of slurry per cubic meter of flue gas. Tower spray density runs roughly 15 to 30 m³ per square meter of cross-section per hour. The slurry itself runs 10 to 20 percent solids at a pH of about 5 to 6, with a density near 1,100 kg/m³.

The nozzle duty follows from those figures. A 300 MW absorber circulates 10,000 m³ of slurry per hour or more. Spray banks therefore use large units: 1 and 2 inch spiral jets at 120 to 170 degrees, around 0.7 to 1.5 bar. The 12 mm free passage of a 2 inch unit matters more than any droplet statistic. Recycled slurry carries scale, grit and debris from the reaction tank.

Three material rules dominate FGD nozzle life. First, 316L is the entry grade for acidic slurry. Second, chloride-rich fuels push the spec to duplex or super-duplex grades. Third, hard chrome or ceramic coating extends life against abrasive scale. Coating thickness is limited by groove tolerance, so verify it does not close the passage.

Wear shows up in the flow number, not the pattern. As the groove edge erodes, K rises and the nozzle passes more slurry at the same pressure. A common replacement trigger is flow drift of about 15 to 20 percent above rated. Track flow per header, and you catch worn units before the absorption efficiency drifts. Which pattern the absorber needs is settled in our full-cone versus hollow-cone comparison.

How to Select a Spiral Jet Nozzle in Six Steps

Selection is a sequence, and the order protects you from the most common mistakes.

1. Fix the cone first. Hollow cone for gas-to-liquid contact in a tower. Full cone for quenching, cooling a surface, or knocking dust out of a volume.
2. Fix the angle from the geometry. Measure the height or distance to target, then pick the angle that covers the area you need without huge overlap. 120 degrees is the default for most duties.
3. Size the flow from the duty. Multiply the area by the required liquid density (liters per square meter per hour) and divide by the number of nozzles. Then read the capacity curve to pick the thread size that passes that flow in the 0.7 to 3 bar band.
4. Check the free passage against the water. Compare the nozzle’s free passage with the largest solid in your stream. If the solid is bigger than the passage, you need a larger size or a strainer.
5. Match the material to the fluid. Brass or PP for clean water, 316 stainless for corrosive or abrasive service, alloy or coated bodies for FGD slurry.
6. Verify the cone on the datasheet. Single-turn versus multi-turn changes the pattern. If the datasheet does not state full or hollow, ask.

That sequence kills the two most common spiral jet failures: buying a full cone when the tower needed a hollow cone, and buying a fine-orifice nozzle for water that carries solids.

Common Mistakes That Cost Money

Three errors come up again and again in the field.

Running spiral jets above their band. At 5 bar and up, the extra pressure adds little coverage and shortens life. The groove edge erodes, the angle widens, and the pattern becomes ragged. If you have pressure to spare, you have the wrong nozzle family, not a tuning problem.

Judging by thread size alone. Two nozzles with the same 1/2 inch thread can differ by a factor of three in flow. The thread is the inlet, not the capacity. Always select from the capacity curve, never from the pipe size.

Ignoring overlap in wide-angle layouts. A 170 degree cone is tempting because it covers so much, but the pattern thins at the edge. Nozzle spacing must overlap the edges or you get dry wedges between cones. For wide-angle spiral jets, space them so each cone overlaps the adjacent one by 15 to 20 percent of its radius.

Start With the Right Cone and the Right Curve

A spiral jet nozzle is a simple machine with a specific job: maximum flow, open passage, and a cone that covers. Get the structure right, size from the capacity curve, and stay in the low-pressure band, and it will run for years in water that kills other nozzles in weeks. Get the cone or the angle wrong, and no amount of pump pressure will fix it.

If you are not sure whether your duty calls for a full cone or a hollow cone, or you need help reading a capacity curve against your pump, send us the application details: fluid, solids loading, available pressure and target coverage. We will spec the spiral nozzles for your exact numbers, and confirm the free passage before it ships. Tell us the flow you need, and we will give you the nozzle that keeps spraying.

If you want to go deeper first, read our guide on spiral nozzles in dirty water to understand clogging, and our full-cone versus hollow-cone comparison to settle the pattern question before you spend money.

If you already know the duty, the liquid, the flow and the solids load, the BoreJet spiral range covers full-cone and hollow-cone spiral jets for scrubber and cooling duty. Send the application details and we will size the right spiral jet from your numbers.

Related reading: spiral nozzle selection for the range, spiral clogging in dirty water for wear, and full versus hollow spiral cones when the pattern is open.