Table of Contents
New spray-pattern options arrive every year, fine atomizing mists, variable hollow cones, air-assisted sheets, and yet the plain flat fan nozzles are still the default on most washing, rinsing and coating lines. Not because buyers are lazy. Because when the job is “cover this flat surface evenly,” a flat sheet is the geometry that matches the problem, and the alternatives each solve a different problem.
This guide is for the engineer or buyer who has to defend a nozzle choice, or who is staring at a header drawing and wondering whether to swap the flat fans for something newer. We will look at what “even coverage” really requires, why a flat sheet delivers it, and exactly where the alternatives beat it, so you can pick with reasons, not habit.
Throughout we will use flat fan nozzles as the working example, but the logic applies to any sheet-forming tip in the family.
What “Even Coverage” Actually Means on a Line
Even coverage is not “wet everywhere.” It is “wet to the same film thickness everywhere you care about, with no dry stripes and no double-dosed seams.” A conveyor belt wash, a panel rinse, an adhesive primer coat. All of them fail the same way when coverage is uneven: a pale streak, a thin patch, or a band that flakes later.
The surface in these jobs is flat and moving, and the target is a band of defined width. That is the key phrase: defined width. You are not trying to fill a tank or penetrate a packed bed. You are trying to paint a rectangle of liquid onto a moving plane. The nozzle that does that best is the one whose output is already a rectangle.
Quantitatively, even coverage means the coefficient of variation (CV) of the liquid application rate across the width stays low: the standard deviation of local deposition divided by the mean. A CV in the single digits reads as a visibly uniform film; double-digit CV is where operators start describing the line as “streaky.” That number is the scoreboard for every nozzle choice in this guide, and it is worth measuring before and after any pattern change, because “looks fine from the catwalk” is not a specification.
Why a Flat Sheet Beats a Cone for a Flat Surface
A flat fan nozzle turns liquid into a thin, flat sheet that opens into a roughly rectangular spray pattern. Fire it at a flat wall and you get a band: dense in the middle, thinning toward the two ends. That band is exactly the shape of the surface you are covering.
A hollow cone throws a round, ring-like pattern built for penetration, good for scrubbing a pipe interior or reaching into a cavity, useless for laying an even film across a belt. A full cone fills a roughly circular volume with droplets, which is what you want when you are cooling a chunk of metal or quenching, not when you are coating a sheet. A spiral nozzle does the same filling job with even more droplet density and reach. None of them produces the clean rectangular band a flat surface wants.
The efficiency gap is easy to state: a round pattern projected onto a rectangle wastes its corners. A cone that covers the same nominal width as a flat fan throws a large fraction of its liquid outside the working band, past the edges of the part, onto the machine frame, back into the tank. On a moving line that lost fraction is pure cost, and it never shows up on a deposition test that only measures the belt. The flat sheet keeps nearly all of its output inside the defined width, which is why it wins even-coverage jobs not by being newer but by being shaped like the answer.
The Edge Profile Is What Makes the Band Even
The most ignored property in nozzle selection is what the pattern does at its edges. Hold a flat fan against a wall and you see the band thinning toward the ends. How it thins decides whether a row of them reads as one continuous sheet or a set of overlapping stripes.
There are two families. Tapered-edge fans fade gradually to nothing at the ends. Even-edge (sometimes called “full-edge”) fans hold near-constant density across almost the whole width and then stop abruptly. Both are legitimate; they belong in different layouts.
When nozzles sit in a row and their patterns overlap, you want tapered edges. The thin end of one sheet lands in the thick middle of its neighbour, and the two blend into a continuous line. Mount even-edge fans side by side instead and each one stops hard at its boundary, leaving a low-density seam between every pair, the repeating banding stripe operators know well. This distinction matters far more than the brand on the body, and it is the first thing to check when a line will not cover evenly.
Why do the edges behave differently at all? The physics is the same in every sheet: the free edges of a liquid sheet contract under surface tension, thickening into a heavier rim. A tapered-edge tip manages that contraction so the deposited density falls smoothly; an even-edge tip is machined to compensate the contraction, holding density constant almost to the cut-off. That is why the two profiles are not interchangeable. The edge shape is built into the orifice, and no amount of pressure adjustment converts one into the other.
Flat fan spray nozzles, flat spray nozzles and the plural variants you see in catalogs all describe this same sheet-forming family. The words overlap in everyday use; what separates a good choice from a bad one is the edge profile and the spray angle, not the label.
Tapered Edge vs Even Edge: Side by Side
| Property | Tapered edge | Even edge |
|---|---|---|
| Density at the ends | Fades smoothly to near zero | Holds to the boundary, stops hard |
| Overlap in a row | 40–50% of pattern width | None: butt edge to edge |
| Failure mode | Seams reopen if spacing drifts | Banding if overlapped at all |
| Best duty | Multi-nozzle headers on moving belts | Single-nozzle zones, defined cut-off |
| Edge trimming | Outer nozzles often angled inward | Not needed: clean boundary |
| Typical jobs | Wash rows, rinse curtains, coating headers | Spot treatment, marking, zoned cleaning |
The decision rule is short: a row of nozzles that must read as one band takes tapered edges; a single nozzle that must own its strip takes an even edge. The most expensive mistake in the family is mixing them on the same header. One even-edge tip inside a tapered row leaves a hard seam at its position for the life of the line.
How Wide a Flat Fan Actually Covers
Coverage width follows basic geometry. On a flat surface it is:
width = 2 × standoff × tan(angle ÷ 2)
where standoff is the distance from the nozzle to the surface. At a 300 mm standoff the common angles land at roughly:
| Spray angle | Coverage at 300 mm | Typical duty |
|---|---|---|
| 15° | 80 mm | Cutting, descaling, deburring |
| 25° | 130 mm | Spot washing, targeted rinsing |
| 40° | 220 mm | General line washing |
| 65° | 380 mm | Coating, general coverage |
| 80° | 500 mm | Wide belt coverage |
| 110° | 860 mm | Low-pressure wide rinsing |
| 145° | 1900 mm | Fogging, dust knockdown |
These assume the nozzle fires straight at a flat surface. The practical point is that widening the angle is the only way to cover more width from a fixed distance. Raising pressure does not widen the sheet: it only pushes more liquid and impact through the same angle. A wide angle spray nozzle or a wide spray nozzle earns its place when the standoff is fixed and you need to span a broad belt without adding nozzles.
Standoff moves every width proportionally, and that is worth a second table because it is the source of most “it used to cover” complaints:
| Standoff with a 65° fan | Coverage width |
|---|---|
| 200 mm | ~255 mm |
| 300 mm | ~380 mm |
| 400 mm | ~510 mm |
| 500 mm | ~640 mm |
For coating and rinsing duties a 65° or 80° flat fan nozzle is usually the starting point: it spreads the film wide without throwing liquid past the part. Narrow 15–25° fans concentrate energy into a thin, high-impact line that is right for cutting or descaling, wrong for an even coat.
Flat Fan vs the Alternatives
Here is the honest comparison buyers actually need:
- Hollow cone: round, hollow ring; built for impact and penetration into cavities. Wins when you must reach into a void, loses when you need a uniform band.
- Full cone: round, filled volume of droplets; wins for cooling solids, dust conditioning in a bin, or quenching. Loses on a flat moving surface where you want a sheet, not a splat.
- Spiral: high droplet density, long reach, excellent clog resistance; wins for gas scrubbing and heavy debris washing. Loses on precision film thickness because the pattern is not a clean band.
- Air-atomizing: produces a fine mist for humidification or very light coating; wins where droplet size matters more than area. Loses on simple even-coverage because it is more complex, more expensive and harder to keep uniform.
- Flat fan: the rectangular sheet; wins whenever the target is a defined-width flat band. That is most wash, rinse and coat lines.
| Pattern | Shape on a flat target | Wins when | Loses when |
|---|---|---|---|
| Flat fan | Rectangular band | Even film on a moving belt | Cavities, penetration duties |
| Hollow cone | Round ring | Pipe interiors, impact | Even coverage on flat surfaces |
| Full cone | Round filled disc | Cooling solids, quenching | Precision film thickness |
| Spiral | Round filled disc, denser | Scrubbing, heavy debris | Clean band geometry |
| Air-atomizing | Fine mist, wide soft plume | Droplet-size-sensitive coats | Simple uniform wetting |
A flat jet spray nozzle and a spray nozzle flat fan are just the same sheet-forming idea under different catalog names. Pick the family first, then the angle and edge.
How Evenness Is Measured
Because “even” is a number, it is worth knowing how the number is produced. In the lab, a flat fan is fired into a patternator, a trough divided into narrow channels, each draining to its own measuring tube. The collected volumes across the channels plot the distribution curve: a bell shape for a tapered fan, a flat-top shape for an even-edge fan. The flatter the top and the more symmetric the fall-off, the better the tip.
In the plant, the same measurement is done with a row of identical collection tubes across the belt, or with a strip of absorbent paper under a stationary header. Run a timed interval, compare the volumes, and you have the CV. Two things are worth asking any supplier: what the distribution looks like across the band (a good tapered fan holds within a few percent of its mean across the middle of its width), and at what pressure the quoted distribution was measured, because distribution curves flatten or distort as pressure moves away from the rating.
A tip that reads perfectly on a patternator can still cover badly on a line, and the gap between the two is exactly the overlap and spacing discipline covered below. Evenness is a system property; the nozzle is the best-behaved component in it.
The Trade-Offs You Accept When You Pick Flat Fan
Flat fans are not perfect, and pretending otherwise gets lines into trouble. The trade-offs are real:
- Narrow operating window on height. Move the header and the covered width changes proportionally. A layout proven at 300 mm standoff drifts a stripe at 200 mm.
- Pressure-sensitive flow. Flow through a fixed orifice scales with the square root of pressure, so a pressure swing changes delivered volume and can distort the pattern.
- Clog sensitivity. Small orifices pick up chips or dried coating; one partially clogged tip breaks the symmetry of the whole row.
- Overlap discipline. A flat fan only looks even when the row is spaced correctly. Bad spacing reopens seams no matter how good the tip.
- Wear changes the edge. Erosion opens the orifice, widens the fan and flattens the edge profile; an old row of flat fans covers differently than a new one, which is why spare rows should be swapped as sets.
None of these is a reason to avoid flat fans. They are reasons to specify them deliberately instead of by habit.
Getting the Overlap Right
Once you know the real coverage width at your standoff, you space the nozzles so their patterns overlap. For tapered-edge fans the common starting point is 40–50% of the pattern width: a nozzle covering 500 mm gets spaced about 200–250 mm from its neighbour. The thin end of each sheet then lands well inside the thick middle of the next, and the row reads as one band.
Below roughly 30% overlap the seams reappear as pale stripes; above roughly 60% the middle of the belt is double-dosed and liquid is wasted. The tolerance band is why 40–50% is the standard home. For even-edge fans the rule inverts: no overlap at all, butt the edges, and keep each zone’s boundary clean. The full spacing procedure, including a worked layout for a 1500 mm belt, is in our guide on flat fan header layout and even coverage.
Fan spray nozzles and fan nozzles follow the same spacing math. Mixing edge types on one header, common when someone tops up a row with whatever was in the bin, is the classic source of banding, so decide the edge type up front and keep the whole row consistent.
Pressure: What It Changes and What It Doesn’t
Pressure is the lever people reach for first, so be precise about it. Flow through a fixed orifice scales with the square root of pressure:
Q ∝ √P
| Pressure change | Flow change | Effect on pattern |
|---|---|---|
| 2.0 bar → 3.0 bar | +22% | Fan opens slightly, droplets finer |
| 3.0 bar → 4.0 bar | +15% | Fan opens slightly, droplets finer |
| 3.0 bar → 6.0 bar | +41% | Fan opens, edges flare, finer spray |
A tip rated 4 L/min at 3 bar delivers about 5.7 L/min at 6 bar, not 8. Impact rises with it, which helps cleaning but does nothing for coverage width. If your problem is a gap at the edge, opening the pump will not close it. That is angle and distance. If your problem is a coat that is too thin, more pressure helps, up to the point the pattern distorts or the pump complains.
When Flat Fan Is the Wrong Call
Be clear about the cases where you should reach for something else:
- Target is a cavity, pipe interior or packed bed → hollow cone or spiral.
- Target is a solid object you cool or quench → full cone.
- Target needs a very fine mist and droplet size is the spec → air-atomizing.
- Target is a broad flat band and you must cover it uniformly → flat fan.
A flat fan air nozzle is a different animal again: it uses flat-jet air for blow-off and drying, not liquid coverage, and it belongs in the drying zone rather than the wash zone.
Picking the Angle for the Job
Narrow angles (15–25°) throw a thin, high-impact line, right for cutting, descaling or spot cleaning, wrong for even coverage. Wide-angle fans (110–145°) lay a broad, gentle curtain that is perfect for low-pressure rinsing and dust knockdown but useless where you need to knock something off the surface. For the even-coverage job, 65–80° is the usual home.
One angle pitfall deserves its own line: the angle is only as real as the pressure behind it. A 65° tip run 30% below its rated pressure throws a noticeably narrower fan, and the row spacing that was right at 3 bar quietly reopens seams at 2 bar. If the line runs below rating, treat the actual width as the design width.
An atlantic directional fan nozzle and similar directional variants simply aim the sheet in a fixed direction for tight spaces; the even-coverage logic is unchanged.
A Worked Example: Coating a 2000 mm Panel
Put the pieces together for a concrete case. A coating line runs 2000 mm panels at 8 m/min on a conveyor, and the process calls for a uniform primer film at roughly 2 L/m² of applied liquid.
Step one: angle and spacing. With a fixed 300 mm standoff, a 65° flat fan nozzle covers about 380 mm. Spacing the row at 45% overlap means about 210 mm between nozzles: 2000 ÷ 210 ≈ 10 nozzles, rounded up to eleven to keep the edges inside the panel. The outer two tips sit at half-spacing from the panel edges, which tucks the tapered ends of the row inside the working width.
Step two: flow budget. Eleven tips at 3 L/min each draw about 33 L/min from the header. The applied area per minute is 2 m × 8 m = 16 m²/min, so the applied rate works out to 33 ÷ 16 ≈ 2.1 L/m², right on the process target. If the panels were 3000 mm wide, the same standoff and angle would need roughly fifteen tips and about 45 L/min; the angle, standoff and overlap discipline stay identical.
Step three: verification. Before committing the line, run the row over a strip of absorbent paper and compare the wet band to the panel width. The CV across the band should land in the single digits with tapered edges at 45% overlap; if it does not, the first suspect is pressure at the header, not the tips.
A Quick Selection Shortcut
Before you re-spec a header, run this down:
- Is the target a defined-width flat band? → flat fan family.
- What is the real standoff, and how wide must one nozzle cover? → pick the angle from the width formula.
- Will nozzles overlap in a row? → tapered edge, spaced at 40–50% of width.
- Single zone, no neighbour? → even edge, butt edge to edge.
- Is the fluid viscous or dirty? → upsize free passage and strain ahead of the smallest orifice.
- Only then choose the tip.
Frequently Asked Questions
Why do flat fan nozzles give more even coverage than cone nozzles? Because the flat sheet is shaped like the target: a defined-width band on a flat surface. A cone projects a round pattern onto that rectangle and wastes the corners; the flat fan keeps its output inside the band.
What is the difference between tapered-edge and even-edge flat fans? Tapered-edge fans fade to nothing at the ends and are made to overlap in rows. Even-edge fans hold density to the boundary and stop hard, for single-nozzle zones. Mixing them on one header causes banding.
How much overlap should flat fan nozzles have? For tapered-edge fans in a row, 40–50% of the real pattern width at your standoff. Below about 30% the seams reappear; above about 60% you double-dose the middle. Even-edge fans take no overlap.
Does higher pressure make the coverage more even? No. Pressure changes flow and droplet size, not the band geometry. Edge gaps are fixed with angle, standoff and spacing. A pump change will not close a seam.
How do I check whether my line is covering evenly? Collect the liquid across the belt in identical tubes for a timed run and compare volumes. The coefficient of variation should sit in the single digits for a properly spaced tapered row.
Why does my flat fan line band even though the nozzles are new? Check edge profile consistency across the row, then overlap, then pressure at the header. A row that mixes edge types, or spaces by habit instead of by width, bands with brand-new tips.
When should I NOT use flat fan nozzles? When the target is a cavity, a packed bed, or a solid to be cooled or quenched, those want hollow cone, spiral or full cone. Flat fans own the flat, defined-width band and little else.
Most “the new pattern is better” swaps end up reinstalling flat fans, because the flat sheet is still the shape that matches a flat surface. If you want a second set of eyes on your header geometry, send us your duty conditions. Line width, standoff, fluid and target flow are enough for us to sanity-check the angle, edge type and overlap you have specified, and to point you to the right flat fan nozzles for the job.
Next Step
Send the Duty. Get Sized Nozzles Back.
Send your flow, pressure, fluid and target coverage. We come back with nozzle options and figures, not a catalogue number.
Written by
Ray ChanIndustrial spray nozzle specialist. I size tank cleaning, atomizing, flat-fan and spiral nozzles against real duty conditions, flow, pressure, fluid and target, rather than catalogue numbers. Every guide here comes from actual sizing work.