Table of Contents
You changed every nozzle on the header, the line is back up, and two shifts later the same pale stripes are running down the panel. It is tempting to blame the supplier or assume the tips wore out already. In most plants the nozzles are fine. The streak is built into how the pattern was specified or laid out. This guide walks through the failure modes that survive a fresh set of nozzles, and how to pin down which one is yours.
A flat fan spray nozzle turns liquid into a thin, flat sheet. That sheet is the most common tool for coating, washing and rinsing a moving surface because it lays down an even film across a defined width. The catch is that “even” depends on how the sheet is shaped at its edges and how neighbouring sheets meet. Get either detail wrong and the defect shows up immediately, on brand-new tips.
The stripe pattern itself is diagnostic. Light stripes that repeat at exactly your nozzle pitch point to the geometry of the row; one off-colour stripe at a fixed position points to one bad unit; bands that drift with pump speed point to the pump, not the nozzles at all. Read the pattern before you touch the parts bin.
What the Pattern Looks Like Up Close
Hold a flat fan against a wall and you see a rectangle-ish band: dense in the middle, thinning toward the two ends. The shape of that thinning is the single most ignored property in nozzle selection.
There are two families. Tapered edge fans fade gradually to nothing at the ends. Even (or “full”) edge fans hold near-constant density across almost the whole width and then stop abruptly. Both are legitimate; they just belong in different layouts. This distinction matters far more than the brand stamped on the body, and it is the first thing to check when a line streaks.
The intensity profile is set by the internal slot and the edge treatment of the orifice, and it is stable: it does not widen when you raise pressure, and it does not flatten with age. It only changes with wear, which is why an edge-type problem is identical on the day the tips ship and two years later. That stability is what makes streaks reproducible, and reproducible defects are findable.
Flat fan spray nozzles, flat spray nozzles and the plural variants you will see in catalogs are all describing 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: The Usual Culprit
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. If instead you mount even-edge nozzles side by side, each one stops hard at its boundary. So between every pair you get a low-density seam, and directly under each nozzle a slightly heavier band. The result is exactly the repeating light-and-dark stripe operators call “banding.”
The reverse is also true. A single even-edge nozzle aimed at one spot, with no overlap to worry about, is perfectly happy. But mix the two types on one header, common when someone tops up a row with whatever was in the bin, and you have created a stripe that no amount of new nozzles will fix, because every new nozzle is also the wrong edge type.
If your streaks repeat at a regular spacing that matches your nozzle pitch, edge mismatch is the leading suspect.
How Overlap Builds a Uniform Film
The reason tapered edges work in a row is arithmetic. Where two sheets meet, the intensity at any point is the sum of the two profiles. A tapered sheet at 30% of its centre intensity meets a neighbour at roughly 70% of its own, and the pair adds up to a flat line. Overlap too little and the sum dips between nozzles, a light seam. Overlap too much and the sum peaks under every nozzle, a dark band. The flat film you are trying to coat with is not produced by any single nozzle; it is produced by the sum of the row.
That puts the practical spacing question in numbers. The pitch between nozzles is usually quoted as a fraction of the pattern width measured at the part:
| Pitch as % of pattern width | Typical result |
|---|---|
| Below 30% | Heavy double-dose bands; coating wasted in the overlap zone |
| 40–50% | Recommended starting point for tapered-edge fans |
| 55–65% | Visible light seams with most tapered profiles |
| Above 75% | Dry stripes between nozzles |
These are starting numbers, not law: the exact profile of your tips shifts the optimum, which is why the collection test described later beats any chart. But the chart explains why “I just spaced them evenly” fails. Even spacing is not the same as overlap spacing.
Coverage Width Is Set by Angle and Distance, Not Pressure
A second, quieter cause of uneven coating is simply aiming the wrong width at the part. Coverage width follows basic geometry: it is twice the standoff distance multiplied by the tangent of half the spray angle. At a 300 mm standoff the standard angles give you roughly:
| Spray angle | Coverage at 300 mm |
|---|---|
| 15° | 80 mm |
| 25° | 130 mm |
| 40° | 220 mm |
| 65° | 380 mm |
| 80° | 500 mm |
| 110° | 860 mm |
| 145° | 1900 mm |
These figures 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 more impact through the same angle. Operators who “open the pump” to fix a gap at the edge are spending energy on a problem the geometry will not solve.
For coating duties a 65° or 80° flat fan spray nozzle is usually the starting point, because 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 better suited to cutting or descaling than to laying an even coat.
Overlap Is a Planned Number, Not a Guess
Even with the right edge type, the spacing between nozzles has to be deliberate. The rule of thumb for tapered-edge fans is to space them at about 40–50% of the pattern width at the work surface, so each sheet overlaps its neighbour substantially. Too little overlap and you get a dry seam between nozzles; too much and you double-dose the overlap zone and waste fluid.
The number that matters is the pattern width measured at the actual part distance, not the catalog width quoted at 300 mm. If your line runs the header at 200 mm the real width is smaller, and your spacing has to shrink with it. Build a quick cardboard test rig, fire each nozzle at a sheet of paper taped to the part, and read the true width before you commit to a layout. A spray nozzle flat fan that looks right on the data sheet can be badly wrong two inches closer to the belt.
A Worked Spacing Calculation
Put the numbers together on a typical line. A coating header runs at 300 mm above a moving panel, fitted with 65° tapered-edge fans. From the table, each nozzle covers about 380 mm at that standoff. At a 45% spacing factor the pitch is roughly 170 mm, so a 2000 mm-wide line needs about twelve nozzles (2000 ÷ 170 + 1, rounded). If each tip runs 4 L/min at 3 bar, the header draws roughly 48 L/min. That total is your pump and feed-line figure, and it is worth writing down before you buy anything.
Now change one number and watch the defect appear. If the header is actually 400 mm off the panel, each 65° fan covers about 510 mm, and twelve nozzles on a 170 mm pitch drop to a 33% spacing factor. The overlap zone gets double-dosed and you get dark bands under every nozzle, on brand-new tips, with the “correct” nozzles. That is the classic case where the fix is spacing, not hardware. The same check applies in reverse: move the header closer and the same pitch that was fine now leaves a dry seam between nozzles. Re-verify the pattern width every time the standoff changes. A standoff change is a layout change, not a tune.
Reference Values for Coating-Grade Flat Fans
As a reference frame for the flows and angles quoted above, here is a representative flat fan line quoted at 3 bar:
| Model ref. | Spray angle | Flow @ 3 bar | Free passage | Edge type | Connection | Material |
|---|---|---|---|---|---|---|
| FF-15 | 15° | 1.2–4.0 L/min | 0.8 mm | Even | 1/8“–1/4“ | 303 / 316L |
| FF-25 | 25° | 1.6–8.0 L/min | 1.0 mm | Tapered | 1/8“–1/4“ | 303 / 316L |
| FF-40 | 40° | 2.4–16 L/min | 1.2 mm | Tapered | 1/4“–3/8“ | 316L |
| FF-65 | 65° | 4.0–30 L/min | 1.6 mm | Tapered | 1/4“–1/2“ | 316L |
| FF-80 | 80° | 6.0–45 L/min | 2.0 mm | Tapered | 3/8“–1/2“ | 316L / PP |
Values are representative ranges, not a binding spec for any single duty. Coating viscosity, solids and temperature all shift the real flow, so quote your duty rather than assuming the midpoint. The free-passage column matters on coating lines: pigmented and filled coatings carry solids, and a small passage plugs sooner than a large one at the same flow. If your coating has a heavy solids load, prefer the wider-passage rows even when a smaller orifice would quote “cleaner.”
Clogging Hides in One Nozzle
Worn tips are obvious; partial clogs are sneaky. A single nozzle in a ten-nozzle header running at 70% flow breaks the symmetry of the whole row. The stripe it causes often looks identical to an edge-mismatch stripe, which is why teams keep swapping tips and never fix it.
Flat spray nozzles resist clogging better when they have a larger free passage, but any small orifice will eventually pick up a chip or a dried chunk of coating. Two habits prevent most of this: strainers upstream of the smallest orifice, and a monthly pattern check where each nozzle is fired at a bare surface and compared to its neighbours. When one fan looks narrower or off-centre, cleaning or replacing that one unit restores the line, no full re-order required.
A partial clog has one signature that separates it from a layout defect: it moves. A geometry stripe is fixed to the header, identical on every shift. A clogged tip degrades gradually, looks different after each cleaning, and sometimes clears itself mid-run. If the stripe “walks” or changes width during the week, treat it as one bad unit, not a re-design.
The Header Feeds the Last Nozzle Last
A stripe that fades or strengthens toward one end of the line points past the nozzles to the header. End-fed bars lose pressure along their length, and the loss is quadratic in flow: at 48 L/min through a 12 mm header the water is moving at roughly 7 m/s and friction is high enough to starve the far nozzles. The same flow through a 25 mm header moves below 2 m/s and feeds the row evenly.
Two rules prevent most header-driven stripes. Keep the header velocity below about 2–3 m/s at full flow. That usually means a larger bar than instinct says. And feed long headers from the centre or from both ends, so the worst-case distance from feed to tip is halved. If the stripe pattern is “fine at the feed end, weak at the far end,” the header is the suspect, and the fix is plumbing, not nozzles.
When Pressure Helps and When It Doesn’t
Pressure is the lever people reach for first, so it is worth being precise about what it does. Flow through a fixed orifice scales with the square root of pressure. A nozzle rated 4 L/min at 3 bar delivers about 5.7 L/min at 6 bar, not 8. The full curve for that tip:
| Gauge pressure | Flow (rated 4 L/min @ 3 bar) |
|---|---|
| 1 bar | 2.3 L/min |
| 2 bar | 3.3 L/min |
| 3 bar | 4.0 L/min |
| 4 bar | 4.6 L/min |
| 5 bar | 5.2 L/min |
| 6 bar | 5.7 L/min |
Impact rises with flow, which is good for cleaning but irrelevant for coverage width. So: if your problem is “the coat is too thin,” more pressure helps, up to the point the pattern distorts or the pump complains. If your problem is “there is a gap at the edge,” pressure will not help. That is angle and distance. Fan spray nozzles and fan nozzles behave the same way; the physics does not care what you call them. The deeper flow-versus-pressure story is covered in the flow rate calculation guide.
Pulsation and Ghost Stripes
Some stripes never touch the nozzles. Diaphragm and piston pumps deliver flow in pulses, and a pulsing header lays down a film whose thickness oscillates along the direction of travel. The stripe spacing then equals line speed multiplied by the pump’s pulse period. Measure the spacing and divide by line speed, and you often get the pump’s stroke frequency exactly.
Ghost stripes have two signatures. They run across the direction of travel (perpendicular to nozzle rows) rather than along it, and they respond to pump speed, not nozzle changes. Fixes are hydraulic, not hydraulic-at-the-nozzle: fit a pulsation damper or accumulator close to the header, confirm check valves are seating, and avoid running the pump at a speed where resonance with the row spacing makes the banding worst. If the stripe moves when you change pump speed, stop swapping nozzles and look at the pump.
Fluid Properties Change the Pattern
The same nozzle produces a different pattern with a different fluid. Viscosity is the big one: a more viscous coating forms a narrower sheet with a thicker centre, because the liquid resists the sideways stretch that widens the fan. Temperature changes viscosity. A warm batch sprays wider, a cold batch sprays narrow and heavy. If your line streaks after a batch change or a season change, re-check the pattern at the real fluid temperature before changing any hardware.
Surface tension and solids play smaller but real roles. A wetting agent or surfactant that lowers surface tension can make the sheet unstable, breaking it into streams or tightening the edges. Settling solids collect in header low points and feed intermittent clogs to the last nozzles. The practical habit is simple: whenever the fluid changes, new batch, new supplier, new temperature setpoint, fire the row at a bare panel and compare. Most “the line suddenly streaks” reports turn out to be a fluid change that arrived quietly.
Measuring Uniformity on the Line
You can argue about profiles, or you can measure. The field method is a collection test: set the row at the real standoff, place a row of graduated cylinders or catch tubes across the coated width, run the line for a fixed time, and compare the volumes. A coating row in good shape holds every tube within roughly ±10% of the average. Tubes that fall outside that band sit exactly where your stripes are. The test turns the stripe pattern into numbers you can fix against.
For a quicker pass, the fired-pattern test works: tape paper or cardboard at the part distance, spray each nozzle for a few seconds, and compare the wet bands side by side. Narrow, off-centre or distorted bands identify the single unit. Neither test needs instruments beyond what a plant already has, and both beat re-ordering tips on a hunch. The collection method is also the only fair way to compare two edge types on your own fluid before you commit a row to one.
A Quick Checklist Before You Re-Order
Before you buy another box of nozzles, run through this on the line:
- Note the stripe spacing. Matches the nozzle pitch? → edge-type or overlap problem.
- Pull one nozzle and compare its fired pattern to a known-good unit. Narrow or off-centre? → partial clog.
- Measure the real standoff and compute the actual coverage width with the formula above. Gap at the edge? → wrong angle or spacing.
- Confirm every nozzle in the row is the same edge type. Mixed? → that is your stripe.
- Look for bands across the direction of travel, not along it. Present? → check pump pulsation.
- Is the stripe worse at one end of the header? → check feed point and header size.
- Changed fluid, batch or temperature recently? → re-test the pattern before touching hardware.
- Only then decide whether you need new tips at all.
Defect Reference Table
| Stripe signature | Likely cause | Fix |
|---|---|---|
| Light–dark bands repeating at nozzle pitch | Even-edge tips in an overlapping row, or mixed edge types | Standardize on tapered edge across the row |
| One narrow or off-centre band | Partial clog or wear in that single unit | Clean or replace that unit; check strainer |
| Film weakens toward one end of the header | End-fed header pressure loss | Centre-feed or step up header size |
| Bands across travel, spacing follows pump speed | Pump pulsation | Add damper/accumulator; check valves |
| Dark band directly under every nozzle | Pitch too tight for the actual pattern width | Increase pitch; re-measure at part distance |
| Streaks at line start after a pause | Dried coating in tips; air in the line | Purge line; clean tips; check strainer |
| Pattern narrows when batch or season changes | Viscosity shift with temperature | Re-verify pattern at real fluid temperature |
Frequently Asked Questions
We fitted brand-new tips and it still streaks. How? The streak is in the layout, not the tips. Edge-type mix, spacing at the wrong standoff, an end-fed header or one partially clogged unit all survive a fresh box of nozzles. Run the checklist before ordering anything.
Tapered edge or even edge for a coating row? Tapered for any row where patterns overlap. The sum of two tapered edges makes the flat film. Even edge is for single nozzles or non-overlapping passes. Mixing the two in one row is a guaranteed stripe.
What spacing should I start with? 40–50% of the pattern width measured at the part distance, then verify with a collection test. Below 30% you double-dose; above 65–75% you open a seam.
Will higher pressure fix a gap at the edge of the pattern? No. Pressure changes flow and impact, not angle. The gap is fixed by angle, standoff or spacing. See the coverage table.
Why is there a dark stripe under every nozzle? The spacing factor is too tight for the real pattern width, so the overlap zone gets more film than the centres. Increase pitch or raise the header (which widens the pattern), then re-measure.
How do I actually measure uniformity? Collection test: catch tubes across the width for a fixed time, compare volumes, target ±10% of the average. The tubes that miss the band are your stripes.
Do I need a strainer on a coating line? Yes, upstream of the smallest orifice. Pigmented and filled coatings carry solids, and one chip in one tip produces a stripe identical to a geometry defect.
When do tips actually wear out? Pattern distortion and a silent flow rise, typically 10–15% over the original rating, are the signals, not a calendar. The nozzle wear guide covers the flow-rise test you can run on the line.
For the common case, a fresh set of tips with the same stripes as the old set, the flat fan spray nozzle is working as specified and the row around it is not. 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 and overlap you have specified. For the broader geometry of fan coverage, the header layout guide and the spray pattern overview cover the same rules from the washing and cleaning side.
Next Step
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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.