A flat fan header looks simple: a pipe, a row of nozzles, liquid in, spray out. The simplicity hides the one number that decides whether the belt comes out evenly wetted or with dry stripes every few inches. That number is the spacing between nozzles, and it has to be calculated from the pattern — not copied from last year’s drawing.
This guide is for the engineer laying out a new wash, rinse or coating header, or fixing one that never covered right. We will treat flat spray nozzles as the working example, but the method applies to any sheet-forming nozzle in the family.
Start With the Standoff, Because It Is Usually Fixed
The first input is not the nozzle — it is the distance from the header to the belt or part. In most machines that distance is dictated by the enclosure, the roller height or the clearance the product needs. You rarely get to choose it; you have to design around it.
Why it comes first: coverage width is a function of standoff and spray angle. Until you know the standoff, you cannot pick an angle, and until you pick an angle you cannot say how many nozzles you need. Pin the distance before anything else.
Coverage Width From Angle and Distance
The width a flat fan covers on a flat surface is:
width = 2 × standoff × tan(angle ÷ 2)
At a 300 mm standoff the standard angles land at roughly 80 mm (15°), 130 mm (25°), 220 mm (40°), 380 mm (65°), 500 mm (80°), 860 mm (110°) and 1900 mm (145°). Move the header to 500 mm and every one of those numbers grows in proportion; pull it to 200 mm and they all shrink.
This is why a “65° nozzle” is not a fixed-width device. The same tip covers 380 mm at 300 mm standoff and about 640 mm at 500 mm. When a layout fails, nine times out of ten someone quoted the 300 mm number while running the header at a different distance.
The Overlap Rule That Removes the Seams
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 to place them at 40–50% of the pattern width. So 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 one, and the row reads as a single continuous band.
If you are using even-edge fans instead — appropriate when each nozzle serves its own zone with no neighbour to blend into — you do not overlap at all; you butt them edge to edge and accept a defined boundary. Mixing the two on one header is the classic source of banding, so decide the edge type up front and keep the whole row consistent.
Fan nozzles and flat fan nozzle spray variants follow the same spacing math; the edge profile and angle are what change the answer, not the name.
Counting Nozzles and Building the Flow Budget
With spacing decided, the nozzle count is just the belt width divided by the spacing, rounded up. A 1200 mm belt at 250 mm spacing needs five nozzles. Then the flow budget falls out: if each tip flows 4 L/min, the header draws 20 L/min. Multiply by the number of headers and you have the pump duty.
This is where layouts quietly fail in the field. A designer picks nozzles for the spray pattern and forgets to check that the existing pump can supply the total. Under-pressure the header and every nozzle in the row drifts off its rated flow, the patterns shrink, and the seams reopen. Size the pump to the summed flow at the working pressure, not to a single nozzle.
Why a Cardboard Test Saves the Rebuild
Catalog widths are measured under ideal conditions: perpendicular aim, clean fluid, the quoted pressure. Your line is none of those. Before you drill the manifold or cut the pipe, build a throwaway rig — a length of rail, the actual nozzles, paper taped to a board at the real standoff — and fire it. Read the true width where the sheets land. Adjust spacing to what you see, not what the data sheet promised.
This ten-minute test catches three problems that otherwise surface after the header is welded in: aim that is a few degrees off perpendicular (which skews the effective width), pressure drop along a long manifold (end nozzles weaker than middle ones), and a pattern that is narrower than quoted because the fluid is more viscous than the test fluid the catalog used.
Matching Angle to the Job, Not the Habit
Narrow angles (15–25°) throw a thin, high-impact line — right for cutting, descaling or spot cleaning, wrong for covering a wide belt evenly. 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 a belt-coverage duty, 65–80° flat spray nozzles are the usual home. They spread the film across a useful width without wasting liquid past the edges. If your current header uses 40° tips and shows dry seams, the fix is often simply switching to a wider angle and re-spacing — not adding more nozzles at the same angle.
A Layout That Survives Commissioning
A header that covers the whole belt on day one still has to cover it in month six. Two things keep it honest: strainers sized to the smallest orifice, so one clogged tip does not break the row’s symmetry, and a spare-nozzle policy where every unit in a row is the same edge type and angle. When a tip is replaced with a different profile, the seam comes back.
A Worked Example: A 1500 mm Belt
Suppose a rinse belt is 1500 mm wide with a fixed 300 mm header standoff. Start from the coverage width each nozzle must deliver. An 80° flat spray nozzle covers about 500 mm at 300 mm, so spacing the row at roughly 45% of that width puts nozzles about 225 mm apart. The belt then needs seven tips (1500 ÷ 225, rounded up). At 4 L/min each, the header draws about 28 L/min.
Now push the same header to 500 mm standoff. The 80° tip now covers about 840 mm, spacing grows to roughly 380 mm, and the same belt needs only five nozzles for the same result. Same belt, same fluid, different standoff — and the nozzle count and pump duty both drop. This is why standoff, not the nozzle alone, sets the layout.
Fluid Properties Change the Real Width
Catalog coverage widths assume water at the rated pressure. A more viscous fluid — a coating, a syrup, a slurry — forms a thicker sheet that can break up differently and land narrower than quoted. Surface tension and any entrained air shift droplet size too. This is exactly why the cardboard test matters: fire the actual fluid, not water, before you fix the spacing. A layout proven on water can drift a stripe once the real fluid flows, and the seam reopens for no reason you can see on the data sheet.
Manifold Pressure Drop on a Long Header
On a long pipe the nozzles nearest the inlet see higher pressure than those at the far end, so their flow and width run larger. Over a meter-long header that gradient can be enough to reopen seams at the tail. Size the feed so pressure drop along the manifold stays small, or feed from both ends on long rows. Otherwise every nozzle in the row is technically the right part, but the row still covers unevenly because the supply is not equal at each tip.
If you are specifying a new line or reworking a streaky one, send us the belt width, standoff, fluid and target flow. We can sanity-check the angle, spacing and pump duty against the geometry above so the first build is the right one.