Table of Contents
A forming fabric that plugs in the middle and streaks at the edges is usually not a nozzle defect. It is a hydraulic sizing fault. The header runs at the wrong pressure, the orifices are worn or undersized, the fans overlap badly, or the oscillator has stalled. Each of those faults shows up as the same symptom: poor cleaning and rising water use.
On a wet end those symptoms stack quickly. Sheet-side felt moisture climbs, the trim edge stays dirty, broke rises, and fabric life falls from months to weeks. This guide treats the wet-end shower as one hydraulic system built from pressure, orifice, angle, standoff and oscillation. Sizing and placement decisions explain most of what operators see.
The Snapshot
- 3 to 4 bar (44 to 58 psi) is a commonly published band for low-pressure forming fabric cleaning showers on modern paper machines.
- 30 to 50 percent is the target overlap between adjacent fan patterns, enough to remove streaks without wasting water.
- Doubling header pressure raises orifice flow by about 41 percent, because flow follows Q = K times the square root of P.
- A 10 percent flow rise above the datum is the usual published trigger to replace worn orifices, long before the wear is visible.
What Each Shower Position Has to Do
Every wet-end shower has one job, and the job sets its pressure, pattern and placement. Confusing the jobs is the most common source of poor performance. A forming fabric cleaning system is not a felt conditioning shower, and neither one is a trim shower.
Forming fabric and wire cleaning
The forming fabric carries the sheet and drains water through its mesh. Fines, fillers and pitch blind the mesh over time. A fan shower crosses the full width and drives contamination back through the fabric. Typical published practice for a wet-end fabric shower is a fan pattern at low pressure, with flow scaled to fabric width. Needle or pencil showers are used where higher impact is needed to open a plugged mesh.
Press felt conditioning
The press felt moves water from the sheet into the felt and then into the uhle boxes. A conditioning shower re-wets and flushes the felt so it stays open and uniform. Felt conditioning usually runs at medium pressure with a fan pattern and a wider standoff. Sheet-side and back-side placement matter here, because the felt must be flooded evenly.
Trim and edge showers
The trim shower cleans the deckle edge and the fabric zone just inside it. Edge contamination builds where the stock hits the fabric and where the trim cuts. A narrow fan or a set of small fans targets that band. Too wide a fan pushes water into the sheet edge and causes edge wetting and breaks.
Uhle box showers
Uhle boxes pull water out of the felt by vacuum. Their showers keep the felt surface lubricated and flush debris so the slots do not blind. Flow here is lower than a felt conditioning shower, and the pattern is chosen to match the uhle box slot. Water quality matters most at this position.
Sheet side versus back side
A shower fitted on the sheet side of the fabric sees the stock. A shower on the back side sees the drainage side only. Published practice usually places high-impact cleaning on the back side, where the water does not disturb the sheet. If a sheet-side shower is used, its pressure and flow are kept lower to protect formation.
The Physics of a Cleaning Shower
Cleaning is done by momentum, not by volume. The impact force of a jet is the mass flow rate multiplied by its velocity, F = rho times Q times v. Water volume alone will not remove a fibre wedged in a mesh. The jet has to transfer enough momentum per unit area to dislodge it.
Two levers raise impact. You can raise flow, or you can raise velocity. Because pressure raises both flow and velocity, pressure is the stronger lever. Flow rises with the square root of pressure, and velocity rises with the square root of pressure as well. So doubling pressure raises flow and velocity by about 41 percent each, and impact by roughly 100 percent.
Impact per unit area is the force divided by the wetted area. A fan shower spreads the same flow over a wide band, so its per-area impact is lower than a needle shower at the same flow. That is why fabric cleaning and felt conditioning use fans, while stubborn mesh plugging calls for a needle or pencil pattern.
Consider a worked number. A fan jet passing 1.5 L/min at 4 bar moves about 0.025 kg of water per second. If its exit velocity is roughly 28 m/s, the momentum flux is about 0.7 newtons. Spread that over a pattern 0.5 m wide and 2 mm thick, and the impact per unit area is modest. A needle jet at the same flow concentrates the same momentum into a 2 mm circle. Its per-area impact can be several times higher, which is why nozzle selection follows the contamination type rather than pressure alone.
Droplet size falls as pressure rises, and finer droplets follow the air stream better. Spray angle stays essentially constant with pressure, so a change in pressure changes flow and impact, but not the footprint. That fact matters when you tune a shower. Raising pressure will not widen the pattern, so overlap must be set mechanically by nozzle spacing and standoff.
Pressure Bands for Different Positions
Published pressure bands vary by machine builder and grade, but the broad bands below are widely quoted. Treat them as starting points, then confirm with a flow test at the header.
| Position | Typical published pressure | Pattern | Notes |
|---|---|---|---|
| Forming fabric cleaning, low pressure | 3 to 4 bar (44 to 58 psi) | Fan | Full width, back side |
| Forming fabric cleaning, high pressure | 15 to 25 bar (218 to 363 psi) | Needle or pencil | Spot cleaning, plugged mesh |
| Press felt conditioning | 4 to 8 bar (58 to 116 psi) | Fan | Medium standoff |
| Uhle box shower | 2 to 4 bar (29 to 58 psi) | Fan, low flow | Matched to slot |
| Trim and edge | 3 to 6 bar (44 to 87 psi) | Narrow fan | Edge band only |
Low-pressure fabric cleaning showers use larger orifices and plenty of water at modest impact. High-pressure needle showers use tiny orifices and very high velocity, but they cover a narrow track and rely on oscillation to sweep the width. Felt conditioning sits between the two. It needs enough flow to flood the felt and enough pressure to flush the void volume.
Grade matters as much as position. A tissue machine running a fine fabric tolerates less impact than a board machine running a coarse one. The same position can sit at different points in the band. Confirm the band against the machine builder data before you set pressure.
Orifice Sizing and Flow per Metre of Fabric Width
Flow per nozzle follows the orifice equation, Q = K times the square root of P. K is fixed by the orifice diameter and discharge coefficient. Once K is known, you can scale flow for any pressure.
| Pressure (bar) | Relative flow (Q = K sqrt P) | Relative impact (force) |
|---|---|---|
| 2 | 1.00 | 1.00 |
| 3 | 1.22 | 1.50 |
| 4 | 1.41 | 2.00 |
| 6 | 1.73 | 3.00 |
| 8 | 2.00 | 4.00 |
Total shower flow is the flow per nozzle multiplied by the nozzle count. On a wide machine the useful number is flow per metre of fabric width. If a header carries 40 nozzles across 8 metres of fabric, that is 5 nozzles per metre. If each nozzle passes 1.5 L/min at 4 bar, the header passes 60 L/min, or 7.5 L/min per metre of width.
Convert to cubic metres per hour by dividing litres per minute by 1000 and multiplying by 60. So 60 L/min becomes 3.6 m3/h. That unit conversion is where most sizing sheets go wrong. A shower quoted at 60 L/min and one quoted at 3.6 m3/h describe the same flow.
Flow per metre of width is set by the cleaning job. Published practice for a forming fabric cleaning system on fine grades runs higher than on coarse grades, because a fine mesh blinds faster. A common starting point is a header sized for a repeatable water loading per square metre of fabric. Then adjust it after a fabric inspection.
K carries the units and the discharge coefficient. For a given nozzle, K is stable until the orifice wears or scales. That stability is what makes Q = K times the square root of P useful for diagnostics. Measure flow at two pressures, fit K, then track it over time. A K that drifts upward signals wear, even when the spray looks normal.
Spray Angle, Standoff and Overlap
The spray width of a fan nozzle is set by its angle and its standoff, width = 2 times standoff times the tangent of half the angle. A 110 degree fan at 200 mm standoff gives a width of 2 times 200 times tan 55, which is about 571 mm. A 65 degree fan at the same standoff gives about 254 mm.
| Angle (degrees) | Standoff (mm) | Pattern width (mm) |
|---|---|---|
| 65 | 150 | 191 |
| 80 | 200 | 336 |
| 95 | 200 | 437 |
| 110 | 200 | 571 |
| 120 | 250 | 866 |
Overlap is the fraction of each pattern that a neighbour repeats. Overlap of adjacent fan patterns should land in the 30 to 50 percent band. Below 30 percent, the uncovered strips show as streaks that run with the machine direction. Above 50 percent, water use climbs and the shower floods areas that do not need it.
Spacing follows from width and overlap. Take two fans 571 mm wide. A 40 percent overlap means a centre distance of roughly 0.6 times the width, about 343 mm. Set nozzle spacing from that number, not from the header pitch you inherited.
Too wide a fan wastes water because most of it lands where contamination is light. Too narrow a fan streaks because the coverage depends on oscillation alone. The right answer sits inside the 30 to 50 percent overlap band, checked with a pattern test on the stopped machine.
Oscillation: Stroke, Speed and Streaking
Many wet-end showers oscillate across the machine to sweep between nozzle tracks. The header traverses a stroke, reverses, and repeats. Traverse rate, fabric speed and stroke length together set the dwell time at any point.
If the oscillator stalls, the shower cleans only where the nozzles sit at that moment. That leaves a continuous streak that runs the length of the fabric. Continuous streaks are the signature of a stalled or seized oscillator, not of a worn nozzle. Check the drive and the stroke indicator before you change anything hydraulic.
Traverse rate should be slow enough to give every point a full pass. It should be fast enough to finish the stroke before the fabric travels far. Published practice usually ties the oscillator cycle to fabric speed so the coverage repeats within a short distance. If fabric speed doubles, the oscillator rate often rises with it to keep coverage stable.
Dwell is the time the pattern spends over any point. A slow traverse raises dwell and impact but can over-wet the felt or the sheet edge. A fast traverse lowers dwell and leaves light contamination behind. Balance the two against what the fabric inspection shows, and log the stroke and cycle time so you can repeat the setting.
Stroke length should cover at least one nozzle pitch plus one pattern width, so every point sees a full pass from two directions. A short stroke leaves the same bands under-cleaned at each end. Published settings often use a stroke of 100 to 300 mm for fan headers on the wire, matched to the nozzle spacing.
Water Quality, Filtration and Plugging
Small orifices plug. A needle shower with a 0.5 mm orifice blocks on a particle that a 3 mm fan orifice passes without notice. Published filtration practice usually keeps the filter mesh well finer than the smallest orifice in the header. Filter opening no larger than one third of the orifice diameter is a common rule.
| Orifice size (mm) | Suggested max filter opening (mm) | Notes |
|---|---|---|
| 0.5 | 0.15 | Needle, very fine |
| 1.0 | 0.30 | Pencil |
| 2.0 | 0.70 | Small fan |
| 3.0 | 1.00 | Fan |
| 5.0 | 1.50 | Low-pressure fan |
Open white water loops carry more fibre and fines than closed loops, so plugging risk rises with open systems. Hardness and dissolved solids scale the orifice and the bore, slowly shrinking the passage and changing the flow long before a nozzle fails. Scale also changes spray quality, because a rough orifice edge splits the fan.
Backwash the filters on a schedule tied to differential pressure, not to the calendar. A rising differential across the filter is the earliest sign that the loop is loading up. Track it and act on the trend.
Published water hardness above roughly 150 mg/L as calcium carbonate raises scaling risk in fine orifices. At that level a needle shower can lose measurable flow within a few weeks of continuous running. Softening or a finer downstream filter, or both, usually keeps the loop inside the filtration rule.
Materials and Wear on a Machine That Never Stops
A wet-end shower runs every hour the machine runs, so wear accumulates fast. Stainless steel bodies resist the corrosion that white water and felt conditioning loops cause. Standard stainless suits the body and the cap. Where fine orifices and high velocity combine, hardened stainless or a ceramic insert holds the geometry far longer.
Orifice wear does not stop the shower. It quietly raises flow. Published practice treats a 10 percent rise in flow above the datum as the point to replace the orifice. By then the pattern has already started to distort. On a machine that runs 24 hours a day, a 10 percent over-application compounds across every position on the wet end.
| Material | Relative wear life | Where it fits |
|---|---|---|
| Standard stainless | Baseline | Fan showers, larger orifices |
| Hardened stainless | Longer than baseline | Medium pressure, fine orifices |
| Ceramic insert | Longest | Needle showers, high velocity |
A worn set drifts together, so flow rises evenly and the fault hides. That is why the wear rule is about flow, not about spray appearance. Replace orifice sets as a set, and re-check header pressure after the change. A clean set at the old pressure will under-deliver water if the old pressure was raised to compensate for wear.
Body and cap material follows the water, not the wear rate. White water carries fines that abrade, so hardened stainless helps where velocity is high. Ceramic inserts cost more per piece but hold the orifice diameter far longer. That keeps flow near the datum across a long campaign.
Commissioning and Operating Checks
Commission a shower position by position, not as a single wet-end system. Start with a flow test at each header, using a catch pan and a timer, and compare the total against the sizing sheet. Measure pressure at the header, not at the pump. Pipe and filter losses between the pump and the header can hide a deficit of a bar or more.
Then run a pattern check with the machine stopped. Use a card or a strip under each nozzle and confirm the fans land in the 30 to 50 percent overlap band. A dry strip shows exactly where the coverage gaps sit. Note any nozzle that sprays off-centre, because a worn or partly blocked orifice deflects the fan.
Finally, log filter differential pressure and oscillator stroke. A rising differential points to loop loading. A short or slow stroke points to a mechanical fault. Both faults look like poor cleaning from the machine floor, but neither is fixed by turning up the pump.
A Worked Example: Sizing a Forming Fabric Shower
Take a machine with a forming fabric 8.0 m wide. The target published range for the low-pressure cleaning shower is 3 to 4 bar. The sizing sheet calls for 8 L/min per metre of fabric width. Start with the flow, then work back to the hardware.
Target total flow is 8 L/min per metre times 8.0 m, which is 64 L/min. Convert to cubic metres per hour: 64 divided by 1000, times 60, gives 3.84 m3/h.
Choose a nozzle count. Spacing of 160 mm across 8000 mm gives 50 nozzles. Flow per nozzle is 64 L/min divided by 50, which is 1.28 L/min. At 4 bar each nozzle must pass 1.28 L/min.
Check the pressure sensitivity. At 3 bar the flow drops by the square root ratio. The factor is the square root of 3 divided by 4, which is 0.87. So flow per nozzle becomes about 1.11 L/min and total flow about 55.5 L/min. The header therefore delivers about 3.33 m3/h at 3 bar and 3.84 m3/h at 4 bar. Size the pump and the filter for the 4 bar figure.
Now set the pattern. Choose a 110 degree fan at 200 mm standoff, giving a width of about 571 mm. For a 40 percent overlap, the centre distance is 0.6 times 571, which is about 343 mm. That is wider than the 160 mm spacing, so a single row would over-overlap. Use a lower angle fan or a larger standoff, or split the header into two staggered rows. Then re-check the overlap band on a dry strip.
Confirm impact is adequate. Each 1.28 L/min jet at 4 bar carries momentum equal to mass flow times velocity. If the fabric inspection still shows blinding in the middle, raise pressure toward 4 bar. Alternatively, add a needle row for the centre zone rather than flooding the whole width.
Frequently Asked Questions
Why does my forming fabric shower leave a continuous streak?
A continuous streak that runs with the fabric almost always points to a stalled or seized oscillator, not a nozzle fault. Check the stroke and cycle time first. If the oscillator is moving, check fan overlap, because a gap below 30 percent also streaks.
How often should I replace shower orifices?
Replace when flow rises about 10 percent above the datum, which is the usual published wear trigger. On a 24/7 machine that can be months rather than years. Replace the full set at once, and re-check header pressure after the change.
Does higher pressure always clean better?
Higher pressure raises both flow and velocity, so impact rises strongly. Doubling pressure raises flow by about 41 percent. But spray angle stays essentially constant, so pressure will not fix a coverage gap. Fix overlap with spacing and standoff instead.
What water quality does a needle shower need?
A needle shower with a 0.5 mm orifice needs filtration with an opening near 0.15 mm, roughly one third of the orifice. Open white water loops carry more fibre, so plugging risk rises. Track filter differential pressure and backwash on the trend.
Should the cleaning shower be on the sheet side or the back side?
Published practice usually puts high-impact fabric cleaning on the back side, where the water does not disturb the forming sheet. A sheet-side shower is kept at lower pressure and flow to protect formation. Felt conditioning is normally on the sheet side of the felt.
How do I convert shower flow from L/min to m3/h?
Divide litres per minute by 1000 and multiply by 60. So 64 L/min becomes 3.84 m3/h. Keep every sizing sheet in one unit set to avoid conversion errors at the pump.
Bringing It Together
Most wet-end shower faults trace back to sizing and placement: the wrong pressure band, worn orifices, poor overlap, or a stalled oscillator. Measure flow at the header, check the pattern on a dry strip, and set overlap inside the 30 to 50 percent band.
If you are sizing a forming fabric cleaning system or replacing a worn set, start with the pressure band for the position. Then work the arithmetic back to orifice size and spacing. You can review fan and needle options on our flat fan nozzle range. Send your fabric width and target flow to our team through the contact page.
For background on the underlying hydraulics, see our guides on spray nozzle flow rate calculation, spray angle and standoff, nozzle wear and silent over-application, and how to choose a spray nozzle.
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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.