The most-asked question in any Coanda screen specification is: which slot width should I choose?
It's the right question to ask. Slot width is the single most influential parameter in a Coanda screen design. It determines what passes through (clean water + everything finer than the slot) and what gets rejected (everything coarser). It influences head loss, fouling rate, slot velocity, sweeping velocity, and ultimately whether the screen self-cleans reliably under your project's operating conditions.
It's also a question that doesn't have a single right answer. The right slot width depends on the application, the debris signature, the fish-protection requirements, and the available hydraulic head: sometimes also on the water chemistry and the operating temperature. There isn't a universal "good" slot width; there's an application-appropriate slot width.
This post is the matrix that EPC (engineering, procurement and construction) contractors and consulting engineers ask for: an application-by-application table of recommended slot widths, plus the reasoning behind each row. Where the matrix shows a range, project-specific factors decide the final value during review by an ADENCO engineer.
Why slot width is the most important parameter
Three things happen at the slot:
1. Filtration cutoff. Particles larger than the slot width can't pass through. Particles smaller can. The slot defines the upper bound of what reaches downstream equipment (turbine, pumps, reverse osmosis or RO membranes, downstream treatment).
2. Slot velocity. At a given flow rate, narrower slots present less open area, which means water has to move faster through each slot. A high slot velocity entrains more fine particles and (critically for fish-protection applications) exceeds regulatory limits on fish entrainment.
3. Self-cleaning hydraulics. The Coanda effect's sweeping action depends on water shearing across the screen surface faster than it permeates through. Slot width influences this balance. Too narrow and you fight head loss. Too coarse and the screen no longer filters meaningfully.
Get the slot width wrong and you have one of three problems. Too coarse: insufficient filtration, debris reaches downstream equipment. Too narrow: excess head loss, faster fouling, high slot velocity. Wrong choice for the chemistry: corrosion at the wedge wire edges accelerates beyond design.
The trade-offs sound complex but they're well-understood. The matrix below summarises 12 years of ADENCO operating results plus the design limits established by the US Bureau of Reclamation (USBR) Coanda Design Guide (Wahl, 2003, report R-2003-03).
The fundamental trade-off
Before the matrix, the underlying physics. Three competing factors influence the slot width decision:
Filtration target pulls slot width DOWN. Drinking water needs finer filtration than industrial cooling water. Equipment with tight tolerances (RO membranes, certain turbine types) needs finer filtration than rugged equipment.
Debris severity pushes slot width UP. Heavy debris loading favours coarser slots that don't foul as quickly. Sediment-rich rivers challenge narrow slots. Flood-event projectile debris (logs, gravel) favours coarser slots with upstream protection bars (stainless steel bars mounted in front of the wedge wire that take the impact of rocks and timber).
Available hydraulic head limits how narrow you can go. Narrower slots produce more head loss at design flow. If your site has marginal head (less than 0.7-0.8 m), you can't push slots below about 1.0 mm without losing self-cleaning function.
These three factors combine into the matrix below. Where the matrix shows a range, expect project-specific factors to push toward one end or the other during engineer review.
The slot-width-by-application matrix
| Application | Recommended slot width | Rationale |
|---|---|---|
| Hydropower: run-of-river, fresh water, moderate debris | 1.0 mm | Standard hydropower application. Self-cleans reliably across the operating flow range. ADENCO's most-deployed slot width across small hydropower (SHP) plants. |
| Hydropower: SHP retrofit (Tyrolean replacement) | 1.0-1.5 mm | Slightly coarser than new mountain-stream intakes because retrofits often inherit a debris environment that the original Tyrolean was already overwhelmed by. The wider slot prefers heavy-debris reliability over filtration. |
| Hydropower: silt-heavy lowland river | 1.0 mm with protection bars upstream | Sediment passes through; protection bars reject the impact-damage debris. |
| Drinking water: municipal, no fish-protection requirement | 0.5-1.0 mm | Narrower end of the range when downstream treatment depends on intake-stage filtration; coarser when downstream has its own filtration step (sand filters, clarifiers, etc.). |
| Drinking water: municipal, fish-protection required (NMFS-aligned) | 0.5-1.0 mm with slot velocity ≤ 0.12 m/s | NMFS criteria require slot ≤ 1.75 mm and velocity ≤ 0.12 m/s. ADENCO defaults to the narrower end of this range to give margin. |
| Raw water: industrial cooling | 1.0-2.0 mm | Cooling water tolerates coarser filtration; the downstream heat exchanger is damaged by fine particles but does not need sub-millimetre precision. |
| Raw water: desalination first-stage (seawater) | 0.5-1.0 mm | RO membranes downstream are expensive; a first-stage Coanda screen followed by sand filtration is the typical multi-stage approach. |
| Raw water: process water with sensitive downstream equipment | 0.5-1.0 mm | Sensitive equipment (small bore exchangers, certain pump types) defines the cutoff. |
| Snowmaking: alpine fresh water | 1.0-1.5 mm | Snow guns tolerate coarser water than potable systems. Wider slot reduces fouling during freeze-thaw cycles. |
| Agricultural irrigation: gravity diversion to canal | 1.0-2.0 mm | Open canals are forgiving downstream. Where drip irrigation follows, choose the narrower end of the range. |
| Agricultural irrigation: feeding drip-emitter systems | 0.5-1.0 mm | Drip emitters are intolerant of debris. A finer slot at the intake reduces emitter cleaning frequency. |
| Geothermal: high-chloride raw water | 1.0-1.5 mm | Geothermal water usually carries moderate debris but corrosive chemistry. Slot choice is secondary to material choice (Super Duplex 2507). |
A few points are worth highlighting:
- The 1.0 mm slot is the most common choice. It works for most hydropower, most drinking water without fish-protection, and most industrial cooling. When in doubt, it's the right starting point.
- The 0.5 mm slot is for fine-filtration applications: drinking water with downstream-sensitive equipment, projects governed by fish-protection rules, fine particle removal.
- The 1.5-2.0 mm slot is for heavy-debris applications: SHP retrofits replacing overwhelmed Tyroleans, agricultural diversions, raw water for tolerant downstream equipment.
The range we offer is 0.5 to 2.0 mm (1.0 mm standard); narrower slots are available on request, though no delivered project has yet needed one. Below 0.2 mm the screen ceases to be a Coanda screen: head loss becomes prohibitive and the self-cleaning hydraulics break down. Above 2.0 mm and you're effectively a passive bar screen with the Coanda surface adding cost without delivering filtration value.
The fish-protection special case
Fish-protection requirements operate in a different regulatory regime from the rest of the matrix. They're worth their own row because the criteria are set by external regulators, not by engineering trade-offs.
The two most-cited sets of criteria are below. They are a starting point rather than the binding text: criteria are revised, and the local agency's licence conditions override the general guidance.
United States: NMFS Anadromous Salmonid Passage Facility Design (2008/2011):
- Slot opening ≤ 1.75 mm (0.069 in) for juvenile salmonid exclusion
- Approach velocity ≤ 0.12 m/s (0.4 ft/s)
- Slot velocity ≤ 0.12 m/s
European Union: Water Framework Directive 2000/60/EC and supporting guidance:
- Slot openings tuned to the local protected fish species, with juvenile-fish criteria usually determining the specification
- Low approach velocities required to allow swim-away behaviour
- Maximum slot openings for eel-protected sites can be up to 4 mm but are more often 1-2 mm depending on site classification
For a Coanda screen designed to fish-protection criteria, the design conversation is:
- Confirm the regulatory regime governing the site (NMFS, EU WFD, IHA Sustainability Standard, regional variants).
- Confirm the local criteria for the target species: often local agencies have specific values that override the broad guidance above.
- Set the slot opening at the criteria value (typical: 1.0 mm for NMFS-aligned sites; 1.0-1.5 mm for EU WFD sites).
- Confirm that the slot velocity at design flow stays under the regulatory limit. This often requires a larger screen area than the design flow alone would need.
- Document compliance in the project's environmental approval package.
ADENCO's fish-protection deliveries include the EU WFD alignment notes as a standard document with the project. Local-regulator coordination is the EPC contractor's or the project owner's responsibility; we provide the technical information they need to demonstrate compliance.
How ADENCO tunes slot width during engineer review
The matrix is a starting point. Project-specific factors push the final slot width up or down by 0.1-0.5 mm during engineer review. The factors we look at, in priority order:
1. Debris analysis from the site. What does the site debris look like at the worst-case event? Photos and operator descriptions go further than generic categorisations. "Heavy debris" for a Black Sea coastal site looks different from "heavy debris" for an alpine stream.
2. Available head loss allowance. How much head loss can the project absorb before power generation or plant operation is affected? Sites with abundant head can use narrower slots; sites with marginal head get pushed toward the coarser end of the application range.
3. Downstream sensitivity. What happens to particles between the slot and the downstream equipment? A Coanda screen feeding a sand filter is a different filtration design than one feeding directly to a turbine.
4. Operating temperature regime. Cold conditions (below 0°C, freeze-thaw cycles, frazil ice risk) tolerate wider slots better than narrow slots. Sub-zero operation pushes us toward the wider end of the application range.
5. Maintenance access. Where the operator can clean the screen during scheduled maintenance, narrow slots are tolerable. Where the operator can't (remote sites, difficult access), wider slots that foul less aggressively are favoured.
6. Future expansion. A project that may add capacity later might want a slightly wider slot now to cope with the future flow range without replacing the screen.
Engineer review takes 1-2 business days. The output is a specific slot width recommendation with the rationale documented, plus an estimated head loss at design flow.
What to send for an engineer-reviewed slot recommendation
When you're past the matrix and want a project-specific recommendation, send:
- Application context: what's the water for, what equipment is downstream.
- Design flow in l/s or m³/h.
- Available weir width in metres.
- Available hydraulic head (drop height) below the weir crest in metres.
- Debris description: photos, operator descriptions, history of clogging events at adjacent sites if applicable.
- Fish-protection requirements, if any. Cite the regulatory regime.
- Water chemistry: at minimum pH, chloride, sediment loading.
- Operating temperature regime across seasons.
- Maintenance regime: accessibility, planned cleaning intervals.
Submit this through the Sizing Tool for an immediate result on the page, or as a free-form request for quotation (RFQ) via the Contact form. Both routes feed the same engineer review.
A note on slot tolerance
Slot width is a manufactured parameter. Tolerance matters.
ADENCO manufactures wedge wire panels with a slot opening tolerance of ±0.1 mm across the full panel. That's the figure to write into your specification.
Requiring a tight tolerance and verifying it through a factory acceptance test (FAT) report is part of getting the slot width "right."
The matrix gives you the starting point. Project-specific factors push the final number up or down by 0.1-0.5 mm. Engineer review documents the rationale. Manufacturing tolerance keeps the manufactured slot at the specified value.
When in doubt, 1.0 mm is the right default choice for most Coanda intakes. When fish-protection rules apply, 0.5-1.0 mm with slot velocity discipline. When debris is the dominant concern, 1.5-2.0 mm with upstream protection bars. Beyond that, ask.