Project Inputs
Formula Used
The calculator uses an effective open-area approach for bar screens. Geometry defines the open area ratio, then clogging reduces it. A discharge coefficient represents bar shape and contraction effects.
- Channel area:
A = b × y - Approach velocity:
Va = Q / A - Open area ratio:
r = s / (s + t) - Effective open area:
re = r × Fc - Headloss:
hL = (Va² / 2g) × [ (1 / (Cd × re))² − 1 ]
Q is flow, b channel width, y water depth,
s clear spacing, t bar thickness, Fc clogging factor,
Cd discharge coefficient, and g is 9.81 m/s².
How to Use This Calculator
- Enter design flow and select the correct flow unit.
- Provide channel width and expected operating water depth.
- Input bar thickness and clear spacing from your screen selection.
- Select bar shape to apply an appropriate discharge coefficient.
- Set a clogging factor to reflect debris and maintenance allowance.
- Click Calculate Headloss to view results above the form.
- Use Download CSV or Download PDF to document the outcome.
Example Data Table
| Scenario | Q (m³/s) | b (m) | y (m) | t (mm) | s (mm) | Fc | Shape | Headloss (m) |
|---|---|---|---|---|---|---|---|---|
| Baseline | 0.250 | 1.20 | 0.90 | 10 | 30 | 0.75 | Rectangular bars | 0.0095 |
| Higher flow | 0.350 | 1.20 | 0.90 | 10 | 30 | 0.75 | Rectangular bars | 0.0186 |
| More clogging | 0.250 | 1.20 | 0.90 | 10 | 30 | 0.60 | Rectangular bars | 0.0164 |
| Wider spacing | 0.250 | 1.20 | 0.90 | 10 | 40 | 0.75 | Rectangular bars | 0.0080 |
| Streamlined | 0.250 | 1.20 | 0.90 | 10 | 30 | 0.75 | Streamlined bars | 0.0068 |
Professional Guidance Article
Screening headloss is a key hydraulic check for intake works, stormwater channels, and preliminary treatment units where debris control is required. A bar screen introduces energy loss because the flow contracts through the clear openings and then expands downstream. If the loss is underestimated, upstream water levels can rise, freeboard can be reduced, and bypassing or overtopping risks may increase. If it is overestimated, screens may be oversized and cost more than necessary. This calculator supports early sizing, option studies, and design documentation by using practical inputs that engineers routinely know at concept and detailed design stages.
The process begins with the approach velocity based on channel width and operating depth. This velocity represents the average flow speed immediately upstream of the screen. The open area ratio is then computed from bar thickness and clear spacing. Because debris accumulation reduces the effective flow area, the tool applies a clogging factor to estimate the remaining open area during normal operation or conservative conditions. Finally, a discharge coefficient is selected to represent bar shape and contraction effects. Streamlined bars typically pass flow more efficiently than sharp-edged bars.
The example table above demonstrates how sensitive headloss is to project assumptions. For instance, increasing flow while holding geometry constant raises both approach velocity and opening velocity, producing a higher loss. Reducing the clogging factor can increase headloss significantly, even when flow does not change, because the effective open area becomes smaller. Increasing spacing improves the open area ratio, reducing opening velocity and headloss, but spacing must still meet debris capture requirements and any safety or equipment limits.
For best results, use a flow rate consistent with your governing design event or operating case, select realistic operating depth, and adopt a clogging factor aligned with maintenance frequency and debris loading. Compare multiple scenarios (baseline, peak, and conservative clogging) and export the CSV or PDF as a traceable record for reviews. For final design, confirm upstream and downstream water levels, screen submergence, and any manufacturer performance curves or project-specific criteria.
FAQs
1) What does the clogging factor represent?
It represents the fraction of open area still available after allowing for debris and fouling. A value of 0.75 means roughly 25% of the opening area is assumed blocked during operation.
2) How do I choose the discharge coefficient (Cd)?
Select Cd based on bar shape and any applicable standard or vendor guidance. Streamlined sections typically use higher Cd than rectangular or sharp-edged bars because they reduce contraction and turbulence.
3) Why is opening velocity important?
Opening velocity indicates the speed through the clear spaces between bars. High values can increase headloss, promote vibration, and accelerate debris attachment, so it is a practical screening performance indicator.
4) Can I use this for fine screens or perforated plates?
This tool is intended for bar screens and similar grids where spacing and thickness define open area. Fine screens and plates may require different coefficients or manufacturer curves for accurate headloss.
5) What if my channel is not rectangular?
Convert your section to an equivalent flow area at the operating depth. Use width and depth values that produce the same area as your channel cross-section for a comparable approach velocity.
6) How can I reduce excessive headloss?
Increase screen area, increase spacing where allowable, reduce bar thickness, improve bar profile, or plan more frequent cleaning. Always confirm debris capture requirements before increasing clear spacing.
7) What example inputs are provided?
The example table includes a baseline case (Q=0.25 m³/s, b=1.2 m, y=0.9 m, t=10 mm, s=30 mm, Fc=0.75) plus variations in flow, clogging, spacing, and bar shape.