Input data
Example data table
| Case | Units | Q | Vallow | s | t | Clog | Angle | Width | Anet | Aface |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Metric | 1.50 m³/s | 0.80 m/s | 50 mm | 10 mm | 0.25 | 15° | 2.0 m | 2.0625 m² | 3.4164 m² |
| 2 | Metric | 0.75 m³/s | 0.60 m/s | 40 mm | 8 mm | 0.30 | 10° | 1.5 m | 1.4375 m² | 2.5023 m² |
| 3 | Imperial | 120 cfs | 2.0 ft/s | 2.0 in | 0.5 in | 0.20 | 20° | 6.0 ft | 66.00 ft² | 109.74 ft² |
Formula used
1) Net open area
Anet = (Q / Vallow) × SF
Q is design flow, Vallow is allowable approach velocity, and SF is a safety factor.
2) Clogging allowance
Aclear = Anet / (1 − C)
C is clogging fraction (example 0.25 means 25% blocked).
3) Open area ratio of bar rack
φ = s / (s + t)
s is clear spacing and t is bar thickness. Gross normal area uses Agross = Aclear / φ.
4) Rack inclination effect
Aface = Agross / cos(α)
α is rack angle from vertical. Larger angles require larger face area to keep the same normal flow area.
5) Headloss estimate
Δh = K · Vface2 / (2g)
K is an adjustable coefficient. This is a simplified estimate for quick comparison and reporting.
How to use this calculator
- Choose a unit system and enter the design flow rate.
- Set the allowable approach velocity based on project criteria.
- Enter bar spacing, thickness, and an expected clogging allowance.
- Provide a rack angle from vertical and a safety factor.
- Optionally enter a rack width; otherwise use an aspect ratio.
- Press Submit to view the results above the form.
- Use the download buttons to export CSV or PDF reports.
Intake trash rack area design overview
Intake trash racks protect pumps, gates, and downstream equipment by screening debris before it enters a conveyance. A rack that is too small can create excessive approach velocities, accelerate clogging, and increase headloss. A rack that is too large can increase costs and complicate cleaning. This calculator helps you balance these tradeoffs by sizing the rack face area from a target approach velocity, then adjusting for bar geometry, inclination, and blockage.
The sizing logic starts with the net open area required to pass the design flow at an allowable approach velocity, with a safety factor for uncertainty. Next, a clogging allowance increases the clear opening area to reflect debris accumulation between cleanings. Because bar racks are not fully open, the open area ratio (clear spacing divided by spacing plus thickness) converts clear opening needs into gross rack area. Finally, a rack installed at an angle has a larger face area for the same normal flow area, which is handled using a cosine projection.
Beyond area, performance depends on how the rack is operated and maintained. Select bar spacing that matches the debris size you must intercept and the cleaning method you can reliably execute. Fine racks reduce debris passage but clog faster during heavy debris periods. If mechanical raking or automated cleaning is limited, increase the clogging allowance and confirm safe maintenance access. Where fish passage is a concern, lower approach velocities and suitable spacing are typically required by local guidance.
In construction planning, confirm the rack frame and supports can resist debris loads, and ensure raking and disposal routes remain accessible onsite.
Worked example (matches the table above): using a design flow of 1.50 m³/s and an allowable approach velocity of 0.80 m/s with a safety factor of 1.10, the net open area is 2.0625 m². With 25% clogging, 50 mm clear spacing, 10 mm bar thickness, and a 15° rack angle, the required rack face area becomes about 3.4164 m². If the rack width is fixed at 2.0 m, the implied rack height is the face area divided by width.
Use the velocity and headloss outputs for quick checks. The rack-face velocity should remain at or below your selected allowable value, and the through-clear velocity indicates how aggressive the flow becomes at openings when clogging is considered. The headloss estimate uses an adjustable coefficient for comparisons; for final design, confirm losses and loads using project standards and site-specific hydraulics.
FAQs
1) What approach velocity should I use?
Use the project’s intake criteria or relevant local guidance. Lower velocities reduce impingement risk and clogging sensitivity, but increase rack size. If unsure, start conservative, then iterate with maintenance capability and available footprint.
2) How do I pick a clogging allowance?
Base it on debris conditions and cleaning frequency. Sites with seasonal leaves, aquatic weeds, or storm-driven debris usually need higher allowance. If manual cleaning is expected, increase allowance to maintain capacity between cleanings.
3) Why do bar spacing and thickness change the required area?
They control the open area ratio. Thicker bars or tighter spacing reduce the open fraction, so more gross area is needed to provide the same clear opening. This also tends to increase headloss and maintenance demand.
4) What does rack angle from vertical mean?
It is the inclination of the rack plane relative to vertical. As angle increases, the required face area grows because only the normal component of the face area conveys the flow. Angled racks can aid debris removal but need more footprint.
5) Is the headloss result suitable for final design?
It is a simplified estimate for option comparison and reporting. For final design, confirm losses using detailed hydraulic methods, including approach conditions, rack type, debris state, and any upstream/downstream transitions.
6) When should I enter a rack width?
Enter width when it is constrained by the channel, gate bay, or structure. The calculator will then compute the required height. If width is unknown, leave it blank or zero to get suggested dimensions from the aspect ratio.
7) Can I switch units after entering values?
Unit switching changes the displayed labels, not the numbers already typed. If you need a true conversion, record your values, switch units, and re-enter converted inputs to avoid mixing unit systems.