Bar Rack Headloss
Bar rack headloss depends on the velocity difference through and upstream of the bars. The 0.7 factor accounts for the bar loss coefficient. Typical clean headloss is 10–40 mm.
h_L = (1/0.7) × (V² − v²) / (2g)
Fine Screen Headloss
Fine screen headloss uses the flow rate, effective open area, and a discharge coefficient. Fine screens have openings of 0.5–6 mm and can remove smaller solids than bar racks.
h_L = (1/2Cg) × (Q/A)²
How It Works
Screening is the first step in a wastewater treatment plant, catching rags, sticks, plastics, and other large debris before they damage pumps and clog pipes. Bar racks use spaced parallel bars (typically 25–50 mm openings), while fine screens have openings as small as 0.5–6 mm. The headloss across each type is the key design parameter for channel sizing.
Example Problem
A bar rack has an approach velocity of 0.6 m/s, a bar-opening velocity of 0.9 m/s, and g = 9.81 m/s². What is the headloss?
- h_L = (1/0.7) × (0.9² − 0.6²) / (2 × 9.81)
- h_L = 1.429 × (0.81 − 0.36) / 19.62
- h_L = 1.429 × 0.0229 = 0.033 m (33 mm)
Clean bar rack headloss is typically 10–40 mm; it increases as debris accumulates.
When to Use Each Variable
- Solve for Bar Rack Headloss — when you know the approach and opening velocities, e.g., checking whether a bar rack design meets maximum headloss criteria.
- Solve for Opening Velocity — when you know the headloss and approach velocity, e.g., verifying bar spacing is adequate for the design flow.
- Solve for Fine Screen Headloss — when you know the flow rate, screen area, and discharge coefficient, e.g., sizing the screen channel depth.
- Solve for Fine Screen Discharge — when you know the allowable headloss and screen area, e.g., determining the maximum flow a screen can handle.
Key Concepts
Screening headloss is driven by the velocity increase through the restricted opening. For bar racks, the Kirschmer equation uses the velocity difference between the approach channel and the bar openings. For fine screens, headloss depends on the ratio of flow rate to effective open area and a discharge coefficient that accounts for screen geometry and clogging.
Applications
- Municipal wastewater plants: sizing bar racks and fine screens for peak wet-weather flows
- Industrial pretreatment: protecting downstream equipment from fibrous or stringy waste materials
- Combined sewer systems: designing screening facilities that handle storm surge without bypassing
- Aquaculture and intake structures: preventing fish and debris from entering water supply systems
Common Mistakes
- Designing only for clean-screen headloss — screens accumulate debris rapidly, and headloss can increase 5-10x between cleanings
- Using average flow instead of peak flow — screens must pass peak wet-weather flows without exceeding the maximum allowable headloss
- Forgetting the 0.7 loss coefficient in the bar rack equation — omitting it underestimates headloss by about 30%
- Mixing velocity units — the equation requires consistent units (m/s or ft/s) for both approach and opening velocities
Frequently Asked Questions
What is the purpose of screening in wastewater treatment?
Screening removes large solids — rags, plastics, sticks, and debris — that could damage pumps, clog pipes, or interfere with downstream biological and chemical treatment processes. It is the first unit operation in virtually every treatment plant.
What is the difference between coarse and fine screens?
Coarse screens (bar racks) have bar spacings of 25–50 mm and catch large debris. Fine screens have openings of 0.5–6 mm and remove smaller solids, sometimes replacing primary clarifiers in compact treatment plants.
How much headloss do bar racks cause?
Clean bar racks typically produce 10–40 mm of headloss. As screenings accumulate, headloss rises and can exceed 150 mm, triggering automatic rake cleaning. Maximum allowable headloss is set during design, usually at 150–300 mm.
Related Calculators
- Activated Sludge Calculator -- design the biological treatment stage downstream of screening.
- Flocculation Calculator -- calculate mixing for chemical treatment after screening.
- Trommel Screen Calculator -- size rotating screens for solid waste separation.
- Pipe Flow Calculator -- determine flow velocity approaching the bar screen.
- Speed Unit Converter -- convert approach velocity between ft/s, m/s, and other units.
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