Technical Calculation for Bar Screens

Technical Calcculation for Bar Screens

The design calculations for bar screens primarily include selection of screen type, dimensioning, hydraulic calculations, and screenings quantity estimation. Figure 2-5 illustrates a schematic diagram for bar screen calculations.

Figure 2-5 Schematic Diagram of Bar Screen Calculation

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1. Screen Channel Width (B)

Where:

B ——Channel width (m)

s ——Bar width (m)

e ——Clear spacing between bars: 

• Coarse screen: e = 50-100mm 
• Medium screen: e = 10-40mm 
• Fine screen: e = 3-10mm

n ——Number of bar gaps

Qmax —— Maximum design flow rate (m³/s)

α ——Screen inclination angle (°)

h ——Upstream water depth (m)

v ——Flow velocity through screen: 

• 0.6-1.0 m/s (at maximum design flow) 
• 0.3 m/s (at average design flow)

√sinα —— Empirical coefficient

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2. Head Loss Across the Screen (h₂ )

Where:

h₂ ——Head loss across screen (m)

h₀ ——Theoretical head loss (m)

g ——Gravitational acceleration (9.81 m/s²)

k —— Clogging coefficient (typically k=3, accounts for increased head loss due to debris accumulation)

ξ —— Resistance coefficient (dependent on bar cross-section shape):

• Calculated as: ξ = β(s/e) 
• For rectangular bars: β = 2

To prevent upstream water backflow (surge), the channel floor downstream of the screen is lowered by h₁ as hydraulic compensation (see Figure 2-5).

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3. Total Channel Height (H)

Where: H ——Total channel height (m)

h ——Upstream water depth (m)

h₁ ——Freeboard in upstream channel (typically 0.3m)

h₂ —— Head loss across screen (determined by Equation 2-3)

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4. Total Channel Length (L)

Where:

L ——Total length of screen channel (m)

H₁ ——Upstream channel height (m)

L₁ ——Length of inlet transition (widening section) (m)

B₁ ——Inlet channel width (m)

α ——Inlet transition angle (typically 20°)

L₂ ——Length of outlet transition (narrowing section) (m)

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5. Daily Screenings Quantity (W)

Where:

W——Daily screenings volume (m³/day)

W₁ ——Screenings yield (m³/10³m³ wastewater): 

• Coarse screens: Lower range (0.01-0.03) 
• Medium screens: Mid range (0.03-0.1) 
• Fine screens: Higher range (0.1-0.3)

Ktotal —— Peaking factor for domestic sewage flow (refer to Table 2-1)

Table 2-1 Peaking Factor (Ktotal) for Domestic Sewage Flow Variation