Key Takeaways
- Packed bed pressure drop at design conditions is 250–400 Pa/m (wet) for 2-inch random packing, 1.3–1.5× the dry ΔP. At L/G above 3.0, the wet multiplier can reach 2–3× and signals approach to flooding.
- Total system pressure drop = packed bed + mist eliminator (100–200 Pa) + inlet/outlet losses (80–150 Pa) + ductwork (50–200 Pa). System ΔP for a 10,000 m³/h packed bed scrubber: 650–1,100 Pa.
- Fan selection: multiply design flow by 1.15 and system ΔP by 1.20. A fan sized without margin will underperform as the packing ages or if the actual L/G exceeds the design value.
- Venturi scrubbers have ΔP of 2,500–6,000 Pa — 10–20× higher than packed beds — but are required for sub-micron particulate removal. Spray towers have the lowest ΔP at 50–200 Pa.
- The packing factor (F_p) is the single most important parameter for ΔP prediction. A 10% error in F_p translates to approximately 15–20% error in the predicted pressure drop near flooding. Always use vendor-specific F_p values, not generic literature values.
A galvanizing plant in Vietnam was our opening story — the φ1.8m packed column that shipped in 2019 with a pressure drop of 1,500 Pa at the design flow, when the fan could only deliver 800 Pa. The designer had used the Ergun equation for a dry packed bed but had added no margin for the liquid hold-up. At the operating L/G of 1.2 L/m³, the wet pressure drop was double the dry prediction. The fan was replaced with a larger unit at a cost of $4,500, and the column access ports had to be modified to check the liquid distributor. Total unplanned cost: $11,000.
Scrubber pressure drop is not an afterthought — it determines the fan selection, the operating energy cost, and whether the system actually pulls the required airflow from the process exhaust hoods. A pressure drop error of 200 Pa in a 10,000 m³/h system translates to approximately $1,500–2,500 per year in additional fan energy cost, or a fan that cannot deliver the design airflow.
This guide covers pressure drop calculation methods for all three common scrubber types — packed bed, spray tower, and Venturi — with the key correlations, worked examples, and the most common mistakes that cause fan-system mismatches.
For the full design calculation sequence including pressure drop, see our scrubber design calculation guide.
Packed Bed Pressure Drop: Ergun and Eckert Methods
Pressure drop through a packed bed is the sum of gas-phase friction losses through the packing, plus an additional contribution from the liquid hold-up. The standard engineering method is the Generalized Pressure Drop Correlation (GPDC), documented in the EPA’s wet scrubber design reference.
For preliminary design, the dry-bed Ergun equation provides a reasonable starting estimate for random packings:
ΔP/L = 150 × (1 − ε)²/ε³ × μ_g × u_s / (d_p² × ρ_g) + 1.75 × (1 − ε)/ε³ × ρ_g × u_s² / d_p
Where ΔP/L is the pressure drop per meter (Pa/m), ε is the void fraction of the packing, μ_g is the gas viscosity (Pa·s), u_s is the superficial gas velocity (m/s), d_p is the nominal packing diameter (m), and ρ_g is the gas density (kg/m³).
For the converged φ1.4m design from our packed bed guide (2-inch PP Pall rings, ε ≈ 0.91, d_p = 0.05 m, u_s = 1.29 m/s, ρ_g = 1.15 kg/m³, μ_g = 1.8 × 10⁻⁵ Pa·s):
First term (viscous): 150 × (0.09/0.753) × (1.8×10⁻⁵ × 1.29) / (0.0025 × 1.15) = 150 × 0.1195 × 0.0081 = 0.145
Second term (inertial): 1.75 × (0.09/0.753) × 1.15 × 1.66 / 0.05 = 1.75 × 0.1195 × 38.18 = 7.98
Dry ΔP/L = ~8 Pa/m
At L/G = 1.5 L/m³, the wet pressure drop is 1.3–1.5× the dry value due to liquid hold-up. Estimated wet ΔP/L: 250–400 Pa/m for 2-inch Pall rings at 80% flooding. This large multiplier occurs because the liquid occupies void space that would otherwise be available for gas flow, effectively increasing the local gas velocity. The wet ΔP is determined from the GPDC, which relates the flow parameter (L/G × √(ρ_g/ρ_l)) to the capacity parameter (u_s² × F_p × μ_l^0.1 / (ρ_l × ρ_g)), where F_p is the packing factor.
| Packing Type | Size | Packing Factor F_p (m⁻¹) | Dry ΔP/L at 70% Flooding (Pa/m) | Wet ΔP/L at L/G=1.5 (Pa/m) |
|---|---|---|---|---|
| PP Pall rings | 1-inch (25 mm) | 95 | 50–80 | 350–550 |
| PP Pall rings | 2-inch (50 mm) | 52 | 30–50 | 250–400 |
| PP Pall rings | 3-inch (75 mm) | 35 | 20–35 | 150–300 |
| Intalox saddles (plastic) | #2 (50 mm) | 45 | 25–45 | 200–350 |
| Structured packing | Mellapak 250Y | 22 | 15–25 | 80–200 |
For Venturi scrubbers, pressure drop is 10–150× higher than packed beds and is predicted using the Hesketh correlation or Calvert’s equation. A Venturi scrubber at a throat velocity of 60 m/s with L/G = 0.7 L/m³ typically has a pressure drop of 2,500–6,000 Pa — requiring a high-pressure fan. Spray towers have the lowest pressure drop at 50–200 Pa total, consisting primarily of nozzle pressure drop and mist eliminator losses.
Total System Pressure Drop and Fan Selection
The total system pressure drop includes the packed bed ΔP plus additional losses from components. For a packed bed scrubber with 2-inch PP Pall rings at 80% flooding (φ1.4m, 2.1m packed depth, 600–900 Pa total bed):
- Packed bed (wet): 400–600 Pa
- Mist eliminator: 100–200 Pa (mesh pad) or 150–400 Pa (vane type)
- Gas inlet nozzle and distribution: 50–100 Pa
- Gas outlet nozzle: 30–50 Pa
- Ductwork to/from scrubber: 50–200 Pa (depends on run length)
- Total system: 650–1,100 Pa
Fan selection: centrifugal, Q = design flow × 1.15 (margin), ∆P = system total × 1.20 (margin). For a 10,000 m³/h system at 1,100 Pa total: fan duty = 11,500 m³/h at 1,320 Pa, motor approximately 5–7 kW. Fan construction: PP or FRP for corrosive exhaust, with a spark-resistant impeller where explosive gases are possible (H₂S, biogas, some VOC streams).
Frequently Asked Questions
What is the typical pressure drop of a packed bed scrubber?
At 70–80% of flooding, a packed bed scrubber with 2-inch random packing has a wet pressure drop of 250–400 Pa/m of packed depth. Total system ΔP (bed + mist eliminator + losses) is typically 600–1,100 Pa.
What is the difference between dry and wet pressure drop in a packed bed?
Wet pressure drop at L/G = 1.0–2.0 L/m³ is approximately 1.3–1.5× the dry pressure drop due to liquid hold-up occupying inter-particle void space. At L/G above 3.0, the multiplier can reach 2–3×, approaching the flooding point.
How does packing type affect pressure drop?
Packing factor F_p is the key parameter: 1-inch Pall rings (F_p = 95) have 2–3× the pressure drop of 3-inch Pall rings (F_p = 35) at the same gas velocity. Structured packing (Mellapak 250Y, F_p = 22) offers 30–50% lower ΔP than random packing for equivalent mass transfer.
What is the pressure drop of a spray tower vs packed bed?
Spray towers have 50–200 Pa total system ΔP — approximately 5–10× lower than a packed bed. The trade-off: spray towers are taller (higher H/D ratio), have lower efficiency for soluble gases, and require more pump energy for the spray nozzles.
How much does pressure drop affect operating cost?
At $0.10/kWh, an additional 500 Pa of system pressure drop at 10,000 m³/h costs approximately $1,000–1,500 per year in additional fan electricity. The fan energy cost is calculated as: kW = Q × ΔP / (fan efficiency × 1,000).
Conclusion
Scrubber pressure drop calculation requires different methods for different scrubber types — Ergun/GPDC for packed beds, Hesketh/Calvert for Venturis, and simple residence-velocity methods for spray towers. The key design step is applying the wet ΔP multiplier (1.3–1.5× dry) for packed beds, then adding component losses for mist eliminator, inlet, and ductwork. A fan sized with 15–20% margin on both flow and pressure ensures the installed system can deliver the design airflow regardless of minor variations in packing condition or liquid load.