Calculating static pressure of an air washer system is a core HVAC design task. You’re basically summing up all resistances the fan must overcome.

 Calculating static pressure of an air washer system is a core HVAC design task. You’re basically summing up all resistances the fan must overcome.

Let’s go step-by-step in a practical design approach (used in industry) 👇
🔷 1. What is Static Pressure (SP)?
Static Pressure = Total resistance to airflow in the system.
For an air washer, it includes:
Duct resistance (supply + return)
Air washer internal resistance
Filters / eliminators
Grills, dampers, bends, fittings
🔷 2. Basic Formula
[
{Total Static Pressure (TSP)} = ({Duct Losses} + {Equipment Losses} + Fittings Losses)
]
🔷 3. Step-by-Step Calculation
✅ Step 1: Calculate Airflow (CFM)
From heat load:
[
{CFM} = {TR x 400} / {{Air washer effectiveness factor}}
]
👉 Example:
For 100 TR system →
[
CFM = 100 x 400 = 40,000 , CFM
]
✅ Step 2: Duct Static Pressure Loss
Use friction loss method:
[
{Duct Loss} = {Friction Factor} x {Length}
]
Typical values:
0.8 – 1.2 mmWC per 10 m (medium velocity ducts)
👉 Example:
Total duct length = 50 m
Friction loss = 1 mmWC / 10 m
[
= 5 , mmWC
]
✅ Step 3: Fittings Loss (Elbows, Dampers, Transitions)
Use equivalent length or loss coefficients.
Typical thumb rules:
Elbow: 0.5 – 1.5 mmWC
Damper: 1 – 3 mmWC
Grill/Diffuser: 1 – 2 mmWC
👉 Example:
5 elbows → 5 mmWC
2 dampers → 4 mmWC
Total = 9 mmWC
✅ Step 4: Air Washer Internal Loss
This is very important 🔥
Includes:
Spray nozzles resistance
Drift eliminators
Air washer chamber
Typical range:
15 – 30 mmWC
👉 Assume:
[
= 20 , mmWC
]
✅ Step 5: Filters (if installed)
Pre-filter: 5 – 10 mmWC
Fine filter: 10 – 20 mmWC
👉 Assume:
[
= 10 , mmWC
]
🔷 4. Final Static Pressure Calculation
[
{TSP} = {Duct} + {Fittings} + {Air Washer} + {Filters}
]
[
= 5 + 9 + 20 + 10
]
[
= 44 , mmWC
]
🔷 5. Add Safety Margin
Always add 10–15% safety factor:
[
44 \times 1.1 = 48.4 \approx 50 , mmWC
]
✅ Final Answer:
👉 Fan Static Pressure = ~50 mmWC
🔷 6. Important Practical Notes (Very Important for You 👇)
Air washer systems are high static systems
Always check fan selection curve
Consider future choking of filters
Use velocity method carefully (don’t oversize ducts)
For large plants → do ESP (External Static Pressure) calculation separately
🔥 Pro Tip (Design Level)
For accurate design:
[{Fan SP} = {External SP} + {Internal (Air Washer) SP}]