How the sizing works
- Airflow from ACH: CFM = (Volume × ACH) ÷ 60, with volume in ft³.
- Required duct area: Area = CFM ÷ Velocity, with velocity in fpm and area in ft².
- Round duct diameter: D = √(4×Area/π) (feet), then convert to inches and select the next standard size.
- Rectangular equivalent diameter: an approximation is used to estimate a round-equivalent diameter for friction.
- Friction and pressure loss: an empirical friction-rate model estimates in.wg per 100 ft, then: ΔP = FR × (TotalLength/100).
Steps for growers and greenhouse operators
- Pick Known airflow if you already have a fan rating or measured flow.
- Or choose Air changes per hour, then enter your space dimensions.
- Set a target velocity. Use lower values for quieter systems.
- Enter duct length and elbow count to estimate resistance.
- If you have a pressure budget, set a max pressure drop.
- Review the recommended size, velocity, and pressure drop. Export results as CSV or PDF to compare options.
Sample inputs and outputs
| Scenario | Method | Airflow / ACH | Velocity | Length & elbows | Suggested size | Estimated ΔP |
|---|---|---|---|---|---|---|
| Small tent exhaust | Known airflow | 400 CFM | 700 fpm | 15 ft + 2 elbows | 10 in round | ~0.05 in.wg |
| Greenhouse bay | ACH | 20 ACH (8×20×10 ft) | 900 fpm | 40 ft + 4 elbows | 18 in round | ~0.07 in.wg |
| Low-profile wall chase | Known airflow | 1200 CFM | 800 fpm | 25 ft + 3 elbows | 24×10 in rectangular | ~0.08 in.wg |
Airflow planning for productive houses
Balanced ventilation starts with a clear airflow target. Use fan ratings at operating pressure, not free-air numbers. Convert your greenhouse volume and desired air changes per hour into required CFM or m³/h. Split that demand across zones so warm, humid pockets do not linger over benches. When crops are dense, prioritize uniform distribution over raw speed. Document assumptions for future upgrades and maintenance scheduling.
Velocity targets that protect crops
Duct velocity controls noise, energy use, and draft risk. Lower velocities reduce friction and help fans maintain flow, but oversized ducts may be hard to route. Higher velocities shrink duct size yet increase pressure loss and can create cold streaks across sensitive foliage. A practical range is often 500–900 fpm, adjusted for crop height and intake temperature differences.
Resistance, fittings, and real-world losses
Straight duct length is only part of the story. Elbows, tees, dampers, filters, and screens add losses that behave like extra feet or meters of duct. Using equivalent length lets you sum all parts into a single effective run. Then estimate total pressure drop from friction rate tables and scale by length, keeping bends gentle and transitions gradual.
Round versus rectangular selection
Round duct is typically more efficient because it has less perimeter for the same area, which lowers friction. Rectangular sections fit wall chases and truss spaces, but they need careful aspect ratios to avoid high losses. If you must go rectangular, keep corners smooth, avoid extreme flat shapes, and use gradual reducers when changing size.
Commissioning and iterative refinement
After installation, verify performance with an anemometer or pitot traverse at accessible points. Compare measured velocity and static pressure to design values and adjust dampers to balance branches. If pressure is high, look for crushed flex, tight elbows, or dirty filters. Small corrections often recover large airflow, improving temperature control and disease prevention. Recheck settings during seasonal shifts, because insect screens and wet pads change resistance over time.