Inputs
Example data table
These scenarios show typical inputs and outputs. Use your own measurements for real planning.
| Scenario | Size | ACH | Duct | Filter | Typical result |
|---|---|---|---|---|---|
| Small tunnel | 6×3×2.5 m | 10 | 6 m, 150 mm, 2 elbows | Light | ≈ 1,500–1,800 CFM @ 0.25–0.45 inH₂O |
| Seedling room | 4×3×2.4 m | 12 | 4 m, 125 mm, 3 elbows | Medium | ≈ 1,100–1,400 CFM @ 0.35–0.70 inH₂O |
| Large greenhouse bay | 18×8×3.5 m | 8 | 12 m, 200 mm, 4 elbows | Light | ≈ 7,000–8,500 CFM @ 0.30–0.75 inH₂O |
| Screened exhaust wall | 10×5×3 m | 6 | 0 m, 0 duct | Heavy | ≈ 2,200–2,900 CFM @ 0.60–1.20 inH₂O |
Formula used
- Volume: V = L × W × H
- Base airflow: Q₀ = V × ACH (m³/h)
- Adjusted airflow: Q = Q₀ × (1 + Safety) × (1 + Leakage)
- Convert: m³/s = (m³/h) ÷ 3600, and CFM = m³/s × 2118.88
- Duct area: A = πD²/4, velocity: v = Q/A
- Dynamic pressure: q = ½ρv²
- Friction loss: ΔPᶠ = f(L/D)q
- Minor loss: ΔPᵐ = Kq (elbows, entry, exit)
- Total static: ΔP = ΔPᶠ + ΔPᵐ + Filter
- Power: P ≈ (Q × ΔP) / η
How to use this calculator
- Select your unit system and enter length, width, and average height.
- Choose a target ACH based on crop heat and humidity needs.
- Enter duct details and elbows, or set duct length to zero.
- Pick a filter level that matches your screening setup.
- Set safety and leakage allowances for real-world uncertainty.
- Click Calculate, then select a blower using its curve.
- Download CSV or PDF to share with installers or suppliers.
FAQs
1) What ACH should I use for a greenhouse?
Many general greenhouses work well around 6–15 ACH, depending on heat load, humidity, and crop density. Start near 8–10, then adjust based on measured temperature, condensation, and air movement at plant level.
2) Why does duct diameter change the result so much?
Diameter controls air velocity. Smaller ducts raise velocity, which increases friction and minor losses, driving static pressure up. Higher pressure forces the blower to work harder and may reduce delivered airflow if the blower curve is limited.
3) Should I include filter pressure even with screens?
Yes. Insect netting, screens, and pads add resistance, and they clog over time. Choose a filter level that approximates your setup, then add a safety margin so airflow stays acceptable between cleanings.
4) What does the safety factor cover?
It covers uncertainty such as warmer days, future crop canopy, extra bends, installation imperfections, and performance drop as filters load. A typical planning range is 10–25% when you want steadier results across seasons.
5) How do I pick a blower after I get CFM and pressure?
Use manufacturer performance curves. Find your target static pressure, then read the airflow the blower delivers at that point. Select a model that meets or exceeds the recommended airflow at the recommended pressure, with some operational headroom.
6) Why is the power estimate only approximate?
Efficiency varies by blower type, motor, and operating point. Real installations also include transitions, dampers, and outlet losses that are hard to model precisely. Use the estimate for budgeting and compare with blower datasheets for final sizing.
Ventilation goals in growing spaces
Good airflow helps control leaf temperature, humidity, and disease pressure in greenhouses, tunnels, and potting sheds. Targeting appropriate air changes per hour supports uniform conditions, faster drying after irrigation, and steadier carbon dioxide levels for photosynthesis. In hot months, higher exchange rates also protect workers and reduce heat stress in enclosed benches.
Turning volume into airflow
The calculator first computes space volume from length, width, and average height. It then multiplies volume by the chosen ACH to estimate required airflow. Safety and leakage allowances increase the target, protecting performance when crops grow denser or doors and vents create extra bypass paths. For propagation areas, consider additional margin because misting and wet media raise latent load and slow drying.
Why static pressure matters
Airflow alone is not enough for blower selection. Duct length, diameter, elbows, and filters create resistance that the blower must overcome. The model estimates velocity from flow and duct area, then uses friction and minor-loss terms to build total static pressure. Higher velocity sharply increases losses and can increase noise, vibration, and uneven distribution across outlets.
Energy and equipment selection
Power demand rises with both airflow and pressure. The calculator estimates shaft power using flow multiplied by static pressure, divided by overall efficiency. Altitude slightly reduces air density, which changes pressure calculations and may affect delivered mass flow. Use the recommended rounded CFM and pressure as a minimum selection point on manufacturer performance curves. A blower operating near mid-curve usually runs cooler, lasts longer, and maintains stable control with speed drives.
Practical field adjustments
After installation, verify performance with an anemometer, pressure gauge, or fan curve measurements. If airflow is low, check for crushed ducts, tight screens, clogged filters, and unsealed joints. If noise is high, enlarge duct diameter, reduce elbows, or add smooth transitions. Balance multiple branches with dampers, and keep filters accessible for cleaning. Recheck settings seasonally as crop canopy, shade cloth, and insect netting change resistance long term.
FAQs
1) What ACH should I use for a greenhouse?
Many general greenhouses work well around 6–15 ACH, depending on heat load, humidity, and crop density. Start near 8–10, then adjust based on measured temperature, condensation, and air movement at plant level.
2) Why does duct diameter change the result so much?
Diameter controls air velocity. Smaller ducts raise velocity, which increases friction and minor losses, driving static pressure up. Higher pressure forces the blower to work harder and may reduce delivered airflow if the blower curve is limited.
3) Should I include filter pressure even with screens?
Yes. Insect netting, screens, and pads add resistance, and they clog over time. Choose a filter level that approximates your setup, then add a safety margin so airflow stays acceptable between cleanings.
4) What does the safety factor cover?
It covers uncertainty such as warmer days, future crop canopy, extra bends, installation imperfections, and performance drop as filters load. A typical planning range is 10–25% when you want steadier results across seasons.
5) How do I pick a blower after I get CFM and pressure?
Use manufacturer performance curves. Find your target static pressure, then read the airflow the blower delivers at that point. Select a model that meets or exceeds the recommended airflow at the recommended pressure, with some operational headroom.
6) Why is the power estimate only approximate?
Efficiency varies by blower type, motor, and operating point. Real installations also include transitions, dampers, and outlet losses that are hard to model precisely. Use the estimate for budgeting and compare with blower datasheets for final sizing.