| Outdoor T (C) | RH (%) | Effectiveness (%) | Supply T (C) | Airflow (m3/h) | Evaporation (L/h) |
|---|---|---|---|---|---|
| 40 | 20 | 85 | 25.3 | 8,830 | 62.0 |
| 36 | 35 | 80 | 26.5 | 37,883 | 171.7 |
| 33 | 55 | 75 | 27.6 | 0 | 0.0 |
- Twb: estimated from dry-bulb and humidity using a standard approximation.
- Tsupply = Tdb − e (Tdb − Twb), where e is cooler effectiveness.
- Q = rho * cp * Vdot * dT then Vdot = Q / (rho * cp * dT) for airflow.
- Humidity ratio: w = 0.62198 * Pv / (P − Pv), using saturation vapor pressure.
- Evaporation estimate: mwater ~= mdry_air * (wout − win).
- Pad face area: Apad = Vdot / vface using your face velocity target.
- Pick units and enter outdoor temperature and humidity.
- Enter effectiveness based on your pad type and thickness.
- Set the indoor temperature target for plants and comfort.
- Choose a load method: area factor or direct kW value.
- Adjust pad face velocity to match your planned pad wall.
- Press Calculate and review airflow, pad area, and water.
- Download CSV or PDF to store your sizing notes.
Cooling potential and wet-bulb limit
Direct evaporative cooling can only approach the outdoor wet-bulb temperature. This calculator estimates wet-bulb from dry-bulb and relative humidity, then applies effectiveness to predict supply air temperature. When humidity rises, wet-bulb rises too, and the available temperature drop shrinks. Use your hottest, most humid design hour to avoid undersizing.
Effectiveness, pads, and face velocity
Effectiveness represents how close the supply air gets to wet-bulb. Many greenhouse pads fall near 60–90% when clean and wetted. Thick cellulose media and stable water coverage typically improve performance, while excessive face velocity reduces contact time. The pad face area result is tied to your selected face velocity, helping you translate airflow into a practical wall or panel size.
Airflow sizing from sensible heat load
Airflow is sized using the sensible relation Q = ρ·cp·V̇·ΔT. You can enter a direct heat load in kW, or estimate it from floor area and a load factor. For greenhouses, load factors often sit around 120–300 W/m² depending on solar intensity, glazing, shading, transpiration, and internal equipment. If the supply temperature is close to your setpoint, airflow needs rise quickly and fans may become impractically large.
Water use and pump flow guidance
Evaporation is estimated from the humidity ratio increase across the cooler, so it scales with airflow and dryness. This value supports water planning, makeup supply, and drain management. The pump guideline is a recirculation target to keep pads uniformly wet, not a fresh-water consumption rate. Increase distribution if you see dry streaks, and decrease it if water carryover occurs.
Operational checks and tuning
After installation, verify pad wetting, airflow balance, and intake filtration. Track supply temperature, humidity, and fan amperage during peak periods. A field check is the approach temperature: Tsupply minus Twet-bulb; smaller is better. If cooling is weak, reduce air leaks, add shading, or improve water distribution before increasing fan size. Export CSV or PDF outputs to document design assumptions, maintenance dates, and seasonal adjustments.
FAQs
1) Why is my supply temperature not much lower than outdoors?
High outdoor humidity raises wet-bulb temperature, limiting cooling. Increase pad effectiveness, improve wetting, reduce face velocity, or raise the indoor setpoint. Shading and sealing leaks also reduce the cooling load.
2) What effectiveness value should I enter?
Use manufacturer data if available. As a starting point, 70–80% suits many medium pads, while clean, thicker cellulose pads may reach 85–90%. Dirty pads, poor wetting, or high velocity can lower performance.
3) Should I size airflow using area factor or direct kW?
Use direct kW if you have a measured or modeled load. Use the area method for early planning, then refine later. If results seem extreme, revisit the load factor, setpoint, and assumed shading.
4) Does the water evaporation result equal pump flow?
No. Evaporation is the water that becomes vapor and is consumed. Pump flow is recirculated water to wet pads evenly; it can be much higher than evaporation and depends on pad design and distribution hardware.
5) What pad face velocity is reasonable?
Many installations target roughly 1.5–2.5 m/s. Lower velocities usually improve effectiveness but require more pad area. If you choose a higher velocity, expect more pressure drop and a larger fan requirement.
6) Can I use this for outdoor patios or small sheds?
Yes, if you enter realistic heat load and conditions. Open structures leak heavily, so add a safety margin. For tiny spaces, airflow may be limited by available cooler sizes rather than calculations.