| Dry-bulb (C) | RH (%) | Pressure (kPa) | Wet-bulb (C) | Outlet at 85% (C) | Max cooling (C) |
|---|---|---|---|---|---|
| 35.0 | 30 | 101.325 | 21.72 | 23.71 | 13.28 |
| 40.0 | 20 | 101.325 | 22.29 | 24.95 | 17.71 |
| 30.0 | 50 | 95.000 | 21.97 | 23.17 | 8.03 |
| 25.0 | 70 | 101.325 | 21.02 | 21.62 | 3.98 |
Evaporative cooling drives air toward its wet-bulb state. The theoretical minimum outlet temperature is the wet-bulb temperature Twb.
- Saturation vapor pressure (Buck):
Pws(T)in kPa. - Actual vapor pressure:
Pv = RH/100 * Pws(Tdb). - Humidity ratio:
w = 0.621945 * Pv / (P - Pv). - Wet-bulb relation (bisection solve):
Pv = Pws(Twb) - A(Twb)*P*(Tdb - Twb), withA(Twb)=0.00066*(1+0.00115*Twb). - Direct evaporative outlet temperature:
Tout = Tdb - e*(Tdb - Twb), whereeis effectiveness. - Moisture gain estimate:
wout = win + e*(ws(Twb) - win). - Enthalpy:
h = 1.006*T + w*(2501 + 1.86*T)(kJ/kg dry air).
This model uses common engineering psychrometric approximations. For critical design work, confirm with a full psychrometric chart or a validated library.
- Enter dry-bulb temperature and relative humidity for the inlet air.
- Choose pressure input or estimate pressure from altitude.
- Set saturation effectiveness to match your cooler design.
- Optionally add airflow to estimate cooling power and water use.
- Press Calculate to view results above the form.
- Download CSV or PDF to save the report.
Evaporative cooling and the limiting temperature
Evaporative cooling typically lowers air temperature by converting liquid water into water vapor. The process consumes latent heat from the air stream, so the air cools while its moisture content rises. The lowest practical temperature is constrained by the air's initial moisture and pressure.
Wet-bulb temperature as the cooling limit
The wet-bulb temperature is the equilibrium temperature reached when water evaporates into air at constant pressure. In direct evaporative cooling, the outlet temperature cannot be lower than Twb. This tool reports Twb as the limit.
Wet-bulb depression indicates opportunity
The difference (Tdb - Twb), called wet-bulb depression, summarizes how much cooling is available. Hot, dry air often shows a large depression (commonly 10 to 20 °C), enabling substantial cooling. Humid air may show only a few degrees, so evaporative cooling has limited impact.
Role of humidity and dew point
Relative humidity changes with temperature, so the calculator also computes dew point and humidity ratio. Dew point is the temperature at which the current vapor pressure would saturate. Humidity ratio w (kg water per kg dry air) tracks absolute moisture and is the best variable for water addition estimates.
Pressure and altitude effects
Psychrometric relationships depend on total pressure. At higher altitude, pressure is lower (for example, around 80 kPa near 2000 m), which shifts the humidity ratio calculation for the same vapor pressure. This tool lets you enter pressure directly or estimate it from altitude.
Effectiveness connects theory to real devices
Real coolers do not fully reach the wet-bulb state, so an effectiveness factor e is used: Tout = Tdb - e(Tdb - Twb). Typical direct coolers operate roughly in the 60 to 90% range, depending on pad design, airflow, and water distribution.
Moisture gain and water requirement
Cooling comes with added moisture. The calculator estimates outlet humidity ratio trending toward saturation at Twb, then reports water added per kg of dry air. When you provide airflow, it converts that moisture gain into an approximate water use rate (liters per hour), useful for sizing pumps and supply.
Interpreting energy and capacity outputs
Sensible cooling per kg of dry air is estimated from 1.006 * ΔT (kJ/kg), where ΔT is the temperature drop from inlet to outlet. With airflow, the tool estimates dry-air mass flow using specific volume, then reports sensible cooling power in kW. Use it for quick scenario comparisons.
1) Is the wet-bulb temperature always lower than dry-bulb?
Yes, except at saturation. When air is 100% relative humidity, dry-bulb equals wet-bulb and no evaporative cooling is available. As air gets drier, wet-bulb drops below dry-bulb, increasing potential cooling.
2) What effectiveness value should I choose?
Use manufacturer data or field measurements. As a starting point, many direct evaporative pads fall between 0.6 and 0.9. Higher airflow or poor wetting can reduce effectiveness, while optimized media and distribution can increase it.
3) Why does altitude change humidity ratio and capacity?
Humidity ratio depends on the ratio of vapor pressure to total pressure. Lower pressure at altitude increases the humidity ratio for the same vapor pressure, which affects enthalpy, specific volume, and the airflow-based kW and water-use estimates.
4) Can outlet temperature go below the wet-bulb limit?
Not for single-stage direct evaporation under steady conditions. Two-stage systems (indirect plus direct) can deliver air below the outdoor wet-bulb by pre-cooling the air without adding moisture first, but the direct stage still trends toward its local wet-bulb.
5) What does humidity ratio represent?
Humidity ratio is the mass of water vapor per unit mass of dry air. Unlike relative humidity, it does not change simply because temperature changes. It's the right variable to estimate how much water is added during evaporation.
6) How are cooling power and water use estimated?
If airflow is provided, the tool converts volume flow to dry-air mass flow using specific volume. Cooling power is then estimated from sensible heat removal, and water use comes from the calculated moisture gain multiplied by mass flow.
7) Why might the two wet-bulb methods differ slightly?
The iterative method solves a pressure-aware psychrometric relation and is generally more accurate over varied conditions. The fast method is an approximation that trades accuracy for speed and can deviate outside its strongest temperature and humidity range.