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How does this calculation work? ▾
What is adiabatic pre-cooling?
A chiller throws heat out through its condenser coils using outdoor air. The hotter the outdoor air, the harder the chiller has to work. Adiabatic pre-cooling sprays a fine mist of water in front of the condenser. The water evaporates and cools the incoming air — like how sweat cools your skin. This lets the chiller work in "cooler" conditions even on a hot day, using less electricity.
Step 1 — Check if adiabatic is active this hour
We go through all 8,760 hourly rows in your weather file. For each hour, we check whether the outdoor dry-bulb temperature (DBT) and/or relative humidity (RH) meet your chosen conditions (e.g. DBT > 24°C AND RH < 80%). If they do, the adiabatic system switches on for that hour. If not, we skip it.
Step 2 — Find the inlet humidity ratio (ω₁)
The humidity ratio ω tells us how many kilograms of water vapour are mixed in every kilogram of dry air. We calculate it directly from the dew point temperature (T_dp) — a raw field in the EPW file that needs no approximation. At the dew point, air is exactly 100% saturated, so its saturation vapour pressure gives us the actual vapour pressure of the air:
P_ws_dp = 610.78 × e^(17.27 × T_dp / (T_dp + 237.3))
ω₁ = 0.622 × P_ws_dp / (101325 − P_ws_dp)
This is more accurate than going through wet bulb, which is itself an approximation.
Step 3 — Calculate the inlet air enthalpy (h₁)
Enthalpy is the total heat energy in the air — both the sensible heat (temperature you can feel) and the latent heat (moisture). We calculate it in kJ per kg of dry air:
h₁ = 1.006 × DBT + ω₁ × (2501 + 1.86 × DBT)
Step 4 — Find the cooled air temperature (T₂)
There are two ways to define the cooling target:
Effectiveness mode — the system cools the air a fixed fraction of the way from DBT toward WBT (e.g. 80% effective):
T₂ = DBT − (DBT − WBT) × η / 100
WBT Approach mode — instead of a percentage, you specify how close to WBT the system cools the air. For example, an offset of 2°C means the system always delivers air at WBT + 2°C, regardless of how hot the DBT is:
T₂ = WBT + offset
If DBT is already at or below WBT + offset that hour, the air is already cool enough — the system does nothing and the hour is skipped.
Step 5 — Find the exit humidity ratio (ω₂) using enthalpy
This is the key step. Because the evaporation happens with no external heat added or removed, the total enthalpy stays constant — this is called an isenthalpic process. So h₂ = h₁. We solve for the new humidity ratio at the cooled temperature:
ω₂ = (h₁ − 1.006 × T₂) / (2501 + 1.86 × T₂)
The enthalpy is unchanged, but the air is now cooler (T₂) and wetter (ω₂ > ω₁) — sensible heat has been converted to latent heat by the evaporating water.
Step 6 — Calculate water consumed this hour
The difference in humidity ratio (Δω = ω₂ − ω₁) tells us how much water was added per kg of dry air. We multiply by the mass flow rate of air through the condenser:
ṁ_air = airflow (m³/hr) × density (kg/m³) → kg dry air / hr
Water (L/hr) = Δω (kg/kg) × ṁ_air (kg/hr) = kg water/hr = L/hr
(water density is 1 kg/L, so kg/hr and L/hr are the same number)
Step 7 — Calculate energy saved this hour
The temperature drop ΔT = DBT − T₂ reduces the condensing temperature of the chiller. For every 1°C reduction, chiller power drops by roughly your entered efficiency gain (default 2%). So:
Energy saved (kWh) = Chiller capacity (kW) × baseline (kW/kW) × (ΔT × gain% / 100)
Step 8 — Sum up all 8,760 hours
We repeat steps 1–7 for every hour of the year and add them all up to get annual totals: total water consumed, total energy saved, and total CO₂ avoided (energy saved × your grid carbon intensity).