About & methodology

Swamp Cooler Math is a small, independent set of free calculators and guides about evaporative cooling (swamp coolers, desert coolers, evaporative air coolers). We are not HVAC contractors and do not sell equipment. We publish every formula and cite our sources so you can check the work. If you find an error, email 55paz.power@gmail.com and we will fix it.

Psychrometrics

All moist-air properties are computed in SI units with equations from the ASHRAE Handbook—Fundamentals (2017), Chapter 1:

• Air pressure from elevation: standard atmosphere, p = 101.325 (1 − 2.25577×10⁻⁵ Z)5.2559, giving p in kPa with Z in metres. The code works in pascals (Pa; 1 kPa = 1,000 Pa); for reference, 1 psi = 6.895 kPa and 1 inHg = 3.386 kPa. Actual pressure varies with weather by a few percent, which the standard atmosphere ignores.
• Saturation vapour pressure: Hyland–Wexler equations (over water and over ice).
• Humidity ratio: W = 0.621945 pw / (p − pw).
• Wet bulb: solved numerically from the psychrometric (wet-bulb) equation. Dew point: inverse of the saturation-pressure equation.
• Specific volume: v = 287.042 (T + 273.15)(1 + 1.607858 W) / p m³/kg dry air, with p in Pa.
• Enthalpy: h = 1.006 t + W (2501 + 1.86 t) kJ/kg dry air.

An earlier version of this site used the Stull (2011) wet-bulb approximation, which assumes sea-level pressure and is accurate to about ±1 °C. The current version uses the full ASHRAE equations and accounts for elevation. Real-world results are still limited by the accuracy of the weather inputs.

Calculators

• Suitability: verdict bands use DOE Building America Solution Center guidance (design wet bulb ≤ 70 °F / 21.1 °C well suited; up to 74 °F / 23.3 °C reduced performance).
• Outlet temperature: Tout = T − ε(T − Twb). The outlet humidity ratio follows a line of constant wet bulb (adiabatic saturation).
• Water use (derived estimate): dry-air mass flow × increase in humidity ratio, plus bleed-off and dump-pump drains. Recirculated water is excluded. This is our own mass-balance derivation from standard psychrometrics, not a published or measured figure.
• Sizing: volume × air changes per hour (DOE: 20–40 ACH). Relief area 2 ft² per 1,000 CFM. Sensible capacity (derived estimate) = ṁ × c_p × (indoor − outlet), with c_p = 1.006 + 1.86 W kJ/kg·K. This is not an equipment rating.
• Operating cost: kWh × tariff, plus m³ of water × tariff, in your currency, with separate run-hours for the cooler and the AC.

How we test

The calculation code runs in automated tests on every build:

• Wet bulb, dew point, humidity ratio, specific volume, enthalpy and pressure are compared with PsychroLib, an independent open-source implementation of the ASHRAE equations (Meyer & Thevenard, Journal of Open Source Software, 2019). Fixtures cover 0–60 °C, 1–100% RH and 0–4,000 m. Our code matches PsychroLib to within 0.01 °C for temperatures. This shows our implementation reproduces the reference equations. It says nothing about real-world accuracy, which depends mainly on your temperature and humidity readings and on equipment condition.
• Results reproduce the worked examples published by the US DOE: 100 °F/10% RH → 63 °F wet bulb; 105 °F with 90% media → 64.5 °F; the 5,333 CFM sizing example.
• Water evaporation is cross-checked against an independent energy balance.
• Invalid inputs (out-of-range humidity, elevation, effectiveness, negative values, indoor target below supply air) are rejected.

Limitations

Results are estimates for steady conditions. They do not account for building heat loads, duct heat gain, air bypassing pads, partially dry pads, or equipment condition. Nothing here is professional HVAC design, electrical, plumbing or financial advice.

Disclosure

The site shows ads (see privacy). Ads never change a calculation. Any future affiliate links will be labelled.