Free Evaporative Cooler & CFM Sizing Tool
Calculate leaving supply air temperature from ambient wet-bulb depression, required airflow CFM for room volume, sensible cooling BTU/hr, and water evaporation rate.
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📊 Supply Air Temp & Airflow Sizing
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The Thermodynamics of Direct Evaporative Cooling
Direct evaporative cooling operates along lines of constant wet-bulb temperature (isenthalpic humidification). As warm, dry outdoor air passes across water-saturated corrugated cellulose or aspen pads, liquid water absorbs sensible heat from the air to supply the latent heat of vaporization (~1060 BTU/lb of water evaporated), causing the dry-bulb temperature to drop while moisture content rises.
Leaving Supply Temperature Equation
The leaving air dry-bulb temperature is dictated by the Wet-Bulb Depression ((T_{db} - T_{wb})) and the media Saturation Effectiveness (( arepsilon_{wb})):
$$T_{supply} = T_{db,in} - arepsilon_{wb} cdot (T_{db,in} - T_{wb,in})$$
In hot, arid climates where ambient dry-bulb is 95°F and wet-bulb is 65°F (a 30°F depression), an 85% effective rigid media pad achieves a temperature drop of (0.85 imes 30 = 25.5^circ ext{F}), delivering fresh 69.5°F supply air at a fraction of the electricity cost of compressor-based DX refrigeration.
Frequently Asked Questions
Why must windows be opened when running an evaporative cooler?
Unlike refrigerated air conditioning which recirculates air, evaporative coolers are 100% outside air systems. If doors or windows are not cracked open to exhaust the moist air, humidity inside the building rises rapidly to 90%+, halting evaporation and making the room warm and clammy.
How is media pad face velocity controlled?
Media face velocity should be kept between 250 and 300 feet per minute (FPM). Velocities exceeding 350 FPM create excess air resistance and shear liquid water droplets off the pads into the ductwork, causing mildew.