Indirect Evaporative Cooler Leaving Temperature
The leaving air temperature from an indirect evaporative cooling stage, which cools without adding moisture.
Example
You enter
- Entering product dry-bulb (°F) 95
- Secondary-stream wet-bulb (°F) 65
- Indirect effectiveness (0-1, ~0.5-0.75) 0.65
You get
- Leaving dry-bulb (dry, no moisture added) 75.5 F
- Temperature drop 19.5 F
- Wet-bulb depression (max drop) 30 F
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method.
Details, formula, and sources
ScopeAn indirect exchanger cools the product air SENSIBLY (no moisture added) toward the secondary (scavenger) stream's wet-bulb: T_out = T_ent - eff x (T_ent - T_wb,secondary), eff ~0.5-0.75. 95 F product air against a 65 F secondary wet-bulb at eff 0.65 leaves at 75.5 F but bone dry, where a direct pad would reach ~69 F near saturation. Because the product air stays dry, a direct pad downstream (a two-stage/IDEC machine) cools below the direct-only result without the humidity penalty. Standard indirect approaches the secondary wet-bulb; a dew-point (Maisotsenko) design is separate. The equipment data govern.
Formula or tableT_out = T_ent - indirect_effectiveness x (T_ent - T_wb,secondary); temp_drop = eff x (T_ent - T_wb,secondary); wet_bulb_depression = T_ent - T_wb,secondary.
Source and editionASHRAE Handbook (HVAC Systems & Equipment, Ch. 41) indirect-evaporative sensible-effectiveness relation, by name; classical psychrometrics.
Free accessThe sensible-effectiveness relation is the direct analog already used for the direct-evap tile; published in engineering texts. The exchanger effectiveness is a manufacturer rating.
What governsEstimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.
API field namesdry_bulb_F, secondary_wet_bulb_F, effectiveness, leaving_db_F, temp_drop_F, wet_bulb_depression_F
- Sensible effectiveness eff = (T_ent - T_out)/(T_ent - T_wb,secondary); user-entered, ~0.5-0.75ASHRAE / manufacturer rating
- No moisture added the product air is cooled at constant humidity ratio (sensible only)psychrometrics of an indirect exchanger
- Secondary wet-bulb floor the leaving dry-bulb approaches but cannot fall below the secondary stream's wet-bulbscope of this tile