Evaporative Cooling

Overview

Engineering reference data for Evaporative Cooling in air psychrometrics.

Key Formulas

Humidity Ratio

$\omega = 0.622 \frac{P_v}{P_a}$

Mass of water vapor per mass of dry air.

Relative Humidity

$\phi = \frac{P_v}{P_{vs}} \times 100\%$

Ratio of actual to saturation vapor pressure.

Wet Bulb Temperature

$T_{wb} = T_{db} - \frac{P_{vs} - P_v}{\gamma}$

Temperature measured by wet-bulb thermometer.

Enthalpy of Moist Air

$h = c_p T + \omega h_g$

Sensible + latent heat per unit mass of dry air.

Variables

Direct Evaporative Cooling Systems

In a direct evaporative cooling system, outside air is pulled through a water-saturated medium (typically cellulose) or air is sprayed with water, and cooled by evaporation. The cooled air is then circulated by a blower.

Effects on Air Properties (Direct System):

• Dry bulb temperature is reduced
• Wet bulb temperature is unchanged
• Relative humidity is increased
• Specific humidity is increased

Indirect Evaporative Cooling Systems

In indirect evaporative cooling, a secondary air stream is cooled down by evaporating water. This secondary air stream is then used to cool the primary air in a heat exchanger. Moisture is not added to the primary air stream.

Effects on Air Properties (Indirect System):

• Dry bulb temperature is reduced
• Wet bulb temperature is reduced
• Relative humidity is increased
• Specific humidity is unchanged

Hybrid Evaporative Cooling Systems

Direct and indirect evaporative cooling units can be combined in hybrid systems with serial-connected indirect and direct evaporative units. Additionally, evaporative units can be combined with DX (Direct eXpansion) compressor cooling units.

Benefits of Evaporative Cooling Systems

• Economical — required size of DX compressor cooling system can be reduced
• Reduces DX system run time
• More environmentally friendly — no refrigerants required
• More effective than DX systems at higher temperatures

System Comparison

⚠️ Legionella Warning: Any water system with an operating temperature between 69 °F (20.5 °C) and 122 °F (50 °C) is a potential legionella source. Ensure proper water treatment and system maintenance protocols are followed.

References

• Original Source