What Is Evaporative Cooling?

Evaporative cooling is a thermal process that lowers the temperature of an air stream by converting sensible heat into latent heat as water evaporates into it, and the devices that exploit it are known as evaporative coolers. Because vaporizing water absorbs roughly 2,450 kilojoules per kilogram at ordinary ambient temperatures, a small quantity of water can remove a large quantity of heat without any refrigerant, compressor, or vapor compression cycle. The approach predates mechanical refrigeration by millennia in the form of porous water jars and wetted screens, and it survives in modern practice because its electrical demand is a fraction of that of a comparable air conditioner.

The governing limit is psychrometric. In an ideal adiabatic saturation process the air moves along a line of constant wet-bulb temperature, so the wet-bulb temperature of the entering air sets the lowest temperature achievable. The available temperature drop is therefore the wet-bulb depression, the gap between dry-bulb and wet-bulb readings, which is large in arid climates and nearly zero in humid ones. Saturation effectiveness, defined as the achieved drop divided by the wet-bulb depression, is the standard figure of merit and typically ranges from about 0.7 to 0.9 for well-maintained media.

Direct Evaporative Cooling

In a direct evaporative cooler, outdoor air is drawn through a wetted porous medium, usually corrugated cellulose or rigid fiber pads kept damp by a recirculating pump, and delivered straight to the conditioned space. The air leaves cooler and considerably more humid, so these units are operated with continuous outdoor air exchange rather than recirculation, and windows or relief openings must be provided to exhaust the moist air. Guidance from the Building America Solution Center at Pacific Northwest National Laboratory places good performance in climates where the 1 percent summer design wet-bulb temperature is below 70 degrees Fahrenheit. Water consumption is a genuine design constraint, since bleed-off and periodic flushing are needed to prevent mineral scale as dissolved solids concentrate in the sump.

Indirect and Two-Stage Systems

Indirect evaporative coolers separate the two air streams with an air-to-air heat exchanger. A secondary or scavenger stream is evaporatively cooled and then absorbs heat from the primary stream through the exchanger wall, so the supply air is cooled without any humidity gain. The penalty is a lower effectiveness for a given wet-bulb depression, commonly around 0.6 to 0.7, because the process is limited by heat exchanger performance as well as by psychrometrics. Two-stage designs place an indirect stage ahead of a direct stage, pre-cooling the air so that the direct stage begins from a lower wet-bulb temperature and adds less moisture for a given drop. Regenerative configurations go further by diverting part of the already cooled product air into the wet channel, allowing the supply temperature to approach the dew point rather than the wet bulb. The Maisotsenko cycle is the best known implementation of this dew point approach.

Cooling Towers and Data Center Cooling

The same physics operates at industrial scale in wet cooling towers, where warm condenser water is distributed over fill material and cooled by an induced or forced draft of air, rejecting heat from power plants, refineries, and chiller plants. Data centers have become a significant adopter of indirect evaporative systems because they need year-round heat rejection and can tolerate relatively high supply air temperatures under current thermal guidelines. A review of evaporative cooling in data centers describes indirect air-side arrangements in which outdoor air cools the recirculated room air across a heat exchanger, with evaporative assist engaged only when dry operation cannot meet the setpoint. The engineering trade is water consumption against electricity, along with control of Legionella risk and mineral fouling in any wetted system.

Applications

Evaporative cooling has applications in fields including:

• Residential and commercial air conditioning in arid climates
• Data center and telecommunications facility cooling
• Power plant and process cooling towers
• Greenhouse and livestock building climate control
• Gas turbine inlet air cooling to raise output
• Produce storage and industrial spot cooling