Evaporative Cooling

Evaporative cooling is a process by which moisture is evaporated into an air stream in order to lower the air’s temperature. The resulting air stream can be used to cool a building, other air streams, or the components of an air-conditioning system. The lower the relative humidity, the greater the possible cooling effect when moisture is added. This technology is a versatile and energy-efficient alternative or adjunct to compressor-based cooling. In favorable climates (most of the western United States and other dry-climate areas worldwide), evaporative cooling can meet most or all building cooling loads using as little as one-fourth the energy of conventional equipment. It can also be applied cost-effectively when integrated with conventional chiller systems. Using evaporative technology can also improve a facility’s load profile because it reduces the load associated with air conditioning, which often sets demand peaks.

What are the options?
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Several evaporative cooling options are available, including direct, indirect, and two-stage evaporative coolers; condenser air precoolers; the DualCool; and the EER+.

Direct cooling. Direct evaporative coolers blow air over a wet surface. Heat in the air evaporates moisture from the surface, thereby lowering the air temperature (Figure 1). Although these systems typically use less than a quarter of the energy that vapor-compression air conditioners do, they’re often restricted to industrial or warehouse applications in drier climates because they add moisture to the building air supply. Their suitability for a particular application depends on the cooling load and the range of outdoor wetbulb temperatures (a metric that incorporates both the temperature and the humidity level of the air).

Figure 1: Direct and indirect evaporative coolers
A direct evaporative cooler adds moisture to the building supply air, whereas an indirect evaporative cooler does not. A two-stage, or indirect-direct, evaporative cooler uses an indirect stage first before passing the building supply air through a direct stage. The darker-colored arrows indicate that moisture has been added to the air stream.

Because evaporative cooling requires a moving air stream, the amount of indoor air that is exhausted from the building must be equal to the amount being supplied. If the amounts are not equal, the building will become pressurized, which leads to insufficient airflow plus difficulty closing doors and air whistling through stairwells and elevator shafts. When comparing direct evaporative coolers, the most relevant metric to use is the effectiveness of the unit. Effectiveness is a term that quantifies, as a percentage, how close to the wetbulb temperature the unit can reach. Air that reaches 100 percent relative humidity in an evaporative process emerges at the wetbulb temperature, the theoretical limit for direct evaporative cooling. The effectiveness of such a (rare) cooler would be 100 percent. Achieving more than 90 percent effectiveness in a direct cooler is difficult, requiring a very thorough mixing of water and air. Most evaporative air coolers operate with 70 to 90 percent effectiveness using wetted fibrous or corrugated pads or media.

Indirect cooling. Indirect evaporative coolers use the evaporative cooling process without adding moisture to the building supply air (see Figure 1). This makes them suitable for a wider range of applications, including offices, and they can be combined with traditional compressor-based systems. Indirect evaporative coolers can take a couple of forms:

  • Self-contained. The building supply air (or primary airflow) flows through a heat exchanger. The building exhaust air (or secondary airflow) is evaporatively cooled and passed through the other side of the heat exchanger, thereby removing heat from the supply air. This approach can be used in many climates because the outdoor humidity levels don’t significantly affect the evaporative cooling process.
  • Tower/coil approach. Often called a water-side economizer, this approach uses a cooling tower to produce cool water that’s fed to a separate finned cooling coil in the supply air stream. The cooling tower could be part of an existing water-cooled chiller plant.

Two-stage cooling. Two-stage evaporative coolers—also called indirect-direct evaporative coolers (IDEC)—employ both indirect and direct stages, as the name implies, and thus can produce air cooler than is possible with either stage alone. The first stage uses an indirect section to cool the air without adding moisture. The air is then directly evaporatively cooled in the second stage. This produces air at a temperature lower than the outdoor wetbulb temperature, which is not possible with direct evaporative cooling alone. Because the two-stage approach introduces less moisture to the air than direct evaporative cooling alone, it can be used in more building types, but because IDECs still rely on the evaporative cooling process, they work best in dry climates. To ensure that peak cooling needs are met, especially on humid days, enlist the help of an HVAC designer to properly specify system components. One packaged IDEC is the OASys from Speakman CRS, which can be used in small commercial and residential applications.

Condenser air precoolers. This type of evaporative cooler has been available for many years for both large and small air-cooled systems. Large units typically use flat, rectangular rigid-media blocks with a sump and pump placed over the intake side of the condenser coil. These types of systems can theoretically provide much of the same savings as evaporative condenser air conditioners, although there’s no independent research to quantify their savings potential.

The DualCool. The DualCool employs two approaches in one design intended for packaged rooftop units of 15 tons or larger. It uses a direct evaporative cooler to precool the condenser air and an indirect evaporative cooler to precool the building supply air. As with other evaporative coolers, the DualCool works better in drier climates. Originally designed by the Davis Energy Group, an HVAC consulting firm, it’s now offered by Integrated Comfort Inc.

A 2003 study by the Heschong Mahone Group consulting firm provides some savings estimates for the DualCool: The study estimates that units in Fresno, California (a hot, dry climate), and Santa Rosa, California (a milder, more humid climate), delivered, respectively, annual energy savings of 24 and 16 percent and demand savings of 0.43 and 0.19 kilowatts per ton.

The EER+. The EER+ is a heat-exchange module that can be attached to both existing air-cooled air conditioners and heat pumps to increase their efficiency. Manufactured by Global Energy Group, the module works by capturing waste condensate water from the rooftop unit and routing it over evaporative cooling pads; exhaust air or outdoor air is blown across the pads (Figure 2).

Figure 2: How to evaporatively cool an air conditioner
In the EER+ module, an evaporative cooling pad uses condensate water to subcool and desuperheat the refrigerant.

The resulting evaporative cooling removes heat from the air-conditioner refrigerant after the compressor and subcools it after the condenser—thereby increasing the efficiency and capacity of the system. The EER+ works in most climates if the exhaust air from the building is used; outdoor humidity will not significantly affect the heat exchangers. However, when using outdoor air in humid climates, the efficiency increase will not be as great as it is in dry climates.

The EER+ system can boost energy savings by as much as 40 to 50 percent, but the efficiency gains depend on the efficiency of the existing system: The lower the efficiency of the existing system, the more benefit the EER+ can offer. The EER+ costs from $400 to $1,100 per ton installed, depending on the size of the unit (smaller units are more expensive per ton). It’s available in capacities from 6 to 100 tons, and larger capacities can be accommodated by connecting multiple units. Paybacks vary based on the cooling load of the building.

How to make the best choice
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Assess the potential for evaporative cooling in your climate. Figure 3 shows average July daily wetbulb and drybulb temperature ranges for 13 U.S. cities. (Drybulb temperature is what a standard thermometer indicates. Wetbulb temperature accounts for the moisture content of the air. The wetbulb temperature is always lower than or equal to the drybulb temperature. When the wetbulb temperature and the drybulb temperature are equal, it means that the air is “saturated”—it has reached the dew point—and cannot hold additional moisture.) In five of the cities shown (Albuquerque, Boston, New York, Salt Lake City, and Tucson), the average wetbulb range is fully below the drybulb range, and in all five the wetbulb range is below 70° Fahrenheit (F). The arid climates of Albuquerque, Salt Lake City, and Tucson make them excellent locations for evaporative cooling. In more humid locations like Boston and New York, evaporative cooling may be used in dry weather, but will need to be supplemented by compressor-based cooling in hot, humid weather.

Figure 3: Daily average wetbulb and drybulb temperatures in July for selected cities
Cities where the wetbulb range is fully below the drybulb range are excellent candidates for evaporative cooling of commercial buildings.

The five cities with wetbulb ranges extending to or below 55°F are all in the West in regions that are ideal for evaporative cooling. (However, Seattle’s low drybulb temperature range means cooling loads can usually be satisfied with outdoor air.) Locations with average July wetbulb temperature ranges extending above 70°F (Atlanta, Houston, and Miami) are not good candidates for evaporative cooling in July.

Consider eliminating compressor-based cooling. Prospects for completely eliminating compressor-based cooling are best in high-altitude climates that have dry air and lower summer daytime temperatures, as represented by Denver in Figure 3. In very hot summer climates like Phoenix (not shown), where afternoon July wetbulb temperatures often exceed 75°F, there are times when even a good two-stage evaporative cooler cannot cool air to desired indoor temperatures without exceeding the relative humidity limits set by ASHRAE (the American Society of Heating, Refrigeration, and Air-Conditioning Engineers). However, in those climates, direct or indirect evaporative cooling (or a combination of the two) can usually satisfy full cooling loads for 10 months of the year and can be applied to ventilation air all year long.

Cost-effectiveness in these locales depends on local utility rates, the duration of the cooling season, cooling load patterns, and ventilation air quantities. In addition, even when a two-stage evaporative cooler cannot meet the entire cooling load, it can reduce the load that is met by the compressor by 50 to 80 percent. For this reason, combined direct/indirect/mechanical cooling systems usually have a much smaller compressorized refrigeration capacity than a system where there is no evaporative system.

Who are the manufacturers?
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More than 40 companies manufacture evaporative cooling equipment. Here are some of the leading suppliers.

Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by E Source, nor does any content the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of your service provider’s various technology-related programs.
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