Evaporative cooling is a process that involves evaporating moisture 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 with the addition of moisture. This technology is a versatile and energy-efficient alternative or adjunct to compressor-based cooling. In favorable climates, which includes most of the western US and other dry-climate areas worldwide, evaporative cooling can meet most or all of a building’:s cooling load using as little as a quarter of the energy of conventional equipment. It can also be cost-effective to implement 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?
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 wet-bulb temperatures, a metric that incorporates both the temperature and the humidity of the air.
Because evaporative cooling requires a moving air stream, the amount of indoor air that’s exhausted from the building must be equal to the amount being supplied. If the amounts aren’t equal, the building will become pressurized, which leads to insufficient airflow in addition to 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% relative humidity in an evaporative process emerges at the wet-bulb temperature, the theoretical limit for direct evaporative cooling; the effectiveness of such a (rare) cooler would be 100%. Although older coolers typically achieved 50% to 80% effectiveness, properly functioning and well-designed systems with thicker media—at least 10 inches—can achieve 93% effectiveness.
Indirect cooling. Indirect evaporative coolers use the evaporative-cooling process without adding moisture to the building supply air (figure 1). This makes them suitable for a wider range of applications, 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, and thus can produce cooler air than either stage can produce 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 isn’t possible with direct evaporative cooling alone. Because the two-stage approach introduces less moisture to the air than direct evaporative cooling does, it can be used in more building types; however, because IDECs still rely on the evaporative-cooling process, they work best in dry climates. To meet peak-cooling needs, especially on humid days, enlist the help of an HVAC designer to properly specify system components.
Condenser air precoolers This type of evaporative cooler has been available for many years for 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. For example, a precooler with 70% effectiveness can deliver 10% total energy savings and 20% peak demand savings; a precooler with 50% effectiveness can deliver 8% total energy savings and 15% peak demand savings.
The DualCool The DualCool, which is intended for packaged rooftop units of 15 tons or larger, employs both direct and indirect cooling approaches. 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 best in drier climates. Originally designed by the Davis Energy Group, an HVAC consulting firm, it’s now offered by Integrated Comfort Inc.
A 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 annual energy savings of 24% and 16% and demand savings of 0.43 and 0.19 kilowatts per ton, respectively.
The EER+ The EER+ is a heat-exchange module that can be attached to 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).
As demonstrated in figure 2, 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 using the exhaust air from the building, as outdoor humidity won’t significantly affect the heat exchangers. However, when using outdoor air in humid climates, the efficiency increase won’t be as great as it is in dry climates.
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 even larger capacities are possible by connecting multiple units. Payback periods vary based on the cooling load of the building.
How to make the best choice
Assess the potential for evaporative cooling in your climate Average July daily wet-bulb and dry-bulb temperature ranges for 13 US cities are shown in figure 3. Dry-bulb temperature is what a standard thermometer indicates. Wet-bulb temperature accounts for the moisture content of the air. The wet-bulb temperature is always lower than or equal to the dry-bulb temperature. When the wet-bulb temperature and the dry-bulb temperature are equal, it means that the air is saturated—it has reached the dew point—and can’t hold additional moisture.
In five of the cities shown (Albuquerque, Boston, New York, Salt Lake City, and Tucson), the average wet-bulb range is fully below the dry-bulb range, and in two others—Denver and Chicago—the wet-bulb range falls just below the dry-bulb range without overlapping; in all seven of these cities, the wet-bulb range is below 70° Fahrenheit (F). The arid climates of Albuquerque, Salt Lake City, Denver, and Tucson make them excellent locations for evaporative cooling. In more-humid locations like Boston, Chicago, 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.
The five cities with wet-bulb ranges extending to or below 55°F are all in the West in regions that are ideal for evaporative cooling, although Seattle’s low dry-bulb temperature range means cooling loads can usually be satisfied with outdoor air. Locations with average July wet-bulb temperature ranges extending above 70°F (Atlanta, Houston, and Miami) aren’t good candidates for evaporative cooling in July.
Consider eliminating compressor-based cooling. The most-promising prospects for completely eliminating compressor-based cooling are in high-altitude climates that have dry air and lower summer daytime temperatures, like in Denver (figure 3). In very hot summer climates like that of Phoenix (not shown), where afternoon July wet-bulb temperatures often exceed 75°F, there are times when even a good two-stage evaporative cooler can’t cool air to desired indoor temperatures without exceeding the relative humidity limits set by ASHRAE. 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 can’t meet the entire cooling load, it can reduce energy use 60% annually compared to the baseline when coupled with a packaged rooftop unit system. For this reason, combined direct/indirect/mechanical cooling systems usually have a much smaller compressorized refrigeration capacity than a system where there’s no evaporative system.
Who are the manufacturers?
More than 40 companies manufacture evaporative cooling equipment. Here are some of the leading suppliers:
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