Modern Evaporative Coolers Not All Evaporative Coolers Are Created Equal
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Modern Evaporative Coolers Not all evaporative coolers are created equal. Modern machines operate more efficiently and with much less fuss than the old swamp coolers did. ADOBE ADOBE Here is an inside view (left) and an outside view (right) of a modern evaporative cooler by Adobe. by Larry Kinney dampers, also known as up ducts, in the fer the better comfort and fresh, clean ceilings of rooms on the top story. The air our systems give them. The $300 we ave Emmitt owns Direct Drive process is controlled by a multifunction save them on their utility bill each sum- Service, a company in Colorado thermostat that has the smarts to throttle mer just pays for a barbeque with 50 of Dthat specializes in efficient low- back the fan speed when the setpoint their closest friends.” mass boilers—and high-efficiency evap- temperature is about to be met, rinses Emmitt’s retrofit business is also doing orative coolers. We’re not talking swamp out the reservoir to keep water and air quite well. Colorado homeowners who coolers on the roof of a mobile home; clean, and partially automates much of have never had cooling systems generally Emmitt’s staff install coolers in the attics the end-of-season maintenance. tend to install conventional A/C of large, site-built homes. The coolers “We install modern evaporative systems—as do those who are pull in air from large gable vents, cool it cooler systems in new homes that cost disenchanted with their old swamp cool- by 30°F or so, and distribute it via several $2 million,”reports Emmitt. “These ers. But if Emmitt gets to them first, they large ducts, typically to a hallway below. people can afford conventional are more likely to upgrade to one of the Cool air is directed by the patterns of compressor-based cooling and the high-quality evaporative coolers he window openings or by backdraft higher bills that go with it, but they pre- installs, which deliver two or three ACH 24 www.homeenergy.org SEPTEMBER/OCTOBER 2004 • HOME ENERGY HVAC and wash the pollen out of that air before tage of evaporative cooling effects, but tures an indirect/direct evaporative it gets inside. Emmitt is enjoying a grow- use an air-to-air heat exchanger to cool cooler, called the OASys, that was devel- ing customer base, fueled largely by word without raising the humidity of the oped by the Davis Energy Group in of mouth from happy customers. cooled airstream. Indirect/direct coolers Davis, California (see Figure 1). cool in two stages. In the first stage, the The OASys system uses a single Evaporative Cooling air passes through an indirect cooler, blower that pulls in outside air and directs Innovations which lowers the temperature without most of it—about 70%—through the dry adding humidity.The air that enters the side of a heat exchanger that uses 14- Emmitt wouldn’t be getting the same second, direct cooling stage is already inch-thick media to efficiently and indi- good word of mouth if he were rectly cool the airstream without selling old-fashioned swamp adding moisture. This partially coolers, but modern evaporative OASys Air Flow cooled air then passes through a coolers are several technological direct-cooling module before leaps ahead of those old being directed into the home. machines.All evaporative cool- About 30% of the outside ers—old and new—rely on the airstream is used in the other, same design principle:Water can wet side of the counterflow heat be used to cool air.Air blowing exchanger, where it is cooled, through a wet medium—a tee gathers moisture, and is then dis- shirt, aspen fibers (excelsior) or charged to the outdoors. Water treated cellulose, fiberglass, or from both the indirect- and the plastic—evaporates some of the direct-cooling processes gathers water and its dry bulb tempera- in a single reservoir, which is ture is lowered (see “Wet’s the purged with a frequency that is Difference: Dry Bulb and Wet tied to the amount of scale-caus- Bulb Temperatures”). The cool- ing minerals in local tap water ing effect of an evaporative and the rate of water use by the cooler depends on two factors. system. The rate of water use in The first is the local difference DAVIS ENERGY GROUP turn depends on the blower between the air’s dry bulb and speed, which is controlled by a wet bulb temperatures; the sec- multifunction thermostat. ond is the cooler’s efficiency This machine incorporates a (see Table 1, p. 27). Exactly how Figure 1. The OASys system employs a single cabinet that houses all its parts. number of improvements over an evaporative cooler takes This simplifies the overall design, helps maintain tolerances, shortens assembly earlier indirect/direct evapora- advantage of water’s chilling time, and ensures a long lifetime. tive coolers designed for effect—the pathway and the residential use. It employs a sin- velocity of the air as it passes gle polyethylene cabinet that over the media combined with the con- substantially cooler than the outside houses all parts of the system. This sub- dition of the media—determines how air, and it can be cooled a good deal stantially simplifies the overall design, efficient it is as a cooling device. more because it’s still largely dry.Com- helps maintain tolerances, shortens Today’s evaporative coolers come in bining these two techniques enables assembly time, and ensures a long one of three general designs: direct, indi- efficient indirect/direct units to deliver lifetime. The OASys also uses an rect, and indirect/direct. Modern direct air that is cooler than the outside wet electronically commutated motor evaporative coolers couple high-perfor- bulb temperature. (ECM) controlled by a smart mance media—generally plastic-coated After losing a good deal of market thermostat, so blower speed can be cellulose—with low-velocity air flow. share to conventional cooling, there’s changed while maintaining high motor This combination maximizes moisture optimism in the evaporative cooling efficiency. This is important, because transfer from the wet media to the hot, industry that higher-quality units will overall system efficiency of the OASys is dry air.The effectiveness with which an lead the way to recovery (see “Copious best at low fan speeds and low air flow appliance makes this transfer—known in Refinements,”p. 28). One particularly rates (see Figure 2). the trade as its direct saturation effective- promising new design is being manufac- Engineers at the Davis Energy Group ness—is a key measurement of the appli- tured by Speakman CRS—short for took these plots of efficiency at different ance’s cooling efficiency. Clean, Renewable, Sustainable—a branch flow rates and other test results—such as Direct evaporative coolers humidify of the Speakman Company,a firm that temperatures delivered under different the incoming air, making them inappro- has been producing showerheads and conditions of wet and dry bulb tempera- priate for use in humid climates. other water-related products for more tures—and performed simulations of a Indirect evaporative coolers take advan- than 130 years. The company manufac- very efficient 1,600 ft2 home in eight of HOME ENERGY • SEPTEMBER/OCTOBER 2004 www.homeenergy.org 25 HVAC California’s climate zones.The results for Fresno are most pertinent for the OASys Measured Performance at Southwest, as Fresno has a hot, arid cli- Three Supply Air Flow Rates mate not unlike many locations in the 160 Southwest (1% dry bulb 101ºF,mean 140 coincident wet bulb 70ºF).The base case home with a conventional DX air con- 120 ditioning system rated at 12-SEER uses 100 1,890 kWh per year with a peak of 3 kW,while the OASys uses 135 kWh per 80 year with a peak of 0.52 kW. This 60 amounts to an annual energy savings of 93% and a peak demand savings of 83%. 40 In another evaporative cooling devel- 20 opment,AdobeAir has recently Btus Delivered/Watt Primary Btus Delivered/Watt Power introduced its own series of efficient 0 evaporative coolers that fit in a sidewall 0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 LARRY KINNEY (see Figure 3).These units are easy to Supply Air Flow (CFM) install either in new dwellings or as retrofits and have clear aesthetic and Figure 2. The data gathered was at entering dry bulb temperatures of 104ºF, with the unit supplying dry bulb tem- mechanical advantages over rooftop or peratures of 68ºF. Power plotted is the sum of fan and pump power. The vertical axis shows overall efficiency in that concrete-pad-mounted coolers. it expresses the ratio of cooling Btu’s out versus the combined power use of the pump motor (20 watts or so) and that of the power consumed by the fan motor. The point of the graph is that spectacular efficiencies are achieved at lower flows, both because of the overall design of the device and the fact that its motor is quite efficient at low Water and Energy Use speeds—unlike most motors used in the evaporative cooling industry. (Source: Davis Energy Group.) There’s no getting away from the fact that an evaporative cooler relies on, and gallon of water would yield roughly needed to flush out the residue of air pol- so consumes, a significant amount of 8,800 Btu of evaporative cooling. If the lutants and scale in the water. In water, but the newer units use up much flow of water and the flow of air are well inefficient units, water that is not evapo- less than the old-fashioned coolers do.