Water is used in two different ways in EAC’s. Primarily water is used as the EAC working fluid, so it is dissipated in the evaporation process. this water use will vary with the type of media used, supply air volume, temperature and humidity content of the outside air and the number of operating hours. When the air is hot and dry, more water will be used and the evaporative cooling effect is the greatest. The second way water is used in an EAC is to control the buildup of solids that are naturally in the water. This bleed-off method will dilute the sump water in order to wash away the dust and other particulate caught by the wet media, and to prevent the buildup of solids that may be in the water supply. As discussed in the Environmental Considerations section, there is some variability of the water use of an EAC system. Factors that cause this are the existence of a bleed-off system, the rate of bleed-off dilution, and the method of controlling solids buildup.
There are three options for water bleed systems. They are presented in order of increasing water use:
1. No water bleed off.
2. Intermittent water bleed off. 3. Continuous water bleed off.
The no-bleed option may be practical in areas with little naturally occurring minerals, and on aspen media EAC’s used in areas that are experiencing a drought. Since the aspen media has a low replacement cost, it can be replaced periodically depending on water conditions. In addition, some thin-pad EAC media is washable as described in the section on EAC media.
A common alternative to a continuous water bleed that can reduce overall EAC water use is using an additional sump pump, which is periodically energized to completely empty the sump. This has the advantage of using fresh rather than the more concentrated sump water, and a more complete flush of the accumulated dust. The manufacturer states that use of a “Clean Machine” sump dump pump with integral controller will “use almost 70% less water than bleed-off systems”21. Another more expensive method of flushing the sump water intermittently uses water conductivity sensors to initiate the flush when a predetermined value is sensed.
The continuous bleed method uses more water but is the simplest method of solids control. An accepted bleed rate for an aspen pad EAC is known as the “beer can” method because it uses 12 oz. per minute. This water can be routed to a plumbing vent on the roof, but is usually routed to the area landscaping for water conservation purposes.
Clean sump water is important for maintaining consistent cooling, good IAQ, and extending the service life of the evaporative media. This is discussed in more detail in the Maintenance Section. The following test information on water use was compiled in New Mexico, and compares EAC water use for seven different systems.
“Mineral buildup on evaporative cooling pads directly decreases life and the saturation effectiveness of the pads. Most direct evaporative coolers recirculate their pan water, while increasing the concentration of dissolved minerals. Typically, a bleed off system is employed with these coolers to remove minerals as they accumulate. Obviously, a bleed off system will also increase overall water consumption of an evaporative cooler. At some point, increased water consumption has little effect on reducing scale. The goal is to optimize scale control while using a minimum of water for such purposes.
Testing of seven direct evaporative coolers with different water supply systems was conducted by the Southwest Technology Development Institute (SWTDI) during the summer of 1991 in Las Cruces, New Mexico. The purpose of the tests was to determine the water consumption and pad-scaling of the different bleed off systems under normal operating conditions for the cooling season climatic conditions of southern New Mexico area under normal operating conditions for a greenhouse and warehouse. The data in this report shows how seven direct evaporative coolers operating at the same time under similar conditions performed with regards to water consumption and total scale deposition.
In Figure 3, measured spot checks of bleed off rates are shown over the summer. Cooler #7 bleed off rates increased over the summer, while cooler #1 bleed off rates dropped over the summer. Both units had identical standard bleed systems and demonstrated how these systems can fluctuate radically. Cooler #6 did not have a bleed off, but the water rates shown were spot checks of water quantities coming off the pad while the solenoid valve was open. Cooler #5 did not have a bleed off and flushing rates for cooler #4 were not measured.”22
Adequate water flow over the evaporative media is an important factor in maintaining the maximum saturation effectiveness and extending media life. Inadequate flow will result in accelerated mineral buildup on the media and shorter media life. “With many fewer pump sizes than cooler models, water flow in pads is extremely variable. Excess wetting raises pumping costs slightly but cools better and helps wash dust into the sump, postponing clogging. Inadequate wetting, however, both reduces saturating efficiency and accelerates clogging. As noted, wherever individual fibers evaporate water faster than their rewetting rate, scale will be deposited. Because evaporating rates vary inversely with the entering air humidity, water-needs change widely with the weather, though pump outputs are almost constant. Optimum water rates are the minimum flows that ensure wetting all strands in driest weather at greatest airflows. They can be determined by simple tests. Because saturating efficiencies are governed by cooler design, adjustment, and condition, not weather, coolers whose saturating efficiencies fall in hot dry weather necessarily have strands that dry out, showing that their water flows are either inadequate or maldistributed. Constricted water lines, dirty troughs, or excelsior-dammed pump inlets may be responsible; if not, larger pumps may be needed. In general, tall, narrow pads show lower relative water needs than do broad ones whatever the air conditions and flow-rate… Thus, needed pump capacity to prevent any related drying in extreme weather is, for the assumed 6000 cfm output … is 6.0 gpm. That flow approximates 0.113 lbs of water per lb. of air, which greatly exceeds common usage. However, it should wash all dust from pads, postpone virtually all lime deposits in them, probably double the pad life, admit much less dust and pollen indoors and guarantee optimum cooling in all weathers, including the worst.” 23