This article was originally published in the May/June 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online May/June 1996
Installing and Maintaining Evaporative Coolers
by Roy Otterbein
Roy Otterbein is president of Otterbein Engineering in Phoenix, Arizona. He holds three patents in indirect evaporative cooling and is a member of the ASHRAE Standards Committees on direct and indirect evaporative coolers.
Evaporative coolers cost only one-tenth to one-fourth as much to operate as refrigeration air conditioning and are much cheaper to buy ($400-$800). This makes them an excellent option, particularly in hot, dry areas of the country.
An evaporative cooler is a simple device consisting of a fan and a water-wetted pad. A small pump recirculates water from a sump (which is part of the cooler cabinet) to keep the pad wet. The fan draws outside air through the wet pad, making the air more humid but colder. This air is blown into the house, forcing the warmer air in the house to be exhausted out open windows or through a vent into the attic (see Figure 1). This is quite different from refrigeration air conditioning, which cools inside air and returns it to the house.
Although the use of outside air is one major benefit of evaporative coolers, it also complicates their installation and operation. The evaporative cooler must be installed outside the house, ducted into the house, and freeze protected and isolated from the house during the winter months.
Figure 1. Evaporative coolers bring outside air into a house and exhaust it through open windows or ceiling-mounted barometric dampers.
Evaporative coolers are most popular in areas with the coldest summer wet-bulb temperatures, which tend to be in the western United States. Figure 2 shows 1% wet bulb isolines--the wet bulb temperature that is exceeded only 1% of the time during the summer months in a given area.
Figure 2. Wet bulb temperature isolines at the 1% level. Summer wet bulb temperatures indicate where evaporative coolers will work best. If an area has a 1% summer wet bulb temperature of 70°F or below, an evaporative cooler should be able to provide most of a house's cooling needs. However, they are still very popular in areas with wet bulbs between 70°F and 75°F.
A quick check for evaporative pad performance is to compare the temperature of the water in the cooler sump (approximately the wet-bulb temperature of outside air) to that of the air entering and leaving the cooler (see Figure 3). The following equation can be used:
Saturation effectiveness = ( TOSA-TLA)/( TOSA -TSUMP ) x 100where TOSA = Temperature of outside air entering cooler
TLA = Temperature of air leaving pad (air inside cooler)
TSUMP = Temperature of sump water
Figure 3. The saturation effectiveness of evaporative cooler pads can be checked with a thermometer. In this example, a cooler has 70°F air leaving the pad during a 100°F day, and has a sump water temperature of 60°F. Saturation effectiveness = (100-70) / (100-60) x 100 = 75%.
An alternate way to determine pad performance is to check the temperature of the air leaving the cooler against the cooler index (see Table 1). The cooler index is the anticipated temperature of the air leaving an aspen pad cooler (described below) and accounts for heat added to the air by the pump and fan motors and a cabinet exposed to the sun. The cooler index has the advantage of enabling the homeowner to check cooler performance by watching the news and checking the temperature of the air leaving the diffuser in the house. If this temperature is 3°F or more higher than the cooler index indicates as normal, the cooler is not operating as well as it should be, probably due to a malfunctioning water distribution system, sagging pads, or poorly manufactured aspen pads.
Table 1. Evaporative cooler index for standard aspen-pad coolers. Enter the table from the left with outside air temperature and the top with outdoor humidity. Where the row and column meet is the temperature of air a typical evaporative cooler will produce. The cooler index can also be used to check the performance of cooler pads.
Homeowners can also use the cooler index to decide when to switch between evaporative cooling and air conditioning. As a general rule, if a cooler produces air colder than 70°F, it will create a comfortable environment; if it produces air hotter than 75°F, it will not. Between 70°F and 75°F is a gray range in which some people are comfortable and some are not.
Evaporative Cooler Types
Fiber Pad CoolersThe most common pads are shredded aspen wood fibers packed in a plastic net. This material (also known as excelsior) was once widely used to ship delicate items like glassware. There are a number of synthetic-fiber pads; however, few perform as well as high-quality aspen pads, which have a naturally wettable surface. These pads are 1 to 2 inches thick. Quality and cost vary substantially; the least expensive pads are usually the thinnest. If thin pads are used, each pad frame should be double-packed, using two pads to improve saturation effectiveness.
Fiber pads must operate at low air velocities to prevent water from being pulled off the pad by the airstream. They are therefore used on coolers that have air inlets on many sides. The pads are simply discarded every year or two and replaced with new ones. Fiber pad coolers usually cost the least and require the most maintenance.
Rigid-Sheet Pad CoolersThe other main type of cooler uses a rigid-sheet pad--a stack of corrugated sheet material that allows air to move through at higher velocities than is possible with aspen pads. These pads are usually 8 or 12 inches thick. Twenty years ago they were found only in large expensive commercial coolers, but they are now common in residential coolers as well.
Coolers using rigid-sheet pads usually have a single air inlet (and are often referred to as single-inlet coolers). The pads have a corrugation pattern that forces water to flood the pad's air inlet side where most of the evaporation of water (and scaling) occurs (see Figure 4). These pads are substantially more expensive than aspen pads, but they can last for many years if water quality is properly maintained with a bleed-off or sump dump system (discussed below). Therefore the life cycle cost for these pads can equal the cost of aspen pads (not to mention the labor savings from not having to change the pads every year or two).
Two-Stage CoolersTwo-stage (also called indirect/ direct) evaporative coolers usually use a rigid pad and have an indirect evaporative precooler. The indirect coolers precool the air without adding humidity to the air going into the house. To understand this concept, imagine blowing air through the core of a pipe. Then sprinkle water on the outside of the pipe, and blow air across the pipe. The air inside the pipe is cooled by contact with the cool pipe, but it is not in contact with the water, so its humidity does not increase. Since this precooling adds no humidity to the air, it can still be subsequently direct-evaporatively cooled. However, because the precooled air cannot hold as much moisture, the result is both colder and drier air.
A rather startling feature of two-stage evaporative coolers is that they can produce air colder than outside wetbulb. Two-stage coolers are the highest priced and best-performing evaporative coolers. They are at their best during extremely hot (110°F-plus), dry days.
Since evaporative cooler pads are designed to provide wet-surface contact with all the air moving through them, they are also remarkably good air filters (hence the term air washer or scrubber). Many rigid-sheet pads can filter out 90% of particles 10 microns (µ) and larger, including most pollens and dust. Fewer data are available on aspen pad filtration; however, in my personal experience, these pads also perform well as filters.
Maintaining Water QualityAs the water in an evaporative cooler evaporates, fresh water (makeup water) is brought into the cooler. A float valve controls delivery. However, the minerals (salts) brought into the cooler with the makeup water do not evaporate, and the water in the sump becomes brackish. Eventually the water becomes saturated with minerals and the minerals precipitate out (usually at the air inlet side of the pad). During operation, most of the water evaporation and filtration occur at the air inlet side, leaving a combination of scale and previously airborne dirt on that surface. When a pad has failed, the inlet face is usually clogged while the downstream face can appear brand new. A trick to lengthen the life of rigid sheet pads is to rotate and turn the pad upside down, so that the previously downstream face becomes the upstream face.
To prevent the water from becoming saturated with minerals, a bleed-off or sump dump system should be installed.
A bleed-off system is simply a tee installed in the water distribution discharge, with a hose to a nearby drain or to the ground. Whenever the pump turns on, a small amount of the water is diverted.
A sump dump system (referred to as a blow-down in cooling tower jargon) evacuates the water from the sump every six hours or so while the cooler is operating. The dumping is done by a second pump (most commonly) or by a power-activated sump drain valve.
Sump dump systems are better than bleed-off systems because they discharge not only brackish water but also some of the enormous amount of filtered dirt that collects on the bottom of the sump. Some coolers have sloped bottoms so that minerals and dirt will gravitate toward the sump dump.
A water treatment system is a good idea for fiber pad coolers. Often it enables the user to keep a set of pads for two years. For rigid-sheet coolers, a water treatment system is essential, because rigid pads cost up to $100 to replace.
In reality, it is rare for water in a cooler not to become saturated with minerals in most desert environments. Hard water is very common in areas where evaporative cooling is used, and maintaining ideal water conditions in a cooler would consume too much water. Bleed-off systems can use as much as 5 gallons of water per hour, but if water is particularly expensive in the area, even 1 gallon of water discharge is substantially better than trapping all the minerals in the cooler.
A technique to minimize the effects of water waste from bleed-off and sump dump systems is to send the discarded water to a consumer of potentially low-quality water. Sending this water to a garden is ideal, because the cooler discharges more water when the weather is hottest and the watering needs of the garden are greatest. (Mineral-sensitive plants could be harmed, but I have watered a standard vegetable garden with no trouble.) Someone should develop a system for using this water to flush toilets.
Sizing Evaporative Coolers
Evaporative coolers are assigned an Industry Standard CFM by the manufacturer. This CFM (cubic feet per minute air flow), which is usually a number between 2,000 and 6,500, is approximately 50% higher than the highest air flow the cooler can actually produce with no ductwork restriction. Although the Industry Standard CFM claims much more air flow than a cooler can deliver, this approach to defining cooler sizes has been used for years by many manufacturers and shocks only the novice specifier. It is not unlike the technique used to define lumber sizes; most people know that a 2 x 4 is actually smaller than 2 inches x 4 inches. Most manufacturers also provide the actual air flow the cooler produces at various duct resistances.
The ideal evaporative cooler installation is an engineered system--a room-by-room heat load is calculated, a cooler is selected, and a corresponding duct system is designed. In reality, this is rarely done, because coolers are so inexpensive. Here's an alternate way to size three basic systems common in residences.
A/C Add-OnAn add-on evaporative cooler blowing into the refrigeration cooling duct system is most common in low elevation desert areas that have high cooling needs. It has a refrigeration cooling system and ductwork sized to meet the needs of that system. The ductwork is smaller than the ideal size for an evaporative cooler, but the system offers many advantages over straight refrigeration air conditioning:
Independent Ducted SystemAnother type is an evaporative cooler blowing into a single diffuser in the hall ceiling or into a dedicated duct system in the ceiling space. This is most common in areas with modest cooling needs and in houses that have floor-based heating-duct systems entirely too small for evaporative-cooling ductwork.
The sizing guide for these systems is to use 2-3 CFM (Industry Standard) per ft2 of floor space in most climates. Use 3-4 CFM per ft2 in hot desert areas.
Window-Mounted CoolersThe third type is a window-mounted evaporative cooler. This is a low-cost installation and is found wherever coolers are common. These coolers should be sized in the same way as an independent ducted system.
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