Evaporative cooling is actually a complicated phenomenon dependent upon air and water temperature, airflow and humidity (the amount of water that already exists in the air). Although the physics behind this phenomenon are complex, it can be explained simply in layman’s terms.

On an afternoon where it is 80°F (26°F) and dry, a slight breeze can give you a chill. On that same 80°F day, however, if conditions are humid, that same breeze will offer little relief. This has everything to do with the ability of the air to accept the moisture that your body is producing through perspiration. On the dry day, the air can easily accept the water your body generates; hence, cooling you through evaporation. Similarly, on the humid day, there is not enough room in the air to evaporate all of the additional water. So, it remains on the surface of your skin, making you feel damp, clammy and anything but cool.

Evaporative cooling can work almost anywhere; however, it is most common and most efficient in dry climates for the reasons stated above. Evaporative cooling is used for both process and space cooling in industrial, commercial and residential applications. Some industrial facilities use evaporative cooling to supplement equipment and space cooling requirements. The introduction of refrigerated air created an industry that could provide adequate cooling regardless of the outdoor environment.

Refrigerated air comes with a cost. The cost of the refrigeration equipment is multiple times that of the cost of its evaporative cooling counterpart. The cost of running a compressor and air handler also can be a several times the cost of running an evaporative cooling system.

An evaporative cooling system is relatively simple. Although there are different types running at various efficiencies, the basic systems are similar in nature. You need the following components in all systems: media, a means to move air and a means to distribute water to the media.

  • Media. This is a material that can  be saturated with water. Air must be able to enter and pass through the media to be cooled via evaporation.
  • Means to Move Air. This is typically a fan or centrifugal blower powered by an electric motor. The air-moving device moves the air through the media and forces it into the space to be cooled.
  • Means to Distribute Water to the Media. Systems vary slightly here. Single-pass systems simply use city water pressure to spread water across the top of the media. Other systems have a reservoir and use a small pump to recirculate the reservoir water over the media.

While no one can discount the effectiveness of a refrigerated system, there is no reason that both evaporative and refrigerated cooling systems cannot be employed in the same location. Many evaporative cooling advocates do just that. For instance, for space cooling applications, the evaporative cooler is used in the spring and fall when thermostat calls for cooling. In the hottest months of the year when high humidity hits, causing the evaporative cooler effectiveness to drop, they switch to a refrigeration system. The evaporative system keeps the conditioned space comfortably cool during the dry season while saving a percentage of the electricity required to run the refrigeration system. In very dry climates such as Colorado, Utah and North Dakota, where the temperatures do not get as high, users may be able to use evaporative cooling as the only space cooling method during the summer.

Water Usage and How It Relates to Evaporative Cooling

Water usage is as necessary to evaporative cooling as electricity is to refrigeration. Water is a resource like many others in that it is 100 percent renewable after use in an evaporative cooler.

See the related web exclusive content, "Media Efficiency and How It Relates to Evaporative Cooling," to walk through the calculations that show how media efficiency affects evaporative cooling efficiency.

When water is evaporated in a cooler, it leaves various solid compounds and elements such as calcium, which is normally found in tap water, in the cooler’s reservoir. To remove those solid compounds and elements from the water cooling system, the reservoir water is purged routinely and is suitable for applications such as watering grass.

All evaporative coolers consume water via evaporation. This is what provides the cooling. The amount of water consumed by any given evaporative cooler can be expressed in a fairly simple equation. In order to begin understanding this concept, you must first understand a few relevant terms:

  • Dry Bulb Temperature (Db). This is the ambient air temperature that surrounds you.
  • Wet Bulb Temperature (Dw). This is lowest temperature the air can attain by evaporating water into the air.
  • Wet Bulb Depression (ΔT). This is the difference between the dry bulb and wet bulb temperature. It is typically expressed as this equation: (Db – Dw = ΔT).
  • Efficiency (eff). This is a ratio of the actual air temperature drop across the media compared to the wet bulb depression, expressed as a decimal percentage. If the wet bulb depression is 40°F and the actual temperature drop measured across the cooling media is 30°F, the cooling efficiency of the media is 75 percent (30/40 = 0.75). This cooling efficiency is also known as the saturation efficiency because it refers to the amount of moisture that the media can evaporate into the air.
  • Cubic Feet per Minute (cfm). This unit of measure is used with a number to express the volume and velocity of air movement.
  • Gallons per Hour (gal/hr). This unit of measure is used with a number to express volume and speed of water evaporated. 

Now that we understand the terms, let’s examine the equation for the rate of evaporation. To find the evaporation rate in gallons per hour, the equation is:

(cfm x ΔT x Eff) / 8700 = Evaporation Rate (gal/hr)

The rate that water evaporates is affected by the speed the air is passing through the media, the actual wet bulb depression and the efficiency of the media itself. For illustration purposes, assume the following conditions:

  • Db is 100°F.
  • Wb is 60°F.
  • ΔT is 40°F.
  • Eff is 93 percent (0.93).
  • Airflow is 6,000 cfm

Given those conditions, the calculation is:

(6,000 x 40 x 0.93) / 8700

Rate of evaporation = 25.66

This is the water that is actually evaporated, but that value does not account for the purge water that is evacuated from the reservoir to maintain a clean media section. Also note that this value represents run time hours, not total hours per day.

Purge water is another necessary use for water in an evaporative cooler. As discussed earlier, when water is evaporated, it leaves behind the solid particles such as calcium and various salts that do not evaporate. If they accumulate in the system, the solids create scaling and corrosion, so they must be evacuated periodically. There are a few methods commonly used to satisfy this need. They include bleed-off, scheduled system purges, timer-based purges and a programmable drain system.

Bleed-Off. This method is one of the oldest and most common methods to reduce the buildup of minerals in the reservoirs. It is simply a capillary tube that allows a certain amount of water from the pump to be diverted from the media-distribution assembly directly to the stand-pipe drain. The typical rate of water relieved through this tube is 12 oz/min of cooler run time, or approximately 4.7 gal/hr of cooler run time. Although using a bleed-off tube is a widely accepted method, it is difficult to regulate the bleed-off rate based on the size of the cooler or water hardness level. This makes it the least economic from a water usage standpoint, and this method may use more water than actually required.

Scheduled System Purge. Another common method used to purge an evaporative cooler of its solids is a scheduled purge. With a timer-based method, a timer is used to log the number of run-time minutes. After a predetermined run has passed, the timer cycles a separate pump to run for a programmed amount of time in an attempt to evacuate the reservoir. This method is more cost effective than the bleed-off method because it is not a constant bleed off. It typically only evacuates the system once in an 8-hour period and then for only 7 minutes. At a flow rate of 4 gal/min, it only evacuates 28 gallons over an 8-hour period, or 3.5 gal/hr of cooler run time. This is a savings of 1.2 gallons compared to the bleed-off method. The timer method is an improvement over the bleed-off method but it still typically purges 28 gallons of water over each 8-hour period of cooler run time.

Programmable Drain System. With a programmable drain system, the user can select how often the system should purge and how long that purge should last. The frequency of how often the system should purge is based upon the quality of the water supplied to the cooler. In areas with very hard water, the frequency of purging should be more frequent; less frequent where the water quality is better.

 In conclusion, consider an evaporative cooling system when you have a space that needs cooling.