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Evaporative Cooling Basics
You may have heard about “swamp coolers” before. Surprisingly, we don’t see too many of them in Florida—even though the state is famous for its swamps.
Contrary to their name, swamp coolers are common in arid (dry) climates. They’re also called evaporative coolers because they use the evaporation process to cool the air.
This article will explain the science behind evaporative cooling. It will also address some of the common applications and concerns of swamp cooler usage.
The chemistry and physics behind evaporation
Considering that HVAC/R techs work on units with evaporators on a daily basis, you probably already know a bit about evaporation. It’s the process in which a liquid becomes a gas. Evaporation occurs when you apply heat, but there’s more to it than that.
When a substance undergoes a phase change, its temperature remains constant until the phase change is complete so long as the pressure stays the same. A certain amount of heat must be added or removed to complete a phase change before it can affect the temperature. We call this amount of energy latent heat. (By contrast, the heat energy that affects temperature is called sensible heat.)
Science textbooks tend to use boiling and freezing points to describe latent heat, so we’ll introduce the topic using water’s boiling point as an example. Water boils at 212°F and becomes water vapor. However, the vapor cannot heat up past 212° until the water has completely evaporated. The amount of heat it takes to complete the phase change from liquid to vapor is called the latent heat of vaporization. Water’s latent heat of vaporization is 970 BTUs per pound.
That’s a lot of energy, especially when you compare it to common refrigerants. For example, R-22 has a latent heat of vaporization that’s just under 87 BTUs per pound (at 40°F saturation).
Water’s high latent heat of vaporization is the key to evaporative cooling. However, no boiling occurs in evaporative cooling. Evaporation and boiling are different. Boiling results in evaporation, but not all evaporation occurs due to boiling.
Evaporation vs. boiling
Think about a puddle of water on the road. That puddle will eventually evaporate, even though the temperature never reaches the water’s boiling point (212° F). How is that possible?
Temperature is only an average measure of molecular activity. Within that puddle of water, some molecules have much more kinetic energy than others. When those individual molecules absorb enough heat, they will move quickly enough to break free and vaporize. It still takes a lot of energy to vaporize individual molecules that are not at the boiling point, but the process is much slower and less noticeable than boiling.
Boiling occurs when we apply a lot of heat to water molecules. Under most circumstances, water does not exist as a liquid beyond 212°F at atmospheric pressure. The suddenness of the heat application raises the water’s vapor pressure until it equals the atmospheric pressure. As a result, boiling forces all of the liquid molecules to vaporize much more quickly than normal, not just the individual molecules with more kinetic energy.
Evaporative cooling does not involve boiling. Some of the higher-energy water molecules will vaporize when they absorb ambient heat, but it’s a slow process. Evaporative coolers may not be vaporizing pounds of water at the boiling point, despite what the latent heat of vaporization may suggest. Still, the amount of energy required for water to evaporate is quite high compared to other substances, no matter how much water vaporizes or at what temperature it does so.
How evaporative coolers work
Cooling, as we usually think of it, is a loss of heat energy. Like vapor-compression refrigeration systems, evaporative coolers remove heat from the airstream, but that heat goes into the latent phase change of water, which is suspended in the same air. That results in LOWER temperature but constant enthalpy when the evaporation/condensation of the water vapor is considered.
Evaporative coolers have two main parts that facilitate the cooling process: a fan and filter pads. They use a fan with a motor to draw in outdoor air. The air then passes through water-saturated filter pads. (These pads are saturated by a pool of water in a tank at the bottom of the cooler.) Colder air exits the system because the moisture absorbs the air’s initial heat. The moisture saturates the air that passes through, and the cooler gives off colder, more humid air.
Warm air contains thermal energy. Evaporative cooling harnesses that thermal energy from the air that passes through it (sensible heat) and makes it vaporize water within the saturated filter pads. That process converts the sensible heat to latent heat, removing it from the air. Since water’s latent heat of vaporization is so high, these coolers can take up a lot of heat from the air before vaporizing the water within them.
The evaporative cooling process only works because the air maintains a constant enthalpy value before and after cooling. That makes evaporative cooling an isenthalpic or adiabatic process. (Remember, iso- means “same,” and enthalpic refers to enthalpy.)
The enthalpy stays the same throughout the process because the humidity and water vapor content also change with the temperature. Everything is proportional.
The amount of sensible heat that becomes latent heat is directly proportional to the temperature drop that occurs. Similarly, the air becomes more humid when it loses heat to the water on the filter pads. The increase in humidity is also directly proportional to the latent heat gain within the system. The energy lost as sensible heat becomes latent heat, and the temperature decrease corresponds to the humidity increase.
In other words, the evaporative process removes heat from the air. However, the enthalpy stays the same because of the increased water vapor content in the air.
Adiabatic coolers and condenser efficiency
We use adiabatic cooling for more than just human comfort, too. Adiabatic cooling systems can increase condenser efficiency by helping control ambient temperatures.
Condensers that use CO2 risk going into the transcritical zone at temperatures above 86°F. It’s not difficult to reach an ambient temperature of 86°, especially in hot, dry places in the Southwestern United States. Transcritical conditions reduce efficiency in CO2 refrigeration plants. That inefficiency can negate the environmental efforts it set out to solve if we aren’t careful.
Adiabatic coolers can reduce the ambient temperature. Lower ambient temperatures reduce the discharge pressure of these refrigeration systems and improve efficiency. Adiabatic coolers activate at around 66° Fahrenheit and perform better with higher ambient temperatures. As such, they are most beneficial during the hot months.
Cortella, D’Agaro, and Coppola published a study on adiabatic coolers, subcoolers, and transcritical CO2 refrigeration. Their study thoroughly explains everything I’ve just covered, and it contains some additional research and field test procedures.
Evaporative cooling drawbacks
As I’ve already mentioned, evaporative coolers increase humidity. They recreate the perfect humid conditions for bacterial growth, especially Legionella bacteria.
Legionella infections can lead to a severe form of pneumonia called Legionnaires’ disease. This disease can be fatal without treatment. Evaporative coolers can foster Legionella bacterial growth. According to the Utah Department of Health, the risk of Legionella growth and infection is even higher in locations with warm water, usually in large plumbing systems and tanks but also in the environment. (That's another reason we don’t use swamp coolers often in Florida.)
You can minimize the risk of Legionnaires’ disease by maintaining your evaporative cooler properly. Attentive cleaning is the best preventative action you can take. We have a podcast about preventing Legionnaire's disease with proper cleaning practices, and you can listen to it HERE.
Before summer, make sure your power is disconnected the entire time. Remove and clean or replace your filter pads before the hot season, scrub all waterways, and make sure you use clean water before you regularly operate your cooler. It’s a good idea to disinfect your entire unit with chlorine bleach.
After summer, make sure you power your unit off. Scrub all the waterways again and drain the water. Once again, use chlorine bleach to clean the pump and tank and remove the filter pads to let them dry. Keeping the parts dry during non-operation is crucial to preventing humid conditions that promote bacterial growth.
These preventative procedures were outlined in the Western Australia Department of Health’s thorough guide for preventing Legionella growth.
It’s also worth noting that humid ambient conditions reduce the effectiveness of evaporative coolers. Cooling occurs because the heat gets transferred to the water within the swamp coolers. When the ambient conditions already have a lot of water vapor, the water within the swamp cooler can’t do much to remove the heat. A lot of the air’s heat has already been trapped in water vapor, so more water vapor won’t take that heat out of the air. Any cooling that occurs will be minimal or nonexistent.
Like any other cooling technology, evaporative coolers have ideal and non-ideal applications. They’re great for cooling and humidifying homes in desert climates, such as Central Australia and the Southwestern USA. Moreover, they can adjust the ambient conditions for commercial refrigeration systems and improve their efficiency. They may hold the key to making the commercial refrigeration sector more eco-friendly.
They may not be useful for residential applications in Florida (and a good portion of the Southeastern USA). However, evaporative coolers certainly have their niche, as they can cool and humidify the air without the need for refrigerants or compression.