Evaporative Cooling: Your Ally in Hot, Dry Weather

In the heat of summer, you need a way to make your living and working spaces more comfortable. In a hot, dry climate, this means adding moisture to the air while you’re cooling it off. People feel most comfortable in 40 to 60 percent relative humidity. Our bodies need a certain amount of moisture in order to function properly, and if the air is too dry it can irritate your eyes and cause sinus and lung problems. Furthermore, viruses have been shown to live longer in very dry environments. Even your houseplants, furniture and electronic devices have a harder time in a very dry environment.

In short, the solution to hot, dry weather is evaporative cooling. This is the method used by misting fans and evaporative air coolers. Here’s a quick overview of how they work and the differences between them, so that you can pick the cooling device that makes the most sense for you.

What is Evaporative Cooling?

Select focus point Water drops on the glass

Liquid water is made of individual water molecules bound together by certain forces. All of these molecules are moving around, even if the water itself looks perfectly still. When some of the molecules on the water’s surface get moving fast enough, they break loose from the bonds that were holding them in liquid form, and they evaporate, or rise up into the nearby air as water vapor.

  • Important Note: It is important to understand the difference between water vapor (which you cannot see) and mist or fog or steam (which is visible). Water vapor refers to individual water molecules that become part of the air itself. These individual water molecules are much too small to see. (In an ordinary drop of water, there are about 13,000,000,000,000,000,000,000 water molecules.) When you see mist or fog or steam, what you are seeing is a lot of small drops of liquid water, each one made up of gazillions of water molecules bound together.

Once the individual molecules become part of the air, they tend to hang around in the vicinity of the water they just broke loose from, unless there’s some wind to blow them away. If a glass of water is left in a quiet place, the air immediately above the glass will become more crowded with water molecules (more humid). Evaporation slows down in a humid environment, because there’s less and less room in the air for new water molecules to enter it.

But Where Does the Cooling Part Come In?

Sun protection

We’re just getting to that. The first thing to know is that when something cools off, it’s actually having heat removed from it. You might think you’re “adding coolness” to your body if you get out of a swimming pool and feel cold all over. What’s actually happening, though, is that your skin is having some of its heat removed by the evaporating water. The heat lost from your skin is called “sensible heat,” because it’s a temperature change that people (and thermometers) can sense.

Back to our glass of water. Some of the water molecules on the water surface are moving faster. They’re almost ready to evaporate, or become vapor. But they need a little kick of heat energy before they can make the leap away from liquid water. They’ll take that heat from any source that’s nearby. That’s why, if you leave a glass of water out in a 70-degree room, the water temperature will be cooler than 70 degrees. If swimming-pool water is evaporating off your skin, each water molecule is grabbing a tiny bit of your body heat in order to make its phase change from liquid to gas. Your body loses heat, and feels cooler. If drops of water are being sprayed in the air from a misting fan, they take heat from the air around them when they evaporate, so the air becomes cooler. The amount of heat that water has to absorb from its surroundings in order to evaporate is called “latent heat.”

Wind Speeds Up Evaporation

If you step out of a swimming pool, you feel cool. But if there’s a breeze, that cool feeling can become downright chilly. The reason for this is that the wind blows away the newly evaporated water molecules, so they can’t just hang around making the air right above your skin more humid. If it’s windy, there’s always fresh dry air passing over your wet skin, ready to absorb more water vapor.

How are Misting Fans and Evaporative Air Coolers Different?

Misting fans and evaporative coolers both make use of the physics of evaporation — but they locate that cooling action in different places. Evaporative coolers cool off the air internally (we’ll explain how in the next paragraph), so they’re useful indoors as well as in garages, porches, and so on. They don’t release any spray or need to be connected to a water supply.

Misting fans, by contrast, produce a fine mist of cool water droplets, which mostly evaporate while they fly through the air. If you’re near a misting fan, some of the moisture will land on you, and cool you off by pulling heat from your skin as it evaporates. Misting fans are perfect for backyard patios, festivals, warehouses and other fresh air environments, but they are too wet to bring into your living room.

Evaporative cooling systems, however, are highly versatile, and in dry regions they can be an excellent energy-efficient substitute for air conditioning systems. It’s worth noting that a few open windows are needed when you use an evaporative cooler, in order to allow for sufficient air movement.

What Happens on the Inside of an Evaporative Cooler

Close up of old dirty brown cellulose paper pad or evaporative cooling pad texture background. Evaporative cooling pad is material of evaporator system.

Evaporative coolers, sometimes called “swamp coolers” have replaceable pads inside them. These cooling pads have many layers, providing a large surface area for evaporation. The pads soak up water from a reservoir inside the cooler. Electricity powers a fan in the cooling tower, which pulls dry warm air from the room along the wet pads. These pads get cold in the breeze, just like you do when you get out of a pool on a windy day. As the air passes over the many layers of these wet pads, its temperature decreases, and the cool air is then pushed by the fan out into the room. Some evaporative coolers have air filters inside them as well. Water reservoirs inside the cooler can be cleaned with warm water and mild soap, as part of regular maintenance.

Energy Savings from Evaporative Coolers

The evaporative cooling process requires only a small fraction of the energy that air conditioners use. The heat transfer is accomplished through the physics of evaporation, rather than by compressing liquid refrigerant into a smaller space and then moving it across a heat exchanger. The only thing that is powered in an evaporative cooler is a fan and sometimes a water pump.

Indirect evaporative cooling systems make use of this same efficiency for building design, and they are gaining popularity with energy policy makers.

Your Region Makes a Difference in the Type of Cooling Technology You Choose

We’ve explained how evaporative cooling works better in dry environments. If the relative humidity is high, the air is already carrying a lot of water molecules and it doesn’t have room for many more. In a hot, humid climate, you can hang a wet towel on a clothesline and it won’t dry, even if it’s 100 degrees F. outside. ln this type of environment, you’ll want to close your windows and use an air conditioner. Air conditioning systems dry and refrigerate the air, reducing moisture and providing relief.

When you’re in a dry environment, however, evaporation is incredibly efficient. Hang that wet towel outside in Phoenix or Las Vegas, and it may be bone dry in half an hour. Many parts of the western United States, such as California, Arizona, and parts of Texas, have very low humidity for much of the year. If you live in one of these arid places, you’ll find that evaporative coolers and misting fans are an efficient way to stay cool. Depending on the humidity in your area and the size of your space, evaporative coolers can reduce the air temperature by up to 30 degrees F.

A Few More Words About Humidity

If you love to geek out over the science behind cooling, let’s take a look at psychrometrics. This has nothing to do with psychology or the mind; it comes from Greek words meaning “cold” and “measure.” Psychrometrics, or psychrometry, is a field of engineering that focuses on how gases and liquids interact with temperature. Specialists in this field design HVAC systems, and keep our environments comfortable by harnessing the latent heat of evaporation. There are two ways that psychrometric engineers measure air temperature: Dry Bulb Temperature (DBT) and Wet Bulb Temperature (WBT). DBT is measured by a thermometer that’s shielded from any rain or wetness. This is considered the true air temperature. WBT is taken by a thermometer which is wrapped in a wet cloth, so that it reflects the maximum possible cooling effect of evaporation. In a very dry environment, wet bulb temperature can be up to 40 degrees cooler than dry bulb temperature. The difference between the two temperature readings is a way to measure relative humidity.

Evaporative cooling units use the elegant laws of physics to maintain a healthful temperature and humidity in your environment. They come in a wide range of models, sizes and features, to efficiently meet your comfort needs.


LEARN MORE:

NewAir Portable Evaporative Coolers

Portable Evaporative Cooler | NewAir EC111W

  • • Bring down the temperature by up to 20 degrees
  • • All-natural eco-friendly cooling uses no harmful chemicals
  • • Uses about the same electricity as a 100-watt light bulb

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NewAir Outdoor Misting Fans

Misting Fans NewAir AF-520B

  • • Refreshing mist significantly decreases outdoor temperature
  • • Three fan speeds cool up to 600 sq. ft. of patio space
  • • Compatible with any standard garden hose

 

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