28 AUG 2020
What Is Evaporative Cooling?
Evaporative cooling is the process of removing heat through evaporation.
It’s one of the most common heat rejection methods that cooling towers use to cool industrial equipment and HVAC systems. As hot water enters the cooling system, some of it evaporates, and the remaining water is cooler than the water that entered the system. This is then recirculated back into the pipes so that it can continue cooling your equipment.
While evaporative cooling processes can represent as much as 50 percent of a facility’s water use, they’re still far more efficient than “single-pass cooling systems” which use water once and then send it into the sewers.
However, even though the water recirculates to keep cooling equipment, there’s still water loss from the evaporative cooling process. There are also ways evaporative cooling systems accelerate the process of evaporation to make the cooling more efficient.
Here’s a more detailed explanation of how it works.
Blowdown, drift, and make-up water
In an evaporative cooling system, there are three sources of water loss: evaporation, blowdown, and drift. For an evaporative cooling system to operate properly, the lost water has to be continually replaced.
As water evaporates, it leaves behind any minerals that were in the water. If the concentration gets too high, it can cause scaling, corrosion, or contamination, so there’s a built-in process to reduce the mineral concentration. This involves removing some of the water that doesn’t evaporate. The removed water is called “blowdown” or “draw-off.”
To accelerate the process of evaporation, evaporative cooling systems also circulate air, usually with a fan. As a result, water droplets are often blown out of the tower along with the water vapor. These lost water droplets are referred to as “drift.” (Some cooling towers use “drift eliminators” to help keep these droplets from escaping.)
Between evaporation, blowdown, and drift, evaporative cooling systems lose a lot of water. To make up for this loss, “make-up” water gets added to the system. Make-up water has to be equal to the system’s evaporation, blowdown, and drift to keep the water flow consistent.
One of the ways evaporative cooling systems accelerate the process of evaporation is by spreading out the hot water into a “spill area.” This allows more water to come into contact with air. Some cooling systems simply let the water drip (this may be referred to as “gravity-fed distribution”), while others use pressurized nozzles to spray it in smaller droplets or mist.
Additionally, evaporative cooling systems use a “fill material” to further increase the surface area of the water in the spill area. It also slows or breaks up the water flow, increasing the time it spends in contact with air. Fill materials come in a variety of forms, but they often fall under two categories: splash fill or film fill.
Splash fill disrupts the water flow and prevents it from splashing. (Think of it like a sponge.)
Film fill is a sheet of material such as PVC that spreads the water into a thin film, like a waterfall.
Each type makes the evaporative cooling process more efficient.
Evaporative cooling systems constantly have air flowing through them to help cool the water. Depending on the type of configuration of the cooling unit, the air may flow horizontally across the water flow (crossflow), or it may flow vertically (counterflow).
Sometimes the design of the cooling unit allows this airflow to occur naturally, using physics. This is known as “natural draft.” Other units use fans, and this airflow is referred to as “mechanical draft.” If the fan is located at the bottom of the structure and propels the air up, it’s a “forced draft.” If the fan is on the ceiling of the structure, pulling the air up, it’s an “induced draft.”
The ideal airflow system depends on each structure’s unique circumstances, such as where it’s located, the local climate, and the type of facility it’s operating within.
Does your facility use evaporative cooling?
Evaporative cooling is used in a wide range of industries. And while it’s highly efficient, organizations often need better visibility into their water infrastructure to get the most from their water-intensive processes. Evaporative cooling can represent as much as 50 percent of your total water use, and a single issue with your cooling tower can easily waste tens of thousands of gallons of water per day.
Our water efficiency as a service program scans your water infrastructure and analyzes your water data in real time across thousands of potential failure points, including scaling, contamination, corrosion, and low-flow events.
Here’s how it works.