The term “adiabatic” means “occurring without gain or loss of heat,” but applying this definition to common industry terms requires distinguishing between heat and temperature. When a gas is compressed under adiabatic conditions, its pressure increases and its temperature rises without the gain or loss of any heat.
Conversely, when a gas expands under adiabatic conditions, its pressure and temperature both decrease without the gain or loss of heat. A good example is the adiabatic cooling of air as it rises in the atmosphere to form clouds.
Adiabatic coolers work by using evaporation to pre-cool the air flowing through a closed loop coil.
Imagine a fluid cooler running fans but with the spray pump off. If the air is cool enough, the loop temperature is low enough to keep the system going whether it is HVAC or process cooling. Now, picture a series of cooling misters in the air intakes of the fluid cooler. When fine mist of water entering the fluid cooler rapidly evaporate, absorb from the air the latent heat required for evaporation and, therefore, cause the temperature of the air to decrease.
Now the air entering the fluid cooler is low enough to deliver the right temperature water back to the process. Adiabatic cooling systems work on the same principle, but use a variety of means to pre-cool the air before it flows through the coil. Wetting pads, thin film fill, plastic mesh and mist nozzles are commonly used to create enough surface air for the water to quickly and completely evaporate.
The air is cool enough (and dry enough) for significant parts of the season throughout most of the United States to make dry coolers work, but it is not practical to stop production or let the building get hot every time the temperature climbs above the point where the dry cooler can satisfy the load.
Traditional evaporative cooling towers get around this by using water which can remove enough heat through evaporation to satisfy even the highest load conditions.
Adiabatic cooling systems combine features of both approaches and save water by operating as a dry cooler whenever the load is light enough or the air is cool enough to do so. Water savings can add up fast.
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Saving water is important, but it must be balanced with the total cost of operation. The main downside is the same downside for most air condensed systems. Because air can absorb less heat than water, adiabatic cooling towers take more energy to accomplish the same reduction in temperature as an evaporative cooling tower. The typical figure is 33% more energy.
While adiabatic cooling systems may save water, the increased cost of energy more than offsets these savings in most applications. There are no clear trends and comparing the cost of operation generally comes down to the quality of the water supply and the cost of electricity. Very hard and alkaline-rich water and inexpensive electricity favors adiabatic systems, where good to moderate water and rising electricity costs favor traditional cooling towers.
While manufacturers downplay the scale problems in their wetted pads and thin film fill, we know from experience that anytime water evaporates to dry, salts of evaporation form.
Traditional cooling towers are engineered to minimize the impact of dry off, but adiabatic systems are designed specifically to evaporate water completely to dry. Calcium carbonate scale will accumulate on the wetted surfaces of the adiabatic system just like it does where water evaporates completely to dry on any cooling tower.
Legionella can be a problem any time warm water is used to create aerosols, including the operation of adiabatic cooling equipment.
Regulations in the United Kingdom do not exempt adiabatic systems in any way and they publish a document which describes the risk in detail.
Much like traditional cooling towers, some adiabatic designs are more prone to Legionella issues than others. Risk often comes down to design and engineering, but it is important to understand that adiabatic system are not free from Legionella issues.
Much like fluid coolers, the main water treatment concern on most adiabatic systems is the closed loop. Water supplying the wetted pad or mist nozzles is generally not recirculated and is designed to evaporate completely. Collection and drain systems mainly address valve or control program failures.
While these features are not sumps to collect recirculating water, system design varies and the water that collects at these points may benefit from treatment to prevent corrosion and microbiological growth. Marketing brochures always show clean systems, but experience proves that they do not stay clean without proper treatment and preventative maintenance.
