Adiabatic cooling towers cool water in two states of operation, dry and adiabatic. When they are in the dry state, they operate by blowing air onto the hot water in the system to transfer the heat into the ambient air. When it is in an adiabatic state, the hot air has water added to it either through wet pads or a humidified chamber. It is in the dry state the majority of the time, but if temperatures rise too much, it will switch to an adiabatic state and require water. Because adiabatic cooling towers operate in a dry state, the majority of the time, they need around 80% less water than traditional cooling towers.
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 a fine mist of water enters the fluid cooler, it rapidly evaporates, absorbing the latent heat required for evaporation and causing 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 ambient air in the United States is dry and cool enough during the year to allow for dry cooling. However, dry coolers do not operate well under high temperatures in the Summer; therefore, the adiabatic state on a cooling tower that adds water cooling will be necessary during this.
The alternative, evaporative cooling towers, utilize significant amounts of water, yet can handle very high heat loads. Adiabatic cooling towers are one of the best options for reducing water usage in a cooling tower.
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-conditioned 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.
It is important to balance water savings with energy savings. Adiabatic cooling towers can indeed consume 80% less water; however, they can cost 33% more in energy to power the system. The cost of energy and water may differ in your local area, which will help decide whether adiabatic cooling towers are cost-effective for your facility.
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 favor adiabatic systems, where good to moderate water and rising electricity costs favor traditional cooling towers.
There are still scale problems in adiabatic cooling tower wetted pads and thin film fill. We know from experience that anytime water evaporates, dry minerals are left behind.
Traditional cooling towers are engineered to minimize the impact of dry off; conversely, 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. The level of scale will rise substantially in the summer months, when the cooling tower enters its adiabatic state and switches to using water cooling.
The issue of Legionella arises when water is at a high temperature, just like it is during heat transfer in an adiabatic cooling system.
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 systems 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.
Adiabatic cooling towers require water treatment programs designed specifically for them. The water treatment chemical usage needs to be dynamic to account for changes in water usage. An effective water treatment program will save water and energy costs by improving the water quality and controlling issues such as bacteria, scale, and corrosion. To learn more about our adiabatic cooling tower water treatment, reach out here.
