Cooling Tower

Warm condenser water from the chiller is pumped to the top of the tower and spread over fill, a material with a large surface area. Fans move air through the fill, and a small part of the water evaporates. Evaporation takes heat from the remaining water, which cools it before it falls into the basin and returns to the chiller.

Because the tower relies on evaporation, its performance depends on the outdoor wet-bulb temperature, not the dry-bulb temperature. A typical HVAC tower is designed to cool water from about 95°F to about 85°F. The temperature drop through the tower is called the range, and the difference between the leaving water temperature and the wet-bulb temperature is called the approach.

Building cooling towers are factory-built or field-erected mechanical draft towers, much smaller than the concrete towers seen at power plants.

• Open circuit (direct contact): the condenser water itself flows over the fill and is exposed to the air. The most common type for HVAC.
• Closed circuit (fluid cooler): the loop water stays inside a coil, and a separate spray water circuit evaporates over the outside of the coil, which keeps the main loop clean.
• Crossflow and counterflow: describes whether air moves across the falling water or upward against it.
• Induced draft and forced draft: fans either pull air through the tower from the top or push it in from the side.
• Natural draft: the large hyperbolic towers at power plants, which move air without fans and are not used for building HVAC.

As water evaporates, the minerals it carried stay behind, so dissolved solids build up in the tower water. Towers drain off some water (blowdown) and add fresh makeup water to keep this under control. The ratio between the mineral concentration in the tower water and in the makeup water is called cycles of concentration, and it is the main setting for balancing water use against scale and corrosion.

Cooling towers can also grow Legionella bacteria, which spread through the fine water droplets (drift) that leave the tower. Water management programs following ASHRAE Standard 188 typically include biocide treatment, regular cleaning and disinfection, drift eliminator inspections, and procedures for towers that sit idle. Some cities and states also require cooling tower registration and routine Legionella testing.

Routine work includes inspecting and cleaning the basin and fill, checking fans, belts and gearboxes, verifying water treatment chemistry and conductivity controls, and inspecting drift eliminators. Many towers have VFDs on their fans, which let the tower hold its leaving water temperature while using less fan energy. Common problems include:

• Scale and fouling on the fill, which reduces heat transfer
• Clogged distribution nozzles or hot water basins, causing uneven water flow
• Fan, belt, gearbox and motor wear
• Damaged or missing drift eliminators
• Freeze damage in cold climates, prevented with basin heaters and controls
• Corrosion of the structure and basin

Useful data includes entering and leaving water temperatures, outdoor wet-bulb temperature, fan speed, basin level, conductivity, makeup and blowdown water volumes, chemical treatment logs and Legionella test results. Makeup water is often metered separately, because some water utilities give credit on sewer charges for water lost to evaporation.

In a digital twin such as Syyclops, the tower's BMS points, water treatment work orders and inspection documents can be kept on the tower as an asset, in the same building model as the chillers.