COOLING TOWERS

Main Types of Cooling Towers

Since water can absorb large amounts of heat when it evaporates, cooling towers normally use that physical principle to reject heat from a building. Units that use water evaporation to achieve their cooling effect are classified as either open circuit or closed circuit cooling towers.

Open circuit cooling towers discharge water carrying the building’s heat directly on the upper side of the unit. Water falls against an upward flow of air, which is established by a fan above the cooling tower. With the combination of forced airflow and water evaporation, the unit releases a large amount of heat in a relatively compact volume. However, there are two important considerations when using an open circuit cooling tower:

• The water is exposed directly to the atmosphere before it returns to the chiller, heat pump or process being cooled. Therefore, it must be treated constantly to prevent contamination with particles or bacteria. Legionella bacteria can be particularly dangerous for occupants and technical staff members, and the issue must be addressed immediately if they are detected.
• Since some water evaporates as it falls through the cooling tower, it must be replenished constantly to sustain the required flow through the HVAC unit or process being cooled.

Closed circuit cooling towers do not expose water to the atmosphere, and instead it circulates through a coil inside the unit.

• This cooling tower design also uses a fan to establish an airflow upward, but water is sprinkled from a separate source. As a result, the water that flows through the chiller plant or cooled processes is never exposed to the atmosphere.
• Closed circuit cooling towers are also known as adiabatic cooling towers, and they can achieve energy savings of around 30% compared with an open circuit configuration.
• These cooling towers also save water, by deactivating the sprinklers when airflow from the fan can provide enough cooling by itself.

An alternative system configuration to prevent water contamination is using an open circuit cooling tower with a plate heat exchanger.

• In this case, the water stream from the chiller or process does not reach the cooling tower directly, and instead it cools down in a heat exchanger connected to the cooling tower.
• This configuration requires an extra water loop and pump, but it prevents contamination with bacteria or particles.

If your building already uses a closed circuit cooling tower, there is no benefit from adding a plate heat exchanger, since the water is already isolated from the atmosphere. A heat exchanger is only a viable addition if you are using an open circuit configuration

There are also dry cooling towers, which are similar to closed circuit cooling towers but without the sprinklers for evaporative cooling. As you might expect, these cooling towers have a lower capacity than an equally-sized unit that also includes evaporative cooling.