Cooling – Cooling Towers
Cooling Tower with a Propeller Fan
Cooling towers are devices used to transfer heat to the atmosphere. Large industrial or power plant cooling towers often rise 600-feet as hyperboloid structures. For HVAC purposes, smaller cylindrical or rectangular rooftop units, available in lightweight heavy-duty plastic, are typical. Water-cooled chillers are paired with cooling towers because heat rejection to tower water at near wet-bulb temperatures makes water-cooled chillers more efficient than air-cooled chillers, which must reject heat to the dry-bulb temperature.
Cooling towers rely on evaporation to draw heat from the water. As air is drawn past a flow of water, it absorbs water vapor leaving less heat in the remaining water flow. Air flow can be generated in several ways.
- Natural draft–the tendency for warm, moist air to rise–creates a current of air through the tower as the warm, moist air rises.
- Mechanical draft uses a power driven fan to move air through the tower. It can do this via induced draft where the fan is positioned at the discharge end of the tower and pulls air through the tower. Forced draft positions the fan at the intake and air is forced into the tower. With forced draft, discharged air is more prone to flow back into the air intake and the fan is more susceptible to complications of freezing conditions, but forced draft can be installed in more confined spaces or indoors.
- A third option is fan-assisted natural draft.
Cooling towers can also be categorized as crossflow and counterflow. In the crossflow design, air and water flow perpendicularly to one another. Air flows in horizontally from one or more sides of the tower and water flows downward and perpendicularly to the air flow. In counterflow patterns, air flows upward and water flows downward.
How It Works
Cool water from the tower is pumped to the chillers where it absorbs heat. The heated water returns to the top of the cooling tower and trickles downward (counterflow design) over a fill material inside the tower. As it contacts ambient air rising upward, a small amount of the water is lost to windage and some of the water evaporates. The heat required to evaporate the water is derived from the water itself, leaving the water cooler and ready to recirculate back to the chiller. Because the evaporated water leaves salts, the accumulated salts are controlled by drawing off some of the water. Fresh makeup water replaces the water lost by windage and the draw-off.
Cooling towers range in size from 10 to 20,000-ton systems. A ton of air conditioning rejects 12,000 Btu/h. An equivalent ton on the cooling tower side rejects 15,000 Btu/h, a chiller coefficient of performance (COP) of 4.0 or an equivalent energy efficiency ratio (EER) of 13.65.