Heating, ventilation and air conditioning (HVAC) cooling towers are auxiliary devices that help very large air conditioning units run more efficiently. They work by evaporating a portion of the water stream, which cools down remaining water in the system. “Wet bulb” temperature, which determines how dry the air is, determines the effectiveness of the unit.
Towers work by following a building’s air conditioning cycle, in which heat from a building is transferred to chilled water, then into refrigerant and ultimately into cooling water. The cooling tower is the last heat-transfer point, where it is released into the atmosphere. HVAC cooling towers are used by large office buildings, hospitals, schools and similar buildings as part of their ventilation systems.
Thermostats that sense water temperature control cooling tower fans. When water temperature rises, fans either start or increase in speed. Controls generally use a fixed temperature setting for operation. The best temperature setting is a combination of the ambient wet bulb temperature and the approach temperature. The web bulb temperature indicates the dryness of the air while the approach temperature indicates how much the water temperature can be lowered toward the wet bulb temperature. Optimal control is achieved by a digital system that continually changes the setting for the fan that cools the water.
As an auxiliary unit to an air conditioning system, the primary job of an HVAC cooling tower is to serve the chiller to which it is connected and make it more energy-efficient. For each degree that a cooling tower lowers the temperature of water, a chiller's kilowatt consumption falls by 1 to 1.5 percent. A reduction of 10 degrees Farenheit, from 70 to 60 can result in a decrease in a chiller's energy demand of 10 to 15 percent.
Types of Towers
Cooling towers are classified either as direct (meaning open circuit) or indirect (closed circuit). Direct systems are enclosed structures with internal workings that distribute the water to be cooled over a labyrinth-like fill. This fill provides an expanded space for water to be cooled. Water is then collected in a cold water basin, from which it is pumped back through the fill to collect more heat. Heat- and moisture-laden air is eventually released to the outside at a remote point.
Indirect towers receive no direct air or water contact. These units have two fluid circuits: An external unit recirculates water on the outside of the second unit, which consists of tube bundles or closed coils. Air is drawn through water running over the outside of these coils to provide cooling to the recirculating water in the second unit. resulting in a process that's similar to that of a direct cooling tower.