Evaporative cooling occurs when water evaporates, changing state from liquid to vapor and requires an input of heat energy - the latent heat of evaporation. The input of heat is drawn as a waste product directly from the server facility. Modern heat rejection requirements employ cooling towers or evaporative condensers as the most efficient and cost effective method, maximizing the contact between air and the water to be cooled.
Cooling towers used in evaporative cooling water systems and domestic hot and cold water systems are a common source of Legionella. The disease is transmitted via the inhalation of mist droplets containing the bacteria. The use of UV water treatment ensures that microbial contaminants are effectively inactivated, including slime formers that impair cooling tower performance. Unlike chemical disinfection systems, organisms do not demonstrate a tolerance or resistance to UV light. Typically, cooling towers require nearly 66% less power to reject a given amount of heat than alternative “dry methods”. In addition, they occupy a smaller footprint and are significantly quieter. Some server farms use reclaimed water for cooling, although all need optimal performance from their cooling loops.
Cooling towers evaporate pure water, leaving any suspended or dissolved solids, such as minerals etc., behind in the retained water. This resultant build-up of solids or concentration factor would leave the water unusable, reducing operating efficiencies and potentially damaging the recirculating system. In an effort to reduce build up, it is necessary to blowdown or bleed a proportion of the system water. In the US, the total dissolved solids (TDS) of the supply water requires that the concentration factor within an evaporative cooling system is maintained at 3 to 3.5 times, requiring an amount of water equivalent to up to 50% of the evaporation losses being bled to waste. For a typical MW (1,000Kw) of heat rejected, this equates to 150 to 200 gallons per hour that is drained to waste. Several novel approaches are being utilized for cooling water systems, including the use of reclaimed or wastewater for cooling in an attempt to reduce the use of potable water. The selection of filtration products that minimize backwash water loss is critical; as a result high efficiency media filters such as the Vortisand® Systems with Cross-Flow Microsand Filtration are specified for the most demanding applications.
Cooling towers are effective air scrubbers. As a consequence of the cooling method, they flush airborne contaminants into the system where they deposit on and foul the heat exchange surfaces. Suspended matter in the cooling water also supplies waterborne microorganisms with a supply of nutrients. Modern UV systems use automatic wipers to keep the optical path free from contamination. Many of these airborne contaminants, as well as iron in solution in the water, will foul the quartz sleeves and prevent optimal disinfection of the cooling water.
Particulates under 5 micron in size contribute to reduced cooling efficiencies by fouling the surfaces of heat exchangers.