Many data centres release their heat through a cooling tower. Warm water from the building is spread through moving air, and a small part of it evaporates, taking the heat with it into the sky. That evaporation is the point of the tower, and it is where most of the water goes. A smaller share is drained away on purpose to stop minerals building up, and a little blows out as fine mist. Everything that leaves has to be replaced with fresh supply, called make-up water.
The loop that carries heat to the tower
Inside the building, the heat from the computers ends up in a loop of water. The US Department of Energy explains that the condenser water loop carries the heat to the cooling tower, and that the system relies on evaporation from a cooling tower as the last stage of heat removal from the facility. It also notes that overall consumption is directly related to the heat load created by the IT equipment: more computing means more heat, and more heat means more water evaporated.
Three ways water leaves a cooling tower
The Department of Energy's guidance on cooling tower management describes the water balance in one line: Make-Up = Evaporation + Blowdown + Drift.
| Where the water goes | What it is | Why it happens |
|---|---|---|
| Evaporation | Water turned to vapour in the tower | It is how the tower moves heat to the air; the guidance calls it the primary function of the tower |
| Blowdown | Water drained from the loop | As water evaporates, minerals stay behind and concentrate; draining some and adding fresh water keeps them in check |
| Drift | Fine mist or droplets carried out by the air | A small quantity escapes as mist; good towers keep it low |
| Make-up | Fresh water added | It replaces everything that left by the three routes above |
Evaporated water and drift leave the site for good. Blowdown leaves the tower as wastewater, which is treated or discharged under local rules.
Cycles of concentration: using the same water longer
Operators measure how hard they work their water with a number called cycles of concentration: the ratio of dissolved solids in the blowdown water to those in the make-up water. The higher the number, the longer each litre stays in use before it is drained. The Department of Energy's guidance gives an example: increasing cycles from three to six reduces cooling tower make-up water by 20% and cooling tower blowdown by 50%. How high an operator can go depends on the quality of the incoming water and on treatment, because concentrated minerals can scale or corrode equipment.
Water that is not drinking water
A cooling tower does not need drinking water. The Department of Energy lists alternative sources for cooling tower make-up, including reclaimed wastewater, harvested rainwater and condensate captured from air handling units. Its guidance notes that reclaimed wastewater is typically treated to non-potable levels for uses such as cooling tower make-up, and that some sources need further treatment before they can be used this way.
What is not certain
- How much water a particular facility uses depends on its climate, its load, its cooling design and its water quality. This article explains where the water goes, not how much any one site uses.
- The 20% and 50% reductions are the Department of Energy's example for moving from three to six cycles; results at a real site differ.
- Rules on blowdown discharge and on using reclaimed water vary by country and region.
Sources and method
- US Department of Energy, Federal Energy Management Program, Best Management Practice #10: Cooling Tower Management, published 8 October 2013, updated 26 May 2026. https://www.energy.gov/cmei/femp/best-management-practice-10-cooling-tower-management
- US Department of Energy, Federal Energy Management Program, Cooling Water Efficiency Opportunities for Federal Data Centers, published 9 January 2019, updated 28 April 2026. https://www.energy.gov/cmei/femp/cooling-water-efficiency-opportunities-federal-data-centers
- US Department of Energy, Federal Energy Management Program, Alternative Water Sources. Read 29 October 2026. https://www.energy.gov/femp/alternative-water-sources
- US Department of Energy, Federal Energy Management Program, Best Management Practice #14: Alternative Water Sources, published 1 April 2019. https://www.energy.gov/cmei/femp/best-management-practice-14-alternative-water-sources
Method: each link was opened on 29 October 2026 and every figure was taken from the page cited beside it. No figure was estimated.
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This article provides general information, not investment advice or an offer of securities.