# Chilled Water Plant The central plant is where a facility's heat finally leaves the building. Pumps circulate water, chillers cool it when outdoor conditions require, heat rejection equipment discards the heat to the atmosphere, and a control system decides moment by moment which of those machines need to run. The plant's design governs how much power the facility spends on cooling, which is the second-largest consumer of electricity on the site. > [!note] The one-sentence version > Make water cold, send it where the heat is, and run the refrigeration machinery only on the days the weather will not do the job for free. ## Why it exists Heat must be moved from thousands of distributed sources to a place where it can be given to the outdoors. Water carries heat far better than air per unit volume, so a water loop is the natural collection medium; the plant is the machinery at the end of that loop. Centralizing it allows large, efficient equipment, redundancy among several units, and a single point at which the facility's entire thermal performance can be measured and optimized. ## Prerequisites - **Chiller:** a refrigeration machine producing chilled water; centrifugal and screw compressors dominate at data center scale, and absorption chillers appear where waste heat is available. - **Coefficient of performance:** heat moved per unit of electricity consumed; a good chiller moves several units of heat per unit of power, which is why cooling costs far less than the heat it removes. - **Primary and secondary loops:** a primary loop through the chillers at constant flow, and a secondary loop to the loads at variable flow, decoupled so each can run at its best point. - **Condenser water loop:** the circuit carrying heat from the chiller to the [[wiki/Cooling Tower|cooling tower]] or dry cooler. - **Economizer, or free cooling:** operating without the compressors when outdoor conditions allow the water to be cooled directly; the single largest efficiency lever available. - **Approach and delta T:** how close the plant can bring water to the outdoor limit, and the temperature difference across the loads; wider temperature differences mean less pumping for the same heat. - **Thermal storage:** a tank of chilled water providing minutes of cooling during a power transfer, and sometimes hours for shifting load off peak. ## How it works Warm return water from the data halls arrives at the plant. If outdoor conditions are cool enough, water passes through a heat exchanger against the condenser loop and is cooled without running any compressor; this is free cooling, and raising the temperature the halls accept widens the hours it is available. When conditions require, chillers run: the refrigeration cycle absorbs heat from the chilled water into a refrigerant, compresses it, and rejects it into the condenser loop, which carries it to the towers or dry coolers. Supply water returns to the halls, to coolant distribution units, rear doors, and air handlers. Redundancy follows the same logic as electrical design: N+1 chillers and pumps, headered so any unit can be isolated, with valved piping so a section can be worked on while the plant runs. Continuity requires more care than electrical continuity, because a liquid-cooled hall loses thermal margin within seconds of flow stopping; pumps and plant controls are therefore supported by uninterruptible power, and thermal storage covers the interval while generators come up. See [[wiki/Uninterruptible Power Supply|Uninterruptible Power Supply]]. ## Stack position - **Upstream:** electrical supply, water supply, and outdoor conditions, which set what is thermodynamically available. - **Downstream:** every cooling device in the building. - **Function and vendor:** mature mechanical engineering with many qualified suppliers; the design choices are chilled-water temperature, economizer strategy, and heat-rejection type. - **Degraded case:** a plant designed for cold supply water runs its compressors far more hours per year, which shows up directly as a worse [[wiki/Power Usage Effectiveness|power usage effectiveness]] and a larger electrical bill for the same compute. ## Lineage and trajectory Data center plants inherited commercial building practice, with cold supply water and continuous mechanical cooling. The great shift was upward in temperature: once equipment was shown to accept warmer air and warmer water, free cooling became available for most or all of the year in many climates, and in the best cases the chillers became emergency equipment rather than everyday machinery. Liquid cooling at the chip pushes the same direction further, because a cold plate works well with water far warmer than an air coil needs; a facility that captures heat warm can often reject it with no refrigeration at all, and can then consider giving it to someone who wants it. See [[wiki/Heat Reuse|Heat Reuse]]. ## Relationships - **rejects heat through:** [[wiki/Cooling Tower|Cooling Tower]] - **serves:** [[wiki/Coolant Distribution Unit|Coolant Distribution Unit]] · [[wiki/Rear-Door Heat Exchanger|Rear-Door Heat Exchanger]] · [[wiki/Air Cooling|Air Cooling]] - **part of:** [[wiki/Data Center Cooling|Data Center Cooling]] · [[wiki/Facilities and Thermal Management|Facilities and Thermal Management]] - **measured by:** [[wiki/Power Usage Effectiveness|Power Usage Effectiveness]] - **enables:** [[wiki/Heat Reuse|Heat Reuse]] - **source articles:** [[articles/Vertically Integrating an AI Superpower from AI Factory to Citizen|Vertically Integrating an AI Superpower from AI Factory to Citizen]] · [[articles/The Gigawatt Is the New Measure of War|The Gigawatt Is the New Measure of War]] - **collection route:** [[collections/American Infrastructure|American Infrastructure]] · [[collections/American|American Collection]] ## Evidence posture - **Established:** chiller types and the coefficient of performance; primary and secondary loop decoupling; economizer operation and its dependence on accepted supply temperature; approach and delta-T effects on pumping energy; N+1 plant redundancy and valved isolation; thermal storage for ride-through; the seconds-scale thermal margin of a liquid-cooled hall after flow loss. - **Established:** warm-water operation permitting compressor-free heat rejection for much or all of the year in suitable climates. ## Sources - ASHRAE design guidance for data center cooling plants and liquid-cooling temperature classes, as current October 2026.