# Facility heat rejection and cooling tradeoffs

Canonical: https://semiaifoundry.com/learn/atlas/facility-heat-rejection/
Revision: 2026-10-07
Rights: https://semiaifoundry.com/ai-atlas/licenses.html

Data-center physical systems · Atlas v1.3.0

# Facility heat rejection and cooling tradeoffs

Transfers collected heat to the outdoor environment under weather-dependent limits.

[Explore in Atlas](https://semiaifoundry.com/ai-atlas/#facility-heat-rejection)Chip voltage rails to facility heat rejection

Learning objectiveConnect rack heat transport to a weather- and resource-dependent final rejection system.

## The mechanism

Liquid cooling moves heat away from chips, but the heat must still leave the facility. Dry coolers exchange heat with outdoor air; evaporative systems use water evaporation; chillers can lift heat to a higher rejection temperature when ambient conditions demand it. Actual systems combine these methods and change operating modes with weather and load. Heat-exchanger approach temperatures and fan, pump or compressor energy affect usable capacity. The Department of Energy’s cooling guidance connects water and energy choices. A favorable annual efficiency figure cannot establish performance on the hottest design day or reveal which watershed supplies consumptive water.

1.  Sum the heat reaching the facility system, including auxiliary equipment included in the boundary.
2.  Choose the rejection route and identify its dependence on dry-bulb or wet-bulb conditions.
3.  Account for heat-exchanger temperature approaches and the required coolant supply temperature.
4.  Calculate fan, pump or compressor demand at the actual operating point.
5.  Check water availability, seasonal limits and fallback modes before assigning continuous cooling capacity.

## Science and assumptions

### Chiller performance

$$
\mathrm{COP}=\frac{\dot Q_{\mathrm{removed}}}{P_{\mathrm{elec}}}
$$

Cooling COP is heat removed divided by the included electrical input power.

Symbols and units

COP

Cooling coefficient of performance at the declared point

dimensionless

Q̇\_removed

Heat removed inside the stated cooling boundary

W

P\_elec

Electrical input included in that same cooling definition

W

Both heat removed and electrical input are watts; COP is dimensionless.

Assumptions A specified operating point; auxiliary loads and heat-rejection equipment must be included or excluded explicitly.

## Engineering example

Synthetic chiller removing 1 MW with 0.2 MW electrical input has COP 5. Its condenser must reject about 1.2 MW when that compressor input enters the same heat path.

## Distinctions and tradeoffs

Direct liquid cooling does not necessarily mean evaporative cooling, zero water use or free heat rejection.

Dry rejection can reduce direct water use but may require more area or electricity in hot conditions; evaporative operation adds local water demand.

Heat-rejection limits can cap sustained cluster operation even when rack cold plates and electrical capacity are installed.

## Specifications and interfaces

Ultimate sink

Outdoor air and/or evaporative rejection

Internal liquid transport is upstream of the final sink.

Operating condition

Weather, load and supply temperature

Efficiency and capacity vary with the stated condition.

### Interfaces

Facility heat loop Input

Heat collected from IT and cooling equipment

Environment Output

Rejected heat, evaporation where used and auxiliary demand

## Check your understanding

### Why can cooling capacity vary with weather?

1.  Heat stops being conserved outdoors
2.  GPU count changes the atmospheric constants
3.  The available temperature difference and evaporation conditions change

Answer and reasoning

The available temperature difference and evaporation conditions change

Heat rejection depends on the environmental sink and equipment operating point, not merely on installed component count.

## Evidence and scope

COP and water values are not manufacturer defaults. Local climate, equipment selection and accounting boundaries determine actual performance.

No current calculator directly models this lesson’s main mechanism. Use the library for related bounds while retaining their stated omissions.

## Sources

-   [Cooling Water Efficiency Opportunities for Federal Data Centers](https://www.energy.gov/cmei/femp/cooling-water-efficiency-opportunities-federal-data-centers)

[Published lesson data](https://semiaifoundry.com/ai-atlas/lesson-data/facility-heat-rejection-41f650cbcecd4500.json) · [Atlas releases and licensing](https://semiaifoundry.com/ai-atlas/licenses.html)

## Continue exploring

### Foundations

-   [Facility cooling and final heat rejection](https://semiaifoundry.com/learn/atlas/cooling-plant/)
-   [Cold plates, manifolds and separated coolant loops](https://semiaifoundry.com/learn/atlas/coolant-loop-separation/)

### Related lessons

-   [Water withdrawal, consumption and cooling circulation](https://semiaifoundry.com/learn/atlas/water-boundary-accounting/)