CRAC, CRAH, Chillers and Liquid Cooling
Trace heat from silicon to the outside environment and match cooling architecture to density, climate, water and operational constraints.
Learning outcomes
- Differentiate CRAC and CRAH heat paths
- Follow chilled and condenser water loops
- Recognize when liquid cooling shortens the thermal path
COEFFICIENT OF PERFORMANCE
COP = HEAT REMOVED ÷ ELECTRICAL POWER USED
A COP of 4 means 4 kW of heat is moved for each 1 kW of cooling electricity under the stated conditions.
CRAC and CRAH are not interchangeable labels
A computer room air conditioner commonly uses a direct-expansion refrigeration circuit in or near the room. A computer room air handler uses a chilled-water coil supplied by a central plant. Both move room air, but the heat travels through different equipment and failure domains.
Names in the field are sometimes used loosely. Read the actual mechanical sequence, refrigerant or water connections and controls rather than inferring the whole design from one acronym.
The plant rejects heat outside
In a chilled-water design, the CRAH transfers room heat into chilled water. The chiller moves that heat to a condenser loop or another rejection path; cooling towers may then reject it to ambient air through evaporation. Pumps, fans and controls are part of the energy and reliability picture.
Economizers can use favorable outdoor conditions to reduce compressor work. Water availability, water quality, climate and local rules shape the best design.
Liquid moves closer to the chip
Rear-door heat exchangers capture rack exhaust. Direct-to-chip cold plates carry liquid to processors. Immersion places equipment in dielectric fluid. These approaches can support high heat flux with less room-air movement, but they introduce manifolds, leak detection, water quality, service procedures and new skills.
There is no universal winner. Density, hardware compatibility, heat-reuse goals, redundancy, maintenance and the existing building determine the practical path.
How it appears in Data Center Fan
Every cooling technology has an authored COP. Moving from window AC to row, chiller, rear-door or liquid systems reduces simulated cooling electricity for the same heat load and improves PUE.
Common misconception
“Liquid cooling removes the need to reject heat.” It changes how heat leaves the IT equipment; the captured heat still needs a final sink or useful reuse path.
Knowledge check
A cooling system removes 400 kW of heat while using 100 kW of electrical power. What is its COP?
- 4.0
- 0.25
- 500
COP = 400 ÷ 100 = 4.0.