Cooling Equipment: CRAC, CRAH, and Chilled-Water Systems

Why Data Centers Need Dedicated Cooling

IT equipment converts nearly all the electrical power it consumes into heat. Unlike a typical office building, a data center must remove that heat continuously, 24 hours a day, every day of the year. Two families of equipment do most of that work: Computer Room Air Conditioners (CRACs) and Computer Room Air Handlers (CRAHs). Understanding how each works—and how they differ—is essential for anyone managing or maintaining a critical facility.

The Direct-Expansion CRAC Unit

A CRAC unit uses a direct-expansion (DX) refrigeration cycle, meaning refrigerant circulates inside the unit itself to absorb and reject heat. The cycle has four major components:

Compressor — The compressor draws in low-pressure refrigerant vapor and compresses it to a high-pressure, high-temperature vapor. Think of it as the pump that drives the whole cycle. The compressor is typically the heaviest electrical load in the unit and the component most likely to require scheduled maintenance.

Condenser — The hot, high-pressure vapor travels to the condenser, where it releases heat and changes state into a high-pressure liquid. In an air-cooled CRAC, the condenser coil is cooled by outdoor air via a remote condensing unit or by internal fans. Heat is rejected to the environment at this stage.

Expansion Valve — The high-pressure liquid passes through the expansion valve (also called a metering device), which causes a rapid pressure drop. As pressure falls, the refrigerant cools dramatically and becomes a cold, low-pressure liquid/vapor mix. This is where the useful cooling potential is created.

Evaporator Coil — The cold refrigerant enters the evaporator coil inside the CRAC unit. Warm air from the data center is drawn across this coil by the blower (fan assembly). The refrigerant absorbs heat from the air, evaporates into a low-pressure vapor, and the now-cooled air is discharged back into the room or under-floor plenum. The low-pressure vapor returns to the compressor, and the cycle repeats.

The blower is the fifth component students must know: it moves air across the evaporator coil and is the primary driver of airflow in the room.

The Chilled-Water CRAH Unit

A CRAH unit contains no refrigerant compressor. Instead, it circulates chilled water supplied from a remote chiller plant through a water coil. The blower draws warm return air across the coil; the water absorbs the heat and carries it away. The CRAH itself is mechanically simpler—essentially a coil and a fan—which reduces on-unit maintenance. However, the chiller plant that produces the cold water adds complexity at the building level.

The Chiller Plant Loop

A chilled-water system has two connected loops:

1. Chilled-water loop (primary/secondary): The chiller cools water to a design supply temperature (commonly around 44 °F / 6–7 °C, though higher temperatures are used in high-efficiency designs). Primary pumps circulate water through the chiller; secondary pumps distribute it to the CRAH units throughout the facility. After absorbing heat at the CRAH coils, the warmer return water travels back to the chiller. A typical temperature differential (delta-T) between supply and return is around 10–14 °F, though designs vary.

2. Condenser-water loop: The chiller must reject the heat it absorbs. It does this by transferring heat to condenser water, which is pumped to a cooling tower (or dry cooler in water-scarce regions). The cooling tower rejects heat to outdoor air, and the cooled condenser water returns to the chiller.

Labeling supply and return temperatures at each stage of this loop—chiller output, CRAH inlet, CRAH outlet (return), and chiller inlet—is a core skill for troubleshooting and efficiency monitoring.

CRAC vs. CRAH: Key Contrasts

| Feature | CRAC (DX) | CRAH (Chilled Water) |

|---|---|---|

| Refrigerant in unit | Yes | No |

| Cooling source | On-board compressor | Remote chiller plant |

| Scalability | Moderate (add units) | High (central plant serves many units) |

| Typical use | Smaller or edge sites | Large enterprise/hyperscale |

| Maintenance complexity | Higher on-unit | Distributed across plant |

ASHRAE Supply Air Temperature Guidance

ASHRAE Technical Committee 9.9 publishes recommended and allowable supply air temperature ranges for IT equipment. The widely referenced A1 class equipment recommendation for supply air is broadly in the range of 64.4–80.6 °F (18–27 °C), though specific values depend on equipment class. Setting CRAC/CRAH supply air setpoints too cold wastes energy by running the refrigeration plant harder than necessary; setting them too warm risks equipment overheating. Operators should consult the current ASHRAE TC 9.9 guidance for their specific equipment classes when configuring setpoints.

Safety Notes

Before working on or near any cooling equipment: follow your site's lockout/tagout (LOTO) procedures (OSHA 29 CFR 1910.147) before performing any maintenance on energized equipment. CRAC compressors and condenser fans operate at line voltage—treat all electrical panels as energized until proven otherwise with a calibrated meter. Refrigerants may cause frostbite on skin contact and can displace oxygen in enclosed spaces; wear appropriate PPE (insulated gloves, eye protection) and ensure adequate ventilation. Chilled-water systems involve water under pressure—verify isolation valves are closed and pressure is relieved before opening any pipe fitting. Slip hazards from condensate are common around CRAC/CRAH units; keep the area dry and clearly marked.