What is a CRAC?
A CRAC (computer room air conditioner) is a precision cooling unit that removes heat from a data center or server room using its own refrigeration cycle. Each unit contains a compressor, an evaporator coil, fans and controls, and rejects heat outdoors through a condenser or a water or glycol loop. CRAC units are built to run continuously, to handle the mostly sensible heat that IT equipment produces and to hold temperature and humidity within narrow limits, which sets them apart from comfort air conditioners. They are common in small and medium rooms, edge sites and older raised-floor facilities.
CRAC vs CRAH
The nearest related term is CRAH (computer room air handler). A CRAH has no compressor. It blows room air across a coil carrying chilled water from a central chiller plant, and a valve controls its cooling output. Large facilities often choose CRAH units with chillers because a central plant can run more efficiently at scale and can use economizers. CRAC units are self-contained, so each one is easier to add and does not depend on a chiller plant.
- Cooling source: a CRAC uses direct expansion refrigerant; a CRAH uses chilled water.
- Compressor: inside every CRAC; in the central plant for CRAH systems.
- Typical use: CRAC for small rooms and edge sites; CRAH for large halls.
- Failure domain: a CRAC failure affects one unit; a chiller plant failure can affect every CRAH it serves.
Target temperature and humidity
Most operators set targets with reference to the ASHRAE TC 9.9 thermal guidelines. The recommended range for IT equipment inlet air is 18 to 27 °C (64.4 to 80.6 °F), with humidity limits expressed mainly as dew point. Measure at the server inlets on the cold aisle side. Readings at the CRAC return can be much warmer than inlet air, and a unit that controls on return temperature may overcool some racks while others run hot.
A worked example: sizing and N+1
Sizing starts with heat load. Nearly all electrical power drawn by IT equipment becomes heat, so a room with 120 kW of IT load needs about 120 kW of sensible cooling, plus allowances for lighting, UPS losses and people. In imperial units that is about 34 tons of refrigeration, since one ton equals roughly 3.52 kW. With CRAC units rated at 40 kW of sensible capacity each, three units carry the load (N = 3). An N+1 design adds a fourth unit, so the room survives one failure or one unit down for maintenance. If the load grows to 130 kW, four units still cover it, but the spare is gone.
Recheck cooling redundancy whenever IT load grows, using measured load instead of design figures.
Airflow management
A CRAC can only cool what its air reaches. Most rooms arrange racks in hot and cold aisles so that supply air enters the front of servers and exhaust returns to the units without mixing. Missing blanking panels, open cable cutouts in raised floors and racks installed facing the wrong way all let hot exhaust recirculate into server inlets. The result is hotspots even when total cooling capacity is sufficient. Containing the hot or cold aisle reduces mixing and lets units run with warmer supply air.
What to monitor on a CRAC
Room temperature can rise quickly after a cooling failure, so CRAC controllers are usually connected to monitoring over Modbus, BACnet or SNMP. Common points include:
- Supply and return air temperature and humidity.
- Compressor status, run hours and high or low pressure alarms.
- Fan status and speed.
- Filter differential pressure or dirty filter alarms.
- Water leak detection under the unit.
- Rack inlet temperatures in the zone the unit serves.
How CloudSino approaches CRAC monitoring
CloudSino DCOS lists precision air conditioners and temperature and humidity devices among the server room infrastructure it supports, alongside UPS units, PDUs and liquid cooling facilities. Its energy and capacity functions include intake and exhaust temperature monitoring and hotspot risk warnings. CloudSino's energy and capacity solution adds per-U-position temperature monitoring and server room heatmaps that help teams locate hotspots and adjust air conditioning. DCOS automated inspection also checks environment monitoring equipment and displays exceptions centrally.
Frequently asked questions
How many CRAC units does a room need?
Enough to carry the full heat load with the redundancy your design requires. Divide the expected sensible heat load by the sensible capacity of one unit at your design conditions, round up to get N, then add spares. N+1 is common. Confirm that airflow from the remaining units still reaches every rack when one unit is off.
Can CRAC units cool high-density racks?
Room-level air cooling gets harder as rack density rises, because delivering enough cold air to each rack becomes difficult. Dense racks often need containment, in-row cooling, rear-door heat exchangers or direct liquid cooling, with CRAC units handling the remaining room heat.
Why do neighboring CRAC units sometimes work against each other?
When units control humidity independently, one can humidify while another dehumidifies, which wastes energy. Drifting sensors and older standalone controls make this more likely. Networking the units so they share setpoints and run as a group, and calibrating their sensors, usually solves it.