computer room air conditioning (CRAC) unit

What is a computer room air conditioning (CRAC) unit?

A computer room air conditioning (CRAC) unit is a device that monitors and maintains the temperature, air distribution and humidity in a data center, network or server room. CRAC units replace the air-conditioning units used in the past to cool data centers. By comparison, CRAC systems have better air filtration, better humidity control mechanisms and higher airflow than typical AC systems.

CRAC units help prevent low humidity and water vapor from forming. Low humidity can cause static electricity buildup, which can damage electronics, and water vapor buildup, which can cause short circuits and corrode equipment.

CRAC units work via a refrigeration cycle where air is blown over a cooling coil filled with refrigerant. Refrigerant in the cooling coil is kept cold by a compressor. Excess heat is expelled as air, water or a glycol mixture. Older CRAC units can only turn on and off, while newer units enable different airflow variations.

There are a variety of ways that CRAC units can be situated. One popular CRAC setup is having cooling air dispensed through an elevated floor. Also called raised flooring, the cooled air rises through the perforated floor sections, forming cold aisles. The cold air flows through the racks, where it picks up heat before exiting from the rear of the racks. The warm exit air forms hot aisles behind the racks, and the hot air returns to the CRAC intakes, which are positioned above the floor.

An example of a raised floor airflow
Raised floor cooling in data centers forces cool airflow upward through the floor, up past servers and into a CRAC intake where the air is cooled.

Components of a CRAC unit

CRAC units include the following components:

  • Refrigerant. This is a chemical compound that is used as a coolant in CRAC and air conditioners.
  • Cooling coils. Also called evaporator coils, refrigerant flows through the cooling coils to cool the air.
  • Compressors. This keeps the refrigerant running through the coil cool.
  • Air filters. These help catch the accumulation of dirt or dust that travels through a CRAC unit.
  • Electronically commutated (EC) fans. EC fans blow cool air while producing less heat with high efficiency.
  • Humidifiers and dehumidifiers. Although these are optional in CRAC systems, humidifiers increase the amount of moisture in the air, while dehumidifiers remove water vapor from the air.

What are the differences between CRAC and CRAH units?

Computer room air handler (CRAH) units are another data center cooling device. The main difference between CRAH and CRAC units is how they cool their surrounding environments. CRAC units use both refrigerants and compressors, while CRAH units use chilled water and control valves.

CRAH units use fans to blow air over cooling coils. Instead of the coils being filled with refrigerant, however, they're filled with chilled water. Warm air from the data center room is drawn back into the CRAH unit, passing over the chilled water coils. The heat transfers from the air to the water, where the water then returns to a chiller.

Because CRAH units don't need a compressor, they use less energy and are more efficient when compared to CRAC units. This makes CRAH units more suitable for sustainable data centers.

CRAC units are ideal for data centers with electrical loads of 200 kilowatts (kW) or less. Meanwhile, CRAH units are designed for data centers with electrical loads of 200 kW or more. CRAH units are also ideal for environments with higher availability requirements.

Learn more about the cooling systems used in data centers, like how liquid and air cooling compare to each other.

This was last updated in August 2022

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