A Computer Room Air Handler (CRAH) unit is a device within a data center that cools the air using a cooling coil filled with chilled water. The water is supplied to the CRAHs by a chilled water plant, also known as a chiller. This differs from a Computer Room Air Conditioning (CRAC) unit as CRACs use mechanical refrigeration. CRAHs can modulate fan speed to maintain a set static pressure, allowing for the stability of both temperature and humidity.
CRAH Unit Installation Configurations
The layout and configuration of the room where the CRAH units are installed determines the most optimal configuration. Some potential configurations include:
- Down flow supply. A unit with a down flow configuration draws air from the space that is to be cooled and passes it through the top of the CRAH unit. Once the air enters the unit, it is drawn downwards through the cooling coil where it is then conditioned before being blown back into the space through the lower frontal part of the unit. This configuration is most common in data centers that use CRAH units with a fan wall.
- Up flow supply. An up flow configuration involves air being drawn from the space that is to be cooled and passed through the lower frontal section of the CRAH unit. The, the air is drawn upwards through the cooling coil where it is then conditioned and blown back into the space through the upper frontal part of the unit. This configuration is most common in electrical rooms, IT rooms, and UPS rooms.
- Frontal flow supply. A unit with a frontal flow configuration draws hot air from the space and passes it through the lower frontal section of the CRAH unit. The air is then drawn upwards through the cooling coil where it is conditioned and blown back into the space through the upper frontal part of the unit. This configuration is most common in electrical switch, UPS, and IT rooms.
- Under floor supply. An under floor supply configuration involved hot air being drawn into the unit through the top and then passed through the cooling coil. The CRAH unit is mounted on a frame installed within the floor to allow conditioned air to pass through and then dispersed. The conditioned air is passed into the space and then conditioned further using floor grilles usually positioned near/in front of the racks.
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