The guide below is to provide some indication on how the settings at CRAC (computer room air conditioning) units can be modified to satisfy ASHRAE recommended range and save energy on cooling and humidification.


How To Guide

Note: This assumes that humidity is not controlled by fresh air AHUs (air handling units).

  1. Reduce fresh air supply to data hall to minimum requirements of pressurisation / ventilation
  2. Measure air inlet temperature to IT equipment
  3. Resolve for each hot spot (that affects air entering other IT equipment) with blanking plates, moving grilles, removing cabinet glass doors, etc. See other blogs on air performance.
  4. Increase CRAC set points. If possible, increase the return air set point, but ensure to maintain IT equipment within recommended ASHRAE ranges (18°C / 64.4°F to 27°C / 80.6°F). Ideally it would be best to change the CRAC control strategy from return air to supply air control so that constant air temperatures are supplied.
  5. The next step is to think about adjusting the Relative Humidity (RH) settings controlled by the CRAC units. For humidification the dew point (DP) needs to be maintained above 5.5°C / 41.9°F. This is done by aligning the return air temperature set point at the CRAC with its RH setting as follows: 24°C / 75.2°F (30%RH), 26°C / 78.8°F (27%RH), 28°C / 82.4°F (24%RH), 30°C / 86°F (21%RH) and 32°C / 89.6°F (19%RH). Note these RH setting are much lower than the legacy values normally used, but will satisfy ASHRAE recommended environmental range.
  6. For de-humidification, the dew point should be below 15C / 59F (for supply air temperatures to IT equipment between 23°C / 74.4°F and 27°C / 80.6°F). Given that most cooling systems will have ADP (apparatus dew point) well below these values, the necessary dehumidification will take place anyway in the CRAC cooling coil. Therefore, no further measures are not normally necessary other than having condensate trays and pipes on CRAC units.
  7. However, if there is an important source of humidification due to high external humidity, and active dehumidification is required the RH set points on the CRAC units should be set to maintain the Dew Point below 15°C / 59°F as follows: 24°C / 75.2°F (57%RH), 26°C / 78.8°F (51%RH), 28°C / 82.4°F (45%RH), 30°C / 86°F (40%RH) and 32°C / 89.6°F (36%RH).
  8. Periodically monitor the supply air temperature dry bulb and dew points to the IT equipment