RECAP | ROUNDTABLE | Understanding Supply Air Control

On 13 Feb 2020, Members of the Institute discussed in a roundtable on the topic of Supply Air Control.

Terry Altom, Uptime Institute, senior consultant, opened a 13 Feb 2020 Inside Track roundtable on supply air control (SAC), by noting its increased use in data center facilities. He said that the use of SAC enabled data operators to increase the chilled water from 50oF to 60oF (10oC to 15.5oC) at three facilities in which he worked by moving temperature sensors, tweaking controls and changing set points. Rebates from the electrical company were about USD $30K.

Altom noted that SAC in chilled water facilities, requires good airflow management, including hot aisle/cold aisle arrangements, sealing gaskets, blanking plates and potentially, aisle containment for higher density rows.

Terry, however, cautioned that Implementing SAC at other facilities can be more difficult, especially in data centers that include direct expansion cooling (DX) systems. Compressors in these systems, particularly older ones may need 2-3 minutes to restart, during which time, the blower is blowing hot air beneath the raised floor. In one instance, a data center was using a 25-year-old DX unit with no quick compressor restart feature, so that switching to SAC may not have been a good risk. Altom also discussed challenges posed to SAC by newer servers with variable speed fans.

Roundtable participants had a number of questions. As a result, Altom explained that programming CRACS to operate on data from external sensors is not difficult, as the supplier provides programming for both the chilled water valve and fan speed. He prefers installing a differential pressure sensor to control the speed of the CRAC’s variable speed fan.

Altom addressed a follow up question by suggesting the further explaining the benefit of using differential pressure to control the fans, particularly in data halls with differing rack densities. It is important to share the standard heat load formula for the servers with the person laying the perorated tiles, so that right number and type are placed. It is also important to have sufficient positive static pressure under the floor.

Containment strategies became a subject of discussion. One participant noted that hot aisle and cold aisle containment have roughly the same efficiency and cost. The participant also noted that in facilities using hot aisle containment systems:

• Personnel servicing equipment will be exposed to the hotter temperatures
  
• Cooler ambient air can serve as a reservoir of cold air.
  
• The hot air return path is particularly important when using return air control systems because the CRAC will perform at a higher efficiency, when the return air is hot.

Altom added that the building design, the mechanisms installed and of the presence or absence of plenums are also considerations to selecting cold or hot aisle containment. Altom also advised that the raised floor should be free of blockages and high enough to have sufficient air volume, especially when used in conjunction with an SAC system.