The Use of Ceiling-Ducted Air Containment in Data Centers

Overview

Using a flow-network model coupled to CFD, the analysis shows that performance is governed mainly by the vacuum the ceiling plenum holds relative to the room, which depends on cooling-to-IT airflow ratio, ceiling leakiness and plenum depth, more than on duct construction. Assumptions are IT airflow of 125 cfm/kW (212 m3/h/kW), racks filled to 30U, HACS clusters of 20 racks and 8 in (200 mm) server-to-rear-door clearance for ducted racks; 'Bad' means at least 10% recirculation.

At a plenum vacuum of 0.03 inH2O (7.5 Pa), individually ducted racks are limited to about 8 kW/rack while ducted HACS supports up to about 16 kW/rack. Vacuum above 0.03 inH2O needs deliberate sealing effort, and above about 0.06 inH2O (15 Pa) typical ceiling tiles begin to lift, capping achievable vacuum. A cooling-to-IT airflow ratio of 1 is generally acceptable for ducted HACS, but ducted racks may need 1.2 or higher. Rack-to-rack uniformity for HACS is good with plenums deeper than 18-24 in (0.46-0.61 m), even at 12 kW racks.

Active ducted racks allow up to about 12 kW/rack but add about 10 in (250 mm) to rack depth. Capital cost per rack is $500-700 for passive ducted, $1,800-2,000 for active ducted and $500-1,000 for ducted HACS.

The recommendation is to fix plenum pressure before adding fans. Implementation guidance: plenum at least 18 in (450 mm) deep, and ducted cooling airflow at least 10% above ducted IT airflow, or 20% for passive ducted racks with leaky ceilings.

Key points

1. At 0.03 inH2O (7.5 Pa) vacuum, ducted racks reach about 8 kW/rack and ducted HACS about 16 kW/rack.
2. Above about 0.06 inH2O (15 Pa), typical ceiling tiles lift and cap the vacuum.
3. Cooling-to-IT airflow ratio: 1 for HACS, 1.2 or higher for ducted racks.
4. HACS uniformity is good with plenums deeper than 18-24 in (0.46-0.61 m), even at 12 kW racks.
5. Active ducted racks reach about 12 kW/rack and add about 10 in (250 mm) of depth.
6. Cost per rack: passive $500-700, active $1,800-2,000, ducted HACS $500-1,000.
7. Plenum at least 18 in (450 mm); ducted cooling airflow at least 10% above IT (20% for passive racks with leaky ceilings).

Using it in design

• Add passive and active ducted racks and ducted HACS to the containment selector, with density limits tied to a ceiling plenum vacuum input (default 0.03 inH2O / 7.5 Pa, warning above 0.06 inH2O / 15 Pa).
• Raise an error for plenum depth under 18 in (450 mm) in ducted designs, and warn when the cooling/IT airflow ratio is under 1.1 (1.2 for passive chimneys).
• Suggest ducted HACS when a passive chimney rack exceeds about 8 kW.
• For active ducted racks, state that CFD verification is required and add 250 mm to rack depth or row pitch.

Who should read it

Cooling and layout engineers designing ceiling-plenum containment.

Related standards

• ASHRAE TC 9.9 Thermal Guidelines for Data Processing Environments
• EN 50600-2-3

Publication details

Authors

Jim VanGilder; Simon Zhang; Paul Lin

Publisher

Schneider Electric

Year

2017

Document number

WP 182

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation