Guidelines for Specification of Data Center Power Density

Overview

The paper argues that specifying density in W/ft2 or W/m2 is ambiguous and misleading. The same hypothetical 500 kW, 100-rack room can be described as anything from about 1,282 to 8,039 W/m2 (119 to 746 W/ft2) depending on which area is used. That spread causes serious misunderstandings in specifications. The proposed method is to specify density per rack, with both an average and a peak value for each row or zone. A peak-to-average rack power ratio of about 2 within a row is recommended (it must be greater than 1). Optional rules-based placement of high-density racks is also proposed.

The numbers make the problem concrete: the typical area allowance is about 2.6 m2 (28 ft2) per rack and the rack footprint about 0.62 m2, so a 50 W/ft2 specification implies only 1,400 W per rack. IT equipment needs 100 to 160 cfm (47.2 to 75.5 L/s) of air per kW. Legacy design density was typically 40 to 80 W/ft2 (430 to 861 W/m2), while actual IT density is 3 to 20 kW per rack.

Raised-floor air delivery practically limits average density to about 5 kW per rack without special measures. Infrastructure space grows sharply with density: at 1.5 kW per rack about 15 percent of floor space goes to power and cooling equipment, and above about 7 kW per rack more than 50 percent. The practical result is to take density inputs per rack: average and peak rack power for each row or zone, with a default peak-to-average ratio of 2. W/m2 should appear only as a derived metric stating its area basis. Airflow should be computed at 100 to 160 cfm per kW, and the user should be warned when average power exceeds 5 kW per rack on a raised floor.

Key points

1. The same 500 kW / 100-rack room gives 119 to 746 W/ft2 (1,282 to 8,039 W/m2) depending on area definition
2. Typical area allowance about 2.6 m2 (28 ft2) per rack including aisles; rack footprint about 0.62 m2 (6.7 ft2)
3. A 50 W/ft2 spec implies only 1,400 W per rack
4. IT equipment requires 100 to 160 cfm (47.2 to 75.5 L/s) of air per kW
5. Raised-floor delivery practically limits average density to about 5 kW per rack without special measures
6. Recommended peak-to-average rack power ratio in a row is about 2 (must be above 1)
7. At 1.5 kW/rack about 15 percent of floor space goes to infrastructure; above 7 kW/rack more than 50 percent
8. Legacy design density 40-80 W/ft2 (430-861 W/m2) versus actual IT density of 3-20 kW per rack

Using it in design

• Make the primary density inputs per rack: avgRackKw and peakRackKw per row or zone, default peak/avg 2
• Show W/m2 only as a derived metric that states its area basis (IT-only, white space, gross)
• Compute required airflow with 100-160 cfm/kW (default 120 cfm/kW, about 56.6 L/s per kW)
• Warn when raised-floor supply is chosen with average rack power above 5 kW
• Optional rule: a rack can exceed the average only by the amount its neighbours are below it

Who should read it

Designers, consultants and client representatives who write power-density specifications.

Related standards

• ASHRAE TC9.9 Thermal Guidelines
• TIA-942

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2014

Document number

WP 120

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