Ten Cooling Solutions to Support High-density Servers

Overview

The early measures are operational hygiene: a cooling health check, maintenance, blanking panels and cable management, clearing underfloor blockages and sealing cut-outs, spreading high-density racks, hot/cold aisle layout, aligning CRACs with hot aisles and placing vents correctly. The last two are fan-assisted airflow devices and a change of air-distribution architecture to containment, row-based cooling or rack/row containment pods. The Uptime Institute found operational deficiencies (dirty coils, undercharged DX, reversed piping, unused economizer modes) in more than 50% of the data centres it visited.

A traditional raised-floor CRAC room without air containment is unlikely to deliver more than 3-6 kW to any rack regardless of CRAC count or floor depth; flooded-aisle hard-floor rooms deliver even less. The example room has a 5,000 ft2 raised floor, 30 in (762 mm) depth, 560 kW UPS load, 200 racks and a 2.8 kW average; cooling is unlikely to exceed 3 kW per rack. Five 10 kW racks grouped in one row cannot be supported, but spreading them lets them borrow neighbours' capacity.

With raised-floor cooling, CRAC units should be aligned with hot aisles (the return path) rather than cold aisles, since CFD shows hot air crossing over racks otherwise. Fan-assisted devices (ADU/ARU, active tiles) can raise rack cooling to between 3 kW and 8 kW and should be UPS-powered to avoid thermal overload before the generator starts.

When average rack density exceeds 3 kW in an uncontained flooded supply/flooded return system, the air-distribution architecture must change; row-based cooling suits smaller data centres of 200 kW or less. For sustained densities above 6 kW per rack over an area, going directly to solutions 9 or 10 may be appropriate; hot-aisle containment pods and rack air containment systems support up to 60 kW per rack.

Key points

1. An uncontained raised-floor CRAC room is unlikely to deliver more than 3-6 kW to any rack regardless of CRAC count or floor depth; flooded hard-floor rooms deliver less.
2. For sustained densities above 6 kW per rack over an area, going directly to solution 9 or 10 may be appropriate.
3. The Uptime Institute found operational deficiencies in more than 50% of data centres visited.
4. Example: 5,000 ft2, 762 mm floor, 560 kW UPS load, 200 racks, 2.8 kW average; cooling unlikely to exceed 3 kW per rack; five 10 kW racks in one row cannot be supported, but spread out they borrow neighbours' capacity.
5. CRACs should align with hot aisles for the return path; otherwise hot air crosses over racks.
6. Fan-assisted devices raise rack cooling to 3-8 kW and should be UPS-powered.
7. Above 3 kW average in an uncontained flooded system the architecture must change; row-based cooling suits 200 kW or less.
8. Hot-aisle containment pods and rack air containment systems support up to 60 kW per rack.

Using it in design

• Set density warnings by air-distribution type: uncontained flooded return above 3 kW/rack average, uncontained raised floor above 3-6 kW/rack, fan-assisted retrofit ceiling 8 kW/rack, HACS/RACS pods up to 60 kW/rack.
• Check layout: all racks in a row face the same way, rows alternate hot and cold, perimeter CRAC/CRAH sit at hot-aisle ends, perforated tiles only in cold aisles.
• Flag clustered high-density racks that exceed local tile or cooler capacity even when the room average is fine.
• Put fan-assist devices and row-cooler fans on UPS-backed circuits in high-density zones, and recommend row-based cooling by default for rooms of 200 kW or less with high-density loads.

Who should read it

Designers and operators adding high-density racks to rooms built for low average density.

Related standards

• ASHRAE TC 9.9 Thermal Guidelines

Publication details

Authors

Peter Hannaford

Publisher

Schneider Electric

Year

2015

Document number

WP 42

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