Cooling Options for Rack Equipment with Side-to-Side Airflow

Overview

The problem comes from three recirculation mechanisms: adjacent equipment feeding its exhaust into the next intake, no partition between intake and exhaust, and enclosures amplifying the effect. In open-frame rows, the intake air of side-breathing equipment can run 10°C (18°F) above ambient because of neighbouring exhaust, and the excess grows rack by rack along the row. Under MIL-HDBK 338, every 10°C above nominal halves equipment lifetime.

Four mitigations are compared on density, floor space, efficiency, predictability and TCO: spaced open frames, low-density enclosures, supplemental fans, and active side air distribution that converts the flow to front-to-back. Rack density limits are 0-1 kW, 0-1 kW, 0-2 kW and 0-6 kW respectively. In a test with a Cisco 6500 switch in a 42U enclosure, rack side air distribution lowered switch inlet temperature by an average of 8.3°C (15°F).

The paper also collects reference limits and vendor restrictions: the ANSI T1.304-1997 telecom operating range of 5 to 40°C, the ASHRAE TC9 recommended inlet of 18 to 27°C, and shutdown of side-breathing equipment above 40°C. Vendor guidelines call for at least 6 inches between a wall and chassis vents and 12 inches between two chassis. Recirculation is said to add over 10% to CRAC electricity cost. The 2011 study predates current product-level side-airflow solutions.

Key points

1. Per MIL-HDBK 338, every 10°C (18°F) rise above nominal temperature halves equipment lifetime.
2. In open-frame rows, side-to-side equipment intake air may be 10°C (18°F) above ambient because of adjacent exhaust, and it accumulates along the row.
3. Recirculation and hot spots can raise CRAC electricity cost by over 10%; cooling often takes nearly half of data-centre electricity.
4. Test in a 42U enclosure with a Cisco 6500: side air distribution lowered inlet temperature by an average of 8.3°C (15°F).
5. Density per rack: spaced open frames 0-1 kW, low-density enclosure 0-1 kW, supplemental fan 0-2 kW, side air distribution 0-6 kW.
6. Quoted limits: ANSI T1.304 range 5-40°C, ASHRAE TC9 recommended inlet 18-27°C; side-breathing gear typically shuts down above 40°C.
7. Vendor restrictions: at least 6 inches wall-to-vent, 12 inches between chassis horizontally, no congested or unventilated enclosed racks, use baffles.

Using it in design

• Add an airflowPattern field to the equipment catalogue (front_to_back, back_to_front, side_to_side, side_to_back) and fill it in for switches and routers first.
• Warn when a side_to_side device sits in an enclosed rack with no side-air-distribution or baffle accessory; cap rack density near 1 kW (open/spaced) or 2 kW (fans) unless such a unit is specified, which allows up to 6 kW.
• Flag adjacent racks where side exhaust lines up with the neighbour's intake; without ducting, reserve U space for air-turning devices and keep 6 inches to walls and 12 inches between chassis.
• Show the 10°C-halves-life rule next to inlet-temperature risk in reliability tooltips.

Who should read it

Designers placing switches, routers and other side-breathing gear in network rooms and data-centre halls.

Related standards

• ASHRAE TC 9.9 Thermal Guidelines
• ANSI T1.304
• EIA-310-D
• MIL-HDBK-338

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2011

Document number

WP 50

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