Impact of High Density Hot Aisles on IT Personnel Work Conditions

Overview

The paper uses CFD to compare heat stress for staff in an uncontained raised-floor room and in a room with an enclosed hot aisle. Heat stress is expressed as WBGT, the metric used in OSHA and ISO work/rest limits. The finding is that containment raises hot-aisle WBGT by only about 1 degF, because IT exhaust air is dry (typically below 20 percent RH) and evaporation from people still works. In the uncontained case the air behind racks averaged about 4.4 C (8 F) above the cold aisle.

OSHA WBGT limits for moderate work are about 30 C (86 F) for continuous work, 87 F for 75/25 work/rest, 89 F for 50/50 and 32.2 C (90 F) for 25/75. Bulk room conditions were historically about 22 C (72 F). At the densities studied, hot-aisle heat stress stays well below the regulatory limits. The results depend on specific assumptions: 3 kW per rack, 45 percent RH control and a 0.91 m (36 in) unobstructed raised floor. For designs that depart from those assumptions, especially shallower or obstructed floors, the conclusions may not hold.

In a design tool a hot-aisle work-environment check can be defined. Hot-aisle dry-bulb temperature is estimated as supply temperature plus rack deltaT, and WBGT is estimated from humidity. Above 30 C WBGT, work/rest scheduling is needed; 32.2 C is the limit of the 25/75 regimen. The check should appear in the containment designer and in operations and maintenance notes.

Key points

1. Containment raised hot-aisle heat stress by only about 1 degF WBGT versus an uncontained room
2. OSHA WBGT limits (moderate work): continuous about 86 F/30 C; 75/25 87 F; 50/50 89 F; 25/75 90 F (32.2 C)
3. Bulk room conditions historically about 22 C (72 F); IT exhaust typically below 20 percent RH
4. CFD study fixed at 3 kW per rack, 45 percent RH control, 0.91 m (36 in) unobstructed raised floor
5. Air behind racks averaged about 4.4 C (8 F) above the cold aisle in the uncontained case

Using it in design

• Add a hot-aisle work-environment check
• Estimate hot-aisle dry-bulb as supply temperature + rack deltaT, and estimate WBGT from humidity
• Warn when WBGT exceeds 30 C (work/rest scheduling needed) or 32.2 C (the 25/75 limit)
• Show this check in the containment designer and in O&M notes

Who should read it

Engineers designing hot-aisle containment and data-centre operations safety officers.

Related standards

• OSHA heat stress (WBGT)
• ISO 7243
• ASHRAE TC9.9 Thermal Guidelines

Publication details

Authors

Jim Fink

Publisher

Schneider Electric

Year

2010

Document number

WP 123

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