Impact of Hot and Cold Aisle Containment on Data Center Temperature and Efficiency

Overview

The paper compares the two containment approaches in the same chilled-water plant. Both reduce mixing of hot and cold air, but with cold-aisle containment the rest of the room becomes the hot return. If that space has to stay tolerable for people, setpoints must be lowered, and that erodes economizer hours. With hot-aisle containment the room stays on the cold side; the heat is concentrated inside the contained aisle, so the work environment does not constrain the setpoints.

The comparison is quantitative. With the uncontained area held at a comfortable 24 C, HACS gives 5,319 economizer hours and a PUE of 1.69, while CACS gives 0 hours and a PUE of 1.98. Holding the room at 27 C, HACS reaches 6,218 hours and PUE 1.65, CACS 2,075 hours and PUE 1.86. The traditional uncontained baseline gives 2,814 economizer hours and PUE 1.84, with airflow at about 149% of IT airflow. Overall, HACS cuts annual cooling-system energy cost by 43% and annualized PUE by about 15%.

Containment quality drives the results: uncontained rooms typically leak or bypass 25 to 50% of airflow, while containment systems leak 3 to 10%. Uncontained rooms often supply about 13 C (55 F) air to avoid hot spots, far below what IT needs; the applicable recommended IT inlet range is 18 to 27 C. For people, a HACS hot aisle can reach about 47 C (117 F) at 45% cold-aisle RH and still stay within the OSHA 25/75 work/rest WBGT limit of 32.2 C. The paper recommends HACS as the default for new builds, and notes that CACS is often easier to apply in existing raised-floor rooms with perimeter units. Because the comparison rests on one chilled-water plant and specific scenario assumptions, the absolute PUE values do not transfer to every site; the direction and mechanism of the difference do.

Key points

1. HACS versus CACS: 43% lower annual cooling-system energy cost and about 15% lower annualized PUE.
2. Room held at 24 C: HACS 5,319 economizer hours, PUE 1.69; CACS 0 hours, PUE 1.98.
3. Room held at 27 C: HACS 6,218 hours, PUE 1.65; CACS 2,075 hours, PUE 1.86.
4. Traditional uncontained baseline: 2,814 economizer hours, PUE 1.84, airflow about 149% of IT airflow (49% cold-air and 20% hot-air leakage).
5. Uncontained rooms typically leak or bypass 25 to 50%; containment systems typically leak 3 to 10%.
6. A HACS hot aisle can reach about 47 C (117 F) at 45% cold-aisle RH and still meet the OSHA 25/75 work/rest WBGT limit of 32.2 C.
7. Uncontained rooms often supply about 13 C (55 F) air to avoid hot spots.
8. The applicable recommended IT inlet range is 18 to 27 C (64.4 to 80.6 F).

Using it in design

• Default to hot-aisle containment for new designs; if containment is deferred, reserve the provisions and space for a future HACS.
• For existing raised-floor rooms with perimeter CRAHs, offer CACS as the recommendation.
• Use airflow oversupply factors of 1.25 to 1.5 for uncontained rooms and 1.03 to 1.10 for contained ones.
• In the economizer and PUE model, let the containment choice shift the allowable chilled-water setpoint and the economizer hours.

Who should read it

Mechanical designers, operations engineers and retrofit planners deciding on aisle containment and cooling setpoints.

Related standards

• ASHRAE TC9.9 Thermal Guidelines
• OSHA heat stress (WBGT)
• The Green Grid PUE

Publication details

Authors

John Niemann; Kevin Brown; Victor Avelar

Publisher

Schneider Electric

Year

2017

Document number

WP 135

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