Airflow Management Considerations for a New Data Center - Part 1: Server Power versus Inlet Temperature

Overview

In this 2017 post on the Upsite Technologies blog, Seaton tests the belief that energy is wasted as server fans speed up when inlet temperature rises. He reviews HP and Dell/APC studies, then builds a model of 1,000 Class A3 servers in an 800 kW facility with chiller, CRAH and server power at several inlet temperatures.

Among the prior studies, the HP study showed PUE of 1.27/1.20/1.19/1.18 at 68/77/86/95 F, but task cost of $510/$501/$565/$601, so PUE and cost diverge. In the modelled A3 800 W server, mean power is 800 W at 68 F and rises to 904 W at 104 F.

In the 800 kW model, total facility power is 994.7 kW at 68 F, 985.2 kW at 77 F, 988.7 kW at 86 F and 1,078.3 kW at 104 F, with a sweet spot of about 86-90 F. The Albuquerque free-cooling analysis finds that an air-side economizer design can be chiller-free if inlets may reach about 99 F, and a water-side design at 86 F, while this is not achievable in Houston or Miami.

The author warns that redundancy of air movers can be exhausted at the highest temperatures. If server fan energy is unknown, about 10% of operating load can be assumed. The work is a model and a blog-level analysis, and needs validation for any specific server or facility.

Key points

1. HP study: PUE 1.27/1.20/1.19/1.18 at 68/77/86/95 F but task cost $510/$501/$565/$601; PUE and cost diverge.
2. Modelled A3 800 W server: mean power 800 W at 68 F rising to 904 W at 104 F.
3. 800 kW model: total facility power 994.7 kW at 68 F, 985.2 kW at 77 F, 988.7 kW at 86 F, 1,078.3 kW at 104 F; sweet spot about 86-90 F.
4. Albuquerque: air-side economizer design can be chiller-free if inlets may reach about 99 F; water-side chiller-free at 86 F; not achievable in Houston or Miami.
5. If server fan energy is unknown, assume about 10% of operating load.
6. Air-mover redundancy can be exhausted at the highest temperatures.

Using it in design

• Include server fan power rising with inlet temperature in the cooling energy tool (for example about +13% from 68 to 104 F for A3) so set-point optimisation minimises total energy, not PUE.
• Flag loss of air-mover redundancy when economizer-only designs operate at high inlet set points.
• In set-point decisions, weigh total facility energy and task cost instead of PUE.

Who should read it

Designers and operations engineers working on cooling set points and airflow management.

Related standards

• ASHRAE Thermal Guidelines for Data Processing Environments

Publication details

Authors

Ian Seaton

Publisher

Upsite Technologies blog

Year

2017

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