Choosing Between Room, Row, and Rack-based Cooling for Data Centers

Overview

The paper compares room-based, row-based and rack-based cooling architectures, with and without containment, on first cost, efficiency, predictability and density capability. The goal is clear guidance on which architecture to choose given project scale and density. Uncontained room cooling works for low, stable densities (about 1 to 2 kW per rack; 323 to 753 W/m2) but becomes unpredictable and wasteful as rack power rises. The recommendation: for new data centres above 200 kW, room-based cooling with hot-aisle containment; below 200 kW, row-based cooling with no raised floor needed. For existing rooms, row-based cooling is advised for pods of 5 kW per rack or more.

Rack-based (closed) cooling reaches the highest densities, up to about 50 kW per rack, and is reserved for extreme density or unstructured layouts. The analysis assumes IT airflow of 120 cfm per kW, and an uncontained room delivers about 125 percent of rated airflow because of mixing and bypass. Close-coupled row cooling can save more than 50 percent of fan power.

A 480 kW case study compares the electrical cost of each approach across 3 to 20 kW per rack. N+1 row-based cooling remains practical up to about 25 kW per rack, while the number of room CRAHs can roughly double above 10 kW per rack to keep N+1. Containment strategies make operating densities of 3 kW per rack or higher practical.

The thresholds convert directly into a cooling-architecture recommender: for a new build, room cooling with hot-aisle containment if IT load is above 200 kW and row-based cooling below it. In a legacy room, a row-based pod is suggested if a zone averages above 5 kW per rack, and rack-based or liquid cooling above 25 kW per rack. The default airflow is 120 cfm per kW.

Key points

1. Uncontained room cooling is effective at about 1 to 2 kW per rack (323 to 753 W/m2, 30 to 70 W/ft2)
2. New data centres above 200 kW: room-based cooling with hot-aisle containment
3. New data centres below 200 kW: row-based, no raised floor; existing rooms use row-based for pods of 5 kW/rack or more
4. Rack-based (closed) cooling reaches up to about 50 kW per rack
5. Analysis assumption: IT airflow 120 cfm per kW; uncontained room delivers about 125 percent of rated airflow due to mixing/bypass
6. Row-based close coupling can save more than 50 percent fan power versus uncontained room cooling
7. N+1 row-based remains practical up to about 25 kW per rack; room CRAH count can roughly double above 10 kW/rack
8. Containment makes operating densities of 3 kW per rack or higher practical

Using it in design

• Drive the cooling-architecture recommender: for a new build with IT load above 200 kW, default to room cooling + hot-aisle containment
• Below 200 kW, default to row-based cooling
• If any zone in a legacy room averages above 5 kW/rack, suggest a row-based pod
• Above 25 kW/rack, suggest rack-based or liquid cooling
• Use 120 cfm/kW as the default airflow factor, with a 1.25 bypass multiplier for uncontained rooms and 1.0 to 1.1 when contained

Who should read it

Data-centre designers and consultants choosing a cooling architecture.

Related standards

• ASHRAE TC9.9 Thermal Guidelines
• ASHRAE 90.4

Publication details

Authors

Kevin Dunlap; Neil Rasmussen

Publisher

Schneider Electric

Year

2012

Document number

WP 130

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