How to Fix Hot Spots in the Data Center

Overview

The paper defines a hot spot strictly: a measured temperature at the IT equipment intake that exceeds the ASHRAE TC 9.9 recommended value. Hot-aisle temperature is not the test, and hot spots occur most often near the top of a rack. The author argues most arise not from insufficient cooling capacity but from poor delivery of existing cooling through leakage, bypass and recirculation. A cited Uptime Institute study found 10% of racks running hotter than guidelines, and rooms with 15 times the required cooling capacity still had hot spots in 7% to 20% of racks.

A real raised-floor case is analysed with an airflow mass and energy balance. Of 42,480 m3/h of CRAH airflow, 54% leaves through perforated tiles and 46% leaks through cutouts; only 52% reaches rack fronts, 2% is bypass and 7.15% of IT airflow (1,700 m3/h) is recirculated. Typical operator responses, namely lower set points, perforated tiles in hot aisles or near CRAHs, pedestal fans, ice, portable or extra cooling units, are classed as ineffective or emergency-only.

The recommended fixes are rack and room airflow management, containment, load relocation, supply-air temperature control and DCIM-driven fan control. Racks belong at least 3 tiles from cooling units, since high discharge velocity lowers static pressure and starves nearby tiles. In the rebalancing check, measured row airflow divided by 125 CFM/kW should roughly equal the row's rack power. Active ducted racks and active tiles support about 12 kW per rack and fan-assisted devices up to 3 kW; an appendix explains why lowering set points reduces plant capacity.

Key points

1. Hot spot: IT intake temperature above the ASHRAE TC 9.9 recommended value, most often near the top of a rack.
2. Case: of 42,480 m3/h CRAH air, 54% leaves via tiles and 46% leaks; only 52% reaches rack fronts.
3. 7.15% of IT airflow (1,700 m3/h) is recirculated; 2% is bypass.
4. Uptime Institute: 10% of racks run hot; rooms with 15x capacity still had hot spots in 7-20% of racks.
5. Place racks at least 3 tiles from cooling units.
6. Tile rebalancing: row airflow / 125 CFM/kW should roughly equal row rack power.
7. Inspect racks above 5 kW with metered rack PDUs.
8. Active ducted racks and tiles support about 12 kW per rack, fan-assist devices up to 3 kW; moving control sensors from return to supply air evens inlet temperatures.

Using it in design

• Evaluate hot spots only at rack inlets against the selected ASHRAE class recommended range, not hot-aisle temperatures.
• Warn when a rack or perforated tile on a raised floor sits within 3 tiles of a CRAH discharge.
• Compare total perforated-tile airflow per row with sum(rack kW) x 125 CFM/kW and flag over- or under-supply.
• Flag perforated tiles in hot aisles, and cooling units controlled on return air when containment is present.

Who should read it

Operations and design engineers resolving cooling airflow problems.

Related standards

• ASHRAE TC 9.9 Thermal Guidelines for Data Processing Environments
• EN 50600-2-3

Publication details

Authors

Paul Lin

Publisher

Schneider Electric

Year

2014

Document number

WP 199

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