Avoidable Mistakes that Compromise Cooling Performance in Data Centers and Network Rooms

Overview

The errors are grouped into five families: missing blanking panels and leaky racks, rows that all face the same way, clustered high-power loads, humidity set too high or CRAC units fighting one another, and misplaced supply and return vents. For each family a table lists the effect on availability and cost, then the remedy. The introduction sets the baseline electricity split at 44% IT load, 38% cooling, 15% power system and 3% lighting.

The mechanism at the centre of the paper is recirculation. When hot exhaust air returns to server intakes, CRAC capacity falls and N+1 fault tolerance is quietly lost, because cooler return air lowers the capacity of the surviving units. Operators typically respond with a chain of compensations: lower the set point, add 20% more CRAC capacity, dehumidify, spend more fan power, then re-humidify. The paper calculates that this cascade raises cooling cost by 10 to 25%.

Layout gets its own discussion. About 25% of data centres face every rack in one direction, and of the 75% that use hot-aisle/cold-aisle, more than 30% do not arrange supply and return correctly. Row positions and orientation should be fixed before the ventilation system is designed. The paper closes with a short policy list, including a hot-aisle label, to align IT and facilities staff; because it dates from 2011, its humidity and temperature advice should be checked against the current ASHRAE edition.

Key points

1. Typical electricity split: IT 44%, cooling 38%, power system 15%, lighting 3%; a 20% loss of cooling efficiency raises total consumption by 8%, about $700,000 over 10 years for a 500 kW facility.
2. Blanking panels are omitted to some degree in 90% of data centres; the resulting recirculation can raise IT equipment temperature by 8°C (15°F).
3. The compensation cascade triggered by recirculation adds 10-25% to cooling cost, wasting $300,000 to $700,000 over 10 years for a 500 kW load.
4. Recirculation compromises N+1 CRAC fault tolerance because cooler return air reduces the capacity of the remaining units.
5. About 25% of data centres face all racks the same way; over 30% of hot/cold-aisle rooms get supply and return wrong.
6. Recommended humidity is 35-50% RH, with every CRAC at the same set point and a +/-5% deadband to stop humidifiers fighting.
7. Rack flaws to avoid: shelves, 23 inch (584 mm) racks without rail brushes, unbrushed cable openings, glass or low-perforation doors, roof fans and shallow racks.
8. Returns belong over hot aisles and supply vents over or in cold aisles only; vents at unloaded positions should be closed.

Using it in design

• In layout validation, raise an error when adjacent rows of front-to-back equipment face the same direction, and a warning for perforated tiles or supply diffusers in a hot aisle.
• Add shelf-mounted, door type (glass or low-perforation), rail brush and roof fan fields to the rack catalogue, and map glass doors and roof fans to warnings.
• Force one humidity set point and a deadband of at least 5% across all CRACs in a zone; default the RH window to 35-50% but keep it configurable to ASHRAE values.
• When recirculation risk flags exist (missing blanking, same-direction rows), annotate that N+1 cooling may not be achieved, and apply a configurable 10-25% cooling-energy penalty in the energy model.

Who should read it

Mechanical designers and operations teams who set hall layout and cooling parameters.

Related standards

• ASHRAE TC 9.9 Thermal Guidelines

Publication details

Authors

Neil Rasmussen

Publisher

Schneider Electric

Year

2011

Document number

WP 49

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