Outdoor air in data centers

Overview

In this article Kosik uses EnergyPlus to simulate a 20,000 sq ft raised-floor room loaded at 150 W/sq ft and examines how envelope performance and outside-air economization interact with indoor humidity. The simulated system uses chilled water with water-cooled centrifugal chillers and VAV air handlers, with 55 F leaving-coil air and 75 F maximum return air. The article is dated 2008, so its discussion of relative humidity limits should be read against the recommended values of that time.

The article shows that envelope heat gain is a meaningful share of cooling only when the room is lightly loaded: the envelope share falls from 8.2% at 20% IT load to 1.7% at 100%, so the envelope matters most during partial build-out. Air leakage, in turn, dominates moisture behaviour; wind-driven infiltration is the main leakage mechanism, and leakage correlates strongly with RH swings and winter RH, weakly with summer RH.

A parametric study examines raised cold-aisle temperatures and wider humidity limits across six global cities. Allowing annual RH swings of about 30-70% instead of 40-55% could be acceptable and unlocks larger economizer savings; humidification raises the RH floor but erodes savings. San Jose and Bangalore give the best savings with few low-RH hours, while Chicago and Reykjavik save significantly but with many hours below minimum RH. A sidebar introduces Telcordia NEBS requirements for telecom equipment: levels 1-3 per SR-3580, with level 3 applying to central-office equipment, and key documents GR-63-CORE and GR-1089-CORE. The article is a freely accessible expert article from 2008 and was written against the recommended RH floor of that time.

Key points

1. Simulated room: 20,000 sq ft raised floor at 150 W/sq ft, chilled water with water-cooled centrifugal chillers and VAV AHUs, 55 F leaving coil air, 75 F maximum return.
2. Envelope share of cooling load falls from 8.2% at 20% IT load to 1.7% at 100% IT load, so the envelope matters most during partial build-out.
3. Wind-driven infiltration is the main leakage mechanism; leakage correlates strongly with RH swings and winter RH, weakly with summer RH.
4. Allowing annual RH swings of about 30-70% instead of 40-55% could be acceptable and unlocks larger economizer savings; humidification erodes savings.
5. San Jose and Bangalore give the best savings with few low-RH hours; Chicago and Reykjavik save significantly but with many hours below minimum RH.
6. NEBS levels 1-3 per SR-3580; level 3 applies to central-office equipment; key documents GR-63-CORE and GR-1089-CORE.

Using it in design

• In the tool's economizer studies, report hours below the humidity floor alongside energy savings, per climate.
• Model envelope gain explicitly for phased or partially loaded halls rather than ignoring it.
• Add a NEBS flag to telecom and edge equipment catalogue entries.
• When widening humidity limits, account for humidification energy eroding the savings.

Who should read it

Cooling and building designers assessing air-side economizers, humidity limits and envelope decisions.

Related standards

• ASHRAE Thermal Guidelines for Data Processing Environments
• Telcordia GR-63-CORE
• Telcordia GR-1089-CORE
• ASHRAE 90.1-2007

Publication details

Authors

Bill Kosik

Publisher

Consulting-Specifying Engineer

Year

2008

Document language

English

Source type

Expert / community

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation