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Technical guideFree to read2022

Understanding the Physics of Airflow in High Density Environments (Julius Neudorfer)

Data Center Frontier · Julius Neudorfer

Part of a series on air cooling high-density IT, this article works through the airflow equation, required CFM versus delta-T, raised-floor tile capacity and how leakage paths undermine containment.

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Overview

Julius Neudorfer's article in Data Center Frontier is part of a series on air cooling high-density IT. It works through the airflow equation, required CFM versus delta-T, raised-floor tile capacity and underfloor pressure. The airflow equation is BTU/h = CFM x 1.08 x delta-T (°F); on that basis 1 kW needs about 158 CFM at 20°F delta-T and 316 CFM at 10°F.

Typical raised-floor plenum pressures average 0.04-0.08 in. H2O and can vary -100% to +100% in operation. Doubling static pressure from 0.04 to 0.08 in. H2O increases tile airflow only about 130-145% (Table 4). Cold aisles are usually two tiles wide, giving about one supply tile per cabinet. These values show that tile selection is sensitive to the pressure assumption.

The article also explains how leakage paths undermine containment: gaps of 0.5-2 in. under cabinets let high-velocity supply air draw hot air in via the Venturi effect, and gaps between cabinets also leak. Because of variable server fan behaviour, delta-T may vary from 10°F to 40°F during normal operation, causing hot spots in rooms not designed for it. As a limit, it is a trade-press article that uses simplified equations and does not replace detailed CFD analysis.

Key points

  1. Airflow equation: BTU/h = CFM x 1.08 x delta-T (°F); 1 kW needs about 158 CFM at 20°F delta-T and 316 CFM at 10°F.
  2. Typical raised-floor plenum pressures average 0.04-0.08 in. H2O and can vary -100% to +100% in operation.
  3. Doubling static pressure from 0.04 to 0.08 in. H2O increases tile airflow only about 130-145% (Table 4).
  4. Cold aisles are usually two tiles wide, giving about one supply tile per cabinet.
  5. Gaps of 0.5-2 in. under cabinets let high-velocity supply air draw hot air in via the Venturi effect; gaps between cabinets also leak.
  6. Server delta-T may vary from 10°F to 40°F during normal operation as fan speeds respond to load and inlet temperature.

Using it in design

  • Implement the CFM = kW x 3412 / (1.08 x delta-T) check per rack and per tile.
  • Warn when required tile CFM exceeds the catalogue tile rating at design plenum pressure.
  • Add blanking and gap-sealing items to the checklist.
  • Allow a delta-T range rather than a single value.

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Who should read it

Cooling engineers designing air-cooled high-density halls and operations teams.

Related standards

  • ASHRAE TC9.9 Thermal Guidelines

Publication details

Authors
Julius Neudorfer
Publisher
Data Center Frontier
Year
2022
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

Julius Neudorfer (2022). Understanding the Physics of Airflow in High Density Environments (Julius Neudorfer). Data Center Frontier. https://www.datacenterfrontier.com/special-reports/article/11427261/understanding-the-physics-of-airflow-in-high-density-environments

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