SoSo CAD
Technical guideFree to read2022

IT Equipment Delta T and Flow Rate: How They Impact Your Data Center

Upsite Technologies blog · Lars Strong

Lars Strong's Upsite article explaining how the temperature rise across IT equipment sets the airflow it needs, and why matching cooling-unit airflow to it is essential to limit bypass.

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Overview

This article, by Lars Strong, appeared on the Upsite Technologies blog in 2022 and is freely available expert writing. The author explains that the temperature rise across IT equipment (delta T) determines the airflow the equipment needs, and that knowing this rate is essential to matching cooling-unit airflow and limiting bypass.

The method rests on the sensible heat equation, q = Cp x mass flow x delta T. Strong uses its normalized imperial form for sea level: CFM = 3.16 x watts / delta-T (F). The formula lets a rack's required airflow be computed directly from power and delta T. The formula is simple enough for quick checks at design stage. The coefficient is for sea level; other altitudes change air density, so a correction may be needed.

In the worked example, two configurations drawing the same power are compared. A 5 kW blade chassis at 35 F delta T needs about 451 CFM. Ten 500 W rack-mount servers (5 kW in total) at 20 F delta T need about 790 CFM, roughly 75% more air. The lower-delta-T configuration needs substantially more air, and this scales into large differences at room level. The article's conclusion is that matching cooling-unit airflow to actual IT airflow is the route to cutting bypass. It offers a practical calculation method, but its rack-level example is limited to two configurations; for real facilities, manufacturer data should be the basis.

Key points

  1. Sensible heat relation: q = Cp x mass flow x delta-T.
  2. Normalized sea-level form: CFM = 3.16 x watts / delta-T (F).
  3. A 5 kW blade chassis at 35 F delta-T needs about 451 CFM.
  4. Ten 500 W rack servers (5 kW) at 20 F delta-T need about 790 CFM, roughly 75% more air.
  5. The lower-delta-T configuration needs far more air, scaling to large differences at room level.
  6. Matching cooling-unit airflow to actual IT airflow is the route to cutting bypass.

Using it in design

  • Rack and server catalogue entries should carry a delta-T (or airflow per kW) field.
  • The tool should compute rack airflow as 3.16 x W / delta-T and sum it against cooling-unit airflow.
  • Warn when supply airflow exceeds demand by a large margin (bypass) or falls short (recirculation).
  • Model blade and rack-mount servers as different air demands even at the same power.

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Who should read it

Mechanical designers and facility operators balancing airflow and cooling.

Publication details

Authors
Lars Strong
Publisher
Upsite Technologies blog
Year
2022
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

Lars Strong (2022). IT Equipment Delta T and Flow Rate: How They Impact Your Data Center. Upsite Technologies blog. https://www.upsite.com/blog/equipment-delta-t-flow-rate-impact-data-center/

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