Air distribution's importance in data processing environments

Overview

Justin Hsiao and Ben Olejniczak's 2022 article in Consulting-Specifying Engineer explains why air distribution matters in data halls. It first reviews the ASHRAE TC 9.9 environmental classes and then sets out the two principles of data-hall air design: deliver supply air to every rack inlet within the envelope and keep supply and exhaust paths separate. The article is freely readable and is an expert article.

The framework rests on the idea that only the inlet temperature of the IT equipment governs equipment performance, and exhaust air matters only where it mixes into the inlets. Classes A3 and A4 were added in 2012 to support free cooling, and class H1 was defined for high-density equipment needing lower inlet temperatures. This information is the starting point when choosing design target temperatures.

The article compares overhead and underfloor supply. Overhead supply needs controlled discharge velocity: too high bypasses the inlets, too low causes premature mixing. Obstructions in an underfloor plenum make the pressure distribution unpredictable, and CFD modelling is recommended to validate raised-floor designs. Rows should be arranged front-to-front and back-to-back, and front-to-back rows are not recommended. Cold- and hot-aisle containment are also described.

The article treats leakage control at racks, containment and floors as essential and gives a quantitative limit: ASHRAE Applications (2019) Section 20.15 recommends that air leakage not exceed 2% of design flow. The article's limit is that, as an expert article, it is short and stays at the level of principle rather than design calculation. Related documents are ASHRAE Thermal Guidelines (5th edition) and the ASHRAE Handbook - HVAC Applications (2019).

Key points

1. Only ITE inlet temperature governs equipment performance; exhaust air matters only where it mixes into inlets.
2. Classes A3 and A4 were added in 2012 to support free cooling; H1 covers high-density equipment needing lower inlet temperatures.
3. Overhead supply needs controlled discharge velocity: too high bypasses inlets, too low causes premature mixing.
4. Underfloor plenum obstructions cause unpredictable pressure distribution; CFD modelling is recommended to validate.
5. Arrange rows front-to-front and back-to-back; front-to-back rows are not recommended.
6. ASHRAE Applications (2019) Sec. 20.15 recommends air leakage not exceed 2% of design flow.
7. Cold- and hot-aisle containment and leakage control at racks, containment and floors are covered.

Using it in design

• In the layout checker, flag rows not in hot/cold aisle orientation and missing blanking panels.
• Estimate containment leakage and warn above 2% of design airflow.
• For raised-floor designs, prompt for CFD or at least a plenum obstruction review.
• Define a control range for discharge velocity in overhead supply.

Who should read it

Mechanical engineers and operations teams doing data hall air distribution, aisle containment and layout design.

Related standards

• ASHRAE Thermal Guidelines for Data Processing Environments (5th ed.)
• ASHRAE Handbook - HVAC Applications (2019)

Publication details

Authors

Justin Hsiao; Ben Olejniczak

Publisher

Consulting-Specifying Engineer

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