4 Steps to Reducing Bypass Airflow

Overview

Strong's 2014 post defines bypass airflow as any cooling airflow in excess of IT airflow, not just leakage through holes. The core finding is that sealing cable openings and managing perforated tiles does not reduce total airflow volume; it merely relocates bypass to the cold aisle. The post is a freely available blog entry.

The method starts by measuring the cooling capacity factor (CCF): the rated capacity of running cooling units (kW) divided by 1.10 times the IT critical load (kW), the extra 10% covering lights, people and similar loads. With fixed-speed or full-speed fans, CCF approximates the room's bypass airflow ratio. Airflow management is then improved in order: raised floor, rack (blanking, under-cabinet and rail gaps) and row (gaps, empty positions, containment).

Finally return set points are raised up to the IT inlet maximum, and units are switched off or fan speed is reduced via VFDs, keeping redundant units running. The process is iterated. The post is a practitioner's view, and CCF should be remembered as an approximate indicator that does not replace detailed measurement and simulation.

Key points

1. Bypass airflow is defined as any cooling airflow in excess of IT airflow.
2. CCF = rated capacity of running cooling units (kW) / (1.10 x IT critical load kW); the 10% covers lights, people etc.
3. With fixed-speed or full-speed fans, CCF approximates the room's bypass airflow ratio.
4. Sealing cable openings and managing perforated tiles moves bypass to the cold aisle rather than reducing total volume.
5. Improve airflow management in order: raised floor, rack (blanking, under-cabinet and rail gaps), row (gaps, empty positions, containment).
6. Then raise return set points up to the IT inlet maximum, switch off units or reduce fan speed via VFDs, keeping redundant units running.
7. The method is iterative.

Using it in design

• Compute CCF for each room from running cooling capacity and IT load and show it as a KPI, with a warning when it is high (oversupply).
• In the efficiency wizard, use a design checklist that enforces floor, rack and row sealing items before allowing set-point increases.
• Verify that redundancy is kept when set points are raised.

Who should read it

Operations and design engineers who want to improve cooling efficiency in air-cooled halls.

Publication details

Authors

Lars Strong

Publisher

Upsite Technologies blog

Year

2014

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