Adoption of Variable Cooling Systems to Optimize Data Center Infrastructure

Overview

The starting point is load variability: server power now varies across roughly 45-106% of peak depending on platform, against about 20% variation two decades earlier. A fixed-speed cooling unit can follow that swing only by cycling on and off, so the paper walks through each cooling-unit component and compares how it can adapt to load.

On compressors it compares multiple fixed compressors, continuously modulating (digital scroll) and variable-speed compressors. In digital modulation, a 15 s cycle with 7.5 s loaded gives 50% capacity and 11 s loaded gives 73%. Variable-speed compressors offer about 7:1 turndown, with a 1,000-7,200 rpm range covering 20-100% capacity.

Fans, valves and pumps add further savings. Variable-speed fans can save up to 76% of fan energy, a 10% airflow reduction saves up to 33% of fan energy, and EC fans averaged 18% savings versus VFDs. Electronic expansion valves can save up to 15% energy compared with thermostatic valves. For chilled-water systems the paper covers pressure-independent control valves and variable-speed pumped economizers.

As control strategy, supply-air temperature control is preferred over return-air control because rising server delta-T makes return-air setpoints unreliable. The paper closes by listing which variable components come standard in each cooling-unit size range. As a vendor document, its figures reflect the manufacturer's view.

Key points

1. Server power varies across roughly 45-106% of peak by platform, versus about 20% two decades earlier.
2. Digital modulation example: in a 15 s cycle, 7.5 s loaded gives 50% capacity and 11 s gives 73%.
3. Variable-speed compressors give about 7:1 turndown; 1,000-7,200 rpm covers 20-100% capacity.
4. Variable-speed fans save up to 76% of fan energy; a 10% airflow cut saves up to 33%; EC fans averaged 18% savings versus VFDs.
5. Electronic expansion valves can save up to 15% versus thermostatic expansion valves.
6. Supply-air temperature control is preferred because rising server delta-T makes return-air setpoints unreliable.
7. For chilled water the paper covers pressure-independent control valves and variable-speed pumped economizers.

Using it in design

• In the cooling-unit catalog record compressor modulation type (fixed staged, digital, inverter), fan type (EC/AC/VFD), valve type (EEV/TXV, PICV) and control mode (supply or return air).
• Use part-load efficiency curves in the energy model rather than full-load EER.
• Recommend supply-air control for contained, high delta-T halls.
• Warn on fixed-speed compressors where the load is expected to vary widely.

Who should read it

Mechanical designers selecting cooling units and energy analysts modelling part-load efficiency.

Related standards

• ASHRAE TC 9.9 Thermal Guidelines
• ASHRAE 90.4
• AHRI 1360

Publication details

Authors

Vertiv

Publisher

Vertiv

Year

2020

Document number

SL-70653

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