Optimize Data Center Cooling with Effective Control Systems

Overview

The paper argues that cooling equipment chosen without regard to how it will be controlled leads to demand fighting, unstable economizer transitions, manual errors and wasted energy. It lists the shortcomings of traditional stand-alone control: per-unit return-air and relative-humidity setpoints, monitoring without actuation, and no view of whole-plant energy. In the example 1 MW facility at 50% load with PUE 1.91, cooling uses about 36% of total facility energy and about 75% of non-IT infrastructure energy.

The proposed hierarchy has four levels (device, group, system, facility/BMS), each described by benefits, limitations, recommended applications and examples. Device-level control alone is recommended only for a small IT room with a single cooling unit; system-level control is recommended for chilled-water, direct and indirect air economizer and glycol-cooled DX architectures. Assessments found economizer modes often seldom used because control instability in partial-economizer transitions led operators to enable them manually only late in winter.

In the group-control example with variable-speed fans on 600 mm row-based CRAHs, cutting fan speed by 20% saves 45% of fan power (cube law), and with VFD fans it is more efficient to run all units, redundant ones included, at lower speed than to switch redundant units off. For humidity the paper recommends a centrally maintained dew point at IT intakes instead of relative humidity at each unit's return, which causes humidify/dehumidify demand fighting. Delta-pressure control across a contained aisle is presented as faster and more precise than delta-T control, and system-level control can raise chiller EER by resetting the chilled water setpoint at light load, reverting to regular control in emergencies.

Key points

1. Example 1 MW facility at 50% load, PUE 1.91: cooling is about 36% of facility energy and about 75% of non-IT energy.
2. Four levels: device, group, system, facility/BMS.
3. Device-level control alone only for a small room with one cooling unit; system-level for chilled water, air economizer and glycol DX.
4. Economizer modes are often enabled manually only late in winter because of partial-transition instability.
5. Cutting fan speed 20% on 600 mm row-based CRAHs saves 45% of fan power.
6. With VFD fans run all units, redundant included, at lower speed rather than switching redundant units off.
7. Control dew point centrally at IT intakes; return-air relative humidity causes demand fighting.
8. Delta-pressure across a contained aisle beats delta-T; system control can raise chiller EER at light load.

Using it in design

• Add cooling catalog fields: VFD fans/compressors/pumps, supported control level, group-control protocol and vendor compatibility, and whether control is on rack-inlet temperature and dew point.
• Warn when several CRAC/CRAH units in one room lack group control, especially with per-unit humidifiers.
• Warn when an economizer architecture is chosen without system-level control that can transition automatically; flag multi-vendor system-level control as a commissioning risk.
• Defaults: setpoints on IT inlet, centralised dew-point humidity control, redundant VFD units run at reduced speed.

Who should read it

Design engineers choosing cooling architecture and control strategy.

Related standards

• ASHRAE TC 9.9 Thermal Guidelines for Data Processing Environments
• ASHRAE 90.4
• ISO 16484

Publication details

Authors

Paul Lin

Publisher

Schneider Electric

Year

2016

Document number

WP 225

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