©2018 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 60, no. 10, October 2018.
By Jeff Stein, P.E., Member ASHRAE; Brandon Gill, P.E., Member ASHRAE
About the Authors
Jeff Stein, P.E., is a principal and Brandon Gill, P.E., is an associate at Taylor Engineering in Alameda, Calif. Stein is a member of ASHRAE SSPC 90.4, Energy Standard for Data Centers. Gill is a voting member of ASHRAE TC 1.4, Control Theory and Application.
For high reliability data centers, there is general agreement in the design community on the need for redundancy for certain mechanical equipment (e.g., N+1 pumps, chillers, and cooling units). For other mechanical equipment (e.g., redundant piping), there is little agreement, but at least the options are fairly clear. However, there is far less agreement on and understanding of the redundancy requirements and options for the controls components used to monitor and control the mechanical systems.
Controls redundancy may be less visible and less well understood than mechanical redundancy, but it is no less important. In fact, in the authors’ experience, more major data center cooling system failures are due to poor controls design and implementation than are due to mechanical equipment failures. Well-designed control systems must recognize and respond to the possible failure or degradation of any device, including any controller, sensor, actuator, variable frequency drive (VFD), fan, pump, chiller, power source, electrical circuit, and controls communication path. This article discusses how to design and commission data center controls for maximum reliability. Five key areas are addressed:
- Control system architecture design and associated controlled device configuration;
- Redundant sensor requirements;
- Fault responsive sequences of operation;
- Alarming and notification requirements; and
- Commissioning.
Controls System Architecture and Controlled Device Configuration
A good data center controls design does not necessarily require controller redundancy. In fact, controller redundancy can reduce reliability due to added complexity and additional points of failure.
Twenty years ago, the most common data center cooling design included constant airflow, air-cooled, direct-expansion, computer room air-conditioning units (CRACs). One advantage of this design is that it requires little if any centralized control; all CRACs operate independently.
Today, most data center cooling designs are far more efficient and cost-effective but require some form of centralized control to coordinate multiple computer room air-handling units (CRAHs), fans, chillers, pumps, etc. For example, supply fan speeds of multiple CRAHs are typically controlled in unison to maintain a common setpoint, such as underfloor pressure or cold to hot aisle differential pressure (ΔP). The proportional–integral–derivative (PID) loop maintaining ΔP at setpoint runs on a single, central controller, which sends the speed command to all the CRAH units. The CRAH units may rely on the central controller not only for fan speed command, but also for start/stop command, supply air temperature setpoint, outside air dry-bulb and wet-bulb temperature (e.g., for economizer operation), etc.
The controls design must account for the potential failure of the centralized controller, i.e. the loss of a single controller should not result in the loss of data center cooling. The basic options are redundant central controllers or distributed/fail-safe controls. Often, both of these strategies are employed within the same data center. For instance, a central plant may be sequenced by redundant central controllers while the air handlers serving the data hall(s) may achieve fault responsiveness using distributed fail-safe control. The following sections present design considerations for each option.
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