At a medical data center in the upper Midwest, cooling-system uptime reliability is ensured with redundant computer-room air-conditioning (CRAC) units. For example, in a room needing eight units, 10 are employed.
“Under normal conditions, we’re using 80 percent of the installed capacity to cool the room, which is about a 30-percent turndown,” the data-center manager said.
The CRAC units are positioned on the same raised floor as the racks of computer servers. The units supply cool air into an underfloor plenum, where it is distributed to cold aisles via perforated tiles. From there, the conditioned air is drawn into computer-rack inlets and ejected on the other side into hot aisles at a constant rate of flow. This design is simple and reliable, but maintaining a constant flow of air 24/7 consumes a lot of fan energy.
When the facility embarked on its 20-year equipment-replacement process, the manager was looking for new technologies that would address energy-efficiency concerns and ensure the Uptime Institute’s Tier III standard for medical data centers—99.982 percent uptime, which translates to no more than 1.6 hr of downtime per year, and an uninterruptible power supply that can handle a power outage of at least 72 hr—was met. An appraisal of the existing cooling system indicated the CRAC units were in good mechanical condition; the motors generally did not need replacement. Where replacement was needed, motors, bearings, and valves were readily available. The parts for CRAC-unit controls, however, were becoming scarce, and the existing panels did not meet the facility’s redundancy and plant operating goals.
Drive Intelligence Replaces CRAC-Unit Controls
When the data-center manager contacted Brian Peterson of Shoreview, Minn.-based commercial HVAC- and cleanroom-equipment dealer SVL (Schwab, Vollhaber, Lubratt) Inc. about automating a cooling tower using a Danfoss VLT variable-frequency drive (VFD), he discovered the drive also incorporates significant control capabilities that can be applied to CRAC units.
“Typically, Danfoss VLT HVAC Drives are used to control fan-motor speed to save energy in cooling applications,” Peterson said. “In this application, the end user took advantage of the VLT Drive’s onboard intelligence to control the CRAC units themselves. As a result, we scrapped the control panels and keypads that came with the units. The VLT Drives provided full stand-alone unit control to improve reliability by handling electrical phase faults and compensate for CRAC-unit failures.
“The VLT HVAC Drive FC102 has a tremendous amount of onboard intelligence,” Peterson continued. “In this case, we could take advantage of that, along with a variable-speed drive’s ability to cut energy consumption using fan affinity laws.”
In theory, the fan affinity law states power is reduced in proportion to the cube of a fan’s revolutions per minute, which means, for example, a 20-percent speed reduction translates to exponential fan energy savings. In this case, applying the drives enabled the data center to reduce its cooling costs by 40 percent.
Peterson said Danfoss’ factory support worked with him to develop the solution.
“I was able to send and receive drive project files from Danfoss application-support experts Bruce Widell and Joe Hammock,” Peterson said. “The right people were in place to tailor the drive intelligence to the job’s requirements.”
One Danfoss VFD was installed at the bottom of each of 40 CRAC units. Each unit also includes a 7.5-hp fan motor, a chilled-water coil, and a valve. The drives regulate the fans to maintain proper airflow rate and underfloor duct static pressure. Also, they modulate the chilled-water valves to control either discharge-air or return-air temperature.
The drive provides floating-point valve control. Unlike on/off control, a floating-point controller responds when temperature rises above setpoint by sending a signal to an actuator to open or close a chilled-water valve slowly to increase or decrease coil temperature. When the setpoint plus dead band is reached, the actuator stops. This relatively simple method of control is more reliable than proportional-integral-derivative control, whereby a valve is capable of moving continually.
Drives Provide Phase-Loss Protection
Utilities supply data centers and industrial facilities with three-phase electrical power. A downed power line, a blown fuse, or equipment failure can cause the loss of one and even two phases.
“From an uptime/reliability point of view, a phase fault is one of the few things that can take out all motors in a data center,” the manager said. “Even if the phase fault doesn’t damage the motors, the resulting thermal overload can trip breakers, requiring local manual reset that can add up to serious downtime.”
Danfoss VFDs provide motor protection with phase-loss/imbalance detection that automatically compensates to continue operation on any two of the three input phases. Load is reduced when any input phase is lost. In that event, the reduced cooling capacity can be replaced by reserve capacity from the redundant CRAC units. These and other fault conditions can be monitored and controlled locally from the drive keypad or though the Modbus TCP building automation system (BAS).
In this case, the BAS is used for monitoring and data collection only. To avoid the possibility of network failure inherent in a centralized control scheme, each CRAC unit is controlled locally. The OEM panel on each CRAC unit was replaced with a Danfoss backlit digital display and keyboard with immediate access to motor power, motor voltage, motor nominal speed, and minimum/maximum frequency.
“When we discovered the capability of these drives, it was inevitable to put as much responsibility on the end controls, rather than a centralized control,” the data-center manager said. “Essentially, the drives are doing the job of programmable logic controllers, as well as a variable-speed-fan motor drive.”
The drive units have a remarkable maintenance and performance record, the data-center manager said.
“In this facility, we have Danfoss drives that are more than 10 years old running 24/7 with zero failures to date,” he said. “As for performance, incorporating the drives lets us reach our full energy-savings potential. For redundancy, there is always significant excess capacity. Every unit is oversized by some margin. Then, every redundant system is oversized again. For example, anywhere a data center needs three units, we install four or more. Taking advantage of this excess capacity and managing it with a drive allows for much more significant energy reductions than a typical HVAC application. It also allows the drive to run cooler, use fewer fan cycles, and be more tolerant to utility faults.”
Peterson, who has been installing drives for over three decades, concluded: “The intelligence of Danfoss drives is incomparable to any other drive on the market. They’ve got the brains to handle CRAC-unit control and the brawn to handle HVAC fan motors to meet uptime requirements—plus, the energy efficiency to reduce a data center’s cooling costs.”