Data centers are spaces specifically designed to accommodate dense arrangements of computer equipment. This currently includes telephone company “central offices” (“telcos”) and computer labs. Any space where dedicated HVAC is installed to handle computing equipment load is likely to be considered a data center.
Data Center HVAC Systems
Data centers in the baseline building will typically be served by a separate system because they trigger the exception to the HVAC mapping requirements having to do with abnormal internal heat gain and schedules of operation. When heat gain differences of more than 10 Btu/h or operation schedule differences of more than 40 hours/week trigger this exception, a separate system is required to serve the data center. Either system 3 (PSZ-AC) or system 4 (PSZ-HP) shall serve the data center (depending on the heating source for the building). See the PRM, G3.1.1, Exception b.
Loads in a data center could range from anywhere as low as 20 W/ ft² up to 400 W/ ft² Load densities above 400 W/ ft² would not typically be handled by an air based cooling system. As a rule of thumb: the initial actual energy use of a new data center is 50% of the design capacity of the cooling system serving the data center, so the equipment is initially oversized by a significant margin. In many cases, the data center load will increase each year, as additional equipment is added, reaching full load after a five year period. However, if there is more precise information on the expected load, this should be used. In most cases, the load for the baseline and proposed buildings should be the same, unless documentation of equipment that is more efficient than standard practice can be provided.
Given the uptime requirements of data center equipment, redundant equipment is a necessity. Purely redundant equipment is ignored in the modeling of the proposed building and the baseline building. This might simply be additional CRAC (computer room air conditioning) units, or might be more complicated to include additional chillers and supporting pumps.
In mixed-use facilities, ventilation for the data center is often provided by the “house air” system, i.e. the system that serves the office or other commercial space, with a separate system providing cooling. In dedicated data center systems, the ventilation air is typically provided by a dedicated outside air system and should be modeled accordingly. Outside air in the proposed and baseline system will be the same, but the baseline building system will typically be modeled with its own supply of outside air.
Humidity Control Systems
The ASHRAE document “Thermal Guidelines for Data Processing Environments” describes the “Recommended” relative humidity (RH) range for Class 1 and Class 2 computing environments as 40% to 55%, and the “Allowable” range for these same environments as 20% to 80%. These RH values apply to the air entering the computer equipment.
CRAC units with on-board humidity control systems typically have the temperature and humidity sensors factory-mounted in the return air opening of the CRAC. For open aisle data centers, the sensors are typically left in this position, and the humidity of the return air is controlled to the ASHRAE recommended range of 40% to 55%.
For ducted return and fully enclosed data center aisles, the practice is to relocate the CRAC temperature and humidity sensors to a cold aisle, or to install additional temperature and humidity sensors in the cold aisles and disable the original CRAC sensors. The aisle-mounted sensors will control the CRAC humidity system to provide the ASHRAE recommended range of 40% to 55% in the cold aisles.
The baseline building system shall be modeled with the same temperature and humidity conditions as the proposed design.
Uninterruptible Power Supply (UPS)
Data centers typically include some type of battery-based system to address power failures. Depending upon the size, configuration and amount of load, the efficiency of these systems will range from 86% to 93%. In determining the data center power usage, this must be accounted for in the building modeling. In addition, there will be additional cooling requirements for the UPS systems that will need to be modeled. Both the baseline and proposed buildings should be modeled with the same UPS configuration and power, unless supporting documentation of a more efficient technology or arrangement can be documented.