HP ML150 Optimizing facility operation in high density data center environment - Page 20

While the "tons" capacity rating is measured at 80˚F, HP recommends that facility managers target an optimal operating range of 68 - 77˚F and 40 - 55% relative humidity (RH). These ranges are in inline with the recommendations of the American Society of Heating, Refrigerating and AirConditioning Engineers (ASHRAE)8. Managers should also be aware that as the operating temperature of the room decreases, so does the cooling capacity of the CRAC unit. Furthermore, the tons rating is very subjective because it is based on total cooling, which is comprised of "sensible cooling" and "latent cooling".9 Computer equipment produces sensible heat only; therefore, the sensible cooling capacity of a CRAC unit is the most useful value. For this reason, CRAC unit manufacturers typically provide cooling capacities as "total kWh" and "sensible kWh" (or "total BTU/hr" and "sensible BTU/hr") at various temperatures and RH values. Customers should review the manufacturer's specifications and then divide the sensible cooling capacity (at the desired operating temperature and humidity) by 3.5 kWh (12,000 BTU/hr) per ton to calculate the useable capacity of a given CRAC unit, expressed in tons of cooling. Cooling capacity is also expressed in volume as cubic feet per minute (cfm). The volume of air required is related to the moisture content of the air and the temperature difference between the supply air and return air (ΔT): Cubic feet per minute = (kWh x 3412) ÷ (1.08 × ΔT) Or, using BTUs/hr Cubic feet per minute = BTU/hr ÷ (1.08 × ΔT) The cooling capacity calculations presented here are theoretical, so other factors must be considered to determine the effective range of a particular CRAC unit. The effective cooling range is determined by the capacity of the CRAC unit and the "cooling" load of the equipment in its airflow pattern. Typically, the most effective cooling begins about 8 feet (2.4 m) from the CRAC unit. The CRAC capacity, equipment cooling loads, and under floor conditions (airflow restrictions) will vary the effective cooling range of a CRAC. NOTE: Many CRAC manufacturers are now using kWh rather than BTU/hr to describe the capacities of their equipment. For that reason both units are included. Placement of CRAC units The geometry of the room and the heat load distribution of the equipment determine the best placement of the CRAC units. CRAC units can be placed inside or outside the data center walls. Customers should consider placing liquid-cooled units outside the data center to avoid damage to electrical equipment that could be caused by coolant leaks. CRAC units should be placed perpendicular to the rows of equipment and aligned with the hot aisles, discharging air into the supply plenum in the same direction (Figure 16). This configuration provides the shortest possible distance for the hot air to return to the CRAC units. Discharging in the same direction eliminates dead zones that can occur beneath the floor when blowers oppose each other. Rooms that are long and narrow may be cooled effectively by placing CRAC units around the 8 "Thermal Guidelines for Data Processing Environments" can be purchased at: http://resourcecenter.ashrae.org/store/ashrae/newstore.cgi?itemid=21074&view=item&page=1&loginid=269 2065&words=thermal%20guidelines%20for%20data%20processing%20environments&method=and& 9 Latent cooling is a result of a phase change (vapor and liquid) and sensible cooling is a result of lowering temperature 20