Issue link: https://cp.revolio.com/i/148754
Figure 10 - "Snapshot" without new UPS, at 45 °F CHWS operating with the existing UPS, which is approximately 87% efficient as reported by the contractor. Each subsequent line of varying colors follows the same pattern in increments of 5 °F, where the GREEN line represents a 65 °F CHWS temperature (75 °F SA) with and without the new UPS. As expected, we see significant PUE improvements when the new UPS is utilized and further improvements when the CHWS temperature is increased. The curve in the line represents the economizer operation. This line effectively flattens as the CHWS temperature is increased and number of "free-cooling" hours is increased. This is what we would expect to see. Figure 11 - "Snapshot" with new UPS, at 45 °F CHWS There is additional data center airflow and temperature related metrics that can be utilized to make comparisons. Three parameters developed by ANCIS, Incorporated will be reviewed briefly here; Rack Cooling Index (RCIHI, RCILOW) and Return Temperature Index (RTI) [1]. The following formulas are used to describe these metrics; Figure 12 - Monthly Average Data Center PUE Variations If the RCIHI = 100%, then all rack inlet temperatures are ≤ the maximum recommended temperatures per ASHRAE TC 9.9. The same holds true for RCILOW. If the RTI > 100% then there is recirculation within the space. If the RTI < 100% then there is bypass air within the space. Table 2 below shows these three metrics for supply air at three conditions; 55 °F, 65 °F and 75 °F. Each of the SA scenarios is also shown with and without hot aisle containment. Notice that the RTI is near 100% for all scenarios. This is because the fans are variable and only supply air to meet demand by the servers. We can see from Figure 12 above that the proposed PUE of about 1.65 (reference line) is good, but still has room for improvement. Some of the efficient design components include; 1) Low central chiller kW/ton efficiency of 0.586 at 100% load and 0.256 at 25% load, and 2) Variable speed fans on all CRAC units. The PUE does improve as the SA temperature is increased (and CHWS). Remember that the chiller operates more efficiently as the CHWS is increased. Further potential improvements include raising CHWS temperature and upgrading the UPS systems. www.datacenterjournal.com The RCILOW is < 0% at 55 °F SA because all rack inlet temperatures are below the ASHRAE recommended 65 °F. The RCIHI holds steady until the SA is raised to 75 °F. At this point ~25% of the servers are above the ASHRAE maximum recommended range of 80.6 °F. This is improved to 100% when hot aisle containment is added and the servers fall back within safe operating ranges. THE DATA CENTER JOURNAL | 25