Phased cooling retrofit under a 900 kW growth mandate
A 480 m² white space operates six rows of 42U racks at 4.5 kW/rack average and 320 kW total IT load. The 2012 design assumed 600 kW, but eight 60 kW CRAC units are arranged 4+4 without containment; measured PUE is 1.85. The business wants 900 kW within 18 months. Constraints: 600 mm raised floor, 3.2 m ceiling, 8 kN/m² floor loading, a 1,600 kVA utility feed, 2×1,000 kVA UPS (N+1), and 1,300 kW spare chilled-water capacity at 12/18°C. No new outdoor chillers are permitted. One vendor proposes rear-door heat exchangers on all racks; another proposes hot-aisle containment plus in-row coolers for 20 high-density racks. Work must be phased with zero downtime, and finance demands payback under four years with no stranded UPS capacity.
1. Evaluate the two proposals against the constraints. Show the cooling and electrical arithmetic, and state which proposal elements you would reject, pilot, or adopt.
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At 900 kW IT and PUE 1.4, total facility load is about 1,260 kW; the 1,300 kW chilled-water spare leaves only 40 kW headroom, so any proposal must be judged on PUE and on not adding parasitic load. Electrical: 2×1,000 kVA UPS N+1 at 0.9 PF gives about 900 kW usable, so 900 kW IT consumes the entire redundant UPS capacity; there is no room for growth or for a UPS module out of service. That is a decision gate, not a vendor detail.
Rear-door heat exchangers on all racks add water connections, weight and maintenance at every rack; on 4.5 kW average racks they are expensive and do not fix bypass airflow. I would reject full deployment. Hot-aisle containment is defensible because it addresses the measured 1.85 PUE and needs only ceiling/row work. In-row coolers for 20 high-density racks are a pilot, not a blanket commitment; confirm floor loading, condensate, and chilled-water delta-T. Adopt containment first, then close-coupled cooling only where rack density exceeds air-cooling limits. The central plant spare must be verified at 18°C return, not just at nominal.
2. Recommend a phased sequence with zero downtime and the release gate for the 900 kW capacity.
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Phase 0: meter rack inlet/outlet, seal floor gaps, CFD, confirm UPS and chilled-water capacity; no construction. Phase 1: containment on two rows at a time, with temporary aisle doors and no rack shutdown; measure PUE. Phase 2: pilot in-row coolers on four high-density racks, then scale if inlet temperatures and delta-T meet targets. Phase 3: upgrade UPS to N+1 with at least 1,250 kVA usable, or accept a lower IT cap; do not release 900 kW until UPS and cooling are proven together. Release gate: 24-hour integrated load test at 900 kW with one UPS module and one chiller unavailable, rack inlet ≤27°C, PUE ≤1.4, and no stranded UPS capacity. Finance payback must include the UPS upgrade, not just containment. If payback fails, cap the hall at about 700 kW and use the containment savings to defer the UPS.