Data Center Chilled Water Loop Expansion in Phoenix

Scenario

A hyperscale data center campus in Phoenix, AZ added a new 12,000 sq. ft. server hall requiring supplemental chilled water cooling alongside existing DX systems. Ambient conditions (115°F peak summer dry-bulb, low humidity) demanded high reliability and redundancy. Constraints included: strict uptime SLA (99.999%), limited available electrical capacity for pumps, and aggressive PUE target (<1.35). Existing chilled water plant had spare capacity but required flow verification for new branch.

Given Data

• Cooling coil load: 420 tons (derived from IT equipment heat density: 180 W/sq. ft. × 12,000 sq. ft. ÷ 3.517 kW/ton)
• Temperature difference (ΔT): 8.5°F (selected to maximize chiller COP under high ambient conditions and accommodate future growth; validated against chiller manufacturer’s partial-load performance maps showing optimal efficiency at 8–9°F ΔT when condenser water is 95°F+)

Calculation

Using the same fundamental formula:

Flow Rate (gpm) = (Cooling Load × 12,000) ÷ (500 × ΔT)

Substituting values:

• Numerator: 420 tons × 12,000 BTU/hr/ton = 5,040,000 BTU/hr
• Denominator: 500 × 8.5°F = 4,250
• Flow rate = 5,040,000 ÷ 4,250 ≈ 1185.88 gpm

The tool reports: 1185.88 gpm.

Result and Decision

The engineering team specified dual 600-gpm parallel pumps (N+1 redundancy) with magnetic-bearing VFDs, feeding a dedicated 8″ insulated carbon steel loop. Flow was verified via ASHRAE Guideline 110 commissioning protocol, confirming ±2% accuracy against calculated value. A secondary temperature sensor at the coil outlet triggered automatic ΔT adjustment if fouling or valve drift exceeded 0.5°F deviation.

Lesson

In high-ambient environments, lowering ΔT increases flow but can significantly improve chiller efficiency and thermal stability—however, it demands rigorous attention to pumping energy, pipe sizing, and control precision. Here, the 8.5°F ΔT saved ~140 kW/year in chiller power but added ~18 kW in pump energy; net gain justified by PUE reduction and extended chiller life.