Cooling Capacity, Loads and Efficiency Metrics
Cooling Capacity, Loads and Efficiency Metrics tests your ability to build a defensible cooling load, size capacity for it, and compute or interpret the efficiency numbers the industry argues about.
Build the load from the inside out
Start with IT load - measured is better than nameplate, and design load should reflect diversity and known growth rather than the sum of every label. Add UPS and PDU losses (roughly 2–6 % of IT load), fan power (all of it ends up in the airstream), lighting and envelope gains, and humidification or reheat where present. Add a design margin, typically 10–20 %; 50 % margins are how halls end up with chillers at 15 % load.
Capacity terms
One refrigeration ton = 3.517 kW. kW/ton = 3.517 / COP, so a chiller at COP 5.5 is 0.64 kW/ton. EER is Btu/h per watt; IPLV and NPLV describe part-load behaviour, which is what actually matters because plants rarely run at design.
Facility metrics
PUE = total facility energy / IT energy; DCIE = 1 / PUE; pPUE applies the same idea to a zone or subsystem. Pair PUE with WUE (litres per kWh), CUE (carbon) and ERF (heat reused). Always state the boundary and the period - an instantaneous winter PUE is not an annual figure.
Worked example
500 kW of IT load; UPS/PDU losses 4 % = 20 kW; CRAH fans 30 kW; lighting and envelope 20 kW. Coil load = 500 + 20 + 30 + 20 = 570 kW. The cooling plant at 0.35 kW per kW of coil load = 200 kW, plus 10 kW of other facility load. Total = 500 + 20 + 200 + 10 = 730 kW, so PUE = 730 / 500 = 1.46. If containment lets supply temperature rise and plant efficiency improves to 0.30 kW/kW, cooling falls to 171 kW and PUE becomes 1.40.
Traps
- Counting lighting energy both as IT load and as cooling load, or leaving fan heat outside the boundary.
- Using nameplate load and calling it capacity.
- Mixing COP, EER and kW/ton in the same comparison.
- Reporting a single-day PUE as the site's efficiency.