Approach temperature
The gap between a coolant's temperature and the medium it rejects heat to; a small approach demands a bigger exchanger.
Current numbers
~10–15 °Ccoolant-to-air approach temperature for liquid-to-air CDU coils (nominal; Vertiv CDU070 15 °C, Delta 10 °C) — the budget that caps warm-coil capacity
~3–5 °Ctypical CDU heat-exchanger approach temperature (lower = warmer facility water, more free cooling)