ASHRAE
The engineering society whose thermal guidelines define the temperature and humidity envelopes for IT equipment.
Current numbers
up to ~45 °C supply / ~65 °C return (W45-class)GB200 NVL72 coolant acceptance envelope — supply / return (ASHRAE W45-class); ~25 °C was NVIDIA's operating example, not a limit
up to ~45 °C supply / ~65 °C return (W45-class)GB200 NVL72 rack coolant acceptance envelope (ASHRAE W45-class) — warm-water operation is the design intent; colder supply buys thermal margin, at chiller cost
45 / 65 °CGB200 NVL72 rack coolant envelope — max supply / return (ASHRAE W45-class; ~25 °C @ ~2 L/s was NVIDIA's operating example, not a limit); rack flow sized from ~1.2–2.0 L/min/kW, <10 °C cold-plate rise; excursions beyond the envelope drive progressive GPU thermal throttling