Rack cooling check
Find which cooling path carries a rack today and at your roadmap rack: air, air with a rear door, or direct liquid with room air or a rear door. Enter rack power, the rack maker's liquid share, the room's air capacity per rack, any rear door and your hottest-day facility water; get the residual air heat, the CDU flow at OCP's 1.25–2.0 L/min per kW, and the coolant temperature at the rack.
Rack cooling check
| Path | Today (135 kW) | Roadmap (228 kW) |
|---|---|---|
| Air only | Not applicable: 121.5 kW of this rack's heat must go to liquid | Not applicable: 205.2 kW of this rack's heat must go to liquid |
| Air + rear door | Not applicable: 121.5 kW of this rack's heat must go to liquid | Not applicable: 205.2 kW of this rack's heat must go to liquid |
| Direct liquid + room air | Works: liquid 121.5 kW, room 13.5 kW of its 20 kW | Fails: residual air 22.8 kW exceeds the room's 20 kW |
| Direct liquid + rear door | Not applicable: no rear door entered | Not applicable: no rear door entered |
How this works: liquid heat is rack power times the liquid share, and the room's air takes the rest, through a rear door if you have one. The coolant reaching the rack is the facility water plus the CDU approach, and it must not be warmer than the rack allows → Ch 5.6. Flow uses OCP's PG25 guideline of 1.25–2.0 L/min per kW of liquid heat, which keeps the coolant rise near 7.5–12 °C → Ch 5.11. This is a screen: before you buy, get the rack maker's flow, pressure and fluid limits and check the room's airflow at the rack → Ch 5.4, Ch 5.3.
Worked decision — stated assumptions
All ledger inputs are assumed teaching values chosen to exercise this chapter’s method; they are not market quotes, OEM performance claims or operating records. Arithmetic uses the full entered precision.
| Input | Value |
|---|---|
| Rack power | 132 kW/rack |
| Heat captured to liquid | 87 % of rack heat |
| Room air capacity per rack | 20 kW/rack |
| Rear-door capacity | 0 kW/rack |
| Facility water to the CDU, hottest day | 30 °C |
| CDU approach | 5 K |
| Warmest coolant the rack accepts | 45 °C |
| Roadmap rack power | 132 kW/rack |
| Roadmap heat captured to liquid | 87 % of rack heat |
Decide whether a GB200 NVL72-class rack fits this hall. Assume 132 kW per rack with 87% of the heat going to liquid (HPE's split), a room that can take 20 kW of air per rack, 30 °C facility water on the hottest day, a CDU with a 5 K approach and a rack that accepts coolant up to 45 °C. Liquid heat is 132 × 0.87 = 114.84 kW, and the room takes the other 17.16 kW, inside its 20 kW. The coolant reaches the rack at 30 + 5 = 35 °C, under the 45 °C limit, so direct liquid with room air works. At OCP's 1.25–2.0 L/min per kW, the CDU must deliver about 144–230 L/min to this rack.
Raise the hottest-day facility water to 41 °C: the coolant arrives at 46 °C and the liquid path fails. The crossover is 40 °C facility water at a 5 K approach; a CDU with a 3 K approach moves it to 42 °C. Get the rack maker's flow curve and pressure limits before you order.
Primary method: ASHRAE Handbook (2023), Applications ch. 20 — thermal paths and CDUs. Method and handoff: Chapter 5.6 — CDUs & the Secondary Loop.
These are transparent screening estimates, not final designs, financial advice, or project approvals. Replace every default with current project data and have the responsible project authorities validate the result. You can save, share a permalink, or export to CSV; see the full calculator suite.