The 385 kW CDU Pump Budget Exists Only at a Rejected 6 K ΔT

The second recomputation of the cooling-distribution pumps does not merely strain the electrical allowance. It overruns it, and that overrun is the first of two reasons the hydraulic basis rejects 6 K. At 6 K and a median 330 kPa, site pump power is 464 kW [MODELED]. The electrical basis has already spent 385 kW [MODELED] on the same machines. The 385 kW did not come from the cooling work. It arrived as an already-used value, eleven units at 35 kW, graded an assumption pending equipment-supplier data, and it is one of the two components of the 585 kW auxiliary allowance inside the 12,711 kW protected load [MODELED]. The hydraulic basis then back-calculates the conditions under which 385 kW could be true, and finds exactly one: a secondary temperature difference near 6 K with a loop pressure drop near 300 kPa. Its second recomputation rejects that temperature difference outright, and does so partly because the pump power there overruns the 385 kW the budget was meant to cover.

385 kW Is an Already-Used Electrical Allowance

The method is short enough to check. Shaft power is flow times pressure drop. Electrical power is shaft power divided by a combined pump, motor and drive efficiency taken as 0.62 [MODELED]. Flow at full nameplate duty falls as the temperature difference rises — 226.8 m³/h at 6 K, 170.1 at 8 K, 136.1 at 10 K, 113.4 at 12 K — and with fixed pipe geometry the pressure drop scales as flow to the power 1.9, an assumed turbulent exponent [MODELED].

Totalled across eight units at full load plus three standby units drawing 1 kW of control power each, the site secondary pump power comes to 144, 284 or 425 kW at 6 K across low, medium and high pressure-drop assumptions, and to 35, 67 or 99 kW at 10 K [SENSITIVITY]. The 385 kW sits only in the corner of that grid. At 10 K, or with pressure drop in the medium-to-low band, the site figure is 286 kW to 350 kW lower [SENSITIVITY].

The 100 kPa band that cannot be reached

The pressure-drop chapter then removes the optimistic half of the grid, which matters more than the headline.

Built up component by component — unit internals, zone ring main, rack branches and hoses, two pairs of dry-break couplings, balancing valves, strainers, and the rack manifold and cold-plate group — the total lands at 235 kPa to 450 kPa [MODELED], above the medium assumption and partly above the high one. The 100 kPa low band is unreachable on a real cold-plate and coupling chain. The largest single term, 100 kPa to 200 kPa for manifold and cold plates, is marked as vendor data the project does not hold [HOLD].

6 K Is Rejected for Pump Power and for Pressure

Recomputed at a median 330 kPa, the per-unit and site figures change character. At 6 K the per-unit full-load pressure drop is 571 kPa and the site total is 464 kW; at 8 K, 330 kPa and 201 kW; at 10 K, 214 kPa and 105 kW; at 12 K, 152 kPa and 62 kW [MODELED].

So the budget is not merely optimistic at 6 K. It is exceeded there. A design point that breaks the electrical side's own auxiliary allowance from below is not a conservative choice. It is an inconsistent one.

The second reason has nothing to do with power. At 571 kPa the secondary loop pressure drop approaches or passes the working pressure ceiling of the cold-plate group and couplings, which the author puts at a typical allowable drop of 250 kPa against an assumed 1,000 kPa system rating [MODELED]. That is an equipment pressure and warranty boundary. A larger pump does not resolve it.

What follows is a recommended design point of 10 K or better, giving 46 kW in normal parallel operation and 105 kW with a unit out [MODELED] — against 385 kW carried, a difference of 280 kW to 339 kW, with the secondary pump component overstated by roughly four to eight times. The recommendation is itself conditional. The allowable temperature difference and maximum return temperature belong to the cold-plate vendor. The facility side may not select them alone.

Offer the 339 kW Pump Difference to Other Auxiliaries First

The instruction attached to that difference is the part most likely to be dropped in transmission. The released allowance may not simply be deducted from the protected load. It is to be offered first to primary pumps, dry-cooler fans and makeup water treatment, which the same 585 kW probably never counted.

Testing that instruction against its own three columns, the heat-rejection schedule confirms it. Its two auxiliary lines sum to 305 kW in the baseline column and 785 kW in the conservative column against the 585 kW original — about 280 kW lower at the favourable end and about 200 kW higher at the unfavourable one [MODELED]. The change candidate stays a candidate. No electrical-basis figure is edited.

Limits and open items

Confirmed: 385 kW is an already-used electrical-side value; it holds only near 6 K and 300 kPa; the bottom-up loop pressure closes off the 100 kPa band; at a median 330 kPa the 6 K site total is 464 kW; 6 K is rejected on both power and pressure. [FACT]

Modelled and not frozen: the 0.62 efficiency, the 1.9 exponent, the flow table, the 144 / 284 / 425 kW and 35 / 67 / 99 kW grids, the 235 kPa to 450 kPa build-up, the 571 kPa and 464 kW pair, the 46 kW and 105 kW recommended-point powers, the 280 kW to 339 kW difference, and the 305 kW / 785 kW auxiliary test. [MODELED]

Open: the rack-manifold and cold-plate pressure drop, still a 100 kPa to 200 kPa placeholder; the allowable secondary temperature difference and maximum return temperature, owed by the cold-plate supplier; the typical 250 kPa allowable drop and the 1,000 kPa system rating. [HOLD]

One condition would change the conclusion. If the cold-plate vendor returns an allowable pressure drop well above 250 kPa and an allowable temperature difference that includes 6 K, then only the first rejection reason survives, and 6 K becomes a live if expensive option whose pump power has to be re-argued against the auxiliary allowance rather than against a pressure boundary. Nothing above reflects Engineer of Record review, equipment-supplier confirmation, commissioning acceptance or measured pump performance. A supplier calculation in the read set reports secondary pump heat to five decimal places on assumed efficiencies, with no pump curve behind it.

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Source: K&K Data Service Inc., “The 385 kW CDU Pump Budget Exists Only at a Rejected 6 K ΔT,” https://www.kkdatasvc.com/lab/direct-liquid-cooling-and-cdu/the-385-kw-pump-budget-only-exists-at-a-rejected-6-k/.