Running Standby CDUs Saves 46% of Pump Energy, Not 56%

The hydraulic basis writes the product in the same breath as the factors. All fluid distribution units in a branch should run in parallel during normal operation rather than only the duty units, so that redundant machines sit warm, observable and immune to the hidden failure of a standby pump that will not start. For the eleven-unit study branch, eleven machines sharing eight units' flow each carry 0.727 of a duty unit's flow, pressure drop per unit falls to 0.545 and shaft power to 0.396, and site pump power therefore becomes 0.436, a saving of about 56% [MODELED].

Eleven multiplied by 0.396 is 4.356. The branch it is being compared against runs eight machines at full duty, which is 8.0. The ratio is 4.356 ÷ 8 = 0.545, a saving of 46% [MODELED]. Recovering 0.436 from 4.356 requires dividing by 9.99 — a ten-unit baseline the branch does not have.

The Exponents Are Right; the Eleven-Unit Baseline Is Wrong

Both intermediate factors close. A flow ratio of 8/11 is 0.7273; raising it to 1.9 gives 0.545 and to 2.9 gives 0.397, so the hydraulic basis is using a turbulent friction exponent near 1.9 and the matching power exponent near 2.9 rather than the textbook squares and cubes [MODELED]. That is the more defensible choice, and it is why the error cannot be dismissed as sloppiness throughout.

Run the ideal affinity laws instead and the answer barely moves: 0.7273 cubed is 0.3847, eleven of those is 4.232, and against 8.0 that is 0.529, a 47% saving [SENSITIVITY]. The conclusion is insensitive to which exponent set is used and sensitive only to the baseline, which is the one thing the printed line got wrong.

The corrected total ratio, 0.545, is the same number already printed in the same sentence as the per-unit pressure-drop ratio. The right answer was sitting in the line under a different name while a different figure was carried into the conclusion.

The 56% Is the Twelve-Unit Table, Not Eleven Units

A later table in the same file, at ΔT = 10 K, gives site secondary pump power as 46 kW with twelve units in parallel and 105 kW with eight units at full load [MODELED]. Forty-six over 105 is 0.438, a saving of 56%. That 56% is a real ratio. It is the twelve-unit story, in kilowatts. It is not the eleven-unit product 11 × 0.396. Copying it onto the eleven-unit branch, or reading 46 kW as 46%, is the same class of slip as writing 0.436 for 4.356.

The table even claims consistency with the 0.436 coefficient. The two stories are being treated as one claim twice. They are not.

The heat-rejection basis makes the same operating argument for the rejection fans: run the spare, do not leave it cold. The idea is right on both sides of the plate exchanger. The percent still depends on which branch and which baseline.

H-08's 1.3 Fallback Matches Neither 46% nor 56%

Item H-08 records the assumption that eleven-unit parallel low-flow operation is feasible, notes that it saves about 56% of pump power, and states the fallback: if the supplier cannot support it, the branch reverts to eight running units with three cold standby and pump energy rises by about 1.3 times [SENSITIVITY]. Reversing a 56% saving means multiplying by 1 ÷ 0.436, which is 2.29; reversing the corrected 46% means multiplying by 1 ÷ 0.545, which is 1.83. Neither is 1.3.

So one document carries three mutually inconsistent statements of the same ratio, and the register entry that a reader would consult when pricing the fallback is the least consistent of them.

Correcting 56% to 46% Does Not Move Protected Load

The base is a band, not a value. Conclusion 4 states that the inherited 385 kW figure for site secondary pump power holds only in the corner case of a temperature difference near 6 K with loop pressure drop close to 300 kPa; if the supplier allows 10 K or more and the loop lands mid-range, the figure is 35 to 99 kW [MODELED]. Conclusion 14 then narrows that from the other side: a bottom-up build-up puts the loop total at 235 to 450 kPa, so the pump power range should be read in its upper half.

The 46 to 105 kW band sits against a 385 kW allowance: 46 kW is about 0.4% of secondary duty, and the 385 kW allowance is about 3.3% [MODELED]. Ten percentage points of a 46 kW design-point quantity is under 5 kW; ten points of the 385 kW corner case is 38.5 kW [MODELED]. The correction matters to the calculation note and to anyone reproducing it, and it does not by itself move the electrical basis. The hydraulic basis forbids deducting that difference from protected load on its own.

Limits and open items

Recommendation R-05 fences the strategy: connections and pumps are selected on nameplate flow, never on the parallel running flow, because parallel operation is a control strategy and may not be allowed to weaken the redundancy it is layered on. The parametric allocation engine holds the opposite convention in machine-readable form — units carrying the standby role are assigned zero heat, zero mass flow, zero volumetric flow and zero load share in all thirty-six scenarios. Running them is therefore not a setpoint change; it redefines a role that another controlled document enumerates as zero.

The controls basis indexes normal-operation zone coordination as a sequence still missing its time and exception slots, blocked on supplier parameters. Conclusion 33 of the hydraulic basis states that parallel low-flow running sits in the set of system-level sequences that falls outside every equipment supplier's standard scope, which is where unpriced scope surfaces at commissioning.

Confirmed on the current record: 46 kW is about 0.4% of secondary duty; the 385 kW allowance is about 3.3%; the 385 kW figure holds only in the 6 K / 300 kPa corner. [FACT]

Recommendation, not confirmed: all units in a branch run in parallel in normal operation so the spare sits warm and observable.

Modelled and not frozen: the printed 0.436 / 56% product, and the draft check that eleven units at 0.396 against eight running units leave about 0.545, a saving of about 46%. [MODELED]

Held open, beside the claim it limits: item H-08 asks the supplier for minimum stable flow, variable-speed lower limit and duty-standby synchronised control capability, and none of the three is held [HOLD]. The eleven-unit and twelve-unit branches are both HELD. Neither is a purchase. If the unit is confirmed stable at 0.727 of nameplate flow, the strategy stands and the saving is about 46%. If its minimum stable flow sits above that, the branch reverts, and the live question stops being the percentage and becomes whether three cold standby machines can be shown to start at all. Nothing above reflects Engineer of Record review, supplier confirmation, commissioning acceptance or observed operation.

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Source: K&K Data Service Inc., “Running Standby CDUs Saves 46% of Pump Energy, Not 56%,” https://www.kkdatasvc.com/lab/hydraulics-and-transients/running-standby-cdus-saves-46-percent-of-pump-energy-not-56/.