N+1 on the CDU List Is Not N+1 in the Pipes

Isolate the second unit in a 3+1 in-row group for a seal change. The racks it served lose 100 percent of their liquid cooling [MODELED]. Three healthy units remain a few metres away with unused capacity that has no pipe path to those racks. The equipment schedule still reads 3+1. Eleven 1.5 MW CDUs grouped 3+1, 3+1 and 2+1 give 16.5 MW installed and 12.0 MW of N capacity [MODELED]. That arithmetic is true about the list. It is silent about whether the spare unit can reach the load. The finding concerns secondary-side hydraulic topology, not CDU quality or nameplate accuracy.

Without a Join, Isolating One CDU Drops Its Racks

An in-row CDU is a proximity machine. Each unit serves the racks beside it. A 3+1 group survives a unit failure only if the remaining three can feed the racks of the unit that just stopped. Without a hydraulic join inside the group, those racks lose their entire liquid path while the spare sits idle. The redundancy then exists on the schedule and not in the pipes [FACT].

The hydraulic basis records that finding as a logical consequence of the architecture, not as a measurement. It applies to the liquid-cooled share of rack heat. Residual air-side heat is handled separately and is not taken as zero.

Only a Shared Ring or Dual Manifolds Make 3+1 Real

Only two arrangements make the 3+1 count real in the pipes. Either every CDU in the group injects into a shared zone ring and the racks draw from that ring, so a unit dropping out is invisible at the rack manifold. Or every rack manifold accepts supply and return from two different CDUs. The second option doubles rack-side pipework, quick-disconnect count and manifold connections. It also depends on a dual-source manifold that the equipment supplier has not confirmed is available [HOLD].

Absent one of those, the failed unit's racks lose 100 percent of their liquid cooling [MODELED] while three healthy units idle at part load a few metres away. Dual inputs and dual CDUs, as recorded in the controlled supplier-package review, are not end-to-end independent redundancy [FACT]. An average balanced pressure across a network also does not demonstrate local differential pressure at a plate, or survival with one pump stopped. The reported average describes the healthy case rather than the failure state under review.

Maintenance Plus Failure Makes Cooling the Availability Cap

Annual work on a liquid-cooling system is not optional. Heat-exchanger cleaning, secondary-pump seal and bearing replacement, filter and side-stream media changes, fluid sampling and top-up, and control-valve actuator calibration all take a unit out of service. A failure does not defer itself because a technician is on site.

Take one unit out of the 2+1 group for maintenance and the group is running at N with no redundancy at all. Take a second and the group loses 1,296 kW of 2,796 kW at a 0.90 [SENSITIVITY] liquid-cooling fraction, which is 46.3 percent of that zone's heat [MODELED]. On the current allocation model the zone carries 25 percent of the IT load [MODELED]. The eleven-unit grouping is one of several branches held in parallel. None is selected, recommended, or frozen.

The electrical study routes under comparison both survive one path in maintenance plus one module failure [MODELED]. For that specific pairing of events, the cooling grouping sets the availability ceiling. Plant availability is then capped by cooling, not by the electrical path count.

A Ring Segment May Contain Only One CDU Injection

Joining a group into a zone ring moves the redundancy into the pipework. The ring then has to be segmented to be useful. Segment isolation is what turns replacing a length of pipe, repairing a weld, or chasing a seep from a one-third-of-the-room outage into a zero-rack outage, because flow reroutes the other way around the ring. The largest single cooling event the three-zone architecture permits is that one-third of site IT, about 4 MW [MODELED].

That capability has a price list: two isolation valves per segment end, eight segments per ring, two rings per zone. Site-wide that is ninety-six DN200 isolation valves plus forty-eight drain and forty-eight vent points [MODELED]. It is an availability purchase with a quantity. It belongs as its own line rather than folded into a pipe unit rate.

Segmentation also carries a rule that is easy to violate on a layout drawing. A ring segment may contain only one CDU injection point. Put two injection points in one segment and isolating that segment removes two units at once. That converts the N−1 case the capacity check was written for into an N−2 case it was not. Group count, unit count and segment count are therefore one coupled decision.

Limits and open items

Confirmed on the current record: an in-row 3+1 group is not hydraulic until the units are joined; dual inputs and dual CDUs are not end-to-end independent redundancy; an average balanced pressure does not demonstrate local differential pressure or survival with one pump stopped. [FACT]

Modelled and not frozen: the eleven-unit grouping, the 16.5 MW installed and 12.0 MW N-capacity figures, the 0.90 liquid-cooling fraction, the 1,296 kW of 2,796 kW shortfall, the 46.3 percent and 25 percent shares, the 4 MW one-third-of-room domain, and the valve, drain and vent counts. The unit count is one of several branches held in parallel. The valve quantities move with the final grouping. [MODELED]

Open: whether a dual-source rack manifold is offered at all; the nameplate calibration fluid behind the 1.5 MW rating; the approach temperature between the primary supply and the supplier's required secondary supply temperature; the customer's maximum acceptable simultaneous-loss-of-cooling domain, which decides whether three zones is the right number; and whether the redundancy level of the cooling system must match the electrical system. That last item is a commercial decision rather than an engineering one. [HOLD]

No Engineer of Record has reviewed or sealed any of this. No unit count is selected. No quantity is released for purchase.

Which drawing shows the spare reaching the racks

The reason this survives review so often is that the equipment schedule and the piping drawing are read by different people at different times. A schedule that says 3+1 is true about procurement and silent about hydraulics. The question worth putting to a cooling supplier early, before the layout hardens, is not how many units are offered. It is which racks keep flowing when unit number two in a group is isolated for a seal change, and on which drawing that is shown.

© 2026 K&K Data Service Inc. All rights reserved. Reproduction or republication is permitted only with clear attribution to K&K Data Service Inc. and a working hyperlink to the canonical URL of this article. Excerpts must preserve the technical context, maturity labels, assumptions, and limitations. No excerpt may imply project approval, field validation, certification, or endorsement that the original article does not state.

Source: K&K Data Service Inc., “N+1 on the CDU List Is Not N+1 in the Pipes,” https://www.kkdatasvc.com/lab/direct-liquid-cooling-and-cdu/n-plus-1-on-the-cdu-list-is-not-n-plus-1-in-the-pipes/.