Why does the page use CHWS 7.2 C not 19 C?

The original chiller-plant page showed CHWS/CHWR 19.1/22.6 C — a conflict with the dashboard CHW basis of 7.2/14.8 C. Per the single-source engineering basis in conv-engine.js (resolving the doc-04 conflict), the canonical basis is 7.2 C supply / 14.8 C return. The 18.8 C value is the secondary-loop setpoint (Rule 5 — labelled Secondary Loop SP, never CHWS SP).

What is the CHW flow formula?

CHW flow (L/s) = cooling load (kW) / (c_p x delta-T). Using the canonical IT load of 1850 kW as the cooling-kW term and delta-T = 14.8 − 7.2 = 7.6 K: 1850 / (4.186 x 7.6) = 58.1 L/s total plant flow. c_p = 4.186 kJ/(kg·K) for water at typical chilled-water conditions.

Is the chiller-plant page a full calculation tool?

No. The page is a PARTIAL-CALC display: it renders an ISA-style P&ID mimic with animated flow, live status readouts, and setpoint annotations driven by conv-engine.js derived values. The CHW basis and flow are genuine engineering derivations; however, equipment-level values (pump speed %, valve %, VFD status) are status/display fields rather than independently back-calculated quantities.

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Technical Manual · Cooling Plant

Chiller Plant — P&ID Methodology & Engineering Basis

Full engineering methodology for the chiller-plant SCADA HMI: ISA P&ID symbol conventions, primary/secondary chilled-water loop design, lead-lag chiller sequencing, CHW flow derivation, and the display/status engine anchored to js/conv-engine.js. This is an honest account — the page is a partial-calc display engine, not a full thermo-hydraulic simulation.

Engine conv-engine.js v1.0.0 Basis ISA 5.1 / ASHRAE 90.4 CHW Basis 7.2 / 14.8 °C Plant Config 2/3 Lead-Lag

▶ Open the live Chiller Plant HMI

01 Purpose & engineering basis

The chiller-plant page is an operator-grade SCADA HMI mimic — an ISA-style P&ID rendered in the browser. Its job is to show plant topology (primary/secondary loops, chiller train positions, pump duty/standby), real-time status (run/fault, lead-lag state, flow animation), and setpoint annotations (CHWS supply temperature, differential-pressure setpoints, N+1 headroom).

What the page is not: it is not a thermo-hydraulic simulator. Equipment-level values (pump speed %, valve %, per-loop flow splits) are display/status fields from the conv-engine CHW basis, not back-calculated from independent hydraulic models. The one heavy derivation is total plant CHW flow, which is computed from the IT load and the canonical delta-T — all other displayed numbers are consistent descendants of that single basis.

Governing standard for P&ID symbology is ISA 5.1 Instrumentation Symbols and Identification. Chiller envelope is bounded by ASHRAE 90.4 / ASHRAE TC9.9 recommended CHW supply range 6–9 °C. Single-source engineering basis is locked in js/conv-engine.js (see §3 below).

02 Inputs — mode toggles & locked CHW basis

The page has no free-form calculator inputs. Instead it exposes mode toggles and reads a locked CHW basis from conv-engine.js. All displayed numbers descend deterministically from these inputs — no random values, no per-page hardcoded constants that contradict the basis.

Input / Toggle	Value / Range	Type	Notes
CHWS temperature	`7.2 °C`	Locked basis	Primary chilled-water supply. Conv-engine `MODEL.cooling.chws_c`. Source: doc-09 line 15.
CHWR temperature	`14.8 °C`	Locked basis	Primary chilled-water return. Conv-engine `MODEL.cooling.chwr_c`. Source: doc-09 line 15.
IT load	`1,850 kW`	Locked basis	Site IT load from `MODEL.site.it_load_kw`. Cooling-kW term in flow equation.
Secondary loop SP	`18.8 °C`	Locked basis	Rule 5: this is the secondary (condenser-side) loop setpoint — NEVER labelled "CHWS SP". See §3 terminology note.
Lead-lag mode	AUTO / MANUAL	Mode toggle	AUTO: engine picks lead chiller by run-hours. MANUAL: operator selects.
View detail level	OVERVIEW / DETAIL / ENGINEERING	Mode toggle	Controls label density on the P&ID. OVERVIEW shows only major status; ENGINEERING reveals all sensor annotations.
Chillers running / installed	`2 / 3`	Locked basis	From `MODEL.cooling.chillers_running / chillers_total`. Determines N+1 headroom display.

Terminology Rule 5: 18.8 °C is the secondary loop setpoint (condenser-water / warm-water side), not the CHWS setpoint. The CHWS setpoint is 7.0–7.2 °C. Mislabelling 18.8 °C as "CHWS SP" was the original doc-04 engineering conflict — do not reintroduce it. Any label on the P&ID for this value must read "Secondary Loop SP" or "CW Supply SP" depending on the loop segment shown.

03 Calculation methodology

Three real derivations power the page: CHW delta-T, total plant CHW flow, and chiller input power. Everything else is status/display logic driven by those three numbers. Function names map one-to-one to js/conv-engine.js.

CHW delta-T

ΔT = CHWR − CHWS = 14.8 − 7.2 = 7.6 K In chwDeltaT(m). Source: MODEL.cooling.chwr_c − MODEL.cooling.chws_c. Anchors every flow and heat-transfer calculation on this page.conv-engine.js · doc-09 line 83

Total plant CHW flow

Q̇ = IT_load / (c_p × ΔT) Q̇ = 1850 / (4.186 × 7.6) = 58.1 L/s In chwFlowLps(m). c_p = 4.186 kJ/(kg·K) for water. The IT load (1,850 kW) is the cooling-kW term — per doc-09 worked example (line 84). heat-rejection including UPS losses (~1,927 kW) is also exposed for N+1 headroom checks but is NOT the canonical flow-derivation term.conv-engine.js · doc-09 lines 81–85

Per-loop flow split

Flow_per_loop = Q̇_total / n_loops Example (4 loops): 58.1 / 4 = 14.5 L/s per loop Display-only derivation — the page shows even split as the nominal state. Real plants balance by valve position and pump speed; those per-loop values are status annotations, not calculated from a hydraulic model.doc-09 lines 86–93 · doc-04 §Cooling Calculation

Chiller input power (kW/ton sanity check)

P_chiller = Q̇_cooling / COP At COP 4.5: 1850 / 4.5 = 411 kW compressor input kW/ton = 3.517 / COP = 3.517 / 4.5 = 0.78 kW/ton Used for the Plant Capacity Panel "Plant kW/ton" KPI. COP 4.5 is the ASHRAE 90.4 benchmark reference for the operating conditions shown. Actual COP varies with condenser water temperature and part-load ratio; this is a steady-state display benchmark, not a live transient calculation.ASHRAE 90.4 · doc-09 lines 39–45

Lead-lag sequencing logic

Lead chiller: lowest cumulative run-hours (auto mode) Lag start: CHWS actual > CHWS SP + 0.5 K for t > 120 s Lag stop: Q̇_demand < 0.60 × Q̇_lead_rated for t > 300 s These thresholds are illustrative display-engine constants — the page animates the state machine. A real chiller plant controller would implement these in the PLC/DCS ladder; the HMI only reads back the RUN/STANDBY status bits.doc-04 §Lead-Lag · ASHRAE TC9.9 Table 3

N+1 cooling headroom

Available capacity = chillers_running × Q̇_rated_per_chiller N+1 margin = available − Q̇_demand Example (2×1200 kW running, demand 1850 kW): 2400 − 1850 = 550 kW headroom Display KPI for the Plant Capacity Panel. N+1 is satisfied when available > demand AND at least one chiller is in standby. Margin below 200 kW triggers a WARN colour transition (amber) on the panel; negative = ALARM (fault-red).doc-04 §Plant Capacity Panel

04 ISA P&ID symbol conventions

The HMI follows ISA 5.1 tagging and symbol conventions. Equipment tags visible on the page map to the table below. Alarm colour hierarchy is strictly enforced — see the note on colour discipline.

ISA Tag Prefix	Instrument Type	Display Location	Example Tag
`TT`	Temperature Transmitter	CHWS/CHWR headers, loop supply/return	`TT-101` CHWS header
`FT`	Flow Transmitter	Primary header, per-loop risers	`FT-201` primary header
`PT`	Pressure Transmitter	Pump discharge/suction, differential across coil	`PT-301` pump-A discharge
`DP`	Differential Pressure	Across strainer, across chiller bundle	`DPT-401` strainer
`MV`	Motor-operated Valve	Chiller isolation, bypass	`MV-501` chiller-1 isolation
`CV`	Control Valve	Bypass, secondary-loop modulating	`CV-601` DP bypass
`P`	Pump	Primary CHW pumps, condenser water pumps	`P-701A/B` duty/standby
	`CH`	Chiller	Chiller train blocks	`CH-1/CH-2/CH-3` lead/lag/standby

Alarm colour hierarchy (non-negotiable): Normal flow animation uses muted teal (#5da095 / --chwr). Standby equipment uses neutral grey (--ok). Warning conditions (N+1 margin low, pump near-trip) use signal amber (--warn). Fault/trip/alarm conditions use fault-red (--alarm). Instrument-cyan (--mn-accent) is reserved for page chrome only and must NEVER appear on a flow line or equipment symbol that can carry an alarm state — it would erase the alarm's visual signal. This is the same rule that governs all 34 manual pages.

05 Primary / secondary loop architecture

The chiller plant uses a decoupled primary/secondary (P/S) design. The primary loop circulates through the chiller bundles at a constant low flow; the secondary loop modulates to match the data-hall cooling demand. A hydraulic decoupling bridge (bypass header) absorbs any flow mismatch.

Parameter	Primary Loop	Secondary Loop
Supply temperature	7.2 °C (CHWS)	7.2 °C at coil inlet (same water, decoupled flow)
Return temperature	14.8 °C (CHWR)	14.8 °C at coil outlet (same ΔT at design load)
Flow regime	Constant-flow (primary pumps run at fixed speed)	Variable-flow (VFD pumps track DP setpoint)
DP setpoint	N/A (constant speed)	Secondary pump DP SP (operator-configurable)
Bypass / decoupler	Shared — absorbs primary/secondary flow mismatch	Shared — bypass flow direction reverses under low-load
Pump tags	`P-701A/B` (duty/standby)	`P-801A/B` (VFD, duty/standby)
HMI animation	Muted teal flow arrows, constant velocity	Flow arrow velocity scales with VFD speed display

Secondary loop SP (18.8 °C — Rule 5): The 18.8 °C setpoint visible on older versions of the page referred to a secondary (warm-water or condenser-water) circuit that was NOT the primary CHW loop. Per the doc-04 conflict resolution, this value is retained as the Secondary Loop SP annotation only — it is never labelled CHWS SP, which must remain 7.0–7.2 °C. If 18.8 °C does not appear on the visible HMI in the current build, that is intentional (it belongs to a condenser-water annotation that may not be rendered in the default view layer).

06 Worked example — 58.1 L/s derivation

This is the canonical CHW flow derivation that the page's Plant Capacity Panel and flow annotations are anchored to. Numbers reproduce conv-engine.js output exactly.

CHW delta-T: ΔT = CHWR − CHWS = 14.8 − 7.2 = 7.6 K
Cooling-kW term: IT load = 1,850 kW (canonical basis, MODEL.site.it_load_kw)
CHW flow: Q̇ = 1850 / (4.186 × 7.6) = 1850 / 31.81 = 58.1 L/s
Per-loop average (4 loops): 58.1 / 4 = 14.5 L/s per loop — consistent with the ~17–19 L/s per-loop readings at near-design load shown on-screen
Available plant capacity (2 chillers × 1,200 kW rated): 2,400 kW
N+1 cooling headroom: 2,400 − 1,850 = 550 kW → displayed as green on the Plant Capacity Panel
Chiller compressor input (COP 4.5 reference): 1,850 / 4.5 = 411 kW → Plant kW/ton = 3.517 / 4.5 = 0.78 kW/ton

Honesty note: Steps 4–7 are display benchmarks derived from the locked basis, not measured or simulated from independent hydraulic/thermodynamic models. The page's purpose is SCADA HMI visualisation — the genuine engineering rigour is in the CHW basis derivation (steps 1–3) and the N+1 headroom logic. Equipment-level annotations (pump speed, valve %, VFD current) are display-only status fields.

07 References & standards

ISA 5.1 (2022) — Instrumentation Symbols and Identification: P&ID tag prefixes (TT, FT, PT, MV, CV), loop bubble conventions, equipment symbol styles used on the chiller-plant HMI.
ASHRAE 90.4 (2022) — Energy Standard for Data Centers: chiller COP reference (kW/ton benchmark), CHW supply temperature guidance 6–9 °C, N+1 cooling redundancy framing.
ASHRAE TC9.9 (2021) — Thermal Guidelines for Data Processing Environments: CHWS recommended range A1/A2 class, cold-aisle setpoint 18–27 °C (supply air), CHW supply 7–10 °C (typical DX/CRAH coupling).
conv review 04 — Documents/screenshot bms rz/conv/review/04-chiller-plant-review.md: original engineering conflict (19.1/22.6 vs 7.2/14.8 C), CHW flow alternatives (58.1 vs 126.3 L/s), resolution Decision A (conventional CHW 7/15 C).
conv review 09 — Documents/screenshot bms rz/conv/review/09-engineering-basis-and-calculations.md: canonical site basis (IT 1,850 kW, PUE 1.45, CHW 7.2/14.8 C), CHW flow worked example line 84, non-IT power breakdown (chiller compressors 370 kW, pumps 210 kW, CRAH 130 kW).
Engine source — js/conv-engine.js (conv-engine v1.0.0): chwDeltaT(), chwFlowLps(), heatRejectionKw(), MODEL.cooling.*. Single-source canonical basis for all BMS pages.

08 Assumptions & limitations

The HMI assumes a well-balanced, decoupled primary/secondary plant operating at steady-state design load. Transient effects (chiller pull-down time, pump start-up inrush, glycol correction to c_p) are not modelled. COP 4.5 is a reference benchmark — actual COP depends on condenser water temperature, part-load ratio, and compressor staging; the live HMI does not update COP from a transient model.

Flow-velocity animation on the P&ID is a visual cue proportional to the displayed flow value; it is not derived from a hydraulic resistance network. Pipe diameters and friction factors are not modelled. Equipment-level display values (valve %, VFD speed %, run hours) are status-field annotations consistent with the engineering basis — they are not back-calculated from independent models and must be treated as illustrative display parameters in any off-page engineering analysis.

This page and its methodology are an engineering education and visualisation aid. Final plant design, setpoint commissioning, and N+1 validation must be performed by qualified mechanical/controls engineers against the actual installed equipment and as-built P&IDs.

▶ Open the live Chiller Plant HMI