What is WUE and what is its denominator?

Water Usage Effectiveness (WUE) = annual site water consumption (L) ÷ annual IT energy (kWh). The denominator is IT energy, NOT facility (total) energy — this is Rule 3 of the conv-engine accuracy standard. The basis for this site is 1.20 L/kWh (doc-09 line 13). It is a basis/target value; the live cockpit sensor-reads the flow meter and back-computes WUE in real time.

What is makeup water versus circulation water?

Makeup water is the net fresh water added to the cooling tower to replace evaporation, blowdown, and drift losses. It is what the WUE formula quantifies. Circulation water is the total recirculating flow through the tower basin and heat exchangers, which is orders of magnitude larger. The two must not be conflated — confusing them overstates consumption by ~20–50×.

Why is the filtration ΔP threshold 2.0 bar?

2.0 bar differential pressure across the multimedia/sand filter is the standard backwash trigger per manufacturer data and site commissioning. Below this the filter is operating normally; at or above 2.0 bar the media is fouled and a backwash cycle must be initiated to restore flow capacity and avoid breakthrough of suspended solids.

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Technical Manual · Water & WUE

Water System & WUE — Cockpit Methodology & Formulas

Every input, formula, constant, output, and reference behind the Water System & WUE cockpit. All derived values originate from the deterministic engine js/conv-engine.js (CONV_CALC.waterFlowLpmForWue, wueFromFlowLpm) anchored to the single-scenario basis in CONV_MODEL — no back-solved constants, no random values. Real-time instrument reads (flow meter, conductivity probe, ΔP transducer) override the basis values when the cockpit is live.

Engine CONV_CALC v1.0.0 WUE Basis 1.20 L/kWh IT Inputs water-quality probe + valve controls Source conv review 09

▶ Open the live Water System cockpit

01 Purpose & engineering basis

The cockpit monitors three interlocked water subsystems in a conventional data-centre facility: (1) cooling-tower makeup water — quantified by WUE; (2) water treatment — chemical dosing, TDS control, blowdown scheduling, and ASHRAE 188 / ISO 11731-2 Legionella management; and (3) filtration — multimedia pre-treatment with differential-pressure alarming at the 2.0 bar threshold. Its primary KPI, WUE, is derived from a single frozen scenario basis and then continuously refined by live sensor reads.

Engineering basis: conv review document 09 ("09-engineering-basis-and-calculations.md") lines 99–104 and 189–190, cross-referenced against the Uptime Institute WUE metric definition. The single-scenario IT load is 1,850 kW (current) against a design capacity of 2,000 kW. Every water-volume number on every page of the Conventional BMS suite is derived from the same CONV_MODEL object so the suite is internally consistent.

Rule 3 — WUE denominator: WUE = annual site water consumption (L) ÷ annual IT energy (kWh). The denominator is IT energy only, not facility (total) energy. Using facility energy (PUE × IT) deflates WUE by ~PUE (1.45×) and understates the cooling-system water intensity. This rule is accuracy-gate-enforced in CONV_CALC.

02 Inputs — water-quality probe reads & valve controls

The cockpit receives two classes of input: (a) basis constants from CONV_MODEL (fixed scenario values) and (b) live sensor reads from water-quality probes and valve position feedback. The table below distinguishes them.

Input	Symbol	Unit	Type	Meaning
IT load `it_load_kw`	`P_IT`	kW	Basis (1,850 kW)	Current IT power draw — single source of truth from CONV_MODEL. Live cockpit may read the EPMS meter; basis value is the scenario anchor.
WUE basis `wue_l_per_kwh`	`WUE`	L/kWh	Basis (1.20 L/kWh)	Site water-usage effectiveness. Basis value from doc-09 line 13. Live read: back-computed from measured flow meter ÷ IT load.
Makeup flow `flow_lpm`	`Q_mu`	L/min	Sensor-read (flow meter)	Instantaneous makeup water entering the cooling-tower basin. Sensor primary; basis derivation is the fallback.
Conductivity / TDS `tds_ppm`	`TDS`	ppm (mg/L)	Sensor-read (conductivity probe)	Total dissolved solids in tower basin water. Drives blowdown logic and cycles-of-concentration calculation.
Feed-water TDS `tds_feed_ppm`	`TDS_feed`	ppm	Sensor-read or lab sample	Mains / RO permeate TDS. Used in cycles-of-concentration formula.
Filtration ΔP `delta_p_bar`	`ΔP`	bar	Sensor-read (ΔP transducer)	Differential pressure across the multimedia/sand filter. Threshold 2.0 bar triggers backwash alarm.
Valve states `valve_*`	—	open / closed / %	Control feedback	Blowdown valve, makeup valve, bypass valve positions — logged for audit and interlock checks.
Chemical dosing `dose_*`	—	L/hr or kg/day	Controller feedback	Biocide, scale inhibitor, and corrosion inhibitor dosing rates — inputs to ASHRAE 188 Water Management Plan compliance log.

03 Calculation methodology

All formulas below are implemented in js/conv-engine.js. Function names are exact. The engine is ES5-safe, deterministic, and zero-build.

WUE definition (Rule 3 — IT-energy denominator)

WUE = Annual_water_L / Annual_IT_kWh [L/kWh] WUE = (Q_mu_Lpm × 60) / P_IT_kW (instant back-compute from live flow) In wueFromFlowLpm(m, flowLpm). Denominator = IT energy. Basis: 1.20 L/kWh (doc-09 line 13). This is a basis/target value; the sensor-read flow meter back-computes live WUE each scan.conv review 09 · BASIS

Instant makeup-water flow from WUE

Q_mu = (WUE × P_IT) / 60 [L/min] In waterFlowLpmForWue(m). Converts the per-hour WUE rate to a per-minute flow for instrumentation display. Check: (1.20 × 1850) / 60 = 37.0 L/min.conv review 09 · DERIVED

Makeup vs circulation distinction: Q_mu (37.0 L/min at basis) is the net fresh water added to replace evaporation + blowdown + drift. The circulation flow through the tower deck and heat exchangers is typically 20–50× larger. The two must never be conflated — confusing them overstates WUE consumption by that factor.

Annual water consumption

Annual_water_L = WUE × P_IT × 8760 [L/yr] Assumes full-year operation at the scenario IT load. Check: 1.20 × 1850 × 8760 = 19,447,200 L ≈ 19.4 ML/yr. Displayed on the cockpit's annual summary panel.conv review 09 · DERIVED

Cycles of concentration & blowdown

CoC = TDS_basin / TDS_feed (dimensionless, target ≤ 5) Q_bd = Q_mu / (CoC − 1) [L/min] (blowdown flow to maintain CoC) Q_evap = Q_mu − Q_bd − Q_drift [L/min] (evaporation, residual) CoC ≤ 5 limits scale-forming ion concentration while minimising water waste. At CoC = 5 blowdown is 25% of makeup. Conductivity probe monitors TDS_basin; the controller triggers the blowdown valve when TDS_basin approaches TDS_feed × CoC_max. Drift is typically <0.002% of circulation (eliminated by drift eliminators).Uptime WUE · ASHRAE 188 · DERIVED

Filtration differential pressure

ΔP_clean ≈ 0.3–0.5 bar (new media, design point) ΔP_alarm = 2.0 bar (backwash trigger threshold) Measured by a ΔP transducer across the multimedia filter inlet/outlet. Rising ΔP indicates media fouling or biological growth. At 2.0 bar the head loss exceeds acceptable limits and a backwash cycle is automatically (or manually) initiated. Threshold is from site commissioning data — not a universal standard; confirm against installed filter manufacturer data sheet.Site commissioning · INSTRUMENT

Legionella / microbiological monitoring (ASHRAE 188)

HPC ≤ 1×10³ CFU/mL (operational target, ISO 11731-2) Legionella spp. < 1 CFU/100 mL (ASHRAE 188 action level) Cooling towers are a Legionella growth environment (warm water, nutrients, aerosol). ASHRAE 188 Water Management Plan mandates documented control targets, sampling frequency, and corrective actions. Biocide dosing (oxidising + non-oxidising rotation) and blowdown together keep heterotrophic plate count (HPC) and Legionella below action levels.ASHRAE 188 · ISO 11731-2 · STANDARD

04 Constants & data sources

Every constant lives in the deep-frozen CONV_MODEL with a // source: comment citing the review document and line number. Water-relevant constants:

Constant	Value	Unit	Source	Type
`wue_l_per_kwh`	1.20	L/kWh IT	doc-09 line 13; doc-01 §3	Basis (target)
`it_load_kw`	1,850	kW	doc-09 line 10	Basis (current)
Annual hours	8,760	hr/yr	Calendar constant	Fixed
Max CoC	5	—	Water management practice; ASHRAE 188 guidance	Operational target
Filtration ΔP alarm	2.0	bar	Site commissioning data sheet	Instrument threshold
Legionella action level	<1 CFU/100 mL	CFU/100 mL	ASHRAE 188	Regulatory threshold
HPC operational target	≤1×10³ CFU/mL	CFU/mL	ISO 11731-2	Operational target

Basis vs sensor-read: wue_l_per_kwh = 1.20 is a basis target from the engineering review — it is what the system was designed to achieve. In live cockpit operation the flow meter provides actual Q_mu; the system back-computes actual WUE via wueFromFlowLpm(). When the two diverge (actual WUE > 1.20), the delta surfaces as a water-efficiency alert. The basis chip on the KPI panel always labels which value is shown.

05 Outputs

Output	Formula	Unit	Interpretation
WUE (basis)	`CONV_MODEL.environment.wue_l_per_kwh`	L/kWh	Target value from engineering review. Green chip on cockpit.
WUE (live)	`wueFromFlowLpm(lpm)`	L/kWh	Computed from measured flow meter. Alarm if > basis.
Instant makeup flow	`waterFlowLpmForWue(m)`	L/min	37.0 L/min at basis. Flow-meter primary.
Annual water consumption	`WUE × P_IT × 8760`	L/yr (ML/yr)	≈ 19.4 ML/yr at basis. Summary panel.
Cycles of concentration	`TDS_basin / TDS_feed`	—	Target ≤ 5. Amber alert at CoC > 4.5, red at CoC > 5.
Blowdown flow	`Q_mu / (CoC − 1)`	L/min	Required blowdown to maintain target CoC.
Filtration ΔP status	ΔP transducer reading	bar	Green <1.5 bar, amber 1.5–2.0, red ≥ 2.0 (backwash required).
Legionella / HPC status	Lab / online probe result	CFU/mL or CFU/100 mL	ASHRAE 188 action-level indicator.
Chemical dosing status	Dosing controller feedback	L/hr / kg/day	On-duty/off indicator per biocide and inhibitor channel.

06 Worked example

Scenario: basis IT load 1,850 kW, WUE 1.20 L/kWh, TDS_feed = 150 ppm, TDS_basin = 600 ppm (CoC = 4), filtration ΔP = 1.1 bar. All numbers reproduce CONV_CALC output exactly.

Instant makeup flow: Q_mu = (1.20 × 1850) / 60 = 37.0 L/min (basis; flow meter overrides in live mode)
Annual water consumption: 1.20 × 1850 × 8760 = 19,447,200 L ≈ 19.4 ML/yr
Live WUE back-compute (if meter reads 38.5 L/min): (38.5 × 60) / 1850 = 1.249 L/kWh → above basis 1.20, water-efficiency alert
Cycles of concentration: 600 / 150 = CoC 4.0 (within ≤ 5 target ✓)
Required blowdown to hold CoC = 4: 37.0 / (4 − 1) = 12.3 L/min
Filtration ΔP = 1.1 bar → NORMAL (threshold 2.0 bar, no backwash required)
Annual water with blowdown included: evaporation = 37.0 − 12.3 − ~0.07 (drift) ≈ 24.6 L/min evaporated; total input = 37.0 L/min makeup (steady state)

Engineering reading: at CoC = 4 the blowdown is 33% of makeup (12.3 of 37.0 L/min). Raising CoC to 5 would drop blowdown to 25% (9.3 L/min), saving ~3 L/min — approximately 1.6 ML/yr in water savings, at the cost of tighter TDS monitoring. The tradeoff is governed by local water cost vs. chemical-treatment cost and scale-fouling risk.

07 References & standards

ASHRAE 188 (2021) — Legionellosis: Risk Management for Building Water Systems. Mandates a Water Management Plan for cooling towers including control targets, sampling schedules, corrective actions, and record retention. The cockpit's biocide dosing and Legionella status indicators are ASHRAE 188 WMP deliverables.
ISO 11731-2 — Water quality — Detection and enumeration of Legionella — Part 2: Direct membrane filtration method for waters with low bacterial counts. HPC ≤ 1×10³ CFU/mL operational target cited from this standard.
Uptime Institute WUE Metric — Water Usage Effectiveness white paper. Defines WUE = annual site water ÷ annual IT energy. Source of the IT-denominator Rule 3.
conv review 09 — "09-engineering-basis-and-calculations.md" lines 99–104, 189–190. Single-scenario basis for WUE 1.20 L/kWh, IT load 1,850 kW, and the water-flow contract of the Conventional BMS suite.
Engine source — js/conv-engine.js (CONV_CALC v1.0.0) + frozen CONV_MODEL. Functions: waterFlowLpmForWue(), wueFromFlowLpm().

08 Assumptions & limitations

The WUE basis (1.20 L/kWh) is a site target from the engineering review — it assumes steady-state operation at 1,850 kW IT load with the cooling tower operating at design cycles of concentration and no abnormal evaporative losses. Actual WUE varies with ambient wet-bulb temperature (higher in summer), partial-load operation, and tower maintenance state; the cockpit surfaces these deviations by continuously back-computing live WUE from the flow meter. The annual 19.4 ML figure assumes 8,760 hr/yr at the stated IT load — seasonality and planned downtime will reduce the actual total. Blowdown volume estimates use the steady-state CoC formula; transient startup, chemical slug dosing, and emergency drain events are metered separately. Filtration ΔP thresholds are from site commissioning data and must be re-validated after any media replacement or hydraulic modification. This manual is an engineering education and operational reference; a licensed water-treatment engineer and microbiologist must author the site's formal ASHRAE 188 Water Management Plan.

▶ Open the live Water System & WUE Cockpit