Utility Sizing

Quick pick (TL;DR)

Cooling water: flow = Q / (cp·ΔT), ΔT usually ~10 °C.
Steam: flow = Q / latent heat (hfg) at the supply pressure.
Instrument air: sum consumer demand (Nl/min per device) + leakage, dew point −40 °C.
Always apply a diversity/simultaneity factor (not everything runs at once) and then a design margin (~20%).
Size headers to a sensible velocity (1.5–3 m/s water, 15–25 m/s air/steam).

Step 1 — Sizing basis

Utility	Sized from	Basis
Cooling water	m = Q / (cp·ΔT)	Sensible heat
Steam	m = Q / hfg	Latent heat
Instrument air	Σ demand	Consumers
Nitrogen	Inert demand	Inerting

Step 2 — Worked example

Cooling water, 1200 kW

Given Q = 1200 kW = 1,200,000 W cp = 4180 J/kg·K , ΔT = 10 °C Step 1 — Mass flow m = Q / (cp · ΔT) m = 1,200,000 / (4180 × 10) m = 1,200,000 / 41,800 = 28.7 kg/s Step 2 — Volumetric flow Q_v = 28.7 × 3600 / 1000 = 103 m³/h Step 3 — Diversity + margin single duty → diversity 1.0 × 1.20 design margin design flow = 124 m³/h

CW is a sensible-heat balance over a chosen ΔT; steam uses latent heat. Apply diversity + margin on headers.

Step 3 — Headers & sources

Size headers to a sensible velocity.
Source: tower/boiler/compressor + treatment.
Add make-up & N+1 spare.

Cooling-water flow calculator

Sensible-heat flow + margin

m = Q/(cp·ΔT); then diversity & margin.

Heat duty Q (kW)

ΔT (°C)

cp (kJ/kg·K)

Diversity

Margin (%)

CW (sensible heat). Steam uses m = Q/hfg. Header velocity 1.5–3 m/s.

Common mistakes

No diversity factor → oversized.
Sensible heat for steam.
Ignoring instrument-air dew point.

Requisition & datasheet

Utility, duty, supply/return, cp/hfg.
Flow, diversity, margin, header, source.

Frequently asked questions

What should I know about Utility Sizing (Steam / CW / Air)?

Cooling water: flow = Q / (cp·ΔT), ΔT usually ~10 °C. Steam: flow = Q / latent heat (hfg) at the supply pressure. Instrument air: sum consumer demand (Nl/min per device) + leakage, dew point −40 °C. Always apply a diversity/simultaneity factor (not everything runs at once) and then a design margin (~20%). Size headers to a sensible velocity (1.5–3 m/s water, 15–25 m/s air/steam).

Which standards or codes apply?

Project utility specifications, ISO 8573 (compressed-air quality), ASHRAE (cooling water), ASME steam tables (hfg), GPSA for general utility design.

Standards — ISO 8573; ASHRAE; ASME steam tables; GPSA.