Overview

Water is the most common process fluid in engineering — used for cooling, heating, steam generation, cleaning, and as a process material. Its physical properties are essential inputs for pump sizing, pipe flow, heat transfer, and hydraulic calculations. Properties vary significantly with temperature, especially viscosity and vapor pressure.

ρ_{water at 4°C} = 999.97 kg/m³ ≈ 1000 kg/m³ (reference)

Properties at Standard Temperature

Density vs Temperature

Temperature (°C)	Density (kg/m³)	Notes
0 (ice)	917	Solid — less dense than liquid (floats)
4	1000	Maximum density
10	999.7
20	998.2	Room temperature reference
30	995.7
50	988.0
80	971.8
100	958.4	Boiling at 1 atm
200 (16 bar)	864.7	Saturated liquid
300 (86 bar)	712.5	Approaching critical point

Water Is Nearly Incompressible

Water density changes less than 0.005% per bar of pressure — for all practical engineering purposes, water is incompressible. Only temperature has significant effect on density. Use 1000 kg/m³ for rough calculations, 998 kg/m³ for precise work at 20°C.

Dynamic Viscosity

Temperature (°C)	μ (mPa·s or cP)
0	1.79
10	1.31
20	1.00
30	0.798
40	0.653
50	0.547
60	0.467
80	0.355
100	0.282

μ_20°C = 1.002 × 10⁻³ Pa·s ≈ 0.001 Pa·s = 1 cP

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Viscosity drops by ~70% from 0°C to 100°C — this strongly affects pressure drop calculations in hot water systems.

Thermal Properties

Property	Value (20°C)
Specific Heat (c_p)	4.186 kJ/kg·K
Thermal Conductivity	0.60 W/m·K
Latent Heat (vaporization at 100°C)	2,257 kJ/kg
Latent Heat (fusion at 0°C)	334 kJ/kg
Prandtl Number (20°C)	7.0
Prandtl Number (100°C)	1.75

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Vapor Pressure

Vapor pressure is critical for NPSH calculations, boiling analysis, and cavitation:

Temperature (°C)	Vapor Pressure (kPa abs)	Equivalent Head (m of water)
0	0.61	0.06
10	1.23	0.13
20	2.34	0.24
30	4.24	0.43
40	7.38	0.75
50	12.3	1.26
60	19.9	2.03
70	31.2	3.18
80	47.3	4.83
90	70.1	7.16
100	101.3	10.33 (boiling!)
120 (2 bar)	199	20.3
150 (4.8 bar)	476	48.5

Hot Water Kills NPSH

At 100°C, vapor pressure equals atmospheric pressure — open systems boil. For condensate at 90°C, vapor pressure head is 7.2m, subtracting directly from NPSH. See [[npsh-calculation-explained]] for details.

Speed of Sound

Temperature (°C)	Speed (m/s)
0	1,402
20	1,482
50	1,542
100	1,543

Relevant for water hammer calculations (see [[water-hammer-analysis]]).

Common Calculations with Water

Heating/Cooling Duty

Q = m × c_p × ΔT (kW) — for 1 kg/s flow with 1°C change, Q = 4.186 kW

Pressure Loss at 20°C

For 100mm pipe at 2 m/s: Re ≈ 1000 × 2 × 0.1 / 0.001 = 200,000 (turbulent)

Pump Power for Water

P(kW) = Q(m³/s) × ΔP(Pa) / 1000 / η = Q(m³/h) × head(m) × 9.81 / (3600 × η)

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Seawater vs Fresh Water

Property	Fresh Water (20°C)	Seawater (3.5% salinity)
Density	998 kg/m³	1,025 kg/m³
Viscosity	1.00 cP	1.08 cP
Vapor pressure	2.34 kPa	2.20 kPa
pH	~7	~8.1
Corrosivity	Low	Significant (chlorides)

Seawater Corrosion

Seawater is highly corrosive to carbon steel and causes pitting/SCC in 304/316 stainless above ~40°C. Use 90/10 copper-nickel, 2205 duplex, FRP, or titanium for seawater piping and equipment.

Summary

Water at 20°C has density 998 kg/m³, dynamic viscosity 0.001 Pa·s, specific heat 4.186 kJ/kg·K, and thermal conductivity 0.60 W/m·K. Density changes modestly with temperature; viscosity decreases significantly (important for hot water pressure drop); vapor pressure rises exponentially — critical for NPSH at temperatures above 50°C. Use 1000 kg/m³ and 0.001 Pa·s as defaults for preliminary calculations; use precise values from steam tables for design work.

Water Properties (20°C)