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.
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 |
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 |
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 (cp) | 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 |
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 |
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
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
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) |
Frequently Asked Questions
What is the density of water at 20°C? 998.2 kg/m³ (0.9982 g/cm³). For preliminary engineering calculations, 1000 kg/m³ is a widely used approximation.
Why does water have its maximum density at 4°C? Water reaches its maximum density of 999.97 kg/m³ at 4°C. Below this temperature, hydrogen bonding expands the molecular structure as it approaches freezing, which is why ice (917 kg/m³) floats on liquid water.
What is the dynamic viscosity of water at 20°C? 1.002 × 10⁻³ Pa·s, commonly rounded to 1 cP (centipoise). Viscosity drops by roughly 70% between 0°C and 100°C, which significantly affects pressure drop in hot water systems.
What is the specific heat of water? 4.186 kJ/kg·K at 20°C — one of the highest of common fluids, which is why water is the preferred heat-transfer medium for cooling and heating duty.
What is the thermal conductivity of water? 0.60 W/m·K at 20°C, roughly 4-5 times higher than most organic liquids, making water an efficient heat-transfer fluid.
Why does hot water cause NPSH problems in pumps? Vapor pressure rises exponentially with temperature — at 90°C it is 70.1 kPa (7.16 m of water head), which subtracts directly from available NPSH. Above ~50°C, vapor pressure must be checked carefully in pump suction design.
How much does water density change with pressure? Almost none — water is nearly incompressible, changing less than 0.005% per bar. Only temperature has a significant effect on density.
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.
Related Guides and Tools
- Pressure Drop Calculator — pipe friction loss for water systems
- Reynolds Number Calculator — flow regime for water in pipes
- Pipe Velocity Calculator — velocity and flow sizing
- Pump Power Calculator — hydraulic power for water pumping
- Heat Transfer Calculator — heating/cooling duty
- Fluid Density Calculator — density of water and other fluids
- Pipe Pressure Drop Guide — Darcy-Weisbach and Hazen-Williams
- Water Hammer Analysis — surge and acoustic velocity
- NPSH Calculation Explained — cavitation and suction head
- Pipe Flow Engineering — pipe sizing fundamentals
- Seawater Properties — comparison for marine systems