Common fluid properties
The properties of fluids determine their behavior in heat transfer, pipe flow, and lubrication applications. Density, viscosity, specific heat, and thermal conductivity vary with temperature and pressure; therefore, reference data are presented at saturation conditions. Water is the most common working fluid; its properties are listed as a design baseline. For other fluids such as oils, typical values at room temperature are given for comparison.
Physical properties
Section titled “Physical properties”The most relevant physical properties for the design of fluid systems are density (ρ), dynamic viscosity (μ) or kinematic viscosity (ν), specific heat (cp), thermal conductivity (k), and surface tension. For water, the properties change noticeably with temperature; at 20 °C / 68 °F, the density of saturated liquid water is 998,2 kg/m³ / 62,3 lb/ft³ and its dynamic viscosity is 1,002×10⁻³ Pa·s / 2,09×10⁻⁵ lbf·s/ft².
Viscosity
Section titled “Viscosity”The dynamic viscosity of water decreases with increasing temperature, from 1,79×10⁻³ Pa·s / 3,74×10⁻⁵ lbf·s/ft² at 0 °C / 32 °F to 0,28×10⁻³ Pa·s / 5,8×10⁻⁶ lbf·s/ft² at 100 °C / 212 °F at saturation conditions. Kinematic viscosity (ν = μ/ρ) follows a similar trend. In common oils, the viscosity is much higher; for instance, an SAE 30 oil has a dynamic viscosity close to 0,29 Pa·s / 6,1×10⁻³ lbf·s/ft² at 20 °C / 68 °F. The Reynolds number, which characterizes the flow regime, depends directly on viscosity.
| Temperature | Dynamic viscosity μ | Kinematic viscosity ν |
|---|---|---|
| 0 °C / 32 °F | 1,79×10⁻³ Pa·s / 3,74×10⁻⁵ lbf·s/ft² | 1,79×10⁻⁶ m²/s / 1,93×10⁻⁵ ft²/s |
| 20 °C / 68 °F | 1,00×10⁻³ Pa·s / 2,09×10⁻⁵ lbf·s/ft² | 1,00×10⁻⁶ m²/s / 1,08×10⁻⁵ ft²/s |
| 40 °C / 104 °F | 0,65×10⁻³ Pa·s / 1,36×10⁻⁵ lbf·s/ft² | 0,66×10⁻⁶ m²/s / 7,1×10⁻⁶ ft²/s |
| 60 °C / 140 °F | 0,47×10⁻³ Pa·s / 9,8×10⁻⁶ lbf·s/ft² | 0,48×10⁻⁶ m²/s / 5,1×10⁻⁶ ft²/s |
| 80 °C / 176 °F | 0,36×10⁻³ Pa·s / 7,5×10⁻⁶ lbf·s/ft² | 0,37×10⁻⁶ m²/s / 4,0×10⁻⁶ ft²/s |
| 100 °C / 212 °F | 0,28×10⁻³ Pa·s / 5,8×10⁻⁶ lbf·s/ft² | 0,29×10⁻⁶ m²/s / 3,1×10⁻⁶ ft²/s |
Density
Section titled “Density”The density of saturated liquid water reaches a maximum at 4 °C / 39 °F with 1000 kg/m³ / 62,4 lb/ft³, and decreases both when heated and when cooled. At 20 °C / 68 °F the value is 998,2 kg/m³ / 62,3 lb/ft³. The density of saturated vapor is several orders of magnitude lower; for example, at 100 °C / 212 °F vapor has a density of 0,60 kg/m³ / 0,037 lb/ft³. The density of typical oils ranges from 850 kg/m³ / 53 lb/ft³ to 950 kg/m³ / 59 lb/ft³ at room temperature.
| Temperature | Liquid density | Vapor density |
|---|---|---|
| 0 °C / 32 °F | 999,8 kg/m³ / 62,4 lb/ft³ | 0,0048 kg/m³ / 0,00030 lb/ft³ |
| 20 °C / 68 °F | 998,2 kg/m³ / 62,3 lb/ft³ | 0,017 kg/m³ / 0,0011 lb/ft³ |
| 40 °C / 104 °F | 992,2 kg/m³ / 61,9 lb/ft³ | 0,051 kg/m³ / 0,0032 lb/ft³ |
| 60 °C / 140 °F | 983,2 kg/m³ / 61,4 lb/ft³ | 0,13 kg/m³ / 0,0081 lb/ft³ |
| 80 °C / 176 °F | 971,8 kg/m³ / 60,7 lb/ft³ | 0,29 kg/m³ / 0,018 lb/ft³ |
| 100 °C / 212 °F | 958,4 kg/m³ / 59,8 lb/ft³ | 0,60 kg/m³ / 0,037 lb/ft³ |
Specific heat and thermal conductivity
Section titled “Specific heat and thermal conductivity”The specific heat of liquid water is high, around 4,18 kJ/(kg·K) / 1,0 Btu/(lb·°F) at room temperature, making it an excellent thermal storage medium. The thermal conductivity of liquid water increases with temperature, from 0,56 W/(m·K) / 0,32 Btu/(h·ft·°F) at 0 °C / 32 °F to 0,68 W/(m·K) / 0,39 Btu/(h·ft·°F) at 100 °C / 212 °F. Oils have lower specific heats, typically 1,5 to 2,0 kJ/(kg·K) / 0,36-0,48 Btu/(lb·°F), and thermal conductivities of 0,10 to 0,15 W/(m·K) / 0,058-0,087 Btu/(h·ft·°F).
| Temperature | Specific heat cp | Thermal conductivity k |
|---|---|---|
| 0 °C / 32 °F | 4,217 kJ/(kg·K) / 1,007 Btu/(lb·°F) | 0,561 W/(m·K) / 0,324 Btu/(h·ft·°F) |
| 20 °C / 68 °F | 4,182 kJ/(kg·K) / 0,999 Btu/(lb·°F) | 0,598 W/(m·K) / 0,345 Btu/(h·ft·°F) |
| 40 °C / 104 °F | 4,178 kJ/(kg·K) / 0,998 Btu/(lb·°F) | 0,631 W/(m·K) / 0,364 Btu/(h·ft·°F) |
| 60 °C / 140 °F | 4,184 kJ/(kg·K) / 0,999 Btu/(lb·°F) | 0,654 W/(m·K) / 0,378 Btu/(h·ft·°F) |
| 80 °C / 176 °F | 4,196 kJ/(kg·K) / 1,002 Btu/(lb·°F) | 0,670 W/(m·K) / 0,387 Btu/(h·ft·°F) |
| 100 °C / 212 °F | 4,216 kJ/(kg·K) / 1,007 Btu/(lb·°F) | 0,679 W/(m·K) / 0,392 Btu/(h·ft·°F) |
Reference table
Section titled “Reference table”The following table integrates the properties of saturated liquid water from 0 °C / 32 °F to 100 °C / 212 °F in increments of 20 °C / 36 °F. The data are essential for heat exchanger calculations, pumping systems, and convection models.
| Temperature | Density ρ | Dynamic viscosity μ | Specific heat cp | Conductivity k | Prandtl number Pr |
|---|---|---|---|---|---|
| 0 °C / 32 °F | 999,8 kg/m³ / 62,4 lb/ft³ | 1,79×10⁻³ Pa·s / 3,74×10⁻⁵ lbf·s/ft² | 4,217 kJ/(kg·K) / 1,007 Btu/(lb·°F) | 0,561 W/(m·K) / 0,324 Btu/(h·ft·°F) | 13,5 |
| 20 °C / 68 °F | 998,2 kg/m³ / 62,3 lb/ft³ | 1,00×10⁻³ Pa·s / 2,09×10⁻⁵ lbf·s/ft² | 4,182 kJ/(kg·K) / 0,999 Btu/(lb·°F) | 0,598 W/(m·K) / 0,345 Btu/(h·ft·°F) | 7,0 |
| 40 °C / 104 °F | 992,2 kg/m³ / 61,9 lb/ft³ | 0,65×10⁻³ Pa·s / 1,36×10⁻⁵ lbf·s/ft² | 4,178 kJ/(kg·K) / 0,998 Btu/(lb·°F) | 0,631 W/(m·K) / 0,364 Btu/(h·ft·°F) | 4,3 |
| 60 °C / 140 °F | 983,2 kg/m³ / 61,4 lb/ft³ | 0,47×10⁻³ Pa·s / 9,8×10⁻⁶ lbf·s/ft² | 4,184 kJ/(kg·K) / 0,999 Btu/(lb·°F) | 0,654 W/(m·K) / 0,378 Btu/(h·ft·°F) | 3,0 |
| 80 °C / 176 °F | 971,8 kg/m³ / 60,7 lb/ft³ | 0,36×10⁻³ Pa·s / 7,5×10⁻⁶ lbf·s/ft² | 4,196 kJ/(kg·K) / 1,002 Btu/(lb·°F) | 0,670 W/(m·K) / 0,387 Btu/(h·ft·°F) | 2,2 |
| 100 °C / 212 °F | 958,4 kg/m³ / 59,8 lb/ft³ | 0,28×10⁻³ Pa·s / 5,8×10⁻⁶ lbf·s/ft² | 4,216 kJ/(kg·K) / 1,007 Btu/(lb·°F) | 0,679 W/(m·K) / 0,392 Btu/(h·ft·°F) | 1,7 |
Application formulas
Section titled “Application formulas”Fluid properties are involved in numerous engineering correlations. Newton’s law of viscosity defines the shear stress τ = μ (du/dy). The Reynolds number, Re = ρ·v·D/μ, characterizes the flow regime (laminar if Re < 2300). The Darcy-Weisbach equation for head loss in pipes, hf = f (L/D) (v²/(2g)), requires viscosity to calculate the friction factor f. In heat transfer, the Prandtl number, Pr = cp·μ/k, relates momentum and thermal diffusivity; Nusselt numbers for forced and natural convection are functions of Re and Pr.
Selection criteria
Section titled “Selection criteria”The choice of a working fluid depends on heat transfer requirements, lubrication, chemical stability, and temperature range. A high specific heat favors energy storage and transport (water, thermal oils). High viscosity improves lubrication but increases friction losses; in hydraulic systems, viscosities between 20 cSt / 2,2×10⁻⁵ ft²/s and 100 cSt / 1,1×10⁻⁴ ft²/s at operating temperature are preferred. In heating systems, the low viscosity and high conductivity of water make it preferable to oils. For temperatures above 100 °C / 212 °F, thermal oils or silicone fluids with stable properties are used.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the viscosity of water at 20 °C?
Section titled “What is the viscosity of water at 20 °C?”The dynamic viscosity of water at 20 °C / 68 °F is 1,002×10⁻³ Pa·s / 2,09×10⁻⁵ lbf·s/ft², corresponding to a kinematic viscosity of 1,004×10⁻⁶ m²/s / 1,08×10⁻⁵ ft²/s.
How does the density of water vary with temperature?
Section titled “How does the density of water vary with temperature?”The density of saturated liquid water decreases from 999,8 kg/m³ / 62,4 lb/ft³ at 0 °C / 32 °F to 958,4 kg/m³ / 59,8 lb/ft³ at 100 °C / 212 °F, while saturated vapor increases from 0,0048 kg/m³ / 0,00030 lb/ft³ to 0,60 kg/m³ / 0,037 lb/ft³ over the same range.
What is the specific heat value of water?
Section titled “What is the specific heat value of water?”Saturated liquid water has an average specific heat of 4,18 kJ/(kg·K) / 1,0 Btu/(lb·°F) between 0 °C / 32 °F and 100 °C / 212 °F, with a maximum of 4,216 kJ/(kg·K) / 1,007 Btu/(lb·°F) at the boiling point.
What is the thermal conductivity of water at 60 °C?
Section titled “What is the thermal conductivity of water at 60 °C?”At 60 °C / 140 °F, the thermal conductivity of saturated liquid water is 0,654 W/(m·K) / 0,378 Btu/(h·ft·°F), allowing high heat transfer rates.
What Prandtl number does water have at room temperature?
Section titled “What Prandtl number does water have at room temperature?”The Prandtl number of water at 20 °C / 68 °F is approximately 7,0; it indicates that momentum diffusivity dominates over thermal diffusivity at this condition.
What density does a typical hydraulic oil have?
Section titled “What density does a typical hydraulic oil have?”A common hydraulic oil has a density between 850 kg/m³ / 53 lb/ft³ and 950 kg/m³ / 59 lb/ft³ at 20 °C / 68 °F; kinematic viscosity can range from 20 cSt / 2,2×10⁻⁵ ft²/s to 100 cSt / 1,1×10⁻⁴ ft²/s.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/water-properties-temperature-equilibrium-pressure-d_2099.html
- thermopedia.com: https://www.thermopedia.com/content/1254/