Engineering task and calculation objective
The SaWa module calculates the properties of seawater as a function of temperature and salinity according to the Heat Exchanger Design Handbook (HEDH, Part 5: Physical Properties, Begell House 1998). The outputs are density, specific heat capacity, thermal conductivity, dynamic and kinematic viscosity, Prandtl number, thermal diffusivity and thermal expansion coefficient. The validity range covers temperatures from 0 to 180 °C and salinities from 0 to 150 g/kg.
Being able to calculate seawater properties is fundamental to the design of seawater-cooled heat exchangers and condensers in power plants and refineries, of desalination plants (MSF and MED evaporators, preheaters of reverse osmosis plants), and of piping and pumps in the cooling water system. Compared with pure water, the salt content changes above all the density and heat capacity and thus enters directly into heat balances, heat transfer and pressure drop calculations.
The wide salinity range up to 150 g/kg covers not only normal ocean water (around 35 g/kg) but also the concentrated brine in evaporative desalination plants — there, viscosity and boiling point elevation rise noticeably with increasing concentration.
Standard and calculation basis: G.F. Hewitt, Heat Exchanger Design Handbook (HEDH), Part 5, Physical Properties, 1998, Begell House Inc., New York
Calculation workflow
- Specify the state: The inputs are the temperature (0 to 180 °C) and the salinity (0 to 150 g/kg). The module checks compliance with the validity range of the HEDH correlations.
- Density and heat capacity: Density and specific heat capacity are calculated as functions of temperature and salinity: the density lies above and the heat capacity below the value of pure water — both changing in proportion to the salt content.
- Transport properties: Thermal conductivity as well as dynamic and kinematic viscosity are determined as functions of salinity and temperature; the viscosity increases with salt content and falls strongly with temperature.
- Derived characteristic quantities: From the basic properties, the module forms the Prandtl number, thermal diffusivity and thermal expansion coefficient — the input quantities for heat transfer correlations and natural convection calculations.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Temperature | ϑ | °C |
| Temperature | ϑ | °C |
| Salinity | x | g/kg |
| Salinity | x | g/kg |
| Density | ρ | kg/m³ |
| Density | ρ | kg/m³ |
| Specific heat capacity | cp | J/(kg·K) |
| Specific heat capacity | cp | J/(kg·K) |
| Thermal conductivity | λ | W/(m·K) |
| Thermal conductivity | λ | W/(m·K) |
| Dynamic viscosity | η | mPa·s |
| Dynamic viscosity | η | mPa·s |
| Kinematic viscosity | ν | m²/s |
| Kinematic viscosity | ν | m²/s |
| Prandtl number | Pr | - |
| Prandtl number | Pr | - |
| Thermal diffusivity | a | m²/s |
| Thermal diffusivity | a | m²/s |
| Coefficient of thermal expansion | β | 1/K |
| Coefficient of thermal expansion | β | 1/K |
Frequently asked questions
Isn't it sufficient to calculate with the properties of pure water?
For rough estimates with normal seawater (35 g/kg) the deviations are moderate: the density is barely 3% higher, the heat capacity about 4 to 5% lower than for fresh water. In heat balances the errors do not necessarily cancel — anyone determining the cooling water mass flow from a heat duty already makes a systematic error of a few percent with fresh water values. For concentrated brine in desalination plants, the deviations become considerably larger and are no longer negligible.
What exactly does the salinity in g/kg mean?
The salinity specifies the mass of dissolved salts per kilogram of seawater (equivalent to a mass fraction in parts per thousand). Standard ocean water has about 35 g/kg; marginal seas deviate — the Baltic Sea is partly below 10 g/kg, the Arabian Gulf above 40 g/kg, and in MSF evaporators the brine is routinely concentrated to about 70 g/kg and beyond. The design value should use the actual site or process value, not a blanket 35 g/kg.
Does the module also cover boiling and boiling point elevation?
The module delivers the single-phase liquid properties up to 180 °C. For evaporator designs, the boiling point elevation of the salt solution relative to pure water is additionally required; it grows with salinity and temperature and reduces the driving temperature difference per effect, and must be accounted for separately.
Why is the temperature range limited to 180 °C?
The HEDH correlations are fitted to measurement data up to this range, which safely encompasses the peak temperatures of thermal desalination plants (top brine temperature typically 90 to 120 °C). Above that, the data basis is lacking; moreover, in real plants calcium carbonate and sulfate scaling limit the brine temperature well before the correlation limit.