Engineering task and calculation objective
The S-DAT module allows the free input of physical properties for media that are not contained in the built-in property databases. Under a freely chosen name, density, specific heat capacity, thermal conductivity, dynamic and kinematic viscosity, Prandtl number, surface tension, coefficient of thermal expansion, thermal diffusivity and specific enthalpy are recorded and made available to the downstream calculation modules just like a database substance.
The values are entered at two temperature or pressure support points, so that the temperature dependence of the properties can be represented by interpolation over the working range; derived quantities such as kinematic viscosity, Prandtl number and thermal diffusivity can be determined consistently from the basic values. This keeps heat transfer and pressure drop calculations self-consistent even for special media.
In practice, free property input is needed for thermal oils from specific manufacturers, salt solutions, suspensions, product mixtures from process analytics, or whenever customer measurement data are to be used as binding. It is thus the bridge between laboratory or data-sheet values and the equipment calculation.
Calculation workflow
- Create the medium: The special medium is created under a unique name; it later appears in all calculations and printouts in place of a database substance.
- Define the support points: The support points are fixed for two temperatures (and associated pressures); they should enclose the working range of the planned calculation so that no extrapolation is required.
- Enter the basic properties: At each support point, density, specific heat capacity, thermal conductivity and dynamic viscosity are entered – the four basic quantities from which the thermal characteristic numbers are formed; surface tension, expansion coefficient and specific enthalpy can be recorded in addition.
- Form the derived quantities: Kinematic viscosity, Prandtl number and thermal diffusivity are calculated from the basic values or can be entered directly; the module thereby keeps the values mutually consistent.
- Use the substance in calculations: The medium defined in this way is available to the heat exchanger, pressure drop and other modules; intermediate values within the temperature range are interpolated from the support points.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Name | Bezeichnung | - |
| Temperature | ϑ1 ϑ2 | °C |
| Temperature | ϑ1 ϑ2 | °C |
| Pressure | p1 p2 | Pa |
| Pressure | p1 p2 | Pa |
| Density | ρ1 ρ2 | kg/m³ |
| Density | ρ1 ρ2 | kg/m³ |
| Density | ρ | kg/m³ |
| Density | ρ1 ρ2 | kg/m³ |
| Specific heat capacity | cp1 cp2 | J/(kg·K) |
| Specific heat capacity | cp1 cp2 | J/(kg·K) |
| Specific heat capacity | cp | J/(kg·K) |
| Specific heat capacity | cp1 cp2 | J/(kg·K) |
| Dynamic viscosity | η1 η2 | mPa·s |
| Dynamic viscosity | η1 η2 | mPa·s |
| Dynamic viscosity | η | mPa·s |
| Dynamic viscosity | η1 η2 | mPa·s |
| Kinematic viscosity | ν1 ν2 | m²/s |
| Kinematic viscosity | ν1 ν2 | m²/s |
| Kinematic viscosity | ν | m²/s |
| Kinematic viscosity | ν1 ν2 | m²/s |
| Thermal conductivity | λ1 λ2 | W/(m·K) |
| Thermal conductivity | λ1 λ2 | W/(m·K) |
| Thermal conductivity | λ | W/(m·K) |
Frequently asked questions
Why are the properties entered at two support points?
Many properties are markedly temperature-dependent – the viscosity of liquids in particular. With two support points, the module can interpolate over the working range instead of calculating with a constant value. The support points should enclose the actual temperature range of the application; extrapolation beyond the support points is unreliable and should be avoided.
Which properties are required as a minimum?
For heat transfer calculations, the four basic quantities density, specific heat capacity, thermal conductivity and dynamic viscosity – from these follow the Prandtl number, kinematic viscosity and thermal diffusivity. Surface tension is needed only for boiling and two-phase flow, the coefficient of thermal expansion for natural convection, and enthalpy for duty balances with phase change.
What should be checked with values from manufacturer data sheets?
Units and reference quantities: data sheets frequently mix kinematic (mm²/s or cSt) and dynamic viscosity (mPa·s), state heat capacities sometimes in kJ/(kg·K), sometimes in kJ/(kg·°C) at different reference temperatures, or give the density only at 15 °C. Before entry, all values should be converted to the support-point temperatures and their consistency checked via the Prandtl number.
Can two phases or the two sides of an apparatus also be described?
The variable structure provides two complete sets of values, so that, for example, the liquid and vapour of one substance or the media on both sides of an apparatus can be stored separately. Genuine phase-change calculations additionally require the heat of evaporation and saturation data, which are covered via the enthalpy inputs.