Engineering task and calculation objective
The S-TAB module is a properties editor for tabulated thermophysical properties: the user stores custom value tables over pressure and temperature for a medium, from which the module then reads and interpolates the required thermophysical properties. It covers the quantities central to heat transfer and fluid flow calculations: density, specific heat capacity, thermal conductivity, dynamic and kinematic viscosity, Prandtl number, thermal expansion coefficient, thermal diffusivity, specific enthalpy and compressibility factor.
In practice, user-defined property tables are needed whenever a medium is not contained in standard property databases – for example product mixtures, thermal oils from specific manufacturers, brines, suspensions or proprietary process media for which only manufacturer data sheets or measured values are available. Instead of typing property values into every calculation module by hand, the table is maintained once, centrally, and retrieved as needed for the operating point in question.
The retrieved properties can be transferred directly into other calculation modules of the ATLAS system, for example into heat transfer, pressure drop or vessel calculations. This ensures that all calculations of a project work with the same, documented property data – an essential point for traceability and verifiability of the design.
Calculation workflow
- Create and name the substance: A name and a detailed designation are assigned to the medium, so that the table can later be identified unambiguously and documented in the calculation report.
- Enter the value table: The properties – density, specific heat capacity, thermal conductivity, viscosity, enthalpy, compressibility factor, etc. – are entered as data points over temperature and, where applicable, pressure, typically taken from manufacturer data sheets or measurement series.
- Specify the operating point: For the actual calculation, pressure and temperature of the operating point are entered; the module interpolates between the data points and delivers the complete set of properties at this point.
- Form derived quantities: From the basic quantities, the derived parameters are formed, for example the kinematic viscosity from dynamic viscosity and density, the Prandtl number from viscosity, heat capacity and thermal conductivity, and the thermal diffusivity.
- Transfer properties to calculation modules: The determined property set is handed over to downstream modules, so that heat transfer, pressure drop or transient calculations work consistently with the same data.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| 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 |
| Specific heat capacity | cp1 cp2 | J/(kg·K) |
| Specific heat capacity | cp1 cp2 | J/(kg·K) |
| Thermal conductivity | λ1 λ2 | W/(m·K) |
| Thermal conductivity | λ1 λ2 | W/(m·K) |
| Dynamic viscosity | η1 η2 | mPa·s |
| Dynamic viscosity | η1 η2 | mPa·s |
| Kinematic viscosity | ν1 ν2 | m²/s |
| Kinematic viscosity | ν1 ν2 | m²/s |
| Prandtl number | Pr1 Pr2 | - |
| Prandtl number | Pr1 Pr2 | - |
| Specific enthalpy | h1 h2 | J/kg |
| Specific enthalpy | h1 h2 | J/kg |
| Compressibility | Z1 Z2 | - |
| Compressibility | Z1 Z2 | - |
| Name of the medium | Zeichen) | - |
| Detailed name | (Bezeichnung) | - |
| Thermmal expansion | β1 β2 | 1/K |
| Thermmal expansion | β1 β2 | 1/K |
Frequently asked questions
When should I use tabulated properties instead of a property database?
Whenever the medium is not contained in any available database, or when binding manufacturer data exist that are contractually governing for the design – for example with thermal oils, refrigeration brines or product mixtures. The table is also the right approach for media with property data measured in your own laboratory, because the calculation is then based exactly on the documented values.
How many data points does a usable property table need?
What matters is that the entire temperature (and, where relevant, pressure) range occurring in the plant is covered, and that strongly non-linear quantities are supported closely enough. The viscosity of oils, for example, changes exponentially with temperature – considerably more data points are needed there than for the almost linear density. Extrapolation beyond the edge of the table should be avoided as a matter of principle.
What are typical sources of error when entering tables?
The most frequent errors are unit mix-ups (e.g. mPa·s instead of Pa·s for viscosity, kJ instead of J for heat capacity) and tables that apply to a different pressure or state of aggregation than the operating point. For gases, it must also be noted that density and compressibility factor are pressure-dependent – a temperature-only table is often not sufficient there.
What is the compressibility factor needed for?
The compressibility factor Z describes the deviation of a gas from ideal gas behavior. At higher pressures or near saturation, Z deviates significantly from 1; density and volume flow calculations with the ideal gas equation would then be in error. A correct Z value is therefore important for the design of compressors, safety valves and gas lines.