Calculation Settings   

For further information, see

• Model options Simulation,
• Model options Validation and
• General Options Calculation.

 Global Calculation Mode

•     Design or
•     Off-Design

Local Off Design

Local Design

For most components it is possible to set the value of FMODE to "local design".

For components where this setting is active, a design calculation is always performed, also independent of the "global calculation mode".

If there are components in this mode in the model, the question is asked in a message box - even after a partial load calculation - whether the nominal values should be taken over.

If you tick the checkbox "Copy all default values", not only the nominal values but all default values are taken over, with the exception of FMODE itself. This eliminates the need for manual copying, for example from ETAI to ETAIN for the steam turbine.

However, this dialogue can also be hidden if you always want to copy the nominal values to the current profile or always to the main profile. For this purpose, there is a selection box "Save calculated nominal values" in the general settings under Calculation/General with the following setting options:

• "No": the nominal values are not automatically taken over and there is no prompt (with a click on the button "Take over reference values", however, the takeover can be done manually).
  
• "Dialogue box": you are asked (see above) whether the nominal values should be taken over and if so, into which profile they should be copied (this is the default setting).
  
• "Write local design data to active profile": the nominal values are copied to the active profile.
  
• "Write local design data to root profile": the nominal values are copied to the root profile.
  
• "Write local design data into reference profile": the nominal values are taken over into the reference profile.
  
• "Write all default values to active profile": all specification values and the nominal values are taken over into the active profile.
  
• "Write all default values to root profile": all specification values and the nominal values are taken over into the root profile.
  
• "Write all default values to the reference profile": All specification values and the nominal values are taken over into the reference profile.

The reference profile can be set in the profile dialogue (Data --> Profiles).

Design and identification

A design calculation is used to determine the nominal values needed in off-design calculations when using characteristic curves.

In a calculation where a component is in an identification mode, values specified outside the component are used to determine, for example, the efficiency of this component or its k*A value (heat transfer capability). These specifications are values specified with measured value components or by other means on the connected lines, e.g. also with neighbouring components or also via components connected to the line with logic lines. 

Such a calculation in identification mode is possible in design as well as in off-design calculations. 

In such a case, the nominal values of this component are taken over in the design mode and also the identified component value(s) (e.g.: efficiency ETAI is copied to ETAIN for component 6, gradient DT3S2 is copied to DT32SN for component 10, pressure drop DP12 is copied to DP12N or pressure drop DP34 is copied to DP34N for component 10).

In case of a calculation in the identification mode in the off-design, no values are taken over. The component values to be identified   hier weiter

Matrix Precision

The solution process of Ebsilon is a matrix solution. This requires the linearization of all dependencies. To take into account influences resulting from non-linearities, a Newton iteration is done afterwards.

This iteration is considered completely, if the deviation from the previous iteration step is smaller than the specified precision value for all matrix cells. It is recommended to choose a value of 10^(-6) or 10^(-7).

Off-design method for component 10

Component 10 is the feed water preheater, which consists usually of a de-heating part and a condensing part. At the boundary between these two zones there is the well-known <pinch-point>. EBSILON^®Professional offers two methods to carry out off design calculations.

The simplified method assumes that the separation zone between de-heating part and condensing part is fixed and does not move in off-design mode. This method may result in <pinch-point> problems (too small values) for very low loads (below 50%).

The second method was published by Prof. Rabek in the sixties and takes into account moving of the separation zone. This method agrees better with reality, but is much more complicated in the calculation.