DimBo – Dimensioning of Boilers

The program DimBo is designed for the simulation of steam generators in steady-state operation.


DimBo is capable of handling complex systems of components for heat exchange and medium conversion.


The steam generator is represented by a mathematical model, a so called flow schematic. For all components usual in steam generator design the corresponding circuit elements are defined and can be adapted to the desired functions by parameters.


Currently the following types of media have been realised:

  • Fuels (solid, liquid and gaseous).
  • Gases (with dust burden).
  • Water/steam.
  • Dust/slag.
  • Additives.
  • Condensate from flue gas.


In detail the program can be used to determine the following parameters:

  • Medium flows (mass flow rates and analyses).
  • Temperatures and specific enthalpies.
  • Heating surfaces.
  • Heat transfer.
  • Process parameters.
  • Mass and energy balances.


DimBo consists of two parts, a dialogue part to enter and modify the input data and a calculation part. The input data are stored in an input file in ASCII format.


The results are stored as tables and diagrams, which can be displayed on the screen or printed out. Type and scope of the result output can be determined by the user.


For input and conversion of fuel and gas analyses, components of the program COMCAL are used.


For non-steady-state simulations the program DynaBO which is based on DimBo may be used.


System requirements: PC with Windows (32/64 bit) operating system.

Changes in DimBo program

  • 2018-02-12
    The extrapolation method for the furnace exit temperature calculation via Q/A functions is modified. Now a smooth transition between parameters inside and outside of the intended parameter range is achieved and function characteristics for extreme parameters are more reasonable.

    The extrapolation comes into effect in the following cases::

    • Extreme high or extreme low adiabatic flue gas enthalpies.
    • Extreme high or extreme low heat input related to the furnace surface area.

  • 2018-01-09
    Error at temperature condition for media with user-defined material properties. Instead of the specific enthalpy the temperature was used as target value. Thus the iteration was directed to the wrong target value. The error was eliminated.
  • 2016-11-06
    New option for fuel analysis data input (refer to ComCal changes).
    Updated material data base for pipe and tube materials.
  • 2016-09-30
    Due to an unit conversion error in the water-/steam side 2-phase flow pressure loss calulation, the steam content at dry-out point was determined to high (factor 1.33). As the local pressure drop decreases significantly after dry-out, the calculated pressure drop in the evaporator section might be too high. This affects especially the evaporator helix of Once-Through Boilers in part load operation. The error was fixed in version 30.05. The changes will be applied independent of the selected version number of the DimBo calculation procedure.
  • 2016-09-30
    If the number of design data groups was higher than 100, an error occured during data transfer from the graphical interface. This error resulted in a complete loss of geometrical data. The error was fixed in version 30.04.
  • 2016-04-06
    An error in the adjustment of the "Initial media flows" for the fluidised bed furnace model resulted in an access violation and thus in an abort of the calculation.
  • 2016-02-09

    Release of DimBo version 30

    Changes related to version no. 30 for the calculation method:

    • For the determination of material temperatures Dimbo calculates maximum heat flux densities for single rows of tube bundles. However the calculation method for the determination of these heat flux densities is based on pure parallel- or counterflow. The more the arrangement of the heating surface deviates from this ideal case, the larger the error will be. Therefore in Version 30 a correction of the heat flux densities is implemented for the special case, that a bundle heating surface consists only of one tube row. The corrected heat flux densities and thus the detemined material temperatures will be higher at the tube inlet and lower at the tube outlet.
    • In the recent program version an error in the calculation of the inlet cross-sectional area of an CFB reactor resulted in wrong results for the suspension density in the hopper.
      If the inlet cross section is not quadratic, the elimination of the error results in significantly deviations of suspension density and heat transfer coefficients for the hopper region.
      The values for inflection height and total bed mass of the CFB, which are displayed in the Dimbo diagnostic output can also deviate significantly. Nevertheless the effect of the program change on the overall heat absorption of the complete CFB reactor is small.
    • The calculation of the outer convective heat transfer for tube-fin tube cylinders (type 241 and 441) has been revised.
      While the calculation for vertical approach flow (inclination of tubes 0°) remains unchanged, the calculation of the characteristic hydraulic diameter for longitudinal approach flow (inclination of tubes 90°) was corrected. As consequence the convective heat transfer coefficient will increase slightly, especially for large inclination angles.
    • The heat resistance of refractory on tube-fin-tube walls is now related to the projected surface instead of the tube-fin-tube surface. Compared to previous calculations this results in smaller heat transition coefficients.
    Changes come into effect if number 30 is entered as version no. for the calculation method.


    • For the CFB furnace model it is now possible to simulate a post-combustion in the cyclone. The fraction of heat release in cyclone can be defined within the process parameters of the circulation fluidised bed.
    • Calculation procedure for heat transfer and pressure loss for circuit element 251 (annular gap around annular gas duct) has been implemented.
    • An error in check of CFB circuitry, which resulted in a unjustified error message, was eliminated.

  • 2016-01-13

    Improvements for graphical input of circuitry and geometry:

    • Properties of one circuit element can be transferred to another circuit element by copy/paste (Ctrl+C/Ctrl+V).
    • Non initialised geometry parameters of one circuit element can be initialised with data from another element by a click on the corresponding circuit element during parameter input.

  • 2016-01-03
    The calculation procedure for the heat transmission coefficient of Drum Preheater (type no. 513) and Drum Cooler (type no. 514) cercuit elements is corrected.

    The recent program version performed under certain circumstances an unjustified modification of the heat transission coefficient. This modification has been removed from the program code.

  • 2015-10-18
    For tube walls and platen type heating surfaces, lined with studding and refractory, two extra tables are provided with the tabular output:
    • Heat transfer
      The influence of the lining heat resistance on the overall heat transition is described in detail by an extra table.
    • Design data
      Geometrie of studs and refractory thickness as well as their materials are listed in an additional exra table.
    Heating surface lined with studding and refractory are frequently used for combustion chambers of fluidesed bed boilers.
  • 2015-02-04
    Release of DimBo version 29


    • CFB furnace model
      Additional facilities for the calculation of fluidized bed combustion were implemented (see DimBo User's manual)
    • New Element "Hood"
      A new element with the type number 123 was implemented for modelling the hood of a circulating fluidized bed combustion.
    • Calculation of cyclone efficiency from geometry
      For cyclones the calculation of efficiency and pressure drop was implemented according to Muschelknautz (VDI-heat atlas, 2006, Chapt.Lcd).
    • For radiational heat transfer in fluidized bed reactors the absorption coefficient of the wall can be specified deviating from the default value of 0.79 via the process parameters of fluidised bed heating surfaces. Different absorption coefficients can be specified for walls with and without refractory.



    • An erroneous calculation of max. gas velocities for gas ducts with 90° lateral bend (type no. 121 and 122), which contain sling tubes (type no. 401), has been corrected.
    • An erroneous heat transfer calculation is corrected for unheated connectig pipes (type no. 481), which are assigned to a furnace section. No heat from the furnace will be absorbed. Instead, as for all other unheated connecting pipes, only participation in the thermal losses is considered.
    • For the heat transfer by condensation of steam flowing inside cooled tubes the calculation method according to Boyko and Krujilin was implemented.

    Modifications come into effect if number 29 is entered as version no. for the calculation method.

  • 2014-10-17
    An erroneous calculation for the transferred heat of drum cooler or drum preheater elements was removed. The error could occur if drum cooler respectively preheater are described with geometric data and therefore the heat tranfer coefficient is calculated and if, at the same time, a change in the sphase condition of the medium within the tube occurs.

    At the wall temperature calculation for membrane walls an error is corrected. Temperatures for back side and fin base have been swapped. Therefore too low temperatures could be indicated.

    For diagram output a smaller character font size is used if otherwise the text length exceeds the size of the corresponding text box.

    Various modifications for easier handling of the graphical user interface for circuitry processing.

  • 2013-01-17
    The correction of an erroneous calculation of calorific values for gaseous fuels containing Chlorine and/or Fluorine within the sub-program COMCAL comes into effect if version no. 28 is used as calculation method for DimBo.
    For details see change history of COMCAL.
  • 2012-09-26

    Release of DimBo version 28 with following changes:

    • Cross-sectional area calculation for bended gas ducts (Type nos. 121 and 122).
      Modifications are required for bends with extraordinary ratios of width to depth or width to length in order to achive realistic velocities for the gas flow and the corresponding convective heat transfer.
      Same applies for the maximum velocities (narrowest cross-section). However, the value has impact on the heat transfer only in case of fluidised bed heating surfaces. For conventional heating surfaces the maximum velocity is just a output value.


    • Maximum gas velocities for furnaces (Type nos. 113 and 114).
      Furnaces with type nos. 113 or 114, variant 2, which include the hopper geometry, had indicated extreme high gas velocities, as misleadingly a flow of the whole flue gas through the hopper opening had been considered.
      With the new version, the narrowing at the hopper is no longer taken into account while calculating the flue gas velocity.
      The maximum flue gas velocity for a furnaces is a pure output value without any impact on other calculation figures.

    Modifications come into effect if number 28 is entered as version no. for the calculation method.

  • 2012-01-09

    Parameters for heating surface evaluation (evaluation factors, fouling figures) can be taken over from a previous calculation to the present calculation.

  • 2011-10-11

    Release of DimBo version 27 with following changes:

    • For calculation of material utilisation the corrosion allowance is considerd on the outer surface.
      Compared with version nos. < 27 the required wall thicknesses are reduced, which results in lower material utilisation.
    • For the cross-sectional area calculation of gas ducts the reduction by membrane side walls is taken into account (half of the wall is inside the duct). For studded and refractory lined heating surfaces the cross-sectional area is further increased by the refractory layer.
      Effect on gas ducts with membrane side walls (type no. 201) and/or studding and refractory lining:
      • Higher velocties
      • Increased pressure losses
      • Increased heat transfer coefficients for heating surfaces located within the duct.
      For ordinary dimension of power boilers the effect is marginal.
    • Coefficients for the statistical analysis of municipal waste are employed from FDBR-Handbuch Wärme- und Strömungstechnik, release June 2010.
      For version nos < 27, coefficients of the former release June 1996 are used.


    Modifications come into effect if number 27 is entered as version no. for the calculation method.
  • 2011-10-11

    For combustion of municipal waste and special fuels the encapsulation of fuel components Sulphur (S), Chlorine (Cl) and Fluorine (F) into ash can be taken into account.

    To this, the individual degree of encapsulation can be specified as parameter for the corresponding combustion section within the process parameters.

  • 2011-06-01

    The allowable number of circuitry sections is increased from 50 to 75.

    A new graphical user's interface for circuitry processing is released with improved functionality.

  • 2011-02-23

    DimBo is extended by a sectional model for furnace calculation.

    The furnace can be divided into up to 2 hopper sections and up to 18 cuboid sections.

    Fuel and air supply will be distributed over the individual sections by user input.

  • 1981 - 2010
    Various milestones in software improvement. Details can be obtained from Steinmüller Engineering GmbH.
  • 01.06.1981
    Start of development for an autonomous thermodynamic software at L&C Steinmüller GmbH.

Steinmüller Engineering GmbH
IHI Group Company
Fabrikstraße 5
51643 Gummersbach / Germany