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Technological design and off-design behaviour


We have been led to distinguish two levels in models:
phenomenological models, these were implemented initially in Thermoptim only to give access to the calculation of thermodynamic cycles, irrespective of the choice of a particular component technology;
technological design and off-design simulation models not only provide the same results as the previous ones, but in addition allow the user to geometrically design the various components and once the technological design is achieved, to study the behavior of the system outside the operating conditions for which it has been sized.
Such models are for example necessary when one wants to evaluate the performance of an existing facility, operated under conditions different from those for which it was designed. This audit issue (particularly with a view to proposing improvements) is of interest to a growing number of organizations, industrial and others.
This second class of models has so far been little studied, although it is the only means to really answer the questions asked by a lot of modelers. This somewhat paradoxical situation arises because such models are much harder to write and solve than those allowing one to calculate cycles, the latter being often already quite complex. Due to lack of appropriate tools, modelers have so far been forced to limit their ambitions in this area.
Let us make clear that what we call off-design analysis corresponds to the stabilized operation of a facility for operating conditions other than those for which it has been designed: it is not to study the fast transient caused by control actuators.
Thermoptim features now allow to perform as well cycle studies as to simulate off-design operation of various systems

Resolution principles for technological design and off-design operation
To perform technological design, we have introduced new screens from Thermoptim versions 1.7 and 2.7, complementary to those that perform phenomenological modeling.
These new screens allow you to define the geometric characteristics representative of the different technologies used and the parameters necessary to calculate them. For a given component, they obviously depend on the type of technology used.
For example, for an exchanger they define flow patterns or free flow area devoted to the fluid, hydraulic diameters, and for a piston compressor, isentropic or volumetric efficiencies.
Note that the estimation of these quantities is not always easy, because the manufacturers rarely provide them as such. They should generally be identified from the data sheets, charts or software made available to customers.
The most significant changes relate to heat exchangers, Thermoptim versions prior to 1.7 or 2.7 determining only the product UA of the overall heat exchange coefficient U by the area A of the exchanger, without the two terms being evaluated separately. To size a heat exchanger, that is to say calculate its surface, we must first, choose a geometric configuration, secondly compute U, which depends on the configuration, thermophysical properties of fluids, and operating conditions.
Compressors and turbines have also been equipped with technological screens that define the characteristics for determining the flow-rate and the isentropic efficiency from the compression ratio.