REF.BOX

Numerical engine:

REF.BOX is equipped with an extremely robust and advanced numerical engine based on a Levenberg-Marquardt type algorithm which solves the problem equations system. This is determined by the energy and mass conservation equations in the different components of the refrigeration units and exchangers starting from the thermodynamic cycle based on system unknowns (e.g. suctions and discharge saturation temperatures). The mathematical problem is constrained by the used-defined boundary conditions and by the thermal or refrigeration load, when assigned. The calculation engine is developed to solve systems resulting from the different market available refrigeration architectures including single or double vapor compression cycles, split-cycle cycles with economiser, cascade cycles and transcritical cycles.

The heat exchangers are simulated with dedicated models. Finite volume schemes allow the correct characterization of the operating fluids thermophysical properties and phenomenological coefficients variations. Two models are available: a) P-NTU for round tube and microchannel finned-coils with complete heat-exchanger geometric description and state-of-the-art phenomenological correlations for heat transfer coefficients, pressure drops and void fraction and b) CDUV (Corrected U-Value design) for all types of heat exchangers, especially those with liquid secondary fluid. This is an original model: the heat exchangers performance is accurately predicted in all operating conditions basing on the rated values alone, any geometric data are supplied.

For finned coil heat exchangers, a model with a three-dimensional finite volume scheme is also available which is able to graphically characterize all the round tube and microchannel coils possible circuits and to evaluate the effect of convergences and divergences in the refrigerant circuits and the frontal air maldistribution in terms of speed and temperature.

The compressors are simulated with polynomial models in accordance with the EN12900 standard. The model includes multi-frequency or capacity compressor characterization. For Danfoss Turbocor centrifugal compressors, an accurate original polynomial model is included for fast compressor simulation in the refrigeration unit. For compressors with frequency dependant operating envelope and dedicated controllers able to manage the minimum and maximum rotation speed as a function of the operating conditions, the unit numerical solver was modified to efficiently describe the real-life operation. The algorithm relaxes the load constraint to keep the solution within the numerical domain as constrained by the envelope itself.