Algebraic modeling and thermodynamic design of fan-supplied tube-fin evaporators running under frosting conditions

Optimal design of fan-supplied tube-fin evaporators subjected to frosting conditions aiming at minimum energy consumption

“…The present paper advances the study initiated in [12], extending the domain of the analysis from the component-level (i.e., the entropy generation in the evaporator) to the system-level (i.e., the amount of energy consumed by the refrigerator as a whole), thus including the compressor, the fan and the defrost heater in the analysis. For this purpose, the evaporator model developed in [12] was conflated to a quasi-steady-state cycle simulation model to account for the changes experienced by the evaporating temperature due to evaporator frosting, and also to a transient cabinet model to assess the temperature and humidity variations inside the refrigerated compartment, thus bringing the compressor runtime into the analysis.…”

“…Essentially, the evaporator submodel follows the approach introduced in [12], where the surface temperature was assumed to be uniform over the coil. The air flow is assumed one-dimensional, and the air temperature and humidity variations are calculated from energy and mass balances, respectively.…”

“…In general, studies of frost accretion on flat surfaces [4][5][6] and channels walls [7][8][9] consider a constant air velocity over time, although in fan-supplied tube-fin evaporators the air flow rate varies as frost accumulates over the coil, blocking the air passage [10,11]. Therefore, a proper evaporator design for frosting conditions must account for geometric (tubes, fins) and operating (air and coil temperatures, air humidity, defrost cycle, fan characteristics) parameters, which not only affect the heat and mass transfer rates, but also the frosting process [12]. To predict the frost accretion on tube-fin evaporators, different evaporator frosting models have been advanced [13][14][15][16][17][18][19].…”

“…The inspection of the open literature reveals that studies concerned with the optimal evaporator design (i.e., number of transversal and longitudinal tubes, number of fins) constrained by a fan-supplied air stream operating under frosting conditions are still scarce, and the ones available consider the cooling capacity as the objective function, disregarding the nonlinear interactions between the evaporator, the refrigeration system and the refrigerated compartments. For instance, in a prior work, Ribeiro and Hermes [12] advanced a fully algebraic model to simulate the thermal-hydraulic behavior of fan-supplied tube-fin evaporators running under frosting conditions, which is able to predict the capacity depletion caused by the air flow rate reduction due to the frost blocking, and the augmentation in the heat transfer resistance due to the reduced thermal conductivity of the frost layer. Geometric optimization of the evaporator geometry aiming at minimum entropy generation at the component-level was performed considering a constant evaporating temperature, and uniform conditions for the inlet air, which are unlikely to take place in real applications, where the evaporating pressure and the psychrometric conditions of the air depend on the cooling capacity and the thermal loads.…”

A finite-volume diffusion-limited aggregation model for predicting the effective thermal conductivity of frost

“…Most of the time, the frost formation on the evaporator surface is associated with thermal and hydraulic performance degradation in such a way that the system demands an increase in the energy input to provide the same refrigerant effect [1]. To mitigate these issues, different efforts have been made in order to improve not only the evaporator design [2], but also the defrost strategy [3]. Evaporator design usually relies on simulation tools to predict the degradation of the cooling capacity, the rise of the pressure drop, and the time between defrosts [4].…”