Evaluating plate freezing of fish using natural refrigerants and comparison with numerical freezing model

Abstract

This paper compares experimentally measured freezing time and predicted numerical freezing time in industrial CO2 plate freezer. The model uses a two-dimensional, heat capacity-temperature, finite difference method with constant temperature boundary conditions. Also, COP was estimated for two different systems using different refrigerants and evaporating temperatures. Freezing time and COP was used to calculate relative energy requirements for fish freezing and freezing capacity for the different system solutions. The numerical model demonstrated good agreement with experiments, with a difference in freezing time of only 3%. Results reveals that low temperature freezing, down to -50o C, require 51% and 20% more energy per kilo fish than for -30o C and -40o C, respectively, using R744. The higher energy consumption is due to lower COP, which are 1.8, 2.3 and 3.0 for the abovementioned temperatures for a two stage R744 system. However, capacity (kg frozen fish per hour) is increased by correspondingly 65% and 33%. Different freezing times results in varying refrigeration capacity, ranging from 47.9 to 25.9 kW for -50o C and -30o C, regardless of refrigerant.