Diseño y simulación de un sistema de ultra congelación con temperatura de trabajo de hasta -90 °C, para un congelador de 1.2 m³ de capacidad.

Abstract

This research project proposes the design and simulation of a cascade system with the capacity to obtain cryogenic temperatures (-90 °C) in a volume of 1.2 m³. The mathematical analysis is based on two refrigeration circuits, the first with hydrofluorocarbon refrigerant R134a, which condenses the refrigerant of circuit 1, with R744. By varying parameters such as speed and pressure, the aim is to reach the proposed temperature, taking into account limitations such as the supercritical point, as well as the triple point of CO₂. A copper coil type evaporator is obtained for R744 and that at the same time will transmit the temperature to the internal stainless steel wall, the dimensions of the coil correspond to 9 meters in length and 7.5 mm in diameter. The working pressure and enthalpy are obtained from the Mollier cycle, with which a heat transfer analysis is carried out. Using the EES software, which has flow bowling analysis, the proposed equations are solved and the simulation is carried out based on a layout with a six-turn coil. Meshing in Ansys Fluent using body sizing resulted in 3078 nodes and 4100 elements. Several simulations are shown with variation of parameters such as enthalpy, behavior of liquid and vapor throughout the evaporator, evaluation of quality and enthalpy of the refrigerant, working pressures and degree of turbulence. The final result shows that the proposed temperature of -90 °C cannot be reached because when the pressure drops below 5.2 bar and therefore the temperature to -56.6 °C, the pipe presents blockages produced by the CO₂ in solid state. In addition to preventing the continuous flow of the refrigerant, they are dangerous because they produce clumps that increase the pressure, causing accidents due to pipe ruptures.