Optimization of Cryosurgical Probes for Cancer Treatment

Abstract

We take for granted the abundance and variety of food that is available to us all year round. Grocery stores around the U.S and the world sell a wide variety of fruits and vegetables throughout the year no matter when the harvesting season took place; this in large part due to cold storage warehouses. Cold storage warehouses place otherwise perishable food items into freezers and coolers where they can be stored for months, or even years, until they are ready for sale. Many food items placed in storage require the temperature to be held at low temperatures. Though the low temperatures are needed to maintain the quality of stored products, it has an adverse effect on the refrigeration system. By operating warehouses at temperatures below the freezing point, water (in the form of frost) will accumulate on the air- cooling evaporators used to cool the space. As frost builds up on an evaporator, the resistance to heat transfer between air and the refrigerant increases, airflow through the evaporator decreases, and the overall efficiency of the evaporator decreases. The goal of this research project is to develop a better understanding of the factors that influence defrost performance as well as the parasitic impacts on system energy consumption by generating a transient computer model of the processes involved in a hot gas defrost cycle on an evaporator coil of known geometry. This study focuses on utilizing different refrigerant temperatures for melting frost accumulated on the evaporator's coils. To evaluate an evaporator undergoing a defrost process; models of a dry coil as well as a frosted coil were developed using EES (Engineering Equation Solver). The models approximated the coil by representing it as a tube with a radial fin. The models themselves are made up of multiple nodes which are defined by energy boundaries. The nodes in the models are transient and are temperature driven. Freezer conditions as well as frost properties are also implemented in the programs to simulate different environments. The frosted fin model was used to record the time to melt a given mass of frost as well as find the distribution of energy associated with defrosting. The dry fin model was created to record the excess amount of energy that goes into a defrost process that lasts longer than the time required to melt the accumulated frost. The last part of the study focuses on the estimating costs associated with the parasitic loads created by initiating a defrost cycle. Since the energy that is lost from convection, evaporation, and stored energy in the metal coils of the evaporator has to be reclaimed, the compressors in the refrigeration system are analyzed to estimate the energy costs associated with operating a compressor to circulate the refrigerant in order to capture the energy from the surroundings. The compressor cost evaluation is based on a single stage and a two stage system using different operating head pressures to simulate the effects of defrosting throughout a typical year.