Numerical investigation of refrigerants' effects on performance in heat pump drying systems

Global warming, ozone depletion and high cost of energy have long attracted attention to environment friendly and high energy efficient systems. Coolants which can be used due to their environmental effects have also been limited by several protocols. Coolants which are used in several heating-cooling systems have been replaced with the new ones. At this stage, CO2 comes into prominence because it does not harm the environment due to being natural and because of low cost and easy supply. Generally, R134a coolants are used in heat pump tumble dryers and its usage has been considerably limited by Kyoto Protocol. CO2 seems very suitable for working in dryer operating conditions due to its thermophysical effects. With the use of CO2 in dryers, both the harm to environment is reduced to minimum and the system performance is increased. In this study, heat pump tumble dryer machine systems which use CO2 and operate according to transcritical cycle were theoretically examined. Then, a model was developed for CO2 heat pump dryer. Model was made into a software which runs under MATLAB, by developing a graphical user interface. The thesis study consists of six main parts. The first part primarily includes a summary of the studies done in literature in relation to the matter. And then the purpose of the thesis and things to be presented are specified. In the second part, basic concepts for each main component in the system (gas cooler, evaporator, compressor, tumbler and fan) are explained and theoretical analysis is made and calculation methodology is presented. In the third part, calculations are transferred to the computer by using MATLAB R2011b program. At this point, fluid properties are determined by using REFPROP v7 program. Since two phase flow occurs in the evaporator?s CO2 side and then one phase flow occurs in the superheat area, and mass transfer occurred due to the condensation of water vapour in the air simultaneously with the heat transfer in the air part; creation of a sensitive model was required. Therefore, the evaporator was divided into a number of segments and calculations were made and the exit conditions of a segment were entered in the computer as the inlet conditions of the subsequent segment. The solution algorithm developed and the components which were theoretically analyzed were brought together and the general model of the heat pump dryer machine system was created. And finally, model was made into a software which runs under MATLAB, by developing a graphical user interface. In the fourth part, system components are validated through the empirical data which is compiled from literature. In this part, heat transfer coefficient and pressure loss comparisons for CO2 inside the tube in the gas coolers are given in graphics. For the heat transfer which occurs in the air part of the gas cooler and pressure loss validations, Colburn (j) and friction (f) factors are given respectively comparatively with literature. And then, in the CO2 flow which occurs inside the tube in the evaporator; the change in the heat transfer coefficient based on quality and the change in the pressure loss based on mass flux is given. In this part, the two phase CO2 flow which occurs inside the tube is evaluated in nine different types based on the change in quality. The changes in heat transfer coefficient are shown together with the changes in flow types. For the heat transfer which occurs in the air part of the evaporator and pressure loss validations; again respectively, Colburn (j) and friction (f) factors are given comparatively with literature. The mass transfer analyses due to the condensation which occurs in the air part of the evaporator are also given according to the change in Colburn factor (jm) Reynolds number comparatively with the literature. In the fifth part, the effects of CO2 inlet pressure to the gas cooler, evaporation temperature, air flow rate, drum efficiency, leakage ratio, temperature and relative humidity of the ambient air on the performance of the dryer system were scrutinized according to the results obtained from the model. While assessing the performance of the system, COP, MER and SMER values? variations were showed. Through the results obtained, the energy consumption and drying time of the dryer have been presented in parametrical terms. The sixth part includes results and suggestions.