A general compression refrigeration system consists of a compressor, condenser, pressure-reducing valve, and evaporator to induce phase changes in the refrigerant. The performance of the compression refrigeration system strongly depends on the type and characteristics of the refrigerant. However, synthetic refrigerants such as chlorofluorocarbon, hydrochloro-fluorocarbon, and hydrofluorocarbon directly affect the environment as greenhouse gases and indirectly affect the environment by generating substances as they decompose. Thus, researchers have been focusing on developing refrigerants with a low or zero global warming potential. R718 (i.e. water) is an ecofriendly refrigerant, but its specific volume rapidly changes between the liquid and gas phases. This requires making the components of the compression refrigeration system larger than normal to accommodate these volume changes, which is inefficient for manufacturing prototypes. In this study, an alternative refrigerant was applied to the design of a centrifugal compressor that uses R718 as the working fluid. To address the volume change of R718, a numerical analysis was conducted by using the similarity method, where the size of the impeller was reduced and the rotational speed was controlled. To ensure the reliability of the similarity model, its results were compared against the experimental data of the original model for verification.
While the prevalence of depression in later life rises, little is known about how depression varies as the older population is divided into age-subgroups. The current study attempts to determine whether psychosocial factors have a varying effect on each of the age-subgroups. A sample of 5,445 individuals aged 55 and over was adopted from the 7th Korea Welfare Panel Study(KOWPs) surveyed in 2012. The sampled subjects were divided into three age subgroups -the young-old, the middle-old, and the old-old. Three hypothesis were as follows. Those having suicidal ideation will be at a greater risk for depression. In times of reduced social status, having lowered self-esteem will also lead to more depressive symptoms. In the traditional Korean family culture where family harmony is greatly emphasized, decreased family life satisfaction will increase depression. The old-old were found to be at greater risk for developing poorer mental health. Those belonging to the old-old showed the highest score for depressive symptoms, the highest score for susceptibility to suicidal ideation, lowest self-esteem and negative family life satisfaction. To achieve a better mental health status, appropriate programs and policy implications for each age group were suggested.
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