• Proceedings of the KSME Conference
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• 2001.06d
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• pp.636-641
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• 2001

In this paper, we constructed four valve type pulse tube refrigerator and found the characteristic of orifice (needle valve) opening for using phase shifter and optimum cycle time - The valve timing was controlled by the digital timers. The experimental results showed the optimum frequence and cycle time at operating conditions. The results showed that the optimum process time existed and the rate was same at each operating frequence. The no- load temperature of the refrigerator was 85K.

• Journal of Advanced Marine Engineering and Technology
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• v.31 no.6
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• pp.723-728
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• 2007

In order to evaluate the performance of carbon dioxide cycle with a sub-cooling loop. a simulation system was developed to predict the steady state of $CO_2$ trans-critical cycle. Mathematical models are derived to describe the relationships between the system's coefficient of performance and other operating parameters The mathematical models are based entirely on the basic mass and energy conservation law and thermodynamic and transport properties of carbon dioxide A parametric study has been conducted in order to investigate the effect of sub-cooling loop and various operating conditions on the cycle performance. An optimal mass fraction of a refrigerant flowing through sub-cooling cycle existed for the given evaporating temperature, high pressure and air inlet temperature through gas cooler.

• Proceedings of the KSR Conference
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• 2009.05a
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• pp.636-642
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• 2009

The application of LCC-techniques is being introduced at the on-going procurement programs of various techniques. LCC-techniques have the common characteristic that all are designed to motivate contractors to design, manufacture and deliver equipment with lower life cycle costs. You may believe that savings may be in the acquisition cost component of life cycle costs. However, primary emphasis is generally on reducing and controlling operating and support costs by transferring more responsibility to the contractor for equipment operating and support cost performance. It has been found that life cycle cost procurement provisions must be individually tailored to each program. In this study, the currently identified LCC procurement techniques including a variety of LCC incentive provisions are introduced. Moreover, verification method, a procedural issue and incentive to application of LCC-techniques are examined.

• Journal of the Korea Institute of Military Science and Technology
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• v.6 no.1
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• pp.30-38
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• 2003

The measurement on the working gas flow in the closed cycle diesel engine is insufficient to Identify its operating condition because of lack of sensors available. For this reason, the determination of optimum operating condition and controller parameters requires heavy experimental efforts. A simulation program is developed to minimize the experimental efforts. For the development of the simulation program, mathematical models are derived based on the physical laws and linear regression. The validity of the simulation program is demonstrated using the measured data of the closed cycle diesel engine.

• International Journal of Air-Conditioning and Refrigeration
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• v.17 no.3
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• pp.100-106
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• 2009

As an effort to prevent environmental problems caused by ozone depletion and global warming, alternative refrigerants are being developed, and one of the candidates is carbon dioxide. To overcome slightly low efficiency of $CO_2$ refrigeration system, air-conditioning cycle using an ejector was suggested. Ejector compensates throttling loss in an expansion device by reducing compression work. In this study, the ejector refrigeration cycle using $CO_2$ as a refrigerant is investigated to understand the effect of the mixing section diameter and refrigerant charge amount on the performance. If mixing section diameter is too large or too small, either cases show low performance. The optimum refrigerant charge amount which gives the best performance is found for standard operating conditions. The air-conditioning cycle was analyzed for several operating conditions.

• Journal of Navigation and Port Research
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• v.39 no.3
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• pp.261-266
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• 2015

Many of previous studies suggested that working capital management is an important component of firm's financial decision and efficient working capital management affects firms' profitability and it's value. Recently, Korean shipping firms have been suffering from financial distress by recession of shipping economy. In this point of view, this study tries to investigate the relationship between working capital management and shipping firm's profitability, using panel data on 46 Korean shipping firms for the period of 2004-2013. As result of panel regression, it proved that average payment period, inventory turnover, cash conversion cycle and operating cycle are significantly associated with firms profitabilities such as profit margin ratio and operating profit ratio, and the manager of shipping firm can increase firms profitability by more efficient working capital management. There are strong positive relationships between average payment period and operating cycle and firm's profitability. These results suggest that managers can create firm's value by increasing average payment period and operating cycle. Otherwise inventory turnover and cash conversion cycle have negative relationships with firm's profitability. It means that managers can increase firm's profitability by reducing these variables.

• International Journal of Air-Conditioning and Refrigeration
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• v.11 no.4
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• pp.209-217
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• 2003

The term “energy saving is economical” is appropriate for the national view point and for design and assessment of one system, but not appropriate when choosing the system by comparing alternative systems in the early design step. Sometimes, non-energy saving system is more economical than energy saving system because of the price of electricity, gas or oil, which are used for operating the air conditioning system. Therefore, when designing a system, we should consider the efficient alternatives through economic assessment of energy saving method. However, research on non-operating number control of the system is not sufficient because it is more common to use operating number control of the system for most economic assessment of air conditioning systems. For this reason, this research can provide the economic operating number control method as basic design data. The data obtained through analysis of life cycle cost based on amount of yearly energy use, are produced by system simulation of HASP/ACLD/8501 and HASP/ACSS/8502 for six alternative heating$.$cooling systems based on seasonal air conditioning system, which is widely used for medium and large size office buildings in Busan.

• Proceedings of the KIPE Conference
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• 2019.07a
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• pp.317-319
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• 2019

In this paper, a three level duty cycle controlled half bridge LLC converter for EV charger application is presented. The topology and operating regions of the converter are discussed. The equations of the converter are derived in time domain. A small signal model of the converter is developed by perturbation and linearization of the steady state model about their operating point using Extended Describing function.

• Transactions of the Korean Society of Mechanical Engineers
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• v.16 no.1
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• pp.77-86
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• 1992

This paper discusses the thermodynamic study on the suction cooling-steam injected gas turbine cycle. The aim of this study is to improve the thermal efficiency and the specific output by steam injection produced by the waste heat from the waste heat recovery boiler and by cooling compressor inlet air by an ammonia absorption-type suction cooling system. The operating region of this newly devised cycle depends upon the pinch point limit and the outlet temperature of refrigerator. The higher steam injection ratio and the lower the evaporating temperature of refrigerant allow the higher thermal efficiency and the specific output. The optimum pressure ratios and the steam injection ratios for the maximum thermal efficiency and the specific output can be found. It is evident that this cycle considered as one of the most effective methods which can obtain the higher thermal efficiency and the specific output comparing with the conventional simple cycle and steam injected gas turbine cycle.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.10 no.4
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• pp.495-507
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• 1998

The transient characteristics of a 4.0㎾ inverter driven heat pump was investigated by theoretical and experimental studies. The heat pump used in this study consists of a high side scroll compressor and $\Phi$7 compact heat exchangers with two capillary tubes. A series of tests was peformed to examine the transient characteristics of heat pump in heating and cooling mode when the operating speed was varied from 30Hz to 102Hz. One of the major issues that has not been addressed so far is transient characteristics during speed modulation. A cycle simulation model has been developed to predict the cycle performance under frequency rise-up conditions, and the results of theoretical study were compared with the results of experimental study. The theoretical model was driven from mass conservation and energy conservation equations to predict the operation points of refrigerant cycle and the performances at various operating speeds. For transient conditions, the simulated results are in good agreement with the experimental results within 10%. The transient cycle migration of the liquid state refrigerant causes a significant dynamic change in system. Thus, the migration of refrigerant is the most important factor whenever An experimental analysis is performed or A simulation model is developed.