• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.28 no.12
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• pp.502-508
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• 2016

An optimized PI controller design method is presented to promote the control performance of an oil cooler system for high precision machine tools. First, a transfer function model of the oil cooler system with a variable rotating speed compressor was obtained by the perturbation method as the first order system with a negligible dead time. Then, the closed-loop control system was described as the second order system with a zero. Its dynamic behaviors are mostly governed by characteristic parameters, the damping ratio, and the natural frequency which is incorporated in PI gains. Next, an optimum integral of the time-weighted absolute error (ITAE) criterion was applied to the second order system. The characteristic parameters can be determined by the given design specifications, percent overshoots and settling times and comparisons with the ITAE criterion. Hence, the PI gains were plainly identified in a deterministic way. Finally, the PI gains were fine-tuned to obtain desirable dynamics in real systems, considering the zero effect and parameter variations. The validity of the proposed method was proven by computer simulations and real experiments for selected cases.

• Journal of Advanced Marine Engineering and Technology
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• v.29 no.3
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• pp.297-305
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• 2005

The evaporation heat transfer coefficient of $CO_2$ (R-744) in a horizontal tube was investigated experimentally. The experiments were conducted without oil in a closed refrigerant loop which was driven by a magnetic gear pump. The main components of the refrigerant loop are a receiver. a variable-speed pump. a mass flow meter. a pre-heater and evaporator (test section). The test section consists of a smooth. horizontal stainless steel tube of 7.75 mm inner diameter. The experiments were conducted at mass flux of 200 to $500\;kg/m^{2}s$. saturation temperature of $-5^{\circ}C\;to\;5^{\circ}C$. and heat flux of 10 to $40\;kW/m^2$. The test results showed the evaporation heat transfer of $CO_2$ has greatly effect on more nucleate boiling than convective boiling. The evaporation heat transfer coefficients of $CO_2$ are highly dependent on the vapor quality. heat flux and saturation temperature. The evaporation heat transfer coefficient of $CO_2$ is very larger than that of R-22 and R-134a. In making a comparison between test results and existing correlations. the present experimental data are the best fit for the correlation of Jung et al. But it was failed to predict the evaporation heat transfer coefficient of $CO_2$ using by the existing correlation. Therefore. it is necessary to develop reliable and accurate predictions determining the evaporation heat transfer coefficient of $CO_2$ in a horizontal tube.

• Transactions of the Korean hydrogen and new energy society
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• v.30 no.3
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• pp.282-288
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• 2019

When liquefied natural gas (LNG) is vaporized to form natural gas for industrial and household consumption, a tremendous amount of cold energy is transferred from LNG to seawater as a part of the phase-change process. This heat exchange loop is not only a waste of cold energy, but causes thermal pollution to coastal fishery areas by dumping the cold energy into the sea. This project describes an innovative new design for reclaiming cold energy for use by cold storage warehouses (operating in the 35 to $62^{\circ}C$ range). Conventionally, warehouse cooling is done by mechanical refrigeration systems that consume large amounts of electricity for the maintenance of low temperatures. Here, a closed loop LNG heat exchange system was designed (by simulator) to replace mechanical or vapor-compression refrigeration systems. The software PRO II with PROVISION V9.4 was used to simulate LNG cold energy, gas re-liquefaction, and the vaporized process under various conditions. The effects on sensible and latent heats from changes to the array type of heat exchangers have been investigated, as well as an examination of the optimum.

• Food Science of Animal Resources
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• v.34 no.4
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• pp.525-532
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• 2014

This study was designed to elucidate the effect of ozone exposure on the bacteria counts and oxidative properties of ground Hanwoo beef contaminated with Escherichia coli O157:H7 at refrigeration temperature. Ground beef was inoculated with 7 Log CFU/g of E. coli O157:H7 isolated from domestic pigs and was then subjected to ozone exposure ($10{\times}10^{-6}kg\;O_3h^{-1}$) at $4^{\circ}C$ for 3 d. E. coli O157:H7, total aerobic and anaerobic bacterial growth and oxidative properties including instrumental color changes, TBARS, catalase (CAT) and glutathione peroxidase (GPx) activity were evaluated. Ozone exposure significantly prohibited (p<0.05) the growths of E. coli O157:H7, total aerobic and anaerobic bacteria in ground beef samples during storage. Ozone exposure reduced (p<0.05) the CIE $a^*$ value of samples over storage time. The CIE $L^*$ and CIE $b^*$ values of the samples fluctuated over storage time, and ozone had no clear effect. Ozone exposure increased the TBARS values during 1 to 3 d of storage (p<0.05). The CAT and GPx enzyme activities were not affected by ozone exposure until 2 and 3 d of storage, respectively. This study provides information about the use of ozone exposure as an antimicrobial agent for meat under refrigerated storage. The results of this study provide a foundation for the further application of ozone exposure by integrating an ozone generator inside a refrigerator. Further studies regarding the ozone concentrations and exposure times are needed.

• Food Science of Animal Resources
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• v.44 no.1
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• pp.146-164
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• 2024

Owing to the residual toxicity and adverse health effects of chemical preservatives, there is an increasing demand for using natural preservatives in food. Although many natural extracts have been evaluated, research on their antibacterial effects remains insufficient. Therefore, this study aimed to explore the possibility of developing Psidium guajava, Ecklonia cava, and Paeonia japonica (Makino) Miyabe & Takeda extracts as natural food preservatives. Further, the effect of mixing these extracts on microbial growth and quality was evaluated during the refrigeration of sausages. Optimal mixing ratios were determined based on the minimum inhibitory and bactericidal concentrations of each mixed extract against the Listeria monocytogenes, Salmonella spp. and Escherichia coli. D-optimal mixing design optimization tool was further used to obtain an optimum mixing ratio of Formulation 1 (F1). The antibacterial activity of F1 increased with increasing concentration, with similar activities at 0.5% and 1%. The sausages with synthetic or natural preservatives showed significantly lower lipid oxidation than those of the control and grapefruit extract-treated sausages after 4 wk of refrigeration. Total plate counts were observed only in the control and treatment groups stored for 3 wk, and no significant effect of ascorbic acid was observed. Compared to the other samples, sausages with added natural extracts showed the highest overall acceptability scores initially and after 4 wk. Therefore, similar amounts of grapefruit seed and natural extracts had the same effect on microbiological analysis and lipid rancidity during sausage storage. Hence, this mixture can serve as a potential natural preservative in meat products.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.24 no.5
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• pp.422-427
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• 2012

This study investigates on the relationship between total load which is caused by infiltration and insulation thickness against compensation effect. As the result of experiment, the PVC(Synthetic resins sash) window frame in airtightness is superior to the AL(Aluminum sash) window frame. In this study, as the increasing of insulation thickness in reference building does not reduce significantly cooling load, the compensation effect due to airtightness against infiltration is very small. But the compensation effect against infiltration can be closely related with heating load. Therefore, the proper thermal insulation thickness can be needed respect to cooling and heating load.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.4 no.4
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• pp.332-341
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• 1992

An absorption heat pump cycle has been modeled and simulated to analyze the system performance of water-cooled, series-flow, double-effect absorption heat pump, which can be applied to a direct gas fired cooling system with the medium range of cooling capacity (15RT level). Effect of absorption cooling system parameters, such as concentration difference, inlet temperature of cooling water, 1st generator temperature, leaving temperature differences of condenser and evaporator and efficiency of solution heat exchanger, has been investigated in the view of system cooling performance.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.14 no.6
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• pp.477-484
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• 2002

A numerical investigation of the performance of the plate heat exchanger with rectangular winglet is conducted to examine the combined effect of vortex generator and louver fins. Velocity and temperature fields and spanwise averaged Nu and friction factor are presented. Enhancement of heat transfer and flow loss penalty is evident. A Parametric study of three factors (Re, angle of attack and louver angle) with levels of 5 (Re= 300, 500, 700, 900, 1100), 4($\alpha=15^{\circ}, 30^{\circ}, 45^{\circ}, 90^{\circ},$), and 4($\beta=0^{\circ}, 15^{\circ}, 30^{\circ}, 45^{\circ}$), respectively, indicates the performance defined by the ratio of heat transfer enhancement to flow loss penalty shows monotonic behavior for each parameter alone but the interactions between parameters is found to be considerable effect on the performance of heat exchanger and should be considered in design. The effect of stamping is also examined.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.10 no.5
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• pp.640-648
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• 1998

A numerical study has been conducted to investigate the effect of inflow supply air temperature and velocity on ventilation effectiveness in an underfloor air conditioning space. A low Reynolds number k-$\varepsilon$ model is implemented to calculate steady state turbulent velocity distributions. A step-down injection method is used to calculate local and room mean ages from transient concentrations based on the concept of the age of air. Results show that there is a significant effect of Archimedes number on ventilation effectiveness especially for cooling conditions. Reynolds number shows relatively minor effect on velocity distribution and ventilation effectiveness especially for isothermal and heating conditions. It can be concluded that underfloor air conditioning system provides good ventilation characteristics for cooling conditions because of temperature stratification in the space.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.21 no.2
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• pp.118-125
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• 2009

The purpose of this study is to evaluate the stack effect reduction alternatives in tall complex buildings in cold climates. The main problems in tall buildings occur in elevators. Such problems as elevator doors that do not close and exhaust airflows result in excessive pressure differences across elevator doors due to stack effect. Under the expected conditions causing these pressure differences, Field measurements are performed in three tall buildings. Each result of the measurements is analyzed in architectural design aspects. With these analysis, building design guidelines are proposed and analyzed by field measurements and computer simulations.