https://doi.org/10.6110/KJACR.2025.37.11.511

Ho-Sung Choi ; Jae-Ou Lee

The recent rise in large-scale earthquakes has highlighted the urgent need to ensure the seismic safety of buildings and industrial facilities. A key focus in this area is the seismic performance of fire protection piping systems, which are crucial for protecting lives and property during fires. This study conducts a static analysis of three common types of piping connections?Grooved Coupling, Welding Joint, and Thread Joint?and compares their structural responses under seismic loading conditions. Notably, the nonlinear behavior of the Grooved Coupling cannot be accurately captured by a typical linear model. Therefore, a bi-linear moment-rotation relationship was developed and integrated into the AutoPIPE analysis model, enabling a detailed evaluation of stress concentration, displacement, and stiffness variation in the joints under seismic loads. The analysis revealed that the Welding Joint demonstrated the highest rigidity and stability, indicating superior seismic performance. While the Thread Joint offers benefits in terms of ease of construction and cost-effectiveness, it exhibited the weakest seismic resistance and necessitates structural reinforcement. The Grooved Coupling, although capable of effectively dispersing and absorbing loads to a certain extent due to its flexibility, displayed nonlinear behavior with a sudden increase in stiffness and stress concentration once the joint gap closed and the tie-link was engaged. By systematically analyzing the seismic characteristics of each connection type, this study provides a comprehensive foundation for selecting appropriate joint methods in the seismic design of fire protection piping systems, taking into account structural stability, constructability, and economic feasibility. Additionally, it underscores the limitations of current technical data on Grooved Coupling, which primarily focus on strain or flexibility factors. The study advocates for the collection of experimental force?displacement data to enhance modeling accuracy. These findings aim to contribute to the development of more reliable seismic analysis models and design standards that reflect real external loading conditions.

https://doi.org/10.6110/KJACR.2025.37.11.521

Jung Hoon Yang ; Jin Kyun Cho ; Joo Hyun Moon

Global warming increases refrigeration demand, impacting electricity consumption as higher ambient temperatures degrade system performance (higher power, lower COP). We simulated a transcritical CO₂ system, varying gas cooler tube diameter (0.004-0.011 m) and fin area (0.06-0.20 m²) from 20-40℃. Enlarging tube diameter (0.004 to 0.011 m) proved detrimental: at 40℃, COP decreased 21.2% (from 2.05 to 1.62) and total cost increased, indicating the optimal diameter is 0.004 m. Conversely, expanding fin area (0.06 to 0.20 m²) markedly improved heat rejection. At 40℃, COP increased 80% (from 1.22 to 2.19) and compressor power dropped 39% (from 148.1 to 90.4 W). While performance improved up to 0.20 m², total cost analysis identified an economic optimum in the 0.14-0.16 m² range. Comparative results demonstrate that fin area adjustments produce more significant performance changes than tube diameter modifications, particularly under high ambient temperature conditions. Therefore, in practical system design, it is more effective to address fin area configuration before tube diameter adjustments, as this approach can maintain higher energy efficiency over different seasons while also reducing operational costs.

https://doi.org/10.6110/KJACR.2025.37.11.536

Yong-Il Kwon

Buffer tanks in data center cooling systems need to maintain thermal stratification to ensure that chilled water remains available during disturbances. We examined two diffuser configurations through transient three-dimensional simulations: Case 1 features a single 360° annular discharge near the upper wall, while Case 2 employs an upper loop manifold with 20 nozzles to distribute the same total flow. We quantified thermal stratification using the mixing index (MI) and the cold-layer volume fraction (????), and we tracked tank and outlet average temperatures over a duration of 600 seconds. Case 2 consistently maintained a sharper and longer-lasting thermocline compared to Case 1, with a lower minimum MI (~0.31) occurring later in the process. Additionally, ???? decreased more slowly in Case 2, retaining approximately 10% of the cold volume at around 400 seconds, while Case 1 was nearly depleted. The rise in outlet temperature was correspondingly more gradual in Case 2, indicating a stronger buffering effect. The multi-nozzle manifold in Case 2 disperses jet momentum into a horizontal wall-jet that attaches to the upper surface, effectively suppressing entrainment across the thermocline. In contrast, the concentrated annular jet of Case 1 promotes wall impingement and substantial recirculation, which increases vertical entrainment and accelerates mixing. In conclusion, for applications where maintaining stratification is crucial, we recommend using an upper multi-port (dispersive) diffuser that is aligned for horizontal discharge and designed to distribute jet momentum. This approach helps preserve the low-temperature layer, delays outlet warming, and enhances overall performance.

https://doi.org/10.6110/KJACR.2025.37.11.545

Hiwon Yun ; Young June Lee

This study explores the causes and challenges of acceleration work in mechanical equipment projects and proposes strategies for improvement. Acceleration work, often triggered by schedule delays, design changes, or client directives, necessitates intensive resource inputs within a limited timeframe. This pressure can lead to quality issues, safety risks, and cost overruns. Through an analysis of relevant legal precedents, a review of existing scholarly works, and insights from expert consultations, the research identifies phase-specific practical measures and highlights the importance of systematic documentation in supporting cost claims. Key policy recommendations include revising government contract guidelines to incorporate provisions for direct labor costs and enhancing subcontract agreements to clarify indirect costs. The goal of this research is to foster safer, higher-quality, and more dispute-resistant mechanical equipment projects.

https://doi.org/10.6110/KJACR.2025.37.11.556

Jun Young Choi ; Ki Won Lee

The importance of international climate change countermeasures and environmental issues has grown significantly in the 21st century. With the emergence of greenhouse gas emissions trading and clean development mechanisms, new rules are shaping the energy market, prompting nations to develop response strategies. Under the Paris Agreement, reducing greenhouse gas emissions from the building sector is essential for achieving carbon neutrality by 2050. In Korea, approximately one-quarter of total energy consumption is attributed to residential and commercial/public buildings, making energy savings in this sector urgent. Heating and cooling account for the largest share of building energy consumption: thermal energy constitutes about 28 percent of total national energy use, while electricity represents 13 percent. Most thermal energy is consumed in homes and buildings?approximately 90 percent residential, 8 percent commercial, and 2 percent public?meaning that improving efficiency in this area can significantly reduce national energy use and lower CO₂ emissions. Heat pumps, which provide high-efficiency heating and cooling, are recognized as a viable replacement for traditional heat sources and are increasingly viewed as a crucial measure in the fight against climate change. Buildings contribute to roughly 25 percent of domestic greenhouse gas emissions (including indirect emissions), with household heating consuming 65 percent of heating energy. Many experts agree that transitioning from fossil fuel heating to heat pumps, including air-source pumps, is essential. However, while many countries classify air heat as renewable energy, there is still debate in Korea regarding its inclusion in renewable energy legislation, highlighting the need for institutional improvements to promote the adoption of air-source heat pumps. This study predicts the CO₂ reduction effect of converting existing heating systems in domestic single-family houses to heat pumps and estimates the reduction potential through 2035.