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Vol 113, No 1 (2024)
Original Study Articles
Study of nonequilibrium regenerative heat exchange in positive displacement compressors
Abstract
BACKGROUND: An important task in the design of thermal engines is the evaluation of power losses in different processes to determine the installation efficiency. One of the processes that generates power losses in the compressor is the process of nonequilibrium regenerative heat exchange (NRHE) of the compressed gas with the working cavity walls. This heat exchange occurs at a significant temperature difference, which can lead to noticeable power losses. Modern compressor design methods do not consider losses from nonequilibrium regenerative heat exchange and do not describe them separately from other types of losses.
AIM: This study investigates the mechanism of losses during regenerative heat exchange between the gaseous working flow out and the working cavity walls and evaluates the scale of these losses.
METHODS AND RESULTS: This work provides a qualitative description of the mechanism for the formation of losses from NRHE. As a result of solving the problem of unsteady thermal conductivity, an analytical expression for calculating such losses is derived based on the Gouy-Stodola theorem.
CONCLUSIONS: The study revealed that power losses from NRHE account for a significant share in the overall balance of machine losses. Parameters influencing the magnitude of these losses were also determined, and recommendations were developed to reduce them.
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Selection of refrigerant for use in chillers
Abstract
BACKGROUND: Chillers (chillers) are essential components in various industrial processes. They are primarily used when direct cooling through direct heat exchange between the boiling coolant and the cooled medium is not feasible. This could be attributed to the cost of the refrigerant, especially when a large amount of expensive substances for long refrigerant lines is needed, or the risks associated with using toxic and flammable refrigerants, which could be hazardous in case of leaks. Chillers are typically classified into three types based on their temperature applications: high-temperature chillers for uses like plastic manufacturing and data centers, medium-temperature chillers for air conditioning systems, etc.) and low-temperature chillers for applications like ice fields and food storage.
Given their widespread use and high production volume, designing efficient chillers is an important task.
AIMS: Substantiating refrigerant use in terms of efficiency and service life of refrigeration equipment.
MATERIALS AND METHODS: Chillers are designed for various industrial applications, with specific inlet and outlet temperatures for the evaporator: +26°С / + 20°С (ВТ); +12°С / +7°С (ST); −10°С / −13°С (НТ)). These chillers operate using refrigerants R134a, R410A, R404A, and R1270, evaluated through an entropy-statistical method of thermodynamic analysis [1]. R1270 is highlighted as a promising refrigerant for monoblock chillers, since there are no filling restrictions for these chillers when installed in open space [2].
RESULTS: Among the refrigerant reviewed, R1270 demonstrates the highest thermodynamic efficiency in BT mode, outperforming R404A by 11.97%, R134a by 2.15%, and R410A by 5.48%.
In CT mode, R1270 also leads,–with a 14.13% advantage over R404A, 3.04% over R134a, and 3.41% over R410A.
Similarly, in HT mode, R1270 shows superior thermodynamic performance, being 21.95% more efficient than R404A, 29.73% more than R134a, and 11.44% more than R410A.
The use of R410A and R134a in NT mode is limited owing to high discharge temperatures during compression, namely 116.94°C for R410A and 114.63°C for R134a, potentially reducing equipment lifespan. The lowest discharge temperature during actual compression, 84.63°C, is achieved using R404A coolant. However, despite its efficiency, this refrigerant results in a relatively high discharge temperature of 96.84°C.
CONCLUSIONS: The analysis highlights specific applications for various refrigerants in chillers and underscores the potential of natural refrigerant R1270, which is produced in the Russian Federation.
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The role of miscible PAG lubricants in ammonia refrigeration systems reduction and compactness
Abstract
Today’s society is going through an expanding use of natural refrigerants, earlier present only in big industrial facilities. This has made necessary the charge reduction and compactness improvement of the equipment, to fit the safety measures and standards requirements.
The refrigerant charge and the complexity of an ammonia refrigeration system are mainly because of flooded evaporators use and the need for an efficient oil separation system, since conventional lubricants and ammonia are immiscible.
The use of ammonia miscible lubricants can address the oil separation problem. A Poly-Alkylene-Glycol
(PAG) can be designed and formulated to be miscible with ammonia, and the plant operated like a conventional halocarbon refrigerant one. The circulating lubricant will be recovered through the compressor suction port of the compressor. This will help in the operation of Direct Expansion (DX) evaporators, reducing to a testimonial size the oil separation circuit.
This article is a translation of the article by Muñoz-Alonso M, Dixon L, Seeton CJ, Karnaz J. The role of miscible PAG lubricants in ammonia refrigeration systems reduction and compactness. In: Proceedings of the 9th IIR Conference on the Ammonia and CO2 Refrigeration Technologies. Ohrid: IIF/IIR, 2021. DOI: 10.18462/iir.nh3-co2.2021.0018 Published with the permission of the copyright holder.
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R744 Refrigeration Solution for Small Supermarkets
Abstract
Application based technology solution is nowadays preferred to achieve the performance of the cooling system configuration at its best. Therefore, benchmarking is an essential criterion which would add value to the optimum system design for various applications. In this study, a case study is carried out for a Milk Bar (small supermarket) to evaluate the potential of R744 cooling system for the similar cooling demand and tropical conditions. Field data collected during a particular time and temperature zone is further used to develop a yearly round performance of the HFC plug-in cabinets for MT and LT applications. The data is further used to develop a centralized R744 booster system that would meet the similar cooling load and demand of all the cabinets in the shop. Based on the cooling loads for the HFC unit, the yearly performance of the R774 booster system is calculated and compared to the existing plug-in solution. It is observed that the yearly electric energy consumption for the R744 centralized refrigeration system is 3.3 M W·h, 24% lower than with the existing solution based on HFCs. Moreover, the economic prospective of the R744 is further discussed to alternative material for the components and centralized unit structure which could empower the system with more reliability and an effective substitute to the synthetic technology for smaller supermarkets.
This article is a translation of the article by Singh S, Pardiñas ÁÁ, Hafner A, Schlemminger C, Banasiak K. R744 Refrigeration Solution for Small Supermarkets. In: Proceedings of the 9th IIR Conference on the Ammonia and CO2 Refrigeration Technologies. Ohrid: IIF/IIR, 2021. DOI: 10.18462/iir.nh3-co2.2021.0030 Published with the permission of the copyright holder.
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Investigation on heat recovery strategies from low temperature food processing plants: Energy analysis and system comparison
Abstract
Industrial food processing plants often have significant thermal requirements at both low and high temperatures. These plants can produce a variety of products including frozen, chilled and grilled/steamed foodstuff, creating thermal demands at several temperature levels. Rapid freezing of the foodstuff at temperatures below -40 °C is required to preserve a high-quality product while steaming/grilling of foodstuff require heat above 100 °C. Heat recovery from the low-temperature refrigeration system provides an interesting opportunity to reduce the overall energy consumption of the plant. This paper presents different strategies to achieve heat recovery from a CO2/NH3 cascade refrigeration system. The low stage of the cascade features pumpcirculated CO2 circuits at -40 °C and -5 °C evaporation levels, while the high temperature stage consists of an ammonia circuit. For this investigation, a case is defined based on requirements for temperature level and heat quantity from the industry. Subsequently, different strategies for the integration and control of the energy systems are examined. Finally, the strategies are compared with selected key parameters and the results are presented.
This article is a translation of the article by Ahrens MU, Selvnes H, Henke L, Bantle M, Hafner A. Investigation on heat recovery strategies from low temperature food processing plants: Energy analysis and system comparison. In: Proceedings of the 9th IIR Conference on the Ammonia and CO2 Refrigeration Technologies. Ohrid: IIF/IIR, 2021. DOI: 10.18462/iir.nh3-co2.2021.0034 Published with the permission of the copyright holder.
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