МЕТОДИ СТВОРЕННЯ СХЕМИ ТЕПЛОВИКОРИСТАЛЬНОЇ ХОЛОДИЛЬНОЇ МАШИНИ З РОБОЧОЮ РЕЧОВИНОЮ ДІОКСИДОМ ВУГЛЕЦЮ

The operating conditions of a refrigeration machine determine its energy efficiency. Some industries, particularly the food industry, are characterized by seasonal (short-term) consumption of cold in the low-temperature range (−40℃ and below) when rapid freezing of the product is required to achieve high quality because of finely dispersed crystallization of moisture and a “shock” temperature for psychrophilic microorganisms. To solve this problem, the periodic use of a low-temperature refrigerant, which, from a rational technical point of view, is not always combined with the constant required temperature of refrigerated storage because of possible operation under vacuum conditions and other difficulties, is advised. The difference between the temperature regime of the required short-term (seasonal or periodic) processing and the constant (main) temperature regime (e.g., refrigerated storage of products) often reaches 20℃ or more, which imposes certain restrictions on the design or operation of the refrigeration machine. In this regard, small and medium-sized refrigeration machines with the capability to operate on two refrigerants (e.g., low and medium temperatures, with periodic automatic mutual replacement) are promising. Such a refrigeration machine is referred to as “bi-refrigerant” in this study. The aforementioned difficulties are also solved by the proposed scheme of a “two-compressor” refrigeration machine, in which the possibility of its operation is realized during periodic change of modes from one stage to two stages, and vice versa, according to the need for the corresponding differences in boiling and condensation pressures.
 BACKGROUND: Environmental conditions and boiling point impose certain requirements on the selection of refrigerant and the design of the refrigeration machine, which often necessitates the division of the design period of operation of the refrigeration machine into two seasons, i.e., summer and winter, or the main operating mode (e.g., refrigerated storage) and nonessential (e.g., freezing seasonal or periodically supplied products to the refrigerator). This need may be especially acute in seasonal refrigerators and refrigerators operating under the conditions of large differences in ambient temperatures that periodically occur in some geographic areas. The need for a two-season (“bi-refrigerant”) refrigeration machine arises not due to fluctuations in the conversion coefficient under different environmental conditions (although this does occur) or a change in operating mode from the refrigeration cycle to the heat cycle (heat pump) but due to the seasonality of the supply of products subject to primary processing (quick freezing).
 AIMS: This work aims to provide theoretical justification for the practical implementation of the “bi-refrigerant” and “two-compressor” refrigeration machine schemes.
 MATERIALS AND METHODS: A theoretical investigation method was used to propose schemes for “bi-refrigerant” and “two-compressor” refrigeration machines, was used. The objects of the study were the diagrams of vapor compression refrigeration machines.
 RESULTS: A schematic diagram of a “bi-refrigerant” refrigeration machine operating by periodically replacing “seasonal” refrigerants has been developed. A schematic diagram of a “two-compressor” refrigeration machine, which provides the possibility of periodically changing a one-stage to a two-stage cycle, has been presented. A diagram of the automatic control of the proposed solutions is presented using the example of a “bi-refrigerant” refrigeration machine. Mathematical support and justification of the developed schemes are also presented.
 CONCLUSIONS: Our investigation revealed the theoretically justified possibility of using “bi-refrigerant” and “two-compressor” refrigeration machine schemes, e.g., in industries with seasonal short-term supply of short-term products subject to rapid (low-temperature) freezing or in other industries that require periodic changes in the boiling point of the refrigerant. Delaying factors in the application of the proposed “bi-refrigerant” refrigeration machine design may be the selection of compressor oil capable of operating on two refrigerants and the need to carefully solve the problem of circulation and its return to the compressor. However, this problem can be solved.

In our country, as well as throughout the world, special attention is paid to finding more modern, energy-efficient and environmentally friendly methods of cooling food products, including animal ones. The basis of most technological processes of refrigeration treatment is working fluids, which can be natural or synthetic substances. Due to the negative impact of synthetic raw materials on the environment, preference is given to substances of natural origin. The greatest interest in the last twenty years in the world community of refrigeration equipment and technology is shown to carbon dioxide. This refrigerant is actively used in traditional vapor-compression refrigeration machines. In Russia carbon dioxide as a working substance in closed cycles of refrigeration machines is found at industrial facilities whose operation is based on cascade cycles. In this case, carbon dioxide acts as a working substance of the lower cascade, since it belongs to high-pressure refrigerating agents. Until 2022, carbon dioxide-based refrigerants were also actively developed at transcritical temperature levels, where the condensation temperature of carbon dioxide is close to ambient temperatures. In this study, carbon dioxide will be considered as a refrigerant when it interacts with the product in an air-gas environment during sublimation, in which carbon dioxide changes its aggregate state from snow-like to gaseous.

The paper presents the results of theoretical and experimental analysis of a vacuum-freeze-drying refrigeration machine based on carbon dioxide. Such refrigeration machines can be equipped with refrigerators for the needs of medicine, where low temperatures are widely in demand in the range from minus 80 degrees Celsius to minus 130 degrees Celsius. The problem of developing alternative refrigeration machines is dictated by modern environmental requirements for working substances, which are gradually being tightened and soon the use of familiar freons will become impossible. For the air conditioning and commercial refrigeration industry, new substances are being actively developed, such as hydrofluoroolefins (HFOs), hydrocarbons can also serve as substitutes for hydrofluorocarbons and their mixtures, but it must be considered that they are combustible and explosive. A carbon dioxide vacuum sublimation unit may be an alternative to modern freon vapor compression refrigeration units for the indicated temperature range, but subject to comparable energy costs during its operation. The article examined the three-stage and four-stage cycles of a vacuum-freeze-drying refrigeration machine, conducted their theoretical comparison in terms of COP with a cascade cycle on hydrocarbon working substances. It can be noted that the COP of the considered cycles are close, but the environmental friendliness, safety and non-combustibility of carbon dioxide gives the advantages of a vacuum freeze-drying refrigeration machine.

Liquid-to-suction heat exchanger (LSHX) has been used for air conditioning and refrigeration machines over years. It was aimed to increase the cooling capacity and coefficient of performance. In this paper, the influence of LSHX utilization on a refrigerating machine with refrigerant R404a was investigated. The tests were focused on the heat exchanger effectiveness under various evaporating temperature. To vary the evaporating temperature, a valve was installed on the inlet side of thermostatic expansion valve. By varying the openings of this valve, the pressure of suction line could be varied from 30 psi to 50 psi, resulting in the evaporating temperatures from -24°C to -10°C. With the LSHX, the liquid refrigerant temperature could be lowered 4°C to 8°C. The reduction of the temperature improves the capacity and coefficient to performance up to 16%. Based on the experiment, the effectiveness of LSHX ranges from 0.18 to 0.25. The lower the evaporating temperature, the higher the effectiveness of LSHX. However, sub-cooling and superheating should be limited to a certain value in order to control the compressor within its safe operating conditions.

Background. Currently, in the cycles of steam-ejector refrigeration machines (SERM) are used monoagent jet compressors, the working body in which is water. Despite their low efficiency, SERM have a number of advantages over refrigeration machines that usemechanical compressors: low cost, ease of manufacture and reliability. The use of SERM is largely limited by the inability of these installations to reach temperatures below 0 °C. This limitation can be overcome by applying a fundamentally new steam compressioncycle using a bioagent (water-freon) jet compressor (BAJC).Objective. The purpose of the paper is experimentally confirm the operability of a refrigeration machine operating on a steam compression cycle using a bioagent (water-freon) jet compressor. Determine the efficiency of the experimental refrigeration machine andfind the optimal geometric characteristics of the jet compressor.Methods. An experimental stand was created to study the refrigeration plant using BAJC. The refrigeration machine efficiency e was determined according to the developed method.Results. Experimental values of the cooling machine coefficient e were obtained depending on the change in the ejection coefficient q of BAJC. The maximum machine coefficient values emax = 1,3 ÷ 1, 4 correspond to those of the ejection coefficient q opt ≈опт 0, 25q 0,25, which also achieves the maximum efficiency h ≈max 0,35 ÷ 0, 45 of the BAJC.Conclusions. The efficiency of a refrigeration unit operating on a steam compression cycle using a bioagent (water-freon) jet compressor has been experimentally confirmed. To expand the temperature spectrum of the cycle in the direction of lowering the boilingpoints in the evaporator (tboilt −≤кип 20 °C), it is planned to modernise the laboratory bench by using aqueous salt solutions as a coolant.

Currently, more and more attention is being paid to the problem of effective application of ozone-safe refrigerating agents used in the cycles of vapor-compression refrigeration machines. This has become especially important when a number of ozone-safe refrigeration agents have a large global warming index, which has recently been unacceptable with respect to the environment worldwide. Therefore, conducting studies to evaluate the performance of new refrigerating agents as a result of replacing some refrigerating agents with others is an urgent task. At the same time it is necessary to try to keep the same equipment as it was before the replacement of the working agent. In this connection the justified efficiency of vaporcompression refrigeration machines at replacement of freons are an integral part of modernization of refrigeration systems. This paper presents a study of the efficiency of vapor-compression single-stage cycles of refrigeration machines when operating with different refrigerating agents. Thermodynamic evaluation of different ozone-safe refrigerating agents was carried out in comparison with forbidden ozone-safe refrigerating agent – dichlorodifluoromethane. In the course of the conducted research the cold-productivity and effective power of the refrigeration cycle for the considered refrigerating agents were determined. An estimation of the refrigerating agent value and the degree of cycle perfection for different refrigerating agents has been carried out. Specific volumetric refrigerating capacity of the investigated refrigerating agents for the considered cycle and parameters has been determined.As a result of this research, the most expedient refrigerating agent was recommended for replacement at the investigated parameters and cycle of the refrigeration machine.

Marine refrigeration machines are responsible for large amounts of electricity consumption, as well as direct emissions of high global warming potential (GWP) refrigerants, so they need to be gradually upgraded. The possibility of using a ship ejector refrigeration machine with two-stage compression, consuming waste heat, as well as a cascade compression-ejector refrigeration machine to obtain the temperatures of minus 28–27 °C was analyzed. It was shown that the traditional indicator of the efficiency of heat-driven refrigeration machines COPtherm ot be recommended for the selection of energy-efficient modes of operation of ejector refrigeration machines that consume waste heat. As a criterion for the energy efficiency of ejector refrigeration machines that consume waste heat, it is proposed to analyze COPmechTot, which takes into account the electric power of pumps, fans, as well as units that provide “feeding” the refrigeration machine with waste heat. A two-stage ejector refrigeration machine that consumes waste heat with a temperature of 95–45 °C and is intended for the operation of a ship ice generator (boiling temperature — 28 °C) loses to a traditional vapor compression refrigeration machine in terms of energy consumption: COPmechTot = 1.266 vs. COPmechTot = 1.52. A cascade vapor compression ejector refrigeration machine that consumes waste heat with a temperature of 85–95 °C and is designed to provide refrigeration (boiling temperature — 27 °C) of ship provision chambers is more attractive than a traditional vapor compression one: COPmechTot = 2.37 vs. COPmechTot = 1.82. An alternative technical solution for the production of sub-zero temperatures on ships, which the authors plan to consider in further research, is a refrigeration machine with two-stage compression. It uses a compressor in the first stage and an ejector in the second stage. Bibl. 26, Fig. 7, Tab. 1.

Carbon dioxide is used in large volumes to produce urea, a highly efficient nitrogen fertilizer. It is compressed in a multistage compressor to a pressure of 15 MPa and fed to the urea synthesis unit. The specific energy consumption for the compression of carbon dioxide by a compressor reaches 0.16 kWh/kg. It may be more profitable to use in the system of compressor-pumping and refrigeration units. They can be used to liquefy carbon dioxide and compress it to pressure 15 MPa before feeding it to the synthesis of urea. In the simplest scheme, an ammonia compression refrigeration machine (ACRM) is included in the system to improve efficiency. The specific energy consumption in such a system for the liquefaction and compression of CO2 is 0.118 kWh/kg. In case of replacement of the ACRM with an absorption refrigeration machine, unit costs can be reduced to 0.09 kWh/kg. These two systems can be used to increase urea production or to ensure stable operation of the units during the summer period of their operation. The analysis showed that further improvement of the technological scheme of the entire system will completely abandon the use of the compressor method of compression of CO2 to pressure 15 MPa before its supply to the urea synthesis unit. To do this, you need to include an additional absorption lithium bromide refrigeration machine in the system. In this scheme, the compressor-pumping unit will provide the simultaneous supply of liquid carbon dioxide and ammonia for the synthesis of urea with a pressure of 15 MPa. To increase the daily production of urea from 1400 to 2000 tons, it is necessary to increase the feed liquid CO2 in the amount of 62 t/hour and liquid NH3 — 47.5 t/hour. Bibl. 14, Fig. 3.

BACKGROUND: Existing refrigeration machines contain four main units, i.e., compressor, condenser, expander, and evaporator that perform the refrigeration cycle. Currently, the liquefaction of the refrigerant occurs in the condenser. The creation of the conditions under which condensation of the working fluid would occur in the compressor unit would make the development of a compact refrigeration machine possible by eliminating the condenser from it.
 AIMS: This work aims to investigate the possibility of creating a compact refrigeration machine with the implementation of the condensation process of the working fluid in the compression chamber of the compressor.
 MATERIALS AND METHODS: The object of the study is a low–speed compressor, in which the pressure ratio significantly exceeds known analogs and simultaneously the temperature is significantly lower than that of high–speed machines because of the creation of conditions under which compression occurs with polytropic indicators below 1.08. The research method involves determining the gas temperature at critical pressure and comparing the obtained result with the critical temperature. The system of equations and assumptions used relates to the model with lumped parameters of the working fluid.
 RESULTS: The results obtained on the compression of such refrigerants as ammonia, carbon dioxide, R12, R22, and Freon–134a showed the possibility of obtaining the required pressures in a low–speed machine at temperatures significantly below the critical temperature.
 CONCLUSIONS: This work examines and proposes to study further those working fluids of refrigeration machines that can go into the liquid phase in a low–speed compressor unit because of their characteristics and operating parameters in the refrigeration machine. Further experimental confirmation of this will enable partial or complete elimination of the condenser unit in the refrigeration machine.

In this work a procedure for easily selecting safe and productive operating conditions of homogeneous semibatch reactors in which exothermic autocatalytic reactions are performed is developed and presented. Such a procedure is based on the boundary and temperature diagram method previously proposed in the literature for nonautocatalytic reactions. In the autocatalytic case an additional parameter appears in the mathematical model of the reactor as well as in the safety criterion, that is, the initial catalyst amount which is directly related to the coreactant accumulation in the system. A number of diagrams for identifying inherently safe operating conditions is provided, together with some rules of thumb for their use, allowing for easily selecting safe and productive operating conditions of homogeneous semibatch reactors in which exothermic reactions with autocatalytic behavior occur.

The safety of vessels navigating in the sea waterway system is ensured by fulfilling the acceptable restrictions called safe ship operation conditions in that system. The assessment of navigation safety is particularly important when the conditions for safe operation of ships in the waterway system are changed concerns increasing the maximum parameters of vessels, increasing the allowable hydrometeorological conditions or changing the minimum tug assistance. The article presents a method for assessing navigation safety when the conditions for the safe operation of vessels in the waterway system get changed. The method uses two indicators, which are difference in navigation risks and relative navigation risk. To determine the navigational risk, algorithms were developed for calculating the probability of accidents caused by the deterioration of navigation conditions and technical failure of ship equipment and tugs. Another algorithm was developed for calculating the consequences of the accidents that involve blocking a waterway by a ship anchoring in an emergency, grounding, impact of the ship against a port structure or moored ship and a collision with another ship in motion. The method developed for assessing navigation safety by means of relative navigation risk can be used in practice when changing the conditions for safe operation of vessels in the waterway system and when the system is modernized. Navigational safety management is a decision process that is implemented in the loop presented in the article. The acceptable risk is determined on the basis of vessel traffic intensity and ship parameters defined by safe operation conditions for a given waterway system. Relative navigational risk may be used in assessment and comparison of various conditions of safe ship operation. The probability of an accident caused by ship's moving outside the available navigable area due to technical failures of ship equipment or tugs is determined, depending on the type of port waterway and the manoeuvres performed.

Safe operating conditions determination for stationary SI gas engines

Теплоиспользующие холодильные машины относятся к энергопреобразующим системам, работающим по обратным термодинамическим циклам. Первичной энергией для этих машин служит тепло. В соответствии с классификацией, эти машины разделены на следующих категории согласно вида проведения компенсирующего процесса: сорбционные (адсорбционные и абсорбционные), эжекторные и компрессорные. В статье проанализированы новые схемно - цикловые решения теплоиспользующих машин с точки зрения энергосбережения и экологической безопасности, а также освещены перспективы совершенствования. Также рассмотрены возможности теплоиспользующих машин в системах тригенерации. В статье дан обзор вклада одесских учёных в создание и развитие теплоиспользующих холодильных машин и тепловых насосов.

Modern heat-using ejector refrigeration machines used in heat recovery systems for power plants based on gas turbine engines and internal combustion engines have many advantages over absorption refrigeration machines: smaller dimensions and weight; the ability to obtain lower temperatures. However, they are inferior in energy efficiency, and the thermal coefficient is much lower and can be 0.2…0.4. The efficiency of such refrigeration machines largely depends on the choice of the working fluid (refrigerant). Hence the need to choose a refrigerant that would provide the maximum heat factor, and hence the maximum efficiency of heat recovery. Given the relatively low efficiency of the ejector refrigeration machine, the search for a working fluid that will provide, on the one hand, higher thermal coefficients, and on the other hand high environmental friendliness, is one of the promising areas of development of heat recovery technologies in power plants. The study used the software complex developed by the authors to calculate the refrigeration cycles of heat-using refrigeration machines, taking into account the properties of many modern refrigerants, ejector characteristics, as well as basic heat exchangers (condenser, evaporator, generator). The efficiency of ejector refrigeration machines when working on the following working bodies was analyzed: R142b, R134a, R600, R600a, R1234ze(E), R1233zd(E), R1234yf, R227ea, R236fa, R245fa. R142b, R600, R600a, R245fa have the largest values of thermal coefficients. It is established that the most profitable in terms of environmental friendliness (ODP, GWP) and energy efficiency is the use of refrigerant R245fa, which has a condensation temperature range is 25…35 oC and boiling in the evaporator is 0…15 oC thermal coefficient is 0.40…1.03.

Gas hydrate formation is considered one of the major problems facing the oil and gas industry as it poses a significant threat to the production, transportation and processing of natural gas. These solid structures can nucleate and agglomerate gradually so that a large cluster of hydrate is formed, which can clog flow lines, chokes, valves, and other production facilities. Thus, an accurate predictive model is necessary for designing natural gas production systems at safe operating conditions and mitigating the issues induced by the formation of hydrates. In this context, a thermodynamic model for gas hydrate equilibrium conditions and cage occupancies of N2 + CH4 and N2 + CO4 gas mixtures at different compositions is proposed. The van der Waals-Platteeuw thermodynamic theory coupled with the Peng-Robinson equation of state and Langmuir adsorption model are employed in the proposed model. The experimental measurements generated using a cryogenic sapphire cell for the pressure and temperature ranges of (5-25) MPa and (275.5-292.95) K, respectively, were used to evaluate the accuracy of this model. The resulting data show that increasing nitrogen mole percentage in the gas mixtures results in decreasing of equilibrium hydrate temperatures. The deviations between the experimental and predictions are discussed. Furthermore, the cage occupancies for the gas mixtures in hydrate have been evaluated. The results demonstrate an increase in the cage occupancy for both the small and large cavities with pressure.