Vol 110, No 4 (2021)

This paper reviews the problem of high humidity in an ice arena in Sochi and ways of solving it. The authors consider the ventilation system structure and determine the causes of the high humidity—moisture in the water vapor forms and a cold surface of ice cools the surrounding space and objects. The consequences of this air state are fogging over the ice surface, namely, condensate formation, which exacerbates the quality of the ice rink, corrosion of steel and iron structures, and mold attack. In addition, the facility microclimate is uncomfortable for people to stay there. During this study, the authors identified several ways to achieve the necessary indoor air parameters. The first method is sorption dehumidification. This system can function at low temperatures and cope with extreme dampness, but it has significant costs. The second method, which is simpler and more effective, is assimilation. This method is based on the ability of warm air masses to hold a larger amount of water vapor than cold air masses. This method for improving the air parameters is more effective when required to modify an existing ventilation system. In this particular case, in the ice arena in Sochi, assimilation was the most effective method. Air coolers with a drift eliminator were installed in the existing ventilation system. The calculation of the selected heat exchangers was performed using ventilation equipment selection software (VESS).

BACKGROUND: Carbon dioxide can be an alternative refrigerant for vapor compression refrigeration systems, particularly air conditioning systems (ACSs). However, its use suffers from the increased pressure in the refrigeration circuit. To solve this, for its reduction, a mixture of CO₂ with a substance that has significantly lower pressures under the same conditions can be developed, for example, dimethyl ether (DME), which has zero GWP and ODP, is inexpensive and readily available. The study of DME, in particular, was conducted by the Department E4 “Refrigeration and Cryogenic Engineering, Air Conditioning and Life Support Systems” of N.E. Bauman Moscow State Technical University. DME is moderately toxic and flammable.

AIM: This study aims to investigate the possible use of a mixture of DME and carbon dioxide for energy-efficient application in ACSs using commercially available compressors for R410A.

METHODS: Comparative calculation analysis of the characteristics of a simple one-stage vapor compression cycle was performed using R410A and a mixture of DME and CO₂ using the calculation packages Mathcad 15, Aspen HYSYS v. 10, SOLKANE8, and REFPROP.

RESULTS: 1. The cycle on pure DME is the most effective in terms of the coefficient of performance: ε = 5.63 at an ambient air temperature of 26°C, ε = 3.07 at 40°C. 2. It is necessary to consider the influence of temperature glides, the average value of which ranges from 10°C to 30°C depending on the concentration of components. 3. At DME/CO₂ ratios of 40/60% and 60/40% (in mole fraction), the discharge pressure corresponds to the discharge pressure in the R410A cycle, with 39.62 bar at an ambient temperature of 26°C and 37 bar at 40°C, respectively.

CONCLUSIONS: An environmentally friendly mixture of DME and CO₂ with low GWP and zero ODP is developed. An increase in the percentage of DME in the mixture increases the coefficient of performance and reduces the pressure range, and at the same time, significant temperature glides arise, which affects the installation efficiency, namely, the transition to a cycle with a receiver tank, i.e., with a recuperative heat exchanger between the fluorinated refrigerant flow leaving the evaporator and the fluorinated refrigerant flow leaving the condenser. The developed mixture is less efficient than the R410A refrigerant in terms of the coefficient of performance and discharge pressure. However, it is possible to further consider a mixture of DME and CO₂ with concentrations of 40% and 60%, respectively, as a replacement for the R410 refrigerant because there is a correspondence of discharge pressures for serial compressors (about 40 bar); however, it is necessary to keep in mind the flammability risk of the mixture.

In the fourth part of the article, the algorithm for applying the newly developed the m-ε-NTU method for calculating air coolers in dehumidifying or frost conditions is described step by step, and its comparison with the method of segmented division of the heat exchanger is also given. This comparison showed good convergence of the calculation results with a multiple reduction in their execution time. The value of the deviation of the calculated value of thermal power calculated using the newly developed method from the same value calculated using the method of segmented division averaged 3.23% modulo and does not exceed 4.5% modulo. When the heat exchanger is divided into 40 segments, the execution time of the calculation programs increases approximately 18 times compared to using the newly developed method, which can be called a significant advantage of the latter. Considering the above, the newly developed method can be widely used for the selection of air coolers, their verification and design calculations.

BACKGROUND: A universal method for calculating air coolers that is applicable to design and verification calculations is necessary. The method should consider the effect of dehumidification and frosting on the heat exchange process and allow the quick performance of many calculations to simulate the operation of refrigeration and air conditioning systems without significant loss of accuracy. However, this example of calculation method that addresses all the above-mentioned criteria is unavailable in the domestic and foreign literature.

AIM: This study develops a universal method for calculating air coolers that is applicable to design and verification calculations. This method considers the influence of dehumidification and frosting on the heat exchange process and allows the quick performance of many calculations to simulate the operation of refrigeration and air conditioning systems without significant loss of accuracy.

MATERIALS: The developed method for calculating air coolers is based on the classical approach of ε-NTU (efficiency– the number of heat transfer units) and is its adaptation, allowing consideration of the influence of dehumidification and frosting on the heat exchange process and performing the calculation (including the combined operating mode of the air cooler) without dividing the heat exchanger into separate segments. The estimation of the error of calculations performed using the developed method was conducted by comparing the calculated values of the thermal power of the device with those using the segmented division method for various operating modes (including combined).

RESULTS: Comparison with the segmented division method of the heat exchanger showed good convergence of the calculation results with multiple reductions in execution time. The deviation value of the calculated value of the thermal power computed using the developed method from that using the segmented division method averaged 3.23% modulo and did not exceed 4.5% modulo. When the heat exchanger was divided into 40 segments, the execution time of the calculation programs increased approximately 18 times compared to using the developed method, which can be called a significant advantage of the latter.

CONCLUSION: The division of the heat exchanger into segments for calculation does not result in a significant increase in accuracy compared with the new method. Thus, the developed m-ε-NTU method can be widely used for the selection of air coolers, their verification, and design calculations.

Attempts to reduce the negative impact on the environment have led, among other things, to the use of so-called natural refrigerants. One of which is SO₂.

At food processing enterprises, this refrigerant is used in two main cycles - cascade and transcritical.

Along with the negative impact on the environment, it is also necessary to take into account the increase in the efficiency of refrigeration plants, which is especially important for transcritical cycles.

The purpose of the study was to develop a method for calculating and analyzing the transcritical cycle of SO₂ with parallel compression and an ejector.

The calculation of the transcritical cycle is based on the fundamental laws of thermodynamics, the analysis method is based on the entropy-statistical method of thermodynamic analysis. Cycle calculation includes analysis of compression loss by system components.

Description of operation of transcritical cycle of SO₂ with parallel compression and ejector with two temperature levels is given, procedure of calculation and analysis of losses in elements of refrigerating unit operating according to transcritical cycle of SO₂ with parallel compression and ejector is given.

The use of this method allows you to identify the elements and processes with the greatest losses and take further measures to improve the efficiency of the refrigeration system.