Vol 110, No 3 (2021)

One of the tasks of the membrane gas separation process is the extraction of low-content highly penetrating impurities from gas mixtures.

The gas separation process is efficiently organized using hollow fiber-type membrane modules.

In this study, an attempt was made to develop an engineering method for calculating the purification of gas mixtures based on the removal of small impurities from a countercurrent membrane module, which would enable the calculations related to the separation mode using analytical dependencies, without the use of numerical integration procedures and iterations.

A mathematical model representing the gas separation process from binary mixtures having low content of a highly penetrating component was analyzed to address the challenges related to cleaning, purifying, and drying gas mixtures. Parametric studies related to the process were carried out and an engineering calculation method that enabled calculating separation in the module without using complex, resource-intensive algorithms having analytical formulas was developed herein. During the parametric study, the separation of a binary gas mixture was considered a limiting case. The proposed method was tested with respect to the example of air drying in a module comprising a (polyvinyltrimethylsilane) PVTMS membrane, which was an approximation of a binary mixture. The numerical and analytical results of the calculation are consistent with each other, and the calculation herein based on the developed method requires several orders of magnitude fewer calculations, that can even be performed manually.

Automated packages for calculations of the engineering systems of buildings, which accelerate the design process and solve complex engineering problems, have been increasing recently. Therefore, comparing traditional methods of calculation and design of microclimatic systems using manual methods with those that use mathematical modeling in known modern packages is important. This comparison is especially relevant for the space industry, which is intended to be advanced. The paper considers the architectural package Autodesk Revit and the cycle calculation package Aspen HYSYS. A calculation model of the building was created in accordance with the drawings, and heat gains and losses were calculated. Manual calculation and comparison were then performed. A pneumohydraulic scheme was illustrated and calculated manually using the calculation package. The discrepancy in the results was evaluated. Results revealed that the use of Revit and HYSYS mathematical modeling systems is promising. Thus, considering the real possibilities of the designed engineering systems of specific buildings and structures, calculations in the automated package Aspen HYSYS agree well with traditional methods of manual calculations, and those in the architectural package Autodesk Revit should be adjusted.

A new calculation method that is applicable to air coolers operating in «dry» (without dehumidification and frosting), «wet» (with dehumidification or frosting on the entire surface), or combined (with dehumidification or frosting on part of the surface) conditions has been developed for counterflow and parallel-flow air coolers without phase transition of the cooling fluid and for cases with phase transition. The advantages of the developed method include the following: its versatility; consideration of the effect of dehumidification and frosting on the heat exchange process; the possibility of application to design and verification calculations; and low algorithmic complexity (due to the absence of the need to divide the heat exchanger into separate segments for calculation, as well as the absence of iterative calculations to determine the proportion of the dry heat exchange surface in the combined operating mode), resulting in the high speed execution of calculation programs.

The third part of the article describes a method for calculating air coolers operating in «wet» and combined conditions. The transition criterion from the «wet» operating mode of the air cooler to the combined mode is provided. In the case of the combined operating mode, for counterflow air coolers, the proportion of the dry heat exchange surface can be directly expressed, which helps to avoid iterative calculations and reduce the execution time of calculation programs.

BACKGROUND: A universal method for calculating air coolers that is applicable to design and verification calculations is necessary. The method considers the influence of dehumidification and frosting on the heat exchange process and allows the quick performance of a large number of calculations to simulate the operation of refrigeration and air conditioning systems without significant loss of accuracy. A calculation method that addresses all the above-mentioned criteria is unavailable in domestic and foreign literature.

AIMS: This study aims to develop 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 a large number of calculations to simulate the operation of refrigeration and air conditioning systems without significant loss of accuracy.

MATERIALS AND METHODS: The developed method of calculating air coolers is based on the classical approach of ε-NTU (efficiency – the number of heat transfer units) and is its adaptation, allowing to consider the influence of dehumidification and frosting on the heat exchange process and perform 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 the same values calculated using the segmented division method for a variety of operating modes (including combined).

RESULTS: Comparison with the segmented division method of the heat exchanger demonstrated good convergence of the calculation results with multiple reductions in their execution time. The deviation value of the calculated value of the thermal power computed using the developed method from the same value computed using the segmented division method averaged 3.23% modulo and did 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 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 lead to a significant increase in their accuracy compared to the new method. Therefore, the developed m-ε-NTU method can be widely used for the selection of air coolers, their verification and design calculations.

BACKGROUND: The selection of refrigerants for modern air conditioning systems (ACS) in ground facilities is a multidisciplinary task. Particularly, meeting the required energy efficiency of the refrigeration cycle as well as ensuring ecological safety of production, operation, and utilization of the refrigeration system. Herein, the working pressure levels of the refrigeration cycle considerably affect the availability, cost, and safety of the refrigeration equipment. The fire safety of the working substance is also important.

AIM: To investigate the feasibility of a mixture of dimethyl ether and carbon dioxide as refrigerant for energy efficient and safe application of ACS in ground facilities.

METHODS: Comparative analysis of a simple one-stage vapor–compression cycle using traditional working substances (R22 and R410A) and the proposed working substance, which is in the form of a mixture of dimethyl ether and carbon dioxide, using packages, such as Mathcad, HYSYS, CoolPack, and REFPROP, was performed.

Results: An ecofriendly mixture of dimethyl ether and carbon dioxide with low global warming potential and zero ozone depletion potential was proposed as refrigerant. Increasing the percentage of dimethyl ether in the blend reduces the temperature glide in the gas cooler, a property of CO₂, and pressures at which the blend operates. The mixture has limited operational properties due to the flammability of dimethyl ether, but its environmental performance makes the material of some practical interest.

CONCLUSION: Fire safety of the proposed working substance was calculated. The concentration of dimethyl ether in the mixture at which it becomes flammable and unsafe for ACS was determined to be 8.3%.

With an increase in the dimethyl ether content in the mixture with CO₂ from 4% to 8%, the refrigeration coefficient of the cycle increases from 2.53 to 2.88, but it is 1.57 times less than that of R410A.

The difference in operating pressures between the used non-ecological refrigerants and proposed mixture was determined. Results indicate that the mixture of dimethyl ether and carbon dioxide is currently inapplicable to mass production compressors, which use R410A as refrigerant. The condensation pressure of the most effective and nonflammable mixture of dimethyl ether and CO₂ (with dimethyl ether concentration of 8%) is 101 bar against 30 bar for R410A.

Therefore, we intend to evaluate test mixtures of dimethyl ether with other substances in the future.

The use of CO₂ as a refrigerant in transcritical cycles requires additional measures to increase the efficiency of the refrigeration system. Transcritical refrigeration systems are widely used at food processing facilities

Different modifications of the base cycle are used to increase efficiency, but the methods for calculating and analyzing such cycles are not sufficiently presented in the literature.

Along with the calculation of cycles and the determination of parameters at the base points, it is necessary to conduct an efficiency analysis in order to determine the optimal parameters.

The purpose of the study was to develop a method for calculating and analyzing the transcritical cycle of parallel compression CO₂.

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.

The operation of the transcritical cycle of the parallel compression CO₂ with two temperature levels is described, the procedure for calculating and analyzing losses in the elements of the refrigerating unit operating according to the transcritical cycle of the parallel compression CO₂ 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.