Modified efficiency-NTU method (m-ε-NTU) for calculating air coolers in dehumidifying or frost conditions. Part III

Abstract

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.