Evaluation of the Ratio of Heat Exchanging Tube Finning on Heat Exchanger Efficiency

. For the purpose of thermal and hydraulic and aerodynamic testing of the heat exchanger tube bundles, various full-scale tube specimens with different finning ratio have been proposed. Formulas that allow to evaluate the effect of the ratio of heat exchanging tube finning on the heat exchanger efficiency are presented.


Abstract.
For the purpose of thermal and hydraulic and aerodynamic testing of the heat exchanger tube bundles, various full-scale tube specimens with different finning ratio have been proposed. Formulas that allow to evaluate the effect of the ratio of heat exchanging tube finning on the heat exchanger efficiency are presented.
Moisture separator-reheaters are heat exchangers where heat-exchanging processes are accompanied by load and temperature variations caused by the action of variable forces in the tube and shell sides, both in the transverse and axial directions. A combination of variations in force factors, existing or new gaps between the mating surfaces, where the sediment penetrates, either result in crevice corrosion followed by depressurization of the heat-exchanging circuits, or in a rapid loss of tube fastening tightness and strength [1]. For this purpose, the development of new designs of heat-exchanging cartridges using advanced materials is a critical task to ensure the strength and reliability of power plants.
Up to date, a lot of works are focused on the study of the heat-exchanging tube bundle strength subject to the specifics of the manufacture and assembly processes. Tube-totubesheet fixation methods, evaluation of the residual stress-strain state of fixing points and their quality are widely covered in [2][3][4][5][6][7]. However, the use of finned heat-exchanging tubes of various designs requires new thermal and hydraulic and aerodynamic studies to determine the optimal dimensions and number of fins, as well as the choice of materials that ensure the up-state of heat-exchanger for the specified service life of all the equipment.
The development of theoretical models that describe the heat and hydrodynamics of the tube bundles, and integral-differential equations are extremely difficult to treat mathematically [8]. Therefore, the development of experimental research methods and adequate models for thermal and hydraulic and aerodynamic testing is a critical task to ensure the tube bundle strength.
For this purpose, a number of research and development problems should be solved. The first task is to develop the methods for simulation of coolant flow in the tube and shell sides based on the coolant choice that adequately implement the thermal and hydraulic processes. The methods for simulating testing should be applicable for typical tube bundle designs. The heat-exchanging tubes with different arrangement of fins (longitudinal and transverse) are taken as models. The tube finning ratio is taken as an evaluation criterion.
The second task is to develop the testing bench design, selecting the process equipment and measuring instruments for the process parameters to ensure the thermal and hydraulic conditions specified.
This approach enables to evaluate the influence of the tube finning ratio on the thermal and hydraulic and aerodynamic characteristics of the heat exchangers, and benchmark the heat-exchanging tube bundle strength.
The following approach is proposed for the first problem.
The heat-exchanging tubes may be bare, longitudinally or transversely finned. Some tube designs are shown in Fig. 1. These designs are characterized by a different heat exchange surface, which provides the equipment heating capacity required at the final temperatures and pressures specified. For the purpose of study, we select tube specimens with typical geometric parameters that include: inner diameter, wall thickness and tube length, considered being equal for all the designs.
F heat exchange surface may be determined as follows [9] cp where Q is heat exchanger heating capacity under rated operation conditions; ∆t cp is average temperature drop; k is heat transfer coefficient; ψ is correction factor; ∆t lg is logarithmic temperature drop.
The heat exchanger performance depends to a large extent on the routing of coolant relative flows and the features of the hydrodynamics and heat transfer processes. The heat MATEC Web of Conferences 346, 03028 (2021) ICMTMTE 2021 https://doi.org/10.1051/matecconf /202134603028 transfer in tube bundles will be different under equal hydrodynamic conditions. The formulas for determining the heat exchange surface of the tube bundle, in relation to the designs specified, Fig. 1, will be as follows.
For bare tubes it is For finned tubes it is Formulas (2) and (3) where Н e is estimated heat exchange surface; k s is safety factor, k s = 1.1÷1.2. The design heat exchange surface for the tube bundle that consists of an equal number of the tubes with different finning ratio is a constant value, other things being equal. It characterizes the heat exchanger performance.
Then subject to (8) it is true that Hence, it becomes possible to experimentally benchmark the influence of heat exchanging tube finning ratio.
Thermal calculation and heat transfer parameters are determined according to the recommendations of [9]. Given that, we set the thermal and hydraulic testing conditions.