Finned Tube Bundles Research Articles

Impacts of Water Flow Rate on Freezing Prevention of Air-Cooled Heat Exchangers in Power Plants

Under cold ambient conditions, the freezing risk of air-cooled heat exchangers, especially the frontal finned tube bundles, has been a critical concern in power plants. Based on the freezing conditions of the cooling deltas under windy conditions, the flow and heat transfer characteristics of natural draft dry cooling system (NDDCS) with 30%, 40% and 50% increased water flow rates are investigated in this work, and the outlet circulating water temperatures of the easily freezing cooling deltas and sectors are obtained. The results show that the deltas in the middle front and rear sectors become free from freezing at all wind speeds when the circulating water flow rate is increased. For the frontal sector with increased water flow rate, the outlet water temperatures of deltas increase conspicuously at 4 m/s and 8 m/s, while as the wind speed rises to 16 m/s, these deltas still face serious freezing risks due to the huge heat rejection to ambient air. Therefore, freezing prevention of air-cooled NDDCS heat exchangers can be achieved by increasing the water flow rates at small wind speeds, while as the wind speed becomes high, the water flow redistribution is suggested for the frontal and middle sectors due to their big performance difference.

Improvement air condensers evaluation model

The results of analysis of the literature on the calculation of the heat transfer coefficient of an air condenser in the flow past a bundle of finned tubes by an air flow. The methods of calculation are disassembled, marked advantages and disadvantages of each. Calculations of the heat transfer coefficient for each method are given; the results compared with the experimental data.

HEAT AND MASS TRANSFER AND RESISTANCE CHARACTERISTICS OF EVAPORATIVE AIR COOLER WITH HIGH FINNED-TUBE BUNDLES

A theoretical model was suggested and experimental analysis of heat and mass exchange mechanism of a high finned-tube evaporative air cooling device was made. Experiments showed that the heat transfer of the upper four rows of tubes was higher than that of the lower four rows. The upper and lower layers of the proportion of water spray experiments were carried out. In the experimental range, the higher the proportion of the upper water spray, the greater the overall heat transfer. Besides, evaporation and heat transfer of water film were not only related to the air flow and spray water intensity, but also were affected by the relative humidity of imported air, with decrease in the relative humidity of imported air decreased, and with evaporation and heat transfer between air and water film strengthened. According to the experiments, the correlations of evaporation loss and heat transfer were fitted by air flow, spray water intensity, and relative humidity, which showed close agreement with the experimental results to within ± 11% and ± 9%, respectively. The correlation of pressure drop in the high finned-tube bundle was related to air flow rate and the intensity of the spray water, and the fitting error was within ± 8%. This study was instructive for the design of wet air cooler with high finned-tube bundle.

Anti-Freezing Mechanism Analysis of a Finned Flat Tube in an Air-Cooled Condenser

In cold winter weather, the air-cooled condensers (ACCs) face serious freezing risks, especially with part load of the power generating unit. Therefore, it is of benefit to investigate the heat transfer process between the turbine exhaust steam and cooling air, by which the freezing mechanism of the finned tube bundles can be revealed. In this work, the flow and heat transfer models of the cooling air coupling with the circulating water, are developed and numerically simulated for the anti-freezing analysis on basis of the finned tube bundles of the condenser cell. The local air-side heat transfer coefficient, condensate film development, and non-condensable gas development are obtained and analyzed in detail. The results show that, the most freezing risk happens at the fin base due to the highest air-side cooling capacity, besides the windward velocity, ambient temperature and turbine back pressure all determine the freezing risk with the constant inlet flow rate of the non-condensable gas. Furthermore, increasing fin thickness and decreasing fan rotating speed are the most effective anti-freezing measures. Additionally, increasing turbine back pressure can also be adopted to avoid ACC freezing, however the adjustment of outlet steam-air flow is not recommended.

Numerical studies on heat transfer and pressure drop characteristics of flat finned tube bundles with various fin materials

The air-cooled heat exchanger plays an important role in the field of industry like for example in thermal power plants. On the other hand, it can be used to remove core decay heat out of containment passively in case of a severe accident circumstance. Thus, research on the performance of fins in air-cooled heat exchangers can benefit the optimal design and operation of cooling systems in nuclear power plants. In this study, a CFD (Computational Fluid Dynamic) method is implemented to investigate the effects of inlet velocity, fin spacing and tube pitch on the flow and the heat transfer characteristics of flat fins constructed of various materials (316L stainless steel, copper-nickel alloy and aluminium). A three dimensional geometric model of flat finned tube bundles with fixed longitudinal tube pitch and transverse tube pitch is established. Results for the variation of the average convective heat transfer coefficient with respect to cooling air inlet velocity, fin spacing, tube pitch and fin material are obtained, as well as for the pressure drop of the cooling air passing through finned tube. It is shown that the increase of cooling air inlet velocity results in enhanced average convective heat transfer coefficient and decreasing pressure drop. Both fin spacing and tube pitch engender positive effects on pressure drop and have negative effects on heat transfer characteristics. Concerning the fin material, the heat transfer performance of copper-nickel alloy is superior to 316L stainless steel and inferior to aluminium.

Thermo-Hydraulic Performance Evaluation, Field Synergy, and Entransy Dissipation Analysis for Hexagon-Like and Circular-Like Pin Finned Tube Bundles

ABSTRACTIn this paper, a comprehensive thermo-hydraulic performance evaluation for air flow across the hexagon-like and circular-like staggered pin finned tube bundle heat transfer surfaces has been numerically carried out by adopting the performance evaluation plot of enhanced heat transfer techniques oriented for energy-saving. In addition, the simulation results have also been analyzed from the viewpoints of field synergy principle and entransy dissipation extreme principle. The results indicate that the heat transfers are all enhanced based on identical pressure drop for the hexagon-like and circular-like pin finned tube bundles within the inlet velocity range from 1 m/s to 10 m/s studied. Moreover, the circular-like pin finned tube bundle offers the lowest friction factor increase ratio for the same Nusselt number increase ratio. Furthermore, the synergy between velocity and fluid temperature gradient has been proved again, having inherent consistency with the dissipation of entransy.

Research of heat transfer of staggered horizontal bundles of finned tubes at free air convection

The study of free-convective processes is important because of the cooling problem in many machines and systems, where other ways of cooling are impossible or impractical. Natural convective processes are common in the steam turbine air condensers of electric power plants located within the city limits, in dry cooling towers of circulating water systems, in condensers cooled by air and water, in radiators cooling oil of power electric transformers, in emergency cooling systems of nuclear reactors, in solar power, as well as in air-cooling of power semiconductor energy converters. All this makes actual the synthesis of the results of theoretical and experimental research of free convection for heat exchangers with finned tube bundles. The results of the study of free-convection heat transfer for two-, three- and four-row staggered horizontal bundles of industrial bimetallic finned tubes with finning factor of 16.8 and equilateral tubes arrangement are presented. Cross and diagonal steps in the bundles are the same: 58; 61; 64; 70; 76; 86; 100 mm, which corresponds to the relative steps: 1.042; 1.096; 1.152; 1.258; 1.366; 1.545; 1.797. These steps are standardized for air coolers. An equation for calculating the free-convection heat transfer, taking into account the influence of geometrical parameters in the range of Rayleigh number from 30,000 to 350,000 with an average deviation of ± 4.8%, has been obtained. The relationship presented in the article allows designing a wide range of air coolers for various applications, working in the free convection modes.

Operation of air cooled condensers for optimised back pressure at ambient wind

The traditional back pressure regulation of turbine is influenced more seriously by ambient wind and temperature, especially the hot air reflow and high temperature effect on the axial fans of the air-cooled condensers. This variation would result in the cooling capacity of the fans decreasing and the power consumption increasing. The grey difference incremental correlation method is proposed for considering the back pressure regulation problem from a single condenser cell perspective, and analyze the influence on the heat rejection when processing the speed regulation of the axial fans, then the relationship between the heat rejection increment and the influence of the back pressure will be obtained. The flow and temperature fields of the air-cooled condensers can be obtained by applying the CFD method, and the different back pressure in the different fan speed will be calculated. The results show that, larger the fans ventilation rates will not necessarily cause higher the heat rejection of the finned tube bundle. Reasonably improve the speed of the higher relational degree fans can effectively reduce the turbine back pressure and save energy.

Performance evaluation of decay heat removal system of PFBR with partial availability of air side dampers

Decay heat in PFBR is rejected to the atmosphere by the air flowing over the finned tube bundle of sodium to air heat exchangers which is part of the decay heat removal system of the reactor. Air flow over the tube bundle is driven by buoyancy forces generated by a chimney. Dampers with two louvers are provided in the air flow path at upstream and downstream of air heat exchanger to facilitate the heat removal through decay heat removal system on demand and minimise the loss of useful heat when the reactor is in normal operating condition. Experimental studies have been conducted to characterise the heat transport capability of the system when the air dampers are partially available. Hydraulic resistance of the finned tube bundle and air dampers were also evaluated for the operating conditions. From the studies it has been verified that the non-availability of one of the louvers in the two louver damper system is not affecting the heat transport capability of the decay heat removal system. It is also quantified that the hydraulic resistance offered by the finned tube bundle of air heat exchanger is 29% of the total hydraulic resistance of the air flow path.

Performance improvement of natural draft dry cooling system by water flow distribution under crosswinds

The cooling performance of natural draft dry cooling system may be deteriorated by ambient winds, so effective measures need to be taken against the adverse wind effects. In this work, a three-dimensional numerical model of a typical natural draft dry cooling system coupled with the condenser is developed to investigate the thermo-flow performance improvement of natural draft dry cooling system by circulating water flow distribution, in which the finned tube bundles are dealt with the heat exchanger model to take both the air and circulating water into account. The flow and temperature fields, heat rejection of each sector, outlet water temperature of air-cooled heat exchanger and the corresponding turbine back pressures are obtained. The results show that appropriate water flow distribution can significantly improve the thermo-flow performances of natural draft dry cooling system, while the improper one may deteriorate the cooling performance more seriously at low wind speeds. Based on the entransy dissipation theory, the optimized heterogeneous water distributions are provided under five certain wind conditions by using the numerical-theoretical iterative method.

A Generalized Dependence for Convective Heat Transfer in Horizontal Bundles of Finned Tubes with Free Air Convection

Results of a study of free convective heat transfer in two-, three-, and four-row horizontal in-line bundles of bimetallic finned tubes with coefficient of finning 16.8 are presented. The steps in the breakdown in the tubular sheets correspond to the applicable standardized steps in air-cooled heat exchangers with cross step S1 = 58–76 mm, and values of the longitudinal step S2 in the range 58–100 mm correspond to each value of S1. A generalizing equation for calculating the average convective heat-transfer coefficient, taking into account geometric parameters studied in the range of Reynolds numbers from 20000 to 70000, is obtained. Through the use of the dependence, it has become possible to design a wide range of gas–liquidtype heat exchangers for different purposes that function in free convection regimes.

Impacts of geometric structures on thermo-flow performances of plate fin-tube bundles

Continuous plate fin-tube heat exchangers are commonly applied to natural draft dry cooling system in power plants due to their excellent thermo-flow performances, but the effects of geometrical structures on the air-side flow and heat transfer characteristics have not been thoroughly clarified yet. In this study, the air-side flow and heat transfer models of plate fin-tube bundles are developed, and the numerical modeling and methods are validated by the experimental test. The thermo-flow characteristics of the plate fin-tube bundles at various parameters of air face velocity, fin oblique angle, fin spacing, fin thickness and tube external diameter are numerically simulated. The buoyancy effect due to natural convection is also investigated. The results show that the performance evaluation criterion (PEC) varies little with the fin oblique angle. As the fin spacing increases, the average PEC will increase despite of a considerable heat transfer reduction. The increased fin thickness and tube external diameter are both beneficial to the thermo-flow characteristics, but the buoyancy effect on the flow and heat transfer performances can be neglected. The fin oblique angle of 30°, fin spacing of 10 mm, fin thickness of 0.3 mm and tube external diameter of 18 mm are recommended within the scope of investigation. The correlations between the Nusselt number, friction factor and the Reynolds number and geometrical parameters are recommended using the orthogonal experimental design method, which can be used to the optimal design of air-cooled heat exchanger in power plants.

Air-side flow and heat transfer characteristics of flat and slotted finned tube bundles with various tube pitches

A three-dimensional numerical investigation was carried out to clarify the air-side flow and heat transfer performances of flat and slotted finned tube bundles, which was validated by the experimental test. For both the flat and slotted fin geometries, fifty models of finned tube bundles at various longitudinal tube pitches (25mm, 30mm, 35mm, 40mm and 45mm) and transverse tube pitches (25mm, 30mm, 35mm, 40mm and 45mm) were developed and analyzed at different air inlet velocities, based on which the correlations for f, Nu and PEC were obtained respectively. The results show that the finned tube bundles with slotted fins are superior to those with flat fins in heat transfer performance, but present a high pressure drop. The friction factor varies little with the tube pitch, however the Nusselt number decreases conspicuously with increasing the longitudinal and transverse tube pitches. The variations of the performance evaluation criterion with the transverse and longitudinal tube pitches show that better thermo-flow performances can be achieved by reducing the tube pitch. From the actual needs of engineering, the optimal configurations of the finned tube bundles with the tube pitches from 35mm to 40mm are recommended for air-cooled heat exchangers by using the system performance criterion.

Research on Performance of Finned Tube Bundles of Indirect Air-cooled Heat Exchangers

Research on Performance of Finned Tube Bundles of Indirect Air-cooled Heat Exchangers

Effects of continuous and alternant rectangular slots on thermo-flow performances of plain finned tube bundles in in-line and staggered configurations

Thanks to the high heat transfer performance and compact structure, the finned tube bundles with enhanced slotted plain fins have received more and more attention in the air-cooled heat exchangers of natural draft dry cooling system in power plants. In this paper, two kinds of rectangular slot configurations, continuous slots and alternating slots, were presented and the effects of the slots on the air-side thermo-flow performances of the plain finned tube bundles in in-line and staggered configurations were analyzed by means of numerical simulations, which were validated by the experiment. The performance evaluation criterion was applied to predict the improvement of the thermo-flow performances of plain finned tube bundles by slot configurations. The results show that the slot on the fin surface can cause a considerable heat transfer augmentation of finned tube bundles with a modest pressure loss penalty when compared to the plain fin. However, the slotted fin only improves the heat transfer for the main-flow zones. Due to the relatively drastic swirled flow generation, the slotted fin with continuous rectangular strips shows a better performance than the alternant upper and lower slotted fin, although the latter has a higher heat transfer capability. For the overall performance, the finned tube bundles in a staggered pattern are superior to those in an in-line one. The staggered fin-and-tube bundles with continuous slotted rectangular strips possess best overall performance, so are recommended to apply to the air-cooled heat exchanger.

A review on the thermal hydraulic characteristics of the air-cooled heat exchangers in forced convection

In this paper, a review is presented on the experimental investigations and the numerical simulations performed to analyze the thermal-hydraulic performance of the air-cooled heat exchangers. The air-cooled heat exchangers mostly consist of the finned-tube bundles. The primary role of the extended surfaces (fins) is to provide more heat transfer area to enhance the rate of heat transfer on the air side. The secondary role of the fins is to generate vortices, which help in enhancing the mixing and the heat transfer coefficient. In this study, the annular and plate fins are considered, the annular fins are further divided into four categories: (1) plane annular fins, (2) serrated fins, (3) crimped spiral fins, (4) perforated fins, and similarly for the plate fins, the fin types are: (1) plain plate fins, (2) wavy plate fins, (3) plate fins with DWP, and (4) slit and strip fins. In Section 4, the performance of the various types of fins is presented with respect to the parameters: (1) Reynolds number, (2) fin pitch, (3) fin height, (4) fin thickness, (5) tube diameter, (6) tube pitch, (7) tube type, (8) number of tube rows, and (9) effect of dehumidifying conditions. In Section 5, the conclusions and the recommendations for the future work have been given.

Upgrading the SPP-500-1 moisture separators-steam reheaters used in the Leningrad NPP turbine units

The specific features of existing designs of moisture separators-steam reheaters (MSRs) and experience gained with using them at nuclear power plants are considered. Main factors causing damage to and failures of MSRs are described: nonuniform distribution of wet steam flow among the separation modules, breakthrough of moisture through the separator (and sometimes also through the steam reheater), which may lead to the occurrence of additional thermal stresses and, hence, to thermal-fatigue damage to or stress corrosion cracking of metal. MSR failure results in a less efficient operation of the turbine unit as a whole and have an adverse effect on the reliability of the low-pressure cylinder’s last-stage blades. By the time the design service life of the SPP-500-1 MSRs had been exhausted in power units equipped with RBMK-1000 reactors, the number of damages inflicted to both the separation part and to the pipework and heating surface tubes was so large, that a considerable drop of MSR effectiveness and turbine unit efficiency as a whole occurred. The design of the upgraded separation part used in the SPP-500-1 MSR at the Leningrad NPP is described and its effectiveness is shown, which was confirmed by tests. First, efforts taken to achieve more uniform distribution of moisture content over the perimeter and height of steam space downstream of the separation modules and to bring it to values close to the design ones were met with success. Second, no noticeable effect of the individual specific features of separation modules on the moisture content was revealed. Recommendations on elaborating advanced designs of moisture separators-steam reheaters are given: an MSR arrangement in which the separator is placed under or on the side from the steam reheater; axial admission of wet steam for ensuring its uniform distribution among the separation modules; inlet chambers with an extended preliminary separation system and devices for uniformly distributing steam flows in the separator; separated layout of the of the separator and steam reheater; and use of transversely finned tube bundles for organizing cross flow of steam over the tubes.

Condensation Characteristics of Flue Gas/Steam Mixtures with Fine Lignite and Ash Particles along Horizontal Finned Tube Bundles

The condensation behavior for a gas/steam mixture with fine lignite particles and lignite ash particles is experimentally investigated as the particles flow over horizontal finned tube bundles. The effects of the gas velocity, inlet temperature of cooling water, excess air coefficient, and particle dimension are discussed. The total mass flow rate of the condensate and the condensation heat transfer coefficient for flue gas including particles are higher than those of flue gas excluding particles when Reynolds number is higher than 2,300. The area covered by ash depositions tends to grow from the leeward toward the windward side with increasing particle diameter.

Design of a natural draft air-cooled condenser and its heat transfer characteristics in the passive residual heat removal system for 10 MW molten salt reactor experiment

As one of the Generation IV reactors, Molten Salt Reactor (MSR) has its superiorities in satisfying the requirements on safety. In order to improve its inherent safety, a concept of passive residual heat removal system (PRHRS) for the 10 MW Molten Salt Reactor Experiment (MSRE) was put forward, which mainly consisted of a fuel drain tank, a feed water tank and a natural draft air-cooled condenser (NDACC). Besides, several valves and pipes are also included in the PRHRS. A NDACC for the PRHRS was preliminarily designed in this paper, which contained a finned tube bundle and a chimney. The tube bundle was installed at the bottom of the chimney for increasing the velocity of the air across the bundle. The heat transfer characteristics of the NDACC were investigated by developing a model of the PRHRS using C++ code. The effects of the environmental temperature, finned tube number and chimney height on heat removal capacity of the NDACC were analyzed. The results show that it has sufficient heat removal capacity to meet the requirements of the residual heat removal for MSRE. The effects of these three factors are obvious. With the decay heat reducing, the heat dissipation power declines after a short-time rise in the beginning. The operation of the NDACC is completely automatic without the need of any external power, resulting in a high safety and reliability of the reactor, especially once the accident of power lost occurs to the power plant.

Can CFD accurately predict the heat-transfer and pressure-drop performance of finned-tube bundles?

A Computational Fluid Dynamics (CFD) model capable of predicting heat-transfer and pressure-drop performance of finned-tube bundles in cross flow is presented. Three helically-wound fin geometries are investigated: solid (continuous); partially-serrated; and fully-serrated. A steady-state approach, with a two-equation turbulence model, is employed to examine Reynolds numbers in the range of 5000–30,000. The external Nusselt number and overall pressure drop predicted by the CFD model are compared with those predicted by published empirical correlations. The CFD results fall within the range of values predicted by the empirical correlations. The CFD results show that the Nusselt number increases by up to 23% between the partially- and fully-serrated fins; this sensitivity to serration geometry is not captured in any currently published empirical correlations.