Air-cooled Heat Exchanger Research Articles

Enhanced performance of air-cooled thermal power plants using low temperature thermal storage

This paper presents the essentials of low temperature thermal storage (LTTS), a novel technique whereby thermal energy storage is employed to achieve sub-ambient condensation in air-cooled Rankine cycle power plants. It summarises work which was undertaken to explore the potential and the range of application of LTTS. The technology is most effective at geographical locations with large average daily temperature ranges, and high summertime temperatures. Hourly normal temperature data was sourced for five potential deployment sites, which provided a representative sample of different climate types. A steam turbine, a condenser, an air-cooled heat exchanger, and a chilled water thermal energy storage tank formed the LTTS configuration – a techno-economic model of which was developed to simulate system behaviour. The size of the air-cooled heat exchanger, the fan speed of the air-cooled heat exchanger, and the hours of charge, discharge, and bypass of the thermal energy storage tank were all modelled as variables to determine the effects of component sizes and operating patterns. LTTS performance was benchmarked by comparison with a direct air-cooled condenser model. Results presented in this paper include daily plant output, annual power output, and payback period. This study shows that LTTS can deliver all the advantages of dry-cooling, without suffering the usual performance degradations. The inherent flexibility of LTTS allows for configurations to be customised to exploit the prevailing site climate, and capitalise on the local power demand pattern. There are also clear indications that, in suitable climatic settings, LTTS outperforms traditional air-cooled thermal power plants by offering up to 10% additional generating capacity. Coupled to this are payback periods as short as 2.5 years, ensuring LTTS can be considered a viable alternative to current air-cooling strategies.

Effect of cooling system design on the performance of the recompression CO2 cycle for concentrated solar power application

The thermal performance of a supercritical CO2 (sCO2) recompression cycle is expressively influenced by main compressor inlet temperature. Design of the cooling system is imperative since the compressor inlet temperature substantially influence the system performance. Due to nonlinear variation of both thermal and transport properties of the CO2 under critical condition, the cooling tower design and selection for the sCO2 cycle power plant is quite different from the power plants with steam cycle. The present work comprehensively investigates the effect of cooling system design on the optimal cycle performance under different operating condition. An iterative section method is applied while designing and optimizing the air-cooled heat exchanger bundles inside the tower. Prior to the design of natural draft dry cooling tower (NDDCT), an optimal operating condition is rectified at which the cycle efficiency is maximal. The tower performance is investigated by demonstrating unit height heat rejection and average heat rejection by each heat exchanger bundle. A detailed economic analysis of NDDCT is performed which takes account of capital cost, maintenance cost, annual cost, and specific investment cost. The thermo-economic assessment of the NDDCT is conducted by the influence of sCO2 inlet temperature inside the tower and variation of ambient air.

Air pre-heating for anti-freezing of air-cooled heat exchanger by spraying flue gas

In cold winter, natural draft dry cooling system (NDDCS) confronts a severe freezing risk due to high cooling capacity of ambient air. In this work, the anti-freezing of air-cooled heat exchanger by spraying flue gas at the inlet of cooling deltas to pre-heat ambient air is proposed. With the coupled heat transfer models of steam condenser and NDDCS, the cooling performance of air-cooled heat exchanger and the minimum outlet water temperatures of various sectors are obtained at three typical low ambient temperatures, based on which the flue gas spraying strategies for the potential frozen sectors are analyzed. At not very low ambient temperature of 268.15 K, the flue gas should be sprayed for the frontal and middle front sectors at the wind speeds of 8 m/s and 12 m/s. At 263.15 K, the frozen sectors differ greatly as the wind speed increases, besides some non-frozen sectors should also be pre-heated. As the ambient temperature drops to 258.15 K, more flue gas should be sprayed at all wind speeds due to the increased air cooling capacity. By spraying flue gas, the pre-heated air can contribute to the safe and energy efficient operation of NDDCS in frozen days.

Topical Problems Concerned with the Thermophysical Characteristics of New-Generation Light Water Reactors: Comprehensive Study Results

The article presents and analyses the results from first-priority thermophysical investigations aimed at assessing the characteristics and safety of water-cooled reactors, including enhancement of the nuclear fuel technical and economic characteristics, design solutions for passive safety features, and stable operation of the passive heat removal system (PHRS) air-cooled heat exchangers under extreme meteorological conditions. It is shown that the fuel assembly (FA) critical power level depends both on the type and number of spacer and mixing grids and on the burnout occurrence spot. The critical heat fluxes that occur when the coolant flowrate drops by 4% are higher in unsteady modes than they are in steady modes of operation at the same flow parameters. Information about the upgraded base of estimated experimental data on the critical heat fluxes in the new FA designs is given. The article also presents data on pressure variations in the main components of the RU-ZO experimental bench (containing the reactor plant and containment models) in the experiment modeling an accident involving reactor coolant pipeline (RCPL) rupture at the reactor inlet (scenario 1) and decrease of the steam generator (SG) model condensing power in the bench (scenario 2). It is found that the drop of SG condensing power is approximately 30% of its level at the onset of the SG condensing operation mode during accidents according to both the scenarios. However, the remaining condensing power level is sufficient for reliably cooling the VVER-1200 reactor core for no less than 24 h with joint operation of the second-stage hydro accumulators (HA-2) and PHRS. Experimental investigations of the external aerodynamic characteristics of the Novovoronezh-2 NPP (NVNPP-2) main building aimed at checking if proper operation of the PHRS is secured independently of the wind load have shown that extreme meteorological conditions, including hurricane winds and dust storms, may manifest themselves in a decreased capacity of part of the PHRS air-cooled heat exchangers. Studies carried out for assessing the design of a water-cooled nuclear power facility (NPF) for an electrical power capacity of 1700 MW operating at supercritical parameters and the experimental VVER-SKD-30 reactor with a thermal power capacity of 30 MW are mentioned. The problems that have to be solved in carrying out further thermophysical investigations of water-cooled reactors for supercritical parameters are listed.

Influencing Mechanisms of a Crosswind on the Thermo-Hydraulic Characteristics of a Large-Scale Air-Cooled Heat Exchanger

For the large scale air-cooled heat exchanger of a natural draft dry cooling system (NDDCS) in power plants, its thermo-flow characteristics are basically dominated by crosswinds. Unfortunately however, the detailed mechanisms of the crosswind effects have yet to be fully uncovered. Therefore, in this research, the local flow and heat transfer performances of the cooling deltas, which are also termed as the fundamental cells of the large-scale air-cooled heat exchanger, are specifically investigated with full consideration for the cell structure and the water-side temperature distribution at various wind speeds. A 3D CFD method with a realizable k-ε turbulence model, heat exchanger model, and porous media model is developed, and the accuracy and credibility of the numerical model are experimentally validated. With the numerical simulation, the overall 3D outlet air temperature of the large-scale air-cooled heat exchanger, and the corresponding local air velocity and temperature fields of the cooling deltas are qualitatively analyzed. Furthermore, the air-mass flow rate and heat rejection are also quantitatively studied at both the global and local views. The results depict that with an increase in the wind speed, the air mass flow rate and heat rejection will increase greatly for the frontal deltas; however, they will drop dramatically for the middle-front deltas. As for the middle- as well as the middle-rear deltas, the thermo-flow performances vary markedly at various wind speeds, which behave in the most deteriorated manner at a wind speed of 12 m/s. The rear deltas show the best thermo-flow performances at a wind speed of 12 m/s, but the worst at 16 m/s. A detailed analysis of the variable fields for each cooling delta may contribute to the performance improvement of the large-scale air-cooled heat exchanger of NDDCS.

Anti-freezing of air-cooled heat exchanger with rolling-type windbreaker

On cold days with even high wind speeds, the excessive cooling capacity of ambient air may result in a freezing risk for air-cooled heat exchanger. In this work, an anti-freezing approach is proposed based on the air flow control by pulling-down adjustment of rolling-type windbreakers, to ensure that the lowest outlet water temperature of each sector in air-cooled heat exchanger is not less than 0 °C. The coupled heat transfer models of natural draft dry cooling system with steam condenser are developed and experimentally validated. The opening degrees of rolling-type windbreakers and air mass flow rates for various sectors are obtained, and the outlet water temperature of each sector with rolling-type windbreakers is calculated. The correlating equations of the opening degree with wind speed at various ambient temperatures are also fitted. The results show that the opening degree of rolling-type windbreaker for each sector varies widely with the ambient temperature and wind speed. At the ambient temperature of −5 °C, only the windbreakers of windward sectors are regulated. While at −10 °C and −15 °C, the windbreakers of lateral and leeward sectors should also be adjusted. The configuration and regulation of rolling-type windbreakers can realize the anti-freezing of air-cooled heat exchanger with the low cost and high energy-efficiency.

Performance Test of the Air-Cooled Finned-Tube Supercritical CO2 Sink Heat Exchanger

This technical paper presents results of an air-cooled supercritical CO2 (sCO2) finned-tube sink heat exchanger (HX) performance test comprising wide range of variable parameters (26–166 °C, 7–10 MPa, 0.1–0.32 kg/s). The measurement covered both supercritical and subcritical pressures including transition of pseudocritical region in the last stages of the sink HX. The test was performed in a newly built sCO2 experimental loop which was constructed within Sustainable Energy (SUSEN) project at Research Centre Rez (CVR). The experimental setup along with the boundary conditions are described in detail; hence, the gained data set can be used for benchmarking of system thermal hydraulic codes. Such benchmarking was performed on the open source Modelica-based code ClaRa. Both steady-state and transient thermal hydraulic analyses were performed using the simulation environment DYMOLA 2018 on a state of the art PC. The results of calculated averaged overall heat transfer coefficients (using Gnielinski correlation for sCO2 and IPPE or VDI for the air) and experimentally determined values shows reasonably low error of + 25% and – 10%. Hence, using the correlations for the estimation of the heat transfer in the sink HX with a similar design and similar conditions gives a fair error and thus is recommended.

To broaden safe operation range of the indirect dry cooling system with internal annular windbreak cloth in extremely cold days

As for the natural draft dry cooling tower, air leakage is inevitable in practical running, even though all louvers in front of the air-cooled heat exchanger closed entirely. To reduce the adverse effects of air leakage on safe operation of the indirect dry cooling system in extremely cold days, a novel annular windbreak cloth is proposed to be arranged on the horizontal cross section of the tower at air inlet height. By numerical simulation, flow fields in and around the tower are presented, and cooling water temperature drop is exported as well, under different sizes of the annular windbreak cloth. By the analysis of flow fields, it was found that the gradual increasing resistance caused by the windbreak cloth and the external airflow intrusion appearing at the top of the tower as ventilation area reduces to a critical value, would greatly reduce thermal performance of the tower. Then, coupling the tower with the condenser, ranges of ambient temperature are determined for safe operation of the indirect dry cooling system.

Hybrid geothermal heat pumps for cooling telecommunications data centers

The technical and economic performance of hybrid geothermal heat pump (GHP) systems supplying year-round cooling to small data centers were analyzed and compared to air-source heat pump (ASHP) cooling systems. Numerical TRNSYS models were used to simulate the operation of five configurations that included GHPs, ASHPs, air-cooled heat exchangers – dry coolers (DCs), and/or waterside economizers (WSEs). The models were validated against data measured from an experimental GHP cooling system in Ithaca, NY, USA. The average coefficient of performance (COP) and the total cost of ownership (TCO) of the optimized systems were evaluated and compared to a base case ASHP cooling system. The results showed that the GHP systems had higher lifetime average COPs than the ASHP system, while the TCOs of the GHP systems were slightly higher than the ASHP system. The reductions in cooling capacity and performance of the geothermal systems due to subsurface temperature increases were calculated and showed that the addition of a DC or WSE mitigated capacity reductions and improved lifetime performance. The economic performance of the GHP systems relative to ASHP systems were analyzed using weather data from several locations across the U.S., with sensitivities to soil conductivity, price of electricity, and cost of drilling the geothermal wells quantified. GHP systems performed best in colder climates with greater temperature variability both seasonally and diurnally.

Exergoeconomic optimisation of an air cooled heat exchanger with copper oxide nanoparticles

In the current study, the effect of suspended CuO nanoparticles in a fluid on the exergoeconomic and thermoeconomic properties of an air-cooled heat exchanger (ACHE) is investigated. Particle swarm optimisation algorithm is employed for multi-objective optimisation where the exergy efficiency and effectiveness versus total annual cost (TAC) are considered as two sets of objective functions for exergoeconomic and thermoeconomic optimisation, respectively. For both cases, the results are compared with the base fluid, which has no nanoparticles. Results show that nanoparticles have no meaningful effect on the exergoeconomic improvement of ACHE while a significant improvement is observed for the thermoeconomic optimisation. Also, it is shown that CuO nanoparticle added into the base fluid can decreases the heat exchanger total annual cost and increase the heat exchanger effectiveness.

Exergoeconomic optimisation of an air cooled heat exchanger with copper oxide nanoparticles

Exergoeconomic optimisation of an air cooled heat exchanger with copper oxide nanoparticles

Experimental and numerical investigation of using pulsating heat pipes instead of fins in air-cooled heat exchangers

The goal of this study is the experimental and numerical investigation of using pulsating heat pipes (PHPs) as substitutes for fins in a typical air-cooled heat exchanger. Due to the small temperature difference between the cooling air and internal airflow in the case study, it was necessary that appropriate PHPs with the capability to work in small temperature difference conditions be investigated and an appropriate working fluid be selected. After reviewing various studies, R134a was selected as the best working fluid from the heat transfer standpoint. Afterward, a testing device was designed and fabricated. Additionally, in order to predict the behaviors of the main tubes with pulsating heat pipes and also finned tubes, the computational fluid dynamics was used and pulsating heat pipes and fins were simulated and compared. Different tests were performed with various air flow rates and temperatures. The results indicated that using pulsating heat pipes as fins have a significant effect on improving the heat transfer. In free convection (without external flow over the tubes), using the heat pipes resulted in a better performance in comparison to fins and in the best case, the overall heat transfer coefficient was improved by 310%. This improvement was approximately 263% for the forced convection condition.

Performance Testing of an Axial Flow Fan Designed for Air-Cooled Heat Exchanger Applications

An axial flow fan developed in the previous study is tested in order to characterize its performance. The M-fan, a 7.3152 m diameter rotor only axial flow fan was designed to perform well under the challenging operating conditions encountered in air-cooled heat exchangers. Preliminary computational fluid dynamics (CFD) results obtained using an actuator disk model (ADM) as well as a periodic three dimensional model indicate that the fan meets the specified performance targets, with an expected total-to-static efficiency of 59.4% and a total-to-static pressure rise of 114.7 Pa at the operating point. Experimental tests are performed on the M-fan in order to determine its performance across a full range of flow rates. A range of fan configurations are tested in order to ascertain the effect of tip clearance, blade angle, and hub configuration on fan performance. Due to the lack of a suitable facility for testing a large diameter fan, a scaled 1.542 m diameter model is tested on the ISO 5801 type A fan test facility at Stellenbosch University. A Reynolds-averaged Navier–Stokes CFD model representing the M-fan in the test facility is also developed in order to provide additional insight into the flow field in the vicinity of the fan blades. The results of the CFD model will be validated using the experimental data obtained. Both the CFD results and the experimental data obtained are compared to the initial CFD results for the full scale fan, as obtained in the previous study, by means of fan scaling laws. Experimental data indicate that the M-fan does not meet the pressure requirement set out in the initial study at the design blade setting angle of 34 deg. Under these conditions, the M-fan attains a total-to-static pressure rise of 102.5 Pa and a total-to-static efficiency of 56.4%, running with a tip gap of 2 mm. Increasing the blade angle is shown to be a potential remedy, improving the total-to-static pressure rise and efficiency obtained at the operating point. The M-fan is also shown to be highly sensitive to increasing tip gap, with larger tip gaps substantially reducing fan performance. The losses due to tip gap are also shown to be overestimated by the CFD simulations. Both experimental and numerically obtained results indicate lower fan total-to-static efficiencies than obtained in the initial CFD study. Results indicate that the M-fan is suited to its intended application, however, it should be operated with a smaller tip gap than initially recommended and a larger blade setting angle. Hub configuration is also shown to have an influence on fan performance, potentially improving performance at low flow rates.

Heat Transfer Investigation of a Tube Partially Wrapped by Metal Porous Layer as a Potential Novel Tube for Air Cooled Heat Exchangers

Laminar forced convection heat transfer from a constant temperature tube wrapped fully or partially by a metal porous layer and subjected to a uniform air cross-flow is studied numerically. The main aim of this study is to consider the thermal performance of some innovative arrangements in which only certain parts of the tube are covered by metal foam. The combination of Navier–Stokes and Darcy–Brinkman–Forchheimer equations is applied to evaluate the flow field. Governing equations are solved using the finite volume SIMPLEC algorithm and the effects of key parameters such as Reynolds number, metal foam thermophysical properties, and porous layer thickness on the Nusselt number are investigated. The results show that using a tube which is fully wrapped by an external porous layer with high thermal conductivity, high Darcy number, and low drag coefficient, can provide a high heat transfer rate in the high Reynolds number laminar flow, increasing the Nusselt number almost as high as 16 times compared to a bare tube. The most important result of thisstudy is that by using some novel arrangements in which the tube is partially covered by the foam layer, the heat transfer rate can be increased at least 20% in comparison to the fully wrapped tube, while the weight and material usage can be considerably reduced.

Estimating Recoverable Performance Degradation Rates and Optimizing Maintenance Scheduling

Many of the components on a gas turbine are subject to fouling and degradation over time due to debris buildup. For example, axial compressors are susceptible to degradation as a result of debris buildup on compressor blades. Similarly, air-cooled lube oil heat exchangers incur degradation as a result of debris buildup in the cooling air passageways. In this paper, we develop a method for estimating the degradation rate of a given gas turbine component that experiences recoverable degradation due to normal operation over time. We then establish an economic maintenance scheduling model, which utilizes the derived rate and user input economic factors to provide a locally optimal maintenance schedule with minimized operator costs. The rate estimation method makes use of statistical methods combined with historical data to give an algorithm with which a performance loss rate can be extracted from noisy data measurements. The economic maintenance schedule is then derived by minimizing the cost model in user specified intervals and the final schedule results as a combination of the locally optimized schedules. The goal of the combination of algorithms is to maximize component output and efficiency, while minimizing maintenance costs. The rate estimation method is validated by simulation where the underlying noisy data measurements come from a known probability distribution. Then, an example schedule optimization is provided to validate the economic optimization model and show the efficacy of the combined methods.

Heat Transfer of Circular Finned Tubes with Nonuniform Operational Fouling of the Interfin Space

This paper presents the results of a theoretical calculation of the heat transfer of finned tubes for air-cooled heat exchangers having external operational foulings. To determine the heat resistance of the fouling layer, a problem on the temperature distribution in the ring layer has been formulated and solved. The minimum and the surface-average temperatures of the fouling, the heat resistance of the ring layer, and the heat transfer coefficient of the finned tube heat exchanger have been calculated. It has been shown that the heat transfer coefficient is the most strongly influenced by the fouling layer thickness at the base of the fins.

Correction to: Fiber Orientation Effects in Fused Filament Fabrication of Air-Cooled Heat Exchangers

Correction to: Fiber Orientation Effects in Fused Filament Fabrication of Air-Cooled Heat Exchangers

Design and performance study of dry cooling system for 25 MW solar power plant operated with supercritical CO2 cycle

Dry natural draft cooling towers are mostly employed in thermal power plants in arid areas with no energy consumption and maintenance compared with the mechanical draft cooling system. Different types and shapes of natural draft dry cooling towers (NDDCT) are available in many thermal and nuclear power plant industries. The main purpose of NDDCT is to generate air flow through the finned tube heat exchanger bundles by the influence of buoyancy due to the density difference between the ambient air and the warm air inside the tower. In the present chapter, a one-dimensional MATLAB code is developed to design a NDDCT with detailed specification of finned tube heat exchanger bundles for a 25 MW solar plant operated with supercritical CO2 (sCO2) Brayton cycle. The tower height, the tower inlet and outlet diameters and the number of heat exchanger bundles are evaluated after accomplishing the design requirements. The performance study of an air-cooled heat exchanger is performed for a range of sCO2 inlet temperature from 71 °C to 91 °C, operating pressure from 7.5 MPa to 9 MPa and ambient air temperature from 20 °C to 50 °C. The sCO2 inlet temperature and operating pressure to the heat exchanger significantly affect the cooling system performance. During high ambient temperature period, the cooling potential of NDDCT is significantly reduced.

Design and comparison of direct and indirect cooling system for 25 MW solar power plant operated with supercritical CO2 cycle

A validated MATLAB code is developed to design a dry cooling unit for a 25 MW solar power plant operated with supercritical CO2 (sCO2) Brayton cycle. Both direct and indirect cooling systems are designed for the power plant and their performance are compared. For direct cooling system, air cooled heat exchanger unit is designed whereas for indirect cooling system, optimum size of shell and tube heat exchanger is selected addressing the retainment of shell side heat transfer correction factor within a reasonable limit. The nonlinear property variation of sCO2 near the critical condition is reasonably well captured by the present code in both shell and tube and air cooled heat exchanger units. The comparative study of direct and indirect cooling system is performed for inlet sCO2 temperature of 71 °C, operating pressure of 7.5 MPa and ambient air temperature from 15 °C to 40 °C. For the same heat rejection, indirect cooling system requires a much higher cooling tower. During high ambient temperature period, direct dry cooling system shows superior cooling performance in terms of lower sCO2 outlet temperature compared to indirect cooling system.

Numerical study on thermal performance of an air-cooled heat exchanger: Effects of hybrid nanofluid, pipe arrangement and cross section

This study is a numerical investigation of a heat exchanger under external flow. Temperature variable properties are used to solve the governing equations. Air is selected as an external fluid and water and MgO-MWCNTs/EG hybrid nanofluid are selected as radiator fluid. In this paper, the efficiency of the horizontal and vertical radiator are studied using experimental data for thermo-physical properties of MgO-MWCNTs/EG Nanofluid. Then, tubes with circular and elliptic sections are investigated for different flows, inlet temperatures and concentrations. The pressure drop and Nusselt number for each simulation are analyzed. Results are presented in terms of temperature distribution, pressure drop, Nusselt number and their sensitivity analysis for different states. The results indicate that radiators with vertical tubes have better efficiency up to 10% than radiators with horizontal tubes. In addition, heat exchangers with circular pipes have 25% lower pressure drop and those with elliptical tubes had 10% higher Nusselt number. The finding shows that an increase in the Nano-fluid concentration improves the Nusselt number and increases the pressure drop.