Air-cooled Heat Exchanger Research Articles

Modeling and analysis of air-cooled heat exchanger integrated with supercritical carbon dioxide recompression Brayton cycle

This paper investigates the thermal performance of the recompression supercritical carbon dioxide power cycle integrated with a direct air-cooled heat exchanger. The desirable attributes for economical concentrated solar power plants are their integration ability with thermal energy storage and to accommodate dry cooling. The expressively influencing parameters such as compressor inlet temperature, pressure, and split mass fraction have been investigated for cycle maximum efficiency under arid climatic conditions. The cooling process of supercritical carbon dioxide, unlike steam condensation, is sensible heat transfer with the non-linear variation of thermophysical properties. Effective, efficient, and affordable heat exchanger technology is crucial for the deployment of supercritical carbon dioxide Brayton power cycles in concentrated solar power plants. A MATLAB code has been developed for an air-cooled heat exchanger with multi-pass sub heat exchanger approach to overcome the inevitable temperature variations at the extreme end of the gas cooler. A two dimensional discretization methodology is used to model the heat exchanger to accommodate the rapidly varying isobaric heat capacity in the critical temperature region. The heat transfer and pressure drop performance are calculated using empirical correlations for the Nusselt number and friction factor for each sub-heat exchanger. With the increased contact time of cooling and process fluid, the desired gas cooler outlet temperature of 33.5 °C is achieved by rejecting 823.76 kW, 359.5 kW, and 219.46 kW amount of heat from each sub-heat exchanger, respectively. The results of this work are of significance for the design of the air-cooled heat exchanger for the supercritical carbon dioxide power generation system.

Corrosion simulation via coupling computational fluid dynamics and NORSOK CO2 corrosion rate prediction model for an outlet header piping of an air-cooled heat exchanger

In this study CO2 corrosion of outlet header pipe of an air-cooled heat exchanger in a natural gas dehydration unit of a gas refinery was investigated by simulation via coupling computational fluid dynamics and NORSOK M-506 CO2 corrosion prediction model. The Euler-Euler approach incorporated with k–ε model for the fluid turbulence was employed to simulate gas-liquid two-phase flow. The simulation results were utilized as the inputs of NORSOK CO2 corrosion prediction model to calculate CO2 corrosion rate. Fluid streamlines, velocity, viscosity, density and temperature profiles and also the distribution of gas and liquid phases through the investigated outlet header pipe were calculated and a three-dimensional corrosion rate map was presented. The results showed that there is a good agreement between the data obtained by corrosion simulation and field observation. It was found here that coupling computational fluid dynamics and CO2 corrosion prediction models can successfully predict regional corrosion rates and determine locations susceptible to CO2 corrosion.

Modelling the structural behaviour of a round nozzle to flat plate interface in pressure vessels

In the field of pressure vessel design, much research has been done to predict the structural behaviour of round nozzles connected to spherical and cylindrical structures. This has been motivated by the fact that the majority of pressure vessels in industry have round bodies, as these are better suited to withstanding pressure. A large demand however also exists for flat-sided pressure vessels with round nozzles, particularly in the air-cooled heat exchanger industry, but there is a lack of research and guidelines on accurately predicting how these structures behave at the nozzle interface. This paper presents a new model that predicts the linearised stresses occurring in these structures in order to gain a better understanding of how they behave.

Water-energy nexus approach for optimal design of hybrid cooling system in direct reduction of iron plant

Direct reduction of iron process in steel industry has special production conditions. Low quality cooling water, low cold and high hot cooling water temperature, space limitation for new equipment installation, high value-added of product and severe effect of cooling water temperature on production rate are of these conditions. Considering technical and economic constraints and limitations, this situation makes this process an attractive case study for converting the existing wet cooling tower to hybrid cooling system. In this paper, based on integration of process, dry and wet cooling system and ambient conditions profiles, a new method for designing hybrid cooling system has been proposed. The calculation program has been developed as a MATLAB code and resulted: 1) Calculation of annual water saving and energy consumption with reasonable accuracy, 2) Control system of dry and wet cooling blocks to maximize water saving under various ambient conditions, 3) Closed loop simulation of cooling system considering thermal feedback of process to cooling system and 4) Obtaining optimum values for 16 design parameters of dry block cooling (consisting air cooled heat exchanger and plate heat exchanger). The optimization has been done via genetic algorithm, considering annual response of each design under various ambient temperature and relative humidity. Considering design standards for new equipment and operational and technical constraints, made the optimization model reliable as well as checking the design parameters with process equipment constructors. In optimum point, converting existing wet cooling tower to hybrid cooling system in direct reduction of iron plant of Mobarakeh Steel Complex located in Isfahan province in Iran, will decrease the annual water consumption by 70.15% and production loss by 98% with payback period of 1 years and 10 months.

Experimental Study for Counter to Cross Flow Air-cooled Heat Exchangerof Annular Tube having Internal Circular grooving at Different pitches - A Review

The objective of this study is to provide modern analytical and empirical tools for evaluation of the thermal-flow performance or design of air-cooled heat exchangers (ACHE) and cooling towers. This review consist various factors which effect the performance of ACHE. We introduced systematically to the literature, theory, and practice relevant to the performance evaluation and design of industrial cooling. Its provide better understanding of the performance characteristics of a heat exchanger, effectiveness can be improved in different operating conditions .The total cost of cycle can be reduced by increasing the effectiveness of heat exchanger.

Thermal design evaluation of ribbed/grooved tubes: An entropy and exergy approach

The smooth circular tubes are typically used in air-cooled heat exchangers systems. In literature, various research toward improving the hydraulic/thermal performance by changing the tubes' inner surface topology is found. In the current work, fluid friction and heat transfer characteristics of air-cooled tube heat exchangers with installed ribs and grooves have been numerically compared. Initially, the numerically calculated data of the ribbed tubes are validated /compared to the performance results in the literature. Performance evaluation criteria (PEC), entropy and exergy are numerically investigated for a tube with a group of ribs and grooves over the range of Reynolds number Re = 6700–20,000. Rib radius (r), number of ribs in the same group (Ng), the distance between two adjacent ribs (SL), and pitch between two adjacent groups (p) are the ribs parameters selected to study their effects on flow and heat transfer criteria. Also, the same evaluation criteria are performed on grooved tube with optimal parameters to make a comparison between results. Moreover, streamline patterns and temperature/pressure/velocity/turbulence kinetic energy contours are visually introduced to explicate the physics of heat transfer and pressure drop phenomena and to assess the flow structures inside different ribbed/grooved tubes. For the tubes with high-performance evaluation criteria (PEC), lower and higher entropy and exergy efficiency are determined. Recapitulation results from the technical comparison supported the use of grooved-tubes instead of ribbed-tubes. This is mainly due to lower flow irreversibility accompanied by higher PEC and exergy efficiency than those of ribbed tubes at the same flow condition.

PREDICTING OF STEAM CONDENSATION HEAT TRANSFER COEFFICIENT IN HORIZONTAL FLATTENED TUBE

The heat transfer coefficient of steam condensation has a significant role in the performance of air-cooled heat exchangers. The purpose of this work is to predict the local/average local steam condensation heat transfer coefficient inside the horizontal flattened tube under vacuum conditions using numerous correlations that were developed by some researches which have been conducted under specified conditions. The results from these correlations have been compared with experimental data of Davies, therefore more investigate for the values are necessary to improve or/and validate the existing correlations. The effect of such parameters like the uniform heat flux and saturation temperature also have been studied on the local steam condensation heat transfer coefficient as the results show that the heat transfer coefficient decrease as the heat flux increase, while it increases as the steam saturated temperature increase.

Design and research of cooling system for 2.5 MW permanent magnet wind turbine

In order to solve the problem of excessive temperature rise caused by 2.5 WM permanent magnet wind turbine in operation, this paper designs a heat dissipation system. The combination structure of the heat exchanger and the heat sink was determined, as well as the heat dissipation method of the internal and external cycle isolation heat exchange. The control method and operation mode with Siemens PLC as the core are put forward. The computational fluid dynamics method is utilized to design the air-cooling heat exchanger and the liquid-cooled heat sink of the heat dissipation system. The results show that the cooling pipe gap has a great impact on the liquid cooling effect. The cooling pipe arrangement and piping arrangement of the air-cooled heat exchanger and liquid-cooled radiator are optimized. Finally, taking a 2.5 MW direct-drive permanent magnet wind turbine as an example, four environmental temperatures were selected to test generators in rated power. Verify that the cooling system is designed to meet cooling requirements.

Numerical Simulation and Field Synergy Analysis of IGBT Air-Cooled Heat Exchanger for EMUs

In view of the heat transfer and ventilation characteristics of the Insulated Gate Bipolar Transistor (IGBT) in the Electric Multiple Unit (EMU), the authors aim to improve the traditional fin structure of the air-cooled heat exchanger (ACHE) and propose the use of corrugated fin structure. Using the computational fluid dynamics (CFD) technology, the ACHE with new fin structure was numerically simulated, and the temperature field, velocity field and pressure field at different fin corrugation angles and the synergy between the fields were analyzed. The results show that the improved fin structure can make the fluid flow in a corrugated flow channel, effectively increase the flow distance, and thus significantly enhance the turbulence performance of the fluid in the ACHE; the fin corrugation angle of 120 degrees is the key design point; compared with the traditional fin structure, the new fin structure improves the synergy between the various fields and increases the heat exchange efficiency. The research findings provide new ideas for the design of this type of ACHE.

Hot air extraction to improve aerodynamic and heat transfer performances of natural draft dry cooling system

A large amount of hot air in dry cooling tower of natural draft dry cooling system can be recovered for many applications, such as the boiler air supply and feed water preheating in power plants. However, the hot air extraction will definitely affect the aerodynamic characteristics of dry cooling tower and heat transfer performance of air-cooled heat exchanger. In this work, the impacts of hot air recovery on the thermo-flow performances of natural draft dry cooling system under crosswind conditions are investigated. The variable fields are presented and the outlet water temperature of air-cooled heat exchanger is calculated with the wind speed, the mass flow rate of recovered hot air and the height of hot air extractions considered. Finally, the turbine backpressure and power consumption of induced fans are compared to evaluate the overall cooling effect of dry cooling system with hot air extractions. The results show that hot air extraction is favorable for the cooling performance of natural draft dry cooling system, and the higher mass flow rate of recovered hot air leads to a stronger cooling capability. The hot air extractions are proposed to set near the tower inlet with the turbine backpressure most reduced by 9.28%, which can contribute to the design and operation of natural draft dry cooling system in need of energy conservation.

Anti-freezing performance and application of air-cooled heat exchanger with different water entry patterns

In practical engineering, air-cooled heat exchangers are commonly equipped with middle, left, or side water entry arrangement, and in winter may be at risk for freezing. This research studies the water flow and the exit water temperature distributions of tubes in different rows and passes, and explores the anti-freezing characteristics for air-cooled heat exchangers with these three typical water entry patterns. The micro-volume transport model is adopted to more accurately determine the air/water-side thermo-flow performance of finned tubes. The results show that air-cooled heat exchangers with different water entry patterns have different water flow distributions that induce different exit water temperature distributions. However, water entry patterns have little impact on the anti-freezing performance of air-cooled heat exchangers because the minimum exit water temperature is not significantly changed. Exit water temperatures of windward row 1 tubes, especially those near the header region of an air-cooled heat exchanger with a middle-type header and far from the header region of an air-cooled heat exchanger with a left/side header, are suggested for monitoring at dry-cooling power plants.

Performance characterization of a novel membrane-based liquid desiccant air conditioning system

Abstract Membrane-based liquid desiccant air conditioning (MLAC) system is one of the energy-efficient alternatives to the conventional vapor compression based air conditioning systems. However, due to inherent resistance for heat and mass transfer in the membrane and heat of absorption generated during the mass transfer, the desired room temperature and specific humidity are not achieved in the MLAC system. Present study proposes a novel MLAC system with an additional air cooling heat exchanger for the efficient control of both temperature and specific humidity of the supply air. The influences of different liquid desiccants, ambient conditions, and design and operating parameters are studied. The performance of MLAC system is presented in terms of room temperature and specific humidity, latent effectiveness of dehumidifier and regenerator, coefficient of performance and exergy efficiency. The results of the present study prove that the proposed MLAC system is capable of achieving better indoor conditions than the conventional MLAC system. The effects of the mass flow rate of desiccant and the effectiveness of the desiccant to desiccant heat exchanger on the performance of the proposed MLAC system are found to be significant.

A numerical study on interactions between three short natural draft dry cooling towers In an in-line arrangement

Natural draft dry cooling tower (NDDCT) are a favourable choice for the cooling system of concentrating solar thermal power (CST) plants located in arid regions with high solar radiation. However, additional cooling towers may be required as the capacity of the CST plants is increased. The geometrical arrangement of the NDDCTs is an influencing parameter on the thermo-flow performance of the entire system. The aim of this study was to investigate the effect of tower spacing and wind speed on the performance of three short NDDCTs in an in-line layout. The simulated tower is representative of an actual steel-membrane cooling tower in a campus of the University of Queensland. The geometry of the cooling tower in this study is a cylindrical shape with a horizontally arranged air-cooled heat exchanger, and is 20 m high with a diameter of 12.5 m. This study investigated the effect of two major parameters: wind speeds (0–8 m/s), wind and tower spacings (1.8D, 2.6D, and 4.2D) on the thermo-flow performance of the cooling towers. The interaction of the towers from the bottom and top of the towers were identified at different tower spacings and wind speeds. At all tower spacings, the windward tower protects the middle and leeward towers by deflecting the upcoming wind. The interaction of the towers happens both from the bottom and top of the towers. Finally, it was concluded that enlarging the towers increase the thermal performance of the towers during windy conditions and this may lead in better protection of the windward tower for the leeward ones.

Heat and mass transfer studies in a finned tube heat exchanger employed with evaporative cooling technique

Fin and tube heat exchangers are air cooled, cross flow heat exchangers, which have shown broad prospects in places where there is lack of water resources. Research is focused on improving the heat rejection capacity of these exchangers and various enhancement techniques have been studied. Evaporative cooling is one of the techniques employed to improve the performance. Air cooled heat exchangers with evaporative cooling techniques perform better compared to conventional fin and tube heat exchangers. In the present work, a numerical simulation has been developed to investigate heat transfer enhancement in a fin and tube heat exchanger incorporated with evaporative cooling technique. An experimental setup has been built to test the modified heat exchanger and to validate the numerical results. Results indicate 40 to 60% increase in heat duty and effectiveness of enhanced heat exchanger, for cooling water flow rates in the range of 150 to 450 LPH and air flow rate of 300 CFM. Experimental values of heat transfer rate and outlet temperatures of hot water and air are compared with those of numerical simulation and the agreement is generally good.

Evaluation of heat-exchange performance of carbon-based nanofluids for air-cooled exchangers with different cross-section shapes

In this study, a vortex trap method was employed to produce carbon-based nanofluids (CBNFs) as working fluids for air-cooled heat exchangers (ACHEs) with rectangular and circular tubes. Poly(sodium 4-styrenesulfonate) was added to the produced CBNFs as a dispersant to enhance their stability, and the CBNF concentration was adjusted to 0.01 wt% (S1) and 0.05 wt% (S2). The viscosity, density, and specific heat of CBNFs were measured and analyzed. Finally, CBNFs were tested in the rectangular- and circular-tube ACHEs to evaluate their heat-exchange capacity, the electricity consumption of pumps, and the system efficiency factor under various temperatures (40 and 55 °C) and flow rates. The results revealed that for S2, a maximum enhancement ratio of 8.17% was achieved for the heat-exchange capacity at a flow rate of 2.0 L/min (LPM) and temperature of 40 °C in the rectangular-tube ACHE, whereas for S1, that of 4.88% was achieved at a flow rate of 2.5 LPM and temperature of 40 °C in the circular-tube ACHE. Moreover, for S1 achieved a maximum enhancement ratio of 8.24% for the system efficiency factor at a flow rate of 2.0 LPM and a temperature of 40 °C in the rectangular-tube ACHE and of 6.37% at a flow rate of 2.5 LPM and temperature of 55 °C in the circular-tube ACHE. Considering the overall trend of system efficiency based on the experimental parameters of the rectangular- and circular-tube ACHEs, S1 was deemed more suitable than S2 to serve as the working fluid.

Failure analysis of an air-cooled heat exchanger in natural gas dehydration unit

Corrosion and erosion can have a synergistic effect on failure related to heat exchangers in gas refineries. This work presents failure analysis of outlet piping of an air-cooled gas cooler in a natural gas refinery. Visual inspection as well as thickness measurements were employed to determine the severity of metal loss. Chemical composition analysis and hardness measurements were performed in order to make sure of authentication of base and weld metals. Metallographic investigation indicated that no metallurgical changes have been occurred. Assessment of service condition revealed that temperature drop of gas stream passing through the gas cooler resulted in water condensation followed by corrosion. On the other hand, calculations of critical velocity of fluid revealed that gas velocity exceeded the critical velocity which intensified the erosion and flow induced damages. Thus, it was concluded that although general CO2 corrosion plays a role on the metal loss, flow induced localized corrosion and erosion were the main causes of failure.

Integration of absorption refrigeration systems into rankine power cycles to reduce water consumption: An economic analysis

This work presents the economic modeling and analysis of a thermoelectric steam power plant (SPP) cooled with an air-cooled heat exchanger (ACHX) and using an absorption refrigeration system (ARS) as an intermediary, as discussed in Salgado et al., 2017. This novel configuration has the advantage of not consuming fresh water in the cooling system, which is of special interest in locations where water is scarce or expensive. Based on the thermodynamic analysis of this SPP configuration (considering three different power ranges: 5 MW, 50 MW, and 200 MW, and different ambient temperatures from 5°C to 40°C), economic and sensitivity analyses were conducted to determine the cost-effectiveness and profitability of the power plant. Finally, a comparative analysis was conducted to test the economic outcomes of the SPPs using the proposed ARS compared with conventional refrigeration devices based on water cooling and dry cooling technologies. The results show that conventional evaporative cooling towers are the best solution from an economic perspective. Nevertheless, in regions where water is not easily accessible, the proposed ARS achieves better economic outcomes than the conventional ACHX when the ambient temperature is beyond the range of 17.5°C–22.5°C, depending on the power rating of the plant.

Aluminum compound forms in the Al–H2O system as applied to operating conditions of Dry Cooling Towers

One of important indicators of reliable operation of the steam-water cycle at power plants is water chemistry. Special chemicals are used to adjust properties of working fluids. At designing a new unit the chemicals are selected on the basis of parameters of water, steam and materials, used in different parts of the cycle.Another indicator is minimizing the cooling water loss. Typical loss of cooling water is about 5% of total water amount. This loss is associated with evaporation, droplet carry-over, and blowdown in cooling towers.For power plants, it is typically recommended to use dry cooling towers for minimizing the cooling water loss. In dry cooling systems, cooling water with low amount of impurities goes to mixing condenser, where turbine steam condensation occurs. Then one portion of mixed water flow at the condenser outlet is supplied to the boiler or steam generator feed water loop. Another portion is returned back to the cooling tower. Inside the cooling tower, water is cooled down by passing through air-cooled heat exchangers manufactured of aluminium or steel alloys.On the basis of Pourbaix diagram it is proposed to evaluate the influence of water chemistry on the forms of existence of aluminium compounds in dry cooling towers. The results of calculations are compared with the experimental data.

Life cycle assessment of novel heat exchanger for dry cooling of power plants based on encapsulated phase change materials

Cooling systems in power plants account for approximately 40% of total freshwater withdrawals in the U.S. Due to dwindling access to freshwater resources worldwide, continued operation of wet cooling systems poses a significant engineering challenge. To reduce water consumption, a novel air-cooled heat exchanger has been developed using encapsulated phase change material (EPCM) for dry cooling of power plants. Compared to traditional finned-tube air-cooled condensers, this novel EPCM heat exchanger improves the heat transfer coefficient and power plant efficiency while reducing the pressure drop and cooling system cost. Life cycle assessment (LCA) and techno-economic analysis (TEA) are used to evaluate the environmental and economic performance of EPCM heat exchangers from cradle-to-grave and to compare them to wet cooling and traditional air-cooled condensers. A thermodynamic model is developed to predict the EPCM heat exchanger performance for plant-scale operations. Equipment and construction costs for heat exchangers are estimated based on design parameters obtained from the thermodynamic model. Both process-LCA and economic-input–output LCA are used to simulate and test the sensitivity of EPCM alternatives with commercial wet and dry cooling technologies. We investigate options for EPCM end-of-life management upon retiring the heat exchanger and construct a process-based LCA model to estimate a greenhouse gas (GHG) emissions credit for recycling the EPCM. The life cycle GHG emission of the novel dry cooling technology is 1.16 kg CO2 eq./MWh compared with the 1.1–4.3 kg CO2 eq./MWh reported for commercial dry cooling technologies and consumes 9.5 L/MWhe of water for cradle-to-gate life cycle, which is significantly lower than that of wet cooling systems. The TEA shows many advantages of EPCM cooling technology over the state-of-art dry cooling solutions. Overall, the EPCM heat exchanger provides a better alternative compared to existing dry cooling and wet cooling technologies.

Characteristics study of foulants on air cooled heat exchangers of a gas processing plant in the Middle East

Fouling of heat exchangers in energy and power plant industry, oil and gas industry, pharmaceutical and food process industry, industrial waste heat recovery is a major problem which needs to be solved urgently. In the United Arab Emirates, a majority of oil & gas processing plants are located in the desert where water accessibility is limited, thus fin-fan type of heat exchangers are used for cooling and/or heating applications. Such harsh environments cause external fouling which becomes an unavoidable problem in these heat exchangers. With increased humidity and high process temperatures, the dust particulates start to stick on the outer surface of heat exchangers which drastically reduces the thermal performance and increases power consumption. In order to identify possible methods to control and/or eliminate such kinds of external fouling, it is important to identify the composition and morphology of such foulants. This study presents the characterization of selected fouling samples collected from several fin-fan type air cooled heat exchangers taken from a gas processing plant. Scanning electron microscope (SEM) tests were carried out to determine the physical characteristics of sample foulant. Variability in sizes and shapes was found between samples due, perhaps, to different working temperature ranges of the selected heat exchangers. X-ray diffraction (XRD) analyses revealed presence of quartz, calcite, and alumina with traces of halite and hematite. Laser flash analysis (LFA) was conducted to quantify the specific heat capacity, thermal diffusivity and thermal conductivity of the fouling samples. The diversity of these fouling samples reflects complexity with respect to their potential removal and effects on heat transfer characteristics. Though, this research work is a case study for a specific petrochemical plant, however, findings in this study can be adopted by other industries involved in similar chemical processing in the Middle East as well as other countries having similar weather and geological conditions.