Number Of Heat Transfer Units Research Articles

Estimation of the direct leakage of rotary air preheaters based on temperature distribution modeling

In thermal power plants, air preheaters are used to preheat the combustion air with residual thermal energy of the flue gas. The air leakage from air to flue gas has negative impact on the heat transfer efficiency of air preheaters and the unit thermal economy. This paper proposes an online applicable approach to estimate the direct leakage of the air preheater based on temperature distribution modeling. To describe the heat transfer process in the air preheater, energy balance equations are established after discretizing the air preheater into a number of heat transfer units, where the unstable heat transfer caused by the rotor rotation is considered. The temperature distribution of the air preheater is obtained through iterative calculation of these energy balance equations. By solving the optimization problems with the objective to meet the conservation laws of mass and energy, the direct leakage of the secondary air and the primary air is estimated. Pseudo-online simulations are carried out with the history data of a 300 MW subcritical unit to verify the approach. The updating period of the calculation results is set to 30 s so that the proposed approach may be applied for online monitoring purposes.

Thermal-exergetic behavior of a vertical double-tube heat exchanger with bubble injection

ABSTRACTThe use of air bubbles as a heat transfer improvement technique for heat exchangers has been proposed by some researchers. The vertical motion of tiny bubbles because of density difference with liquid provides extra vibration, eddies, turbulences, and consequently further heat transfer rate. The variety of affected parameters, such as injection method, air mass flow rate, bubbles size, number of perforations that forms bubbles, etc., has added to the complexity of this phenomena so that any change in the said parameters significantly influences the thermal-exergetic behavior of the heat exchanger. The quality and quantity of the impact of bubbles on the thermal performance of heat exchangers are different for any type of them. Moreover, each type of heat exchanger requires a specific injection method depending on the heat exchanger structure. In the present research, an injection way is proposed for vertical double-tube heat exchangers, and the effect of bubbles on thermal-exergetic characteristics is experimentally studied and discussed, which have not been performed before. Nondimensional exergy destruction, number of heat transfer units, and effectiveness are the main evaluated parameters in the present paper. Results showed a significant thermal improvement of the heat exchanger under the bubble injection.

Experimental and Numerical Study of Heat Transfer Enhancement in a Shell and Tube Heat Exchanger using Helical Coiled Wire Inserts

An experimental work on heat transfer enhancement in a shell and tube heat exchanger by insertion of helical coiled wires into the tube bundle was conducted. Four different pitch sizes were used (4.8, 6.4, 8.0, 9.6) mm. The working fluid for both shell and tube sides of the exchanger was water. Flow arrangement used was parallel and laminar for both sides with a Reynolds number for the tube inside ranging (100-920). Variables were taken into account, including; convection heat transfer coefficient, number of heat transfer units, thermal effectiveness, and pressure drop. CFD analysis was done in order to validate the experimental results. The study results showed an increase in heat transfer coefficient on the tube side by 54% due to the use of 9.6 mm pitched inserts. Highest thermal effectiveness obtained was in the case of 9.6 mm pitch inserts. Pressure drop rose up to 260% due to inserts.

Heat Transfer Enhancement and Optimization of Lean/Rich Solvent Cross Exchanger for Amine Scrubbing

The lean/rich amine cross exchanger is one of the cost centers in the amine scrubbing process, and accounts for 20–30% of the capital cost. To minimize the cross exchanger cost, shortcut methods that determine optimum LMTD and fluid velocity were developed. The optimum LMTD is a function of heat transfer coefficient, temperature change and the capital cost of heat exchanger. A greater LMTD should be used to prevent excessive capital cost when the number of heat transfer units (NTU) is large and the heat transfer coefficient is small. The heat transfer performance can be enhanced by increasing pressure drop and reducing solvent viscosity. The corrugation angle is the primary design geometry for plate-and-frame exchanger (PHE). Based on the empirical correlations for PHE, the heat transfer coefficient at 60° is almost double that at 30°; however, the pressure drop at a large corrugation angle is also greater. The dependence of the pressure drop per unit length on the heat transfer coefficient is 0.35–0.40, which implies that the heat transfer coefficient will increase 30% by doubling the pressure drop per unit length. The cost associated with the optimization of the cross exchanger has been developed as a function of the fluid velocity, the physical properties, the exponents of the empirical correlations and the pricing parameters. The optimum velocity is independent of the solvent rate, the temperature change of the cross exchanger, and the cross exchanger LMTD. To stay at optimum fluid velocity, the plate number needs to increase as the solvent rate increases while the plate length will increase as the NTU increases. Viscous solvent will result in a lower optimum velocity since it causes higher pressure drop. Typical optimum fluid velocity is at 0.32–0.42 m/s for 8 m PZ. It is worthwhile to utilize higher fluid velocity and pressure drop when the heat transfer can be effectively enhanced by turbulence.

Performance Comparison between Selected Evaporative Air Coolers

The aim of this study is to determine which of the heat exchangers is characterized by the highest efficiency in different applications. Various types of evaporative air coolers were compared: a typical counter-flow unit, the same unit operating as a heat recovery exchanger, a regenerative unit and a novel, modified regenerative exchanger. The analysis includes comparing the work of evaporative heat exchangers during summer and winter season. The analysis is based on the original mathematical models. The numerical models are based on the modified ε-NTU (number of heat transfer units) method. It was established that selected arrangements of the presented exchangers are characterized by the different efficiency in different air-conditioning applications. The analysis faces the main construction aspects of those evaporative coolers and also compares two above-mentioned devices with modified regenerative air cooler, which can partly operate on cooled outdoor airflow and on the exhaust air from conditioned spaces. This solution can be applied in any climate and it is less dependent on the outdoor conditions. The second part of the study focuses on winter season and the potential of recovering heat with the same exchangers, but with dry working air channels. This allows establishing their total potential of generating energy savings during the annual operation.

Effectiveness Analysis of Double Pipe Heat Exchanger with Curls Band in Various Angles

In the modern learning, effectiveness-NTU studies in a dual tube heat exchanger armed with curly band in view of numerous angles were experimentally deliberate. Additionally, discrepancy of the Dual effectiveness with heated water Reynolds numbers for dissimilar cold water flow rates were obtainable. These turbulators with unlike angles of 45 L, 60 L, 90 L, 120 L and 150 L were made of electrified plates with thickness of 1 mm and were attached in the internal tube of heat exchanger. The experimentations were conceded out at Reynolds numbers of 3000–13,400 at turbulent flow establishment. All over the research, hot and cold water rolled finished the internal pipe and the interplanetary between the pipes (annulus), correspondingly. It was strained to keep the bay hot and cold water temperatures at constant values. Effectiveness-NTU studies were made for the circumstances with and without curly bands including their different angles and associated to each other. Results displayed the substantial decision of turbulators on effectiveness (e) and number of heat transfer units (NTU) of dual tube heat exchanger. In totalling, some experiential associations uttering the domino effect were also industrialized constructed on curve fitting.

Theoretical and numerical analysis of counter-flow parallel convective exchangers considering axial diffusion

We perform a systematic analysis of heat transfer in a counter-current three dimensional convective exchanger, when the inlet/outlet influence is fully taken into account. The analysis, carried out for constant fluid properties, considers the various influences of the fluid/solid conductivity, the imposed convection, inlet/outlet far-field conditions, and lateral boundary conditions. Using a generalized Graetz mode decomposition which permits to consider, both transverse and longitudinal diffusion influence in the exchanger as well as in the inlets/outlets, we put forward several salient generic features of convection/conduction heat transfer.In all cases we found an optimal Péclet number for the cold or hot effectiveness. Even if, as expected, the larger the Péclet the larger the Nusselt number, high transfer performances are found to be poorly efficient and/or to necessitate non-compact elongated exchangers. Performance degradation arising at high Péclet number are found to be related to “convective leaks” taking place within outlets. A fully developed regime occurs at large Péclet and/or for long exchangers, which is fully determined by the first eigenvalue of the generalized Graetz mode decomposition, which is an extension of classical Graetz analysis. Numerical results are found consistent with a generalized linear relation between effectiveness and the number of heat transfer units asymptotically established in the convection dominated regime. This study opens new perspectives for micro-heat exchangers where moderate convection provides the best effectiveness and compactness. This contribution is also useful for giving reference solutions to counter-flow exchangers with realistic inlet/outlet boundary conditions.

Effectiveness-NTU analyses in a double tube heat exchanger equipped with wavy strip considering various angles

In the present study, effectiveness-NTU analyses in a double tube heat exchanger equipped with wavy strip considering various angles were experimentally studied. Moreover, variation of the effectiveness with hot water Reynolds numbers for different cold water flow rates were presented. These turbulators with different angles of 45°, 60°, 90°, 120° and 150° were made of galvanized plates with thickness of 1mm and were installed in the inner tube of heat exchanger. The experiments were carried out at Reynolds numbers of 3000–13,500 at turbulent flow regime. Throughout the experiments, hot and cold water flowed through the inner pipe and the space between the pipes (annulus), respectively. It was tried to keep the inlet hot and cold water temperatures at constant values. Effectiveness-NTU analyses were made for the conditions with and without wavy strips including their different angles and compared to each other. Results showed the considerable effect of turbulators on effectiveness (ε) and number of heat transfer units (NTU) of double tube heat exchanger. In addition, some empirical correlations expressing the results were also developed based on curve fitting.

Erratum to Effectiveness and humidification capacity investigation of liquid-to-air membrane energy exchanger under low heat capacity ratios at winter air conditions

In this research, a novel small-scale single-panel liquid-to-air membrane energy exchanger has been used to numerically investigate the effect of given number of heat transfer units (4.5), different cold inlet air temperature (1.7, 5.0, 10.0 °C) and different low heat capacity ratio (0.4, 0.5, 0.6, 0.7, 0.8, 0.9) on the steady-state performance of the energy exchanger. This small-scale energy exchanger represents the full-scale prototypes well, saving manufacturing costs and time. Lithium chloride is used as a salt solution in the system and the steady-state total effectiveness of the exchanger is evaluated for winter inlet air conditions. The results show that total effectiveness of the energy exchanger decreases with heat capacity ratio in the mentioned range. Maximum numerical total effectiveness of 97% is achieved for the energy exchanger. Increasing the heat capacity ratio values on given inlet air temperature, the humidification capacity of energy exhanger is also investigated in this paper. The humidification performance increases with heat capacity ratio. The highest humidification performance (4.53 g/kg) can be reached when inlet air temperature is 1.7 °C, and heat capacity ratio is 1.0 in winter inlet air conditions in the range of low heat capacity ratio.

Thermo-hydraulic design of single and multi-pass helical baffle heat exchangers

A shortcut sizing approach for single and multipass heat exchangers with helical baffles is presented. An alternative approach for the determination of the correction factor of the logarithmic mean temperature difference as a function of the number of heat transfer units is introduced. Multipass arrangements give rise to complex internal heat flow paths; in this work, such flow paths are modelled using a simplified methodology that breaks up the complex flow and represents it as a network of simple arrangements such as counter flow and parallel flow. The network is solved to determine the outlet temperatures from where the correction factor is determined. The methodology is demonstrated on case studies from the open literature.

Experimental study on heat pipe assisted heat exchanger used for industrial waste heat recovery

Steel industry plays an important role economically in China. A great amount of hot waste liquids and gases are discharged into environment during many steelmaking processes. These waste liquids and gases have crucial energy saving potential, especially for steel slag cooling process. It could be possible to provide energy saving by employing a waste heat recovery system (WHRS). The optimum operation condition was assessed by integrating the first and the second law of thermodynamics for a water–water heat pipe heat exchanger (HPHE) for a slag cooling process in steel industry. The performance characteristics of a HPHE has been investigated experimentally by analyzing heat transfer rate, heat transfer coefficient, effectiveness, exergy efficiency and number of heat transfer units (NTU). A specially designed on-line cleaning device was used to clean the heat exchange tubes and enhance heat transfer. The results indicated that the exergy efficiency increased with the increment of waste water mass flow rate at constant fresh water mass flow rate, while the effectiveness decreased at the same operation condition. As the waste water mass flow rate varied from 0.83m3/h to 1.87m3/h, the effectiveness and exergy efficiency varied from 0.19 to 0.09 and from 34% to 41%, respectively. In the present work, the optimal flow rates of waste water and fresh water were 1.20m3/h and 3.00m3/h, respectively. The on-line cleaning device had an obvious effect on the heat transfer, by performing the device, heat transfer rate, heat transfer coefficient, effectiveness and exergy efficiency were improved by 6.11%, 9.49%, 7.19% and 7.93%, respectively.

Dehumidification performance investigation of run-around membrane energy exchanger system

Liquid-to-air membrane energy exchanger is a novel membrane base energy exchanger, which allows both heat and moisture transfer between air and a salt solution. It uses semi-permeable membrane to eliminate entrainment of liquid desiccant as aerosols in air stream and allow simultaneous heat and moisture transfer between salt solution flow and airflow. The heat and mass transfer performance of a single liquid-to-air membrane energy exchanger is significantly dependent on two dimensionless parameters. They are the number of heat transfer units (NTU) and the ratio of heat capacity rates between solution flow and air flow (Cr*). The liquid-to-air membrane energy exchangers can also be applied in a run-around membrane energy exchanger system, which is mainly comprised of two liquid-to-air membrane energy exchangers and a closed loop of aqueous desiccant solution and used as a passive energy recovery system to recover the energy (both heat and moisture) from the exhaust air to precondition the supply air in air conditioning systems. In this study the dehumidification capacity of a run-around membrane energy exchanger is investigated numerically at different exhaust air temperatures and Cr* values. Increasing the exhaust air temperature or the Cr* would enhance the dehumidification capacity of the a run-around membrane energy exchanger system under Cr*?1, but the improvement is limited. The dehumidification capacity at low Cr* is much lower than that under the optimal Cr* value (Cr*=3.2) where the maximum latent effectiveness is obtained.

Exergy analysis and optimization of charging–discharging processes of latent heat thermal energy storage system with three phase change materials

In this paper, a mathematical model for the overall exergy efficiency of combined charging–discharging processes of three phase change materials (PCMs) named PCM1, PCM2, PCM3 and different heat transfer fluid (HTF, the solar field HTF and thermal energy storage (TES) system HTF are different) has been developed. The model takes into consideration the effects of inlet temperatures and the number of heat transfer units (NTUs) of the solar field HTF and the TES system HTF on the maximum overall exergy efficiency and the optimum melting temperatures of PCM1, PCM2 and PCM3. The analysis is based on a lumped model for the PCMs which assumes that a PCM is a thermal reservoir with a constant temperature of its melting point and a distributed model for the solar field HTF and the TES system HTF which assume that their temperatures vary in their flow path. The results show that the maximum overall exergy efficiency can be improved by increasing the NTUs of either the solar field HTF or the TES system HTF which is not more than 5. It is found that, for the TES system HTF, increasing its inlet temperature can increase the maximum overall exergy efficiency, however, for the solar field HTF, only when the NTUs of the solar field HTF and the TES system HTF are both more than 2, increasing its inlet temperature can increase the maximum overall exergy efficiency. It is also found that, compared to increasing the NTUs of the TES system HTF, increasing the NTUs of the solar field HTF is more efficient in improving the maximum overall exergy efficiency. Considering actual application of solar thermal power, we suggest that inlet temperature ranges of the solar field HTF and the TES system HTF should be 800–1200K and 350–400K, respectively, and the ranges of the NTUs of PCM1, PCM2 and PCM3 should be 4–5, correspond to the ranges of Tm1.opt, Tm2.opt and Tm3.opt are 750–850K, 550–600K, 450–500K, respectively. The present analysis provides theoretical guidance for application of three PCMs storage system for solar thermal power.

Experimental Effectiveness Investigation of Liquid-to-air Membrane Energy Exchangers under Low Heat Capacity Rates Conditions

The liquid-to-air membrane energy exchanger is a novel membrane base energy exchanger that allows both heat and moisture transfer between air and a salt solution using a membrane. The heat and mass transfer performance of a single one is significantly dependent on two dimensionless parameters: number of heat transfer units and the ratio of heat capacity rates between solution flow and airflow (Cr*). In this study, the effectiveness of a small-scale liquid-to-air membrane energy exchanger under low Cr* conditions (i.e., Cr* < 1) that has been not investigated previously is experimentally and numerically tested.

Performance investigation of liquid-to-air membrane energy exchanger under low solution/air heat capacity rates ratio conditions

Liquid-to-air membrane energy exchanger is a novel membrane base energy exchanger, which allows both heat and moisture transfer between air and a salt solution. The heat and mass transfer performance of a single one is significantly dependent on two dimensionless parameters: number of heat transfer units and the ratio of heat capacity rates between solution flow and air flow (Cr*). The performance of liquid-to-air membrane energy exchanger under high Cr* (i.e. Cr* ≥ 1) has been comprehensively investigated experimentally and numerically in previous research. In this study, the effectiveness of a small-scale liquid-to-air membrane energy exchanger under low Cr* conditions (i.e. Cr* &lt; 1) is experimentally tested. Good agreement between the experimental and numerical results is achieved under low Cr* cases. Practical application: The ideal energy exchanger is one that can transfer both heat and moisture because during hot and humid conditions such an exchanger is capable of transferring up to four times as much energy as an exchanger that can transfer sensible heat only. It is beneficial if the exchanger can transfer heat and moisture also between remote supply and exhaust airstreams, as this may minimize the ducting required and reduces contaminant transfer from one airstream to the other. This is very important for applications such as hospitals, laboratories, and manufacturing facilities, where slight cross contamination can cause serious health effects.

Investigating similarity between a small-scale liquid-to-air membrane energy exchanger (LAMEE) and a full-scale (100 L/s) LAMEE: Experimental and numerical results

Liquid-to air membrane energy exchangers (LAMEEs) are novel membrane based energy exchangers that allow simultaneous heat and moisture transfer between salt solutions and air streams. The similarity between a small-scale (2.5L/s) LAMEE and a full-scale (100L/s) LAMEE is investigated, both experimentally and numerically, in this paper. Dimensional analysis of the coupled heat and mass transfer equations in the air-side, solution side and membrane of the LAMEE shows that five important dimensionless parameters affect the performance of LAMEEs: (1) number of heat transfer units, NTU, (2) thermal capacity ratio, Cr∗, (3) number of mass transfer units, NTUm, (4) mass flow rate ratio, m∗, and (5) operating factor, H∗. The differences in the small-scale and full-scale LAMEEs effectivenesses are numerically investigated for these five important dimensionless parameters. The results show that in most cases the differences between the small-scale and full-scale LAMEE effectivenesses are less than 2%. The comparison between the experimental and numerical sensible, latent and total effectivenesses of the small-scale LAMEE and the full-scale LAMEE shows that there is a similarity between the small-scale and full-scale LAMEEs, and that the small-scale LAMEE effectiveness results can be used to predict the performance of the full-scale LAMEE.

Power from a hot gas stream with superheater and reheater in parallel

In this paper we determine the fundamental relation between global performance and flow configuration (constructal design) in the case of steam power generation with superheater and reheater placed in parallel in the same stream of hot gases of combustion. The superheater heats the steam for the high pressure turbine, and the reheater supplies steam to the low pressure turbine. Both turbines operate irreversibly. We consider superheaters and reheaters with balanced counter flows and unbalanced counter flows. The total heat transfer area (or the overall number of heat transfer units) is finite. We show that the heat transfer area can be allocated to the superheater and reheater such that the overall power output is maximum. The optimal area allocation ratio is reported for two scenarios: designs with and without a maximum allowable steam temperature.

Investigating a Prototype Heat Exchanger for Steam Storage

A coiled tube single pass counter flow heat exchanger was designed and fabricated using locally available materials and its capacity to transmit heat to water to make steam was tested. The heat exchanger was part of components used in solar thermal power production using a parabolic trough solar concentrator. The design of the heat exchanger storage system was done using Auto CAD 2010 software. Higher temperatures of steam were realized when the heat exchanger was used as a steam storage system. The heat transfer fluids used were locally available and they were water, sunflower oil, Rina oil, used engine oil, unused engine oil, 2 M sodium chloride salt solution, 4 M sodium chloride salt solution and 6 M sodium chloride solution. For each of the heat transfer fluids, heat exchanger operating points were obtained and it was operated between 1 Nm -2 to 1.0 Nm -2 of pressure. In the study the number of heat transfer units of the heat exchanger obtained was 0.61 and the thermal efficiency was found as 0.91. The average rates of heat transfer were 68.4 J for 6M sodium chloride solution, 62.8 J for 4 M sodium chloride solution, 57.9 J for used engine oil. Thermal conductivity of the salt solutions was better than for other heat transfer fluids although they were more corrosive on the system. Thermal efficiencies of the heat transfer fluids were 6 M sodium chloride solution; 0.89, for 4 M sodium chloride solution ; 0.84, for 2 M sodium chloride solution ; 0.80, water ; 0.78, sunflower; 0.69, Rina oil ; 0.66, unused engine oil ; 0.75 and used engine oil; 0.71. Storage of solar thermal energy will address the problem of low solar density and its variability. The heat exchanger storage system achieved a thermal capacity of 3.26 kJ at a maximum temperature of 249.4 o C and at a pressure of 7.2 Nm -2 . The use of heat exchangers in solar thermal storage will increase the overall efficiency of total

Second Law Efficiency Analysis of Heat Exchangers

Analytical analysis of unbalanced heat exchangers is carried out to study the second law thermodynamic performance parameter through second law efficiency by varying length-to-diameter ratio for counter flow and parallel flow configurations. In a single closed form expression, three important irreversibilities occurring in the heat exchangers—namely, due to heat transfer, pressure drop, and imbalance between the mass flow streams—are considered, which is not possible in first law thermodynamic analysis. The study is carried out by giving special influence to geometric characteristics like tube length-to-diameter dimensions; working conditions like changing heat capacity ratio, changing the value of maximum heat capacity rate on the hot stream and cold stream separately and fluid flow type, i.e., laminar and turbulent flows for a fully developed condition. Further, second law efficiency analysis is carried out for condenser and evaporator heat exchangers by varying the effectiveness and number of heat transfer units for different values of inlet temperature to reference the temperature ratio by considering heat transfer irreversibility. Optimum heat exchanger geometrical dimensions, namely length-to-diameter ratio can be obtained from the second law analysis corresponding to lower total entropy generation and higher second law efficiency. Second law analysis incorporates all the heat exchanger irreversibilities. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library (wileyonlinelibrary.com/journal/htj). DOI 10.1002/htj.21109

Automatic Synthesis of Alternative Heat-integrated Water-using Networks

A novel sequential approach is developed in this work to identify alternative heat-integrated WUNs with four mathematical programming models. The proposed synthesis method can be implemented in two stages. In the first stage, the objective is to identify all alternative optimal configurations featuring the minimum freshwater usage, the minimum energy consumption and the minimum total interconnection number respectively. These structures can be determined by solving two distinct models in sequence. The possible WUN designs are identified exhaustively by adopting the ‘solution pool’ technique provided by Solver CPLEX in GAMS. The HEN designs are then generated in the second stage. From each candidate obtained in the first stage, the stream data can be readily extracted and utilized in the transshipment models for determining the minimum utility usage and the minimum number of heat transfer units. The advantages of this approach are twofold: (1) more than one alternative designs can be found, and (2) all model scales can be kept to manageable levels. A case study is provided in this paper to illustrate the effectiveness of the proposed method.