Number Of Transfer Units Research Articles

A comparative study of waste energy recovery from a hot-oil central boiler in a textile finishing factory

ABSTRACT Energy consumption in the textile industry is a significant part of the production cost. A shell-and-tube heat exchanger (STHX) was designed and manufactured for waste energy recovery from a hot-oil boiler in a textile finishing factory. An STHX with a single pass, including finned tubes and no baffle, was manufactured. By providing waste energy recovery to the applied system, air is released to the environment at 323 K instead of 525 K. Two scenarios were analyzed: (i) the designed STHX, alternatively with finned and unfinned-tube designs; and (ii) a double-pipe heat exchanger (DPHX) under the same operating conditions. These two cases were compared in terms of thermal performance. The applied system achieved an energy recovery of approximately 74.2%. The application of finned-tubes has an effect of between 11% and 24% on the total heat transfer coefficient. Applying the DPHX would provide thermal energy recovery of approximately 13% under the same operating conditions. The reason for the greater waste energy recovery when applying STHX is also shown by this analysis. Abbreviations: NTU: number of transfer units

Formulation of Coefficient of Performance Characteristics of Water-cooled Chillers and Evaluation of Composite COP for Combined Chillers

The Coefficient of Performance of an ordinary water-cooled chiller is presented as a relationship with the chiller load factor and cooling water temperature. However, the cooling water temperature fluctuates according to the processed heat of the cooling tower originating in the cooling energy of the chiller and to the outside temperature and humidity. It is therefore difficult to obtain the cooling water temperature under the processed-heat and weather conditions at the time of evaluation. This, in turn, makes it difficult to determine the Coefficient of Performance of a water-cooled chiller at the evaluation time. In this research, we formulated the Coefficient of Performance of a water-cooled chiller as a relationship with the chiller load factor and specific enthalpy of outside air. Specifically, we used the Number of Transfer Units (NTU) model of a cooling tower to calculate the cooling water temperature corresponding to the cooling-tower load factor targeting a counterflow cooling tower for a range of values of outside-air specific enthalpy. This technique makes it possible to evaluate the Coefficient of Performance of a water-cooled chiller without determining the cooling water temperature. Furthermore, for the case of installing multiple units of chillers, it becomes possible to calculate the composite Coefficient of Performance of those chillers without having to determine the cooling water temperatures for the different operation load factors of those chillers. Moreover, since the composite Coefficient of Performance can be calculated by combining the different installation capacities of these chillers, the energy consumption of multiple chillers can be calculated at the basic planning stage.

Comparative Study for Thermal and Fluid Flow Peculiarities in Cascading Spiral Inner Tube Heat Exchanger with or without Diverse Inserts over Spiral Tube

Present work deals with experimental analysis in which various observation takes place related to performance characteristics like HTR, effectiveness and fluid flow peculiarities like Nu, Re, f, NTU, TPF etc in a cascaded spiral concentric tube type heat exchanger consisting of a GI shell and a cascaded spiral copper inner tube having conical shape inserts, hemispherical insert and plane profile unit for parallel flow. Five spiral cascades with three turns in each are used for the inner side copper tube unit. Water enters at 304°K from the innermost turn along the cascaded spiral tube, and leaves at the outermost turn. The hot fluid inlet for heat exchanger is from bottom of one end of the shell at 333°K, flowing radically across the cascades, and then leaving the exchanger from the top of another end. This paper based on comparative experimental study of the behavior of such system for parallel flow with different shape of inserts, and then compared with the simulation results for same. In this analysis comparative approach is considered for evaluating optimum fluid flow and thermal properties on the basis of geometrical parameters. The results predicted that out of three profiles of insert, plain, conical and half cut elliptical profile, which located over inner copper tube of exchanger, half cut elliptical profile insert shows best results on fluid flow and thermal peculiarities, for mass flow rate range 0.012 to 0.049 Kg/s and Reynolds number range 4236 to 18540, heat transfer rate varies from 1128.06 to 2374.77, Nusselt number varies from 75.15 to 144.05, Friction factor varies from 0.00376 to 0.00305 and effectiveness varies from 0.77 to 0.40 and Number of transfer unit varies from 2.08 to 0.68 for half cut elliptical profile.

Performance Comparison of Enthalpy Recovery Devices Using Liquid Desiccant and Desiccant Wheel

Abstract Heat recovery between outdoor air and indoor exhaust air is an effective approach for energy-saving in the air-conditioning system. Performances of enthalpy recovery devices using liquid desiccant (LD) and desiccant wheel (DW) are compared in the present study. Effects of key factors including desiccant flow rate, number of transfer units (NTUm), and air inlet parameters on recovery performance are analyzed. There exists the same ideal recovery efficiency with a given NTUm for these two kinds of devices. However, an optimal solution flow rate could be achieved for enthalpy recovery device using LD, while a higher rotation speed leads to a higher recovery efficiency for that using DW. Recovery efficiencies of the two devices increase with the increase in NTUm, while they have different ranges of NTUm. NTUm of the DW is usually superior to that of the LD, due to the material difference. Then, the optimum methods to improve recovery efficiency of the two devices are clarified, i.e., increasing NTUm for an enthalpy recovery device using LD and choosing a reasonable rotation speed for that using DW, respectively. The present research will be beneficial to cast light on the relation between enthalpy recovery devices using LD and DW.

Accuracy of lumped element model for cyclic sensible thermal energy storage systems

The sensible thermal energy storage system proposed in the present study is composed by a set of flat parallel plates and is meant to work as a thermal rectifier, continuously powered by a cyclical thermal stream in order to retrieve a conditioned output. The energy storage system is modeled by a lumped element model, but its accurateness needed to be ascertained. Computational fluid dynamic is used as a reference modeling to assess the lumped model deviations following a Design of Experiments planning, performed by the Box–Behnken method. Nine controlled and independent parameters are identified to operate the energy storage system, equally representative of the heat storage material, the plate geometry and the working fluid. The temporal evolution of the fluid temperature at the system discharge is used to observe the amplitude and phase deviations in respect to reference modeling. Amplitude deviations are found to be below 6% and phase deviations below 4% for 130 simulated cases assigned by the Box–Behnken method. Heat conduction along the plate axial direction is prevailing in respect to the transversal transfer, meaning that significant deviations from the lumped model to the reference one are only noticed when the storage system combines high values for the number of transfer units and the plate thermal conductivity. The present study shows that thermal energy storage systems like the one proposed in this paper can be simulated with lumped element models.

Intensified regeneration performance of spent caustic from LPG sweetening by HiGee reactor

Spent caustic from the desulfurization of liquefied petroleum gas has been proved to be effectively regenerated in a rotating packed bed (RPB) with classic wire mesh packing. However, the influence of the packing’s surface wettability on regeneration performance in the RPB was not clear with this oil-water two-phase system. In this work, the surface-modified packing was proposed to be used in the RPB to estimate the regeneration performance, including oxidation and separation efficiency. Three different surface-modified packings of hydrophilic surface-modified wire mesh packing (ISP), non-surface-modified wire mesh packing (NSP), and hydrophobic surface-modified wire mesh packing (OSP) were prepared and employed in the RPB for regeneration experiments. Results show that regeneration performance is in order of ISP＞NSP＞OSP. Compared with OSP, the NaSR conversion, RSSR removal efficiency, and number of transfer units of ISP increased nearly by 34%, 13%, and 21% at 1000 r/min rotational speed and 25 mm packing thickness, respectively. For the view of different packing combination, not only in the inner layer but also in the outer layer, ISP had noticeable superior regeneration performance, which displays the bright future of advanced surface-modified packing used in the RPB for process intensification of reaction and separation.

Thermal performance of a Divided-flow type Shell and Tube Compact Heat Exchanger

Shell and Tube heat exchanger is widely used as heat-transfer equipment in various process industries. The paper attempts to understand the flow, heat transfer characteristics and the effectiveness of the divided-flow type heat exchanger. The study is based on the CFD based simulation methods and the experimental methods. The study provides the flow and heat-transfer characteristics of the divided-flow type heat exchanger by using validated Computational Fluid Dynamics (CFD) model. Results are obtained for the temperature, velocity and turbulence intensity distributions in the flow domain. The results from the simulation and experimental study are used in determining the effectiveness (ξ) of the heat exchanger using Logarithmic Mean Temperature Difference (LMTD) and Number of Transfer Units (NTU). The simulation and experimental performance are found to be in good agreement highlighting the minimization of losses in the experimental setup. This confirms the accuracy of the flow-based heat and turbulence distributions. The effectiveness value shows that the heat exchanger can be used for compact applications

Prediction of outlet water temperature from cooling towers

This paper presents a very accurate method for water temperature prediction of outlet water from counter flow of cooling tower. This method was derived from the method of [hc A/Cpm] which is called NTU (Number of Transfer Units). The effect of the water and air mass flow rate, wet bulb temperature of the inlet air, outlet water temperature, tower range, and temperature approach have been studied. The results of comparison between the presented and the NTU methods show a very good match when the outlet water temperature is between 29 and 45°C and a difference reaches to 7.23% between 26 and 28°C. Another very good match has been noticed when the air mass flow rate is between 15.6 and 16.5 kg/s with difference reaches to 2.5 % when the air mass flow rate is between 10 and 15 kg/s. The last very good match has been noticed when the water mass flow rate is between 17.5 and 19 kg/s with the difference reaches to 1.2% when the water mass flow rate is (10 - 17 kg/s).

Numerical analysis for enhancing transferred heat in porous counter flow heat exchanger

The numerical analysis is accomplished to simulate the heat transfer in double tube heat exchanger with and without porous media by utilizing ANSYS FLUENT 14.1. A geometry system is used to predict the heat transfers and pressure drop characteristics of the flow. Alumina (2.5mm diameter) as a porous media were added in to: inner tube (IP), outer tube (OP) and both tube of heat exchanger (IOP) were tested with the variation of hot and cold fluid inlet temperature, mass flow rate ratio (mr) to evaluate their influence on effectiveness of heat exchanger, Nusselt number, number of heat transfer unit. The evaluating has been performed in the steady-state. Water was used as a working fluid in a double tube heat exchanger. The study was conducted at the hot and cold water mass flow rates between ( 0.0166 - 0.0833kg / s), ( 0.05-0.116 kg/s) respectively. The inlet temperatures of cold and hot water were (20, 25 °C), (47, 55 °C) respectively. Noted from results that adding pad of porous increases effectiveness of the heat exchanger and the increasing rate of transferring heat as mass flow rate ratio increases and the highest value are obtained when alumina is in double pipe. Effectiveness decreases with increase in mass flow rate ratio but increases when using alumina as a porous media and the better value is obtained when alumina is in IOP, IP, OP and NP respectively. Effectiveness increased as the NTU increases.

Experimental Investigation on Airflow and Water Production in the Integrated Rooftop Solar Thermal System

Abstract Solar chimney is a passive renewable technology, adopted and followed widely for the application of room ventilation and power production. It requires a wide space for operation. Hence, an effective heat transfer mechanism is required within the available space to improve the performance. The solar chimney is integrated with a solar still using an external condenser to effectively utilize the energy released during the condensation of water vapor in the external condenser. The external condenser acts as an additional heat source besides the solar collector in the solar chimney. The experiments were conducted with the solar chimney of heights 1 m and 2 m, by considering the effects of an external condenser in both summer and winter. Heat transfer studies on the external condenser are also made to determine the effectiveness and the number of transfer units. The temperature and mass flow rate of vapor in the still are the influential parameters on the effectiveness of the external condenser. The condensation energy released from the external condenser increased the daily average air velocity by 14.9% and 22.4% in summer and winter, respectively. However, the overall solar chimney efficiency was improved by 37.1% in summer and 14.5% in winter for the integrated system with the chimney of height 2 m.

Thermal Performance and Energy Saving Analysis of Indoor Air–Water Heat Exchanger Based on Micro Heat Pipe Array for Data Center

According to the temperature regulations and high energy consumption of air conditioning (AC) system in data centers (DCs), natural cold energy becomes the focus of energy saving in data center in winter and transition season. A new type of air–water heat exchanger (AWHE) for the indoor side of DCs was designed to use natural cold energy in order to reduce the power consumption of AC. The AWHE applied micro-heat pipe arrays (MHPAs) with serrated fins on its surface to enhance heat transfer. The performance of MHPA-AWHE for different inlet water temperatures, water and air flow rates was investigated, respectively. The results showed that the maximum efficiency of the heat exchanger was 81.4% by using the effectiveness number of transfer units (ε-NTU) method. When the max air flow rate was 3000 m3/h and the water inlet temperature was 5 °C, the maximum heat transfer rate was 9.29 kW. The maximum pressure drop of the air side and water side were 339.8 Pa and 8.86 kPa, respectively. The comprehensive evaluation index j/f1/2 of the MHPA-AWHE increased by 10.8% compared to the plate–fin heat exchanger with louvered fins. The energy saving characteristics of an example DCs in Beijing was analyzed, and when the air flow rate was 2500 m3/h and the number of MHPA-AWHE modules was five, the minimum payback period of the MHPA-AWHE system was 2.3 years, which was the shortest and the most economical recorded. The maximum comprehensive energy efficiency ratio (EER) of the system after the transformation was 21.8, the electric power reduced by 28.3% compared to the system before the transformation, and the control strategy was carried out. The comprehensive performance provides a reference for MHPA-AWHE application in data centers.

Study on the Relationship between the Theoretical Plate Number and the Mass Transfer Unit Number in Chemical Unit Operation

Study on the Relationship between the Theoretical Plate Number and the Mass Transfer Unit Number in Chemical Unit Operation

HIGH TEMPERATURE COMPACT CERAMIC REGENERATIVE HEAT EXCHANGER

The subject of the research is the high-temperature compact ceramic heat exchanger of a regenerative type with a honeycomb structure. In relation to high-temperature processes of basalt smelting and glass melting, the possibility of its application in a system of high-temperature air combustion is considered. It is noted that heating of the air pursues both energy goal – reduction of natural gas consumption, and technological goal – increasing the amount of free oxygen in the basalt processing zone. The methods of determining the transient temperature field in the wall of the solid heat-storing material of the regenerator and the operating time of the heat exchanger from the condition of ensuring the permissible surface temperature of the heat regenerator are given. It is established that for the range of the Fourier number Fo≤2.5, it is possible not to take into account the temperature distribution in the wall of the solid heat-storing material of the regenerator and to operate with its average temperature. Features of compact ceramic regenerative heat exchangers are given – heat exchanger efficiency, heat recovery coefficient, number of transfer units, power – as applied to the conditions of the heat engineering process. The heat exchanger efficiency is confirmed. The operating time of the heat exchanger and its effect on the surface temperature of the solid heat-storing material are determined. It is shown that in relation to the conditions of the basalt melting and glass melting processes, the material of the regenerative heat exchanger – mullite – is not an optimal solution. Only for one of the considered variants of the technological process, the surface temperature of the regenerative heat exchanger was lower than the permissible temperature of prolonged use of mullite. In other cases, the resulting temperature of the regenerator exceeded the permissible value of long-term use. It is advisable to use a material with a permissible temperature of application of 1525 °C and above as of the solid heat-storing material of the regenerator. Testing of the results of the research was carried out on pilot industrial samples of bath melting furnaces equipped with HiTAC (high-temperature air combustion system).

Analytical solution of the governing equations for heat and mass transfer in evaporative cooling process

In this work, the governing equations of the heat and mass transfer in evaporative cooling process are solved by using the power series method. The physical properties, including the Lewis factor, are considered constant along the cooling process. The water loss from water stream vaporization is taken into account. An iterative procedure is developed for calculating the expansion coefficients of the humidity ratio, the air enthalpy, and the number of transfer units. In all study cases, the power series solution is convergent for the heat and mass transfer equations, except for the number of transfer units equation. Thus, Gauss quadrature technique is implemented as an alternative method for determining the number of transfer units profile. As a comparison, the study cases are also solved numerically by the Dormand-Prince Runge-Kutta method. The numerical and analytical results are found to be in excellent agreement when the mass flow-rate ratio between water and dry air is low. The computational execution time of the analytical solution is 50 times faster than the numerical solution. Furthermore, the proposed technique was applied to a study case previously reported and the results were properly represented with an average error of 3%.

Effect of wired nails circular–rod inserts on tube side performance of shell and tube heat exchanger: Experimental study

In this paper, the thermal, hydraulic, and thermodynamic performances in a shell and tube heat exchanger fitted with wired-nails circular–cut rod inserts on tube side are investigated experimentally. Thermal performance is expressed in terms of overall heat transfer coefficient (U), effectiveness (ε), and number of transfer unit (NTU), while hydraulic and thermodynamic performances are expressed in tube-side pressure drop and exergy efficiency, respectively. The tube side flow rate of hot water varies from (13–18) LPM while a constant flow rate of cold water on the shell side (i.e. 18 LPM) is considered. The experiments are performed for two configurations of wired-nails circular–cut rod inserts with different numbers of nails, varied space between them, and five different configurations of insert distribution in tubes. Also pressure drop is measured and exergy analysis is performed. All data obtained is compared with the conventional plain tube heat exchanger. The experimental results reveal that the proposed inserts configurations have a significant improvement on heat exchanger thermal and thermodynamic performances and a drawback in hydraulic performance. Percentages augmentation in U, NTU, ε, and exergy efficiency are (210–280%), (132–149%), (185–224%), and (130–210%), respectively over the conventional design.

Investigation of effectiveness and pumping power of plate heat exchanger with rough surface

In the present paper, an experimental study of the effectiveness and pumping power of plate heat exchanger (PHE) with rough surface was performed. The experiments were developed for different values of relative roughness, Rr, of 2.6 * 10−4 (smooth surface), 3.4 * 10−4, 7.5 * 10−4 and 9.8 * 10−4 for Reynolds number, Re, varied from 500 to 5000. The average heat transfer rate, Qavg, hence the effectiveness of PHE, ε, and number of transfer units, NTU, as well as pressure drop, Δp, consequently pumping power, P, and specfic heat rate, SHR, were estimated. The results show that Qavg and NTU are enhanced with increasing Rr; while, they are decreased with increasing Re. At Re = 5000, the effectiveness of PHE with relative roughness of 3.4 * 10−4, 7.5 * 10−4 and 9.8 * 10−4 is greater than that of the smooth surface by 11.5%, 16.6% and 18.6%, respectively. Furthermore, Δp and P are increased by increasing Rr and Re. Moreover, at Re = 500, SHR is enhanced with increasing Rr and its improvements compared with the smooth surface are 39.5%, 23.4% and 6.9% for Rr of 3.4 * 10−4, 7.5 * 10−4 and 9.8 * 10−4, respectively. In addition, empirical correlations are proposed to estimate SHR as a function of Rr and Re with a standard deviation of 5.1%.

Regeneration energy analysis and optimization in desiccant wheels using purge mechanism

As the desiccant wheel has been commonly used for dehumidification in drying process and cooling systems, purge mechanism has been commercially applied to reduce the energy consumption and improve its performance. Accordingly, a general optimization study would be essential to maximize this mechanism's advantages for desiccant wheels in various dimensions and design parameters. This study aims to use a non-dimensional model to propose best purging configurations for various desiccant wheel designs. This model applies non-dimensional conservation equations and finite volume as the solving method to simulate the dehumidification effect of rotatory desiccant wheels with the purging capability. Based on non-dimensional parameters, two optimization schemes have been designated to find effective and optimum purge angles. Effective purge angle corresponds to the purge set up which minimizes the humidity ratio of process outlet. The optimum purge angle makes the desiccant wheel have maximum coefficient of performance (COPDW). The optimum and effective purge angles for different dimensionless dehumidification angles, rotational speeds, numbers of transfer units (NTU) and regeneration temperatures are determined. In addition, the regeneration energy reduction made by purging mechanism has been evaluated. It is found that the energy consumption in the regeneration section reduced by applying the purge mechanism. Increasing regeneration temperature and NTU leads to higher energy saving. Besides, the optimum purge angle is observed to be higher than the effective one in all configurations.

What dominates heat transfer performance of hybrid nanofluid in single pass shell and tube heat exchanger?

Influence of nanoparticle volume concentration and proportion on heat transfer performance (HTP) of Al2O3 – Cu/water hybrid nanofluid in a single pass shell and tube heat exchanger is analyzed. Multiphase mixture model is adopted to model the flow. Three-dimensional governing equations and associated boundary conditions are solved using finite volume method. The numerical results are validated with the experimental results. Results indicate that optimized nanoparticle volume concentration and proportion dominate HTP of hybrid nanofluid. Heat transfer coefficient and Nusselt number are monotonic increase functions of nanoparticle volume concentration and proportion. The percentage increase in heat transfer coefficient of hybrid nanofluid is 139% than water and 25% than Cu/water nanofluid. At higher Reynolds number, the increment in Number of Transfer Units (NTU) between water and hybrid nanofluid is close to 75%. Maximum enhancement in Nusselt number for hybrid nanofluid exceeds 90% when compared to nanofluid (Al2O3/Water nanofluid). Consequently, highest heat transfer performance is attained for hybrid nanofluid systems. Effectiveness of heat exchanger increases almost to 124% when hybrid nanofluid is employed. We show that it is higher than water as well (conventional coolant). Results are expected to be helpful in further industrial-scale deployment of nanofluids, which is an area that is currently relevant for ongoing academia-industry partnership efforts worldwide.

Methods for forming interfacial heat-mass exchange surface in flowing air contact with water

A review of methods for forming interfacial heat-mass exchange surface in contact devices for air steam treatment is given. An effective method for creation of heat-mass exchange surface in air flow contact with water in the course of adiabatic wetting in air conditioning systems is proposed: by use of a plate head-piece with a mechanical vibration exciter. A method of defining area of the interfacial heat-mass exchange surface for these conditions is developed on the base of number of transfer units. The criterion equations to describe heat and mass transfer which occur when a plate head-piece with a vibration exciter is used for adiabatic air wetting are obtained. Use of a plate head-piece with vibration exciter as a contact device reduces energy consumption due to reduction of aerodynamic resistance of the air path and process intensification.

Steady and Transient Behavior of Cross-Flow Three-Fluid Heat Exchanger With Temperature Nonuniformity in All the Fluids—A Detailed Investigation

Abstract Cross-flow three-fluid plate-fin heat exchanger is analyzed under both steady-state and transient conditions with a nonuniform inlet temperature of all the three fluids. The influence of the longitudinal heat conduction and axial dispersion in the separating sheets and three fluids, respectively, is also considered. Five different combinations (modes) of temperature nonuniformity in the three fluids have been considered and compared for the performance. An important phenomenon of temperature cross between/among the fluids has been observed and presented for certain modes of temperature nonuniformity and operating conditions. The effect in the performance has been presented on the basis of mean exit temperature and deterioration factor. Implicit finite difference technique has been used for the numerical solution. The heat exchanger's performance is found to be dependent on the mode of temperature nonuniformity, number of transfer units, and the operating parameters.