Horizontal-tube Falling Film Evaporator Research Articles

Wetting behaviour of different tube materials and its influence on scale formation in multiple-effect distillers

Multiple-effect distillation (MED) plants with horizontal tube falling film evaporators for sea water desalination exhibit relatively high heat transfer coefficients achieved by film flow under clean surface conditions. However, they are susceptible to heat transfer deterioration and scale formation accompanied by film breakdown. Thus, maintaining all heated tubes in a fully wetted state is one of the key issues to be considered in designing falling film evaporators. Surface tension, hydrophilicity, hydrophobicity and wettability of materials play an important role in scale formation. The wetting rate, that is the brine mass flow rate on one tube per unit tube length, is one of the most important parameters in the design and operation of MED plants. It has an impact on heat transfer, tube bundle design, scale formation, the capital and operational costs of the plant and its operational flexibility. Thus, it is very important to acquire a better understanding of the wetting behaviour and the impact of different tube materials, various process parameters and sea water properties on tube wetting. The wetting behaviour of different tube materials such as aluminium brass, copper–nickel 90/10 and aluminium alloy AlMg2.5, which are commonly used in MED plants, was investigated in a horizontal tube falling film evaporator in pilot plant scale. In tests with stepwise increasing and decreasing wetting rate, pictures of the film flow over the horizontal tubes were taken and the percentages of the wet surface area were evaluated by means of an image processing software. The tube surfaces were characterized by measuring contact angles and determining surface free energies. Furthermore, the horizontal tube falling evaporator was used to study the effect of the wetting rate on composition and quantity of mixed salt scale. Experiments were performed with artificial sea water at wetting rates from 0.14 kg/(s m) down to 0.025 kg/(s m) for different tube materials such as aluminium brass, copper–nickel 90/10 and AlMg2.5. New insights into the wetting behaviour of different tube materials and the influence of the wetting rate on scale formation in the horizontal tube falling film evaporators are given. The effects of various process parameters such as wetting rate, temperature and salinity on the wettability of typical tube materials and the effect of the wetting rate on composition and quantity of mixed salt scale are presented and discussed.

Experimental investigation on heat transfer in horizontal-tube falling-film evaporator

AbstractAn experimental platform for horizontal-tube falling-film evaporation was set up to measure heat transfer characteristics. Experiments were carried out to show how the heat transfer coefficient (HTC) was affected by different parameters including spray Reynolds number (Re), saturation temperature, salinity, and tube arrangement. The results revealed that the HTC increased first and then decreased with growth of Re, and the HTC of seawater decreased with increasing saturation temperature. The results also showed that the HTC of rotated square pitch was higher than triangular pitch, rotated triangular pitch, and square pitch, but the heat transfer capacity per unit volume of triangular pitch was the highest. Meanwhile, the HTC decreased during the increase of salinity.

Analysis of a mechanical vapor recompression wastewater distillation system

This paper presents a comprehensive design model of single-effect mechanical vapor recompression (MVR) system. The analysis focuses on the prediction of the specific power consumption of the compressor and the heat transfer areas of the evaporator. The MVR unit, with a capacity of 20kg/h and horizontal tube falling film evaporation, is designed and installed in the laboratory. The performance of the system was investigated using high salinity wastewater containing Na2SO4. Except for the compressor consumption, experimental data shows good agreement for the predictions. The specific power consumption for the vapor compressor is 55.6kWh/m3. The experimental and theoretical results indicated that the evaporation rate increases by increasing the evaporate temperature from 70°C to 85°C. The evaporation rate, compressor consumption and heat transfer area of evaporator depend strongly on the temperature difference of heat transfer. The evaporation rate and specific power consumption increase linearly with temperature difference. On the other hand, the heat transfer area of evaporator goes down with the increase of temperature difference. This inferred that there is an optimum value of temperature difference that will lead to the optimum system with lower power consumption and smaller heat transfer area.

Heat transfer characteristics of horizontal tube falling film evaporation for desalination

An experimental study on heat transfer characteristics of falling film evaporation outside horizontal tube was carried out at low evaporation temperature. The Al-brass tubes with 19 mm outer diameter and 1,600 mm effective length were adopted and arranged vertically in the evaporator. The electric heater was fixed tightly in the test tube for generating uniform heat flux. And thermocouples were installed in the surface of the test tube to measure surface temperatures. The experimental fluids are water and seawater with salinity of 1.5, 3, and 4.5%. The variations of heat transfer coefficient h were studied in detail with saturation temperament ranging from 50 to 70°C, the spray density Γ ranging between 0.017 and 0.087 kg m−1 s−1, and the heat flux q varying from 7.75 to 12.30 kW m−2. The results show that the average heat transfer coefficients of both water and seawater increase with the spray density until about 0.052 kg m−1 s−1. The heat transfer coefficient of fresh water increases with evaporation temperature whereas that of the seawater decreases slightly with the evaporation temperature. The average heat transfer coefficient decreases with salinity. In addition, the circumferential distribution of local heat transfer coefficients and the effect of defoamer on heat transfer are studied.

Numerical analysis on flow and heat transfer of a tube bundle in a horizontal-tube falling film evaporator

In order to get a better understanding of distribution characteristics of the dynamic and heat transfer performance in a horizontal-tube evaporator, a numerical calculation was developed to simulate the internal film condensation and external falling film evaporation in a tube bundle. The temperature field, film thickness and local heat transfer coefficient were predicted along the tube length and in-between tubes. In view of a good agreement of the simulated predictions with the data of the practical desalination plant, the theoretical model was proved to be valid and accurate. The results show that the flow field and heat transfer rate are improved by means of optimizing the flow density distribution of liquid film outside a tube bundle on basis of the variation of the internal condensation process. The internal vapour condensation temperature reduces sharply at the outlet of the second pass. The local overall heat transfer coefficients tend to decrease from the inlet of the tubes to the outlet and approach the maximum at the bottom of the bundle.

Experimental evaluation of an ejector as liquid re-circulator in a falling-film water chiller

To experimentally evaluate the performance of an ejector working as a liquid re-circulator in a horizontal-tube falling-film evaporator with R134a, experimental tests are performed using a horizontal-tube falling-film water chiller prototype. Experimental observations on intertube liquid flow pattern of tube bundle validate the feasibility of the liquid re-circulation system using a liquid–liquid ejector. The analysis results show that the influence of the motive flow rate on the entrainment ratio of the ejector is small, and the average entrainment ratio of the ejector is about 2.03. With the increase of the valve opening of the regulating valve, the evaporating capacity of the falling-film water chiller rises 4.8%, from 940.2 kW with the re-circulation ratio of one, to 985.5 kW with the re-circulation ratio of 1.135. The COP of the falling-film water chiller reaches a maximum and then drops down with the increase of the re-circulation ratio, and the optimal re-circulation ratio is 1.135.

Heat transfer performance and bundle-depth effect in horizontal-tube falling film evaporators

Abstract A set of experimental facilities were set up to measure overall heat transfer coefficient of horizontal-tube falling film evaporator with square-pitch bundle. Effect of spray density, saturation temperature, total temperature difference, and inlet steam velocity on the overall heat transfer coefficient K is studied. The tubes are made of HAL77-2A aluminum brass with an outer diameter of 25.4 mm. Fluids inside and outside the tubes are steam and fresh water respectively. The results indicate that growth of spray density and saturation temperature helps to increase the K. The K could also be increased by reducing the total temperature difference. However, the impact of the inlet steam velocity on the K is less significant. Furthermore tube bundle-depth effect and space distribution of local overall heat transfer coefficient in the evaporator are also presented. Based on this investigation, basic engineering design information will be provided to establish the governing parameters for horizontal-tube falling film evaporator in the field of seawater desalination.

Effect of flame spray coating on falling film evaporation for multi effect distillation system

Abstract Horizontal tube falling film evaporators find various applications like multi effect distillation for sea water desalination, power and process applications, refrigeration applications, etc. In this system, latent heat released inside the tube due to condensation is transferred to the falling film on the tube surface resulting in convective evaporation. Among many heat transfer enhancement techniques, thermal spray coatings enjoy diverse applications with economic advantage for commercial applications. This paper focuses on Computational Fluid Dynamic(CFD) analysis of falling film evaporation of water on horizontal tubes under the influence of gravity at vacuum. ANSYS Fluent with VOF two-phase model is used for the falling film studies on the horizontal tube. Half section of the tube is modeled with gravitational flow of sea water from the top. This model is used to predict the enhancement in convective evaporation on thermal spray coated tube surface with varying roughness. These CFD results are...

Experimental study of falling film evaporation heat transfer coefficient on horizontal tube

Abstract The horizontal-tube falling film evaporation is a widely adopted technique in multiple-effect distillation desalination plant. It has a high heat transfer coefficient under quite small temperature difference. In this paper, an experimental equipment for horizontal-tube falling film evaporation was set up. Experiments were carried out to show how the heat transfer coefficient is affected by different parameters including heat flux, circumference direction of tubes, spray density, evaporation temperature, and experimental fluid. Results indicate that the heat transfer coefficient decreases after a little increase with growth of spray density. The heat transfer coefficient decreases along the tube circumference, but at the bottom of the tube, it shows increasing trend. In addition, a simple comparison between seawater and fresh water in heat transfer coefficient is also provided.

Numerical Simulation of the Liquid Flowing and Heat-Transfer outside the Tube of the Horizontal-Tube Falling Film Evaporator

This paper mainly studies the falling film evaporator in the field of water desalination. Using the method of fluent simulates the process of the liquid flowing and heat-transfer on the horizontal-tube falling film evaporation. The author analyses the distribution of the liquid film, and obtain the rule that spray density, evaporation temperature, temperature difference and pipe diameter affect the performance of heat-transfer in a certain range. So the paper plays a guiding role in heat transfer enhancement in the falling film evaporator.

Study on the Intervascular Two-Phase Flow Characters of Horizontal-Tube Falling Film Evaporator

In this paper, the mathematical model of a horizontal-tube falling film evaporator that used in air condition and refrigeration system was founded. The evaporative tubes of the evaporator were cold and staggered arrangement. The numerical simulation of the intervascular flow of the oil-bearing refrigerant R134a was carried out, under the conditions of varies flow mode and varies structures of distributed equipment. Under the standard condition of air conditioning, the study of the intervascular two-phase flow characters of the horizontal-tube falling film evaporator were analyzed in the last. The research achievements can provide theoretical laws for relief the effects of the intervascular vapor shear stress on the quality of the liquid film formed outside the evaporative tubes. And the conclusions have practical and guide means for the horizontal-tube falling film evaporator design and evaporative tubes arrangement optimization.

Numerical simulation and performance analysis of horizontal-tube falling-film evaporators in seawater desalination

A comprehensive distributed parameter model for simulating the steady-state performance of a practical horizontal-tube falling-film evaporator has been developed and validated. This model is capable of predicting the distributions of thermal parameters in the tube-side and shell-side, which provide important information of heat and mass exchange processes. The fluid flow and heat transfer characteristics in tubes are analyzed in detail. The computational time is reduced significantly in comparison with the Computational Fluid Dynamics. Based on the numerical results, it is found that the steam is not evenly distributed in the horizontal tubes of each tube pass, which is favorable for parallel channels with uneven heat fluxes. The mass and heat flux of steam are mutually matched, indicating that the self-compensation characteristic appears among the tubes. In addition, the overall heat transfer coefficient reaches the maximum value of about 3300 W/m 2 K at the entrance region of each tube pass, and then decreases gradually along the flow direction. As liquid film falls downward from tube to tube, the liquid flow rate of seawater continually decreases from 0.063 kg/ms to 0.04 kg/ms with the corresponding salinity gradually increasing from 36 g/kg to 56 g/kg.

NUMERICAL INVESTIGATION OF FILM DISTRIBUTION IN HORIZONTAL-TUBE FALLING FILM EVAPORATOR

The technique of horizontal-tube falling film has been used in the cooling and heating industries such as refrigeration systems, heating systems and ocean thermal energy conversion systems. The comprehensive performance of evaporator is directly affected by the film distribution characteristics outside tubes. In this paper, numerical investigation was performed to predict the film characteristics outside the tubes in horizontal-tube falling film evaporator. The effects of liquid flow rate, tube diameter and the circular degree of tube on the film thickness were presented. The numerical simulation results were compared with that of the empirical equations for calculating the falling film thickness, and agreements between them were reasonable. Numerical simulation results show that, at the fixed fluid flow density, the liquid film is thicker on the upper and lower tube and the thinnest liquid film appears at angle of about 120°. The results also indicate that, when the fluid flow density decreases to a certain value, the local dryout spot on the surface of the tube would occur. In addition, the film thickness decreases with the increases of the tube diameter at the fixed fluid flow density.

Thermodynamic performance of a low temperature multi-effect distillation experimental unit with horizontal-tube falling film evaporation

A five-effect distillation experimental unit was designed and fabricated and the thermodynamic performance was measured and evaluated. The mechanical design and process operation parameters of a low temperature multi-effect distillation experimental unit were optimized. This nit featured horizontal-tube falling film evaporation. Its designed performance was achieved and maintained, and its heat loss was effectively reduced. The variation of temperature and pressure in each evaporator effect and the thermodynamic performance of the unit were tested and ana lyzed. The results indicated that the measured and calculated vacuum and evaporating pressure in each effect coincided. Water production rate and gained output ratio (GOR) decreased slightly with an increase in flow rate of feed seawater. With an increase in flow rate of heating steam the water production rate linearly increased and GOR slightly increased. The calculated results were found to be fairly close to the experimental observation, which justified the expectation that the analytical model developed for the multi-effect distiller was reliable, and the overall design of the experimental system was correctly done.

Scale Formation of Mixed Salts in Multiple-Effect Distillers

In multiple-effect distillers (MEDs) with horizontal tube falling film evaporators, difficult-to-control scale is formed on the outside of the tubes, causing considerable capital, operating, and maintenance costs. Because seawater is a multicomponent electrolyte solution, scale formation is primarily caused by coprecipitation of inorganic salts, such as calcium carbonate, magnesium hydroxide, and calcium sulphate, which are insoluble at elevated temperatures. Therefore, solubility data and rate constants obtained under single-salt precipitation are not applicable. The presence of another precipitating salt in the solution affects the precipitation thermodynamics and kinetics, as well as scale structure and strength. Because the scaling process is complex, research primarily has been restricted to single-salt precipitation. A horizontal tube falling film evaporator in pilot-plant scale was used to study scale formation under conditions approximating industrial MEDs at an evaporation temperature of 75 °C, exceeding the top brine temperatures currently used in MEDs. To investigate the interactive effect of mixed salts, experiments were conducted with artificial seawater and model-solutions based on artificial seawater with varying calcium or magnesium concentrations. Scale on the outside of horizontal tubes was analyzed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, wide-angle X-ray diffraction, and atomic absorption spectroscopy to obtain structural, chemical, and quantitative information.

Prediction of the Performance of Falling Film Evaporator with Horizontal Tube Bundle Based on a Distributed Parameter Model

A distributed parameter model is developed for predicting the performance of a horizontal-tube falling-film evaporator. In this model, the variation of heat transfer performance along the tube length and array, as well as the effect of the dry patch on the performance are considered. The model is applied to predicting the performance of a commercial falling film evaporator, and the influences of bundle layout, pass layout, refrigerant mass flow rate and the flooded level of refrigerant on the evaporator performances are studied. The results show that the simulation result agrees well with the experimental data, and it is possible to decrease or avoid the dry patch area on the tube bundle and therefore improve the evaporator performance by rationallly designing the layout of the tube bundle and the flooded level of the refrigerant.

Effects of process parameters and anti-scalants on scale formation in horizontal tube falling film evaporators

In multiple-effect distillers with horizontal tubes, which are increasingly employed in seawater desalination, scale formation on the heat transfer surfaces has a highly deleterious effect on specific energy consumption and production capacity. The allowance for potential scale formation by over-sizing the heat transfer surface, scale prevention methods, cleaning methods as well as production losses during plant shutdown for cleaning create considerable capital, operating, and maintenance costs. It is therefore essential to incorporate a cost-effective scale control technique. In multiple-effect distillers scale is formed on the outside of the tubes. Mechanical cleaning methods cannot be employed. Achieving effective scale control by suitable anti-scalants is a major challenge for the desalination industry. Scale formation and control in multiple-effect distillers with horizontal tubes for seawater desalination have barely been investigated. The effects of the process parameters and of the anti-scalants on the properties of the scale, e.g., the composition, structure, and morphology, are inadequately understood. Clarifying these aspects of scale formation would facilitate further improvement of anti-scalants and thus the efficiency of multiple-effect distillers. In order to study scale formation and control under conditions that are close to those prevailing in industrial multiple-effect distillers, a horizontal tube evaporator at pilot-plant scale was employed. In the test rig the process conditions in any stage and at various operating conditions of multiple-effect distillers can be provided. Thus, the effects of the thermal field, the flow field, the fluid properties, and the substrate properties can be studied. Experiments with artificial seawater were performed at different operating conditions. Novel aspects of the formation and reduction of scales formed from falling seawater films on the outside of horizontal tubes are presented. The effects of various process conditions such as fluid temperature, heat transfer surface temperature, heat flux, and seawater salinity on the composition and structure of scale layers are shown. The beneficial effects of polymeric anti-scalants on the quantity, composition, and morphology of the scale are discussed. For example, the use of only 0.5 ppm of a specific anti-scalant effectuated a very strong reduction of the calcium carbonate content in the scale as well as a notable decline of the magnesium hydroxide content.

Experimental study on a horizontal tube falling film evaporation and closed circulation solar desalination system

A specifically designed solar desalinization system with a solar collector (about 2.01 m 2 in area) has been developed and tested under practical weather conditions. In this system, a considerable fraction of the latent and sensible heat is successfully recycled and utilized for preheating the feedstock and recycling air via a condensation cavity and heat exchangers. The thermal performance of the system is greatly improved because of the falling film evaporation technology used. As a result, the yield is about two to three times more than that of a conventional single basin type solar still under the same conditions. The transient-state performance of the system, the relationships with the solar radiation, the operating temperature, the feedstock flow rate and the productivity are presented. Other factors influencing the freshwater are also discussed.

Analytical analysis of heat transfer in liquid film dripping around a horizontal tube

In this paper we present a theoretical investigation on heat transfer in a liquid film flowing around a horizontal tube in non-saturation conditions. In fact, we completed the models of Parken and Fletcher [1] and Solan and Zfati [2] by an analysis of the stagnation region located at the top of the tube. Finally, the model developed is validated by a comparison between the numerical predictions and experimental data [3]. The results of this work will be used directly to model a horizontal-tube falling-film evaporator for water desalination [4].

Modelling of heat and mass transfer in a horizontal-tube falling-film evaporator for water desalination

In this study, heat and mass transfer in a horizontal-tube falling-film evaporator, destined to be used in a desalination plant functioning by aero-evapo-condensation process, is studied theoretically. The present exchanger is made of polypropylene and it was designed to work at low temperatures (60°C–90°C) utilising geothermal energy. The one-dimensional model developed, uses basic aerodynamic, hydrodynamic and heat/mass transfer information to predict the performance of the exchanger. The predicted transfer characteristics obtained from the simulations are compared with experimental data. From this comparison it can be learnt that the model is well able to predict the trends of heat and mass characteristics of the evaporator. However, in low liquid film flow rate conditions, the model was found to overpredict the transfer characteristics slightly and the reasons for the differences are discussed. The influence of the different thermal, hydrodynamic and geometric parameters on the evaporator performances was investigated. The variations of the distilled water amount inside the exchanger are analysed. The results are used to determine the operational conditions corresponding to a maximum evaporative performances. The model was also used to optimise the different components of the exchanger.