Falling Film Evaporation Research Articles

Effect of fouling on falling film evaporator performance in industrial conditions of fruit juice concentrate production

In food processing, falling film evaporators are commonly used and their energy consumption is a significant operating cost for the production of juice concentrate. This cost is influenced not only by the evaporator design but also by the method of operation, particularly the fouling of the heat exchanger walls formed during the operation of the evaporator on the inner surface of the tubes from the juice side. These deposits are a significant component of the total wall thermal resistance, thus hindering evaporation. This issue results in a production decrease and requires evaporator cleaning. The publication presents an analysis of the calculation methods of fouling formation and their thermal resistance based on a literature review on evaporators for the production of sugar and milk. The obtained results were then processed by adjusting the coefficients to the experimental results obtained during the actual production of fruit juice concentrate. The decrease in actual evaporator efficiency was used to experimentally verify the theoretical calculations. This analysis resulted in a final simple formula for the estimation of the fouling influence on evaporator performance under industrial conditions. Simulations have shown that a sharp drop in evaporator efficiency may occur with insufficient recirculation or prolonged operation without cleaning. In this case, the amount of sediment can increase and cause a change in the boiling character and thus a rapid decrease in evaporation. • Effect of fouling on falling film evaporator for fruit juice concentrate production is estimated. • Fouling build-up speed on the inner surface of the tubes for fruit juice concentrate production is estimated. • Industrial fouling conditions in the production of fruit juice concentrate are analysed.

Dynamic modeling of a multi-effect vertical falling-film evaporator for water reuse in CSP plants

This work presents the dynamic modeling of a vertical multi-effect evaporator plant designed and manufactured for its installation at a commercial concentrating solar power (CSP) plant, within the framework of EU H2020 project SOLWARIS (Solving Water Issues for CSP plants). The model has been developed using Modelica computational language and implemented in Dymola® software environment. The results from the validation show a good agreement against the design data, obtaining relative errors lower than 5%. The dynamic response of the plant against external disturbances of the motive steam mass flow rate, feedwater mass flow rate and condenser pressure has been investigated. The main results reveal that increasing the motive steam flow rate by 5% produces a similar increment of the water recovered (5.2%), although the concentrate salinity is raised to an unsafe operation zone (106%) that could lead to scaling issues in the evaporators. The same effect occurs when the feedwater is decreased by 5% from its nominal value, causing a significant rise in the concentrate salinity (163%). In those cases, the simultaneous and proportional variation of the motive steam and feedwater mass flow rates allows maintaining the outlet concentrate salinity far from scale formation limits.

Numerical study on the flow and heat-transfer characteristics of horizontal finned-tube falling-film evaporation: Effects of liquid column spacing and wettability

The horizontal finned-tube falling-film evaporator has been successfully used in desalination and other industries. An effective three-dimensional Computational Fluid Dynamics (CFD) model was established to investigate the falling film and heat transfer characteristics of the horizontal finned-tube, and the numerical results were verified by experimental data. The influences of the liquid column feed spacing (S) and Reynolds number (Re) on the hydrodynamic characteristics, film thickness distribution and heat transfer characteristics for the finned-tube falling-film evaporation were explored. The fluid behavior and heat transfer performance with the variation of contact angle (α) were further revealed. At low Re, the liquid films fail to fully cover the tube surface with appearance of dry wall, causing the substantial decrease in wetting ratio and heat transfer performance. When Re = 684, the wetting ratio and average heat transfer coefficient were greatly improved, reaching 0.96 and 7922 W/m2·K, respectively. Heat flux has little effect on heat transfer performance. Additionally, as α increases, both the wetting ratio and the average heat transfer coefficient decrease. The average heat transfer coefficient drops from 8052 W/m2 K at 0° to 3692 W/m2 K at 80°. Furthermore, under different Re and α, smaller S exhibits better wetting properties and heat transfer characteristics.

Experimental study on horizontal tube spray falling film evaporator in a low temperature geothermal binary cycle power system

The horizontal tube spray falling film evaporator is an efficient evaporation equipment, which has the advantages of good heat exchange effect, small equipment volume and low cost. In this paper, R245fa is used as the working fluid to build a heat transfer test platform for the horizontal pipe spray falling film evaporator of the Organic Rankine Cycle(ORC) power generation system. The effect of factors such as the spray density of the organic working fluid, the initial temperature of the geothermal water and the flow on the heat transfer coefficient outside the tube is studied. The experimental results show that with the increase of the spray density of the organic working fluid, the initial temperature of the geothermal water, and the flow of the geothermal water, the heat transfer coefficient first increases and then decreases.

A comprehensive review on computational studies of falling film hydrodynamics and heat transfer on the horizontal tube and tube bundle

Falling film evaporation is a promising technology widely applied in refrigeration, desalination and beyond applications owing to the great advantages. However, the liquid film hydrodynamics and heat transfer performance are studied insufficiently. Compered with experiments, numerical simulations are economical and efficient especially in the study of liquid film, through which some microscopic understandings are expected to be extracted. In this paper, a comprehensive review for a large pool of computational studies about falling liquid film flow and heat transfer on the horizontal tube and tube bundle is presented. Computational methods of liquid–gas two-phase flow and mass transfer model, as well as relevant researches were first introduced. Then, the studies on the falling film hydrodynamics, film thickness, sensible heat transfer, flow pattern, evaporation and boiling outside the single tube, tube bundle, special-shaped tube and entire evaporator studied with 2D and 3D models are respectively reviewed in order. Then, the investigations on the falling film breakout and dryout are reviewed. And then, the numerical predictions of falling film thickness and heat transfer coefficient are involved. Afterthat, the benchmark data for falling film numerical simulation are summarized. Finally, some future needs and recommendation relating to crucial technologies that must be solved are proposed.

Prediction of dryout in evaporation of falling films on horizontal plain tubes

Accurate prediction of dryout during falling film evaporation is important to ensure good performance as post-dryout heat transfer coefficients are very low. No prediction method verified with a wide range of data is presently available and there has been a need for one. A new correlation is presented for dryout of saturated fluids on plain horizontal tubes. It has been verified with data for seven fluids from eight sources. The fluids are R-11, R-134a, R-236fa, R-410A, isobutane, and propane. Other parameters in the database are: heat flux from 0.2–95 kW m−2, reduced pressures from 0.00164 to 0.15, and liquid Reynolds numbers from 13 to 970. The new correlation predicts the data from single tubes and arrays of tubes with mean absolute deviation (MAD) of 15.8%. The same data were also compared to other correlations and those had much higher deviations. The new correlation and results of comparison with test data are presented and discussed.

Numerical investigation of the falling film thickness and heat transfer characteristics over horizontal round tube

The numerical investigation was performed to analyze the film thickness and heat transfer characteristics over the horizontal round tube of the falling film evaporator. A three-dimensional model for the falling film flow and heat transfer was established, and the volume of fluid (VOF) method was applied to track the gas-liquid interface. The effects of different spray densities, saturation temperatures, and longitudinal inter-tube spacings (Γ, Tsat, s) on the falling film thickness (δ) and local heat transfer coefficient (h) were analyzed. Asymmetric circumferential distribution of δ is captured by the simulation results. The h reduces first and then rises slightly as the φ increases, and the maximum value appears in the impact point. Moreover, in the circumferential direction, φ=70∘ is the critical value for the variation of δ under different s, and the increment of Γ and Tsat leads to the increase in δ and h. In the axial direction, with increasing the Γ, the δ and the h increase, and the axial size of the crest zone tends to narrow. In addition, the increase of s promotes the rise of h. Meanwhile, the δ varies slightly in the crest zone, and it decreases obviously in the stable zone.

Control of falling-film evaporators with modeling of fouling

Fouling is an undesirable phenomenon in milk evaporation, which affects the cost of production and the quality of the product. This phenomenon can be reduced by using different approaches, where process control is a feasible alternative. There are a large number of proposals for the control of a multi-effect falling-film evaporator (MEFFE), but the effect of process control on fouling has been omitted. In this work, a PID control strategy for MEFFE is developed under a dynamic model with computing of fouling and falling-film thickness. The strategy proposes several control configurations, from which the best option is selected by considering control stability, low fouling thickness and low demand of motive steam. The control strategy was applied to two case studies; the results were validated with experimental data and compared with other control techniques. The results show that this strategy is suitable for MEFFE, and guarantees the quality of the concentrated product.

CFD Modeling of Crystallization Fouling with CO2 Desorption Incorporated for Falling-film Evaporator in Thermal Desalination

CFD Modeling of Crystallization Fouling with CO2 Desorption Incorporated for Falling-film Evaporator in Thermal Desalination

A novel interferometric method for simultaneous measurement of film thickness and film interface temperature for a horizontal tube falling film evaporator for MED systems

Horizontal tube falling film evaporation spans a lot of applications, especially in multi-effect desalination application. Hence, the improvement of its gain output ratio is of prime importance, and for that, the studies on the characteristics associated with the flow, mainly falling film thickness and falling film interface temperature, are inevitable. As the falling film flow is a delicate fluid structure, non-intrusive measurement techniques have to be preferred for the experiment. In this work, we introduce a novel interferometric method to calculate film interface temperature and a shadowgraph approach to estimate film thickness simultaneously. A Mach-Zehnder interferometer with infinite fringe setting was used to capture the thermal fluctuations around the horizontal tube falling film evaporator. Interferometric method implemented in this work also serves as an image visualization technique and does the purpose of qualitative as well as quantitative analysis. An in house designed experimental setup was fabricated for producing a thin and uniform falling film with a fully wetted flow over the horizontal copper tube. A heater controller setup was incorporated into it for accurate temperature control. This method holds the advantage of scanning falling film thickness over circumferential angle for the range 5∘≤θ≤175∘, which is an improvement to state of the art. Furthermore, dynamic film thickness and instantaneous film interface temperature studies were also incorporated to prove the efficacy of the technique and to test the robustness of the algorithms implemented. CFD analysis was also carried out for the entire process by means of VOF scheme and was found to be in good agreement with the experimental results, which proves our technique to be a good one. A standard error of mean analysis was performed on every data set, which validates the reproducibility and the dynamic measurement capability of the technique used.

Influence of operating temperature range on the performance of multi-effect desalination (MED) plant

Desalination technology provides a long-term option for supplying freshwater to water-scarce coastal areas, such as the region between Karachi and Gwadar in Pakistan. Desalination through reverse osmosis (RO) is a low-energy process. Recent advancements in multi-effect desalination (MED) technology, such as integrating adsorption/absorption vapor compression systems and the synthesis of novel antiscalants, have made it a viable option. The current top brine temperature (TBT) for the MED unit is 65 °C (due to salt precipitation in the evaporators), which may accommodate 8 – 10 falling film evaporators. However, with to advancements in antiscalants, the TBT can now be extended to 85 °C, indicating the inclusion of more evaporators and hence increased freshwater yield. A mathematical model of the MED unit is created in Engineering Equation Solver (EES) for this purpose, and the MED unit's performance is evaluated at increased TBT. The mathematical model is validated using a literature-based empirical model as well as a commercial desalination facility in Qatar. This work presents and discusses the influence of the number of effects, TBT on performance ratio, specific heat transfer area, and specific cooling water requirement.

Falling film evaporation experiment and data processing method of R1234ze (E) on horizontal enhanced tubes

For two kinds of specially made enhanced tubes with outer diameters of 19 and 25 mm, the falling film evaporation experiment was carried out by using environmental refrigerant R1234ze (E) under the experimental condition that the water velocity in the tube was controlled from 1 to 3.5 ms−1. "The Wilson-Gnielinski Separation Method" and "The Wilson-Gnielinski-Heat Flux Correction Method" were used to process the experimental data. The comparative analysis showed that when the heat fluxes of all the test points outside the tube were constant, the results of two calculation methods were almost the same. When the heat fluxes of all the test points outside the tube were fluctuant, the results of two calculation methods were different and the heat flux correction method should be applied to reduce the error. The enhanced tube with outer diameter of 19 mm was used to investigate the heat transfer performance of falling film evaporation, with spray density ranging from 0.03 to 0.1 kgm−1 s−1, heat flux ranging from 15 to 45 kWm−2 and evaporation temperature ranging from 0 to 25 °C. And the enhancement rate inside the tube was obtained by using the heat flux correction method. Finally, the variation trends of the overall heat transfer coefficient and the outside tube heat transfer coefficient under different working conditions were both obtained experimentally and analyzed theoretically.

R-1233zd(E) 냉매를 적용한 유하액막식 증발 열전달 특성에 관한 실험적 연구

Along with the development of low global warming potential (GWP) refrigerants to prevent global warming, the development of a falling film type evaporator is required to reduce the charge amount of refrigerant in an evaporator. Unlike existing flooded evaporators, the development of a falling film evaporator using refrigerant is still in an incomplete state. In this study, factors affecting performance characteristics of falling film evaporation using R-1233zd(E) refrigerant were experimentally confirmed and effects of film Re and heat flux on heat transfer were investigated. As film Re of each tube increased, heat transfer coefficient also increased. It then converged after a certain film Re. The relationship between the heat flux and the heat transfer coefficient was examined.

Experimental Study on CaCO3 Fouling Characteristics During Falling Film Evaporation

Abstract Falling film evaporation is widely used in solar desalination systems. Fouling is an important problem to be addressed in many applications involving heat transfer including processes involving the utilization of solar energy in desalination applications. In the research upon which this paper partly reports, an experimental investigation was carried out on a vertical tube in falling film evaporation to determine the effects of temperature, velocity, the use of a porous-sintered tube, and the use of Teflon coating on calcium carbonate deposition characteristics. During the fouling experiments, the pressure inside the test tubes was maintained constant at 101.3 kPa, and the inlet temperature was maintained at 373 K, while allowing the water mass velocity to vary from 0.42 to 1.05 kg m−1 s−1. Results show that the fouling in the test tube becomes more serious as the temperature increases and the flowrate decreases. Compared with stainless-steel tubes, porous-sintered tubes can significantly reduce fouling resistance, but at the same time they bring about a decrease in the heat transfer coefficient. The Teflon coating also has anti-fouling performance but does not affect the heat transfer coefficient in stainless-steel tubes. Through the weighing of local fouling deposits, it has been found that the mass of the fouling deposits in the lower part of the tested tubes is greater than that in the upper part.

Experimental study on hydrodynamics of ionic liquids systems in falling film evaporator

Ionic liquids (ILs) have been proved as novel excellent solvents in gas separation and so on. Due to its excellent heat and mass transfer performance, falling film evaporator is often used in the applications of high viscosity systems and has the potential to be the regeneration equipment for IL-based systems. However, there are few reports on the falling film evaporation of ILs system. The relevant experimental data are scarce. Moreover, the traditional models cannot predict the hydrodynamics of ILs well due to the specific properties of ILs. In this study, an experimental method for studying the ILs falling film flow was established. The falling film hydrodynamics of three ILs (BmimBF4, BmimPF6, OmimBF4), including film flow pattern and film thickness were investigated. The effects of streamwise flow distance and the physical property of ILs on the hydrodynamics were analyzed. A calculation model and a dimensionless empirical model of ILs film thickness were established based on experimental data. The average relative deviation of new dimensionless model is 2.83%, which is much lower than that of Nusselt model (13.48%). The measurement of key parameters and the establishment of models provide experiment data and basis for designing the falling film evaporator of IL-based systems.

Prediction of heat transfer in evaporation of saturated falling films on bundles of horizontal tubes

Falling film evaporators consisting of bundles of horizonal tubes are widely used in many applications including refrigeration, chemical processing, desalination, etc. While many experimental studies have been done, as yet there is no well-verified method for the prediction of heat transfer. The research reported here was done to fulfill this need. A correlation is presented for heat transfer before occurrence of CHF which has been verified with a wide range of data from 12 sources. The data include plain and enhanced tubes of many types and materials, diameters 15.9 to 25.4 mm, P/D 1.1 to 2.0, liquid film Reynolds number from 43 to 6449, and heat flux from 1 to 94 kWm −2 . Included are eight diverse fluids (water, halocarbon refrigerants) with reduced pressures from 0.000046 to 0.172. The correlation predicts 2708 data points with mean absolute deviation of 17.4%. The correlation and its verification are presented and recommendations are made for its use.

Experimental and numerical study on the falling film flow characteristics outside circular tube applied in floating liquefied natural gas (FLNG) under offshore conditions

Spiral wound heat exchanger (SWHE) relying on falling film evaporation and boiling is often used for FLNG. The performance of SWHE can be impacted strongly by the motion of the FLNG caused by the wave and typhoon. The falling film characteristics of SWHE outside circular tube are studied experimentally and numerically by a visualization experimental device based on the high-speed camera and a numerical model based on the dynamic grid. The results show that the wave crest of the liquid film moves to the titled side under offshore conditions. The evolution process of falling film flow pattern outside circular tube with the tilt angle of 9° can be divided into four stages: droplet formation and migration, liquid column formation and migration, liquid column coalescence, liquid sheet formation. A correlation permitting the prediction of the falling film flow pattern outside circular tube and the other one permitting the prediction of the average film thickness of circular tube are developed respectively based on the experimental and numerical data.

The coupling process of steam condensation and falling film evaporation with non-condensable gas

For the steam condensation process inside a horizontal tube under small temperature difference, the operating parameters such as steam flow rate and temperature difference are narrowed in limited variation ranges to guarantee the high heat transfer performance. When the steam is mixed with non-condensable gas, the effective temperature difference shows more complexity compared with pure steam condensation. In this paper, the condensation process of the mixed steam in the tube cooled by the falling film evaporative seawater outside the tube is calculated with the steam mass flow rate of 2–9 kg/(m2·s), the evaporation temperature of the seawater of 50–70 °C, the total temperature difference of 2–4 °C and the inlet non-condensable gas (NCG) mass fraction of 1–4%. The mixed steam condensation heat transfer model combined with distributed parameter model is established and validated to couple the heat transfer on both sides of the tube. The local saturation temperature depression of the mixed steam condensation is divided into two factors, one caused by the flow resistance and the other caused by the non-condensable gas mass fraction. Both types of saturation temperature depression increase with the increase of the inlet non-condensable gas mass fraction. The mass flow rate and the temperature difference are found to have more significant impact on the total temperature depression on the saturation temperature. Besides, the saturation temperature is found to have no significant impact on the condensation heat transfer of the mixed steam. The mass flow rate corresponding to the maximum heat transfer of the heat transfer tube is presented.

A general correlation for heat transfer during evaporation of falling films on single horizontal plain tubes

Falling film evaporators are widely used in many industries including desalination, petrochemical, and refrigeration. Hence ability to correctly predict heat transfer in these evaporators is important. While many prediction methods have been proposed, none has been validated with data from many sources covering a wide range of parameters. Hence there is a need for a well-verified general correlation. This research was done to fulfill this need. A correlation has been developed for heat transfer coefficient of single horizontal tubes with falling film evaporation. It is shown to be in good agreement with data from 22 sources that cover a very wide range of parameters. Included in the data are 11 fluids (water, ammonia, halocarbon refrigerants, hydrocarbons), tube diameters from 12.7 mm to 50.8 mm, heat flux from 1 to 208 kWm−2, and liquid film Reynolds number 19 to 10,734. A total of 1237 data points are predicted with a mean absolute deviation of 17.4%. The new correlation is presented together with details of its verification.

A new multivariable control concept for the falling film evaporator process

The paper presents a new multivariable control concept for falling film evaporators (FFEs). Our concept solves the major challenges encountered in modern FFE control: large transport delays, additional control of the output mass flow, coupling of controlled variables, and disturbances due to time-varying input dry matter content. The challenges are addressed together, for the first time, by the following control design. Based on a dynamic nonlinear input–output model, we consider a linearizing output transformation to enable application of classical linear control methods composed of feedforward design, disturbance rejection, and a decoupling network. Due to these features, we are able to design robust PID and PI controllers that substantially compensate plant-model mismatches. Connecting our concept to a digital twin of the plant yields good performance, which encourages future application of the design in the real-world process.