Falling Film Evaporation Research Articles

Energy efficient spray drying by increased feed dry matter content: investigations on the applicability of Air-Core-Liquid-Ring atomization on pilot scale

Spray drying is the main process for production of powdered products from liquid sources. One way to reduce energy consumption of spray drying processes is preconcentration of feed liquids by using more energy efficient techniques, such as membrane reactors or falling film evaporators. As feed viscosity increases with dry matter content, efficient atomization at high viscosities is necessary. In contrast to standard pressure swirl (PS) atomizers, Air-Core-Liquid-Ring atomization may be an option. Experiments on a pilot scale show its high potential for application in spray drying processes as feed dry matter content may be increased up to 50% compared to PS atomization. The theoretical energy savings associated with the 50% feed solids increment is about 29%. Further investigations are required to assess the complexity, the practicality and the potential benefits for industrial-scale operations.

Innovation of Cooking Juice in Process Production of Red-Cane Sugar in Lawang Village of Agam District

Cane refining activities which produce red-cane sugar has traditionally been the primary source of income in Lawang villagers of Agam district since hundred years ago. Three main weaknesses found in the process traditionally conducted are lower evaporation rate caused in longer time production and need more fuel, lower product quality caused in lower price, and flue gas pollution. Heat transfer mechanisms involved as long as evaporation takes place in traditional cooking process of juice, namely conduction, convection, and radiation, are not well evaluated yet. Meanwhile, falling film evaporation is not found and gummy cakes formed and accumulated in the surface of basket made of woven bamboo placed in the juice pot. The influence of evaporator like device made of a series of diameters of aluminium circular plates to evaporation process has been studied in traditional cane refining in Lawang village. The device was introduced to the juice pot which significantly faster evaporation process, namely reached about 45% faster then what usually run by villagers. The influences of introduction of lids of aluminium plates as well as any variation in circular plates to faster evaporation process had also been studied. It showed that the numbers of circular plates in a series and introduction of lid of aluminium plate contributed dominantly to faster evaporation process till about 56.67% faster, while the incline of circular plates in a range of 0 to 20 degree and the existence of some holes in the inner plates for circulation of juice cooking are not significantly shorten the time needed for evaporation process. It was also found in this study that falling film evaporation occurred in the evaporator like device introduced that contributed to faster evaporation process. The special channel added at the outsider of circular plate could play a role to accumulate gummy cakes formed.

Characteristics of a Falling Liquid Film Flow Using Periodic Jetting Flow

This work tried to form a falling thin film by supplying liquid to the disk attached to the upper part of the device, rotating the disk and jetting liquid onto the wall surface in order to develop a new falling film evaporator without contamination. Heat transfer enhancement is expected by the surface renewal effect of unsteady and/or periodical flow due to the rotating disk with ditches. The influence of the shape and rotation speed of the disk on the flow of the falling thin film was investigated. Four kinds of rotating disk having small two or four ditches with a depth of 1 mm were used. The apparatus consists of a rotating disk, a cylindrical glass tube and a reservoir. An aqueous solution of uranine is supplied from the reservoir to the upper part of the rotating disk by a pump. The flow rate of the aqueous solution was 88 L/h, and the rotation speed of the disk was set at 200 or 300 rpm. The time-dependent flow condition was observed by irradiating a UV lamp from the side of the apparatus and photographing the front test section with the vertical length of 200 mm by a digital camera. The correlation between film thickness distribution and heat transfer characteristics suggested that thinner film thickness, nonuniform film thickness distribution with higher intensity due to large-scale waves, and higher circumferential velocity of liquid film were three major factors to enhance heat transfer.

Improved film evaporator for mechanistic understanding of microwave-induced separation process

Microwave-induced film evaporation separation process has been reported recently to separate the polar/nonpolar mixture. However, the efficiency of the separation is still too low for practical applications, which requires further enhancement via different strategies such as optimization design of evaporator structure. In addition the depth understanding of the separation mechanisms is great importance for better utilization of the microwave-induced separation process. To carry out these investigations, a novel microwave-induced falling film evaporation instrument was developed in this paper. The improvement of the enhancement effect of microwave-induced separation was observed based on the improved film evaporator. The systematic experiments on microwave-induced separation with different binary azeotropic mixtures (ethanol-ethyl acetate system and dimethyl carbonate (DMC)-H2O system) were conducted based on the new evaporator. For the ethanol-ethyl acetate system, microwave irradiation shifted the direction of evaporation separation at higher ethanol content in the starting liquid mixture. Moreover, for DMC-H2O system microwave-induced separation process broke through the limitations of the traditional distillation process. The results clearly demonstrated the microwave-induced evaporation separation process could be commendably applied to the separation of binary azeotrope with different dielectric properties. Effects of operating parameters are also investigated to trigger further mechanism understanding on the microwave-induced separation process.

Establishment of the falling film evaporation model and correlation of the overall heat transfer coefficient.

In order to study the heat transfer of the falling film evaporator with phase change on both sides, in this paper we built the mathematical model and the physical model where the liquid film inside the tube is laminar and turbulent. The film thickness of the condensate at different axial positions, total condensate volume and velocity distribution, and temperature distribution of condensate outside the tube can be obtained by calculating the proposed model. Meanwhile, the liquid film thickness, velocity distribution and temperature distribution inside the tube were obtained by numerical simulation by considering the influence of the liquid film with different compositions on the heat transfer during fluid flow. With ethanol–water as the system, the overall heat transfer coefficient and heat transfer quantity of the falling film evaporator were obtained by the calculation of the model. The accuracy of the proposed model was confirmed by experiments. The model and the calculation of heat transfer proposed in this paper have enormous significance for the basic data and theoretical guidance of the heat transfer performance prediction and operational optimization of the evaporator.

Falling film evaporation of R1234ze(E) and R245fa on a horizontal smooth tube

Falling film evaporation on a horizontal smooth tube with outer diameter of 19 mm was investigated using two refrigerants, R1234ze(E) and R245fa. The heat transfer coefficients were obtained at a film Reynolds number range of 100–1000, heat flux range of 2.5–40 kWm−2, and saturation temperature range of 10–20 °C. The flow patterns of falling liquid film between tubes were observed under adiabatic and heating conditions. The effects of film Reynolds number, heat flux, saturation temperature, and refrigerant properties on the heat transfer were clarified. The heat transfer of falling film was characterized by nucleate boiling and convective heat transfer. Heat transfer was enhanced by increasing heat flux at high heat flux conditions, while heat flux at low heat flux condition had no significant influence. The measured heat transfer coefficients were compared with previous correlations, and a new prediction model that considers both nucleate boiling and convective heat transfer was developed for falling film evaporation.

Evaporative performance of Single Tube and Multi Tube Falling Film Evaporator Prototypes for Molten Butter: An Experimental Study

This paper presents experimental evaluation of evaporative performance of single tube and multi-tube falling film evaporator prototypes for moisture evaporation from molten butter. For single tube falling film evaporator, steam pressures (1.9 kgf/cm², 2.3 kgf/cm², and 2.8 kgf/cm²) and film thicknesses (2 mm, 3 mm and 4 mm) were varied for evaporative performance i.e. steam economy, specific steam consumption, evaporation rate and evaporation ratio. The performance parameters were found statistically significant (p≤0.05) for film thickness and steam pressure and their interactions. The average steam economy (1.75 to 1.80) and evaporation rate (0.18 to 0.21 kg/min) was highest whereas average evaporation ratio (0.83 to 0.86) was lowest for 2 mm film thickness for the studied steam pressures. For multi-tube falling film evaporator, having five tubes in one shell, steam pressure and pump speed were varied for evaporative performance on molten butter. For studied steam pressures, evaporative performance parameters were statistically significant (p≤0.05). For pump speed, evaporation rate and evaporation ratio were statistically significant whereas steam economy and specific steam consumption were statistically non-significant (p≤0.05). At 2.8 kgf/cm2 steam pressure and 320 rpm pump speed, highest steam economy and evaporation rate whereas lowest specific steam consumption and evaporation ratio were obtained. It was observed that falling film phenomena may be used for butter oil making with some improvements.

Study on the performance of a scale inhibitor during horizontal-tube falling film evaporation of seawater

Fouling on heat transfer surfaces is a severe problem and a complex phenomenon in multiple-effect distillation plants with horizontal tube falling film evaporators. In general, the scale on heat transfer surfaces is effectively prevented by adding scale inhibitors in large LT-MED seawater desalination plants. This paper presents an experimental study on the performance of a scale inhibitor with different seawater spray temperatures and salinities during falling film evaporation outside the horizontal tubes. The scale inhibitor is developed by the Institute of Water Treatment of Nanjing Tech University. Experiments are performed under the seawater temperature of 70°C, 80°C and 90°C and the salinity of 30g/kg, 60g/kg and 80g/kg. The dosage of the scale inhibitor is 6mg/L. The titanium tubes are used inside the evaporator and Dalian local seawater is selected as the working agent. The scale growing curves under different conditions are obtained through weighting the amount of the scale formed at different periods. The seawater temperature and salinity have a certain influence on the performance of the scale inhibitor during the horizontal-tube falling film evaporation. As the seawater spray temperature rises, the scale inhibition efficiency decreases. With the increase of salinity, the scale inhibition efficiency decreases.

A study of pool boiling and falling-film vaporization with R-245fa/oil mixtures on horizontal tubes

This paper presents the effect of oil concentration on the pool boiling and falling-film vaporization of oil/R-245fa mixtures with 1%, 2%, and 5% lubricant mass fraction. The saturation temperature ranges from 5 to 20 °C, and the heat flux varies between 6 kW/m2 and 55 kW/m2. Two types of tubes with a 19-mm outer diameter are tested: a smooth tube and a fin tube with 0.4-mm fin height, 60 FPI (fins per inch), and a 0.233-mm fin gap. For both the pool boiling and falling-film vaporization tests, the local oil concentration measured by the refractive index sensors is in good agreement with the average oil concentration obtained from the weight measurement. In case of pool boiling, the heat transfer coefficient for the smooth tube decreases with increasing oil concentration. In contrast, it increases with increasing oil concentration for the fin tube because of a more significant bubbly foam, which facilitates bubble nucleation. For the falling-film evaporation, the heat transfer coefficient increases with increasing oil concentration for both the smooth and fin tubes because the bubble nucleation is enhanced by the foam at a high oil concentration. However, the heat transfer coefficient for the second fin tube decreases with the oil concentration at a high heat flux (>20 kW/m2) because of the partial dry out caused by the liquid held on the top tube.

Review on CFD analysis of horizontal falling film evaporators in multi-effect desalination plants

Review on CFD analysis of horizontal falling film evaporators in multi-effect desalination plants

Heat transfer correlations of refrigerant falling film evaporation on a single horizontal smooth tube

Falling film evaporation heat transfer of R134a and its potential substitutes R290 and R600a outside a single horizontal plain tube is experimentally investigated, and the effects of the saturation temperature, film flow rate and heat flux on heat transfer coefficient are studied. Heat transfer performance of R290 is slightly superior than that of R134a, while R600a is inferior than that of R134a. The threshold film Reynolds number is determined to separate the variation trend of HTC with film Reynolds number into full wetting and partial dryout regimes. Increase of heat flux benefits the heat transfer in both full-wetting and partial dry-out regimes. New heat transfer correlations based on the present data and data for R32 and R1234ze(E) in the authors’ group are suggested for two regimes. The correlation for full wetting regime fits 96.7% of the total 542 correlated data within ±30% while fits 73.4% of the total 289 data in references from −30% to +15%, the correlation for partial dryout regime fits 97.5% of the total 162 correlated data within ±30% while fits 76.8% of the total 95 data from references within ±30%.

Effects of tube shape on flow and heat transfer characteristics in falling film evaporation

Horizontal tube falling film evaporators are widely used in refrigeration and chemical industry. Two-dimensional numerical model with different tube shapes is established in this paper to simulate heat and mass transfer process with CFD method, and the heat transfer coefficient of simulation is also validated by experimental data. Flow and heat transfer characteristics of falling film evaporation on a circular tube and three flat tubes are investigated, respectively. It is confirmed that the liquid film becomes thinner and thinner as height-width ratio increases and average film thickness of three flat tubes is nearly 2.9–16.5% smaller than that of circular tube 1. Additionally, the cross sections of flat tube have a lower dimensionless temperature and a thinner thermal boundary layer, which means a better heat transfer performance compared with the circular tube 1. The average heat transfer coefficient of three flat tubes is approximately 2.2%, 4.2% and 11.2% higher than that of the circular tube, respectively.

Comparative Study of Phenolic Profile and Content in Infusions and Concentrated Infusions of Buddleja Scordioides Treated by High-Intensity Pulsed Electric Fields (HiPEF)

The effect of high-intensity pulsed electric fields (HiPEF) has been reported on the microbial resistance of fruit juices and beverages. However, the influence of HiPEF on bioactive compounds in herbal infusions is still limited. The objective of the present work was to evaluate chemical stability of polyphenols of infusions from Buddleja scordioides or Salvilla under thermal processing (concentrates) followed by HiPEF treatments. Buddleja infusions were prepared at 1% w/v of salvilla, heated, filtered and concentrated in a thin falling film evaporator. Three different HiPEF treatments were applied to Buddleja scordioides concentrated beverages. The percentage of pulse rate was 25 and 90%; output temperature, 18.3 ± 1 °C; and the frequency range, 100, 300 and 400 Hz. The feed flow was 0.5 L/h. DPPH radical scavenging assay, inhibition of Nitric Oxide activity and analysis of phenolic acids and flavonoids by UPLC-ESI-MS/MS were determined. ANOVA one-way analysis and Tukey test (p < 0.05) were used to analyze results. Concentration process increases the amount of flavonols; however, the use of HiPEF produces a minor reduction on antioxidant capacity. The use of HiPEF at 1000 kJ/kg and 1100 kJ/kg displays a similar profile on phenolic acids between HiPEF-treated beverages and concentrates, showing that the use of HiPEF may be a promissory technology in the processing practices of herbal infusions.

Falling film evaporation and nucleate pool boiling heat transfer of R134a on the same enhanced tube

Falling film evaporation and pool boiling of R134a outside a typical reentrant enhanced tube was investigated with an experimental approach. Experimental data from literature with other refrigerants were also compared. The saturation temperature was 11 °C. It was found that the overall heat transfer coefficient for the enhanced tube was as much as 3 times higher than smooth tube. Shell-side falling film evaporation heat transfer coefficient increased by a factor of 2.1–4.9. Pool boiling yielded a higher heat transfer coefficient at higher heat flux for the same enhanced tube. The dependence of falling film evaporation heat transfer coefficient on the heat flux was also different from pool boiling for the same tube. Although the bubbles in pool boiling were pushed up by buoyancy and in falling film evaporating were driven by the flow of films, the transmission of energy were both dominated by the phase change heat transfer. The contribution of forced convection to falling film evaporation heat transfer coefficient is weak compared with phase change heat transfer.

Numerical study of thermal hydraulics behavior on the integral test facility for passive containment cooling system using GASFLOW-MPI

A dynamic film model is developed in GASFLOW-MPI to study the thermal-hydraulic behaviour of the containment with external film cooling, i.e. the passive containment cooling system (PCCS). Former researches on PCCS are mainly conducted with lumped parameter computer codes, hence the CFD study coupling both sides of the containment is necessary. In this study,the dynamic film model of GASFLOW-MPI is validated with both a separate effect test EFFE (Experiments on Falling Film Evaporation) facility and an integral test facility. Firstly, the 2D model of the EFFE facility is built and analyzed in both dry cases and wet cases. The results agree well with the experimental data indicating that the dynamic film model of GASFLOW-MPI can provide reasonable predictions for both heat and mass transfer between the film and gas. Then the 3D model of the scaled down containment test facility is built with the cylindrical coordinate, and a mesh sensitivity analysis is conducted with one section of the 3D model. The 3D model results show that the steady state analysis of GASFLOW-MPI has good agreement with experimental data. Furthermore, GASFLOW-MPI manages to capture the thermal stratification in the containment which is underestimated in former research. The heat transfer analysis on both sides of the containment shell shows that the steam condensation in the dome contributes almost 50% to the total steam condensation, while the film evaporation in the cylinder part contributes about 90% for the heat removal of PCCS. This is the result of heat and mass transfer on both sides of the containment shell coupling with each other. The validation of these two cases indicates that the GASFLOW-MPI can provide reasonable predictions for the thermal hydraulics behavior on both sides of the containment when implementing PCCS. Further research will focus on transient analysis and the model for film coverage.

Experimental investigation on an adsorption desalination system with heat and mass recovery between adsorber and desorber beds

An experimental equipment for adsorption desalination system with falling-film evaporator and heat recovery between adsorber and desorber beds was designed and established. The running performance of the system was investigated experimentally in steady condition. The yield and quality of product water were tested. The results show that the specific daily water production and performance ratio are 4.69 and 0.766, respectively. The heat transfer between tube and shell side in falling-film evaporator is mainly influenced by the amount of vapor desorbed from adsorbents. The utilization of heat recovery cannot increase the water production, but it can save energy. The heat recovery efficiency is 74.7% in every process of heat recovery. All indexes of produced water meet the standard of drinking water in China.

Calcium citrate insolubilization drives the fouling of falling film evaporators during the concentration of hydrochloric acid whey

When dairy powders are produced, the mineral fraction undergoes strong modifications during the vacuum concentration step, leading to the fouling of falling film evaporators. The objective of this study was to determine the nature of the deposits formed during the vacuum concentration of two fouling and highly mineralized products: hydrochloric acid whey and lactic acid whey. These products mainly differ in terms of their mineral composition: lactic acid whey contains a high level of lactic acid and traces of citrate, whereas hydrochloric acid whey contains citrate and no lactic acid. Concentrates at different concentration factors were produced using a pilot-scale falling film evaporator. The compositions of the fouling deposits as well as the precipitates present in the concentrates were deduced from the analytical determination of the composition of the concentrates and their respective diffusible phases. The behavior of the mineral fraction of both acid wheys during concentration was shown to be very different. In the case of hydrochloric acid whey, experimental results suggested a deposition of calcium and citrate ions in the evaporator as well as their precipitation in the highly concentrated products. On the contrary, neither mineral deposition nor precipitation occurred during the concentration of lactic acid whey. This study underlined the key role of citrate ions in the fouling of evaporators during the concentration of hydrochloric acid wheys.

Falling film evaporation of aqueous lithium bromide solution at low pressure

Horizontal tube bundles are often used as falling film evaporators in absorption chillers, especially for systems working at low pressure with H2O/LiBr. The heat and mass transfer processes in these apparatuses are affected by several physical effects and the geometrical design of the heat exchanger, for example the material properties of the working fluid or the surface structure of the heat exchanger. Several influencing parameters are summarized and evaluated. Correlations for the prediction of the heat transfer coefficients of falling film evaporators from the literature are discussed concerning their applicability in dependence of the working conditions of the evaporator in an absorption chiller. In this work, experimental investigations are carried out in a falling film evaporator consisting of a horizontal tube bundle with eighty horizontal tubes installed in an absorption chiller because of a lack of consistent data for heat and mass transfer in the literature. The heat and mass transfer mechanisms and the flow pattern in the falling film are analysed and compared with correlations from literature. The deviations of the experimental data from those of the correlations are within a tolerance of 30%. These deviations may be explained by a change of the flow pattern at a lower Reynolds number than compared to the literature.

Experimental study on heat and mass transfer of falling liquid films in converging-diverging tubes with water

To improve the heat and mass transfer performances of falling films and develop a new type of falling film evaporator, the heat and mass transfer characteristics of falling film evaporation and sensible heating in four different sizes of converging–diverging (CD) tubes were experimentally explored in this study. By analyzing the relationship between the heat transfer coefficient and the falling film flow rate, it was found that the CD tube is suitable for falling film evaporation and sensible heating with a large Reynolds number. For different sizes of tubes, the key factor affecting the heat transfer performance in falling film evaporation and sensible heating is the rib height. At the same rib height and rib pitch, the longer the converging segment of the CD tube, the better the heat transfer performance. According to the comparative analysis on heat and mass transfer performances of falling film evaporation and sensible heating inside the four CD tubes, the CD tube 3# is the best. The evaporation heat transfer coefficient (when the Reynolds number of the liquid films is 2356), evaporation mass transfer rate (when the perimeter flow rate of the liquid films is 0.173 kg/(m·s)), and sensible heating heat transfer coefficient (when the Reynolds number of the liquid films is 1635) of CD tube 3# are 62%, 38% and 63% higher than that of the smooth tube, respectively. For the same CD tube, the evaporation heat transfer coefficient is greater than the sensible heating heat transfer coefficient, but both increases as the flow rate increases. The heat transfer correlations of falling film evaporation and sensible heating in the experimental range were obtained. It will provide a useful reference for future engineering design and industrial applications.

Performance study on a mechanical vapor compression evaporation system driven by Roots compressor

The thermal performance of a mechanical vapor compression (MVC) system was investigated. This work presented describes the mathematical and experimental study of the MVC system, focusing on mathematical models that were established based on the energy and mass balance equations as well as correlations of the thermophysical properties and heat-transfer coefficients. As a result, a MVC experimental platform, which can handle 100 kg/h evaporation rate was designed, with a falling film evaporator heater area of 10 m2. A Root compressor was selected as a vapor compressor, utilizing a power of 18 kw. This paper has studied the feed temperature, evaporation pressure, compressor speed and vapor pressure increment effect on evaporation rate, Specific Moisture Extraction Rate (SMER) and power consumption, as to evaluate the MVC system performance. It was found that feed temperature reach the optimal conditions for saturated liquids. Evaporation pressure should be controlled in low level to ensure that SMER can reach high values and it was furthermore observed that the compressor speed determines the evaporation rate whilst SMER variations showed very subtle effects.