Falling Film Plate Research Articles

Numerical modeling of a confined falling liquid film sheared by a gas flow in a plate heat exchanger of an NH3/H20 absorption chiller

In recent years, the demand for cooling has seen a substantial increase, primarily due to rising summertime temperatures. Conventional mechanical compression air conditioners have played a predominant role in fulfilling this demand, albeit at the cost of significant electricity consumption. In response, absorption chillers have emerged as an eco-friendly alternative, powered by sustainable heat sources like solar energy or industrial waste heat. The present work is conducted within the framework of developing compact absorption chillers incorporating plate heat exchangers. It builds upon prior research by improving an existing numerical model for falling film plate heat exchangers within an NH3/H2O absorption chiller. This former numerical model evaluates the coupled heat and mass transfer between the liquid film and the vapor flow while excluding hydrodynamic interactions at the liquid-vapor interface. The current investigation aims to address compactness and cost-efficiency limitations in absorption chillers. The consequences of operation within confined spaces were numerically evaluated. An innovative analytical model was developed to quantify the influence of the interactions at the liquid-vapor interface. Shear stress and interfacial perturbations were considered compared to the classical Nusselt model, enabling the evaluation of the changes in the average film thickness and the flooding phenomenon. This analytical model was then added to the previously developed heat and mass transfer model. The numerical model was applied to both the desorber and absorber units. The results show that increased confinement has minimal effects on the transfer coefficients. Additionally, an approach is suggested for estimating flooding in confined spaces and the results are compared with various correlations available in the literature.

Numerical simulation and modeling of the heat and mass transfer in a grooved flat falling film evaporator

Falling film plate exchanger are used in a wide range of systems thanks to their high heat and mass transfer performance for a low cost. In this study, a grooved plate is considered for its ability to improve the wetting of the surface compared to a smooth plate. To characterize the heat transfer and evaporation rate of the system, a numerical model has been developed to take account of the new geometry and has been confronted to experimental results. Despite the high aspect ratio of the falling film, the fin effect is very high on the heat transfer and on the vaporization process due to the triple contact lines. The principal aim of this study is to develop a simple and light model able to accurately describe the heat and mass transfer behavior of film while it flows down in the grooves of the vertical plate of the evaporator. The influence of the operating conditions on the evaporator effectiveness are evaluated thanks to numerical investigations.

Research on falling film dehumidification performance of microencapsulated phase change materials slurry

In the process of liquid desiccant dehumidification, the temperature-rise of the desiccant solution caused by moisture absorption will seriously affect the dehumidification capacity and performance. It is expected that the temperature rise of solution can be restrained by adding microencapsulated phase change materials (MicroPCMs) into the liquid desiccant solution. In this paper, based on computational fluid dynamics (CFD) method, two-dimensional models of flat plate and corrugated plate falling film dehumidification of the microencapsulated phase change material slurry (MPCMS) were developed and validated. The dynamic boundary method was used to determine the interfacial mass transfer coefficient. The influences of several factors on the dehumidification performance, including the concentration of MicroPCMs, the inlet temperature of the MPCMS and the falling film plate configuration, were investigated. The results show that, adding MicroPCMs into the liquid desiccant solution is beneficial to the dehumidification performance improvement. The inlet temperature of MPCMS is a very critical variable. When the inlet temperature of the slurry is in the phase transformation temperature range of MicroPCMs, with the adding of the microcapsule, the temperature rise in dehumidification process can be restrained, and the moisture removal performance can be improved significantly. When the MPCMS inlet temperature is at the phase transformation onset point, and the concentration of MicroPCMs is 5%, the water condensation rate is 14.2% higher than that of the base solution. Moreover, it’s not that the more microcapsules added, the larger mass transfer coefficient is. There is an optimum concentration of MicroPCMs to achieve the maximum interfacial mass transfer coefficient. The corrugated plate configuration is more conducive to the dehumidification enhancement by the MPCMS.

Experimental investigation on heat recovery from flue gas using falling film method

The discharge of the flue gas is rich in water vapor and its latent heat, which leads to the main heat and moisture loss for the natural gas. In this article, based on the falling film spray technology, a new test rig was set up to test the heat and moisture recovery performance from flue gas. The effects of water–air ratio and inlet air temperature on the heat transfer effectiveness were examined in a systematic manner. Four case studies, i.e. empty tower, conical baffles staggered arranged in the tower, falling film plates and baffle plates fixed on the falling film plate were designed for the current system. Experimental results showed that: (i) increasing the water–air ratio or decreasing the inlet air temperature can improve the heat transfer effectiveness; (ii) the case with conical baffles shows the highest heat transfer effectiveness. In comparison with the falling film plates, the arranged baffle plates on the falling film plate can further improve the heat transfer effectiveness; (iii) for the cases with falling plates, the heat transfer effectiveness increases with the increase of the number of the falling film plates. The maximum heat transfer effectiveness is obtained at 5 falling film plates; (iv) for the case with conical baffles, increasing the floor of the conical baffles can improve the heat transfer effectiveness. In the range of the present work, the larger central angle of the spiracle is beneficial to increase the heat transfer effectiveness. For the optimal case in the range of the present work, the heat transfer effectiveness is 41.0% higher than that for the empty tower.

Optimal Number of Effects for Multi Effect Evaporation Seawater Desalination Process Using Solar Thermal Energy-Plate Evaporators

Seawater desalination using Multi effect-plate evaporators and solar energy is one of very promising ways to deal with fresh water shortage and pollution in the same time. Indeed, falling film plate evaporators are becoming more used in the desalination applications. This is because of their multiple advantages compared to tubular evaporators such as high heat transfer rate and low investment cost. This study presents the methodology used for sizing and optimization of plate evaporators for seawater desalination unit using multi effect evaporation and solar thermal energy. The pilot plant unit operates with hot water provided by solar vacuum tubes. The unit was designed for a production capacity of 5 to 7 m3/day of fresh water obtained from evaporation of 28 to 30% of feed seawater. The modeling equations are given at steady-state conditions and they are based on mass, heat balance and heat transfer equations, and thermodynamic and physical properties of each stream. MATLAB programming software is used to resolve the developed algorithm. This work focuses more on studying and analyzing the impact of the number of effects on the total required heat transfer area, the required thermal energy and therefore on the specific cost of water. The obtained results show that it is more beneficial to use nine effects, which gives the key solution for minimizing water production cost. The optimal design shows that the total required heat transfer area of evaporators and the last condenser is approximately equal to 42 m2 and the thermal energy needed to drive the pilot unit is about 64 kW.

Numerical Simulation of Water-Oil Flow on the Falling Film Plate

Dimethyldichlorosilane hydrolysis devices are now faced with problems of incomplete reaction and emulsion between the mixture of siloxane and the hydrochloric acid. This paper adopted ideas of falling film and designed a inclined plate to avoid the problems. CFD software was used to simulate the water-oil falling film on the plate.The simulation results show that using a inclined plate within the reactor can expand the contact area between water and oil effectively. The hydrolysis reaction is completed and there is no emulsified phenomenon happening by using the plate.

PEEK film heat transfer surfaces for multi-effect distillation: A mechanical investigation

Abstract Multi-effect distillation (MED) is a well-established process in seawater desalination. Heat transfer surfaces used in MED-plants are exposed to highly corrosive process conditions due to direct contact to evaporating seawater. The use of polyetheretherketone (PEEK) might be a low cost and less corrodible alternative to expensive metal alloys. PEEK seems to be best suited for application in MED-plants, because it possesses an excellent mechanical and chemical resistance. One drawback of polymers is their poor thermal conductivity. Typical MED process conditions require a wall thickness of about 25 μm for PEEK to get overall heat transfer coefficients comparable to metallic heat transfer surfaces. Therefore, it is necessary to investigate the mechanical behavior of PEEK films, to ensure that they can withstand the mechanical and thermal loads in MED-plants. Stress-strain and creeping behavior of PEEK films in water were investigated at elevated temperatures and described by empirical correlations. Mechanical calculations were carried out to determine tensile loads of the polymer film in a falling film plate evaporator at typical MED process conditions. Parameter variations reveal the limitations of use for PEEK film heat transfer surfaces, but the results also show that PEEK films can withstand the mechanical loads in MED-plants.

Falling film evaporation with polymeric heat transfer surfaces

The corrosive process conditions in common multi-effect distillation (MED) plants require heat transfer surfaces consisting of high-grade metal alloys. However, corrosion resistant polymers can be a reasonable alternative to expensive metals. But it is necessary to use thin polymer films, which must be mechanically stabilized by a spacer grid, to compensate the low thermal conductivity of polymers. A falling film plate evaporator with heat transfer surfaces made out of the high performance polymer polyetheretherketone (PEEK) was already developed based on those considerations. Experimentally measured overall heat transfer coefficients with the prototype heat exchanger at MED process conditions are presented in this publication. They are comparable to typical values of metallic falling film heat exchangers. Furthermore, the heat transfer within the prototype heat exchanger was modeled and compared with the obtained experimental results. It will be shown that correlations valid for falling film heat transfer on a vertical wall are not applicable for a spacer stabilized polymeric heat transfer surface, but they can be used after modifications.

Novel polymer film heat exchangers for seawater desalination

Multi-effect distillation (MED) plants with co-current flow show moderate process conditions with respect to temperature and pressure differences between consecutive effects. Thus, polymers seem to be best suited as a low-cost and less-corrodible alternative to metal alloys. Furthermore, polymer surfaces are less susceptible to scaling. In order to proof this concept, a falling film plate evaporator with heat transfer surfaces made of polyetheretherketone (PEEK) films was built. One of the main challenges is to assure a sufficient mechanical stability of the polymer film and simultaneously provide a low thermal resistance for heat conduction. Calculations and experiments show that a polymer film thickness of 25 μm, in combination with an appropriate spacer geometry, is sufficient for mechanical stability in MED plants. This low film thickness results in a thermal resistance for heat conduction comparable to 1.5 mm thick stainless steel. In order to demonstrate the applicability of polymer films for heat transfer surfaces, experimental investigations of the overall heat transfer coefficient were carried out at different operating conditions with a pilot plant scale polymer film heat exchanger.

Thermal analysis of plate condensers in presence of flow maldistribution

Flow maldistribution in plate heat exchangers causes deterioration of both thermal and hydraulic performance. The situation becomes more complicated for two-phase flows during condensation where uneven distribution of the liquid to the channels reduces heat transfer due to high liquid flooding. The present study evaluates the thermal performance of falling film plate condensers with flow maldistribution from port to channel considering the heat transfer coefficient inside the channels as a function of channel flow rate. A generalized mathematical model has been developed to investigate the effect of maldistribution on the thermal performance as well as the exit quality of vapor. A wide range of parametric study is presented, which shows the effects of the mass flow rate ratio of cold fluid and two-phase fluid, flow configuration, number of channels and correlation for the heat transfer coefficient. The analysis presented here also suggests an improved method for heat transfer data analysis for plate condensers.

Mathematical Modeling, Simulation, and Experimental Verification of CO2 Removal in a Turbulent Contact Absorber

A highly efficient technique of contaminant gas reduction, Turbulent Contact Absorber (TCA), is applied to CO2 removal from a typical flue gas. Aqueous K2CO3 sorbent was evaluated as a regenerable sorbent for CO2 from the flue gas. In order to identify the system, momentum and mass balance equations were written for the TCA tower. A flat plate falling film model was employed to simulate the TCA tower and the effect of turbulence was included in mass and momentum transfer coefficients. To check the accuracy of the model, a pilot scale TCA was built and operated. A Testo type gas analyzer was used to detect gas concentrations at the inlet and outlet of the rig. The model was validated successfully with pilot plant data. The effect of velocity and K2CO3 concentration on the TCA performance has also been carried out. It was found that the bed pressure drop increases linearly with gas velocity and then remains constant. An increase in the liquid flow rate increases liquid holdup, which leads to a rise in bed pressure drop. Higher turbulence within the TCA causes a velocity peak to shift from hypothetical gas-liquid interface towards the falling film plate. An increase of the K2CO3 concentration from 1.0 g mol/L to 2.0 g mol/L was found to give an increase in CO2 removal by about 4 %.

Performance of falling film plate evaporators in reconstructed multiple-effect evaporation station in sugar factory

General trend of free trade in regional level as well as in the direction of European Union has motivated sugar factories located in Serbia to invest into technologies that are more efficient in order to make their products more competitive at the markets in Europe. The aim of this work was to evaluate effects of falling film plate evaporators on the energy consumption of evaporation plant, as well as to validate performance of this type of evaporators. It was found that this type of evaporator decreased energy requirements and in the same time evaporation process was more effective due to high values of heat transfer coefficients. .

Heat recovery from sulphuric acid plants for seawater desalination

It is a well known fact that sulphuric acid production is a highly exothermic process. In that sense, combined with the increased energy cost, surprisingly little heat is recovered from the process. Most often the heat is just cooled off by means of seawater or in air coolers. When recovering heat from the absorption and drying tower coolers, only minor changes to the process are needed. With plate heat exchangers (PHEs), the heat in the sulphuric acid can be recovered with negligible temperature losses and minimum space requirements. The PHE can take crossing temperature programs temperature pinches of less than 5°C and still having k-values of about 2500 kW/(°C·m 2). The heat recovered can be used for district heating of cities, boiler feed water preheating or as heating media in connected processes or adjacent plants. A very interesting use of this heat is for the production of fresh water by desalination of sea or brackish water, when the sulphuric acid plants are situated close to the coast such as several installations in Middle East and Northern African countries. The thermal duties in the acid coolers are very well adapted for heat recovery in combination with production of fresh water by means of multi effect distillation (MED). Evaporation temperature in a MED system is typically lower than 80°C, and the heating energy required is in the range of 60–360 kWh per produced m 3 of fresh water, depending on the number of effects. There are many advantages, both installation-wise and operation-wise, using PHE for highly efficient sulphuric acid heat recovery and falling film plate evaporators for the thermal surface in the desalination system. The high k-value of the plate system gives smaller heat transfer areas and compact installations.

Experimental studies on the dehumidifier and regenerator of a liquid desiccant cooling system

Liquid desiccant cooling systems are being explored as an alternative to the conventional vapor compression systems. The dehumidifier and the regenerator form the heart of these systems. A falling film tubular absorber and a falling film plate regenerator have been studied experimentally. The experimental results have been compared with the predictions from theoretical models. Two wetness factors to account for the improper wetting of exchanger surfaces have been defined and estimated. After taking these factors into account, the theoretical models predict the experimental data within ±30%.