A Unified Scaling-Up Technique for Pneumatic Conveying Systems

A major challenge facing the designers of pneumatic transportation systems is how to scale up reliably based on the results from pilot-scale test facilities. Further, even if dense phase flow condition prevails at the start of the conveying system, it may be a dilute phase flow condition at the end of the pipeline. Hence, any scaling-up technique should be able to address the dynamic change of flow condition along the pipeline. The scaling-up technique presented here using the pressure drop prediction models based on modified Darcy-Weisbach equation successfully addresses these dynamic changes. It has been shown that the pressure drop coefficient ‘K,’ as defined by the models, is independent of the pipe diameter. Further, in the case of vertical conveying, ‘K’ has been shown to be independent of particle size distribution for a given material. The predicted pressure values were found to be in reasonably good agreement with the experimental results varying from 3.5% to 19.9%.     

A Possible Scaling-up Technique for Dense Phase Pneumatic Conveying

In this article experimental findings have been presented to show that the pressure drop coefficient (K) for vertical and horizontal pneumatic conveying for a given bulk material follows a certain pattern. The pressure drop coefficient for vertical pneumatic conveying for a given material has been found to be independent of any variation of particle size distribution, within experimental limits. The pressure drop prediction technique proposed by the authors previously has been validated with the test results of alumina and bentonite.     

A Pressure Drop Model for Positive Pneumatic Conveying of Flour Through a Vertical Pipeline

A differential equation of motion for gas-flour two-phase flow in a vertical pipe was first derived based on the momentum conservation and by adopting two empirical expressions for the velocity ratio of flour to gas and frictional coefficient between flour and pipe wall, and then a pressure drop model for dilute positive pneumatic conveying of flour through a vertical pipeline was developed by employing the continuity and state equations for gas. The conveying tests were conducted on a positive pneumatic conveying system of flour in a flour mill. Under each of the six different flow conditions, the conveying parameters, such as the flour and gas mass flow rates and the pressure drop between two selected cross sections on the vertical pipeline were measured. The pressure drop between the two selected cross sections was evaluated using the pressure drop model for each of the six flow conditions. The calculated values of pressure drop agree well with the measured data, and it is demonstrated that the model is applicable to vertical positive pneumatic conveying systems of flour.     

A Simple Technique for Scaling Up Pneumatic Conveying Systems

A brief survey has shown that although scaling-up techniques in pneumatic conveying systems have generally been based on laboratory-scale test data, there still exists a divergence of opinions about the right choice of certain basic parameters such as solids friction factor and air friction factor. In this article, a simple model for pressure drop calculation has been proposed based on the classical Darcy's equation with some modifications. A parameter K, called pressure drop coefficient, has been shown to be independent of pipe diameter and hence suitable for scaling up to pipe sizes different from those used in laboratory-scale tests. For each of the bulk material and pipe size combinations used in this study, we calculated the standard deviation of predicted pressure values from the experimental values along the central 45° line passing through the origin; it varied from±165 mbar to a maximum±285 mbar. It has been shown that the model can be used for both horizontal and vertical pneumatic conveying.     

A Simple Technique for Scaling Up Pneumatic Conveying Systems

A brief survey has shown that although scaling-up techniques in pneumatic conveying systems have generally been based on laboratory-scale test data, there still exists a divergence of opinions about the right choice of certain basic parameters such as solids friction factor and air friction factor. In this article, a simple model for pressure drop calculation has been proposed based on the classical Darcy's equation with some modifications. A parameter K, called pressure drop coefficient, has been shown to be independent of pipe diameter and hence suitable for scaling up to pipe sizes different from those used in laboratory-scale tests. For each of the bulk material and pipe size combinations used in this study, we calculated the standard deviation of predicted pressure values from the experimental values along the central 45° line passing through the origin; it varied from±165 mbar to a maximum±285 mbar. It has been shown that the model can be used for both horizontal and vertical pneumatic conveying.     

Modeling Dense-Phase Pneumatic Conveying of Powders Using Suspension Density

This article presents results of an investigation into the modeling of pressure drop in horizontal straight pipe section for fluidized dense-phase pneumatic conveying of powders. Suspension density and superficial air velocity have been used to model pressure drop for two-phase solids-gas flow. Two applicable models formats (developed by other researchers using two different definitions of suspension density) were used to represent the pressure drop due to solids-gas flow through straight pipe sections. Models were generated based on the test data of conveying power-station fly ash and electrostatic precipitator (ESP) dust (median particle diameter: 30 and 7 µm; particle density: 2300 and 3637 kg m^?3; loose-poured bulk density: 700 and 610 kg m^?3, respectively) through a relatively short length of a smaller diameter pipeline. The developed models were evaluated for their scale-up accuracy and stability by using them to predict the total pipeline pressure drop (with appropriate bend model) for 69 mm I.D. × 168 m; 105 mm I.D. × 168 m and 69 mm I.D. × 554 m pipes and comparing the predicted versus with experimental data. Results show that both the models with suspension density and air velocity generally provide relatively better prediction compared to the conventional use of solids loading ratio and Froude number. For fly ash, the two formats result in considerable different predictions, whereas they provide relatively similar results for ESP dust.     

The effect of oscillating flow on a horizontal dilute-phase pneumatic conveying

ABSTRACT

A horizontal dilute-phase pneumatic conveying system using vertically oscillating soft fins at the inlet of the gas–particle mixture was studied to reduce the power consumption and conveying velocity in the conveying process. The effect of different fin lengths on horizontal pneumatic conveying was studied in terms of the pressure drop, conveying velocity, power consumption, particle velocity, and intensity of particle fluctuation velocity for the case of a low solid mass flow rate. The conveying pipeline consisted of a horizontal smooth acrylic tube with an inner diameter of 80 mm and a length of approximately 5 m. Two types of polyethylene particles with diameters of 2.3 and 3.3 mm were used as conveying materials. The superficial air velocity was varied from 10 to 17 m/s, and the solid mass flow rates were 0.25 and 0.20 kg/s. Compared with conventional pneumatic conveying, the pressure drop, MPD (minimum pressure drop), critical velocities, and power consumption can be reduced by using soft fins in a lower air velocity range, and the efficiency of fins becomes more evident when increasing the length of fins or touching particles stream by the long fins. The maximum reduction rates of the MPD velocity and power consumption when using soft fins are approximately 15% and 26%, respectively. The magnitude of the vertical particle velocity for different lengths of fins is clearly lower than that of the vertical particle velocity for a non-fin conveying system near the bottom of the pipeline, indicating that the particles are easily suspended. The intensities of particle fluctuation velocity of using fins are larger than that of non-fin. The high particle fluctuation energy implies that particles are easily suspended and are easily conveyed and accelerated.     

Prediction of Pressure Drop at the Entry Section from Top Discharge Blow Tank in a Pneumatic Conveying System

Although some literature can be found on the behavior of blow tanks, very few studies could be found on the pressure loss at the entry section to a pipeline (henceforth called entry pressure loss) from a top discharge blow tank in a pneumatic conveying system, even though its magnitude can be significant as compared to the total system pressure drop. This article presents the results of an experimental study carried out to assess this entry pressure loss. The results indicate that it is possible to scale up the entry pressure loss based on laboratory-scale tests with a reasonable degree of accuracy.     

Investigating Straight-Pipe Pneumatic Conveying Characteristics for Fluidized Dense-Phase Pneumatic Conveying

This article presents results from an investigation into the pneumatic conveying characteristics (PCC) for horizontal straight-pipe sections for fluidized dense-phase pneumatic conveying of powders. Two fine powders (median particle diameter: 30 and 55 µm; particle density: 2300 and 1600 kg m^?3; loose-poured bulk density: 700 and 620 kg m^?3) were conveyed through 69 mm I.D. × 168 m, 69 mm I.D. × 148 m, 105 mm I.D. × 168 m and 69 mm I.D. × 554 m pipelines for a wide range of air and solids flow rates. Straight-pipe pneumatic conveying characteristics obtained from two sets of pressure tappings installed at two different locations in each pipeline have shown that the trends and relatively magnitudes of the pressure drops can be significantly different depending on product, pipeline diameter and length and location of tapping point in the pipeline (indicating a possible change in transport mechanism along the flow direction). The corresponding models for solids friction factor were also found to be different. There was no distinct pressure minimum curve (PMC) in any of the straight-pipe PCC, indicating a gradual change in flow transition (change in flow mechanism from dense to dilute phase). For total pipeline conveying characteristics, the shapes of the PCC curves and the location of the PMC were found to be significantly influenced by pipeline layout (e.g., location and number of bends) and not entirely by the dense-to-dilute-phase transition of flow mechanism. Seven existing models and a new empirically developed model for PMC for straight pipes have been evaluated against experimental data.     

Axial and Radial Pressure Drops of Dense Phase Plugs

An experimental technique to measure various characteristics of plug flow in dense phase pneumatic conveying systems based on the unique characteristics of plug flow, i.e., the fluctuation of axial pressure drop along a pipeline and pressure difference in the radial direction at the back of a plug, was developed by Li et al. (2002). Based on this work, a further experimental study combined with numerical modeling was carried out to describe the structure of plugs through the analysis of the measurements of pressure difference in both axial and radial directions. A theoretical explanation of these pressure differences was proposed and agrees very well with the recorded signals of pressure difference from differential transducers. This explanation will prove useful in understanding plug structures in industrial applications.     

Modeling Solids Friction for Dense-Phase Pneumatic Conveying of Powders

This article results from an ongoing investigation aimed at developing a new validated test-design procedure for the accurate prediction of pressure drop for dense-phase pneumatic conveying of powders. Models for combined pressure drop coefficient (“K”) for solids-gas mixture were derived using the concept of “suspension density” by using the steady-state “straight pipe” pressure drop data between two different tapping locations of the same pipe and also for two different diameter pipes. It was observed that the derived models were different depending on the location of tapping points (for the same pipe) and selected pipe diameters. The derived models were then evaluated by predicting the pressure drop for pipelines with various diameters or lengths (69 mm I.D. × 168 m, 105 mm I.D. × 168 m, 69 mm I.D. × 554 m) for the conveying of power station fly ash. A comparison between the predicted pneumatic conveying characteristics (PCC) and the experimental plots showed that the models resulted in significant over-predictions. In the second part of the article, the “system” approach of scaleup was evaluated. “Total” pipeline pressure drop characteristics for test-rig pipelines were scaled up to predict the PCC for larger/longer pipes. It was found that the “system” approach generally resulted in grossly inaccurate predictions. It was concluded that further studies are needed for a better understanding of the solids-gas flow mechanism under dense-phase conditions.     

An Investigation into Pressure Fluctuations and Improved Modeling of Solids Friction for Dense-Phase Pneumatic Conveying of Powders

The aim of this paper is to investigate into flow mechanism with the help of pressure signal fluctuations analysis and modeling solids friction in case of solids–gas flows for fluidized-dense-phase pneumatic conveying of fine powders. Materials conveyed include fly ash (median particle diameter 30 µm; particle density 2300 kg m^?3; loose-poured bulk density 700 kg m^?3) and white powder (median particle diameter 55 µm; particle density 1600 kg m^?3; loose-poured bulk density 620 kg m^?3). These were conveyed in different flow regimes varying from fluidized-dense-to-dilute phase. To obtain information on the nature of flow inside pipeline, static pressure signals were studied using technique of Shannon entropy. Increase in the values of Shannon entropy along the flow direction through the straight-pipe sections were found for both the powders. However, drop occurred in the Shannon entropy values after the flow through bend(s). Change in slope of straight-pipe pneumatic conveying characteristics along the flow direction is another factor which provided indication regarding change in flow mechanisms along the flow. A new technique for modeling solids friction factor has been developed using a solids volumetric concentration and ratio of particle terminal settling velocity to superficial air velocity by replacing the conventional use of solids loading ratio and Froude number, respectively. The new model format has shown promise for predictions under diameter scale-up conditions.     

Evaluation of Scaleup Procedures Using “System” Approach for Pneumatic Conveying of Powders

This article presents results from an ongoing research effort aimed towards developing a validated scaleup procedure for pressure drop for the dense-phase pneumatic conveying of powders. Two existing/popular forms of the “system” approach for scaling up of diameter were evaluated. The validity of the current technique for length scaleup using a “system” approach was also examined. The existing method showed good potential for dilute-phase flow, but resulted in appreciable under-predictions when predicting for dense-phase flow. The effect of bends on the accuracy of the method was also investigated. In this study, steady-state conveying data of four different powders conveyed in various pipes (diameter/lengths) were used for the purpose of scaleup investigations.     

An Experimental Study on Degradation of Maize Starch During Pneumatic Transportation

Although attrition during pneumatic conveying is a common problem, very few publications can be found in the open literature on this subject. The particle-to-wall impact is perhaps the predominant cause of degradation since the particle impinges the wall surface at high velocities in dilute phase pneumatic conveying. The most important factors appear to be the conveying air velocity and moisture content. This article presents the experimental findings of a study on degradation of maize starch during pneumatic conveying process. The tests were carried out in a conveying setup having a pipe length of approximately 50 m and a pipe inner diameter of 50 mm in order to find out the breakage of particles under various airflow velocity conditions and temperatures. Dehumidified air was used during the experimentation, and the air temperatures used during these test were 100°C and 25°C. The experimental results indicated that for a given air temperature condition, the variation of attrition rate was a complex function of air velocity and solids loading ratio. Further, for any start pressure condition, the attrition rate was found to increase substantially with increase in air temperature.     

An Experimental Study on Degradation of Maize Starch During Pneumatic Transportation

Although attrition during pneumatic conveying is a common problem, very few publications can be found in the open literature on this subject. The particle-to-wall impact is perhaps the predominant cause of degradation since the particle impinges the wall surface at high velocities in dilute phase pneumatic conveying. The most important factors appear to be the conveying air velocity and moisture content. This article presents the experimental findings of a study on degradation of maize starch during pneumatic conveying process. The tests were carried out in a conveying setup having a pipe length of approximately 50 m and a pipe inner diameter of 50 mm in order to find out the breakage of particles under various airflow velocity conditions and temperatures. Dehumidified air was used during the experimentation, and the air temperatures used during these test were 100°C and 25°C. The experimental results indicated that for a given air temperature condition, the variation of attrition rate was a complex function of air velocity and solids loading ratio. Further, for any start pressure condition, the attrition rate was found to increase substantially with increase in air temperature.     

吸排气管管径变化对空调系统制冷性能的影响

采用系统仿真的方法研究吸排气管管径变化对空调系统性能的影响。研究结果表明：吸排气管管径变化直接影响吸排气管饱和温降或压降,且吸气管内制冷剂压降直接降低吸气压力,排气管内制冷剂压降直接增加排气压力。其中,吸气管压降变化主要影响低压侧热力参数、制冷剂质量流量、制冷量和EER,对系统耗功影响较小,而排气管压降变化主要影响高压侧热力参数、系统耗功和EER,对系统制冷量影响较小。对于本文研究的系统,吸气管和排气管饱和温降控制在1 K以内时,系统性能相对吸、排气管路制冷剂压降为零的系统降低幅度在2%以内,可作为吸排气管选型标准。     

Effect of Close-Coupled Bends in Pneumatic Conveying

Pressure drop in a close-coupled double bend in pneumatic conveying of fly ash is studied. Tests are carried out with a 6.35 cm (2.5 in) diameter 169.8 m (557 ft) long pipeline with various combinations of airflow, ash flow, phase density, and conveying velocity. The pressure drop across two close-coupled 90-degree bends is compared to the pressure drop in an isolated single 90-degree bend. Six ash samples of different physical and chemical compositions are used in the tests. Resulting bend pressure drops are correlated to the corresponding phase density and superficial air velocity at the bend inlet. The correlation pattern represented by the relationship {\Delta P_{solids} \over {SLR}} = Y1 \cdot V^{Y2} is established and found to vary with ash properties. For both single and close-coupled double bends and operating test conditions with \Delta {\rm P}_{\rm solids} / {\rm SLR} 0.15 at the bend entry, 86% of the measured test points fall within the range of - 20% of the \Delta {\rm P}_{\rm solids} / SLR calculated point. Below this threshold, the test results show that the pressure drops due to solids flow through a close-coupled double bend and single bends are often indistinguishable. Consequently, the loss through a close-coupled double bend cannot be considered as the cumulative effect of two isolated single bends.     

Numerical and experimental studies of heat and flow characteristics in a laminar pipe flow of nanofluid

Thermal performance of energy systems can be improved by adding metal or metal-oxide nanoparticles to a base fluid, thereby increasing heat-transfer efficiency. Laminar pipe flow of a Cu–water nanofluid was studied using discrete phase model numerical simulation and experimental methods. The forces including thermophoretic and Brownian forces were considered to solve the particles governing equation. A two-step method was employed in the preparation of the nanofluid. The influences of Reynolds number, fluid temperature, and particle volume fraction on the flow pressure drop and convective heat-transfer coefficient of the nanofluid have been studied. The results demonstrated that adding nanoparticles to a base fluid significantly enhanced convective heat transfer in a pipe and increased energy loss. The pressure drop increased with increasing Reynolds number. A critical nanoparticle volume fraction existed, beyond which the pressure drop changed from increasing to decreasing with increasing nanoparticle volume fraction. This is attributed to competition between slip of particles on the pipe wall and the effect of a drag force on the particles. The deposition efficiency of nanoparticle changing with the particle size and volume fraction also has been illustrated.     

Modelling of plate finned tube evaporators and condensers working with R134A

A model to predict the behaviour of finned tube evaporators and condensers working with R134a has been developed. For modelling of the refrigerant phase change, evaporation or condensation, the heat transfer and the pressure drop for the two-phase flow have to be calculated. Therefore, a number of correlations, the most recommended ones in the reviewed literature, have been analysed and compared. The results of this comparison are presented for the evaporation and condensation heat transfer coefficients and for the evaporator frictional pressure drop. Once the correlations have been implemented in the model and compared with the experimental results, the ones that work best for the studied heat exchangers have been ultimately selected.The experimental study to validate the model has been carried out in a small airconditioning unit with cross-flow air-refrigerant type heat exchangers. The results are compared with model predictions for thermal capacity, refrigerant superheat or subcooling, and tube-side pressure drop.     

Three-dimensional effects of wave-like flow in horizontal pipelines

Wave-like gas-solid flow in a horizontal pipe has been investigated experimentally. The aim of the investigation was to develop a non-intrusive measuring technique for monitoring the transition from a dilute phase flow to wave-like flow and to measure the properties of wave-like flow. When a gas-solid mixture flows in a wave-like manner though the pipeline of a pneumatic conveying system the solids concentration varies, both axially and radially, with time. As a result, the pressure measured at any location will fluctuate and the difference between measuring points, both axially and radially, can be used to determine the nature of the flow. Previous workers have concentrated on individual pressure measurements and axial pressure differences. In this work the radial pressure difference has been examined. This has been compared with axial pressure measurements and those obtained using capacitance sensors. These techniques have been used to determine both wavelength and velocity. A three-dimensonal numerical model, which is based on the two-fluid theory, was also used to obtain a better understanding of the flow field characteristics.