Peter W Wypych

Pressure drop and slug velocity in low-velocity pneumatic conveying of bulk solids

Low-velocity slug flow pneumatic conveying is being applied increasingly in industry and to a wide range of bulk solid materials. However, to date, most investigations have focused on bulk solid materials with regular shape (e.g. plastic pellets, wheat) and very little work has been undertaken with irregular-shaped materials (e.g. muesli, maize germ). In this paper, a new test-design procedure is presented. Based on particle properties and data from a simple vertical test chamber, the pressure drop and slug velocity in low-velocity slug flow can be predicted accurately in large-scale systems. This procedure can be applied to bulk solid materials with regular, irregular and/or unusual physical properties (e.g. different shape, density, size and size distribution), as long as they are good candidates for this mode of pneumatic conveying.

Minimum transport boundary for horizontal dense-phase pneumatic conveying of granular materials

Successful pneumatic conveying of granular materials can be achieved only when: the air mass flow rate is controlled within a certain range; the pipe size is selected appropriately; and the operating boundaries are well assessed. This paper presents results from investigations into the capacity limitation of low-velocity slug-flow pneumatic conveying. The mechanism for the formation of the unstable zone also is explored experimentally and theoretically. A new theoretical model based on observed unstable flow mechanisms and stability criteria is presented for the purpose of predicting transport boundaries. The modelling results agree very well with the experimental data obtained on poly pellets and different sizes of conveying pipeline.

Pressure drop prediction in low-velocity pneumatic conveying

Abstract Low-velocity slug-flow pneumatic conveying is being applied to an increasing number of applications due to reasons of low power consumption and low product damage. In order to investigate improved design and scale-up procedures, several granular products are conveyed initially in a low-velocity pneumatic conveying test rig comprising various combinations of length and diameter (e.g. L = 52, 96 m; D = 105 mm). Based on these experimental investigations and a force balance of the moving slugs, a semi-empirical model is developed to predict the overall pipeline pressure drop in the horizontal slug-flow of cohesionless bulk solids. Model predictions compare well with additional experimental data obtained on 105 and 156 mm internal diameter horizontal pipelines. A method for determining the optimal operating point for low-velocity slug-flow is also presented.

Investigations into wall pressure during slug-flow pneumatic conveying

Abstract During low velocity slug-flow pneumatic conveying, the wall pressure, that is the pressure that is exerted on the pipe wall by a moving slug of particles, causes a resistance to material flow. Most of the conveying pressure energy is consumed to overcome this resistance force. However, very little information is available for the determination of wall pressure. This paper studies the distribution of wall pressure along the length of a moving slug and then measures on a test rig the total pressure (the sum of the static air pressure and wall pressure) during slug-flow pneumatic conveying under different conveying conditions. Various values of the stress transmission coefficient (such as the ratio of radial stress to axial stress) in particle slugs are obtained from the wall pressure measurements and the calculated axial stress of the slug. Based on the principles of particulate mechanics and the experimental values of the stress transmission coefficient, a semi-empirical expression of stress transmission coefficient is presented for slugs flowing in pipes with rigid and parallel walls.

On improving scale-up procedures for pneumatic conveying design

Abstract The method of scaling-up test rig data to full-scale installations, previously used quite extensively in the design of pneumatic conveying systems, is shown to be inadequate in particular applications. Two popular forms of definition and three existing empirical relationships for the solids pressure drop component are modified to demonstrate the possible extent of this inadequacy. Steady-state pipeline conveying characteristics obtained from three products (fly ash/cement mix, PVC powder, screened coke) and four test rigs are used in the development of an improved scale-up procedure. Suggested methods to predict air-only pipeline pressure drop (for both single-and stepped-diameter pipelines) and to generalise pneumatic conveying characteristics for a particular material (applicable to any system of different length and/or diameter) are also included.

Discrete element simulations of granular pile formation: Method for calibrating discrete element models

Purpose – The purpose of this paper is to examine several calibration techniques that have been developed to determine the discrete element method (DEM) parameters for slow and rapid unconfined flow of granular conical pile formation. This paper also aims to discuss some of the methods currently employed to scale particle properties to reduce computational resources and time to solve large DEM models.Design/methodology/approach – DEM models have been calibrated against simple bench‐scale experimental results to examine the validity of selected parameters for the contact, material and mechanical models to simulate the dynamic and static behaviour of cohesionless polyethylene pellets. Methods to determine quantifiable single particle parameters such as static friction and the coefficient of restitution have been highlighted. Numerical and experimental granular pile formation has been investigated using different slumping and pouring techniques to examine the dependency of the type of flow mechanism on the D...

An Investigation into Modeling of Solids Friction for Dense-Phase Pneumatic Conveying of Powders

This article presents results from an investigation into the modeling of solids friction factor for fluidized dense-phase pneumatic conveying of powders. A fundamental design approach was pursued by employing “straight pipe” and “back calculation” techniques for modeling and using two types of power function formats. The “straight pipe” models were found to be unexpectedly different depending on the selected location of pressure-measuring tapping points (even for the same product). An attempt to explain this variation by studying the “straight pipe” conveying characteristics suggested significant changes in flow mechanisms along the pipe. The derived models were evaluated for scaleup accuracy and stability by predicting for larger and longer pipes. The results showed significant variations in predictions. One format of power function model was found to result in more stable predictions than the other. Possible explanations for the causes of such variations are provided. Physical observations of the flow phenomena of dense-phase conveying for different powders showed the products were mostly conveyed as a dense non-suspension liquid-type-layer along the bottom of the pipe. This mechanism does not seem to be correctly represented by the existing design approach of using a Froude number term in the solids friction factor models, thus initiating a search for suitable alternative dimensionless grouping(s) that can adequately represent the non-suspension flow phenomena. In this study, Steady-state conveying data of three different powders conveyed in various pipes (diameter/lengths) were used for the purpose of modeling and scaleup investigations.

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.

Pneumatic conveying of powders over long distances and at large capacities

This paper reviews design and operating requirements for long-distance and large-throughput pneumatic conveying. Emphasis is placed on: blow tank design to provide an efficient and controlled discharge of material; semi-empirical techniques to predict pressure drop with good accuracy; optimisation of stepped-diameter pipelines to minimise air flow, pressure, wear and power; reliable valves for abrasive and high temperature conditions; back-pressure unblocking techniques. These developments are advancing considerably the reliability and future potential of large-throughput and long-distance pneumatic conveying technology.

Experimental Investigation of Air Entrainment in Free-Falling Particle Plumes

Powders and granulated solids are widely used in industrial bulk solids storage, handling, and transportation systems. Such bulk materials handling operations frequently involve a falling stream of material. During such a process, the surrounding air is induced to flow with the falling particle stream, forming a particle-driven plume. Herein, experimental research results are reported on this fundamental problem, focusing in particular on the velocity profile of air entrained by the free-falling particles. This investigation shows that the velocity profile of the induced air can be modeled as a Gaussian distribution. The radius of the particle plume is found to increase linearly with increasing drop height, and it also increases with increasing bulk solid mass flow rate. Comparisons are made with other entrainment flows, such as jets and plumes, and it was found that air entrainment and hence the angle of spread of the particle-driven plumes was much less than for the other entrainment flows. The angles of spread of the particle-driven plumes were found to be in the range 1.3° < θ s < 1.8° as compared to θ s ≈ 5.7° for miscible plumes arising from sources of heat, for example. In addition, the centerline velocity of the induced air in the particle plumes was found to increase significantly with increasing drop height. Results from high-speed digital video records show that the bulk material does not dilate in a uniform manner as it falls, and a series of distinct particle clouds form in the core of the particle-driven plume. These clouds eventually disperse over a sufficiently large drop height.