Material on the move

Getting resin from point A to point B involves more than you might think, and optimizing every pellet''s journey can save you money.

When designing a material handling system, it''s necessary to be aware of the conveying conditions to optimize performance and reliability of the system. The use of a terminal velocity philosophy will minimize particle velocity and maximize material throughput. The secret is to manage the air-to-solids mixture ratio throughout the conveying path. The system load rate will define the pump and line size selection. A larger pipe size will reduce the velocity and maintain the throughput rate. If the pressure drops are lower in the system, the pellets will accelerate less. Lower acceleration means lower end-point velocities, which will have less impact on the material and the system contact surfaces. Saltation velocities dictate the minimum velocity to prevent slugging and plugging.

Conveying system performance suffers from the intuitive familiarity syndrome. Often, installers and designers strive to make the system aesthetically pleasing and neat-looking, neglecting critical design configurations. The results are lower throughput and operational problems. The ends do not justify the means. The most frequent transgressions are pipe routings that make compromises to good practices. A common example would be vertical drops to horizontal runs. This orientation exemplifies one of the worst routing scenarios and is very susceptible to saltation with slugging and plugging as a result because of velocity drops.

The best system design uses a minimum number of bends to reach the objective as each bend is equivalent to as much as 50 ft of linear horizontal pipe, depending on the bend''s radius. In addition, flow friction from improper coupling installation and long flexible-hose sections should be minimized. Long radius bends or blind Ts can be used to reduce friction when changing the material conveying direction is necessary. Also, systems designed with unnecessarily high velocities impart too much momentum into the plastic pellets, causing pellet deterioration and system damage.

The basics

A successful pneumatic materials conveying system provides an economical means to transport plastic pellets to the processing equipment from silos, bins, or other storage containers and achieve lower part costs through improved materials management, increased machine uptime, and prevention of resin contamination or loss. The process uses air to blow or vacuum convey pellets in a controlled flow without damage to either the resin or system components.

When pellets come from their source in a silo, bin, or other storage container, we note that the pick-up velocity must be sufficient to entrap the pellets in the air stream. This ratio of pellets to air volume, which is defined by the geometries, distances, and orientation of system components, can be adjusted to achieve the optimum throughput. The conveying history of the material affects its integrity (heat degradation, angel hair, Texas pellets) and the material handling system components (dust, abrasion, wear).

There are two fundamental configurations for pneumatic conveying of solids-dense phase and dilute phase. Dense phase conveying usually uses higher pressures and solid concentrations, upwards of 50 lb solids/lb of air. Dilute phase conveying uses lower pressures or a vacuum and operates at a much lower solid concentrations of 5 lb/material/lb of air transported. Note that this is a ratio of masses where air mass per volume or density is a function of the altitude, water content, and temperature of the air being used. High-altitude air is thinner than low-altitude air; moist air has more mass than dry air; and hot air is less dense than cool air. The conveying system''s design must take all these conditions into consideration.

Successful pneumatic conveying system operation depends strongly on the saltation velocity and choking velocity (see diagrams 1 and 2). Saltation velocity is defined as the minimum velocity that will prevent solids from dropping out of the solids-air mixture to the bottom of the pipe in a horizontal pipe section. Choking velocity is described as the minimum velocity for uniform vertical transporting of the pellets. The system''s saltation velocity is always greater than the choking velocity. Therefore, designing to prevent saltation from occurring will also keep choking from happening.

The area to the left of the saltation velocity (see diagram) can be described as dense-phase conveying where as the area to the right represents the dilute phase. Note that as the phase becomes more dilute, the velocities increase exponentially. Air that is less dense can cause a system to become more likely to have saltation. Air that has more mass per unit volume will cause the ratio of material to air masses to become less, increasing the particle velocities.

Plastic pellets are accelerated through the lines and will strike the pipe, fittings, and the hopper components during conveying. Consequently, many plastic materials are stressed and degraded during conveying. This is especially prevalent with olefins and heat-sensitive compounds. In addition, some plastic materials have abrasive contents that quickly wear through piping and erode contact surfaces of components in the system. It is important to ensure the material velocity reduces quickly as the air is drawn through the screen to the filter and vacuum pump. The receiver shown has a screen to prevent particle carry-over, yet permit passage of fines to be collected by the central filter.

When considering a new system, or changes to an existing system, working with a qualified systems supplier is worth the investment to ensure the design accounts for the your plant''s individual requirements.

Jim Horne, systems project manager, Wittmann Inc. www.wittmann-ct.com