Pneumatic conveying

by Ernest George Phillips

It is impossible to give specific details concerning the discharger because, in all cases, the conditions under which the plant has to work affect the whole design. For instance, where the working is only intermittent, e.g. the removal of ashes from a boiler-house, the discharger can be eliminated, provided that the ash bunker is large enough to hold the quantity of ashes to be dealt with at each operation. In such a case, the ash container would be capable of being exhausted, and the material entering as before would simply drop by gravity into the container and remain there until the pump was shut down; it would then be allowed to gravitate into the truck or other conveyance for disposing of the ashes.

Hand-holes for cleaning, and easy access to the interior are essential in the design of a discharger, especially if the material to be handled is liable to “pack” when entering at a high velocity.

Pipe Lines. One of the most important points in the designing of a pneumatic conveying system is the correct lay-out of the pipe line.

Fig. 9.—Sturtevant Patent Junction.

A fatal mistake often made in low pressure and exhausting systems is that the numerous branch pipes are added to or altered after the makers have left the original installation. Almost invariably, branches thus added are made to approach the main trunk at too great an angle, with the result that eddies and whirlpools are created within the pipe, seriously reducing the output of the main trunk. So essential is it that this junction should be correct and that the two streams should run as nearly parallel as possible, that the Sturtevant Engineering Co. has patented a special junction (Fig. [9]) to bring together the two streams of air in the main and the branch pipe practically parallel, as shown.

Bends. In connection with high pressure systems, the following points are of great importance. All vertical and horizontal straight lengths may be of a light section constructed in steel. Bends should be avoided whenever possible, and those which are inevitable should be made in hard cast iron, with every possible provision for easy replacement of wearing parts.

The wear takes place at the point of actual contact which, in elbows, is practically confined to one place only. The material rushes to the end, strikes the bend, and—suddenly changing its direction of travel—whirls off down the next straight length. The impact and the resulting wear on the pipe, as well as the breaking of the material conveyed, are naturally much greater in elbows than in easy bends, but if the breaking of the material is not detrimental, elbows should be employed, as they are less costly and can be replaced more quickly and easily.

Certain raw materials—such as salt, soda, lime and various chemicals—which have to be ground before use, may be prepared to a considerable extent for this operation by the use of elbows. On the other hand, easy bends should be employed for material which it is desired to convey without damage, e.g. malt, coal, and granular substances, which are finally required in granular form and not as powder.

The wear in bends is only on the external radius of the bend, and then is inclined to be localized at certain points rather than distributed over the full sweep of the bend (see Fig. [1]0); this being so, it is often desirable so to construct the bends that the back is in segments which can be renewed easily (see Fig. [11]). Alternatively, the bend may be constructed on the “lobster” principle (Fig. [12]), only the worn sections being replaced when overhauling. It is not necessary always to take a bend at an angle of 90°, and if the small short angle sections are interchangeable, then almost any angle can be constructed by building up with the necessary number of sections.