SEPTEMBER, 1938
INDUSTRIAI, AND ENGINEERING CHEMISTRY
aluminum hydroxide, sodium sulfite, and others. What dmt is carried out of the dryer cylinder when handling such materials is collected by comparatively simple and inexpensive means by reason of the small volume of air to be handled by the dust-oollecting apparatus. Exhaust steam is often used in the steam-tube dryer, but it is of interest to point out that the use of higher pressure steam permits considerably higher drying capacities. The capacity ohtained from 90 pounds per square inch of steam should theoretically he approximat.ely 67 per cent greater than that from 5 pounds per square inch of steam but in actual practice is 150 per cent greater. By using the steam directly in a steam-tohe dryer, thermal efficiencies better than average fire-dryer practice are obtained. Evaporative capacities better than the average fire dryer are also realized even when the latt.er is operated a t considerably higher temperatures than ohtairr in t,he steam-tuhe dryer. I3&C&IVEDh l s y l ,
1’138.
VERTICAL TURBODRYERS A. WEISSELBERG 113 West 42nd Street, New York, N. Y.
T
HE vertical turbodryer represents a recent and im-
portant development in the drying equipment field. In the five years since its development, over two hundred installations have been made, chiefly in Europe. The first turhodryer was of the horizontal type in which a central fan was mounted on a horizontal axis, and the material was conveyed around it on large wheels. The horizontal type was first applied to the drying of textiles and has since been used for t.he drying of hoards (paper and fiber), pulps, and pastes. The largest of these dryers ineas{ires 30 feet in diamet.er. One disadvantage of the horizontal type is the small holding capacity compared to that of other types of dryers. Despite tiis, the drying thne is so greatly reduced for many materials that it is still possible to obtain the same output per cubic foot of space as that in other dryers of modern design. The vert.ical turhodryer (Figure 1) consists of a series of annular shelves vertically arranged as shown in Figure 2 . Fans are mounted on a central vertica.1 axis within the open core formed by the annular sholves. Shelves arc formed by individual trays with gaps in hetween through ~vhichblie material is swept by wiper arms onto the next tray below, after it completes one revolution in the dryer. The wet mat.eria1 is fed to the top shelf and moves by gravity downward through the dryer. Tho only power required for material mo~ementis that for wiping i t off tbe tray and for lerreling the piles formed to a layer of uniform thickness at each shelf-to-shelf trans-
Diarnotei of dryer at top. ifeet 6 indies: at bottom, 30 feet, diameter
999
INDUSTRIAL AND ENGl:NEERING CHEMISTRY
1000
AIR EXHAUST
FIGURE2. TRANSFER TYPETURBODRYER
fer point. The repeated piling and leveling action is unique with this dryer. I n terms of drying action, i t means that new surface is exposed to the drying air on each shelf. An important feature of the vertical turbodryer is the high specific holding capacity. To illustrate, a large dryer installed in this country has a volumetric capacity of only 1.1 cubic feet per square foot of drying area. A dryer 23 feet in diameter and 33 feet high has a tray area of 8000 square feet in forty-five shelves, which is equivalent to a
VOL. 30, NO. 9
drying area of 360,000 square feet. Another feature of the vertical type is that it is practically self-cleaning. It can run for an indefinite length of time without accumulations which make periodical cleaning necessary. The air flow distribution in the dryer is uniform and permits a high uniformity in drying results to be obtained. The fact that the air flow is relatively unrestricted results in a low air horsepower. By virtue of the dryer's vertical chimney-like construction, natural draft assists the movement of the air in counterflow operation. A valuable advantage of this type of dryer when drying fine powders is that very little dusting occurs. The heating elements are generally arranged insid ethe dryer, and this makes a large number of reheating cycles possible. This in turn permits the temperature to be held within close limits a t various levels in the dryer without temperature controls or zoning baffles, and also makes for a high thermal efficiency. Large plant dryers may be readily and accurately designed on the basis of results obtained from laboratory tests. A recently developed variation of the vertical turbodryer is the spiral type, which has not yet been introduced in this country. This type is especially applicable to materials which must be dried without being disturbed and which require long drying time. Such materials are usually dried on long conveyor belts, and then only relatively small capacities are obtained. An illustration of the spiral type of dryer is shown in Figure 3. As in the case of the vertical type, the spiral turbodryer offers a large drying area per unit volume of dryer. For instance, in a unit in which the conveyor belt is spaced 8 inches vertically and moves in a screw spiral 16 feet in diameter, a 1500-foot length of conveyor belt may be accommodated in a little more than 20 feet of housing height. The total horsepower for conveying such a belt would only be about 1.0 horsepower. There will also be very little wear on the conveyor because, within the dryer, it is standing still with respect to the cylinder. Instead of a conveying belt, it is possible to use a conveying chain from which trays, frames, etc., may be hung. RECEIVED May 4 , 1938.
A
r
_--
FIGURE3. SPIRALCONVEYO TYPETURBODRYER
SECTION AA
u-TURBINE
DRIVE
