INDUSTRIAL AND ENGINEERING CHEMISTRY
996
ROTO-LOUVRE DRYER JOHN L. ERISMAN Link-Belt Company, Chicago, 111,
T
HE roto-louvre dryer was developed in Sweden and England about twelve years ago. In Sweden it was used primarily on wood chips and refuse in the pulp, paper, and lumber industries. In England it had a more general application and has been employed in the food, chemical, coal, and cement industries. Approximately two years ago its manufacture was begun in this rountry, where,
1. CROSS SECTION OF ROTO-LOUWE DRYER, ILLUSTRATLNO FLOW OF GASES TAROUGH BED OF MATERIAL FIGURE
VOL. 30, NO, 9
as in England, i t has been adapted for the drying of a variety of materials. The general principle of the roto-lourre design is shown in Figure 1. I n an outer shell are fitted the louvre fin plates dividing the inside of the drum into a number of m a l l CODpartments. To these fin plates are attached the tangential louvre plates. This combination of plates forms the air channels for the introduction of the hot air or gases to the drum. The space between the overlapping louvre plates permits the gases to pass through these channels and up through the bed of material into the drum. The admission of the hot gases is controlled so that they can be admitted only to that portion of the louvres underneath the bed of material. This is done by the specially shaped inlet casting, which is 80 constructed that it covers a predetermined number of louvres, and admits the gases to them in a manner somewhat like the familiar slide-vallve action of admitting s t e m into the ports of a steam engine cylinder. As shown in Figure 2, the louvres are tapered, being larger on the feed end of the dryer where the hot gases are also admitted, and smaller on the discharge end. The taper serves the purpose of permitting a g r a t e r volume of gases to be supplied on that end of the dryer where most needed-that is, the wet end. The tapering of the louvres to a smaller cross section on the dry end cnts down the volume of gases at this point where it is not needed. The conical-shaped interior also facilitates t.he passage of the material through the drum so that it is not necessary to incline the drum for that reason. The drum is set horizontal and revolves a t about 1 to 4 r. p. m., depending on the size. The design of the hot-air inlet casting is such that the greatest volume is presented to the center of the bed where the bed is thickest and where the resistance is also greatest. Tlie exhaust gases arc drawn off through the exhaust pipe, which is mounted in the feed-end plate of the dryer, and extends into the interior of the drum for about one-quarter to one-half of its lengt,li. This permits the vapors t o be collected from both ends simultaneously and thus draws them away from the dried material. It is not necessary for them to traverse the full length of the drum.
FIGURE 2. DRUMSHELL ClE AWAYTO ILLUSTRATE LOUVRES AXD THE BETTER DRYINR OF GMNULAR MATERIALS, COAL,
PoSITIoV OF MATERIALS IN TEE ROTO-L~UVRE DRYER FOR CHENICALS, CRYSTILS, POWDERS, REFUSE, ETr.
SEPTEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
I n some cases it has been found advantageous to draw off the exhaust gases through the louvres by using a special connection similar to the hot-gas inlet connection. The construction thus outlined gives the advantages of both parallel and counterflow methods of drying in one machine. The objective sought in the development of this design was to provide a more intimate contact between the hot gases and the material being dried, with its resulting increase in efficiency of the heat transfer.
TABLEI.
PERFORMANCE OF ROTO-LOUVRE DRYERS
Inlet Gas Exhaust Temp. Gas Temp. Coal
Sand
Salt Dicalcium phosphate Oxalic acid Casein Seed Cheese Apple poma ce
Material Temp.
Initial Moisture
F.
F.
F.
%
900 950 900
175 180 200
125 145 160
18 5 1.8
700 190 280 450 250 620
130 115 125 110 100 200
88 103 105 110 84 180
35 3 60 6.7 27 79
Final Moisture
% 4 2 0.26 11 0 3 3 14 0
The increase in efficiency is reflected in various ways. The dryer can be made in short lengths with resulting saving in floor space. The drying periods are necessarily shorter, and consequently there is less chance of overheating the product. Very little heat is transferred in the roto-louvre dryer by either conduction or radiation, inasmuch as the louvres are in contact with the hot gases for only a small part of a revolution, and for the remainder of the revolution they are subjected to the lower temperature of the exhaust gases so that the temperature of the louvres is much lower than that of the incoming gases. The heat transfer is almost entirely by convection, since the heat is transmitted by contact of hot gases with the material being dried. The hot gases can be supplied from various sources; if pure air is desired, it can be obtained by using steam coils as a heating medium. When higher temperatures are desired, it is customary to use the products of combustion from either gas, oil, or various kinds of coal-fired furnaces; and in this case the products of combustion are tempered by the mixing of cold air so as to bring the temperature down to the desired figure. The maximum temperature used, so as to avoid expensive alloy construction, is about 900" to 1000" F. Because of the nature of the material, it may be necessary to use lower temperatures than these. It is found possible to use higher inlet temperatures and a t the same time obtain lower temperatures of the discharged product than is customary in rotary dryers. This final temperature of the product depends on several factors-for example, what the material is and how readily it gives up its moisture, whether the moisture is simply on the surface, whether it can be brought to the surface by capillary action, or whether it is held inside by a hard outer layer as in the case of seeds and grains. Fibrous materials, unless long and stringy, are usually dried easily. There is a considerable difference between bringing a hightemperature gas in contact with a material and submitting the same material to the contact of a hot plate of the same temperature. The action of the roto-louvre dryer will, in general, dry the material a t a comparatively low temperature. The temperature of the material will be held at approximately that of the wet-bulb temperature of the exhaust gases, as long as the moisture is evaporating freely. This is especially true when the proper depth of bed is used, so that the hot gases do not pass through too freely. If the material is fairly coarse so that the hot gases pass through faster than
997
they can absorb the moisture, then a relatively higher exhaust temperature and a correspondingly higher material temperature will be obtained. A comparison of gas and material'temperatures in drying various materials is shown in Table I. The capacity of the roto-louvre dryer, as in other dryers, depends generally on the amount of air that it is permissible to use with a given material. It is found that some very fine concentrates which did not look very favorable were actually easy to dry because they formed crystals in the dryer. The velocity of the air up through the bed of material in the dryer is usually from 2 to 5 feet per second, so that there is not the tendency to dust, as might a t first be supposed. It is obvious that with the louvre design liquid materials cannot be dried, unless it is permissible to return a portion of the dried material and mix it with the liquid, so that it will not run into the louvres. Tests along these lines have produced a satisfactory product with some materials. The roto-louvre principle of heat transfer can also be used in the reverse direction; that is, it can be applied for the cooling of materials. Air a t room temperature can be introduced into the drum to permeate up through a bed of hot material and thus lower its temperature. This has been done for cooling soap beads, sand, and hot clinker. RECEIVED May 4, 1938.
ROTARY STEAMTUBE DRYER C. E. BILL Louisville Drying Machinery Company, Louisville, Ky.
I
N APPEARASCE the rotary steam-tube dryer resembles very closely the direct-heat rotary type. It is continuous
in operation, and, as its name implies, the heat for drying is supplied by steam directly rather than by hot gases. It is essentially a cylinder carried in approximately a horizontal position on riding rings or tires and rotated slowly on rollers affixed to a base or bed plate (Figure 1). The material to be dried is fed continuously and a t a uniform rate into one end and is tumbled and showered by the rotation of the cylinder. Because of the slope or pitch of the cylinder and the head of the material, the latter progresses a t a more or less rapid rate toward the discharge end. During its travel through the cylinder the material is heated by contact with and radiant heat from the surfaces of the steam tubes, which are pipes or boiler tubes arranged parallel to the axis of the dryer cylinder and extending its entire length. These tubes rotate with the shell (Figure 2). The water evaporated from the material is removed from the dryer cylinder by a current of air. The steam tubes are supplied with steam through a stuffing box and manifold a t the discharge end of the cylinder. The heat for drying is supplied by the condensation of steam in the steam tubes, and the condensate is generally removed a t the same end a t which the dryer is supplied with steam and through the same stuffing box. Continuous and complete removal of the condensate from the interior of the tubes and manifold is of paramount importance in the development of maximum evaporative capacity.
