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.
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INDUSTRIAL AND ENGIN’EEIIING CHEMISTRY
As mentioned sbove, the steam-tube dryer closely resembles the direct-fired rotary types in general appearance, but there the similarity ends. The actual meclranism of drying in the two types is quite different. One fundamental characteristic of the steam-t.uhe dryer explains to a large degree its peculiar suitability to many drying operations and differentiates it sharply from all other types of internally heated rotary dryers. Direct and indirect lieat types, whether high- or low-temperature, are variaiilotemperature and constant-input machines. In ot,her words, unless suitable controls are provided, the temperatures to which the material is subjected in the dryer are directly dependent upon the rate of feed and tlie moisture content of tlie material being dried. The st.eam tube dryer, on the contrary, is exactly the oppositwa constant-tunperatwe and an antoinatiaally
VOL. 30, NO. 9
variable heat-input machine. The maximum temperature to wliicli the material can he heated is determined by the steam pressure employed, regardless of the amount of material in the dryer or its moisture content. Also steam is condensed in the tubes automatically in proportion to the heat required, up to tlie limit of capacity of tlie dryer. This is an important feature of the steam-tube dryer. By taking advantage of this characteristic, a long list of organic materials are dried in steam-tnbc dryers economically, and a dried product of prime quality is produced with the minimum of supervision, vith no autnmatic control instruments, and witliout danger of burning or scorching the ma.terial during drying. Inimediately upon entering tlie dryer cylinder, the wet material is rapidly heated by contact with the hot steam tubes in an atmosphere of high humidity. Lumps and granules are heatcd throughout, as the cooling effect of evaporation from their surfnee is inhiirited, and a condition is attained that is ideal for subsequent rapid and uniform evaporation of the contained moisture without case-hardening. As the material progresses towards the dBcliarge end of the dryer, it comes in cont,act with air of progressively lower humidity, and near the discharge end, with air of coilsiderahly lower temperatiire, The entire drying operation takes place at comparatively low tcmperatnres, determined as explained above by the pressure of the steam used. It is this condition of low temperatore and hi& humidity at the start of the drying operation that enables the steam-tube dryer to be used in the drying of match splints, for instance, because it approximates the conditions present in a welldesigned lumber dry kiln. The same conditions adapt it to the handling of distillers’ slop, chicken feathers, fish scrap, poultry manure, granulated cork, fruit and vegetable pulps, and a long list. of other organic materials. Tho air which is passed through the. dryer cylinder, generally cvuntercurrcnt to t,lie travel of the material, has nothing to do with tlie heating of the material bnt is used simply as a vehiclc to carry off the evaporated moisture. Consequently its amount .is only a fraction of what would be required to supply the lieat fnrnislied by t,lie hot steam tubes, and therefore its reloc!it,y is very low. This characteristic enables the steam-t&e dryer to be employed in drying many finely divided materials, such as paint pigment filter cakes, precipitated FIGURE2. CROSS-SECTIOSAL V m v o r A DEMONBTRATION MODELWITH DUMMY ‘rUsEs chalk, certain washed clays, water-ground mica and silica,
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
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