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veloped, so that the formed materials may be air-dried on the screen conveyor. The metals used in the combruetion of those parts of the fin drum coming in contact with the material have so far been bronze or stainless steel. Bronze has the advantage of less cost, casier maehiilability, and much better heat transfer. Plain steel has been avoided on account of corrosion. Aluminum is not sufficiently durable to last; tbe hardened grades lose their superior physical qualities under heat and working conditions. Platiiig, &sameans of protection on the fin drum, is impractical. For the screen conveyor, micromesh hinge, side guards, and supporting girts, the following metals have been used: galvanized iron, special electroplated iron, aluminum, bronze, and stainless steel. R ~ c m v e oMay 4, 1938.
DRUM DRYING D.S. VAN MARLE BuffaloFoundry and Machine Company, Buffalo, N. Y.
D
RUM drying is that process where the material is dried on the surface of an internally heated revolving drum. Various forms of drum dryers and their application were discussed by Harte (8) and IIarcourt ( I ) , and tlie effect of variation in operating conditions, by Reavell (4); IIirscli (S), in addition, gave a theoretical treatment of the subject. A twili-drum dryer is illust,rated in Figure 1. Major connideratiori and problenis in the use of drum drying have to do primarily with (u) nature of the product to he dried, (b) the drum dryer as an inst.rument of heat transfer, (c) design and construction of the dryer, (d) adaptation of the dryer to t.he product io hand, (e) conditioos under which the dryer operatas, and (f) economies to be effected witahin the drum dryer itself, as well as in co~ineetionwith the maririfactnriiig process in which it is being used.
Product to Be Dried
the boiling point of the solvent as long as the material is completely wetted and then increases during the final stage of the drying cycle. The capacity of a drum dryer depends not only on the drying rate of a thin layer of the material but also on the degree of adherenee of the product to the drying surface. In this respect there is such a wide variation in different products as to make impossible any definite prediction of drum dryer performance. The amount of liquid adhering to the drum surface varies, to a considwable extent, with the type of feed used, as well as with the steam pressure and drum speed. Feed temperature and concentration frequeiitly can be regulated; preheating and preconcent.ration are desirable wherever possible since they result in a reduction of the amount of heat to be transferred per unit weight of dry material. Concentration caniiot always be carried to the limit, because dhere is often an optimum concentration above which it becomes difficult to apply a uniform coating, and adherence becomes poor. Other properties, such as viscosity, surface tension, and wetting power, are undoubtedly important in governing layer formation but are generally fixed because any change in these properties may affect the quality of the dry product. Sometimes it is possible, however, to improve drying properties a great deal by pH control, removal of impurities, or addition of a small percentage of a foreign nubstance. In view of the many advantages of drum drying it is worth while to consider these possibilities whenever a difficult drying problem is to be solved.
Heat Transfer Viewed as a heat transfer problem, drum drying involves heat transfer from condensing steam through a metal wall of considerable thickness to a thin layer of material, from which the moisture is removed by direct evaporation a t or above the boiling point. Over-all rate of heat transfer varies greatly, largely on account of the difference in resistance to the removal of moisture with decreasing moisture content and the variation in film resistance of individual products. It is often particularly difficult to eliminate the last few per cent of moisture, and to do so would mean the installation of a greatly increased amount of drying surface. Under these circunrstances i t is more eeonomid to use the drum dryer for drying to an advantageous moisture content and to remove the balance of the moisture in a more suitable dryer. In one case it was actually found possible to triple the rapacity of the drum dryer by this method.
The produet to be dried is usually in fluid form, with solids either in solution or suspension, and the material is spread in a thin layer on the surface of the drum. Since the layer of material is coniparatively thin, the dryi n e rate is at, no time eovernrd bv " the ~diEusion of^ the vapor through the layer. The drying of solutions involves a change of state of the sohito in addtion to the evaporation of water. Three stages in this change may he observed: a first stage during which the material remains a liquid while its temperature is raised to the boiling point of the solution and becomes higher as the eoneentra.tion of the solute increases; a second stage, during which there is a gradual change from a liquid to a solid state, a t a eonstant temperature if the solute separates a t a definite concentration, or at an increasing temperature if the change is gradual; and a final stage, where thc material is in a solid condition and in which the temperature gradually approaches that of the heating surface. In the case of slurriesand susprnsions the temperature remaiirs a t FEU~E I. TWIN-DHUM DRYER WITH SPLASX FEEDAND ~~
CHROMITIM-PIAT~DDRUMS
SEPTEMBER, 1938
INDUSTRIAL AND ENGINEEAING CHEMISTRY
1007
means of their data and of data o n commercial operation indicate that only for very high rates of evaporation does the steam-side coefficient become of any practical importance. It has been found in practice that in this case the removal of condensate and noncondensable gases must be given special attention. Resistance to heat flow through the metal wall is very considerable and is a larger factor than in most industrial heat transfer problems. Unfortunately, metals of high conductivity are so soft that they do not lend themselves readily to use in drum construction. Evaporative capacity is available in drum drying if conditions are favorable to high heat transfer. Far greater capacity can be obtained if a few per cent of residual moisture is left in the dry product. The presence of this moisture reduces the temperature of the material and thereby increases the temperature difference on the liquor side; a t the same time the liquor-side coefficient increases on account of decreased film resistance. Calculations show that under optimum conditions over-all heat transfer on a double-drum dryer, illustrated in Figure 4, is about 360 B. t. u./hour/square foot/" F. in the area between the drums, and about 220 between the point of closest approach of the drums and the knives. Steam-side coefficients are about 2650 and 450, respectively, liquid-side coefficients, 1500 and 1800, respectively, and temperature drop through the metal, 65" and 35" F., respectively.
FIGURE 2. SINGLE-DRUM DRYERWITH DIP FEED
Maximum rate of evaporation is obtained with dilute solutions which evaporate readily. Under favorable conDesign and Construction ditions this may be as high as 18.5 pounds/hour/square foot drum surface, but rate of evaporation is not necessarily the Considering that in most drum-drying problems actual determining factor in the economy of drum drying because evaporation does not nearly approach the possible limits, it the amount of dry material produced in most cases is the prime remains to adopt the most suitable dryer construction and consideration. the best operating conditions. The simplest design is the I n regard to film coefficients, practically no data are availfeed pan, placed below or against the drum, which holds the able, largely because temperature distribution and heat transliquor into which the drum dips. Figure 2 illustrates a singlefer around the circumference of the drum are not a t all unidrum dryer with dip feed. There is no mechanical control form. Part of the drum surface, between the points where over the amount of liquor adhering to the drum surface other the layer is applied and where it is removed, is notu%lized. In this area there is no heat transfer other than radiation loss; consequently surface temperature of the drum is a t a maximum. At the point of application of the material to the drum a rapid drop in surface temperature takes place. If film resistance a t this point is high, this temperature will remain fairly close to the steam temperature. At low film resistance, however, as is the case with moderately concentrated solutions, heat transfer will be so great that the temperature drop through the metal becomes the controlling influence. Surface temperature and temperature difference in that case will both be comparatively low. At the point of removal of the material, heat transfer FIGURE3. TWIN-DRUM DRYERWITH SPLASHFEED is low and surface temperature high, but temperature of the material also will be than the depth of the liquor in direct contact with that surhigh and will closely approach the surface temperature of face. As concentration increases, this amount generally the metal if residual moisture content is slight. Temperature becomes greater until i t is more than can be dried in one difference a t this point, therefore, will be small unless sufrevolution of the drum, and the excess must be removed by ficient moisture remains in the material to keep the tema spreader bar or other cqntrol device. To prevent conperature down. centration in the feed pan the latter may be placed at a disTemperature of the drum surface has been measured by tance below the drum and the liquor applied to the drum surRoeser and Mueller (5)on a double-drum dryer used for dryface by pumping, splashing, or spraying. Figures 1 and 3 ing milk under actual operating conditions. They found show a twin-drum dryer with splash feed. For heavier that the temperature of the milk film near the point of reliquids a transfer roll which dips in the liquid and contacts moval is within 3" F. of the temperature of the metal. The the drum may be quite effective. Frequently the layer is not individual film coefficients of heat transfer calculated by
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Literature Cited Harcourt, G . N., Chem. & Met. Eng., 45, 179 (1938). Harte, C. H., Jr., IND.ENQ.CHEM.,28, 7 (1936). Hirsch, M . , “Die Trockentechnik,” Berlin, Julius Springer, 1927. Reavell, J. A., Trans. I n s t . Chem. Engr8. (London), 6 , 115 (1928). Roeser, W. F., and Mueller, E. F.. Bur. Standards, Research Paper 231 (1930).
FIGURE4. DOCBLE-DRUM DRYER WITH TROUGH FEED sufficiently even to produce a uniformly dry product, and spreader bars or spreading rolls must be installed. While most of these designs can be applied equally well to dryers with one and with two drums, there is an advantage to the two-drum type. By setting the drums close together and regulating the clearance between them, a layer of uniform thickness can be obtained without removing any of the partially dry material from the drum surface. This serves to increase capacity and eliminates a source of trouble. A double-drum dryer with trough feed is illustrated in Figure 4. All the designs described so far suffer from the disability that a residue always remains in the feed pan which has to be disposed of when the dryer is stopped a t the end of the day. With two drums set close together and feeding the liquid to the reservoir formed between the drums, all available material can be dried without residual loss. In addition, the liquid is preheated to the boiling point and a considerable amount of concentration takes place in this reservoir, which in this case is a distinct advantage. Products over a wide range of concentration and viscosity can be handled without the necessity of providing circulating systems, and spreading devices are rarely required.
Operation and Economics Operating conditions of the drum dryer are of great importance and can be readily adapted to the product. h-aturally steam pressure in the drums is set as high as possible. Frequently temperatures can be higher than that to which the product may be subjected without injury to quality or color, because the time element is exceedingly short, and rapid cooling takes place as soon as the material is removed from the drum surface. The influence of steam pressure on capacity is very complicated and is by no means in proportion to over-all temperature differepce. In general, increase in capacity on account of increase in steam pressure is more than might be expected, because with most products the layer adhering to a revolving heated surface increases in thickness, the higher the temperature of the surface. Increase in drum speed reduces the time of contact between the drum and the liquid and tends to reduce the thickness of the layer of ad-
RECEIVED .May 4, 1938.
ROTARY VACUUM
DRYING L. H.BAILEY F‘. J. Stokes Machine Company, Philadelphia, Pa.
T
HE rotary vacuum dryer consists of a stationary cylindricalsteam-jacketed vessel in which a n agitator revolves. A modification is the rotating vacuum dryer in which the agitation is produced by revolving the cylindrical jacketed shelI. The first mentioned type is used when possible, as it is much more convenient to operate (Figure 1). Several types of agitators are in use in rotary vacuum dryers. Fixed flat paddles, usually set a t a slight angle to the radius, are the least expensive. Flat paddles may also be mounted on hinges and be equipped with springs to keep them in contact with the shell, although these paddles tend to lift and drop the charge in the dryer instead of producing thorough mixing and agitation. Various modifications have been made to produce better agitation and to decrease power requirements, with better mixing and rapid automatic discharge. One of the most satisfactory types consists of narrow spiral or helical shaped paddles mounted on arms of two different lengths (Figure 2). The long arms carry sectional spirals which tend to move the material towards the center of the dryer; the shorter arms carry continuous spirals of the *oppositehand carrying the material away from the center and maintaining a fairly even depth of charge in the dryer. T h e outside spirals provide rapid discharge as soon as the outlet of the dryer is opened. If the dryer is over 15 feet long, two outlets are used and the arrangement of the spirals is modified to suit the need. I n conjunction with a single inlet this double outlet arrangement of the spirals facilitates charging,
