DRUM DRYERS Equipment for the
Chemical Process Industries CHARLES R. HAFZTE, JR. P. 0. Box 917, Buffalo, N. Y.
OF A P A G E IN FIGURE 1. FACSIMILE ADVERTISING SECTION OF T. G . MARLOW’S “DRYING MACHINERY AND PRACTICE”(CROSBY LOCKWOOD &I SONS,LONDON,1910) THE
(Reproduced By courtesy of Bertrams Ltd., Edinburgh, Scotland succe8sor to Jamea Milne & bo, Ltd.)
The drum dryer has long been used for drying sheets of paper or cloth and, more recently, for drying liquids and pastes. Drum dryers in general-and the various types of drum dryers (atmospheric single-drum, vacuum single-drum, and the double-drum dryers) in particularare to be selected or rejected for any given drying requirement on the basis of their individual operating characteristics and costs. Examination of these factors is the object of the present article.
than a century ago in the development of drying rolls for paper and cloth. Problems related to use of the drum dryer to dry continuous sheets of paper or cloth escaped many of the perplexities encountered when this type of dryer is used for liquids and pastes. For liquids and pastes, a host of variables must be considered-viscosity, surface tension, corrosive action, heat and oxidation sensitivity, and rigid chemical and physical specifications for the h i s h e d product. The use of drum-type dryers for drying liqtids and pastes has attracted considerable attention in the process industries since about the beginning of the present century. Commercial application of drum dryers for drying liquids and pastes apparently had its beginning toward the latter part of the nineteenth century, for there were British patents as early as the 1860’s; certainly the use of the drum dryer for such materials was being advertised early in the present century. (Figure 1). During the past twenty years or so, application of the drum dryer has been extended and new types, as well as improved features for the older types, have been developed. A few of the many liquid, slurry, and paste materials for which drum dryers have been used successfully are: milk products, soaps and detergents, dyes, vegetable glue, breakfast cereals, soluble coffee, tannin extract, sodium acetate, calcium *chloride sodium phosphate, calcium and lead arsenates, clay, caustic mud, etc. Types and modifications of the drum dryer which have been developed to meet diverse processing requirements include atmospheric and vacuum machines, of the single-drum and double-drum types with various methods for applying the solution, slurry, or paste (including dip, spray, and top feeding). The drums may be constructed of the following materials: cast iron, bronze, nickel alloys, carbon steel, stainless steel, etc. ; chromium-plated drums have proved excellent under certain special conditions.
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HOLLOW metal cylinder, with an exterior surface that is smooth and true, is so supported that it may turn on its axis; steam is fed into, and condensate taken out of the hollow interior of the cylinder. Such an arrangement is the basic idea of the drum dryer, and it provides a rotary heated surface on which various materials may be dried. Supplement this basic idea with such additions as are necessary for the solution of problems encountered in applying wet material to the drum, removing the dried product, minimizing corrosion of the machine, applying vacuum to the drying material, making the product meet rigid physical and chemical specifications, and, above all, keeping the cost of drying within the range of economic feasibility,-and we have the various drum-drying machines that are available for the process industries today. Historical Background The drum-type dryer dates back to the Industrial Revolution; it made its appearance during the mechanization of the paper and textile industries toward the close of the eighteenth century. Problems related to drum fabrication and drive mechanism, and to the introduction of steam and removal of condensate, were encountered and solved more
Atmospheric, Single-Drum Dryer The drum dryer of simplest construction is the atmospheric, single-drum type (Figures 2 and 3). The drum is usually fabricated by casting (some drums are made from a sheet by 7
to remain in a smooth sheet. Sometimes the dip method (and also other feeding methods) is assisted by use of a spreader roll; this roll may he used to smootli the film and regulate its thickness after it has been picked up on the drying drum, or the spreader roll may dip into the feed and pick up a film which is then transferred from the spreader roll to the drying drum (Figure 3). An alternate method of feeding is by splashing, spraying, or pumping the liquid against the drum. This method permits a wide-range control of the thickness of the layer of material applied to the drum but requires rather skillful operation to take full advantage of this possible control. Very careful design of this sort of feed system is necessary, since the cold liquid gives “wet spots” on the drum unless the feed is uniform over the length of the drum. Some liquids which form drops with the dip feed will remain in a smooth layer when applied by spray. Stili another niethod of feeding is to pour the liquid onto the drnm from a rqservoir at the top; an adjustable weir is sometimes used as a means of regulating t.lie thicknese of the layer thus applied to the drum. Tiis method is useful where the material "bubbles" badly. The removalof dried product from the drum is accomplished by means of a knife, shsrpened and set a t an angle that best Courtesy, F . 1. S t d e s Machine Conipnnii serves the removal of the particular product being dried. FIOTJXE 2. A T M O ~ P ~ E RSIRULE-DRUM IC DRYERFOR DRYING With chromium-plated drums, a flexible knife may be used BONDING CLAY to prevent injury to the plating. (Note suriliades. pmtiouiarly the varisble-speed drive.) Important characteristics of the atmospheric, single-drum dryer are as follows: (1) Frequently great importance is attached to properties rulling and welding), followed by machining or grinding the of the drum-dried product which are acquired during the surface until it is smooth and true. The supports and drive process: The product comes o f f the drum in t,hin flakes, a for the drum offer no special difficulties; their design is desirable form for materials which are to be redissolved where commonplace to the expert. machine designer. The design rapid dissolving is an advantage (soaps, detergents, water problems involved in providing suitable means for continusofteners, etc.); again, the flake form may appeal to the ous introduction of steam (or hot water or oil which are ultimate consumer (breakfast cereals); the drum-dried sometimes used as heating media) and continuous removal of product frequently has a particular color or appearance which condensate through the trunnions of the revolving drum, have is of decided interest in marketing the product; the product been solved in a variety of ways. The even distriliution of delivered from the drying drum is usually quite uniform for gases and steam. so that it will sween out noncondnnsable an entire run of material and seldom requires subsequent ~~-~~ prevent stagnant regions, is important and is arindinr: or blendina. met by a suitable manifold arrangement which .. (‘2) The p r o d u c t i s exfeeds steani t o a number of points within the posed to drying conditions d m . The problem of condensate removal in (temperature) for a comparasuch a way that it does not accumulate in tively short time, a matter of the drum and thereby retard heat transfer. seconds compared to hours has provoked much engineering thought and (or even days) for some other has been solved by various arrangements of methods of drying; this is baffles and condensate piping. important when m a t e r i a l s Feeding the wet material to the drum and a r e d r i e d which h a v e a removing the dried product from the druni tendency to deteriorate a t often call for special design to meet specific redrying temperatures. quirements of a particular process. (3) The dryer is pracOne of the simplest methods of feeding a tically automatic, usually reliquid material is by means of a pan into which quiring only the part-time atca~l.iexo, ~ ~~~~~~~~~i~~ ~ i co., i rnc. the lower part of the drum dips. Often an tention of one operator. adjustable metal blade, sometimes called the FIGURE 3. FINE-F~LM Damn (4) The steani economy “bubble breaker,” is situated just beyond Noto paitionimiy the two-atage feedinp, rolls (tiie is high compared to many Grat ie $early vieible in the lower ioreground. the where the drum rises from the feed pan. This types of dryers. To evaposeoond IS intermediste between the first and the drying drum). Material i s pieked from the Seed b l a d e provides a means of controlling the rate one pound of water retroiuh b y the first xoll, transferred to the eeoond t h i c k n e s s of the layer t o be dried, scmes roll, and srain tisnsieired fo the dryirip drum. quires only from 1.25 to 2.00 t o b r e a k b u b b l e s a n d s m o o t h fhe feed pounds of s t e a m . (Stearn layer, a n d t h u s t e n d s t o p r e v e n t “ w e t r e o u i r e d v a r i e s with the spots.” This method is simple, reliable, and satisfactory temperature of the feed material, a h with the size and radiafor many liqiiids. Possible disadvantages of the dip nrethod tion losses of the dryer.) are: (1) It does not lend itself to fewling a stiff paste or a ( 5 ) Capacities of 2.5 to 4.0 pounds of water evaporated liquid containing heavy crystals, although it may be used for a per square foot of drum surface per hour are obtained for fairly thick slurry; and (?) with sume liquids the layer of dryers operating on Wponnd (gage) steam; these capacities liquid picked u p by the drum tend5 to form drops rather than increase with increasing steam pressure, generally to a &
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JANUAIIY, 1936
INDUSTHIAL A \ D ENGlNEENI\G CHEMISTRY
greater extent than corresponds to the increase in temperature difference between steam and material because film thickness is affected also. (In general, capacity depends primarily upon the characteristics of the material being dried, and the extent to which i t adheres to the surface of the hot drum; and only secondarily on the steam pressure and the feed method.) (6) The capital cost per square foot of drying surface ior this type of machine wili vary with the size of the unit, the special design features required for the particular job, and the level of machinery prices in general. However, a rough estimate of installed cost is $75 to $100 per square foot of drying surface for small dryers (20 square feet or less), and $33 to $50 for the larger machines.
Vacuum, Single-Drum Dryer1 Since some materials are lea&sensitive, they must be dried a t comparatively low temperatures; other materials tend to oxidize while drying and must be dried in tbe absence of air. To meet these requirements, the drum dryer may be incased in a vacuum chamber (Figures 4 and 5 ) . In many respects the vncuum drum dryer resembles the atmospheric type. Steam introduction, condensate withdrawal, feeding methods, and product removal are all essentially the same as ioor the atmospheric dryer, except that removing the dried product from tbe vacuum chamber offers a new problem. Sometinics the product is allowed to fall into a receptacle within the vacuum chamber from which it is removed periodically, but each rcmoval necessitates breaking the vacuum and interrupting the operation of the dryer. Sometimes the product falls into a screw conveyor which carries it to a storage chamber provided with appropriate valves so that the product may be unloaded from storage without breaking tile va.cuum on the drpcr. In this latter setup it is conimon practice to provide two storage chambers to be used alCernately. A convenient arrangement is to have one chember a t each elid of the scren conveyor, which is provided with a reversible drive to permit dlie prodirct to he conveyed in either direction.
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(1) Like the atmospheric dryer, the vacnnm dryer yields a uniform, flaky product, with a color or appearance which may he of special interest to the consumer. (2) As in the case of the atmospheric dryer, time of exposure to drying condit,ions is only a matter of seconds; also, the vacuum dryer (thanks to boiling-point lowering) permits drying to be accomplished a t a temperature much lower than is feasible on the atmospheric machine, an important consideration when heat-sensitive materials are dried. (3) It is essentially automatic, hut, with the added feature o i vacuum faciiities, it requires more attention than an atmospheric dryer of the =me size. (4) Steam economy is essentially the same as for the atmospheric dryer. (5) Capacity per square foot of drying surface per hour for a given steam pressure is greater for the vacnnm dryer than for the atmospheric dryer. (6) The Capital cost of the vacuum machine, with its heavy vacuum casing and accessory v m m m facilities, is necessarily greater than that of the atmospheric machine of corresponding size (square feet of drying surface) and may run four or five times as much. Welded steel construction for the casing and receivers may reduce this ratio to as low as three times and also reduces the weight of the machine.
Double-Drum and Twin-Drum Dryers Prequently two drimis are mounted in a single machine (Figure 6). In the double-drum type there is close (hut adjustable) clearance between drums, and the material !ills the space between the drums. In the twin-drum type the two drunis are spaced apart so that t.hey fiiiiction as two single drums. The two-drum construction permits certain economies hecause the two drums may be served by a single supporting framework, single driving motor, and single piping for parts of the steam and condensate system, may save some floor space, or may be attended by a single operator, etc.
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ISDUSTRIAL AND ENGINEERISG CHEMISTRY
The double-drum set-up permits feeding from the top in such a way that the material pmsses between the drums which, by their spacing, coutrol the thickness of the layer to bedried. I3y means of accurately fitted end plates, a reservoir is formed, Iminded by the drums and the end plates. This reservoir holds a supply of feed liquid from which the rotating drums carry material t,hrough the nnrrow space between them and around in a thin layer on each drum until a point is reached where knives (one for each drum) remove the dried product. While it is held in the reservoir between the drums, the feed liquid is heated to the boiling point by the hot drunis and preconcentrated to a considerable extent. The twindrum set-up is applicable to almost anything that may be dried on the single-dmm dryer. I t is particularly adapted for drying mnterials, such as solutions of inorganic salts, which are crystal-bearing or crystal-forming; the generous clearance between drums avoids trouble from any crushing effect. Dip, spray, and top feeding are all possible with the twin-drum dryer; frequently, also, spreader rolls are used to advantage. The characteristics of these two-drum dryers are similar to those of the single-dnim machine. Reduction in the attention required per unit of capacity, reduction in capital cost per unit of capaoity, and an improved control of feeding, for materials which are top-fed on the double-drum, are points in favor of the tyo-drum construction. Although possibly a minor consideration, with top-feed there is no residual feed liquid a t the end of a run.
Economic Considerations The question, as to whether the application of a drum dryer to perform a particular processirrg function will bo profitable, must he decided on the basis of a coinparison of the cost of obtaining the desired results by use of a drum dryer with t.he costs of all other possibilit.ies. Important points in the e8se for the drum dryer follim : (1) If t,lie uses to which the dried product is to be put are served by the particular physical form of product which is obtained by drum drying (the drum-dried product, may have 5 high dissolving rate, 8 particularly attractive appearance. etc.), justification may be found on that basis. (2) If the material to be dried tends to deteriorate while being dried, the fact' that the drum dryer exposes any given
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particle of the material to drying conditions for a matter of seconds only, may justify its selection over another method which involves longer exposure. (3) Usually, justification will require sufficientproduction so that the machine does not stand idle for an appreciable part of the year. Under conditiom of fairly continuous operation, savings due to good st,eam economy and reduced labor costs (a result of the almost automatic nature of the machine) vi11 have a chance to offset the charges due to capital investment in the machine and yield a profit. (4) If alternative methods involve a loss of an expensive product, due to handling, the drum dryer may find its justification in the elimination or reduction of this loss. A few prominent characteristics of other drying methods may be mentioned: The spray dryer often yields a product which has properties (density, dissolving rate, color, etc.) peculiar to spray drying. Where consumer requirements demand these properties, spray drying is the only choice. The rotary vacuum dryer, with its accessory vncuum facilities, has a comparatively high initial cost. Since it is essentially s batch dryer, its labor costs may run higher than for the continuous dryem (However, for some nonabrasive materials v~cuuinvalves have been developed which allow continuous operation.) The rotary vacuum dryer is somewhat inconvenient to clenn and is best used for long runs of a single product, or for products which may be contaminated with each other without undesirable consequences. It should be given consideration wherever vacuum drying with apitation is desirable, or where the recovery of a volatile solvent is necessary; it is unlikely to find justification where atmospheric drying is permissible. The atmospheric cabinet dryer usually involves a slow batch process, which may require several dnys. In this dryer the niaterial is dried in shallow trays placed on shelves in the cabinet; heat is supplied by the shelves, which may be s t p m pipes, or by a current of heated air which is passed over the trays. The same conditions exist in the vacuum sheE dryer rvhioh consists of trays and heated shelves incased in a vacuum chamber. The atmospheric cabinet dryer is comparatively low as to initial cost; the vacuum shelf dryer, with its heavy vacuum casing, has a much higher initid cost. Both of these dryers have comparatively high oporating costs. Labor costs (loading and unloading of pans) are high; steam economy, particularly for the atmospheric dryer, is couparatively poor, and from 4 to 10 p ~ & of steam may be consumed per pound of water evaporated. Justification of these typesof dryers, particularly theatmospheric type, usually depends upon a situation (often encountered in dye manufacture) where there are a great many different products to be dried, where perhaps only a small production of each prodJict is involved, and where each may be required this week or next and not again for possibly six months. Or justification may be found, particularly for the vacuun~type, where the material to be dried does not require or will not stand agitation. Such a material may demand Ion--temperature drying and sometimes requires Dhe absence of oxygen.
Acknowledgment The author wishes to thank those who assisted in the preparation of this article, particularly the Blaw-Knox Company, the Buell Engineering Company, the Buffalo Foundry and Machine Company, the 3. P. Devine Manufacturing Company, and the F. 3. Stokes Machine Company, for suggest,ionsmade and material furnished. Recmvso October IO, 1935.
