OCTOBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
sideration of all the factors involved, he reaches the conclusion that southern production will quadruple within a decade:
Writing and book papers Wrapping papers Newsprint Board Tissue Miscellaneous
Present (1938) Southern Capacity Tona/day 460 2,000
...
3,000 50 250
5,750
Expected Additional Southern Capacity Tons/day 3,550 2 500 2,000 3,500
700
2,000 -
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proximately 500 million dollars of capital-60 per cent in new mills, 16 per cent in forests, 8 per cent in convertingplants, and 16 per cent in housing for employees. The entire southern branch of the industry thus forecast will consume annually an estimated total of 800 thousand tons of salt cake, 300 thousand tons of chlorine, and fuel equivalent to 3 million tons of coal. Obviously t>herealization of any substantial part of this expectation will result in an important industry.
14,250
Moon further forecasts that the production of this additional 14,250 tons per day will require the investment of ap-
RECEIVED August s, 1938.
DRYING MACHINERY FRED KERSHAW Proctor & Schwartz, Inc., Philadelphia, Pa.
T
such a splendid job of improving their product. The wet HI3 need for good drying equipment today is great. strength of rayon, for instance, is 80 per cent greater today than Chemicals, ceramics, casein, tobacco, textiles, soap, it was 15 years ago. Its elasticity, dye absorption, uniformity leather, pharmaceuticals, plastics, rayon, gun powder, of denier, and other qualities necessary for its manufacture into and a multitude of similar products must be dried one or more finished products have likewise been greatly improved. The times in the course of their manufacture; and they must be refinement of the drying operation has played an important dried properly and economically. The drying operation for part in this improvement of quality. Rayon dryers today are these products is a more vital operation today than ever bescientifically designed for the particular plant in which they fore. With the extended use of brand-name products, standare to operate. The velocity, direction, and distribution of air ards of quality in all these industries have been raised to a flow necessary to give the best results have been scientifically point which makes it imperative that the drying system be determined for each type of rayon. Complete, accurately intelligently and scientifically engineered ; otherwise the controlled air-conditioning equipment is made an integral quality of the product may easily be brought below that part of the dryer. The rayon in its travel through the dryer of its competitors, with a resulting loss of sales. Cigarets, passes through various zones; each zone is of a predeterfor instance, are vastly better today than those made 15 mined length, and in each of them definite dry- and wet-bulb or 20 years ago, and consequently they have a wider contemperatures and air flow are maintained. All of these sumer acceptance. Modern drying systems, scientifically factors in drying rayon have a decided effect on its shrinkage, engineesed, have played an important part in this improvestrength, elasticity, uniment, keeping step with formity of denier, dye abthe exacting requirements sorption, and other qualiof the cigaret manufacties. turers in their constant Twenty-five years ago, drive for better quality and when competition was not economy. When tobacco so keen as it is today, when is being dried, a gum or the standards of quality of sap comes to the surface commercial articles were of the leaf under the acnot so rigid or so high as a t tion of the heat. If this present, when products gum condenses on the leaf were much simpler in their a n d is n o t c o m p l e t e l y construction and make-up, evaporated, the tobacco will and when labor costs were have 9. bitter and unnot such vital factors, the pleasant taste. Modern drying machines are now drying operation was much simpler and was not cond e s i g n e d t o a e r a t e and sidered of sufficient imporseparate the leaves so completely while they are betance to warrant the exing dried that none of this tensive scientific study that bitter sap remains to spoil is given to these problems the flavor. today by the leading deIn the past 15 years the signers and builders of dryworld production of rayon i n g m a c h i n e r y . Consehas increased 1500 per cent. quently, many companies This would not have been a t that time either used possible if the manufachomemade dryers or bought ONE TYPEOF FAN WHICHIs USED IN MANY DRYERS Is turers of rayon had not done standardized ready-made MOUNTED DIRECTLY ON THE MOTOR SHAFT
dryers, and obtained results that today would be ruinous in the face of existing standards of quality and costs. For years a well-known company which manufactures white lead used a homemade rack dryer on which they dried thcir product in pans. It took them 7 days to dry it. Becauseof this long drying time there was considerable discoloration froni one cause or another. To keep in step with their competition, they wanted to improve their product. They recently called in a well-known organization of drying experts and put the problem up to them. As a result they are now producing a much whiter and cleaner product, they dry their lead in an hour and R half instead of 7 days, they are saving 60 per cent of the floor space formerly occupied by their homemade dryer, and the handling of their product is completely automatic from storage tank to finished-product storage bin. Their dust problem has also been greatly minimized
Diversity of Drying Problems Seldom are any two drying problems exactly alike. A slight difference in processing a ma. terial will sometimes make it either harder or easier to dry. Ordinary dyed rayon skein8 without any oil treatment will dry in about I hour, whereas dyed rayon skeins that have been processed with oils, waxes, or gums will take from 2 to 4 hours to dry, depending on the kind and amount of oil used. A tallow-bwe toilet soap dries much fwter and easier than an olive oil soap. It takes twice as long to dry Paris green in one plant as in anothcr because of a slight difference in the method of processing, although the drying systems are exactly alike. Veneer from a gum tree grown in the lowlands of Mississippi takes 50 per cent longer to dry than veneer from the same kind of a gum tree grown on land farther north. The textile fibers and fabrics dyed with dark colors take longer to dry than those dyed with lighter shades. Many bittcr disappointroents and failures have been recorded when someone without sufficientexperience and knowledge of drying has assumed that, because a material in one plant could be dried in a certain time,
(Readingfrom top to bottom)
BATCH-TYPE DRYER FOR DRYINO AND STRETCHINO SKINS BATCH-TYPEDRYER FOR DRYING P A C ~ AOF G CLOSELY E~ WOUNDCOTTON AETER DYEING SEMIAUTOMATIC DRYERFOR DRnNQ Sxh~lrP~RCELAIN OBJECTSPRIORTO FIRING AFTER RAYONIS SPUN, IT IS DESULFURED, WASRED,AND THENDETED IN THE SEMIAUTOMATIC MODERN RAYON DRYER
a eimilar material in another plant could be dried in the same time. Consequently, in order to obtain the most efficient machine for a particular plant, it is necessary t o engineer the job from beginning to end. A careful study must he made covering the peculiarities of the m a t e d to he dried with relation to its sensitivity to heat, the rate at which it gives up moisture, the effect of drying under varying conditions on the color, feel, taste, and odor of the product, the amount of moisture to be removed, the capacity of dried material required, the most economical method of handling the material before and after drying in conjunction with the method of handling through the dryer, the character of heat and power supply, the value of material per pound, the permissible amount of labor a t the existing mt, and a great many other details which must be analyzed before the type of machine which will he most efficientfor that particu. lar joh can be decided. Drying machines are built of steel, stainless steel, aluminum, brass, copper, wood, brick, cement, and concrete. The type of material used for the construction can be determined only after a careful study of the conditions under which the machine is to be operated with regard to the effect of acids, moisture, etc., on the metals used and the contamination that might result from the effects of corrosion. Drying machines must he well insulated to prevent loss of heat; otherwise steam consumption will be high and drying costs comespondingly high. The insulation utilizod depends upon the temperature inside the dryer, the cost of heat used in the dryer, and the return on the investment represented hy a saving in heat. Some machines are built without insulation, others are built with various forms of air-cell aabcstos,others with rook wool or glass wool, some with cork, some with aluminum foil, and others with magnesium covering. A complete study of the factors surrounding the particular installation under consideration is neceasary to deter-
(Readingfrom top to boltom) STICK AND APRUN TOBACCO DRYER FOR DRKING AND CONDITION~Q BUNDLES OF TOBACCO LEAVB~ BEFORE T ~ S K ARE PACKED IN HODSAEADS Eog&ads of tobseco %reaged for 2 yB~csI.
DRYINQ RAYON IN SKEINS rn A MonERN AUTOMATIC DRYER,WITB CONTROLLED TEMPERAT~E AND HWIDITT A ~ O M A T I CTYPEOF DRYERWnicH TAKESHOT LIQUID SOAP FROM TAB h T T L E , SOLIDIFIES, CElXPS, AND DRIES IT IN 15 MINUTES,READYFOR PACKAQINQ
DRYER FOR HANDLINQ BLICACAED CELLULOSE
AUTOMATIC TYPE OF
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INDUSTRIAL AND ENGZNEERING CHEMISTRY
1118
mine the most efficient type of insulation for the job. After a certain point is reached in insulation, the law of diminishing returns makes it unwise to spend money for additional insulation; likewise it is costly to try to save money by not having sufficient insulation. costs
The variables affecting drying costs are so diverse and numerous that it is difficult to present them in the form of a table. I n general, the estimated cost is approximately 0.1 to 0.3 cent per pound of water evaporated, though there are ’ many cases above and below these limits. I n the cost of drying must be included steam, power, labor, floor space, depreciation, maintenance, and interest on the investment. The expression of costs in terms of pounds of water evaporated is less variable than on the basis of pounds of material dried. For example, a thousand pounds of feathers requires a dryer with a much larger holding capacity than would be required for a similar weight of white lead. Therefore the cost of drying a pound of feathers would probably be much higher than that of drying a pound of white lead, but the cost of removing a pound of water from the feathers by a drying process might be less than for the lead. It is obvious also that the various factors entering into the total cost of drying vary from one plant to another, Depreciation, maintenance, and interest on investment will vary especially with the type of machine required for a particular job. As an example of an extreme case, clay pots used in production of glass may be cited. About 2 xeeks are required to dry these clay pots in a mechanical dryer, and about 6 months are necessary if the pots are allowed to dry on the floor where they are molded. The necessity of carefully drying such pots to avoid cracking them contributes to the high drying costs. Another example is glue, which must be dried at low temperatures; the drying cost is therefore higher than is the case with the average material which may be dried in the neighborhood of 200” F.
Research and Preliminary Engineering The building of modern and scientific drying machines is a complex engineering business, and a modern drying machinery company must of necessity maintain a large research department. I n this department tests are made to determine the maximum temperature the material will stand without adversely affecting the quality. The exact percentage of moisture contained by the customer’s material must be determined, and tests at various air velocities must be made to find the lowest possible drying time. This research department usually makes tests also on the customer’s material under various conditions of air flow-first, through the material, both up and down, then across the material from right to left, from left to right, and, in some cases, over a combination of directions. I n the case of loose materials the research department also makes various tests to determine the best thickness of loading the wet material to give the most efficient drying. The research department then turns over to the preliminary engineering department a detailed report of the ideal conditions under which the material should be dried. The preliminary engineering department studies the customer’s floor layout, his labor conditions, and the preceding and subsequent operations, and determines what type of dryer would suit these conditions best and would, a t the same time, meet the conditions set forth by the research department under which the customer’s material can be most efficiently dried. I n some cases a batch dryer, in which the material is placed on trays or trucks manually and left in the dryer for a stated
VOL. 30, NO. 10
period, is the type which best meets all conditions. I n other cases a semiautomatic dryer, in which trucks of material are automatically progressed through a drying compartment, is needed. I n the vast majority of cases, a completely automatic machine, in which the material is automatically fed to the dryer from some preceding operation, such as a rotary filter, automatic press, etc., and is conveyed on some form of conveyor through the drying compartment, is the type of dryer that gives the greatest economy and best quality. After the preliminary engineering department has made a study of all the conditions, a preliminary plan is made showing the general type of dryer and its location in the customer’s plant, with respect to other equipment preceding and following the drying operation. This preliminary layout is then discussed with the customer’s engineers and production men, and any changes which seem desirable are then made.
Design of the Machine After the preliminary plans have been approved, the data from the research department and from the preliminary engineering department are given to the production engineering department, and a machine is designed to fit the requirements exactly. At this point a long and varied experience in the design of drying machinery is essential. I n order that a machine may be designed which will fill the customer’s requirements exactly and a t the same time be efficient and economical, it is necessary that the designing engineer be able to interpret commercially the findings of the research and preliminary engineers. A test in the laboratory is more or less an ideal situation which is seldom reached in practice. It is vitally important, therefore, that the engineer designing the machine shall have had sufficient experience to be able to transform the findings of these preliminary studies into a machine which will give the customer the capacity, the quality of drying, and the economy of operation and maintenance he wants. He must determine the best type of fans to use for the purpose, the number and size of fans, and their most efficient location and speed. He must also determine the best type of heating medium, keeping in mind the source of heat supply. He must properly design all the mechanical parts of the machine so that there will be no undue wear or friction, and the conveying means must be such that the material being dfied will not be damaged in its travel through the machine. He must also see that the heat generated by the heating medium is used with the greatest efficiency, and to do so he must determine the proper amount of air to exhaust from the dryer, the corresponding amount of intake fresh air, and the amount to be recirculated. He must design the framework of the machine in such a way that there will be no vibration and must make proper allowances for expansion and contraction; otherwise mechanical parts will be thrown out of line and wear unduly. Literally hundreds of other points too numerous to mentionmust be borne in mind when designing a drying machine if an efficient machine is to be produced, and the answers t o all of these questions can be known only to those who have had years of experience in this kind of work. RECEIVED April 21, 1938.
CORRECTION. In our June issue some statistics were given in an editorial entitled “A Witness for t h e Defense.” Through an oversight, the price quoted for sulfanilamide tablets failed t o indicate the fact that this was for 1000 tablets. Correspondence indicates the desirability of giving this further information.
