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SYMPOSIUM ON
DRYING AND AIR CONDITIONING 2. The products of combustion first pass through a central
ROTARY DRYERS
flue axial with the cylinder and then return through the annular space, usually counterflow to the passage of material (Figure 3). 3. By a special arrangement the products of combustion of a fuel pass through longitudinal ducts secured to or forming a part of the interior wall of the cylinder, and then return through the central portion of the cylinder. D. Direct-contact dryers of the brick-lined rotary kiln type, in which the fuel is burned within the refractory-lined cylinder itself.
B. A. SMITH The C. 0. Bertlett & Snow Company, Cleveland, Ohio
I
N THE vernacular of the chemical industry a rotary dryer is a revolving, cylindrical, slightly inclined tube,
through which the material to be dried may be passed continuously. Since the purpose of drying is the elimination of water by vaporization, a supply of heat must be continuously provided in some manner and in sufficient quantity to accomplish the purpose.
Analysis of Heat Transfer Mechanism In the case of those dryers specified in A as being directcontact machines, it is apparent that heat transfer by convection is the dominant factor. Conduction and radiation do play small parts in conveying heat into the material, but they also account for no small part of the unavoidable heat losses. To employ convection heat transfer to maximum advantage, means must be provided within the rotating cylinder to disperse the drying material in the heating fluid as fully and as uniformly as possible. Usual means for this purpose are lifting flights or shelves fastened to the inner wall of the cylinder and so rtrranged*ast o successively pick up and shower the material down through the flue gases or air traversing the cylinder. I n this type of dryer the fluid agent being the sensible heat of flue gas or warm air, and the amount of heat entering the cylinder per unit of time being dependent on the number of pounds of such gas or air flowing into the cylinder, it follows that extreme care should be exercised in the choice of cylinder proportions; otherwise, in order to crowd enough heat into the cylinder to do the drying, the velocity of the gases may be such as adversely to influence the movement of the drying material in the cylinder, and may even cause loss of much of the fines through entrainment. * The principal factor in the effective utilization of heat in direct-contact dryers is the temperature drop in the heating fluid as it passes through the cylinder. Consequently, we find high-temperature dryers of this txpe yielding a maximum effective heat utilization of about 80 per cent, and very lowtemperature dryers using effectively as little as 30 per cent of the heat supplied. Typical examples of the performance of high-, moderate-, and low-temperature direct-contact dryers are given in Table I.
Classification The ordinary rotary dryers may be classified as follows: A . Dryers in which the material is directly contacted by the heating fluid, usually air or flue gases, passing either in parallel flow or countercurrent to the travel of material being dried. There are two general classes of direct-contact dryers : 1. High-temperature types, employing hot flue gases. 2 . Moderate- or lowtemperature types, usually employing clean air heated by some type of heat exchanger (Figure 1). B. Dryers in which the heat of the fluid medium is transferred through a metallic wall to the material being dried. There are three general classes : 1. The rotating cylinder is enclosed in a brick or insulated metal housing so arranged that the heat and products of combustion contact the exterior of the cylinder (Figure 2 ) . 2. Several flues pass axially through the cylinder and connect to a plenum chamber in the end of the cylinder; the heat is transferred t o the material through the metal walls of these flues. 3. The cylinder encloses a simple longitudinal bank of tubes so disposed as to contact the material; through these tubes may pass hot flue gases or simply warm air of any convenient temperature. C . Dryers in which the heat is transferred t o the material t o be dried both by direct contact with it, and by transfer through metallic walls. Such dryers are combinations of those mentioned in classifications A and B . There are three usual arrangements : 1. The products of combustion from a fuel first pass around the exterior of the rotating cylinder, yielding a portion of their heat, and then are passed through the cylinder (Figure 2 ) . 993
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLEI. PERFORMANCE OF DIRECT-CONTACT ROTARY DRYERS Per Cent Initial and Final Moistures, Wet Basis High-Temperature Range 10-0.5 6-0.5 6-0.5 40-15 3-0.04
Material to Be Dried Sand Stone Fluorspar Chewing tobacco Sodium chloride (vacuum salt) Green garbage Copperas Ammonium sulfate Sodium nitrate Cellulose acetate Sodium chloride (grainer salt)
7340 Moderate-Temperature Range 7-1 (moles) 3-0.10 5-0.10 60-0.5 25-0.06
Oxalic acid Crystal-clear synthetic resin Sugar-Glucose candy mixture
Low-Temperature Range 5-0.2 6-0.2 4-0.75
Over-all Per Cent of Effective Heat Utilization 61 65 Not known 76.5 70-80 65 54.8 50-60 41 51 35
VOL. 30, NO. 9
use of empirical constants along with the Stefan-Boltzman equation it is possible to approach the solution of a n indirectheat dryer problem with fair technical accuracy. Since the heat in these indirect dryers is not carried by air or flue gas going through the cylinder, the flow of air and vapors is usually a t very low velocity, and the danger of disturbance of material passage through the dryer or the entrainment of fines is almost nil. However, indirect-heat dryers must be chosen with care, for it is not uncommon t o find the damp material sticking to the inside of the cylinder and sometimes baking on. The effective use of heat in indirect-heat dryers is relatively low, seldom exceeding 50 per cent and frequently being as low as 30 per cent. Typical examples of materials dried in indirect-heat dryers and the reduction in moisture are as follows:
29 35 Approx. 30
Considering the wholly indirect-heat type, where the heat transfer is through a metal wall as mentioned in B, heat transfer by convection plays but a small part in heating the material to be dried. The burden of the heat transfer is carried by radiation, with conduction also playing a part. By conduction the heat crosses from one side of the metal wall to the other but thereafter aids only a little in getting the heat from the metal wall into the mass of material. Analysis of many indirect-heat dryers indicates that, once the heat gets into the metal wall, the limiting factor in its subsequent transfer to the material is by radiation. By the
Material t o Be Dried Kaolin d a y Dustless carbon black
Per Cent Initial and Final Moistures 15-2.5 65-0.75
Reported Per Cent of Effective Heat Utilization 65.5 40
Indirect and direct heating are often combined in a single rotary dryer using the so-called double-pass principle (Table 11). The mechanisms of heat transfer are complex; radiation, convection, and conduction all play significant parts. Because the heat transfer is complex and requires arduous analysis, many fantastic claims have been made for rotary dryers of this type. Of the two usual constructions (briefly classsed in C ) the most difficult to analyze accurately is the dryer which is encased in brickwork, subjected externally to the products of combustion and the radiant heat from a bed of fuel, and ar-
FIGURE 1. SIMPLE COUNTERFLOW DIRECT-HEATLOW-TEMP ER ATURE DRYER,STEAM-HEATED AIR
FIGURE2. GENERAL ARRANGEMEN OF INDIRECTOR OF INDIRECT-DIRECT HEAT, PLAIN CYLINDRICAL ROTARY DRYER, BRICK-ENCASED TYPE
SEPTEMBER, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
ranged for the passage of the combustion products through the cylinder after they leave the space between the cylinder and brickwork setting. Simpler to analyze is the heat transfer mechanism in the double-shell dryer, because the heat that warms the central flue must come from but two sources-convection and nonluminous gas radiation. This heat must then be transferred through the flue walls by conduction and radiated to the material being dried. The flue gases leaving the central flue and returning through the annular space yield heat by convection, as in the simple counterflow rotary dryer.
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proportions of such a kiln-type dryer can be made with reasonable accuracy through radiation formulas, using empirically determined constants. The utilization of heat is rather low, seldom exceeding 50 per cent and often being under 30 per cent.
Passage of Solids through Rotary Dryers Frequently overlooked is the i(conveyorll function of a rotary dryer. Improper choice of the speed and slope of a rotary dryer frequently mars the performance of an other-
FIGURE 3. INTERIOR (LOOKING FROM DISCHARGE END)OF AN INDIRECT-DIRECT-HEAT DRYER, USINGCENTRAL FLUE
wise well-proportioned machine. In this connection, work of the United States Bureau of Mines1 has shown (a) that the time of passage of a granular material through a revolving inclined cylinder is a direct function of the length of the cylinder and the square root of the angle of repose of the drying material and (b) an inverse function of the speed of rotation, the diameter of the cylinder, and its slope to the horizontal. Because of the fact that the experimental work TABLE11. PERFORMANCE OF IXDIRECT-DIRECT-HEAT ROTARY and subsequent mathematical analysis of the investigators DRYERS were done only on rotary kilns, some changes must be made Per Cent Per Cent of in the formulas presented to permit accurate evaluation of Initial and Effeotjve Heat Construction Material to Final Moistures Utilization of Dryer Be Dried the action of all rotary dryers in which the material is casBrick-encased Crushed lignite 36-10 81 caded, since this cascading action eliminates the stated in8-2 85 Crushed bituminous coal fluence of the angle of repose and also introduces an advancCrushed coke 6-1 61 Central flue type ing or retarding influence due to the velocity of the air or 9-2 78 (double shell) Bituminous coal flue gases traversing the cylinder. Bank sand 8-0.5 80.2 In simple cylindrical dryers and in the so-called doubleshell dryers it can be shown experimentally that there is a definite maximum amount of material which a t any instant The effective heat utilization of indirect-direct-heat dryers can be carried safely in a given cylinder, even with the most varies. Under good conditions of operation in a properly advantageous location and proportions of the lifting flights. chosen dryer, the percentage of the total heat which is effecIf the charge within the cylinder exceeds this maximum, the tively used may reach 85 per cent. The double-shell dryers time of passage of the drying material will be erratic and the usually show from 70 to 80 per cent effective use of the fuel material will be nonuniformly dried, particularly if the feed heat. rate fluctuates. If the charge is safely within this limit, the Rotary kilns are used on occasion for drying materials intime of passage will be relatively constant over a rather wide sensitive to heat, especially when extremely large and heavy range of feed rates, and the drying material will be uniformly lumps are mixed with a mass of wet fines, and it is impractical treated. to sort out the lumps previous to drying. As in ordinary higher temperature kilns, the heat transfer to the drying RECEIVED May 4,1938. material is principally radiation and conduction from the hot kiln walls and fuel flame. The technical approach to the 1 Bur. of Mines, Tech. Bull. 384 (1927). Analysis of many such dryers indicates that, of the total heat transferred to the material being dried, 15 to 20 per cent is about all that can come through the central flue walls; 80 to 85 per cent is transferred by the direct contact of the returning flue gas with the drying material.
