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termined by experiment. For low rates of evaporation, t h a t is, low wet bulb depressions, the theory of evaporation for free moisture and hygroscopic moisture holds very exactly. For high rates of evaporation of heavier materials, such as ceramics, for example, there is a maximum rate for any temperature at which a n increase of velocity will have no appreciable effect in increasing t h e rate of drying. Under these conditions,
FIG.?-VARIATION
OF
RATEO F
EVAPORATION WITH MOISTURE CONTENT
the dry bulb temperature plays a n important part for t h e reason t h a t t h e temperature of the material is not at the wet bulb temperature, but a t a n intermediate temperature between the wet and dry bulbs, depending upon the evaporation determined by diffusion. The
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higher the temperature the more rapid is the diffusion, for the reason t h a t t h e vapor pressure of internal moisture increases rapidly with t h e temperature. I n such materials, the rate of evaporation per degree depression decreases as t h e surface of the material dries. This variation of rate of evaporation with moisture content is well illustrated by the curves in Fig. 7. These results are from actual tests of ceramic materials i n commercial dryers, and serve very well t o illustrate this practical point. The 100 per cent line indicates t h e rate of evaporation with free moisture. It will be seen that this holds up fairly well until about one-half of the moisture is removed, and then falls off rapidly as the material is dried out. The average rate of evaporation is almost exactly 67 per cent or two-thirds of t h e theoretical free evaporation from a moist surface. I n Material 1 i t is about 30 per cent of the free evaporation from a moist surface. I n applying the foregoing theory these practical considerations must always be borne in mind, and for certain classes of materials experiments must be made on a small scale t o obtain accurate data as t o the rate of drying as affected by diffusion. The general theory, however, has its practical value, since i t indicates very well the effects of arrangement of material and of velocities, temperatures, and wet bulb depressions, SO t h a t from any known operating condition comparative results may be calculated for some other desired condition. I n this a knowledge of the fundamental theory is of great assistance and value.
The Compartment Dryer By W. H. Carrier and A. E. Stacey, Jr. CARRIER ENGINEERING CORPORATION, 39 CORTLANDT ST.,N e w YORK,N.Y.
The art of successful air drying, or, more properly, air processing, is coming t o be appreciated more and more as a process of chemical and physical treatment apart from the mere removal of moisture. There are numerous classes of materials which require special treatment with respect t o (1) temperature, (2) relative humidity, and (3) rate of moisture removal. Most such materials are of animal or vegetable origin, and usually possess exceptional hygroscopic or absorption properties. Frequently they are of a colloidal nature. Among such materials t o which air processing is being successfully applied, the following may be mentioned: *Greenlumber Macaroni Developed films Textiles (natural and artificial) Cured tobacco Coated paper Milk Green tobacco (in curing for cigar wrappers) Tea Washed rubber Writing paper (after sizing) Photographic films Gelatin capsules Certain chemicals Chicle (for chewing gum) Certain industrial ceramics Painted and varnished surfaces, etc.
The optimum temperatures and humidities for t h e above vary over a wide r a n g e f r o m 75" t o 180" F. in temperatures, and from 90 per cent t o 15 per cent in relative humidities. The temperature and humidity requirements usually vary considerably in accordance with a definite established schedule. A
vigorous air circulation is usually important t o secure uniformity and maximum allowable effect. The time element is best regulated by controlling t h e wet bulb depression and temperature. I n some processes there are certain chemical or biochemical changes t h a t must be accurately timed with respect t o t h e percentage of moisture removed. I n these, i t must be kept in mind t h a t the velocity of chemical reaction a t a given moisture content of t h e material depends upon t h e temperature of t h e material (which corresponds t o t h e wet bulb temperature of the air). As chemists will appreciate, this velocity of reaction doubles approximately for every 18' F. increase in temperature. Since the vapor pressure also approximately doubles with each 18" F. increase in temperature, the velocity of chemical reaction is practically in proportion t o the variation in vapor pressure, as produced by variation in wet bulb temperature. On this account, there are certain critical temperatures, as well as humidities, in the processing of such products as green tobacco, macaroni, etc., where certain definite chemical changes are necessary, but where further chemical action must be prevented. I n drying many hygroscopic substances, there are two critical points with respect t o relative humidity. The first is where the free or nonhygroscopic moisture
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May, 1921
is being removed. I n this stage, the high relative humidity must be carried t o prevent hardening and shrinkage of the exterior, which would injure or spoil t h e product, A relative humidity of 80 per cent or more is usually necessary for this purpose, this being approximately the critical point a t which all hygroscopic substances retain their maximum elasticity or plasticity. I n this stage of drying, the free moisture alone is being removed. Again, in t h e final removal of hygroscopic moisture, a certain required amount of residual moisture must be retained, the removal of which would injure t h e product physically or reduce its value. This is especially true of materials containing colloids. The regulation of t h e final or retained moisture is accomplished b y maintaining in the dry room a t t h e end of t h e process, a definite relative humidity, depending upon t h e final moisture content desired. It is also often found necessary, between t h e initial and final moisture removal, t o graduate t h e relative humidity in stages, as the product dries, t o prevent a n excessive rate of drying a t any stage, which, in some cases, is found t o injure the product. This accurate processing of the material in dehydration is made possible and practicable by recent improvements in drying equipment and in the design and application of automatic temperature and humidi t y control. Especial attention will be devoted t o these later developments, both on account of their novelty and of their great practical importance in many industries. Process drying, in which air drying, as distinguished from vacuum drying, is essential, can be accomplished equally well in either t h e continuous or t h e compartment dryer, with which latter type this paper deals. The compartment dryer, however, frequently permits of manipulation and control of conditions which are difficult, if not impossible, with the progressive type of dryer, and entirely out of the question with t h e tunnel type of dryer. When the continuous dryer is used for such processing, i t is really divided up into a series of consecutive compartments, which are handled independently; so, in fact, i t is treated exactly as several Compartment dryers in series, and should be classed as such, except for the matter of handling of material. FIELD
OF
THE
COMPARTMENT
DRYER-ADVANTAGES
AND LIMITATIONS
Solid or plastic materials are usually best handled either by t h e vacuum system or t h e atmospheric system of air drying. The vacuum dryer, of course, recommends itself from the standpoint of speed, but it cannot readily be used for processing, the requirements for which have already been pointed out, nor can it be used successfully where the physical or chemical properties of the material are affected undesirably. Systems of atmospheric drying usually have t h e advantage in first cost and frequently, where properly designed, may also have a slight advantage in cost of operation. As a rule, however, the efficiency of the atmospheric type of dryer is considerably If the mechanlower than t h a t of the vacuum dryer.
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ical and physical problems in the drying and handling of a material indicate an atmospheric drying system, then the choice lies between three types: 1-The continuous automatic type, in which the material is conveyed mechanically through the dryer, or from compartment to compartment of the dryer, and a transverse circulation of air is usually maintained. 2-The tunnel type of dryer, in which the material is passed through on trucks and heated air is blown in at one end and exhausted from the other, usually in opposite direction t o the movement of the material. Material is usually handled on trucks, one truck being put in a t one end while another truck is removed a t the opposite end. 3-The compartment dryer, which may be entirely selfcontained, as a unit dry kiln, all Parts being furnished and set up in the building independent of the building construction, or it may be simply a room or compartment in the building, this fitted with coils, fans, or other apparatus used in the drying process.
The continuous automatic type of dryer is usually indicated wherever the drying period is less than 6 hrs., or wherever the process is continuous for 24 hrs., even though the time of drying may be considerably longer. I n other words, with a continuous dryer material must be fed continuously a t one end and unloaded continuously a t the other end. If the material requires more than 5 or 6 hrs. t o dry, i t will not be practicable t o operate the apparatus continuously a t full capacity. This will reduce t h e overall efficiency of the installation and will render i t extremely difficult to control uniformly the conditions within the dryer. I n other words, the continuous dryer is inefficient and expensive whenever it is not operated t o full capacity. The compartment dryer, on the other hand, can be loaded during t h e production period, and the drying can be carried on a t night, the dryer being unloaded t h e following morning, ready for a fresh charge. I n certain cases, t h e drying process may require several days or even weeks. I n these cases, of course, the continuous dryer is not applicable. The compartment dryer, on t h e other hand, is n o t applicable where a continuous process of handling material is desired. The efficiency, also, tends to4 be lower than t h a t of the continuous dryer. Uniform drying is more difficult t o obtain than with the continuous dryer, but the compartment dryer usually has the advantage of first cost over the continuous mechanically operated dryer. However, under certain conditions of production, more labor is required in handling t h e material with t h e compartment dryer than with t h e continuous mechanical dryer, which will more than offset the saving in first cost. On the other hand, where the material has t o be carried t o and from continuous dryers, on trucks, there may be actual saving in labor with the compartment dryer, in addition t o a saving in first cost, since t h e material can remain on the trucks until dry. The use of the compartment dryer in preference t o t h e continuous mechanically operated dryer is usually advantageous whenever the period of drying exceeds 5 or 6 hrs., and where production is not continuous during the 2 4 hrs.
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The compartment dryer also has a great advantage in intermittent or interrupted production as contrasted with the requirements of a continuous dryer for uniform and steady production. It also has the advantage of being adaptable t o the drying of different articles in small quantities a t the same time. While it is true t h a t there is more difficulty in obtaining uniform drying in the room dryer t h a n in the continuous dryer, this is true only on account of faulty design of the compartment dryer, and various means have been proposed and tried t o increase the uniformity and efficiency of the compartment dryer. It is these features t h a t are of chief interest in the study of the compartment dryer. * Considerable effort has been spent in perfecting the compartment dryer in work requiring accurate processing, and it has been found easier t o maintain a constant condition of temperature and humidity in the compartment dryer t h a n in the progressive dryer, because the control is unified. Control of drying is becoming a n important consideration in many industries, and its value is just beginning t o be appreciated. For many types of controlled drying, the compartment dryer has been found superior t o the continuous dryer, especially where constant control for definite periods is desired. CLASSIFICATION OF COMPARTMENT DRYERS
Compartment dryers may be classified under three general headings : 1-As t o the methods of loading and handling. 2-As t o the methods of supplying heat. 3-As t o the methods of moisture removal. The various classifications may best be shown by the following tabulation, important examples of which are illustrated by accompanying figures and diagrams. TYPES OP COMPARTMENT DRYER A-CLASSIFICATION AS TO MGTHODS OF LOADINQ AND HANDLING (1) Method of Sugport of Material ( a ) Trays (fixed or movable) ( b ) Movable pallets (c) Sticks ( d ) Hooks or clasps ( 2 ) Method of Loading ( a ) Cabinet dryers with drawers or sliding trays ( b ) Rack dryers (c) Truck dryers B-METHODS OF HEATING (1) Heating Medium Hot water Steam Superheated steam Electricity Heated oil Heated air Products of combustion ( a ) Direct application (b) Heated flues Latent heat of absorption (2) Method of Heat Apglication ( a ) Direct heat (radiation or convection) WaU coils Floor coils Distributed coils ( b ) Indirect heat Gravity circulation Mechanical circulation Direct fans (disk fans) Housed fans with distributing ducts Induced circulation Water sprays Steam jets Air jets
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C-METHODS OF MOISTURE REMOVAL ( 1 ) By Ventilation ( a ) Gravity of air supply and exhaust (b) Mechanical (fans or blowers) Exhaust Plenum Combination (2) By Condensation ( a ) Direct surfaces (water or brine) ( b ) Spray Fresh water supply and recirculated water artificially cooled (refrigeration) (3) By Absorption (a) Chemical (calcium chloride or sulfuric acid) ( b ) Physical (silica gel) (4) Transpiration (through cloth walls)
Dryers may also be classified as low temperature dryers, or high temperature dryers. Low temperature dryers are heated either by steam, hot water, or indirect steam, using hot air, and the temperatures in the dryer are usually below the boiling point. I n high temperature dryers, t h e temperatures are usually carried above $he boiling point, and the heat is supplied directly or indirectly by electric resistance, b y heated oil having a maximum temperature of about 600' F., by direct introduction of the combustion gases into the dryer, or by flue air heaters, where t h e air is conducted through tubes exposed t o the combustion gases. I n these high temperature dryers, the object is t o secure a n extremely rapid rate of drying, or t o procure certain chemical .and physical changes in the material, which may or may not be associated with the true drying process. However, i t is the method of applying the heat required for evaporation t h a t is of chief interest, and in which the various types of dryers vary most widely one from another. HEATING
The earliest types of dryers depended upon radiation and convection from internal heat sources for their heating effect. Little or no attention was paid t o the arrangement of t h e material so t h a t uniform heat distribution could be effected, the air moving,
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