Drying Processes with Gas Heat 1 DON D. BEACH Atlanta Gas Light Co., Atlanta, Ga.
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n DRYING, the process of removing water from a body or from the surface of the body being dried, water is evaporated and usually leaves as a vapor. Speed of drying can be increased through greater air circulation and lower relative humidity. Air circulation is attained simply. The relative humidity may be lowered by increasing the temperature of the air or body or by removing moisture from the air before it comes in contact with the work. Raising the temperature of air increases its capacity to take up water. Extracting moisture from the air with a dehumidifier or dehydrator produces lower relative humidities of the air before it is circulated. T w o general methods of lowering the relative humidity of air used for drying have applications within certain well-defined limits. Many materials such as foods, chemicals, drugs, and leather are harmed by excessive heat and must be dried at low temperatures by the dehumidification method. Silica gel and activated alumina are widely used adsorbents. Adsorbed moisture is, a t the proper time and with suitable equipment, driven off by heat. Where a considerable lowering of the relative humidity of the air is required, the direct dehumidification method is preferred. Where a smaller change in relative humidity is required, a liquid type of absorbent agent such as lithium chloride brine or so-called Kathene solution is suitable. T h e density of the solution and the amount of heat applied affect the effectiveness as an absorbent. The field for direct dehumidification is expanding rapidly, principally because much wartime production is dependent on controlled atmospheric conditions. For reasons of economy, however, such methods are not t o be used where higher air temperatures can be used for drying. Most drying is accomplished by the warm air convection method, although it is sometimes possible t o use radiant heat. Advantages of infrared or radiant drying are said to be speed of drying, light weight of drying ovens which makes portability possible, and low initial cost. A few of the disadvantages are unfavorable drying and curing of certain finishes and, where polished bases lie underneath, unequal drying of parts not of uniform thickness. T h e Nation, as never before, is dependent on adequate and satisfactory supplies of food. Gas is doing its part in processing vital foods for consumption at home, in England, Russia, Libya, and the Western Pacific. Gas is widely used in humidity 1 Abstract of a paper presented before the American Gas Association, Industrial and Commercial Gas Conference, at P i t t s b u r g h , Penna., March 12 and 13, 1942.
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and temperature control for the curing of sweet potatoes b y new processes developed by the U. S. Department of Agriculture. Eggs, milk, fruits, coconuts, tomatoes, molasses, and even bananas are dehydrated b y gas heat. T h e resulting powders occupy far less ship space than normal and often eliminate the need for refrigeration. In clothing production gas serves an equally important role. Gas is widely used in the ginning of cotton, as drying prior to ginning increases the quantity and quality of lint obtained by increasing the length of staple. Practically every article of clothing, especially shoes, requires drying or carefully controlled humidity conditions in its manufacture. Gas, because of the close control possible, is widely used in treatment of lumber. Atmospheres surrounding the lumber are first kept highly humid to prevent excessive end checking. As drying progresses the relative humidity of the air is lowered, and the drying is accomplished in hours compared with days formerly required. Furniture is only one of many items requiring this preliminary raw material treatment. In the construction field gas drying also plays an important part. Brick and tile for building must first be dried before they are fired at higher temperatures. Hardware, iron sheets, wire for nails, and countless other shapes are all dried quickly after pickling and washing to speed the production program. Walland plastic-board production requires enormous amounts of heat for drying and forming, and here too gas is extensively used. In the production of tanks, trucks, automobiles, guns, and planes gas drying is essential. Cores as small as a pencil and those literally as big as a barn are all needed, and gas is unsurpassed for this work. In some installations a movable crane carries a gas-fired air heater that efficiently dries the cope and drag of the huge floor mold, necessary in the production of a tank body. At the same time a gas fire dries the pouring ladle subsequent t o pouring steel in this and other similar molds. Newspapers and magazines require drying. Gas drying heats as high as 350° F. are used in spud presses to prevent the smear of ink and colors. Humidity conditions must be right for fine printing and engraving. More than 75 per cent of the kaolin used in this country is mined, processed, and dried in Georgia, and 80 per cent or more of this with gas heat. The drying methods used, however, are varied and C H E M I C A L
undoubtedly parallel drying processes in other lines. In 1741, when production and shipment of kaolin from Georgia to England for the famous Wedgewood potteries was first recorded, it was air dried on wooden poles in sheds b y the sun and wind. This method doubtless continued for many years and ruins of such a drying arrangement still remain near Macon, Ga. Later wood- and coal-fired boilers generated steam that heated pipe coils on which the crude clay or filter cakes were placed. N e x t tunnel dryers through which cars of clay were pushed, and later indirect rotary dryers through which the clay moved b y gravity, came into use with coal, both hand- and stoker-fired, as the source of heat. When natural gas was made available in 1930, efforts were first directed toward replacing solid fuel by converting the existing drying equipment t o gas. This proved not unwise for m a n y of the installations made then are still operated. However, it became apparent during the depression of the early 30's that, if this business was t o be held without rate reductions and expensive services, some more economical method of drying must be developed. Because of the whiteness of the clay, direct heat drying was believed by the kaolin industry to be impossible. However, the necessary trials, m a d e after persistent experiments, were so successful in every case that today the big majority of the clay produced where gas is available for drying is dried by direct-gas applications. Rotary dryers, such as those manufactured b y Christie, Ruggles, and Louisville, are fired indirect and semidirect by gas. Air is partially recirculated and is in some cases exhausted into direct drying ovens for the final extraction of heat. Most of these dryers are used for reducing the moisture content of the clay from 20 or 30 per cent t o 5 or 8 per cent. Where moisture content as low as 1 per cent is required, the clay is often pulverized and dried in a Raymond impact or roller mill with the air heated in a direct-fired air heater supplied at t h e bottom of the mill, and with the drying being completed as the minute particles of clay are carried upward t o the separator. This is an excellent method of drying since the last few per cent of moisture are difficult t o extract when the clay is in lumps, for in thorough drying there is danger of burning or calcining. Tunnel dryers are often operated by using steam coils for heating the air and fans for circulating it. While gas-fired boilers are used for generating the steam, A N D
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other than through the fan and oven itself. Some clay drying is done on large, internally steam-heated drums around which are wrapped cords that run through a v a t of slip. A.s the clay is dried by the hot drum, the strings flex and the dry clay is released, to be carried away in a screw conveyor. A recent adaption to gas of a dryer for clay, hitherto largely used in the textile industry for drying yarns, is one made by Proctor & Schwartz. T h e steam coils in this are replaced with direct gas-fired air heaters of a large size. T h e clay is extruded through a drilled • plate some 6 feet in width and is then carried on a steel mesh belt through the dryer. The clay lies on this belt in the size and shape of macaroni, piled to a depth of 3 to 4 inches, and the heated air readily passes through it for quick and efficient drying.
the most economical operation seems to be obtained by replacing the boiler and steam coils with direct gas-fired air heaters. Most of these air heaters are of corrugated culvert pipe, lined with firebrick and insulating material. These have a heat input rating of 10,000,000 to 20,000,000 B. t. u. per hour. It has been found advantageous to use a preheating chamber for slowly heating the cold clay with nearly saturated air taken from the main body of the dryer. This preheating prevents case hardening of the clay with subsequent slow release of moisture from the center of the clay. Temperatures of air are controlled automatically a t the point of entry to the tunnels, and temperatures in the oven itself are observed as necessary. Sail switches installed in the fresh air intake to the air heaters guard against power failures that might destroy the heaters, since no vent is provided
In one of the most recent installations clay in about the consistency of milk is to be sprayed through specially designed nozzles in the roof of a spray dryer, as made by Tur-bulaire. As this slurry is atomized, heated air furnished by a direct gas-fired air heater and mixed with stack gases from a 400-horsepower gas-fired boiler instantly dries the clay. By the time gravity has pulled it to the floor of the dryer, the clay is in powder form, ready to be removed through the outlet doors provided. While this particular unit is not yet in operation, it is believed that the drying will be economical and satisfactory. Another novel kaolin-drying process is that of intimately mixing superheated steam, produced in a gas-fired boiler and separately gas-fired superheater, with pulverized clay, but this is more a means of obtaining a special quality of clay than of drying.
Industrial Gas Goes to W a r ! HENRY M. HEYN Heat Treating Division, Surface Combustion Division of General Properties, Co., Inc., Toledo, Ohio
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HE 30-ton tank rumbling across the battlefield, the airplane rocketing through the sky, shell after shell plumeting on its way of destruction, the precise instruments which calculate its path, the ships, cars, communication systems—all these and many more could not be possible in the enormous quantities in which the Nation needs them so badly if, during our all too brief period of peace, we had not utilized our greatest ingenuity and energy toward the solution of problems arising in the normal furtherance of peacetime pursuits. Scientific research of the practical type has in the past 24 years been the most important item in the advance of industrial gas. We were not thinking in terms of guns and armor plate when, in the steel industry, radiant-tube annealing covers helped make possible the success of the continuous strip mills. We did not think in terms of shells when the problem of improved combustion for forging furnaces was the problem of the hour. When modern tube mills, with their controlled atmosphere, bright annealing and normalizing furnaces were developed, we were not concentrating on the production of planes. N o thought of siege guns or blitzkreig entered our minds when t h e short cut using industrial gas made possible the production of malleable iron in 110 hours less than just a few years ago. Few, if any of us, had ever imagined an airplane flying and fighting at 25,000 feet when we concentrated our efforts on the wire in-
dustry and by direct gas-flame annealing made possible wire with qualities thought a wild dream just a decade ago. The gas furnace that this very moment is turning out 82 miles of 50-caliber machine gun bullet clips a d a y was never conceived with that idea in mind. Controlled atmosphere furnaces in the brass and copper industry for the annealing of non ferrous metals, so important in the manufacture of cartridge cases, were only a short time ago little more than interesting theoretical discussions. Because it is a principle of Americanism to support private enterprise, these wonders of processing were made possible. We wanted better steel for household appliances, automobiles, surgical instruments, tools, and all of the products which are no longer luxuries t o our civilization. We wanted wire—better, stronger, cheaper— t o extend our lighting systems, our communication systems. We delved into the secrets of nature, into the secrets of the treatment of metal that controlled atmospheres made possible, because we wanted our world to be a better place in which t o live. Yet with all these peacetime accomplishments we could not forestay greed and avarice and so our problem of the hour, at t h e challenge of the hour, becomes one of a peculiar character. N o w we must apply those research a n d engineering accomplishments of peacetime years to the mechanics of war. A good example is the rotary gas furnace, designed during World War I for shell nosing. After the forging operation, t h e next step in making shells is the tapering of one end t o reduce air resistance and
1 Abstract of a paper presented before the American Gas Association, Industrial and Commercial Gas Conference, at Pittsburgh, Penna., March 12, and 13, 1942.
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increase penetrating power. Some projectiles are nosed cold, but all the large projectiles are tapered under high heat. For these a temperature of about 1,850° F. is necessary for forming. In the rotary furnace of World War I many advantages are to be had, particularly because this furnace ensured a definite time-temperature cycle. The only disadvantage in the older, original furnace was the scaling. It was diffiult not to g e t a hard, tight scale on the shell which was injurious t o the dies and sometimes worked into the projectiles themselves. With this old furnace equipped with the recently developed gas variable-flame burner, firing directly and vertically down into the furnace, this objection is completely overcome. During World War I, the cost of 155-mm. shell was somewhere in the neighborhood of $45. Today the same shell, of better quality and produced in much greater numbers, is being made for less than 50 per cent of the old cost. Another example is the manufacture of cartridge cases. One of the earliest studies sponsored by the American Gas Association Committee on Industrial Gas Research dealt with clean annealing of brass and now this peacetime industrial gas research is bearing fruit. Cartridge cases require three, four, or even five drawing operations, depending on the process and size. These operations harden the brass, and after each draw an annealing or softening process at temperatures from 1,200° to 1,250° is used to prevent cracking of the metal in the subsequent draw. Knowledge gained in the early research on clean annealing of brass was applied to the present-day manufacture of cartridge cases, 741
