DRYING BY STEAM. Methods. There are four common methods of drying by steam. First, by bringing the materials to be dried into contact with revolving, polished, steam-heated cylinders. Second, by placing the materials in drying closets heated by high-pressure or low-pressure steam circulating through coils of pipes, the air in the drying room only moving by the force of gravity. Third, by placing the materials in steamheated drying closets having one or more horizontal disk fans near the ceiling of the drying chamber to agitate the air therein. Fourth, by placing the materials in drying closets and evaporating the moisture by means of large quantities of air forced over the pipe coils and through the drying room by means of a fan. A fifth method involves the use of a condenser through which the moisture-laden air is passed to lower its temperature and remove part of its moisture, the air thus treated being circulated over and over between the drying closet and the condenser compartment by means of a fan. Drying by Evaporation.-If a body of warm air, of ordinary pressure and humidity, be brought into contact with water, evaporation will begin at once. The vaporization of the water will continue until the air attains the maximum percentage of humidity, or becomes saturated. The rapidity of evaporation will be proportional to the difference between the actual humidity of the air and the possible humidity, or the point of maximum saturation. The capacity of air for absorbing moisture depends on its temperature and its relative humidity. The higher its temperature and the lower its percentage of humidity, the greater will be the quantity of moisture it can absorb. Should the air be cooled after becoming saturated, part of the moisture will be deposited in the form of fog, which will condense into dew on any object colder than the air. If the temperature of the air is again raised, the air will take up more moisture, which will again be deposited in the form of dew when the temperature falls. It is this moisture-absorbing property of air that makes air valuable as a drying medium. The efficiency of a drying apparatus in which hot air is used as the drying agent depends on several factors, among which are the following: First, the amount of moisture in the air before it is heated; second, the temperature imparted to the air; third, the amount of surface of wet material from which evaporation can readily take place; fourth, the volume of air circulated through the dryer; fifth, the distribution of the fresh dry air over the evaporating surfaces; sixth, the promptness with which the moistened air is removed. The accompanying table shows that the quantity of moisture 1 cu. ft. of air will take up increases greatly as the temperature rises. An increase of about 25° F. doubles its capacity for absorbing moisture. Hence, all other conditions remaining the same, the time required to accomplish the drying in a drying chamber will be reduced about one-half for every 25° F. increase of temperature. This proves the value of a high temperature in the dryer. MOISTURE IN AIR. Percentage of Saturation. Temperature. Weight of Moisture, in Grains per Cubic Foot of Air. The volume of air required to pass through a laundry drying room depends on the size of the dryer, the temperature maintained therein, the number of changes of air per hour, the amount of moisture in the goods to be dried, and the temperature and humidity of the incoming air. The range of temperature in the United States may be taken from 0° F. to 100° F., and the range of relative humidity from 40 to 100 per cent. It will thus be seen that a much greater volume of air will be required in a dryer on a hot, humid day in summer than on a clear, cold day in winter. Since one change per minute gives good results in ordinary practice, an easily remembered simple empirical rule is as follows: Rule.-Multiply together the length, depth, and height of the drying room, in feet; the result is the volume of air required per minute. Another method of determining the volume of air required is to calculate it on the basis of the amount of moisture to be evaporated. Each square foot of wet goods taken from the centrifugal extractor will contain, approximately, 2 oz. of water for thin goods, such as sheets, towels, pillow cases, etc., and from 2 to 3 oz. of moisture per sq. ft. for single thicknesses of blankets and other woolen fabrics. The temperature of the air in the dryer being maintained at, say, 120° F., 1 cu. ft. of air at that temperature is capable of absorbing 34.115 gr. of moisture. The number of grains of moisture in an ounce is 437.5. Therefore, 437.5 34.115 = 12.82 cu. ft. of air is required to absorb 1 oz. of moisture. In order that the surcharged air shall be disposed of quickly and before it reaches the point of maximum saturation, it is good practice to provide for 1.5 times the theoretical amount of air to be delivered to a drying chamber, in order to dry all kinds of materials in the same length of time, and to provide for other contingencies, such as radiation of heat and leakage of air through the walls and crevices of the chamber, imperfect circulation of air, etc. To compute the volume of air to be delivered to a dryer per minute, on the basis of evaporating the moisture contained in the wet fabrics, the following rule may be applied: Rule.-Multiply the number of square feet of wet material in each rack by the number of racks in the dryer. Multiply this product by the number of ounces of moisture in each square foot of wet material and by the number of cubic feet of air required, at the temperature maintained in the dryer, to absorb 1 oz. of moisture; then multiply by 1.5 and divide the product by the time, in minutes, required to accomplish the drying. The result will be the number of cubic feet of air required in the drying room per minute. A dryer with forced circulation is more efficient than one with natural circulation, accomplishing the drying in about one-third the time required by the natural-draft dryer, the difference in time depending on the height and size of the natural-draft vent stack and flues. The vent flue and stack should have cross-sectional areas of .34 sq. in. for each cubic foot of the contents of the drying closet. When air is admitted through holes at the base of the closet, the total area of the holes should be equal to the area of the flue. The areas of the flues and air openings, in square inches, may be found by the following rule: Rule. Divide the number of cubic feet of air supplied to the drying closet per minute by the velocity of the air-current, in lineal feet per minute, and multiply by 144. Should it be desired to accomplish the drying in less time than 1 hr., a larger quantity of air and a corresponding increase in the sizes of the flues and air openings would be necessary. The velocity in the air flues may be 600 ft. per min. The velocity at the air openings of the dryer should be about 300 ft. per min. It has been found, by experience, that approximately 1 lin. ft. of 1" pipe per cu. ft. of space is usually required in the dryer. A more accurate method that is suited to any case arising in practice is given in the following rule: Rule. Ascertain the amount of heat, in B. T. U., to be supplied to the dry room per hour; next, find the difference in temperature between the air to be heated and the steam in the heating coil. Find the coefficient of heat emission by referring to table headed "Heat Emission," and multiply this coefficient by the |