10 CIRCULAR 619, U. S. DEPARTMENT OF AGRICULTURE 



in contact with the material in a reservoir beneath it. The dried 

 material either flakes off or is removed by a scraper (11). 



SPRAY DRIERS 



For spray drying, the material to be dried must be in solution or 

 in a state of very fine subdivision. Two types of spraying" devices 

 are common. The first forms the spray by forcing the liquid through 

 a nozzle at high velocity. The aperture of these nozzles may be 

 varied to increase or decrease the size of the particles of mist, or two 

 nozzles may be opposed to increase the fogging. 



The second type makes use of a rapidly revolving disk upon which 

 the material is dropped. The centrifugal force throws it from the 

 disk as a fine mist. This type, having no nozzles, does not require 

 the material to be in so fine a state of subdivision as where nozzles 

 are used. 



In both types, the spray is thrown into currents of warm air passing 

 through a vertical drying compartment. The air current is usually 

 blown into the compartment so as to create a swirling effect, and thus 

 maintain the spray in suspension and free from the sides until it has 

 become solid. It then falls to the lower part of the apparatus where 

 it collects in an unheated compartment. Spray driers are usually 

 supplied with a dust catcher where the air leaves the drying com- 

 partment; otherwise a part of the dried material would be lost. 



Juices and pulps may be dried in this type of drier. Owing to 

 their high content of hygroscopic sugars, it is necessary to mix many 

 of them with dextrin or other material in order to insure a satisfactory 

 and more or less permanent powder. 



Heat 



Heat plays an important part in the evaporation of moisture, first, 

 in supplying the sensible heat needed to bring the temperature of the 

 water to the point to which the material is raised during drying, and 

 next, in furnishing the latent heat of evaporation, or the heat required 

 to convert water into vapor at the temperature level reached by the 

 drying material. The sum of the sensible heat and the latent heat 

 of evaporation is called the total heat of evaporation. Heat also 

 facilitates the transmission of water through the cell walls, thereby 

 assisting its passage from the interior to the surface of the material; 

 it increases the vapor pressure of water, thus increasing its tendency 

 to evaporate; and it increases the water-vapor-carrying capacity of 

 the air. 



In the United States, the unit of heat customarily used is the British 

 thermal unit (B.t.u.), which for practical purposes is defined as the 

 heat required to raise the temperature of a pound of water 1° F. 



Heat is commonly produced through the combustion of oil, coal, 

 wood, or gas. Heating by electricity is seldom practicable because of 

 its greater cost; but where cheap rates prevail, it is one of the safest 

 and most efficient, convenient, and easily regulated methods. The 

 average numbers of heat units (B. t. u.) furnished by electricity and 

 various fuels are given in table 3. With the exception of those for 

 electricity, these average values will vary with the origin and character 

 of the fuel. 



