112 MISC. PUBLICATION 5 4 0, U. S. DEPT. OF AGRICULTURE 



the heat necessary for its evaporation. Under these conditions it 

 may be assumed that rapid evaporation maintains the particle tem- 

 perature at or near that shown by a wet-bulb thermometer until dry- 

 ing is substantially complete, after which the powdered product is 

 removed from the heated zone so quickly that thermally induced 

 damage is avoided even when high dry-bulb temperatures are em- 

 ployed. 



The necessary situation is attained in practice through the use of 

 a large drying chamber, usually cylindrical in form with a cone bot- 

 tom, but sometimes in the shape of a box. The cylindrical form 

 ranges in commercial practice up to about 20 feet in greatest diam- 

 eter and 40 feet in over-all height. Collectors that accumulate the 

 powder are attached to, or built into, the drying chambers. Fans, 

 heaters, and baffles induce a rapid whirling flow of hot air or flue 

 gas through the drying chamber, collector, and exhaust duct. The 

 product to be dehydrated is introduced into the hot-air stream near 

 its inlet in the form of a fine mist produced by a spray nozzle. 

 Moisture is removed in the space of a few seconds and the resulting 

 fine powder is deposited in the dust-collection system, from which 

 it is removed, cooled, and packaged, usually in a continuous operation. 



The spray drying process is particularly adapted to che production 

 of dried milk and eggs. Certain vegetable purees are readily dehy- 

 drated for use in soups, baby foods, and similar products. Tomatoes 

 and most of the fruits have not thus far been successfully spray-dried 

 on a commercial scale, presumably because of their high content of 

 low-melting, hygroscopic sugars which impart to the dried product a 

 tendency to stick to the equipment rather than to flow smoothly 

 through it. 



Vacuum Driers 



Water can be evaporated from moist substances at relatively low 

 temperatures in a vacuum, and substances that are extremely sensitive 

 to heat, such as serums, are accordingly dried under these conditions. 

 If the vacuum is sufficiently high the temperature may be lower than 

 the freezing point. The process has the additional advantage that 

 there is little or no contact with oxygen during the drying. Equipment 

 for vacuum processing must be of heavy construction to withstand the 

 pressure of the atmosphere, and all closures must be fitted with extreme 

 precision to avoid leaks. For this reason equipment is expensive. 



Most types of dehydration equipment can be built to operate under 

 vacuum. Rotary-drum driers and rotary-kiln driers have been so 

 used for some time. A type of cabinet dehydrator adapted for vacuum 

 operation is perhaps used more extensively for food products than 

 others. It consists of a cast-iron shell in which shelves internally 

 heated by steam, hot water, hot oil, or electricity are placed. Trays 

 loaded with material to be dehydrated are placed on the shelves. Heat 

 transfer is slow and inefficient in the absence of a vehicle such as mov- 

 ing air, and time for complete dehydration is therefore relatively long. 

 A recent development in this field employs a specially designed fan 

 to circulate the attenuated water vapor present in the dehydrator over 

 a heater and then over the charge. In this way evaporation is 

 greatly accelerated and the equipment, though operating batchwise, 

 may prove to have a substantial capacity in terms of dried product. 

 Another recent development is a continuous tunnel operating under 



