82 



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



tunnels, since hot air flows into both ends and cool, moist air is 

 exhausted somewhere near the center of the tunnel. Inspection of 

 the diagrams will make it evident that whatever gain in cost of 

 operation there may be is at least partly offset by "increased com- 

 plexity of construction and control. 



FRESH-AIR INLET 

 ^ ///// 



EXHAUSTER AND 

 EXHAUST-AIR STACK 



WET 



MATERIAL 

 IN 





FRESH-AIR INLET 

 \\\\\ «^- 



DRY 



MATERIAL 



OUT 



Figure 44, — Center exhaust tunnel dehydrator that combines parallel-flow and 



counterflow drying. 



Whether the parallel-flow and counterflow sections are separate or 

 combined in a single tunnel, the typical course of the air temperature 

 and the moisture content of product in passing through the system are 

 illustrated in figure 46. The maintenance of effective drying condi- 

 tions throughout the entire time is reflected in rapid drying and in high 



WET 



MATERIAL 



IN 



/ 2 : 



$ 



— 0DQ[ 



" r\ 



PARALLEL-FLOW SECTIO 



^□DDDDDDD 

 k HGDDDDDnD 



DRY 



MATERIAL 



OUT 



COUNTERFLOW SECTION 



Figure 45. — Multiple-section tunnels. 



capacity secured from equipment of a given size. Suggested tempera- 

 tures and humidities suitable for any of these combination arrange- 

 ments may be found in later sections of this manual. 



Compartment Arrangement 



A less common arrangement of a continuous truck-and-tray dehy- 

 drator is known as a combination compartment and tunnel. The 

 characteristic feature is that air flow through the trucks is transverse 

 to the long axis of the tunnel. This arrangement makes it possible to 

 circulate the same air several times through each truck, and to reheat 

 the air at each stage. For example, if several cabinet dehydrators (see 

 later section on cabinet dehydrators) are placed end to end, with doors 

 between units omitted, a truck load of wet material may start at one 

 end of the group and progress through one compartment after an- 

 other, emerging completely dry at the other end. Then if a blower is 

 used to supply fresh air to the dry-end compartment, and air exhausted 

 from each compartment is used to supply the "fresh-air" requirement 

 of the next one, there is a general countercurrent flow of air through 

 the whole group. Each compartment can be provided with a separate 

 heater, which makes possible a wide freedom of choice with regard 

 to air temperature at successive stages of drying. Since the air can 

 be exhausted at high absolute humidity, good heat economy can be 

 obtained without undue sacrifice of drying rate. 



