80 



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



counterflow tunnel if the air flow were so high or the amount of 

 water evaporated were so small that little drop in air temperature 

 occurred. 



160 



140 



120 



100 



80 



^5 



>- 



l- CE 



z q 4 



IxJ I 

 H LU 



§l 3 



O CD 



i3 

















/ * 



\r Temperature 



















Wet- Bulb 











Temperature-^ 







10 20 30 

 DISTANCE FROM WET END (FEET) 





















































FlGUKE 40. 



10 20 30 



DISTANCE FROM WET END (FEET) 

 -Drying conditions in a lightly loaded counterflow tunnel drier, 



Time, 



6.35 hours ; relative capacity 0.71 ; final moisture content, 5 percent. 



Combination Arrangements 



The parallel-flow arrangement provides the highest possible rate 

 of evaporation at the wet end of the tunnel, while the counterflow 

 arrangement provides the highest possible rate at the dry end. Com- 

 binations of the two have therefore come into increasingly wide 

 use for the dehydration of vegetables. 







BLOWER 















HEATER 







r ..~>t- 



""•—---,--. 









T 



^^--' 



-*""" 



\\,\\\ 





































WET 

 MATERIAL 

























L'._ . J 





IN 







fe — zr 







i* » 

















FRESH- 

 — AIR 

 INTAKE 



DRY 



MATERIAL 



OUT 



TRUCKS EXHAUST-AIR STACK 



Figure 41. — Parallel-flow arrangement in the tunnel-and-truck dehydrator. 



The simplest combination consists of two separate tunnels, as 

 illustrated in figure 43. The parallel-flow tunnel may be relatively 

 short and is designed to accomplish a large proportion of the total 

 evaporation very rapidly, but to discharge a product which stili 



