62 



MISC. PUBLICATION 340, U. S. DEPT. OF AGRICULTURE 



5 

 2 































0.5 



02 

 01 



















\ 



_A 



ir Velocity 



1 

 et per Minute ) 



/ 500 













' . 675 



C^O 855 

















^ — ^ 



^ 





1 2 3 4 5 6 7 



TIME (HOURS) 

 Figuee 29. — Drying curves obtained with varying velocities of air. 



the higher the wet-bulb depression that will be maintained in the 

 dehydrator. 



Manner of Exposure to Air Stream 



In commercial dehydrators the pieces of vegetable are not suspended 

 separately in the air stream, but must be piled on one another, or on a 

 support. Contact with the air stream is thus hindered. The lower 

 pieces in a deep layer will dry very slowly if air flows only across 

 the top. 



In some types of dehydrator the air is caused to flow through the 

 layer of pieces, which are held on a perforated support. This is 

 known as through circulation. It exposes all pieces, even in a deep 

 layer, to contact with the entire flow of air. If the air flow is across, 

 or substantially parallel to, the surface of a layer of pieces, the arrange- 

 ment is known as cross circulation. A relatively thin layer must be 

 used in a cross circulation drier and an effort is usually made to expose 

 the bottom of the layer at least partly to the air stream by using some 

 form of perforated support. 



The experimental results presented above (figs. 24 to 29) all come 

 from cross-circulation runs. The air velocity referred to is the velocity 

 in the main air stream, above the layer of material. 



Direct comparison with a through-circulation experiment is diffi- 

 cult, because there will usually be several differences in conditions, not 

 just the difference in arrangement. It is practically impossible, for 

 example, to run air through a deep layer as rapidly as it is usually 

 blown across a tray ; indeed, the very meaning of the term "air velocity" 

 must be defined carefully or the comparison means nothing. The com- 

 monest way of expressing air flow through a layer is to give the cubic 

 feet of air flowing per minute through a square-foot cross section of 



