28 



BULLETIN" 1141, U. S. DEPARTMENT OF AGRICULTURE. 



inclination of the trays. In consequence there is a rapid and un- 

 impeded air movement throughout the whole apparatus. 



The temperature of the air falls rapidly as it passes through the 

 tunnel, as a result of the heat expenditure in vaporizing water, while 

 the amount of water vapor carried by it of course increases. The 

 difference in temperature at opposite ends of the tunnel is usually 

 from 25° to 30° F. In consequence, the fruit at the lower end of the 

 tunnel dries rapidly, while the rate of drying decreases steadily with 

 decrease in air temperature toward the upper end. When the tunnel 

 is first charged with fruit the trays nearest the air inlet become dry, 

 while those next them still contain much moisture, and those at the 

 upper end are scarcely well started to dry. While this delays start- 

 ing, it is of advantage once operation is well under way. The dry 



Cut. 



Hutu*} 



One. //e*rcA. JtKvci 



5 Colo Ara. OpcHM&a . 





& 



6"Co*t*e.rm 



Fig. 



11. — A section through the tunnel drier. 



trays are removed at the lower end, the whole series moved down 

 the length of one tray, and trays of fresh fruit inserted in the spaces 

 thus made at the upper end. The heat supplied is now increased 

 until the temperature at the lower end of the tunnel becomes as high 

 as is safe to employ in completing the drying, since the fruit nearest 

 the air inlet has lost the greater part of its moisture. The operation 

 now becomes continuous; an exposure of one to three hours to the 

 maximum temperature completes the drying of the fruit on the trays 

 directly exposed to it, and as they are removed the whole series is 

 moved down by the insertion of fresh trays at the upper end. 



For maximum efficiency, it is essential that the baffle-platelike ar- 

 rangement of the trays over the air inlet, termed by operators " bank- 

 ing " or " offsetting," be carefully maintained and that the trays on 

 each runway be pushed closely together, so that the air is forced to 

 move between successive tiers of trays. By so doing a uniform dis- 

 tribution of air movement through all parts of the drier, with a cor- 

 responding uniformity of the rate of drying, is effected. Until the 

 device of banking came into use, the runways were completely filled 

 from top to bottom, with the result that entrance and exit of air 

 were greatly hindered, air movement was mainly through, rather 



