﻿EVAPORATION -QF FRUITS. 27 



while there are no projecting ends of wire to injure the hands of 

 workmen or cause difficulty in moving the trays along the runways. 

 The number of trays provided should be 40 or 50 per cent greater 

 that the capacity of the tunnels. This will enable the day force to 

 spread a sufficient number of trays of fruit to keep the tunnels filled 

 during the night. The night attendant can keep the fires going, 

 remove trays as the fruit becomes dry, and keep the tunnels filled, 

 but he should not be expected to perform these tasks properly if no 

 surplus trays are provided and his time is largely occupied with un- 

 loading and reloading trays. 



THE OPERATION OF THE TUNNEL EVAPORATOR. 



The construction of the tunnel gives it several features to which 

 it owes its superiority over other commonly used types. Fruit 

 is exposed in thin layers to fairly rapid air currents which flow freely 

 over both upper and lower surfaces of the layers, instead of being 

 forced to pass through a single thick layer of fruit, as is the case in 

 the kiln, or through many superposed trays, as is the case in cabinet 

 driers. Fruit is exposed at the beginning of the drying process to 

 air of relatively low temperature and high humidity, thus avoiding 

 injury from overheating, but is automatically transferred without 

 rehandling into air of lower moisture content and higher temperature 

 as the drying proceeds. The operation of loading the drier is 

 continuous, since trays which have become dry are constantly being 

 removed, thus permitting insertion of fresh material as rapidly as 

 it is prepared and keeping the apparatus working at capacity. This 

 is accomplished by an arrangement of the trays upon the runways, 

 which the following description will assist in making clear. 



Freshly prepared fruit is introduced only at the higher end of 

 the tunnel. In charging the tunnel with fruit for the first time a 

 rather moderate fire is started in the furnace, and trays are inserted 

 one after another on the lowest runway and pushed down until the 

 front edge of the foremost tray is just flush with the air inlet in 

 the tunnel floor. If the tunnel is 23 feet in length as recommended in 

 an earlier paragraph, the runways will hold five 4 by 3 foot trays, 

 leaving the 3- foot opening of the air inlet unobstructed. The second 

 runway is next loaded with five trays, but these are pushed down 

 until the edge of the foremost tray projects 2 inches beyond that 

 of the one on the first runway. The other runways are loaded in 

 similar fashion, each tier of trays being made to project at its 

 lower end 2 inches beyond that just beneath it 3 and consequently 

 leaving a corresponding free space at the upper end of the runways. 



When the tunnel is filled the edges of the tiers of trays project over 

 the warm-air inlet, thus forming a series of baffle plates which break 

 up the ascending column of warm air and force it to enter the spaces 

 between the trays. Since the successive trays with their loads of ma- 

 terial form partitions through which the air can not readily pass, it 

 necessarily flows between the trays, thus coming into contact with 

 the fruit above and below until it reaches the upper end of the tunnel, 

 where its free escape into the ventilator is facilitated by the fact 

 that the edge of each tray projects 2 inches beyond its neighbor next 

 above. The movement of the warm air is also aided by the upward 



3 As a matter of convenience a diagonal line should be drawn or marked with paint on 

 the side walls at either end of the tunnel, to indicate the proper position of the trays. 



