PRESERVATION OF FRUITS AND VEGETABLES 19 



35 pounds of dried prunes will be obtained from 100 pounds of fresh 

 prunes. The tunnel drier will be designed, therefore, to embody 

 these characteristics. 



SPREADING AREA 



The following equation gives the spreading surface in square feet: 



Pounds fresh product dried X drying time (hours) 

 24 hours X pounds load per square foot of tray. 



Assuming an average load of 3 pounds of fresh prunes per square 



foot of spreading surface, the spreading area will be — ' ^ b t or 



o X^4 

 4,861 square feet. 



NUMBER OF CARS 



If trays 3 feet square are used on trucks, each holding 2 stacks of 

 trays 25 high, the spreading area per truck will be 3X3X2X25, or 

 450 square feet, and the nearest number of trucks furnishing 4,861 

 square feet of spreading surface will be 11, furnishing 4,950 square 

 feet, 



FREE CROSS-SECTIONAL AREA OF TUNNEL 



If 3 inches of vertical space be allowed each tray, and 1 inch of 



this represents the thickness of the tray, there will be 2 inches of 



.. n 2 inchesX36 inchesX2X25 ,. ., 

 open vertical space per tray, or — ^ , which 



is 25 square feet of free cross-sectional area, through which the air 

 can pass, provided all spaces on all sides of the trucks are occupied 

 by baffles. This figure (25 square feet) will be used later in calcula- 

 ting the air velocities through the tunnel. 



QUANTITY OF WATER EVAPORATED 



If the tunnel is to have a capacity for drying 7 tons, or 14,000 

 pounds, of prunes each 24 hours, assuming that 35 pounds of the 

 dried product will be obtained from every 100 pounds of the fresh 

 fruit, there will be evaporated 14,000X0.65, or 9,100 pounds of water 

 each 24 hours — an average of 6.32 pounds a minute. A definite 

 amount of heat will be required to bring about the evaporation of 

 this quantity of water. 



HEAT REQUIREMENTS FOR EVAPORATION OF WATER 



The requirement for sensible heat will be 1 B. t. u. per pound of 

 water evaporated from the material per degree increase in its tem- 

 perature. The actual temperatures of the products during drying lie 

 above the wet-bulb temperature and approach the dry-bulb tem- 

 perature as the drying progresses and the rate of evaporation decreases. 

 For safety in calculation it may be assumed that the product becomes 

 heated to the dry-bulb temperature at the hot end of the tunnel. In 

 the example considered, increasing the temperature of the fruit from 

 60° F. outside temperature to 160° in the tunnel makes the sensible 

 heat requirement 100 B. t. u. per pound of water evaporated. 



