DEHYDRATION OF FRUITS AND VEGETABLES yd S 
WATER EVAPORATED DURING A GIVEN CHANGE OF TEMPERATURE AND HUMIDITY 
Example: Air enters the drying chamber at_160° F. and 20 per 
cent relative humidity and leaves at 120° F. and 65 per cent relative 
humidity. | 
According to Figure 9, air at 160° F. and 20 per cent relative 
humidity contains 0.044 pound of water vapor per pound of dry 
air, while at 120° F. and 65 per cent relative humidity it contains 
0.050 pound. . Then there has been evaporated into the mixture 
0.050 —0.044, or 0.006 pound of water vapor per pound of dry air. 
Since 1 cubic foot of the original mixture at 160° F. contained (fig. 8) 
0.060 pound of dry ai, there has been evaporated 0.060 x 
0.006, or 0.00036 pound of water per cubic foot of the original mix- 
ture at 160° F. and 20 per cent relative humidity. If the air was 
entering the drying chamber at the rate of 1,000 cubic feet a minute, 
theoretically the evaporation would be 0.36 pound of water a minute. 
ENGINEERING CALCULATIONS FOR DESIGNING A DRIER 
The characteristics of a drier may be determined approximately 
by calculations based on the nature and quantities of the materials 
to be dried. When the drier is to be used for several materials 
separate computations must be made for each, so that the drier will 
fulfill the requirements for all. Such calculations are useful in 
designing a new drier and in remedying the defects of one already 
in operation. 
To illustrate the computations involved, let it be assumed that a 
tunnel drier equipped for recirculation and employing the counter- 
current system of air circulation is to be built with a capacity for 
drying 7 tons of fresh prunes daily. It will be assumed that the tem- 
perature of the outside air is 60° F. General experience in drying 
prunes indicates that if air is heated to 160° F., contains about 
20 per cent relative humidity, and has a temperature drop of 35° F. 
in passing through the tunnel and a humidity at the discharge end 
not exceeding 60 to 65 per cent, the drying period will not exceed 25 
hours and about 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 per 24 hours x drying time (hours) 
Pounds load per square foot x 24 hours 
Assuming an average load of 3 pounds of fresh prunes per square 
9 = 
foot of spreading surface, the spreading area will be arr 
b] 
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 33 x2 25, or 
450 square feet, and the nearest number of trucks furnishing 4,361 
severe feet of spreading surface will be 11, furnishing 4,950 square 
eet. 
aia 
