68 MISC. PUBLICATION" 54 0, U. S. DEPT. OF AGRICULTURE 



on the arrangement of the dehydrator, but a simple example will illus- 

 trate the principle involved : 



Suppose that in a simple counterflow tunnel dehydrator air is circu- 

 lating at the rate of 2,000 pounds per minute. Its dry -bulb temperature 

 at the point of entrance is 165° F. Suppose that 18,000 pounds of 

 prepared carrots enter the tunnel during 24 hours ; original moisture 

 content is 8.5 pounds of water per pound of bone-dry matter (7 T =8.5) , 

 and the product is dried in the tunnel to a final moisture of 4.76 percent, 

 (T f = 0.05). On the average, 12.5 pounds of carrots per minute enter 

 the tunnel, and the weight of water evaporated (see p. 54) is 



12.5 X * — =11.1 pounds per minute. Then the rise in absolute 



8.5 t~ 1 



humidity of the circulating air is Q nn0 = 0.0055. If the fall in air 



temperature is 5° for each 0.001 increase in absolute humidity, the total 

 temperature fall through the tunnel will be 27.5°, and the temperature 

 of the exhaust air will be 137.5°. 



Furthermore, the temperature fall between any two points in the 

 tunnel will be proportional to the change in moisture content of the 

 product between the same two points. What will be the temperature 

 at the point in the tunnel where the moisture content has fallen to 

 T=1.01 In a counterflow dehydrator the hot air enters the dry end 

 of the tunnel. Evaporation between the dry end and the point where 



T=1.0 is 12.5 X — ' = 1.25 pounds per minute; the rise in absolute 



humidity is 0.000625, and the fall in temperature is about 3° F. The 

 temperature will be about 162°. While the application of this principle 

 may be difficult because of the design of a particular dehydrator, it is 

 valid for any arrangement and will usually give a reasonably satis- 

 factory answer regarding temperature changes in the equipment. 



Maximum Evaporation Capacity of a Dehydrator 



If the two principles regarding temperature change are considered 

 together, a useful criterion for the maximum evaporative capacity of 

 any given dehydrator can be formulated. Note first that when the 

 air temperature falls until it is equal to the wet-bulb temperature, the 

 air is saturated and no further evaporation can occur. In fact, the 

 air could never cool quite that far through evaporative cooling alone, 

 because additional evaporation becomes slower and slower as the air 

 nears saturation. Then, since the wet-bulb temperature of the air re- 

 mains substantially constant, the maximum amount of evaporation 

 that can take place can be predicted at once if the original dry-bulb 

 and wet-bulb temperatures and the weight of air circulating are 

 known. The maximum possible fall in temperature is from the origi- 

 nal dry -bulb temperature to the original wet-bulb temperature (if 

 the usual slight fall in wet-bulb temperature is neglected). 



Suppose that in the preceding example the wet-bulb temperature 

 of the air is 100° F. Then the maximum evaporative capacity of that 

 particular counterflow tunnel would correspond to a fall in air tem- 

 perature from 165° to 100°, or 65°. The corresponding increase in 

 absolute humidity of the air will be about 0.013, and if the air flow is 

 2,000 pounds per minute the maximum evaporation will be about 26 

 pounds of water per minute. 



