VEGETABLE AND FRUIT DEHYDRATION 69 



Commercial dehydrators always represent a compromise between 

 high evaporating efficiency and high drying rate. If the air is dis- 

 charged nearly saturated, as it must be to attain full value from the 

 air circulation, the drying rate throughout a large part of the equip- 

 ment will be very low. Choice of the weights to be given to these 

 opposing factors will be based partly on relative costs and partly on 

 considerations of product quality. The standard fruit dehydrators 

 which are common in the West, generally operate at high air efficiency 

 and are correspondingly slow driers; air is usually exhausted from 

 them at 65 percent relative humidity, or even higher. Modern vege- 

 table dehydrators, on the other hand, may exhaust air at only 30 to 40 

 percent relative humidity, since both the economic and the quality 

 factors favor such a course. 



A half or more of the theoretical maximum evaporative capacity 

 will purposely be left unutilized, so as to promote reasonably rapid 

 drying throughout the entire time the product is in the dehydrator. 

 This purpose is attained by maintaining a high ratio of air flow to rate 

 of input of wet material. Note that in the examples just given, the 

 rate of evaporation was to be only 11.1 pounds of water per minute, 

 while the theoretical maximum was 26 pounds per minute; only 43 

 percent of the evaporating capacity would be utilized. This would 

 favor rapid drying. 



Experimental Determination of Drying Time 



The foregoing principles make it possible to obtain a reasonably 

 precise estimate of the drying time for any particular product in a given 

 dehydrator through not more than three experiments in a small cabinet 

 drier (see subsequent section on cabinet dehydrators). The drier 

 should be designed so that the ratio of air flow to evaporation will be 

 very large, since then the change of air temperature in passing across 

 the moist product will be negligible. Air temperature and wet-bulb 

 temperature must be controllable by the operator at all times. The 

 moist material will be carried in the same manner as it will be in the 

 commercial dehydrator (for example, on the same type of tray), and 

 the air velocity will be made the same. The tray will be so arranged 

 that it can be quickly and accurately weighed at intervals, in order 

 to keep a running check of the moisture content of the material on 

 the tray. 



Suppose it is desired to estimate the time required to dry carrots 

 in the counterflow dehydrator mentioned previously. Since the wet- 

 bulb temperature in the tunnel was to be 100° F., that wet-bulb tem- 

 perature will be maintained in the experimental cabinet throughout 

 the run. The dry-bulb air temperature at the exhaust or wet end 

 of the tunnel was"to be 137.5°, and at the dry end 165°. The experi- 

 mental run, then, will start at 137.5° and end at 165°. The tempera- 

 ture will be raised during the progress of the run in proportion to the 

 fall in moisture content of the product, as illustrated in figure 34. In 

 practice the adjustment of temperature will be made in small steps, as 

 shown in the diagram. The time required for drying will then ap- 

 proximate the time that would be necessary in the commercial dehy- 

 drator. 



If the dehydrator is of a more complex type, three such experimen- 

 tal runs may be necessary. For example, in a center-exhaust tunnel 



