VEGETABLE AND FRUIT DEHYDRATION 87 



time has elapsed — the set temperature is raised to 140°+ (160° — 

 130°/3) or 150°. At the end of 3 hours, corresponding to 30 percent 

 of the estimated drying time, the temperature is again raised to the 

 normal hot-end dry-bulb temperature, 160°. 



During this starting-up period, loaded trucks are introduced into 

 the drier at their normal rate. The wet-bulb temperature within the 

 drier should be maintained at or near its normal operating value. 

 This means that a large proportion of the air must be recirculated 

 at the beginning, decreasing in amount as more trucks are introduced. 



Ending a run on a counterflow tunnel dehydrator normally does not 

 present any difficult problems. The only effect of terminating the 

 supply of raw product is a gradual decline of the wet-bulb temperature. 

 This condition will probably not result in injury but will carry the 

 product to a lower moisture content than desired. To prevent this, 

 the recirculation damper should be readjusted to maintain normal wet- 

 bulb conditions. This may also be accomplished by speeding up the 

 removal of the final trucks or slightly lowering the tunnel tem- 

 peratures. 



Parallel-Flow Tunnels in Multistage Driers 



The parallel-flow tunnel is never used as a single-stage vegetable de- 

 hydrator, because of its poor drying characteristics at the finishing end. 

 Therefore it will be considered only as one stage of a multistage de- 

 hydration unit. The product is usually dried in that stage to a mois- 

 ture content of about 50 percent (T= 1.0). For various products, this 

 would correspond to removal of 75 to 93 percent of the original mois- 

 ture and hence, if we were to refer to the time-temperature drying 

 curve as shown in figure 48, the range would be confined to the fairly 

 steep part of the curve which indicates the relatively rapid drying 

 range. 



In starting up a parallel-flow tunnel, the hot -end tunnel temperature 

 is brought up to normal operating temperature, and then the loaded 

 trucks are placed in the tunnel on the normal operating schedule. 

 For temperature-sensitive products and especially those that are 

 susceptible to case hardening, care should be exercised by the operator 

 to adjust the recirculation damper so as to maintain normal operating 

 wet-bulb conditions. During the initial loading, the drying load is 

 light. Hence, greater than normal recirculation must be practiced to 

 keep the wet-bulb temperature at a reasonably high level or else the 

 product may be permanently injured as a result of excessive drying of 

 the outer product surfaces. 



When operation is to be stopped, care must be exercised to prevent 

 damage due to excessive drying and scorching. When a run is ended 

 and filled trucks are replaced with empties, the wet-bulb temperature 

 will begin to decline if countermeasures are not taken. This is oc- 

 casioned by the decrease in drying load. The dry-bulb temperature, 

 being automatically controlled, will remain constant; hence, the last 

 filled car will be subjected to a constant high temperature and a con- 

 tinually falling wet-bulb temperature during its passage through the 

 tunnel. The effect, unless controlled, will be to produce higher and 

 higher drying rates within each successive car of the last tunnel load. 

 Scorching, discoloration, case hardening, and other injury may result. 

 At best the product will not be uniformly dried. 



Adjustment is carried out in the following manner. The hot-end 



