86 



MISC. PUBLICATION 5 4 0, TJ. s. DEPT. OF AGRICULTURE 



this method is predicated upon accurate estimation of hot- and cold- 

 end temperatures. 



In preparing a schedule, a suitable hot-end dry-bulb temperature 

 is selected and a cold-end temperature is computed or estimated. 

 (See section on principles of dehydration.) A normal retention time 

 is estimated, based on past performance ; this time is the same as that 

 required to fill the tunnel under normal scheduling. The initial 

 charging temperature is then located on the curve as equal to the 



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Figure 48. — Typical drying curves for multistage dehydration. 



expected cold-end temperature plus a third of the expected over-all 

 tunnel-temperature drop. A second control point is similarly located 

 on the curve in a position corresponding to a lapse of 10 percent of 

 the retention time. The temperature at this point will be equal to the 

 initial charging temperature plus another third of the difference 

 between the expected hot-end and cold-end temperatures. Similarly 

 a third point is spotted at a time position corresponding to a 30 per- 

 cent lapse of the retention time. At this point, the tunnel temperature 

 is brought up to full normal hot-end dry-bulb temperature. 



In operation this seemingly complicated program is easily carried 

 out. For example, consider a 10-truck counterflow dehydrator being 

 used to dry potatoes. Previous runs under similar conditions in- 

 dicate that the drying time will be approximately 10 hours when the 

 hot-end temperature is 160° F. and the cool-end 130°. The initial 

 temperature should therefore be 130°+ (160°-130°/3) or 110°. At 

 the end of one hour — that is, after 10 percent of the estimated drying 



