EVAPORATION OF FRUITS. 5 



would occur, and this dry layer would retard the movement of water 

 outward to the surface from the interior of the material, thus slow- 

 ing down the drying process. The maximum temperature which 

 can be employed without producing these injurious effects varies con- 

 siderably with the different fruits, since it depends in every case upon 

 the physical structure and chemical composition of the particular 

 fruit, but it is in all cases very considerably below the boiling point 

 of water. 



There must also be a careful adjustment between the amount of 

 heat applied and the volume of air passing through the apparatus. 

 The heat required to convert the water evaporated from the liquid 

 condition to vapor is very considerable; the evaporation of 1 pound 

 of water absorbs a quantity of heat which would reduce the tem- 

 perature of 65,000 cubic feet of air by 1° F., or 1,000 cubic feet by 

 65° F. It would seem at first glance that the greatest efficiency in a 

 drying apparatus would be effected by allowing the heated air to 

 expend most of its heat in vaporizing water and to permit it to be- 

 come saturated before allowing it to escape from the drier. This 

 is not the case, for several reasons. Air at any given temperature 

 takes up water vapor quite rapidly until it has absorbed about half 

 the quantity it is capable of carrying at that temperature, after 

 which absorption goes on at a rapidly diminishing rate. At the 

 same time the air is losing heat through the vaporization of water, 

 every reduction of 27° F. resulting in the loss of half its moisture- 

 carrying capacity, this loss also operating to reduce the rate of 

 absorption of moisture. Consequently, it is not practicable to secure 

 saturation of the air before allowing it to escape, as this would make 

 the drying exceedingly slow. The best practice aims at permitting 

 the air to vaporize and absorb such an amount of moisture as will 

 reduce its temperature by not more than 25° to 35° F. during its pas- 

 sage through the apparatus, thus effecting rapid drying at the ex- 

 pense of the loss of about half of the theoretical drying efficiency 

 of the heat used. As a matter of fact this heat is not wholly lost, 

 since the expansion and resulting buoyancy of the warm air main- 

 tains a current through the apparatus, if provision for its escape 

 at the highest part of the drier is made. Without such provision, 

 the air can not escape and will quickly become saturated, with the 

 result that the escape of water vapor from the material is stopped 

 and the fruit is cooked in its own juices. In most evaporators in 

 common use the construction is of such a type that the air is ad- 

 mitted at the lowest portion of the apparatus and is allowed to escape 

 at the highest point, the arrangement of material in the interior 

 being such as to offer a minimum of obstruction to its flow upward. 

 In other types, the air movement is made independent of gravity 

 by placing fans in the air inlets or outlets or in both. Some driers 

 of this type have the defect that the air movement is so rapid that 

 the air can take up only a portion of the moisture it is capable of 

 carrying, thus giving a low return for the fuel employed. Other 

 driers obtain greater efficiency from the heat used by the employ- 

 ment of devices for the recirculation of the air, which is forced to 

 pass repeatedly over the material before being discharged from the 

 apparatus. As contrasted with driers in which the circulation is 

 wholly dependent upon the buoyancy of the warm air, driers of 

 this type gain somewhat in capacity as a result of the shorter time 



