EVAPORATION OF FRUITS. 31 



closed, and air which has passed through the tunnels is consequently 

 forced to return through the ducts to the fan, whence it passes over 

 the furnace, to be again heated and driven over the fruit. As the 

 temperature and humidity of the returning air rises, the fresh-air 

 inlet into the housing of the fan and the outlet to the ventilator are 

 partially opened, so that some fresh air is continually entering, while 

 a portion of the moist heated air is allowed to escape. With proper 

 attention to the adjustment of these openings, the temperature in the 

 apparatus mav be kept quite constant. It should not be allowed to 

 exceed 160° F. 



The additional cost of the fan and power for operating this form 

 of tunnel evaporator is more than offset by the reduction in fuel con- 

 sumption and the increased capacity of the plant, In drying prunes 

 with air recirculation there is a rather serious disadvantage in that 

 the fresh fruit at the upper end of the tunnels becomes overheated 

 and consequently cracks and drips unless the temperature is kept at 

 or below 150° F. This may easily be done either by moderating the 

 firing or increasing the quantity of fresh air admitted to the system. 



THE TUNNEL-EVAPORATOR WORKROOM AND ITS EQUIPMENT. 



The size of the building necessary and the nature and amount 

 of equipment needed in a tunnel evaporator depends primarily upon 

 the nature of the fresh material to be handled. If apples are to 

 be dried in considerable quantities, the workrooms must contain 

 the equipment for handling, paring, bleaching, and slicing the 

 fruit described in the section on the workroom of the kiln evaporator, 

 page 17. To this must be added the apparatus necessary for pre- 

 paring prunes, apricots, berries, and such other materials as it may 

 be proposed to dry. If the plant is solely for the drying of prunes, 

 or of prunes with the small fruits just named, as will be the case in 

 districts in which apples are not grown, the equipment will, of course, 

 be restricted to that required for handling these fruits. In the de- 

 scriptions and plans which follow, complete equipment for prepar- 

 ing applies as well as other materials has been included; omission 

 of apple-handling equipment in any particular case gives additional 

 storage space and involves no rearrangement of the other equip- 

 ment. 



The number of tunnels to be constructed must, of course, be deter- 

 mined b}^ the volume of fruit to be handled. By reason of the smaller 

 floor space required by tunnels as compared with kilns, a plant con- 

 taining 9 tunnels can be installed in a building 59^ by 42 : ^ feet in 

 size. Figures 11, 12, and 13 give plans for such a plant. As a single 

 tunnel will accommodate approximately 2,000 to 2,500 pounds of 

 fresh prunes or an equal weight of prepared apple slices (equivalent 

 to 3,500 pounds of apples) at one loading, the capacity of a 9-tunnel 

 drier is 9 to 11 tons of fresh prunes or 12 to 15 tons of apples at a 

 charge. The plans presented may be modified to give the plant 

 larger or smaller capacity, as desired, without alteration of the 

 workroom arrangement. 



On the first floor of the building the workroom contains the par- 

 ing and trimming table, as indicated in Figure 12. The remaining 

 space is in part occupied by the furnace rooms of the tunnels and in 

 part by storage space. The furnace rooms are equal in width to 



