26 BULLETIN 1141, U. S. DEPARTMENT OF AGRICULTURE. 



partitions between individual tunnels should extend upward 3 or 4 

 feet into it, in order that winds may not interfere with the air move- 

 ment through a part of the group of tunnels. The doors are made to 

 fit accurately and are supplied with latches which close them tightly 

 to prevent air leakage. Their height and width are equal to the 

 inside dimensions of the tunnel, and they must swing back far enough 

 to clear the opening completely. 



In most tunnel evaporators the trays are supported by wooden 

 runways made of strips 1 inch square, nailed to the partitions paral- 

 lel with the floor of the tunnel, and extending from one end of it 

 to the other. The strips are placed 3-J inches apart from center to 

 center, and the upper edges are planed smooth and carefully lined 

 up with a straightedge in order that trays may be pushed along the 

 runways with a minimum of effort. The lowest runways are placed 

 4 inches above the floor, while the last pair is 6 inches below the 

 ceiling. This arrangement gives 18 tiers of trays. Each tier will 

 accommodate 5 trays, each 3 by 4 feet in size, or a total of 90 trays, 

 with a drying surface of 1,080 square feet for each tunnel. When 

 properly spread, each tray will accommodate about 25 pounds of 

 fresh apples or prunes, 16 to 20 pounds of berries, or 12 to 15 pounds 

 of peaches or apricots. The capacity of a tunnel, therefore, ranges 

 from approximately 2,250 pounds of prepared apple slices to 

 about half this weight of prepared apricots or peaches at a single 

 charge. 



In a number of large plants the labor involved in handling trays 

 is somewhat reduced by substituting trucks with a skeleton frame 

 upon which trays are loaded for the runways just described. The 

 trucks are made of such height as just to clear the tunnel ceiling 

 and are provided with low wheels which run upon light iron tracks. 

 They have not come into very general use for the reason that it is 

 difficult to construct a truck which permits the trays to be 

 " banked " or "offset " in the manner described in the section on the 

 operation of the tunnel evaporator, while it is impossible to secure 

 uniformity of drying at different levels of the truck unless this ar- 

 rangement can be made. 



THE CONSTRUCTION OF TRAYS. 



The best type of tray is one made of wire netting and 3 by 4 

 feet in size, as larger trays can not be conveniently handled when 

 loaded. The wire netting should be the best grade of galvanized 

 screening obtainable, with meshes one-fourth inch square; a 

 screen with one-fifth inch meshes is preferable if berries are to be 

 handled. The frame should be made of wooden strips 1 inch square 

 and should be double, that is, four strips should be nailed together 

 to form a rectangular frame : a strip of metal box strapping or a 

 piece of heavy wire should then be drawn tightly across the frame 

 at the middle and nailed in place, and the wire netting is then 

 nailed to the frame. A second strip of box strapping is then nailed 

 in place over the first, and the edges of the netting are folded back 

 and hammered down so that they do not project beyond the frame. 

 A second set of wooden strips are now nailed to the first, thus giving 

 a tray which can be used either side up. The box strapping re- 

 inforces the middle of the tray and prevents sagging of the wire, 



