﻿EVAPORATION OF FRUITS. 25 



owes its survival and present popularity to the fact that it originally 

 embodied two or three principles essential to the successful drying 

 of prunes, and the expiration of the patents has resulted in gradual 

 modifications and improvements at the hands of users. 2 



In contrast with the kiln, which is intended for use with apples 

 and is not well adapted to the drying of most other fruits, the tunnel 

 evaporator is an excellent general-purpose drier. The distinctive- 

 features of its operation which adapt it to the drying of prunes make 

 it equally well suited to the handling of peaches, apricots, berries, 

 apples, and pears, and it is quite generally employed for drying these 

 fruits wherever they are commercially dried in prune-growing terri- 

 tory. That it has not come into use in districts in which apples alone 

 are dried is due to the larger expenditure of labor involved in drying 

 apples on trays. 



In its essential features the drying chamber of the tunnel evapo- 

 rator consists of a long, narrow compartment, with the floor and ceil- 

 ing inclined uniformly from end to end, with a furnace placed below 

 the floor at the lower end. The room is cut into a series of narrow 

 chambers, the " tunnels," by parallel partitions extending from floor 

 to ceiling. Warm air is admitted to each tunnel through an opening 

 in the floor at the lower end and escapes through a ventilating shaft 

 at the opposite end. The two ends of the tunnel have doors opening 

 the full width and height. The material to be dried is spread on 

 trays which are inserted on parallel runwaj'S at the upper end of the 

 tunnel, pushed gradually along as the drying proceeds, and removed 

 dry at the lower end. The inclination of the tunnel, aided by an 

 arrangement of the trays to be presently described, facilitates uniform 

 flow of air over the trays in all parts of the tunnel. 



DETAILS OF CONSTRUCTION OF THE TUNNELS. 



Tunnels are generally built in groups of three, heated by a single 

 furnace. The prevailing size of the individual tunnel has been 20 

 feet in length by 6^ in height and 3 in width. For reasons which 

 will be stated in considering the operation of the tunnel evaporator, 

 it is strongly recommended that the length be increased to 23 feet, 

 leaving the other dimensions unchanged. The floor and ceiling are 

 inclined, as already stated, the inclination that gives the best results 

 being one of 2 inches per foot of length. Walls and ceilings are of 

 wood or galvanized iron, and the floor should be of galvanized iron, 

 in order to reduce the fire risk. At the lower end of the tunnel an 

 opening in the floor 3 by 3 feet in size admits heated air from the 

 furnace room beneath. This opening is provided with a sliding door 

 of sheet iron which can be closed when ashes are being removed from 

 the furnace, in order to keep dust from rising into the tunnel. The 

 ventilating shaft is located at the opposite higher end of the tunnel, 

 extends entirely across the series of tunnels, and is 2 feet in width. 

 A ventilator of the type already described in the section on the 

 kiln evaporator, page 11, is very effective, The ventilating shaft 

 should extend 6 or 8 feet above the roof of the building, and the 



2 The tunnel evaporator in one of its earlier forms came into somewhat general use in 

 France under the name of the American, or Ryder, evaporator about 1S90. and a some- 

 what improved form was patented in France under the name of the Tritschler evaporator 

 prior to 1893. (Nanot, Jules;, and Tritschler, L. Traite Pratique du Sechage des Fruits 

 et des Legumes, p. 83-91, fig. 12-13. Paris, 1893. 



25497°— 23 4 



