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



floor at this point should be 8 feet ; as the ceiling of the furnace room 

 is formed by the tunnel floors, which are inclined 2 inches per foot of 

 length, the height of the ceiling at the opposite end of the room is 11 

 feet 10 inches. The walls of the furnace room may be of stone, brick, 

 concrete, or metal lath and plaster; they should not be of wood, by 

 reason of the nearness of the side walls to the furnace and the conse- 

 quent danger of fire. Two air inlets 4 feet in length and 12 inches in 

 height are provided in each of the side walls just above the ground 

 level, with single inlets of the same size in the end walls and beneath 

 the door of the furnace room. 



The furnace is placed immediately beneath the floor opening of 

 the central tunnel. The prevailing type of furnace in use in prune- 

 growing districts where wood is the only available fuel is a large box 

 stove, known as a hop-kiln furnace, equipped with heavy linings and 

 of such size as to take 4-foot lengths of cordwood. Brick furnaces 

 lined with fire brick are also used to some extent, and hard-coal fur- 

 naces of the type used in apple kilns may be employed where coal is 

 available. Whatever the type of furnace, it is fitted with a length 

 of heavy 10-inch or 12-inch pipe which rises to within about 4 feet 

 of the floor level immediately beneath the center of the floor opening 

 of the middle tunnel. It is fitted with a tee to which two lines of pipe 

 are attached. These are utilized for heating the two lateral tunnels. 

 Each line is carried from the tee to the center of the opening of the 

 tunnel, where a drum 18 or 24 inches in diameter is sometimes used 

 to give increased radiating surface. From this point the line of pipes 

 is carried parallel with the side wall of the furnace room and beneath 

 the floor of the tunnel to the opposite end of the room, where the two 

 lines may be brought together before entering the chimney. The 

 pipe is given an upward inclination toward the flue equal to that of 

 the floor, and it is usually placed 18 to 20 inches below the sheet-iron 

 floor. This arrangement of the pipes makes them effective in heating 

 the side tunnels. 



A MODIFIED TUNNEL EVAPORATOR. 



A modified form of the tunnel evaporator which has been developed 

 at the Oregon Agricultural Experiment Station 4 has, it is claimed, 

 a considerable advantage over the ordinary form in point of economy 

 of fuel and increase in capacit}^ for a plant of a given size. Its dis- 

 tinctive feature is an arrangement whereby the air is repeatedly re- 

 heated and recirculated over the fruit. This is accomplished by cut- 

 ting an opening 2 feet square in the floor of each tunnel at its upper 

 end and building a duct leading from this opening to a housing sur- 

 rounding a fan, which is so placed in front of the furnace that it 

 forces a current of air over the furnace and piping and into the air 

 inlets at the lower ends of the tunnels. The air intakes in the walls 

 of the furnace room and the ventilator at the upper end of the tunnels 

 are provided with trapdoors, which may be closed or opened to any 

 desired degree at will, and similar provision is made for admitting 

 fresh air into the housing surrounding the fan. The furnace and 

 piping are examined and made tight by cementing or stripping the 

 joints. When the fan is started the trapdoors in the ventilator are 



4 Weigand, Ernest H. Improved Oregon tunnel drier. In Better Fruit, v. 17, no. 7, 

 p. 7-8, illus. 1923. 



