PRESERVATION OF FRUITS AND VEGETABLES 9 



lation duct, to be either reheated or discharged from the drier without 

 passing through any other compartment. (See fig. 1, p. 13.) 



TUNNEL DRIERS 



In tunnel driers the drying is done in a long chamber or tunnel, 

 in which both the materials and the air move horizontally for the 

 most part (6, 16). A concrete-slab floor- hollow-tile walls, and a 

 reinforced-concrete ceiling make a durable and fireproof drier. Many 

 driers, however, have wooden floors and double walls and ceilings of 

 tongue-and-groove flooring nailed to a skeleton framework of 2 by 

 4's. Although the dimensions and capacities of the tunnels vary, 

 a typical tunnel is about 40 feet long, 6 feet wide, and 6 feet high 

 and is capable of holding a single line of 10 trucks, each truck carrying 

 a double stack of trays 2% feet square. Handling the trays on trucks 

 is more economical than conveying them through the tunnel on slides. 

 The trucks and trays should fit snugly in the tunnel, so that all the 

 air will pass between and across the trays. 



The loaded trucks are introduced through a door at one end of the 

 tunnel, and the trucks of dried product are removed through a similar 

 door at the other end. Air locks may be built around these doors to 

 conserve heat during loading and unloading. The doors may be in 

 the side wall at each end of the tunnel, with the air ducts connected 

 at the ends, or vice versa. Flexible movement of trucks is facilitated 

 by transfer trucks, turntables, or pivoted truck wheels. 



The course of the air through the tunnel is usually opposite to that 

 of the material to be dried, sometimes called the counter-current 

 system of circulation. Some operators advocate circulating the air 

 in the same direction as the material, in what may be called the 

 concurrent system. Tunnels wide enough to hold several parallel 

 lines of trucks have been built. In these the air is usually circulated 

 across from one side to the other. Screens or vanes are sometimes 

 installed in tunnels at connecting points between air ducts and the 

 drying chamber, so that inequalities in air distribution may be 

 corrected. 



ROTARY, OR DRUM, DRIERS 



There are two types of rotary driers; in one the dry material is 

 within a revolving drum; in the other it is on the outer surface. 

 The first type is usually direct-fired, the hot gases from the oil or 

 gas furnaces being carried either directly through the revolving 

 cylinder or between it and an outer shell. The food to be dried is fed 

 into one end of the cylinder and is tumbled about as the apparatus 

 revolves, until it is gradually delivered at the other end by a system of 

 spiral flanges. The material must be sufficiently dry at the intake to 

 prevent sticking to the hot surface of the drier, and the temperature 

 within must be carefully regulated in order to prevent scorching. 

 The second type differs from the other in that the material is deposited 

 on the outer surface of the drum in a thin film which dries in some- 

 what less time than it takes the drum to make one revolution. The 

 drums are heated from within by steam. The film of liquid or semi- 

 solid material is either deposited on the revolving drum by a feed- 

 ing device or is picked up as the drum revolves with its lower surface 



327697°— 41 2 



