DEHYDRATION OF FRUITS AND VEGETABLES 13 
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 simi- 
lar 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 
Fic. 6.—Tunnel drier (exterior) 
in tunnels at connecting points between air ducts and the drying 
chamber, so that inequalities in air distribution may be corrected. 
HEAT 
_ Heat plays an important part in the evaporation of moisture, first, 
in supplying the ‘“‘sensible”’ heat needed to bring the temperature of 
the water to the point to which the material is raised during drying, 
and, next, in furnishing the “latent heat of evaporation,” or the heat 
required to convert water into vapor at the temperature level reached 
by the drying material. The sum of the sensible heat and the latent 
heat of evaporation is called the ‘‘total heat of evaporation.” Heat 
also facilitates the transmission of water fiigh the cell walls, 
thereby assisting its passage from the interior to the surface of the 
material; it increases the vapor pressure of water, thus increasing its 
tendency to evaporate; and it increases the water-vapor-carrying 
capacity of the air. 
