DEHYDRATION OF FRUITS AND VEGETABLES 11 
. combine or modify the characteristics of endless-belt, compartment, 
or tunnel driers in their design. Aside from several such types which 
are proving efficient, the ordinary forced-draft driers of the com- 
partment and tunnel types are the most satisfactory for general 
dehydration. This bulletin deals primarily with driers of this sort; 
other types are discussed only by way of comparison. 
VACUUM DRIERS 
Materials can be dried more rapidly and at lower temperatures in 
vacuum than at atmospheric pressure. Such foods as meats, which 
are extremely susceptible to damage by heat, therefore, are more 
safely dried in vacuum. In drying fruits and vegetables vacuum 
driers can not compete commercially with forced-draft hot-air driers, 
because of the high cost of construction and operation and the limited 
capacity of vacuum-drying units. 
NATURAL-DRAFT DRIERS 
Natural-draft driers depend for their circulation upon the ex- 
pansion of air when heated and its consequent uplift by adjacent 
cooler and denser air. Such driers cost less to build than forced- 
draft driers, but their operation is less efficient and more expensive. 
It is difficult to obtain satisfactory drying conditions within them, 
and a long drying pericd is required. Recirculation is rarely prac- 
tical with natural draft; consequently heat is wasted unnecessarily 
in the discharged air. The greater efficiency and economy of forced 
draft combined with recirculation has been clearly demonstrated by 
the Bureau of Chemistry and the Oregon Agricultural College. Asa 
result, many operators cf Oregen tunnel driers are substituting forced 
draft and recirculation for the natural draft previcusly used. 
FORCED-DRAFT DRIERS 
The essential characteristics of all ordinary forced-draft driers are: 
(1) A single or multiple-unit drying chamber; (2) an air-heating unit; 
(3) a power-driven fan; (4) an air duct for ccnveying heated air from 
the heater to the drying chamber; and (5) a dampered recirculation 
duct for returning any part or all of the used air from the drying 
chamber to the heater. 
CONVEYOR-BELT DRIERS 
In driers of the conveyor or endless-belt type the material is carried 
through a drying hood or chamber on one or more moving endless 
belts made of slats or wire mesh. These driers are used successfully 
in completing the drying of partially dried raisins and in drying sweet 
corn. This method is not suited to fruits that stick to the belt or to 
fruits likely to be injured by dropping from one belt to another when 
multiple staggered belts are used. Neither are such driers adapted 
to irregular or discontinuous loading and drying operations. Be- 
cause of these limitations and their small loading capacity, conveyor- 
belt driers are less practicable than tunnel and compartment driers 
for general use. 
