DEHYDRATION OF FRUITS AND VEGETABLES 15 
ating over a comparatively long season on a variety of materials 
where the steam is needed to run auxiliary machinery or to process 
vegetables. 
AIR 
Large volumes of air are required to carry to the products the heat 
needed for evaporation and to carry away the evaporated moisture. 
Insufficient air circulation is one of the main causes of failure in 
many dehydrators, and a lack of uniformity in the air flow results in 
aneven and inefficient drying. 
FANS 
Propellor-blade fans are not efficient in delivering large volumes 
of air against the high frictional resistance in commercial driers. 
Centrifugal or turbine fans are used almost universally. ‘Turbine 
fans with 48 to 64 or more blades of short radial length are more 
satisfactory than the standard type centrifugal fan having only six 
or eight large blades. 
The fan may be installed to draw the air from the heaters and 
blow it through the drying chamber, or it may be placed in the return 
air duct to exhaust the air from the chamber. An advantage of the 
first installation is that the air from the heaters is thoroughly mixed 
before it enters the drying chamber. It has been claimed that 
exhausting the air from the chamber increases the rate of drying by 
reducing the pressure, but the difference is imperceptible in practice. 
Either location for the fan is satisfactory, and the chief consideration 
in any installation should be convenience. 
AIR DUCTS 
Close contact between the air and the heaters and between the 
air and the material is necessary in order efficiently to transfer the 
heat to the air and from the air to the material and to carry away 
the moisture. The increased pressures or resistance against which 
the fan must operate because of such contact is unavoidable and 
must be provided for, but at other points in the system every effort 
should be made to reduce friction. ‘To this end air passages should be 
. large, free from constrictions, and as short and straight as possible. 
Turns in direction should be on curves of such diameter as will allow 
the air to be diverted with the least friction. The general rule in 
handling air is that a curved duct should have an inside radius equal 
to three times its diameter. 
MOISTURE IN THE AIR 
The water vapor present in air at ordinary pressures is most 
conveniently expressed in terms of percentage of relative humidity. 
Relative humidity is the ratio of the weight of water vapor actually 
present in a space to the weight the same space at the same tempera- 
ture would hold if it were saturated. Since the weight of water 
vapor present at saturation for all temperatures here used is known, 
the actual weight present under different degrees of partial saturation 
is readily calculated from the relative humidity. 
Relative humidity is determined by means of two thermometers, 
one haying its bulb dry and the other having its bulb closely covered 
