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16 BULLETIN 1335, U. S. DEPARTMENT OF AGRICULTURE 
by a silk or muslin gauze kept moist by distilled water. Tap water 
should not be used because the mineral deposits from it clog the wick, 
retard evaporation, and produce inaccurate readings. The wick 
must be kept clean and free from dirt and impurities. The two ther- 
mometers are either whirled rapidly in a sling or used as a hygrometer 
mounted on a panel with the wick dipping in a tube of water and the 
bulbs exposed to a rapid and direct current of air. The relative 
humidities corresponding to different wet and dry bulb temperatures 
are given in Figure 7. | 
RELATION OF DRYING CONDITIONS TO DRYING RATE AND QUALITY 
OF PRODUCT 
As a general rule, the more rapidly the products have been dried 
the better their quality, provided that the drying temperatures used 
have not injured them. Some fruits and vegetables are more sus- 
ceptible to heat injury than others, but all are injured by even short 
exposures to high temperatures. The duration of the exposure at 
any temperature is important, since injury can be caused by pro- 
longed exposure at comparatively moderate temperatures. 
The rate of evaporation from a free water surface increases with 
the temperature and decreases with the increase of relative humidity 
of the air. The complex cellular structure and chemical nature of 
fruit and vegetable tissues retard evaporation, so that under no condi- 
tions of temperature and humidity does the rate of evaporation from 
them equal that from a free water surface. When conditions are 
such that surface evaporation from the tissues exceeds the rate 
of moisture diffusion to the surface, the surface becomes dry and hard 
and seals in the moisture. This condition, known as casehardening, 
is overcome by reducing the temperature of the air or by increasing 
the humidity. The maximum rate of drying, then, is attained by 
using the highest temperature which will not injure the product, the 
humidity being sufficient to prevent casehardening. In general 
practice, the temperature of the air entering the drying chamber 
should not exceed 160° to 170° F. The humidity at the air-outlet 
end of the drier should not greatly exceed 65 per cent. In driers 
employing recirculation the conditions of temperature and humidity 
may be largely controlled by varying the recirculation. : 
The velocities of air flow which produce the most efficient results 
in the drying chamber depend upon several conditions. In general 
the rate of drying increases with the velocity of air movement. Low 
air velocities tend to bring about slow and uneven drying. Exceed- 
ingly high velocities may not be used profitably, because a point is 
reached at which the materials will be blown from the trays or at 
which the increased speed of drying will not offset the cost of operating 
a larger fan. Velocities of 600 to 800 feet per minute through the 
drying chamber are satisfactory in tunnel driers; lower velocities are 
permissible in compartment driers. 
RECIRCULATION 
The most practical means of removing moisture from the air, and 
at the same time conserving heat, is through the steady discharge of a 
portion of the air leaving the drying chamber. The rest dries effi- 
ciently when mixed with fresh air from the outside and reheated. 
