12 CIRCULAR 619, U. S. DEPARTxMENT OF AGRICULTURE 



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 for efficient transfer of heat to the air 

 and from the air to the material, and to carry away the moisture. 

 The increased pressure or resistance against which the fan must 

 operate because of such contact is unavoidable and must be pro- 

 vided 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 con- 

 veniently 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 having its bulb dry and the other having its bulb closely covered 

 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 

 thermometers are either whirled rapidly in a sling or used as a hygrom- 

 eter 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 ascertained from charts furnished by the instrument 

 makers, or published in engineers' handbooks. 



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 



