4 CIRCULAR 443, U. S. DEPARTMENT OF AGRICULTURE 



through the exhaust fan and is blown to a collector from which it may 

 be sacked or conveyed to a place of storage. 



In the single-drum drier (fig. 2) the chopped hay passes directly 

 from the wet end (feed end) to the discharge end (dry end). In 

 the triple-drum drier (fig. 3) the material moves through the center 

 shell, back through the second, and out through the third, thus travel- 

 ing approximately three times the length of the drier before it passes 

 to the exhaust fan. 



Portable driers are usually smaller in size and capacity than sta- 

 tionary machines. In the double-drum drier (fig. 4) high-moisture 

 forage enters the rotating drum at the center of one end. The forage 

 moves down the center and back through the outer drum. The forage 

 is passed from the drying-drum through two cyclones for cooling. The 

 action of the drying-air through the drum aided by the baffles or flights 

 in the drum moves the forage through the drier. The exposure of the 

 forage to the drying-air is less than one half the distance of travel of 

 that in the stationary drums. 



Figure 4. — Revolving-drum type portable forage drier, double-drum design. 



The apron-conveyor drier is a relatively low-temperature drier in 

 which the material remains for a considerable period of time. The 

 temperature of the drying air or furnace gases entering the machine 

 is usually held at from 250° to 350° F., and the period of exposure 

 may vary from 15 to 45 minutes, depending in part upon the initial 

 moisture content of the forage. The temperature of the hay remains 

 much lower than that of the surrounding gases because of the cooling 

 effect of evaporation. The temperature of the forage also tends to 

 coincide with the wet-bulb temperature of the drying air, which, in 

 a hay drier, is much below the dry-bulb temperature. 



Apron-conveyor driers are usually much larger machines than drum 

 driers, partly because of the longer period of exposure required. Single 

 apron-conveyor driers for hay are frequently 150 to 200 feet long, 

 10 feet wide, and 10 feet high. 



Drying the forage without chopping, or chopping it in long pieces, 

 also tends to increase the time during which the material must remain 

 in the machine. The leafy portions and fine stems dry quickly, 

 whereas considerable time is required for drying the long, coarse 

 stems. The overdrying of some parts of the plant and heat losses 

 from the long tunnel may result in a low thermal efficiency for some 

 machines of the apron-conveyor type. 



