4 CIRCULAR 443, U. S. DEPARTMENT OF AGRICULTURE 



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 mois- 

 ture content of the forage. The temperature of the hay remains much 

 lower than that of the surrounding gases due to 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, due in part to the long period of exposure. Single apron- 

 conveyor driers for hay are frequently 150 to 200 feet in length with 

 the equivalent of 10 by 10 feet in section. 



Drying the plants without chopping or chopping them 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. 



The drum drier when used for hay may be classed as a high tem- 

 perature machine, necessitating a short period of exposure to prevent 

 charring or burning of the hay. The temperature of the furnace gases 

 entering the machine is usually held at from 1,000° to 1,500° F., by 

 means of adjustments on the secondary air inlet to the furnace or its 

 quantity of fuel supplied to the burner. The material remains in a 

 drier of the drum type for approximately 1 to 3 minutes, or even less, 

 depending somewhat upon the initial moisture content of the forage. 

 However, as is the case with the apron-conveyor type, the tem- 

 perature of the hay is much below that of the drying gases. 



The chopping of the hay into short pieces enables it to dry quickly 

 and thereby makes possible the use of high temperatures in drum 

 driers. As the air is used for moving the material through the drier, 

 the light particles move more rapidly than the stemmy material, 

 which is helpful in preventing overdrying. 



The use of high temperatures and a short period of exposure in 

 drum driers makes possible a rather small, compact unit in comparison 

 with the apron-conveyor type. Single-drum driers are usually 40 to 

 60 feet in length and from 7 to 9 feet in diameter. Double- or triple- 

 drum driers may be somewhat shorter than single drum units, as the 

 use of multiple drums increases the length of the drying zone with 

 reference to over-all length. 



In drum driers there seems to be a definite relation between the 

 temperature of the exhaust gases and the moisture content of the 

 hay as it is discharged from the machine. An exhaust-gas tempera- 

 ture of about 200° to 250° F. is usually maintained by controlling 

 the rate of feed of undried hay. If the moisture content of the forage 

 entering the drier is high (65 to 75 percent) the exhaust temperature 

 should be kept at the high point of the range. With moisture contents 

 around 50 percent the exhaust temperature can be held at the lower 

 end of the range. The high exhaust temperature represents a con- 

 siderable loss of heat which tends to reduce the thermal efficiency 

 of drum driers. 



TESTS OF DRIERS 



Investigational work by the United States Department of Agricul- 

 ture was begun in 1930 in a study of the mechanics of artificial drying 



