394 THE TIMBERS OF THE WORLD 



In the Sturtevant. progressive kiln, air heated by contact with steam 

 pipe coils is driven into one end of the dryer by means of centrifugal 

 fans, and the timber after drying passes out at the other end. The 

 air gathers moisture as it passes over the timber, and cools down. 

 Green timber is entered at the air discharge end, where the temperature 

 is low and the moisture high, and moves progressively through zones of 

 increasing temperature and decreasing humidity, and finally emerges at 

 the hot-air entry end. 



The American Blower Company then developed a progressive kihi 

 in which the circulation of the air was obtained by thermal means. Fans 

 and motive power were not needed, and these kilns, being milder and 

 more moist in their action, proved most useful for hard woods. The 

 A.B.C. kiln, in its latest improved form, is known in this country as the 

 " Erith " moist air dryer. 



As progressive kilns are not adaptable for a mixed output, that is, 

 for small quantities of varying kinds of thicknesses, such as are required 

 for some works, compartment kilns were then developed. B. F. 

 Sturtevant used fan-controlled ventilation, while with other processes 

 fans or thermal circulation were the methods employed for ventilating 

 the kilns. With thermal circulation each compartment is a separate 

 unit, with its own radiator pipes, chimney, and other apparatus, and a 

 drying plant consists of several of these units. With fan circulation, 

 one fan and steam heater serves any number of compartments, all the 

 machinery being in a separate chamber at the side of the drying rooms. 

 The fan kiln has certain advantages over the thermal kiln. Firstly, 

 less steam piping is required to radiate the same amount of heat, and 

 consequently there are fewer joints to keep tight. As the steam pipe 

 is outside the dryer it is under better observation. Finally, as the 

 circulation is under mechanical control, the intensity of ventilation can 

 be varied independently of alterations in the temperature, and " return 

 air," or warm, moist air from the various compartments, gives a humid 

 atmosphere suitable for the starting process, and therefore greater thermal 

 efficiency. 



In B. F. Sturtevant's compartment kilns the temperature in any one 

 compartment was regulated by reducing or increasing the amount of hot 

 air admitted, while the humidity was controlled by altering the pro- 

 portion of return air taken by the fan. The disadvantage of this system 

 was that any alteration to suit conditions in one room naturally altered 

 those in others, while for very low temperatures the volume of air passing 

 was so small that stagnation occurred in parts and mildew developed. 



After many years of little appreciable progress, the British Sturtevant 

 Company brought out their " Triple Duct " dryer. This, a great improve- 

 ment on previous types, will be more particularly described later. 



