4o6 THE TIMBERS OF THE WORLD 



gave independent control in each compartment, and the three factors of 

 ventilation, temperature, and humidity could be varied at will. As the 

 name implies, there are three main air ducts, which respectively convey 

 hot dry air, cold dry air, and moist air. The dry air, both hot and cold, 

 is taken from the outside by a centrifugal fan which injects some part 

 directly into the cold duct, and the remainder to a steam pipe radiator 

 and thence to the hot duct. The wet air is drawn from the drying rooms, 

 and is therefore generally termed " return air." The three main ducts 

 run across the ends of the various compartments, and any required 

 mixture of the three kinds of air can be admitted to the compartment 

 by dampers. By having hot air and cold air on supply to each compart- 

 ment the temperature can be varied without interfering with the ventila- 

 tion. Humid air is also available, and the amount to be admitted is 

 regulated according to requirements. 



In theory, this system gives ideal control, but there are certain 

 mechanical disabiUties. For instance, if the hot-air damper does not 

 shut hermetically, the small leakage of very hot air makes it difficult to get 

 a low starting temperature. Again, if the construction of the ducts is 

 not perfect, or if they are in damp ground, there is a serious loss of heat, 

 and frequently the air in the cold duct gets too hot owing to heat radiat- 

 ing from the hot duct. 



To eliminate these difficulties, a variation of the triple duct dryer 

 was devised by the writer. With this system there are only two main 

 ducts, one for return air and the other for cold dry air. As no hot air 

 is moved through ducts, there are no radiation losses. Then, instead of 

 one main heater for the whole plant, a separate steam pipe heater for 

 each compartment is used. Instead of the temperature being controlled 

 by an air damper, it is varied by the steam stop-valve on the heater. 

 This can be entirely shut off when no heat is required, or opened to give 

 any desired degree of heat. A steam valve is more reUable in closure 

 than an iron air damper, and can also give finer adjustment. 



Very accurate working can be effected with these " Multiple Heater " 

 dryers, and a predetermined course as to temperature and humidity can 

 be followed whilst maintaining a constant ventilation. 



Compartment dryers can be in much smaller units than can the 

 progressive type for a given length of plank, and for some kinds of work 

 it is more convenient to have three or four small rooms than one large one ; 

 where, for instance, supplies of several kinds and thicknesses of timber 

 are required, and where the toted output would not warrant having 

 several progressive dryers. The reason that small compartment units can 

 be made, lies in the fact that the lengths need only be that of the plank, 

 whereas in a progressive dryer greater length is necessary for giving a 

 difference in temperature. The width of a progressive dryer has to be 



