396 



THE TIMBERS OF THE WORLD 



numerous water sprays are so directed as to assist the movement of the 

 air in the desired direction. As the spray water is cooler than the hot 

 wet air from the dryer, the moisture is condensed and the air returns to 

 the heater, containing less moisture, and so after re-heating, is capable of 

 doing further drying work. With this system fans and motors are 

 unnecessary, but a pump system with automatic controls is required. 

 The humidity is well under control, but the ventilation can only be 

 varied within very small Umits. The direction of movement of the 

 circulating air is vertical. This means comphcated pihng, either edge- 

 pihng, which is only possible with square-edged timber, or an air space, 

 broad below and decreasing towards the top, in the middle of the pile. 

 These conditions present Uttle difficulty in the laboratory, but are not 

 so easy of execution in the timber yard. 



There are other methods of extracting the water from timber. It can 



AtiDRFWS Kiln. 



TIMBER 



TlEMAN KiLM 



Types of Condensing Kilns. 



be baked out by putting the wood in contact with steam pipes. This 

 can be done without splitting, if steam vapour is used to restrain the 

 drjdng action. This treatment is, however, very drastic, and uniform 

 drying is difficult unless the wood is absolutely desiccated. There is 

 also the Hausbrand system of drying with superheated water vapour. 

 In this, wet air is at first circulated, and the temperature is gradually 

 raised. After reaching boihng point the air is allowed to escape, and 

 the water vapour only circulates at such a temperature as to cause 

 evaporation of the moisture in the wood. Wood can be dried by such 

 means, but few people would risk their material under such a drastic 

 process. 



The various systems may, therefore, be said to be reduced to the two 

 main types of dryer, the progressive and the compartment, and to two 

 ways of obtaining the circulation, that is, either by mechanical or thermal 

 means. Accessories for working, such as water or steam sprays or 

 hygrometers, can be appUed to any of these. 



