CONTROL OF DRYING PLANTS 



While it is perhaps possible to work a drying plant by rule of thumb, 

 with the aid possibly of a few thermometers or a hygrometer, it has been 

 proved that consistently good results can only be maintained if the plant 

 is worked in a scientific manner with proper instruments to show what 

 is happening inside the dryers, and to keep autographic records. Record- 

 ing instruments have a twofold use : firstly, to ensure that attendants 

 pay proper attention to their work ; and secondly, to find out and record 

 the best processes for different timbers, so that it is possible to repeat 

 or to improve on these which are successful, and discard 

 those which are unsatisfactory. 



The ordinary mercury thermometer is the basis of all 

 readings, both directly for temperature and indirectly for 

 humidity. The degree of humidity is indicated by the dif- 

 ference in readings between two thermometers, one of which 

 is of the normal type, while the bulb of the other is covered 

 with muslin, which is kept moist by a wick dipping into a 

 water tank. 



This combination is called the " wet and dry bulb " 

 hygrometer. The dry bulb gives the temperature direct. 

 On the wet bulb there are two influences, the heat of the 

 surroundings which would raise the mercury to the same 

 level as in the dry bulb thermometer, and the cooling effect 

 caused by the evaporation of the moisture in the muslin 

 round the bulb. If the air is completely saturated with 

 moisture, this e\'aporative effect will be absent, and the two thermo- 

 meters will read equal. If the air is dry, the evaporation will be con- 

 siderable and the wet bulb will show a lower reading than the dry bulb. 

 The amount of the difference between the two readings, together with 

 the dry bulb reading, gives a correct indication of the humidity and 

 temperature in the dryer. 



By means of calculations, tables or charts (hygrodyks), it is possible 

 to work out the humidity in percentages of saturation, but there is no 

 advantage to be gained by this. There are also instruments which give 

 the percentage readings direct. These use the tightening or slackening 

 of a bunch of hairs, or the curling or uncurling of a spiral of fibre, owing 

 to variations of moisture, to operate an indicating needle on a dial. 

 Such instruments may be correct when new, but as they are very delicate 

 and soon get out of order, they are quite unsuitable for practical work. 



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