FrOjiiiary 10, 1921 



HARDWOOD RECORD 



After the heated air lias iloiie its work of evaporatioUj it has lost 

 some of its heat and picked up a corresponding amount of evaporated 

 iiKiisture. Ill an absolutely tight kiln this process would eventually 



order to obtain most rapid drying and at the same time avoid check- 

 ing, easehardening and honeycombing. Such humidity control is best 

 obtaineil in the ordinary kiln with a thermostatic valve on the steam 



Lumber Piled Crosswise in Compartment Kiln 



increase the moisture in tlie air to a point wliere no more drying 

 would take place. Consequently, some way must be available for 

 removing the excess moisture before the air returns to the lumber. 

 In the ventilated type of kiln this is done by removing some of the 

 moist air through openings or stacks and replacing it with fresh air 

 from the outside. In what are known as "re-circulating kilns," 

 moisture is condensed out of the air like dew, either by cold water 

 pipes, cold water sprays or canvas curtains where the air outside of 

 the curtain is colder and drier than that inside the curtain. In the 

 ordinary kiln the cold air which enters through cracks and holes 

 around the door is more than enough to provide for removing the 

 excess moisture, and the common problem is to add moisture to the 

 kiln air rather than to remove it. 



More or less easehardening will usually develop in spite of precau- 

 tions. It can be relieved by softening the outer surfaces of the 

 boards with hot, moist air. This is done by injecting a large amount 

 of steam into the kiln air for a few hours up to the point where the 

 wood absorbs moisture or even until the air is saturated. Either 

 live or exhaust steam will serve this purpose, but live high-pressure 

 steam injected through an open steam pipe gives the quickest and 

 most satisfactory results. 



Having first the building, second, the heat, third, the means of 

 moistening the air and steaming the lumber, fourth, the circulation 

 of air, and fifth, the provision for removing excess moisture, the kiln 

 is complete, and only requires intelligent operation for good results. 



Proper Piling Is Indispensable 



Proper piling of the lumber is tlie first requirement of correct kiln 

 operation. A level and solid foundation is necessary and enough 

 stickers should be used to prevent sagging and warping and support 

 the ends of the boards. The stickers should be of uniform thickness 

 and placed one on top of another in straight vertical rows. It is also 

 very important to place the lumber in the kiln in such a position that 

 the air can circulate freely between the boards without interference 

 from the stickers. That is, if circulation is crosswise of the kiln the 

 lumber should be flat piled lengthwise of the kiln. Openings between 

 the boards or chimneys in the interior of each truck load should .also 

 be provided to allow the heated air to rise upward through the 

 lumber. 



Tem]>erature control is the second essential of good kiln operation. 

 Some woods will safely stand much higher temperatures than other 

 woods. For certain uses, such as in airplanes, where maximum 

 strength is required, temperatures must be limited to not higher 

 than 135 or 14.5 degrees Fahrenheit. For table tops, the same woods 

 might be satisfactorily dried at temperatures as high as ISO degrees 

 Fahrenheit, or more. Temperatures must be varied also to suit the 

 moisture in the wood. With green wood much lower temperatures 

 are necessary than with wood that is nearly dry. Most satisfactory 

 temperature control is obtained by thermostatic valves placed on 

 steam heating pipes. With close attention on the part of the oper- 

 ator, however, good results can often be obtained by hand-operated 

 check valves and pressure-reducing valves. 



The moisture or luimiditv of the air must also be under control in 



Lumber Piled Endwise in Similar Kiln 



spray line operated by a wet wick. It may also be obtained by a 

 hand-operated steam spray valve or by varying the temperature of 

 the water used in water sprays or condensing pipes. 



Since the temperatures and humidities to be used will vary with 

 the amount of moisture in the lumber, good operation requires tests, 

 to determine what the per cent of moisture is, both at the beginning 

 of the drying and at intervals during the drying until the lumber is 

 fully dry. Such tests are made by weighing sections cut several inches 

 from the exposed end of the board, then drying the sections until no 

 further loss of weight occurs and then dividing the amount of loss by 

 the final weight to get the percentage. These moisture tests must 

 in many cases show not only the average moisture, but also the 

 amount in the center of the boards as compared with the outside. 

 Too great a diiference is a danger signal indicating the development 



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Lpmber Piled to Form Chimney or Flue Near Center of Stacks 



