Part 15 (2/2)

3. To prepare the wood for its ultimate use and improve its qualities.

When wood will stand the temperature without excessive checking or undue shrinkage or loss in strength, the first object is most readily attained by heating the wood above the boiling point in a closed chamber, with a large circulation of air or vapor, so arranged that the excess steam produced will escape. This process manifestly does not apply to many of the hardwoods, but is applicable to many of the softwoods. It is used especially in the northwestern part of the United States, where Douglas fir boards one inch thick are dried in from 40 to 65 hours, and sometimes in as short a time as 24 hours. In the latter case superheated steam at 300 degrees Fahrenheit was forced into the chamber but, of course, the lumber could not be heated thereby much above the boiling point so long as it contained any free water.

This lumber, however, contained but 34 per cent moisture to start with, and the most rapid rate was 1.6 per cent loss per hour.

The heat of evaporation may be supplied either by superheated steam or by steam pipes within the kiln itself.

The quant.i.ty of wood it is necessary to carry in stock is naturally reduced when either of the other two objects is attained and, therefore, need not necessarily be discussed.

In drying to prepare for use and to improve quality, careful and scientific drying is called for. This applies more particularly to the hardwoods, although it may be required for softwoods also.

Drying at Atmospheric Pressure

Present practice of kiln-drying varies tremendously and there is no uniformity or standard method.

Temperatures vary anywhere from 65 to 165 degrees Fahrenheit, or even higher, and inch boards three to six months on the sticks are being dried in from four days to three weeks, and three-inch material in from two to five months.

All methods in use at atmospheric pressure may be cla.s.sified under the following headings. The kilns may be either progressive or compartment, and preliminary steaming may or may not be used with any one of these methods:

1. Dry air heated. This is generally obsolete.

2. Moist air.

_a._ Ventilated.

_b._ Forced draft.

_c._ Condensing.

_d._ Humidity regulated.

_e._ Boiling.

3. Superheated steam.

Drying under Pressure and Vacuum

Various methods of drying wood under pressures other than atmospheric have been tried. Only a brief mention of this subject will be made.

Where the apparatus is available probably the quickest way to dry wood is first to heat it in saturated steam at as high a temperature as the species can endure without serious chemical change until the heat has penetrated to the center, then follow this with a vacuum.

By this means the self-contained specific heat of the wood and the water is made available for the evaporation, and the drying takes place from the inside outwardly, just the reverse of that which occurs by drying by means of external heat.

When the specimen has cooled this process is then to be repeated until it has dried down to fibre-saturation point. It cannot be dried much below this point by this method, since the absorption during the heating operation will then equal the evaporation during the cooling.

It may be carried further, however, by heating in partially humidified air, proportioning the relative humidity each time it is heated to the degree of moisture present in the wood.

The point to be considered in this operation is that during the heating process no evaporation shall be allowed to take place, but only during the cooling. In this way surface drying and ”case-hardening” are prevented since the heat is from within and the moisture pa.s.ses from the inside outwardly. However, with some species, notably oak, surface cracks appear as a network of fine checks along the medullary rays.

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