temperature of heated storage would coincide with none of these temperatures. Now, however, the laboratory has produced a more complete chart, partly by interpolation from the original chart and partly from additional tests. This chart (fig. 63) has not been previously published.

A hygrometer is used for taking air readings. The dry-bulb reading is read on the scale of vertical lines at top or bottom of the chart, and the wet-bulb depression, or difference between its reading and that of the dry bulb, is read on the curved lines according to the scale on the interior of the chart. Where the vertical line and the curve cross the height indicates the relative humidity, which may be traced by the horizontal lines to the right-hand scale and there read. Thus far the chart corresponds with the usual hygrometer chart; its difference lies in the addition of the straight lines which slope up gradually from the left-hand scale of moisture percentages in wood; and by tracing down these lines from the junction point already established there will be found on the left-hand scale the moisture content which the air conditions will maintain in wood.

The following table has been compiled from the chart and can be read more conveniently and accurately. The figures of wet-bulb depression run along the side by lines and of dry-bulb temperatures along the top by columns; and at the interior crossings of lines and columns in the chart are shown the corresponding equilibrium moisture contents which the air conditions will maintain in wood. The ange of the chart is sufficient to cover any condition that will be met with in heated storage.

TABLE 1.—Variation of equilibrium moisture content of wood with air temperature and wet-bulb depression1

1 A variation may occur in the figures shown in this table, since samples, even if of the same species, may differ in equilibrium moisture content by at least 1 per cent. The figures shown are based directly upon the values derived in the chart.

TEMPERATURE OF AIR OR DRY-BULB THERMOMETER IN DEGREES FAHRENHEIT

TEMPERATURE OF AIR OR DRY-BULB THERMOMETER IN DEGREES FAHRENHEIT

It may happen that when the reading is found on the preceding chart it may not be the reading desired. The air conditions may correspond with an equilibrium moisture content in wood of 10 per cent when it is desired to heat the storage sufficiently to keep the wood at 7 per cent. How much more heat will be needed? It should be remembered that as a given body of air is heated its relative humidity declines, though its absolute humidity, the actual amount of moisture in a given volume, changes very little, and this only because the air expands and thus changes in volume.

The chart in Figure 63 does not give the answer to such questions, because it does not have the other set of curved lines often shown on hygrometric charts, corresponding with absolute vapor pressures (which nearly correspond with absolute moisture content). The following table will, however, give the information. Each column represents identical air moisture (absolute), but each successive column represents moisture air, corresponding to the number of grains of moisture per cubic foot of air at dew point shown at the top of the column. This moisture does not change anywhere in the column, though the cubic foot of air becomes a little more than a cubic foot by expansion, at higher temperatures; this difference, however, is not great and does not affect the accuracy of the table.

If therefore the heated storage at 68° produces an equilibrium moisture content of 9.9 per cent in the wood and it is desired to reduce this to 7 per cent, follow the line corresponding to that temperature through to the column whose figure corresponds with 9.9, which will be the fourth column; follow down the column until 7 per cent or the figure closest to it is found, which will be in the line corresponding with 78°, the temperature to which it will be necessary to raise the heated storage to get the desired effect.

The table is also useful in discovering the effect of differences of temperature in different parts of the storage room. If tests where the dry-bulb reading is 70° correspond with an equilibrium moisture content in wood of 7.2 per cent, and parts of the room are 4' cooler and part 4° warmer, what will be the corresponding range of effect on the wood? Locating 7.2 in the third column on the 70° line and looking opposite 66° in this column, it will be found that this cooler portion of the room will hold a moisture percentage in wood of 8.2; and opposite 74° it will be found that this warmer

corner of the room is suitable for storing flooring or any other product in which it is desirable to maintain 6.3 per cent of moisture. The table follows:

TABLE 2.-Variation in equilibrium moisture content of wood with variation in air temperature1

1 A variation may occur in the figures shown in this table, since samples, even if of the same species, may differ in equilibrium moisture content by at least 1 per cent. The figures shown are based directly upon the values derived in the chart.

APPENDIX

KILN-DRYING IN A VACUUM

Kiln-drying at less than atmospheric pressure is not practiced in the United States, except occasionally in creosoting cylinders at woodpreserving plants. The process is being developed and applied in Europe, where vacuum pumps for this purpose are also available. Among the latest of these is the Bennett high-vacuum pump produced in Stockholm, Sweden, for which it is claimed that it is capable of producing a vacuum of 99.9 per cent in regular commercial practice and on an economical consumption of power.

In the vacuum-drying process for wood developed for use with this pump the wood is placed in a rectangular iron retort, being run in on cars on a track, and is then heated with a mixture of hot air and steam to a temperature of 131° F. The specifications state that higher temperatures should not be used and that the wood should be uniformly heated throughout, adding, "It is just by reason of no proper regard having been taken to these circumstances that previous methods of vacuum-drying wood have produced unsatisfactory results in one respect or the other."

After heating, which it is stated takes about one hour for each inch of thickness in the stock, the retort is closed and a vacuum applied, and the retort is left with the pump maintaining the high vacuum until the fall of temperature produced by evaporation reached 50° F., when the stage is complete and the retort is again opened and reheated for another stage. It is stated that for 2-inch spruce the evacuation period is about five hours. As many stages of heating and evacuation are used as will produce the desired seasoning.

Assuming that the vacuum applied in the process is 98.8 per cent, the boiling point of water in such a vacuum is at 50° F., or 81° below the point of 131° F. to which the lumber is heated in the preliminary heating. Assuming that the lumber contains 68 per cent of moisture, the specific heat of this amount of moisture per pound of wood would be 0.68 and of the wood itself 0.33, a total of 1.01, and this multiplied by the temperature difference of 81 gives 81.81 British thermal units available for evaporating water in the vacuum stage.

To evaparate 1 pound of water at 212° F. and at atmospheric pressure requires 969 British thermal units; but this evaporation is done at 98.8 per cent vacuum and 50° F. boiling point, where the latent heat absorption amounts to 1,064.3 British thermal units per pound; and this is the proper conversion factor. The available units of heat in 1 pound of wood and its contained water will therefore evaporate 81.81 divided by 1,064.3 pounds, or 0.0768 pound, equal to a reduction of the moisture in the wood amounting to 7.68 per cent of the dry weight of the wood and leaving in the wood 60.32 per cent of moisture.

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