Seasoning of Wood Part 25

[Ill.u.s.tration: Fig. 85. An Asbestos-lined Kiln Door of the Hinge Type.]

In Figure 86 will be seen the Twin Carrier type of door hangers with doors loaded and rolling clear of the opening.

[Ill.u.s.tration: Fig. 86. Twin Carrier with Kiln Door loaded and rolling clear of Opening.]

[Ill.u.s.tration: Fig. 87. Twin Carriers for Kiln Doors 18 to 35 Feet wide.]

In Figure 87 will be seen the Twin Carrier for doors 18 to 35 feet wide, idle on a section of the track.

In Figure 88 will be seen another type of carrier for kiln doors.

In Figure 89 will be seen the preceding type of kiln door carrier in operation.

In Figure 90 will be seen another type of carrier for kiln doors.

In Figure 91 will be seen kiln doors seated, wood construction, showing 3-1/2" 5-3/4" inch-track timbers and trusses, supported on 4-inch by 6-inch jamb posts. "T" rail track, top and side, inclined shelves on which the kiln door rests. Track timber not trussed on openings under 12 feet wide.

[Ill.u.s.tration: Fig. 88. Kiln Door Carrier engaged to Door Ready for lifting.]

In Figure 92 will be seen kiln doors seated, fire-proof construction, showing 12-inch, channel, steel lintels, 2" 2" steel angle mullions, track brackets bolted to the steel lintels and "T" rail track. No track timbers or trusses used.

[Ill.u.s.tration: Fig. 89. Kiln Door Carrier shown on Doors of Wood Construction.]

[Ill.u.s.tration: Fig. 90. Kiln Door Construction with Door Carrier out of Sight.]

[Ill.u.s.tration: Fig. 91. Kiln Door Construction. Doors Seated.

Wood Construction.]

[Ill.u.s.tration: Fig. 92. Kiln Door Construction. Doors Seated.

Fire-proof Construction.]

SECTION XIV

HELPFUL APPLIANCES IN KILN-DRYING

The Humidity Diagram

[Ill.u.s.tration: Fig. 93. The United States Forest Service Humidity Diagram for determination of Absolute Humidities.

Dew Points and Vapor Pressures; also Relative Humidities by means of Wet and Dry-Bulb Thermometer, for any temperatures and change in temperature.]

Some simple means of determining humidities and changes in humidity brought about by changes in temperature in the dry kiln without the use of tables is almost a necessity. To meet this requirement the United States Forestry Service has devised the Humidity Diagram shown in Figure 93. It differs in several respects from the hydrodeiks now in use.

The purpose of the humidity diagram is to enable the dry-kiln operator to determine quickly the humidity conditions and vapor pressure, as well as the changes which take place with changes of temperature. The diagram above is adapted to the direct solution of problems of this character without recourse to tables or mathematical calculations.

The humidity diagram consists of two distinct sets of curves on the same sheet. One set, the convex curves, is for the determination of relative humidity of wet-and-dry-bulb hygrometer or psychrometer; the other, the concave curves, is derived from the vapor pressures and shows the amount of moisture per cubic foot at relative humidities and temperatures when read at the dew-point. The latter curves, therefore, are independent of all variables affecting the wet-bulb readings. They are proportional to vapor pressures, not to density, and, therefore, may be followed from one temperature to another with correctness. The short dashes show the correction (increase or decrease) which is necessary in the relative humidity, read from the convex curves, with an increase or decrease from the normal barometric pressure of 30 inches, for which the curves have been plotted. This correction, except for very low temperatures, is so small that it may usually be disregarded.

The ordinates, or vertical distances, are relative humidity expressed in per cent of saturation, from 0 per cent at the bottom to 100 per cent at the top. The abscissae, or horizontal distances, are temperatures in degrees Fahrenheit from 30 degrees below zero, at the left, to 220 degrees above, at the right.

Examples of Use

The application of the humidity diagram can best be understood by sample problems. These problems also show the wide range of conditions to which the diagram will apply.

EXAMPLE 1. To find the relative humidity by use of wet-and-dry-bulb hygrometer or psychrometer:

Place the instrument in a strong circulation of air, or wave it to and fro. Read the temperature of the dry bulb and the wet, and subtract. Find on the horizontal line the temperature shown by the dry-bulb thermometer. Follow the vertical line from this point till it intersects with the convex curve marked with the difference between the wet and dry readings. The horizontal line pa.s.sing through this intersection will give the relative humidity.

Example: Dry bulb 70, wet bulb 62, difference 8. Find 70 on the horizontal line of temperature. Follow up the vertical line from 70 until it intersects with the convex curve marked 8. The horizontal line pa.s.sing through this intersection shows the relative humidity to be 64 per cent.

EXAMPLE 2. To find how much water per cubic foot is contained in the air:

Find the relative humidity as in example 1. Then the nearest concave curve gives the weight of water in grains per cubic foot when the air is cooled to the dew-point. Using the same quant.i.ties as in example 1, this will be slightly more than 5 grains.

EXAMPLE 3. To find the amount of water required to saturate air at a given temperature:

Find on the top line (100 per cent humidity) the given temperature; the concave curve intersecting at or near this point gives the number of grains per cubic foot.

(Interpolate, if great accuracy is desired.)

EXAMPLE 4. To find the dew-point:

Obtain the relative humidity as in example 1. Then follow up parallel to the nearest concave curve until the top horizontal (indicating 100 per cent relative humidity) is reached. The temperature on this horizontal line at the point reached will be the dew-point.

Example: Dry bulb 70, wet bulb 62. On the vertical line for 70 find the intersection with the hygrometer (convex) curve for 8. This will be found at nearly 64 per cent relative humidity. Then follow up parallel with the vapor pressure (concave) curve marked 5 grains to its intersection at the top of the chart with the 100 per cent humidity line.

This gives the dew-point as 57.

EXAMPLE 5. To find the change in the relative humidity produced by a change in temperature:

Example: The air at 70 Fahr. is found to contain 64 per cent humidity; what will be its relative humidity if heated to 150 Fahr.? Starting from the intersection of the designated humidity and temperature coordinates, follow the vapor-pressure curve (concave) until it intersects the 150 temperature ordinate. The horizontal line then reads 6 per cent relative humidity. The same operation applies to reductions in temperature. In the above example what is the humidity at 60? Following parallel to the same curve in the opposite direction until it intersects the 60 ordinate gives 90 per cent; at 57 it becomes 100 per cent, reaching the dew-point.

EXAMPLE 6. To find the amount of condensation produced by lowering the temperature:

Example: At 150 the wet bulb reads 132. How much water would be condensed if the temperature were lowered to 70?

The intersection of the hygrometer curve for 18 (150-132) with temperature line for 150 shows a relative humidity of 60 per cent. The vapor-pressure curve (concave) followed up to the 100 per cent relative humidity line shows 45 grains per cubic foot at the dew-point, which corresponds to a temperature of 130. At 70 it is seen that the air can contain but 8 grains per cubic foot (saturation).

Consequently, there will be condensed 45 minus 8, or 37 grains per cubic foot of s.p.a.ce measured at the dew-point.

EXAMPLE 7. To find the amount of water required to produce saturation by a given rise in temperature:

Example: Take the values given in example 5. The air at the dew-point contains slightly over 5 grains per cubic foot. At 150 it is capable of containing 73 grains per cubic foot.

Consequently, 73-5=68 grains of water which can be evaporated per cubic foot of s.p.a.ce at the dew-point when the temperature is raised to 150. But the latent heat necessary to produce evaporation must be supplied in addition to the heat required to raise the air to 150.

EXAMPLE 8. To find the amount of water evaporated during a given change of temperature and humidity:

Example: At 70 suppose the humidity is found to be 64 per cent and at 150 it is found to be 60 per cent. How much water has been evaporated per cubic foot of s.p.a.ce? At 70 temperature and 64 per cent humidity there are 5 grains of water present per cubic foot at the dew-point (example 2).

At 150 and 60 per cent humidity there are 45 grains present. Therefore, 45-5=40 grains of water which have been evaporated per cubic foot of s.p.a.ce, figuring all volumes at the dew-point.