down to one or two car capacity, uniformity in the original moisture content is not required. In the operation of compartment kilns it is not necessary to have a continuous flow of stock. Although many of the compartment kilns are of the natural circulation type, most of the fan and blower kilns are found under this classification. The water spray, condensation, and superheated steam ideas have also been applied to compartment kilns. Those having equipment for forced movement of air usually are designed for cross circulation rather than for the natural upward and lengthwise circulation commonly found in progressive kilns. Unless compartment kilns are thus equipped the circulation is usually upward through the center of the stacks, then crosswise to the right and left, and downward along the kiln walls.

Variations in kiln types.-Many variations in progressive and compartment kilns have been patented and placed on the market. Others are constantly being evolved. The descriptions of the various types of kilns below were taken largely from information furnished by the manufacturers of these kilns and are not the results of investigations conducted by the National Committee on Wood Utilization.

Reversible cross circulation kilns: This kiln is recommended by its manufacturer for the drying of hardwood dimension stock because of rapid and reversible circulation and fast drying at low temperatures and high humidities. Internal fans are placed at 8-foot intervals underneath the kiln stacks. Baffles are located above and below the kiln loads to force the circulation between the layers of stock. Stock is placed edge to edge in each layer, and the usual chimney, or flue, is eliminated, thus increasing the kiln capacity and simplifying stacking.

With this kiln hickory billets have been dried, green from the saw, with a degrade of only 3 per cent; 12/4 gum automobile body stock has also been dried with very little degrade.

Mechanical conditioning kilns: Another kiln manufacturer recommends a natural circulation compartment kiln, equipped with an electrically operated conditioning system, for the drying of oak and gum squares green from the saw. After an initial steaming treatment the stock is allowed to dry as long as moisture is given off rapidly. When the humidity of the kiln drops to a predetermined point, the steam sprays are suddenly opened automatically and the stock is given a conditioning treatment which remoistens its surface. It is claimed that this system produces rapid drying and prevents serious casehardening.

Natural circulation kilns with patented heating systems: This type of kiln has been recommended for the drying of green oak and gum squares and other ready-cut stock. Separately controlled units of heating coils are located underneath each kiln stack. Thus, the warm air passes up through the flue, is distributed across the various courses of the kiln stack, and passes out and downward from the edges of the stack. Steam sprays pointing downward between each stack increase the rate of circulation. Control of the tempera ture and humidity of each kiln stack separately is said to be possible with this kiln. A special heating unit is provided at each kiln door to compensate for heat leakage.

Steam-jet blower kiln: This type of kiln has also been successfully used in drying small dimension stock. The steam-jet blowers humidify the air, preheat incoming fresh air, and also increase the rate of circulation. The blowers are usually arranged for circulation upward through the stack flues, crosswise through the stack, and downward along the sides of the kiln. Baffles control the direction of circulation. Steam-jet blowers may be used in both compartment and progressive kilns.

External fan kilns: Several manufacturers are producing this type of kiln and strongly recommend it for the drying of small dimension stock cut green from the saw. It gives flexibility of control and makes it possible to dry almost any kind of stock. It is reported that this type of kiln has successfully dried 8 and 10/4 gum with an original moisture content of 80 per cent down to 5 per cent in 20 days.

These kilns may be of either the compartment or progressive type, and are usually recirculating. The heating units and fans are generally located in an operating room at one end of the kiln. However, the heating units may be placed at other points within the kiln. Circulation may be lengthwise of the kiln for crosswise piling, or the heated air may be forced from below into the chimneys of the kiln stacks, move to the right and left through the piles, and downward along the kiln wall. In a third method of air movement graduated delivery and suction openings are distributed along ducts which run lengthwise of the kiln, thus causing crosswise circulation. Metal air diffusers are often placed along the sides of the kiln to distribute air evenly across each course of the kiln stacks. Endwise piling is used in connection with the two latter methods of circulation.

CONTROLLABLE FACTORS IN KILN DRYING

Heat, humidity, and circulation in kiln-drying control the rate of drying, or, as explained on page 46, the steepness of the moisture gradient curve. These three factors are in turn controllable. However, the exactness of the control depends on the type of kiln used, the efficiency of the equipment, and the ability of the operator.

Heat.-In all modern kilns temperature is controlled by means of air-operated, electrically operated, or self-contained thermostats. Most automatic controls operate more successfully at high temperatures than at temperatures of 110 to 130° or lower, since with a low steam pressure passing through the coils and steam traps the tendency is for them to become water-logged. When the heating coils are controlled in several units, the use of a single unit for low temperatures will partially eliminate this difficulty. Increased pitch of the heating system will also facilitate drainage at low steam pressure. A vacuum steam trap has been recommended as a preventive of water-logged coils. With direct steam pressure this trap operates independently of the vacuum. When the trap is flooded with hot water, a thermostat applies the vacuum and the heating system is drained of condensation.

Since temperatures are usually controlled from only one or two points within the kiln, it is evident that the accuracy of control decreases with the size of the kiln. Individual kilns of the same type and construction often show variations. For these reasons it is im

portant that the operator study each of his kilns very carefully for the purpose of learning temperature conditions at various points. in the interior. When this information is available, the temperature readings on the recorders may be reconciled with actual conditions in the kiln. These precautions are especially desirable in drying refractory stock or stock of large dimension.

Humidity-Relative humidity is referred to in kiln drying as "humidity" and designates in per cent the amount of moisture in the air at a given temperature as compared with the amount which the air could hold if saturated. For example, 1 cubic foot of air at 125° F. will hold a maximum of 40 grains of water. Therefore, air containing 20 grains of water per cubic foot at the same temperature will have a relative humidity of 50 per cent.

Heat is necessary to produce the surface evaporation which starts the drying process. Controlled humidity is necessary to keep the rate of drying below the point at which the material will be damaged in drying. In kiln drying the problem is to maintain the balance bctween temperature and humidity which will produce the most efficient results when applied to the material being treated.

The most common method of increasing the humidity is to inject live steam into the drying chamber. This may be done by sprays, blowers, jets, or similar devices, which not only inject the steam into the kiln but may also distribute the steam and increase the rate of circulation.

Circulation.-Only within the past few years has the question of the rate and direction of air movement been considered to have an important bearing on the process of kiln drying. Recently the tendency has been toward the installation of kilns and equipment which give rapid and controlled rates of circulation. Rapid and uniform circulation produces faster and more even drying than slow, irregular circulation and decreases the difficulty of controlling drying conditions. However, it becomes increasingly difficult to secure uriformity as the rate of circulation increases. At the present time, with reliable instruments available for controlling temperature and humidity, we can turn our attention to the subject of increased rates of circulation.

Optimum conditions of temperature and humidity have been determined for all commercial species under all conditions of moisture content. However, the most desirable rates of circulation for these conditions have not been determined. The Forest Products Laboratory recommends 25 feet per minute as a minimum rate of circulation through the interior of the piles for all difficult hardwood drying which requires a uniform moisture content. When requirements are less exacting, much lower rates of circulation may be used. Present practice in forced circulation softwood kilns calls for much faster rates of circulation than 25 feet per minute. Rates of circulation through the lumber piles of 50 to 150 feet per minute have been reported. It is agreed that, under the same conditions of temperature and humidity, increased circulation increases the rate of drying. It is also agreed that a higher rate of circulation is necessary during the first part of a kiln run than later when the rate of drying has decreased. This is due to the fact that during the conditions of high humidity which obtain at the early part of a

kiln run the kiln air does not absorb and carry off the evaporated moisture as rapidly as does air containing a lesser amount of vapor.

KILN TESTS

A number of valuable and helpful tests have been evolved to assist the kiln operator in his work of turning out stock which has been well kiln dried. A summary of these tests is given below.

Moisture-content tests.-Every kiln operator should at all times know the moisture content of the stock being dried, so that the factors of temperature, humidity, and circulation may be controlled accordingly. The common method of obtaining this information is as follows: The original moisture content is determined by selecting a piece from the material to be dried which is representative of the stock having the highest moisture content. This test piece is crosscut at the center to produce a cross section 1/2 to 3/4 inch thick. This section is carefully weighed immediately after cutting, placed in a drying oven regulated at approximately 212° F., and dried until it fails to lose weight. The moisture-content percentage is computed by using the following formula:

For example, the original weight (O.W.) equals 50 grams, and the oven-dry weight (O.D.W.), equals 38.4 grams. Substituting in the above formula we have

11.6 38.4

or 0.30 X 100=30 moisture content percentage.

When it is necessary to make a large number of moisture-content tests, the operator will find that the moisture percentages may be determined quickly and accurately by using a slide rule. In the example given above, 500 is divided by 384 and 100 subtracted from the result, 130. Rules having a C1 scale will be found to be most efficient for this purpose, since the right index of the C1 scale may be set over 500 on the D scale, the glass indicator set over 384 on the C1 scale and the result, 130, read under the indicator on the D scale. In kiln drying small dimension it is not customary to use sample boards such as are often used in drying lumber, since a piece of stock may be removed easily from the pile whenever testing is necessary. However, it is quite practical to crosscut a piece of dimension, determine the moisture content, end paint the remaining section, weigh, and use as a sample piece. The oven-dry weight of the sample piece may be determined by the formula given below. Thick and irregular shaped material, such as maple last blocks, may be prepared for this purpose by boring a hole through the center of the piece and determining the moisture content by carefully weighing the shavings resulting from the boring. The opening is then

filled with paint, stoppered with corks, the piece weighed and used for testing purposes.

Assuming that the original moisture content is 30 per cent, as in the previous example, and that the original weight of the sample is 5.10 pounds, the oven-dry weight will be computed as follows:

To compute the moisture content of the sample at any time, use the formula given on page 62 for determining original moisture content. Assume that two days after this particular stock was placed in the kiln the sample was removed and was found to weigh 4.90 pounds. Its moisture content is computed as follows:

These calculations may also be made directly by means of slide rule. Several devices have been patented for obtaining the moisture content directly from a section of wood without cutting test pieces, oven drying, or calculation. A mercury-filled maximum-reading thermometer, the bulb of which is a section of ox's intestine, has been developed at the Forest Products Laboratory. This membrane is very sensitive to the presence of moisture. A hole is bored in the stock of the exact size to permit the insertion of the bulb of the instrument. The bulb is then inserted and given several minutes in which to adjust itself before the reading is made. The accuracy of this instrument is limited to moisture contents ranging from 0 to 25 per cent.

An instrument has been patented by T. E. Heppenstall, research engineer, Longview, Wash., which gives directly the moisture content of any piece of wood which has dried below the fiber saturation point. This device is an electrically operated detector based on the fact that, below the fiber saturation point (about 25 per cent), the electrical conductivity of wood varies directly with the quantity of moisture present. The manufacturer of this meter claims that the results obtained by it are in error by only 1 or 2 per cent. The value of the detector in determining the moisture content on 8/4, 10/4, and 12/4 inch stock has not been determined.

Casehardening tests.-Casehardening can be determined by the appearance of stress sections after they have been thoroughly dried. These sections are cut in a manner similar to moisture content sections, excepting that they are approximately 1 inch wide. The sections are then resawed as illustrated in Figure 28. A in Figure 28 illustrates the appearance of green stock or stock which has been perfectly seasoned. B shows tension in the surface layers, a condi