METHODS OF CONTROLLING HUMIDITY 

IN WOODWORKING PLANTS 

Revised August 1953 

No. R1612 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

Madison 5, Wisconsin 

In Cooperation with the University of Wisconsin 
METHODS OF CONTROLLING HUMIDITY IN WOODWORKING PLANTS 

By M. E. DUNLAP, Engineer 

Forest Products Laboratory,1 Forest Service 

U. S. Department of Agriculture 

---

Operators of wood-processing plants in many parts of the United States experi­ ence production difficulties ascribable to swelling or shrinking of wood with 

seasonal changes in relative humidity, These difficulties, which may result 

in imperfect products, give rise to the problem of maintaining conditions in 

the shop and storage rooms that will hold such seasonal fluctuations to a 

minimum. Most commonly, the conditions arise in areas where buildings are 

heated during the winter, when the relative humidity is considerably lower 

inside the building than outdoors and, in consequence, the dry atmosphere of 

the shop causes the wood to dry and shrink excessively. Subsequent exposure

of finished products to higher relative humidities results in swelling, warp­ ing, and other difficulties. To control such shop conditions, heated build­ ings require some sort of humidification equipment. 

Throughout a large part of the United States, the moisture content of wood 

stored or used outdoors and protected from rain averages about 12 percent. In 

the drier areas of the Southwestern States the average moisture content of wood 

is about 8 or 9 percent. Coastal zones in the Southeatern States, in the 

Pacific Northwest and Southwest, and along the Gulf of Mexico, average slightly

higher than 12 percent, 

In most States, outdoor relative humidities during the summer are usually

sufficiently high so that satisfactory conditions of equilibrium moisture 

content can be maintained indoors with adequate ventilation. When cool 

weather comes in the fall and buildings are heated, however, an important

change takes place; lower relative humidities and lower equilibrium moisture 

content values are encountered. As the outdoor temperature decreases, the 

atmosphere can hold less and less water vapor per cubic foot than it could at 

higher temperatures. When heated to normal temperatures, without changing

its water content, this outside air has a greater capacity for moisture and, 

since the relative humidity is the ratio of the quantity of moisture present

in air to that which could be held at saturation under a given temperature,

it is correspondingly lowered, This causes drying of wood stored or housed 

in heated buildings and workrooms. 

1 Maintained at Madison, Wis., in cooperation with the University of 

Wisconsin, 

Rept. No. R1612 (revised) -1- Agriculture-Madison 
An illustration of the operation of this principle will be found by a study of table 1. Assume an average relative humidity of 75 percent for outdoor 

conditions and that no water is added to the inside space. This latter 

assumption, although not strictly true where people are working and manu­ facturing operations are in progress, is nevertheless sufficiently accurate 

to show approximately how conditions inside a building may change as out­ door conditions vary and why wood dries during the winter in a heated build­ ing. 

To prevent the moisture content of material. being processed from becoming 

too low, it is desirable to maintain a relative humidity of about 45 percent

during the winter. This condition prevents the moisture content of the wood 

from falling below about 8 percent and introduces a less serious condensa­ tion problem than if a higher humidity were maintained. Condensation on 

windows and skylights can be minimized by double glazing, by heating the 

glass with steam coils, or by circulating warm air across the glass surfaces. 

By proper placement of humidifiers in woodworking plants, much can be gained

in the reduction of condensation on windows, walls, and other cold surfaces. 

Drip from condensing surfaces should be prevented wherever it is likely to 

damage materials being processed. 

New buildings in northern States can be designed with features that will 

readily avoid condensation, but in older buildings this cannot be done so 

easily, Windows of new buildings can be double glazed; or, for partial avoidance of condensation difficulties , glass blocks can be used. Insula­ tion and moisture barriers can be installed in walls to prevent subsequent

damage to the building. Such alterations can be made in existing build­ ings only with difficulty. There is less need for these special precautions,

of course, if manufacturing plants are located in more moderate climates. 

Methods of Humidity Control 

As a rule, high relative humidity is not a problem in wood-processing plants

in the northern part of the United States and, consequently, conditioning

equipment is needed only to increase the humidity during the heating season,

On the other hand, in manufacturing plants situated in very damp climates,

such as prevail along the Gulf Coast, dehumidification may be required to 

prevent excessive humidity during prolonged periods of humid weather. 

Air Conditioning 

Where wood is to be stored so that a definite moisture content is maintained, 

a room inside a building, preferably with no exterior walls, floor, or roof 

and equipped with air-conditioning apparatus, can be used at any time during

the year with satisfactory results. The apparatus and controls can be 

designed to meet particular needs. 

For the conditioning of a large wood-using plant, any one of several methods 

may be employed. A central plant can be designed to condition the air 

Rept. No. R1612 -2-
completely and equipped with refrigeration or absorption apparatus as well 

as heating and humidifying facilities. The air is distributed by means of 

a system of ducts throughout the building. Such equipment is expensive, 

Another method of conditioning the air in a plant completely is to install 

a number of small air-conditioning units throughout the plant and use no 

distributing system. These units, if fully equipped for year-round con­ trol, are likewise expensive and perhaps nonessential in most cases, 

Heating 

In localities where natural humidity is higher than necessary to give the 

required indoor condition, a definite moisture content in lumber awaiting 

use in storage buildings can be maintained by controlling the temperature

of the space within the building by means of heaters controlled with a 

hygroscopic element, Such control is effective only as long as the 

natural humidity remains high. Even in humid localities indoor conditions 

may sometimes become too dry, If such conditions exist fox any great

length of time, devices can be installed to admit water vapor to the space. 

Humidification 

To accomplish humidification, as distinct from complete air conditioning,

only relatively simple apparatus is needed. Steam jets (fig, 1) may be 

distributed throughout the plant and controlled automatically by a moisture­ sensitJive instrument. The chief objection to the use of steam jets is that 

water is lost from the steam-generating plant and must be replaced, Another 

disadvantage is that heat is added to the space. In winter this may not be 

objectionable, but where humidification is required during a dry summer 

season, the heating effect of steam and the lack of evaporative cooling

make this method unpopular, The simplicity of the steam-jet system and 

its freedom from dust residues commend it. But there is usually an odor 

with the discharge of steam and in some plants steam is not always avail­ able. 

Another method of humidification is by the use of water at room temperature

atomized by a jet of compressed air (fig. 2). With this simple method, 

water is discharged into the air as a finely divided mist. The slight

noise that accompanies this spray generation is not usually objectionable

in an industrial plant where there are machines operating, When properly

designed humidifiers of this type are used, all issuing spray is evaporated

without coarseness or drip, Furthermore, no heat is added to the air from 

such sprays. On the contrary, a marked cooling results from the evaporation, 

Target sprays are also a possibility for this purpose, They discharge a 

fine stream of water against a plate. The bulk of the water striking the 

target is broken up into a fine mist that floats out into the room. Adequate

filters must be provided to prevent plugging of the fine openings, together

with a drain to remove drip. 

Rept, No. R1612 -3-
A number of mechanical devices on the market operate with a rotating disk 

upon which water is allowed to flow (fig, 3). This disk rotates at high

speed, so that the water is driven to the edge of the disk by centrifugal force and thrown against sharp metal teeth. The water is thus broken up

into a fine mist and blown out into the room by a fan located behind the 

disk. Residues resulting from evaporation of the water/accumulate on the 

apparatus and must be removed periodically. Usually, these evaporators are 

controlled with a hygroscopic element that opens or closes a water valve in 

the water feed line or starts or stops the humidifier motor. A number of 

them can, however, be controlled from a single humidistat, 

Where water containing quantities of organic substances or minerals is broken 

up into a mist by pneumatic, target, or mechanical humidifiers, small 

particles of dust are left suspended in the air and eventually settle. This 

is not usually objectionable in industrial plants where dusts are frequently

generated by wood. processing. 

Humidifiers should not be located too close to the shavings exhaust systems,

because released moisture may be carried away before it effectively raises 

the relative humidity. 

Dehumidification 

If dehumidification is a requirement, the space to be conditioned should be 

kept as small. as possible and ventilation kept at a minimum, In some cases, 

it is possible to use drying agents, such as calcium chloride or silica gel, 

to maintain lower relative humidity, particularly if the space to be condi­ tioned is small. Another alternative, previously referred to, is the use of 

heat to lower relative humidity, 

Partial List of Manufacturers of Apparatus for Humidifying Factories 

American Blower Co., Detroit, Mich. 

American Moistening Co., Providence, R. I. 

Armstrong Machine Works, Three Rivers, Mich,

Bahnson Co., Winston-Salem, N. C. 

Carrier Corp., Syracuse, N. Y. 

Clarage Fan Co., Kalamazoo, Mich. 

Parks-Cramer Co., Fitchburg, Mass. 

Vilter Mfg. Co., Milwaukee, Wis. 

This list has been prepared for the information of correspondents. The 

inclusion of names in the list implies no endorsement by the Forest Products 

Laboratory as to quality of service or cost. 

Rept. No. R1612 -4-
Table 1.--Effect of outside temperatures on inside relative 

humidity and equilibrium moisture content when 

the outside relative humidity is 75 percent 

1

Less than 1.1. 

Rept. No. R1612 

Figure 2. --Direct humidifier of the atomizer type operated by com­ pressed air . 

ZM 93492 F (Courtesy Parks-Cramer Co., Fitchburg, Mass.) 
Figure 3.--Centrifugal humidifier (indicated by arrow) located in veneer 

storage room. 

ZM 93490 F (Courtesy Bahnson Co. , Winston-Salem, N. C. )