Fiberboards 



2863 



Humidification. — Pressed hardboard, whether heat treated, tempered or un- 

 treated has a moisture content of essentially zero percent. In service hardboard 

 will equilibrate with the surrounding air, which for the practical range of relative 

 humidities is a moisture content between 3 and 10 percent (fig. 23-91). Such 

 moisture changes cause linear expansion, which can buckle wall panels or 

 siding, and when abrupt can bow, twist, and otherwise distort unrestrained 

 sheets. Controlled preconditioning of the board to a moisture content midway in 

 the range of expected service conditions minimizes buckling and other difficul- 

 ties related to moisture content changes. 



Humidification of hardboard is therefore standard procedure. The most com- 

 mon types of humidifiers are continuous- or progressive-type chambers follow- 

 ing, and often integrated with, the heating oven. A wicket- type humidifer is 

 diagrammed in figure 23-92. 



A humidifier is like a dry kiln operated in reverse; high-humidity air is forced 

 through the stacks of hardboard where it will give up some of its water vapor to 

 the boards. (See fig. 23-93.) The air is then heated over steam coils, moisture is 

 added by steam spray, and the air is cooled almost to saturation by water spray. 

 The entire process is regulated by a dry- and wet-bulb controller. 



Air conditions in the humidifier are limited by the danger of condensation 

 which is destructive because the condensate is corrosive. Temperatures and 

 relative humidities are, therefore moderate (140°F and 70 to 80 percent relative 

 humidity). Higher temperatures and relative humidities would increase the 

 moisture transfer rate but would require extremely well insulated chambers. 

 Liquid water collecting on board surfaces causes water spots. Humidification 

 cycles vary between 6 and 9 hours, depending on board thickness. 



30 40 50 60 70 



RELATIVE HUMIDITY (PERCENT) 



100 



Figure 23-91. — Equilibrium moisture content of y4-inch tempered hardboard manufac- 

 tured by various manufacturers. (Drawing after McNatt 1974.) 



