Drying by Superheated Steam
What this term really signifies is simply water vapor in the absence of air in a condition of less than saturation. Kilns of this type are, properly speaking, vapor kilns, and usually operate at atmospheric pressure, but may be used at greater pressures or at less pressures. As stated before, the vapor present in the air at any humidity less than saturation is really "superheated steam," only at a lower pressure than is ordinarily understood by this term, and mixed with air. The main argument in favor of this process seems to be based on the idea that steam is moist heat. This is true, however, only when the steam is near saturation. When it is superheated it is just as dry as air containing the same relative humidity. For instance, steam at atmospheric pressure and heated to 248 degrees Fahrenheit has a relative humidity of only 50 per cent and is just as dry as air containing the same humidity. If heated to 306 degrees Fahrenheit, its relative humidity is reduced to 20 per cent; that is to say, the ratio of its actual vapor pressure (one atmosphere) to the pressure of saturated vapor at this temperature (five atmospheres) is 1:5, or 20 per cent. Superheated vapor in the absence of air, however, parts with its heat with great rapidity and finally becomes saturated when it has lost all of its ability to cause evaporation. In this respect it is more moist than air when it comes in contact with bodies which are at a lower temperature. When saturated steam is used to heat the lumber it can raise the temperature of the latter to its own temperature, but cannot produce evaporation unless, indeed, the pressure is varied. Only by the heat supplied above the temperature of saturation can evaporation be produced.