14 BULLETIN 509^ V. S. DEPAETMENT OP AGRICULTURE. 



EVAPORATION IN THE ABSENCE OF AIR. 



In vapor alone, no air being present, evaporation from a surface 

 of water takes place at the dew point, but when the water is inti- 

 mately contained in other substances the temperature must be higher 

 than the dew point. If air is present it retards the rate of evapora- 

 tion from a free surface of water, so that the surface is warmer than 

 the dew point, depending upon the degree of relative humidity in 

 the air. ^A'liile the surface of wood is wet its temperature will not 

 I'ise above that of the wet bulb in the presence of air, nor above the 

 dew point in superheated vapor alone. As it becomes drier, how- 

 ever, its temperature wall rise, due to its affinity for retaining mois- 

 ture. In the former condition there is no danger of too rapid drying, 

 but in the latter condition, if the humidity is too low or the superheat 

 too high, the drying from the surface may become more rapid than 

 the rate at which the moisture is transmitted from the center, and 

 casehardening results. 



In considering the manner in which drying takes place in super- 

 heated steam, suppose the pressure is atmospheric and that a wet 

 piece of wood has been heated in saturated steam to 212° F. No 

 evaporation will take place until additional heat is added. Now, 

 suppose steam superheated to 232° F. or 20° of superheat is in- 

 troduced. The portion immediately in contact with the surface of 

 the wet wood will be cooled to 212° F., and in so doing it will 

 vaporize a certain portion of water from the surface. As the specific 

 heat of this steam is, in round terms, one-half, and as it requires 

 about 1,000 thermal units to vaporize one unit of water, to vaporize 

 a single molecule of water at 212° F. will require contact of 100 of 

 the molecules of superheated steam at 232° F. We will then have 101 

 molecules of steam in the saturated condition at 212° F. Evapora- 

 tion must then cease unless this saturated steam is replaced by some 

 fresh superheated steam. Evaporation from a free surface of water 

 in the absence of air (in superheated steam) always takes place at 

 the boiling point (which in this case is the same as the dew point). 

 If, however, there is a deficiency of water in the wood more heat will 

 be required to separate it and to vaporize it, and evaporation will 

 take place at a higher temperature than the dew point. In fact, 

 evaporation may cease altogether in the superheated steam, and a 

 higher degree of superheating be required (which is equivalent to a 

 lower humidity) to get the moisture out of the w^oocl. In the case 

 of a surface of free w^ater the rate of evaporation depends entirely 

 upon the amount of heat transmitted to the w^ater, whether by in- 

 creasing the circulation or by increasing the degrees of superheat. 

 In the latter case, when the moisture is intimately contained in the 



