urated vapor at a pressure usually
above atmospheric. We may just as truly have live steam at pressures
less than atmospheric, at a vacuum of 28 inches for instance. Only in
the latter case its temperature would be lower, _viz._, 101 degrees
Fahrenheit.

Superheated steam is nothing more than water vapor at a relative
humidity less than saturation, but is usually considered at pressures
above atmospheric, and in the absence of air. The atmosphere at, say,
50 per cent relative humidity really contains superheated steam or
vapor, the only difference being that it is at a lower temperature and
pressure than we are accustomed to think of in speaking of superheated
steam, and it has air mixed with it to make up the deficiency in
pressure below the atmosphere.

Two things should now be clear; that evaporation is produced by heat
and that the presence or absence of air does not influence the amount
of evaporation. It does, however, influence the rate of evaporation,
which is retarded by the presence of air. The main things influencing
evaporation are, first, the quantity of heat supplied and, second, the
relative humidity of the immediately surrounding space.

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 he