KILN DRYING HANDBOOK 31 
For velocities higher than the average, such as those usually occur- 
ring in the flues of natural-circulation kilns and in the interiors of 
some forced-circulation types, the Biram type of anemometer 1s 
suitable. This anemometer is in essence a disk fan mounted upon 
pivot bearings and provided with a revolution counter. ‘The counter 
’ is ordinarily in the form of a dial and pointer, one revolution of the 
pointer usually representing an air movement of 100 feet. A watch 
is necessary to determine the time corresponding to a certain air 
movement. It is customary to let the anemometer run a definite 
number of minutes, and then to divide the number of feet recorded 
by the number of minutes, the quotient being the velocity expressed 
in feet per minute. Since the velocity in any duct varies through- 
out the cross section, commonly being greatest at the center and least 
along the sides, a single reading will probably fail to represent a 
true average, and for accurate results the cross section of the duct 
should therefore be divided into squares about equal to the diameter 
of the anemometer, and a reading taken on each square. This will 
seldom be necessary, however, in ordinary work. In using anemom- 
eters in open places especial care must be exercised to set the ane- 
mometer with its axis truly parallel with the air movement, because 
otherwise it will register less than it should. Smoke may be used 
to indicate the direction of the air movement. 
Anemometers are imperfect in that the speed of the fan is not 
.truly proportional to the air velocity over the entire range of useful- 
ness of the instrument, and a correction factor accordingly is neces- 
sary. This correction factor is determined at the factory by actual 
trial, and a calibration curve showing the amount of correction to be 
applied at different velocities should accompany the instrument. 
DRYING AND DRYING STRESSES 
MOISTURE GRADIENT 
The moisture in wood tends to distribute itself equally by flow- 
ing from spots of high moisture content to those of low. If it is 
desired to produce a flow of moisture in a piece of wood of uniform 
moisture content the uniform condition must first be upset. This may 
be done by removing some of the moisture from the surface by 
circulating air of proper temperature and humidity around the piece. 
As soon as evaporation from the surface commences, a “ moisture 
eradient ” is established, that is, the wood has then been made drier 
_ on the surface than in the interior, and thereby a movement of the 
| moisture from the interior toward the surface has been started. 
| A moisture gradient is usually thought of as a curve. Figure 6 
| shows some possible moisture gradients in a wood block of substan- 
@ tial size. The full line, A, represents a typical variation in internal 
moisture conditions when the surface of the green wood has just been 
| dried to equilibrium with the temperature and the humidity of the 
| surrounding atmosphere. The horizontal distances between the ver- 
_ tical axis (at the left) and points on the curve represent distances 
'/ measured directly inward from the surface of the block. Similarly 
) the vertical distances between the horizontal axis (at the bottom) 
| ~and these same points on the curve represent the corresponding values 
_ of the moisture content at the points... 
