34 BULLETIN 80, U. S. DEPARTMENT OF AGRICULTURE. 
practically completed before the end of the cooking period (see 
analogous curves in figs. 12 and 13). This would result in relatively 
higher amounts of condensation, since no heat of reaction would be 
generated during the later stages of cooking. 1 The same explanation 
could apply to the cooks made at the higher pressures. 
The influence of higher cooking pressures on steam consumption 
results from the greater amount of steam required to heat the digester 
and its contents to the higher temperatures and the greater loss of 
heat by radiation at such temperatures. The initial volumes of 
cooking liquor did not vary. The condensation curve indicates that 
this effect was comparatively small in -the tests. 
Like the tests varying the amount of caustic soda, those varying 
the initial concentration influence the steam consumption principally 
by the amount of liquor in the charge, which varies as shown by the 
true hyperbolic curve in figure 17. Hence, increasing the initial 
concentration decreased the condensation, as shown by the corrected 
curve in figure 17, which takes into account the overflow of the 
digester in cooks 25 and 26. 
In considering these results from a commercial standpoint it should 
be kept in mind that the experimental apparatus was comparatively 
small. On this account the heat or steam required for raising the 
temperature of the digester and for replacing heat lost by radiation 
per unit of digester capacity was far greater than would be experienced 
in mill operation. Hence, much less steam per pound of chips would 
be required in commercial operations than is shown by these curves. 
The effects of increased duration of cooking and increased pressures 
especially would be much less pronounced, since with these radiation 
is the more important factor. 
Aside from the direct cost of steam, the condensation is of impor- 
tance in another way. The tests have shown that decreased initial 
concentrations, other cooking conditions being constant, result in less 
severe cooking. It is to be expected that the decrease of concentra- 
tion throughout the cooking period, due to condensation, also tends 
to minimize the cooking effects in a similar manner. 2 
The use of superheated steam in cooking, the installation of larger 
digesters, the insulation or lagging of digesters, and the use of the 
minimum volume of cooking liquors at the start of cook are means 
frequently employed by pulp mills to reduce the condensation. 
i The condensation curve (liquor in digester— gallons) in fig. 3, which is typical for most of the individ- 
ual cooks in these experiments, also shows a greater rate of condensation at the end of the cook than at 
earlier periods except during the first hour when the pressure was being increased. This can be accounted 
for only by the fact that heat, other than from the steam alone, was supplied to the digester during the 
earlier stages of cooking. As the cooking reaction is most vigorous at the beginning, it seems probable 
that the heat supplied was heat of reaction. 
2 It is evident that the effects obtained in the tests varying the initial concentrations are much less pro- 
nounced than would have been the case if the diluting effect of condensation had been absent. The auto- 
clave tests, for which data are given in Table 15, afford fairly conclusive proof of this. 
