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. 



1 The condensation curve (liauor 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. 



