3i6 



CHAPTER XVin 



formed a chamber separated from the rest of the body. This chamber, 

 which is known as the calandria, receives the steam which causes evaporation 

 in that body. The space beneath the lower tube plate, within the tubes, 

 and a small distance above the upper tube plate is filled with the juice under- 

 going evaporation ; the space above the level of the juice is called the vapour 

 space, and it communicates with the calandria of the next succeeding cell 

 by means of a conduit, b, known as the vapour pipe, and by means of an 

 opening in the side of the shell of the body. The vapour pipe from the last 

 cell leads to a condenser, where it is condensed by means of a continuous 

 supply of cold water, combined with the removal of air by means of a pump. 

 By this means a very low pressure is obtained in the condenser, and a pressure 

 only a httie greater in the last vapour space. The calandria of the first body 

 communicates with a source of steam by a pipe, b, corresponding to the vapour 

 pipes in the other vessels. The juice is introduced into the first body by the 

 pipe a, and continuous communication, controlled by valves, is made by 



Fig. 183 



the pipes a to the last body through the intermediate body. From the 

 calandrias of the second and third bodies small pipes, d, also pass to 

 the condenser, directly or through the last body as shown. These 

 pipes, known as the incondensible gas pipes, are provided with valves. 

 Suppose such a system filled with juice in each body to the level of the upper 

 tube plate. By means of the pump the air is exhausted as far as possible 

 from the last body, and by means of the incondensible gas pipes a less degree 

 of exhaustion can be obtained and controlled in the other two bodies. Let 

 steam at a temperature sufficiently elevated be introduced into the first 

 calandria ; its condensation will cause the juice there to boil at the tempera- 

 ture corresponding to the pressure in that cell. The steam or vapour given 

 off here will pass on to the second cell, and condensing will cause the juice 

 there contained to boil, since a lower pressure prevails. A similar process 

 takes place here as between the second and third body. When once started 

 the pressures and temperatures adjust themselves as long as there is a con- 

 tinuous supply of steam and juice, as long as the vacuum or reduced pressure 

 is maintained in the last body, and as long as the incondensible gases are 

 removed. 



