362 CHAPTER XVIII 



I and d being the length and diameter of the pipe in inches, li being the value 

 disregarding friction, and h^ the finally accepted value. 



For obtaining the diameter of the fall pipe, a head of i, 2, 3, etc., feet 

 above that due to the excess of atmospheric pressure over that in the con- 

 denser is assumed ; and a computation on the lines above is made. 



Dimensions of Condensers. — The height of a condenser is governed by 

 the time that it is necessary for the water to remain in contact with the 

 steam, and this in turn is controlled by the time taken for the water to fall 

 down the condenser. Experience has shown that condensers with an un- 

 broken fall for the water require a height reaching to as much as 15 ft. for 

 efficiency. By the use of plates forming cascades, the time taken for the 

 water to fall is increased, diminishing the necessary height of the condenser. 

 The cascades, have, however, another function. Water is a very bad 

 conductor of heat, and consequently, when the outer layer of a film of water 

 has been heated, the rate of condensation of steam also decreases. At each 

 cascade, however, a fresh surface is offered to the steam, and a more rapid 

 condensation begins again. It is also evident that the height of the con- 

 denser is not connected with the quantity of steam to be condensed. The 

 accumulated experience of engineers seems to have led to an over-all height 

 of twelve feet, with four cascades, as affording an efficient condenser. The 

 area of cross-section is evidently proportional to the quantity of steam to 

 be condensed, and practice seems to incline to an area of 1-5 sq. ft. per 

 ton of steam to be condensed per hour for conditions as occur in the 

 tropics. 



Central Condensation. — ^In place of allowing each unit its own condenser, 

 one central condenser may be installed for the whole house ; the advantage 

 claimed is the reduction in the number of cylinders and reduced first cost. 

 The writer is inclined to believe that these advantages are of small moment, 

 for, whatever the number of condensers, they may all draw from the same 

 tank, and their united discharge may be removed by one pump. The re- 

 duction in units only takes place then in the air pumps, but these again 

 can easily be grouped to be operated by one steam cylinder. An objection 

 in large houses is the excessive weight of only one condenser, the placing of 

 the load of which is a much more difficult problem than the distributed 

 load of a number of small condensers. The lay-out of a number of individual 

 vapour pipes to one condenser is also unsatisfactory. 



The use of one pump also causes irregularities when throwing in or cutting 

 out a pan, and sugar boilers much prefer to have each unit independent 

 of the others, as thereby the control of the boiling is unaffected. Regulation 

 of the individual vacua in pans all connected to a central condenser can, 

 however, be obtained by installing valves in the pipe lines to the condensers, 

 whereby the area of the passage may be restricted at will. 



Vacuum Pumps. — The pumps used in sugar factories for the maintenance 

 of the vacuum may be classed first of all as wet or dry, the former class in- 

 cluding such which remove the water and air conjointly, the latter treating 

 the air only after its separation from the water used for condensation. Most 

 designs of wet-air pumps are also capable of functioning at a moderate 

 efficiency as dry-air pumps, and indeed do so when evacuating a vessel in 

 commencing operations. On the other hand many designs of pumps classed 



