3/0 CHAPTER XVIII 



Cubic Feet Displacement per Minute. 



With wet pumps the displacement required is from 2 '5 to 3 times as 

 much. 



Cooling of Water, — In many districts, as for example in Cuba and in 

 Mauritius, the supply of water is not sufficient for the needs of the condensers. 

 It is therefore necessar}' to continuously cool and use over again the available 

 supply. 



The means adopted for doing this is the exposure of the water to the 

 air in such a form as to expose as great a surface as possible. Cooling takes 

 place by radiation, by contact through contact with the air, and also by 

 means of the heat abstracted through evaporation. 



The appliances used to this end are either towers or sprajdng systems. 

 Towers may be either enclosed shafts, to the top of which the hot water 

 is delivered, and down which it flows over a series of trays designed to 

 expose as much area as possible. A fan may force a current of air upwards 

 through the tower, or natural draft may be used. This tyj)e of cooler is 

 not to be found in sugar districts, and its place is taken by open towers. 

 These consist of a. framework, usually about thirty feet high, on which at 

 vertical intervals of about four feet are laid open horizontal platforms ; 

 on these are frequently set faggots or brushwood, so as to increase the cooling 

 area. The. hot water is delivered to a gutter or s^^stem of distributing 

 gutters on the top of the tower. The sides of these are provided with saw 

 cuts, and their inclination is such that an even distribution of the water is 

 obtained. The horizontal cross section of the towers assumes various forms ; 

 it may be circular or a long rectangle, ten to twenty times as long as broad ; 

 or again a very efficient form takes the shape of three sides of a hollow square, 

 the perimeter of which will be about fifteen times the width of the tower 

 itself. Such a cooling tower is indicated in Plate XXVII. Expressions of the 

 coefficient of transmission under these conditions can be obtained, but the 

 assumptions necessary to be made are so broad that the results are very 

 unsatisfying. Actual experience gives the following as a satisfactory basis 

 of design : — Platform area, 300 sq. ft. per ton-cane-hour ; cubic contents 

 of tower 1,200 cu. ft. per ton-cane-hour ; capacity of cistern, 200 ca. ft. per ton- 

 cane-hour. 



The general Cuban practice in connection with cooling towers is to 

 place the condensers at a level so high that the overflow from the barometric 

 seal wUl gravitate to the distributing system on the top of the tower. 



Recent practice in Cuba has tended towards the substitution of spray 

 nozzles for the cooling tower, and such a system is illustrated in Plate XX VI I. 

 As usually installed each spraj^ head is made up of five nozzles, each nozzle 

 under a head of twenty-five feet having a capacity of 40 U.S. gallons per 

 minute. Allowing twenty tons of cooling water per ton of cane this reduces 

 to two nozzles per ton-cane-hour. As, however, a certain number of nozzles 

 are out of commission being cleaned, and the capacity of those in action 

 is reduced due to the presence of dirt in the water, three nozzles per ton- 

 cane-hour would be a more suitable allowance. The pipes carrying the 



