336 CHAPTER XVIII 



If under these conditions the back pressure be raised so as to give better 

 service from the steam, the admission of steam to the cyHnders will also have 

 to be increased, and a greater production of exhaust steam obtains, a result 

 which again limits the economy, unless there is sufficient heating surface 

 in the vacuum pans and heaters. In such cases as these two methods of 

 obtaining increased economy are possible : the initial boiler pressure may be 

 increased, so that a high back pressure may be used without augmenting 

 the actual quantity of the exhaust steam ; or a pre-evaporator may be in- 

 stalled, which produces steam at the normal back pressure of the factory, 

 and if there be not already an excess of back pressure steam the production 

 of steam from the pre-evaporator may be carried to the point where the low 

 pressure steam begins to be in excess of that which can be utilized. 



The most rational and complete system of the economical utilization 

 of steam is obtained by carrying the exhaust pressure high enough to do the 

 required work at all stations, and at the same time increasing the boiler 

 pressure, so that a surplus of exhaust does not result. With an initial 

 pressure of 150 lbs. and a back pressure of 30 lbs. per sq. in., the consumption 

 of steam per H.P. will not be more than with 90 lbs., and 5 lbs. in the live 

 and exhaust lines. With this pressure a quintuple effect with steam separ- 

 ated from the first and second bodies, as in case 3 above, is indicated as the 

 most convenient scheme, for all the steam used in heating and boihng will 

 be delivered to the first cell of the quintuple and all single effect operations 

 eliminated. In this scheme a high pressure battery of boilers and a low 

 pressure battery become convenient, the low pressure main and the engine 

 exhaust being united. 



The position of the pre-evaporator is also sometimes misunderstood. 

 It may be regarded as a device for producing exhaust steam in such quantity 

 that the pre-evaporator exhaust added to the engine exhaust is just sufficient 

 to do all the requisite heating and evaporation. The more work that is 

 done bv the pre-evaporator the less is the total steam consumption, and hence 

 if the consumption of steam in the engines is reduced, the greater is the 

 opportunity for economy by the use of the pre-evaporator. In such a case 

 the statement that engine economy is useless since all the exhaust is utilized 

 loses its validity. 



The whole of the above section has been written with reference to raw 

 sugar manufacture, and does not refer to plantation white sugar making. 

 The steam consumption here increases, and Bolk^- has estimated this increase 

 as 21 per cent, over and above that required in raw sugar manufacture. 

 The items where an increase is shown are in the vacuum pans following on 

 the use of water in washing the sugars, in steam used in purging the sugar 

 at the centrifugals, in filtration of the syrup, and in the increased number of 

 centrifugals necessar5^ 



Step-up Heating. — The exigencies of manufacture require that the juice 

 be heated to 212° F., a temperature which usually only the steam from the 

 first effect can reach. The juice may, however, be heated in a series of steps, 

 say from 80° F. to 120° F., by steam separated from the last body, from 120° 

 F. to 150° F., by steam from the penultimate body, and so on. Although 

 the economy is real, complication of apparatus and increased heating surface 

 necessary have prevented any extended development of the method. The 

 use of juice, however, in surface condensers attached to the last body of the 

 evaporator is quite common in Mauritius ; not only is heat economy obtained. 



