328 CHAPTER XVIII 



the temperature difference by decreasing the temperature at the cold end 

 (decrease of VS^ or increase of the vacuum) leaves the value of K,n fairly 

 constant, so that the rate of evaporation is approximately in direct proportion 

 to the value of Cj — VS^. 



5. Between such limits of pressure and vacuum as are found in 

 practice, the value of K^ in one and the same apparatus may lie between 

 180 and 350, and the capacity of the apparatus may vary from 4 to 9 lbs. 

 water evaporated per sq. ft. per hour. 



6. The value of C^ — Cg, which can be easily observed, forms a rough 

 index of any change in the rate of evaporation. 



The Different Methods for the Utilization of Steam. — The different sys- 

 tems found installed in recent cane sugar houses may be classed as follows : — 



1. The heating, evaporation, and graining systems are entirely dis- 

 connected. 



2. Steam is separated from a cell or cells of the multiple effect evaporator 

 and used for heating or in graining. This system is known generally as 

 the Rillieux-Lexa combination, and is that shown in Rillieux's classical 

 patent (U.S. 4879, 1846). 



3. An independent evaporator is installed operating under a higher 

 pressure, and the steam generated is used for heating, evaporating or graining. 

 The independent evaporator may be operated at single or multiple effect. 

 The most convenient, and the generally adopted arrangement, is to allow 

 the steam generated from the independent evaporator to discharge into the 

 exhaust steam main. This evaporator is known as the pre-evaporator, 

 fore-boiler, or 0-cell ; the system is usually referred to as the Pauly-Greiner 

 combination. 



4. Regeneration of low pressure steam b}' thermo-compression, or by 

 mechanical pressure. 



All of these systems may be combined and combinations of the second 

 and third systems are frequent. 



An analysis of the heat consumption in the first three systems follows. 



Let H and G be the quantities of steam necessary for heating and graining, 



and let E be the quantity of steam necessarv for the concentration to syrup, 



E 

 referred to single effect, then S ~ H -{- G -| where 5 is the total quantity 



of steam required and n is the number of vessels in series in the evaporator. 



The Rillieux-Lexa combination has for its object the separation of steam 

 from a cell of the evaporator, and the use of this steam for heating or graining : 

 these operations may be referred to as obligatory single effect processes. 



Let be the consumption of steam required for obligatory single effect 

 work, and let E be that required for potential multiple effect work expressed 

 in terms of single effect ; then, at single effect throughout, the consumption 

 of steam is a maximum and isO -{- E. HE is done at n effect, the consump- 



E E 



tion is H , and the saving in steam is E . 



n n 



Now let there be n effects, and let p^, p^, etc., steam be separated from 

 the first, second, etc., vessels, and used towards doing the work represented 

 b}' ; lei Z be the steam supplied to (or water evaporated from) the first 



