250 ANIMAL PROTEINS 



up to octuple effect. The great advantage of multiple-effect 

 evaporation is in the saving of costly steam. Reavell gives 

 the following figures to illustrate the economy thus obtained: 



WATER EVAPORATED PER IOO UNITS STEAM. 



There is naturally a limit beyond which the capital cost 

 of the machine neutralizes the advantage of steam economy, 

 and it is seldom that octuple effects are used. There are 

 probably more triple effects in use than any other machine. 



An essential and important part of the modern evaporator 

 is the " condenser," in which the vapour from the last 

 effect is conducted into water (jet condensers) or over 

 cooled surfaces (surface condensers), with a view to pro- 

 ducing and maintaining the vacuum. 



A lasting vacuum cannot be maintained without an air- 

 pump, as air is often introduced (i) with the steam, having 

 entered the boiler dissolved in the feed water ; (2) by leakage 

 from the atmosphere into the condenser and the connected 

 vacuous spaces ; and (3) in jet condensers, in solution with 

 the circulating condenser water. That from the first two 

 sources may be reduced, but the third is beyond control : 

 hence if high vacua are necessary, surface condensers are to 

 be preferred. Dissolved air is usually 5-20 per cent, of the 

 water volume, and is least for sea-w r ater. It should be noted 

 that water leaving a surface condenser is in a very air-free 

 state, and therefore particularly suitable for boiler supply. 

 Apart from the capital cost of a condenser the chief cost of 

 maintaining a vacuum is in pumping the circulating water, 

 of which up to 70 Ibs. is usual per Ib. of steam condensed. 



If W=weight of steam condensed (Ibs. per hour) ; 

 Q== weight of cooling water circulated (Ibs. per hour) 

 T i =inlet temperature ( F.) of cooling water ; 

 T =outlet temperature ( F.) of cooling water ; 

 then 



