Concentration in open pans has now been superseded by evaporation in vacuo. The subject of the gradual development of the modern efficient evaporating plant from the vacuum pan, originated and successfully applied by Howard in 1813 in the sugar industry, is too lengthy to detail here, suffice it to say that the multiple effects now in vogue possess distinct advantages—the greatest of these being increased efficiency combined with economy.
The present type of evaporator consists of one or more vessels, each fitted with a steam chamber through which are fixed vertical hollow tubes. The steam chamber of the first vessel is heated with direct steam, or with exhaust steam (supplied from the exhaust steam receiver into which passes the waste steam of the factory); the treated lyes circulating through the heated tubes is made to boil at a lower temperature, with the reduced pressure, than is possible by heating in open pans.