GB796331A - Method of and apparatus for evaporating in down-flow evaporators - Google Patents

Abstract

<PICT:0796331/III/1> In carrying out evaporation of a liquid in a tubular evaporator of the down-flow type, the liquid is preheated to a higher temperature than that corresponding to the pressure at which the evaporation is carried out, and the preheated liquid is expanded, in an expansion chamber disposed between the preheater and the tubular evaporator, in a plurality of pressure stages whereby a small fraction of the liquid evaporates and the vapours entrain the residual liquid so that it is evenly distributed in a finely-divided state into the individual tubes of the evaporator. The method is advantageously used for the evaporation of milk, fruit juice and blood; and it may be applied in multiple-effect apparatus. Milk is pumped to preheater 2 where it is heated to 75 DEG C.; and passes from there, at a pressure of at least 0.4 atm. absolute, through throttle 19 into expansion chamber 3, which is disposed on the upper tube-plate 4 of downflow evaporator 5. The temperature inside the tubes 11 of the evaporator is maintained at about 60 DEG C. by steam supplied to the evaporator-shell through inlet 12. The lower ends of the tubes discharge into liquid/vapour separator 6 which is nonnected to a condenser 8 and a steam-jet ejector 9, whereby the separator is maintained at a pressure of 0.2 atm. absolute. Chamber 3 is divided into three compartments by partitions 13 and 14 which have four apertures and six apertures respectively. Approximately 2 per cent of the preheated milk evaporates in chamber 3 in three stages, the absolute pressures in the three compartments being, respectively, 0.35, 0.30 and 0.23 atm. abs., the temperature falling to 63 DEG C. The vapours formed in chamber 3 entrain the remaining liquid so that it enters tubes 11 in a finely-divided state. The main evaporation takes place in these down-flow tubes; and the concentrated milk is withdrawn from the base of the separator 6 by a pump.