Construction : A long tube evaporator consists of a long tubular heating element incorporating tubes, 25 mm to 50 mm in diameter and 4 to 8 metres in length. The tubular heating element projects into a vapour space/separator for removing an entrained liquid from the vapour. The upper tube sheet of tubular exchanger is free and a vapour deflector is incorporated in the vapour space just above it. A return pipe connecting the vapour space to the bottom of the exchanger is provided for natural circulation of a unvaporised liquid. It is provided with inlet connections for feed, steam and outlet connections for vapour, thick liquor, condensate, etc. In this evaporator, the solution/liquor to be concentrated is in the tubes and condensing steam surrounds the tubes. Fig. shows a long-tube vertical
evaporator.
Fig. Long-tube vertical evaporator
Working :
In this evaporator, feed enters the bottom of the tubes, gets heated by the condensing steam, starts to boil part way up the tubes and a mixture of vapour and liquid issues from the
top of tubes and finally impinges at a high velocity on a deflector. The deflector acts both as a primary separator and a foam breaker. Some part of the separated liquid is removed as a product and the remaining part is returned to the bottom of the evaporator.
This type of evaporator is widely used for handling of foamy, frothy liquors.
It is used for the production of condensed milk and concentrating a black liquor in the
pulp and paper industry.