Vertical Tube Evaporator/ Short Tube Evaporator Construction Construction :
It consists of a vertical cylindrical shell incorporating a short vertical tube bundle at the lower portion with horizontal tube sheets bolted to the shell flanges. Vapour outlet is provided at the top cover while a thick liquor discharge is provided at the bottom. Usually the tube bundle is not more than 150 cm high and tube diameter (outside) not more than 75 mm (25 mm to 75 mm). A downtake is provided at the centre of the tube bundle having a flow area about 40 to 100 per cent of the total cross sectional area of the tubes for circulating the cooler liquid back to the bottom of the tubes. In this evaporator, the solution to be evaporated is inside the tubes and steam flows outside the tubes in a steam chest. Baffles are incorporated in the steam chest to promote uniform distribution of steam. The condensate is withdrawn at any convenient point near the lower tube sheet, while the non-condensable gas such as air is vented to the atmosphere from a point near the top tube sheet.
Fig. Calendria type evaporator/ Short Tube Evaporator Diagram
Vertical Tube Evaporator/ Short Tube Evaporator Working :
Thin solution/liquor is introduced to the tube side and steam into the steam chest. The liquor covers the top of the tubes. Heat transfer to the boiling liquid inside the tubes takes place from condensing steam on the outside of the tubes. Vapours formed will rise through the tubes, come to a liquid surface from which they are disengaged into a vapour space and removed from the vapour outlet. Circulation of the cold liquor is promoted by a central downtake and the concentrated solution / thick liquor is removed from the bottom of the evaporator.
Advantages of Vertical Tube Evaporator/ Short Tube Evaporator :
(i) Relatively inexpensive.
(ii) As scaling occurs inside the tubes, it can be easily removed by mechanical or chemical means.
(iii) Provides moderately good heat transfer at a reasonable cost.
(iv) Can be put into more rigorous services than horizontal tube evaporators.
(v) High heat transfer coefficients.
(vi) Requires low head room.
Disadvantages of Vertical Tube Evaporator/ Short Tube Evaporator:
(i) Floor space required is large.
(ii) Amount of liquid hold up in the evaporator is large.
(iii) Since there is no circulation, these units are not suitable for viscous liquid.