A horizontal tube evaporator is frequently found to be the most adaptable choice for simple evaporation wherein liquids are not viscous and do not deposit scale or salt on the surface.
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Principle of Horizontal Tube Evaporator
The principle mechanism involved in this type of evaporator is that steam is passed through tubes arranged horizontally. Heating causes evaporation of the feed outside the tubes discharging concentrate at the bottom and vapours passed out from the outlet at top. The vapour is removed from the top of the chamber and the product circulation take place by natural circulation over the heating coil.
Construction of Horizontal Tube Evaporator
Horizontal-tube evaporators are designed with either rectangular or circular cross sections, with tubing of stainless steel, aluminium, nickel, carbon, spellerized iron pipe, lead covered copper, or special bronze, Fig. 5.2 (a). The tubes are extended between two steam chests and arc is fastened to tube. Four-hole packing plate’s force down conical gaskets around the tube ends into counter-sunk holes in the tube sheets. Secure sealing is obtained, with a facility for quick and easy renewal. Horizontal tubes of 2 to 3 cm diameter are extended across the bottom of a cylindrical chamber with a 1 to 3 meters diameter and 2.5 to 4.5-meter height. In the case of vertical tube evaporators, the tubes are arranged vertically in calendria, Fig. 5.2 (b). A calendria is a heating part in evaporator consisting of large number of smaller diameter tubes wherein the liquid is concentrated while rising or falling.
Working of Horizontal Tube Evaporator
In the horizontal-tube evaporator, steam is fed into the steam chest and is directed through the horizontal tubes to heat the liquid surrounding the tube in the bottom of the evaporator body. The definite path followed by the steam assures that all non-condensed gases and condensate are swept to the opposite steam chest, where they are withdrawn. The velocity and paths of circulation of the liquid depend upon the distribution, size, anti shape of the heating surface in the liquid compartment.
Applications
(i) It is used in the manufacture of the cascara extract.
(ii) It is used in the manufacture of caustic soda.
(iii) It is used in the manufacture of salts.
Advantages
(i) A number of units can be joined to obtain more efficient effect.
(ii) It has low cost per unit of heating surfaces.
(iii) It has extreme simplicity.
(iv) Easy renewal of heating surfaces.
(v) Sectional construction with low maintenance cost.
(vi) Ease of operation.
(vii) Ability to carry large volume of liquor in the body.
(viii) It requires low headspace.
(ix) Small cargo space required for shipment.
Disadvantages:
(i) Cleaning and maintenance are difficult when compared with steam jacketed kettle.
(ii) During operation the pressure inside the evaporator increases that reduces the effective temperature gradients and may affects heat-sensitive materials.
(iii) It may be used only when rigorous boiling can be obtained with natural circulation.
(iv) It is not suitable for viscous liquids.
(v) Since the boiling liquid is outside of the tubular heating surface, it is not easily cleaned by mechanical means.
(vi) It is not suitable when scaling or salting liquids are involved.
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