Page 175 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.3 Evaporator types 173

They are best suited for handling clear liquids, or foamy liquids with large evaporation load and
their advantages are

• low residence time
• high heat transfer rate
Advantages and Limitations
• lower cost
• low hold-up
• smaller floor space requirements
• good heat transfer over a wide range of services
• large units can be manufactured
However, these evaporators require high head-room. In some cases, they also require recirculation
of the liquid to ensure wetting of the tube surface. These are not suitable for salting or severely fouling
fluids.
Thin-layer or wiped-film evaporators, also known as thin-film evaporators are expensive and used
specifically for highly viscous liquids and for solvent removal down to
very low concentration. It consists of a vertical tube, with a lower
jacketed part containing the heating medium and the upper part acting
Wiped Film Evaporator
as a separator. An external motor drives a shaft with blades mounted
with a small clearance (w1.5 mm) between blade tips and the inner
surface of the tube, thus extending nearly to the bottom of the tube. The
liquor to be concentrated is picked up by the rotating blades as it enters and gets thrown against the
tube wall to form a thin, well-agitated liquid film even with very viscous liquids. The film gets
concentrated as it flows down by gravity. The concentrated liquor is drawn off from the bottom by a
pump and the vapor leaves the top of the unit to a condenser. Scale formation is minimal on the scraped
heating surface. Large units are uneconomic, and therefore, these are used in laboratory-scale or for
highly valued products.
Plate Type Evaporator consists of a series of plates spaced by gaskets, the entire unit mounted
within a support frame. These can be climbing-film, falling film or a
combination of both and can be operated under vacuum, as well as at
under pressure. Fig. 6.12 shows the flow and plate arrangement of a
Plate Type Evaporator
falling film plate evaporator. Each unit has a product plate and a steam
plate, and the required heat transfer area is provided by multiple such
units. Capacity augmentation by addition of units, if required, is a
major flexibility for plate type evaporators. Product flowing down each side of the plate in series is
adopted when this is advantageous in terms of wetting rates. Both the vapor evaporated from the
boiling film, and the concentrated product is discharged from the evaporator to a vaporeliquid
separator. The product is pumped, and the vapor passes to the next effect (or to the condenser from the
last effect). They also have lower residence time, that is, they operate with high velocity and achieve
high heat transfer coefficient. This allows heating with a low-temperature difference across the heating
surface and makes the plate type evaporators particularly suitable for heat-sensitive materials like fruit
juice, milk, and pharmaceutical products. Low concentration ratios between feed and product normally
require single-pass, while higher ratios require recirculation of part of the exit stream. Satisfactory
mixing of the recycled and the fresh feed stream often use static in-line mixers.
Compared to tubular evaporators, the plate evaporators offer advantages in terms of headroom,
floor space, accessibility, and flexibility. They also have lower residence time. In plate units, boiling on
the heating surfaces is avoided by increasing the pressure on the heating surface. A “restriction orifice”
between the plate pack and the separator ensures this. Plate type evaporators are, therefore, (A) well