Page 169 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.3 Evaporator types 167

Arrangement in a vertical tube evaporator is illustrated in Fig. 6.6B. The tube bundle (calandria)
is vertical and has a large central downtake with an area between 75%
and 150% of the total tube cross-sectional area. The liquid flows
inside the tubes with steam condensing outside. The boiling liquid
Vertical Tube Evaporator
flows up the tubes and returns through the central downtake. The
condensate is removed from a suitable location on the bottom tube-
sheet, and the noncondensed gases are vented from heating (steam)
chest through the vent nozzle. The vent nozzle is located close to the upper tube-sheet, where the
noncondensable gases tend to accumulate. Fig. 6.6B shows the typical location of the feed and
concentrate discharge. The heating tubes are 37e75 mm in diameter and 1e2 m long, with a length to
diameter ratio between 20 and 40. The liquid level in the evaporator has a significant influence on
circulation and heat transfer. The highest heat-transfer coefficients are achieved when the level is about
half-way up the tubes. Lower than the optimum level results in incomplete wetting of the tube wall,
increase in fouling and rapid reduction in capacity. With a salting or scaling product, this type of
evaporator is usually operated with the liquid level appreciably higher than optimum and usually above
the top tube sheet. It is not used for temperature-sensitive materials and is unsuitable for crystalline
products unless agitation is provided. Due to better circulation of liquor in the tubes, the vertical tube
evaporator is widely used in sugar and salt industries, where throughputs are very large.
The advantages of short-tube vertical evaporator include
• low head-space
• fairly high heat-transfer coefficients with low viscous liquids (up to 5e10 cP)
• relatively inexpensive to manufacture
• suitable for liquids with moderate scaling tendency since the product is in the tube side, which is
accessible for cleaning
In the basket-type evaporator (Fig. 6.7), the tube bundle is centrally suspended in the body, thus
forming an annular downtake. This allows the heating unit to be easily removed for cleaning and
repairs and ensures that crystals formed in the downtake do not break up.
In Forced Circulation Evaporators, a propeller or other type of impeller is mounted in the
central downtake, or a circulating pump is mounted outside the
evaporator body to provide higher liquor velocity through
the tubes, which result in high liquid-film heat transfer coefficient.
Forced Circulation Evaporator
Typically, velocity in the tubes is maintained at 1 m/s for salt
evaporators containing less than 5% solids and around 3 m/s for
lower concentration. Higher velocity operation reduces fouling,
thereby maintaining capacity and reducing downtime. Fig. 6.8 shows an arrangement with an external
circulating pump. Centrifugal pumps are used when crystals are present; otherwise, vane types may be
used. The liquor is introduced either at the bottom and pumped through the calandria, or introduced in
the separating section. In most units, the hydrostatic head of liquid suppresses boiling in the tubes and
boiling (flashing) occurs only in the separator.
The heating element is located at a lower elevation, much below the liquid level or the return line to
the flash chamber to avoid vaporization. This makes the heating surface relatively immune to salting
and scale formation. Nevertheless, the highest heat transfer coefficients are obtained when the liquid is
allowed to boil in the tubes, but these are seldom used except for headroom limitations or where the
liquid neither salts nor scales.