The increased demand for large multi-effect
evaporators requiring bigger heating surface in order to obtain better
specific consumption figures, can be met by using an increased amount of tubes.
This would, however, mean that less liquid reaches each tube, and the produced
film is too thin. At high solids contents the viscosity will increase, the film
will not flow any more, and there is a risk of burnt deposits. This will result
in a concentrate with small jelly lumps, often discoloured and found in the
powder as "scorched particles", as these won't dissolve when the powder is
reconstituted. In extreme cases the tubes will be completely blocked and manual
cleaning is necessary.
The designer therefore operates with the so-called coverage coefficient
defined as:
Product kg/h at the lower end of the tubes / Periphery of the
tubes (10)
Evaporator calandrias with external
preheaters
MANUFACTURING OF THE CALANDRIA
The trend has therefore been to
manufacture the calandria with longer tubes in order to obtain more heating
surface, maintaining the coverage coefficient at the same level. About thirty
years ago the evaporators were equipped with 3-4 m tubes and operated with a
temperature difference of about 15șC, whereas evaporators 10 years ago had
tubes with a length of up to 14 m and a temperature difference down to 2șC.
Today most new evaporators have tube lengths up to 18 m. The advantage is that
less product passes are needed to obtain sufficient coverage, fewer pumps, and
reduced residence time.
MODERN EVAPORATORS
The requirement to a modern evaporator is also
flexibility and thus an ability to operate with various products and therefore
with different capacities. The problem is a different solids content in the
product to be evaporated, and that the spray dryer will have a different
capacity when drying different products. Furthermore, the evaporator will have
different evaporation capacities because of different K factor for the various
products.
When designing an evaporator/spray dryer the main product is
therefore always selected, and the evaporator calandrias are designed, so that
optimal coverage coeffi-cients are ensured, also for the other products.
As the K value is approx. 20% lower for whole milk than for skim milk, the
evaporation capacity will be about 20% less on whole milk. As the solids
content is also different in skim milk and whole milk, the feed input to the
evaporator decreases when whole milk is evaporated. This requires special
attention to the design of the calandrias, as the coverage coefficient will be
too low, especially in the first effect due to the lower amount of feed input.
If, on the other hand, the evaporator is designed for whole milk as main
product, and skim milk has to be evaporated, the coverage problem occurs mainly
in the last effect due to the low solids content in the product yielding less
amount of concentrate of 48% TS. The coverage problem was some years ago
overcome by recirculating part of the feed from the outlet of the calandria to
the inlet of same, thus increasing the amount of liquid sufficient to cover all
the tubes. See Fig. 16.
Fig. 16 Falling-film recirculation evaporator
From a technical point of view this is the ideal solution, as it
is cheap and simple, but from a product point of view it should not be
tolerated, as it means that part of the product is exposed to the high
temperature for a long uncontrollable time. This means that the final
concentrate will get increased viscosity and possibly protein denaturation,
both resulting in a powder with an inferior solubility.
In modern falling-film evaporators, the so-called "single-pass"
evaporators, the problem is solved by dividing the effects with low coverage
coefficient in two or more separate calandrias with same boiling temperature
and often one combined separator.
Another method is to split the calandria by dividing it into two or more
sections in a "multi-flow" evaporator. The product is pumped to one section,
from the outlet of which it is pumped direct to the next section, and so forth.
Having passed through the last section it is pumped to the next effect, see
Fig. 17. This system is almost as cheap as the recirculation, but has the
advantage of the divided calandria and no circulation is necessary.
Fig. 17 Evaporator calandria
split in two sections