Evaporators are used for concentration or
crystallization of liquids. In most of the cases evaporators are used to
increase the solid contents of the liquid product prior to drying. This is a
cost-effective method of removal of moisture. Depending on the number of
effects used in an evaporator the quantity of water evaporated per kilogram
of steam increases. In case of multiple effect evaporators, steam jet
ejectors or thermo-compressors are used to increase the thermal efficiency.
An evaporator consists of either plate type or shell and tube type heat
exchanger. The liquid feed is passed through the heat exchanger and
indirectly heated with the help of steam. This operation is either done at
atmospheric pressure or under vacuum. Evaporation under vacuum is most
energy efficient and also ensures that the product does not get over heated.
The different types of evaporators are:
A falling film evaporator is a industrial device to concentrate solutions,
especially with heat sensitive components.
In falling film evaporators the liquid product usually enters the
evaporator at the head of the evaporator. The product is evenly distributed
into the heating tubes. A thin film enters the heating tube and it flows
downwards at boiling temperature and is partially evaporated. In most cases
steam is used for heating the evaporator. The product and the vapor both
flow downwards in a parallel flow. This gravity-induced downward movement is
increasingly augmented by the co-current vapor flow. The separation of the
concentrated product from its vapor is undergoing in the lower part of the
heat exchanger and the separator.
Falling film evaporators can be operated with very low temperature
differences between the heating media and the boiling liquid, and they also
have very short product contact times, typically just a few seconds per
pass. These characteristics make the falling film evaporator particularly
suitable for heat-sensitive products, and it is today the most frequently
used type of evaporator.
However, falling film evaporators must be designed very carefully for each
operating condition; sufficient wetting (product film thickness) of the
heating surface by liquid is extremely important for trouble-free operation
of the plant. If the heating surfaces are not wetted sufficiently, dry
patches and incrustations will occur; at worst, the heating tubes will be
completely clogged. In critical cases the wetting rate can be increased by
extending or dividing the evaporator effects, keeping the advantages of
single pass (no recirculation of product) operation.
The proper design of the product distribution system in the head of the
evaporator is critical to achieve full and even product wetting of the
tubes.
Because of the low liquid holding volume in this type of unit, the falling
film evaporator can be started up quickly and changed to cleaning mode or
another product easily.
Falling film evaporators are highly responsive to alterations of parameters
such as energy supply, vacuum, feed rate, concentrations, etc. When equipped
with a well designed automatic control system they can produce a very
consistent concentrated product.
The fact that falling film evaporators can be operated with small
temperature differences makes it possible to use them in multiple effect
configurations or with mechanical vapor compression systems in modern plants
with very low energy consumption. Features of RAJ F. F. E.
- They result in vapor with very little entrained liquid
- They provide high rates of heat transfer
- They require lower liquid circulation rates (smaller pumps)
- They are suitable for operation at low temperature differences.
Working Principle:
The process fluid to be vaporized is feed to the evaporator at the top of
the tube sheet. A suitable distribution unit is necessary in order to
achieve an even liquid distribution. It is paramount that for this process
minimal wetting rate is achieved.
Application :
Falling film evaporators are used extensively in chemical process industry,
food and paper industry. Due to the absence of static head effect caused by
liquid column as in other types of evaporators, evaporation can take place
at very small effective mean temperature differences. The temperature
difference are typically between 3 - 8oC. This is significantly less than in
other devices used for evaporation, e.g. forced reboilers or kettle
evaporators, here the effective mean temperature difference is between 15
and 300C. The film heat transfer coefficients are in general high, and
characterised by surface boiling.
The absence of hydrostatic head allows this type of evaporator to operate
at very low absolute pressures.
Product residence time can be very short, especially in one through
operation. These characteristic of short retention time low operation
pressure and small required effective mean temperature differences makes
this type of evaporator particularly suitable for concentration of heat
sensitive liquids. The absence of nucleate boiling under normal operation
conditions, and low temperature differences also reduces possible fouling
tendencies.
Forced circulation evaporators are most suited for the liquids which tend
to crystallize upon concentration and which have tendency to scale.
Evaporators in which circulation is maintained, regardless of evaporation
rate or heat duty, by pumping the liquid through the heating element with
relatively low evaporation per pass are suitable for a wide variety of
applications.
The forced circulation system is the easiest to analyze and permits the
functions of heat transfer, vapor-liquid separation, and crystallization to
be separated. Forced circulation systems are generally more expensive than
natural circulation systems and are therefore used only when necessary.
A choice of forced circulation can be made only after balancing the pumping
energy cost, which is usually high, with the increase in heat transfer rates
or decrease in maintenance costs. Tube velocity is limited only by pumping
costs and by erosion at high velocities. Tube velocities are usually in the
range of 5 to 15 feet per second.
Factors which must be considered when establishing the pumping rates
include:
- Maximum fluid temperature permitted
- Vapor pressure of the fluid
- Equipment layout
- Tube geometry
- Velocity in the tubes
- Temperature difference between the pumped fluid and the utility
fluid, and
- Characteristics of pumps available for the service.
Features of RAJ Forced Circulation Evaporators:
RAJ Forced circulation evaporator offer following advantages :
- High rate of heat transfer.
- Positive circulation.\
- Relative freedom from salting, scaling, and fouling.
- Ease of cleaning and a wide range of application.
Applications
- Chemicals
- Food processing
- Effluent treatments
- Dyes
Falling
Film Evaporators
