The name evaporator refers to the evaporation process occurring in the heat exchanger. In an
evaporator, the refrigerant boils or evaporates and in doing so absorbs heat from the substance
being refrigerated. An evaporator must be consist of a pump and condenser, heat exchanger, valves
and a heated bath. Evaporator consists of a heat exchanger for boiling the solution and a means to
separate the vapor from the boiling liquid. Different types are categorized by the length and
alignment (horizontal or vertical) of the evaporator tubes. The evaporation tubes may be located
inside or outside of the main vessel where the vapor is driven off.
Figure 1 Evaporator
There are three main measures of evaporator performance:
Capacity (kg vaporized / time)
Economy (kg vaporized / kg steam input)
Steam Consumption (kg / hour)
Used in pharmaceuticals, foods, pulp and paper, chemicals and polymers, inorganic salts,
In pharmaceuticals industry it is used in the production of drugs, yeast extract, glycerin
and sweet water.
In chemical industry it is used in the manufacture of Calcium Chloride, Caustic Soda,
Pigments, Paints, Sodium Nitrate, and Magnesium Chloride.
The most common types of evaporators are given as follow;
Falling Film Evaporators
Rising Film Evaporators
Thin Film Evaporators
Multi Effect Evaporators
FALLING FILM EVAPORATOR
In Falling Film Evaporator the liquid flows downwards in the form of a film along with the vapors
at the central in a parallel flow. The mechanism of film forming and low residence time makes this
evaporator suitable for heat sensitive, non-salting and non-crystallizing solutions. The liquid to be
evaporated is initially pre-heated to the boiling temperature and enters the heating tubes through
distribution plates in the top bonnet of the evaporator. In falling film evaporator there is sufficient
wetting of the heating surface and it is very important for the heat economy and trouble free
operation. The wetting rates can be increased by extending or splitting the individual evaporator
effects. Vapor liquid separation is performed at the bottom of the heater in a separated vapor body
for obtaining very low entrainment.
FEATURES OF FALLING FILM EVAPORATOR
Appropriate for low temperature applications (heat sensitive applications)
Steam economy is good
Multi-effect combination possible
Not very expensive
Well-designed distribution plate.
Optimum selection of variable parameters like temperature and pressure.
Accurate calculation of wetting surface area.
Proper design of vapor separator.
Hold up time is small.
Heat transfer co-efficient at all temperature is good.
RISING FILM EVAPORATOR
In rising film evaporator the vapor bubbles produced during boiling causes the liquid and vapors
flow upwards in a parallel flow. Simultaneously the vapor quantity also increase forming a
"Climbing" film on the walls of a tube. Due to this there is a high turbulence in the liquid and these
evaporators are beneficial for heat sensitive, highly viscous and moderately scaling liquids. The
temperature difference in the rising film evaporators should be high between the heating chest and
boiling chamber to ensure the circulation and formation of a climbing film. These evaporators are
flexible enough to be converted to Forced Circulation Evaporators.
FEATURES OF RISING FILM EVAPORATOR
Ideal for liquids which have high viscosity and have fouling tendency.
Suitable for high temperature application.
Steam economy is better as compared to falling film evaporator.
Not very expensive
Easy removal of deposits which are achieved by increasing the feed rate or reducing steam rate.
Effluent Treatment Plants
Thin Film Evaporator
Thin film is categorized by external condensers that operate at high pressure (1-100
mbar) and can be used for effective separation of large quantities of components with
FEATURES THIN FILM EVAPORATOR
These are used for reducing the volume of water from the solids and to separate a mixture
of liquids having different boiling points. These can be used to increase the concentration
of liquids that are highly viscous or in paste and syrup forms.
Heat sensitive, viscous, fouling and foaming products
The Thin Film Evaporator consists of a jacketed or coated shell having a machined surface on
the inner side. It also has a vapor nozzle that is typically arranged at the upper end of the
evaporator in order to facilitate the vapors to the external condenser. The liquid flow can be of
both bottom to top or top to bottom and this flow pattern depends on the fluid properties, chances
of scale or crystal formation.
Further, the rotating blades of thin film evaporators make sure that no material stays on the heat
transfer surface for long duration thus preventing thermal degradation.
The Multi Effect Evaporators are designed to economize the consumption of energy. These are
designed in such a way that the steam from an outside source is condensed in the heating element
of first effect. Further for second effect the boiling temperature of first effect is just too high that
evaporated water can serve as the heating medium. The formed vapors than send to condenser, if
it is the double effect evaporator But in the multi effect evaporators, the feed is generally
transferred from the one effect to another, which leads to ultimate product concentration which
further reaches to one effect of evaporator.
There are two feed operations in Multi Effect Evaporator
BACKWARD FEED OPERATION
In feed backward operation, the process of entering the raw feed into last effect takes place and
its discharge works as a feed for the next to last effect. It is very commercial technique as the
cold feed in case the feed is cold, as much less liquid must be heated to the higher temperature
existing in the early effects. The same process is used to balance the viscosity of good and also to
produce good heat transfer coefficients
FORWARD FEED OPERATION
In forward operation, the raw feed goes through parallel to steam flow to effect and the product
comes through the last effect. This technique is useful when the feed is hot and is also useful for
the high temperature deposit scale and concentrated product change
Efficiently uses heat from steam to evaporate water
Uses distillation process
Used for concentration of liquids in the form of solutions
Sugar beet factories
FIRED TUBE HEATER
Fired tube heater is also known as furnace. Furnace is a thermal enclosure used to heat the materials
below or above their melting points. Furnaces and kilns are very similar in design, and are closely
related to dryers. The major difference is that dryers only have an outer metal shell, whereas
furnaces and kilns have refractory bricks for insulation. Several industries like iron and steel
making, nonferrous metals production, glass making, manufacturing, ceramic processing,
calcination in cement production etc. employ furnace.
Figure 2 Fired Heater
COMPONENTS OF A FIRED HEATER
Fire box (it consists of open flames and combustion gases. Temp is about 1800 °F)
Radiant Tubes (receive direct heat from the flames OR burners)
Convection Tubes Convection tubes (process fluid inside them, i.e. oil, etc.) are in the
roof of the furnace so NOT in contact with the direct flames in the fire box.
Damper and Stack
Burners and Air Registers
COMMON PROBLEMS OCCURRING IN FIRED HEATERS
Process Fluid Feed Pump Failure
Fuel Composition Changes
TYPES OF FIRED HEATERS
There are two types of furnaces
Oxy fuel furnace
Electric arc furnace
OXY FUEL FURNACE
Uses a fuel gas such as propane, natural gas or acetylene
Oxygen or atmosphere is blown in and combined with fuel
Heat from combustion melts the charge
A current passed through a material resulting in ohmic heating
Radiation heats charge material
Some convection and conduction occurs
ELECTRIC ARC FURNACE
A current is passed from separate electrodes creating arcs of ionized gas
Heat is transferred from the arc into the charge material
Can be either DC or AC
Figure 3 Different Classifications
Metal injection molding
Annealing, quenching, tensile testing, brazing