CHAPTER TWO
BOILERS
General Principles Of Boilers and Steam Generation
The purpose of a boiler is to convert the chemical energy contained in a fuel into heat and
transfer the heat by steam to a turbine or an engine which will convert the heat into work.
Therefore a boiler is a closed pressure vessel in which a fluid is heated for use external to itself
by the direct application of heat resulting from the combustion of fuel or by the use of electricity.
In a fuel fired boiler, chemical energy in the fuel is converted into heat in the furnace and the
function of the boiler is to transfer this heat to the water in the most efficient manner. This heat
is transmitted to the water in the boiler by radiation, conduction and convection.
In the boiler furnace, the radiant heat waves are transmitted from the hot flame and gases, to
the boiler heating surfaces such as the plates and tubes. On striking these surfaces, most of the
radiant heat is absorbed and passes by conduction to the water inside the tube. Convection
currents in the water itself complete the transfer of the heat from the burning fuel to the water
and steam. Hence all three forms of heat transmission occurs simultaneously. The amount
and arrangement of heating surfaces determine the size, shape and form of the boiler. The
gases can be made to flow through flues or tubes surrounded by water, hence the name firetube boiler, or they can be arranged to flow over the tubes with water circulating through them,
hence the name water-tube boiler.
Before describing various boiler designs, however, it will be very useful to discuss the formation
of steam in a boiler and the methods employed to improve its thermal efficiency. Now , whatever
the type of boiler, steam will leave the water at its surface and it passes into what is called the
steam space. This is the space in the water container directly above the water. Steam formed
above the surface of the water is always wet and will remain wet so long as there is water
present. This is because the steam, rising from the surface of the turbulent boiling water will
carry away with it some minute droplets of water.
Since the water container will and must, always contain water, then the steam in the steam
space is always wet. If wet steam is all that is required, then the steam as it is formed is piped
out of the steam space away into the steam main. If however, superheated steam is required,
then the wet steam is removed from the steam space and is piped into a super heater. Normally
the Super heater consist of a long tube or series of tubes which are suspended across the path
of the hot gases from the furnace.
As the wet steam progresses through the tube or tubes it is gradually dried out and eventually
super heated. From the super heater it passes to the steam main. If a control of the degree of
superheat is required ,as in some of the larger boilers , then attemperator is fitted. The control of
the degree of superheat is obtained by the injection of water or steam into the superheated
steam. If an attemperator is fitted, the superheater is generally divided into two parts. The first
part is called the primary superheater, then comes the attemperator followed by the second part
of the Superheater called secondary Superheater.
Now, the flue gases, having passed through the main boiler and then the superheater, will still
be hot. The energy in these flue gases can be used to improve the thermal efficiency of the
boiler. To achieve this thermal efficiency, the flue gases are firstly passed through an
economizer. The economizer is really a heat exchanger in which the feed water being pumped
into the boiler at a higher temperature than would be the case if no economizer were fitted.
Hence, less energy is required to raise the steam or if the energy is supplied, then more steam
is raised resulting in higher thermal efficiency.
Having passed through the economizer the flue gases are still moderately hot. Further thermal
efficiency improvement can be obtained by passing them through an air heater. This is again, a
heat exchanger in which the air being ducted to the boiler furnace hotter than if the air heated
were not fitted. This results in a higher furnace temperature which thus increases the furnace
potential for steam raising. Thermal efficiency improvement results.
A still further thermal efficiency of the boiler is obtained by the installation of a reheater. The
reheater will often appear in the flue gas path before the economizer. In some of the larger
steam turbines in power stations for example, steam is removed from the turbine after partial
expansion. This steam is fed back to the boiler to the reheater. Here it is reheated to a higher
temperature and then passed back to the turbine where it completes its expansion in the latter
stages.
The object of reheating steam in a turbine plant is to preserve the steam quality in the low
pressure stages of the turbine. If there was no reheat then the steam in the low pressure stages
would become too wet. Wet steam has erosive and corrosive effect on turbine blades . By
returning the steam to the boiler , after partial expansion , the quality of the steam is improved
and wet steam in the low pressure stages is therefore largely avoided.
By reheating there is also greater potential work output steam in the low pressure stages. There
may also be slight improvement in thermal efficiency by the process of reheating.
Fire-tube boilers usually consists of a shell with tubes fitted inside it, while water tube watertube boilers consists of a drum with tubes fitted on the outside and these tubes are also
connected to a mud-drum or headers. The furnace of a fire-tube boiler can either be inside or
outside the shell, while the furnace of a water-tube boiler must always be outside the drum.
A boiler is designed to do this as efficiently as possible.
In designing a boiler, the aims are;
1)
2)
3)
4)
To burn the fuel efficiently
To reduce all heat losses to a minimum
To achieve maximum steam temperature
In some cases to control the Steam temperature.
STEAM BOILERS IN MOTORSHIP APPLICATIONS
The marine use for boilers in motor vessels is confined to auxiliary services and heating
needs. In recent years, reutilization of engine exhaust in a boiler and running an auxiliary
turbine which can be coupled to the main propulsion through the gearing has widened the
scope of steam machineries in a dominant diesel environment.
For conventional cargo vessels, a small fire tube boiler would be sufficient and for tankers
with higher port steam demand ,an aux. boiler of higher capacity water tube variety would be
more common.
Generally speaking, a fire tube unit would produce steam within 10bars with an evaporation
rate of around 2000kg/hr. These are not efficient as their water tube counterparts but can be
run with less stringent feed water quality and water level control.
The water tube boilers are more efficient and can produce steam far more quickly than the
fire tube units. They are manufactured up to a very high capacity range from 10,000-80,000
kg/hr depending on the steam requirement and the pressure may vary between 10 to 40
bars. These boilers require strict feed water level controls; the water contained in these units
are less and with high steam generating rate through the thin tube walls, very correct water
level surveillance and correct feed quality with minimal corrosion and scale forming potential
are required at all times.
Auxiliary boilers are boilers which can supply, steam for auxiliary machineries essential for
propulsion.
Donkey boilers are those used for ‘hotel’ and port services.
Tank boilers are referred to their large tank like capacity for water and most fire tube
auxiliary boilers fall into this category.
The packaged variety of both fire tube and water tube types are available and has, by far,
the best automatics built-in with the package module. These are capable of being fitted
anywhere and can be added to a running plant system without much convenience. Package
boilers produce steam at low capacity around 1000-3000kg/hr are finding popular
application on tankers as a port boiler for ’hotel’ needs and a back-up for the Waste Heat
Recovery units (WHR) at sea; they are also very common in container ships where the
steam demand is only limited.
The other boilers used, are the waste heat recovery units using heat from M.E (Main
Engine) exhaust to produce steam when the vessel is at sea. These could be in the ‘tank
type’ design but more commonly made in ‘coil’ or ‘tubular’ fashion.
CONSTRUCTION OF BOILERS
DRUMS: The steel for boiler drums is made by the open hearth; Oxygen or electric furnace
process. The plates are rolled from the high-grade bottom ends of the ingest. Plates over 38mm
thick are normalized after heating. Plates are mechanically, chemically and visually tested and
approved before rolling. Plates are rolled hot on three roll plate formers. The longitudinal welds
are forged or fusion welded.
The drum ends are welded by the fusion process.
Plates must be flame cut with machine weld preparation. Weld test pieces must be sub-mitted for
examination and undergo rigorous test.
Non-destructive testing of the welds is carried out in a furnace.
TUBES: Tubes are made of mild steel usually the same as the boiler and may be electrical
resistance welded or solid drawn.
ECONOMIZERS: Economizers are fitted to recover waste heat and improve the efficiency of
the boiler. Their design is governed by the flue gas temperature. They are tubular type with feed
water passing through the tubes. The tubes are usually 5cm outside diameter.
REFRACTORY AND INSULATION: After the boiler has been built, it is lifted on board
where the refractory and insulation are fitted. The refractory is for;
1.
2.
3.
4.
Maintaining a high furnace temperature to aid combustion
To protect the furnace from over heating and the escape of gas
To distribute the heat evenly
Protect part of the drum
The refractory must have a high melting temperature; it is usually composed of Alumina, silica
and Quart or chrome ore.
The front walls may be built of fire brick with molded firebrick quart, tiles or firebricks may be
held in steel hooks or bolt.
A gas tight insulated casing will be built around the boiler. This may be double. The boiler is
supported on fix and sliding feet to allow for expansion. Allowance is also made for expansion in
casings and uptakes.
F.D AND I.D FANS: Centrifugal force and induced draught fans are fitted. The heated air is
usually carried via double casing or double fronts to the registers.
SAFETY VALVES: Boiler Safety Valves protect the boiler from over pressurisation. As per the
requirements, at least two safety valves should be fitted to the boiler and both are mounted on a
common manifold with a single connection to the boiler. Boiler with super heater, normally three
safety valves are fitted; two to the boiler drum and one to the superheater. The superheater must
be set to lift first to ensure a flow of steam through the superheater.
The most important fitting on the boiler is the safety valve. Safety valves may be;
1. Full Bore, L=D/4 These safety valves have four times the discharge capacity of an
ordinary spring loaded valve.
2. High lift, L=D/16
3. Improved high lift,L=D/12
Where L=Lift, D=Diameter/bore
Apart from usual rule governing safety valves fitted to boilers, water tube boilers should have;
1. Not less than two valves fitted to each boiler
2. Each saturated drum and each super heater fitted with at least one safety valve.
3. Valves fitted in such a manner that the super heater will not be starved (including
emergency stop conditions)
WATER GLASS GUAGES:
1.
Regulations requires that at least two water gauges must be fitted to record the water
level in the boiler. Two are fitted incase one breaks.
2. The bottom of the gauge must be above the highest tube when cold
PRESURE GUAGES: Are usually bourdon tube type with a red mark showing the blow-off
pressure. They should be regularly checked for accuracy and records the pressure of the saturated
steam formed in the steam space.
FEED CHECK VALVES AND REGULATORS: Regulations requires that;
1. Two independent feed checks are fitted
2. At least one feed system is to be provided with a feed regulator. Feed check valves are of
the screw down non return type.
BLOW DOWN VALVES: At least one must be fitted directly to the water drum on each boiler
or by means of suitable piping.
LOW WATER ALARMS: Low water alarms are fitted to warn of dangerous conditions. Oil
supply must be cut or shut off.
MAIN STOP VALVE: They are designed to ensure slow opening to prevent rapture of
pipelines. On water tube boilers theses are of the screw down Non-return (S.D.N.R) type to
prevent backflow from another in line boiler.
MICELLANEOUS: Various valves and fittings are fitted to facilitate the running of the boiler
1.
2.
3.
4.
Vent for air removal
Various drains
Screw valves
Sampling valves etc.
SOOT BLOWERS: These are fitted at various point throughout the boiler. They remove soot
which;
1. Improve heat transfer.
2. Prevent overheating.
3. Prevents fires.
OIL BURNERS
REGISTERS: The design aims of a water tube boiler register are;
1.
2.
3.
4.
5.
6.
7.
8.
9.
To atomized the fuel.
To supply sufficient air for combustion
To give correct flame shape
To mix the air and fuel
To shut off air when the burner is not in use
To control the burner tip to prevent carbonization
To provide lighting up facilities, hand or automatic
To redraw burner when not in use
To incorporate safety features.
ATOMIZERS
The atomizers in common use are:
1.
2.
3.
4.
5.
Pressure jet
Spill type pressure jet
Variable swirl chamber pressure jet
Spinning cup
Steam or air assistant pressure jet
PRESSURE JET: The oil is pumped at a high pressure (20bar) to the burner tip where it is spun
by a swirl plate and issues through a nozzle as a cone of finely divided particles.
SPILL TYPE PRESSURE JET: Similar to pressure jet, bleed off from swirl chamber reduces
pressure and hence quality of oil delivered without seriously affecting atomization.
VARIABLE SWIRL CHAMBER
PRESSURE TYPE: The oil pressure actuates a spring-loaded piston which opens or closes
orifices in the swirl chamber.
SPINNING CUP: A motor driven cup is spun at high speed; oil is admitted to the cup and is
finally divided by centrifugal force and the addition of air.
STEAM OR AIR ASSISTED PRESSURE JET: In this case a supply of swirling steam or air
is supplied beyond the pressure jet to overcome the drawback of poor atomization at low
pressure in the simple PJ atomizer.
AIR SUPPLY: Heated air is supplied through double fronts to the register. Air adjustment and
shut-off may be by sliding tube, radial vanes, axially hinged vanes which assist in swirling the
air. Swirling air is required to mix fuel and air. Air supply also cools the burner up.
FLAME SHAPE: The flame shape is important, consistent with the shape of the furnace. This is
achieved by nozzle and air supply angle and furnace temperature. Observation glasses should be
fitted for observing flame shape and condition.
LIGHTING UP: Electrical ignite for control room operation. Older type of boiler had holes for
the admission of a hand torch.
BURNER WITHDRAWAL: Burners be withdrawn when not in use to prevent overheating.
SAFETY FEATURES: Oil may not be admitted until burner is in correct position. Burner may
not be removed until oil is shut-off. Plate-electric flame detector for control room operation.
OIL SUPPLY UNITS: It is requirement to provide oil at suitable pressure and viscosities for
good combustion. This is achieved by the oil burning unit which;
1.
2.
3.
4.
Filters the oil
Pumps the oil usually with gear or screw (2.0-3.3bar)
Regulates the oil pressure (adjustable relief valves)
Heats the oil (steam heat exchanger) between 90-120 degrees Celsius
BOILER OPERATION
FILLING UP:
1.
2.
3.
4.
Check through boiler to see that all tools have been removed.
Fit a new manhole door joint.
Fill up boiler to bottom of gauge glass with high quality feed water
Add initial charge of chemical treatment.
LIGHTING UP: Start oil pumping and heating and recirculate until oil has reached correct
viscosity. Emergency pump and heater should be used if no steam is available. Before any
attempt is made to light a burner, the following points should be checked;
1.
2.
3.
4.
5.
6.
Water in the boiler
All stop valves shut
Super-heater drains open
All casing door on
No leakage at oil burner or pipes
Vents open
One burner can now be lit, if the following precautions are taken;
1.
2.
3.
4.
Furnace thoroughly purged with air by F.D. fans
Oil supply at correct pressure and viscosity
Burner set in correct position
Air supply adjusted.
All safety devices should be in good working condition order on burners.
All burners should be checked that when they are shut off no oil may dribble into furnace. The
rate of firing may be adjusted to suit the condition of the boiler e.g.: if new refractory has been
fitted a drying out period is required. As steam pressure rises on the boiler, checked;
1.
2.
3.
4.
Joints
Manhole doors
Low level alarms
Correct combustion
When sufficient steam pressure is generated the boiler is put on line. Precautions to be taken are;
1.
2.
3.
4.
Line drains are opened
Valves cracked only for warming through
Valves opened very slowly
Good watch kept on water level
BOILER STEAMING, UNDERWAY:
Log kept, so that any unusual temperatures or pressures are detected early;
1. Feed water tests taken regularly
2. Boiler water tests taken regularly
3. Chemicals added and blow down to maintain correct balance
4. Burners checked and cleaned regularly
5. Filters checked and cleaned regularly
6. Correct combustion maintained
7. Watch hot spot on casing
8. Glasses blown every watch
9. Soot blowers operated regularly
10. Feed water consumption checked every watch.
CONDENSERS
Functions of Condensers
The main purposes of the condenser are to condense the exhaust steam from the turbine for reuse
in the cycle and to maximize turbine efficiency by maintaining proper vacuum. As the operating
pressure of the condenser is lowered (vacuum is increased), the enthalpy drop of the expanding
steam in the turbine will also increase. This will increase the amount of available work from the
turbine (electrical output). By lowering the condenser operating pressure, the following will
occur :
(a) Increased turbine output (b) Increased plant efficiency (c) Reduced steam flow (for a given
plant output)
It is therefore very advantageous to operate the condenser at the lowest possible pressure (highest
vacuum).
Condenser Types
There are two primary types of condensers that can be used in a power plant : (a) Direct Contact
(b) Surface
Direct contact condensers condense the turbine exhaust steam by mixing it directly with cooling
water.
Steam surface condensers are the most commonly used condensers in modern power plants. The
exhaust steam from the turbine flows on the shell side (under vacuum) of the condenser, while
the plant‟s circulating water flows in the tube side. The source of the circulating water can be
either a closed-loop (i.e. cooling tower, spray pond, etc.) or once through (i.e. from a lake, ocean,
or river). The condensed steam from the turbine, called condensate, is collected in the bottom of
the condenser, which is called a hotwell. The condensate is then pumped back to the steam
generator to repeat the cycle.
Surface Condenser
The surface condenser is a shell and tube heat exchanger in which cooling water is circulated
through the tubes. The exhaust steam from the low pressure turbine enters the shell where it is
cooled and converted to condensate (water) by flowing over the tubes as shown in the diagram.
Such condensers use steam ejectors or rotary motor-driven exhausters for continuous removal of
air and gases from the steam side to maintain vacuum.
Diagram of a Typical Water-cooled Surface Condenser
For best efficiency, the temperature in the condenser must be kept as low as practical in order to
achieve the lowest possible pressure in the condensing steam. Since the condenser temperature
can almost always be kept significantly below 100oC where the vapor pressure of water is much
less than atmospheric pressure, the condenser generally works under vacuum. Thus leaks of noncondensable air into the closed loop must be prevented. The condenser generally uses either
circulating cooling water from a cooling tower to reject waste heat to the atmosphere, or oncethrough water from a river, lake or ocean.
The diagram depicts a typical water-cooled surface condenser as used in power stations to
condense the exhaust steam from a steam turbine driving an electrical generator as well in other
applications
Condenser Components and their Functions
The diagram depicts a typical water-cooled surface condenser as used in power stations to
condense the exhaust steam from a steam turbine driving an electrical generator as well in other
applications
Shell
The shell is the condenser's outermost body and contains the heat exchanger tubes. The shell is
fabricated from carbon steel plates and is stiffened as needed to provide rigidity for the shell.
When required by the selected design, intermediate plates are installed to serve as baffle plates
that provide the desired flow path of the condensing steam. The plates also provide support that
help prevent sagging of long tube lengths. For most water-cooled surface condensers, the shell is
under vacuum during normal operating conditions.
Hotwell
At the bottom of the shell, where the condensate collects, an outlet is installed. In some designs,
a sump (often referred to as the hotwell) is provided. Condensate is pumped from the outlet or
the hotwell for reuse as boiler feedwater.
Vacuum System
For a steam ejector, the motive fluid is steam.
For water-cooled surface condensers, the shell's internal vacuum is most commonly supplied by
and maintained by an external steam jet ejector system. Such an ejector system uses steam as the
motive fluid to remove any noncondensable gases that may be present in the surface condenser.
The Venturi effect, which is a particular case of Bernoulli's principle, applies to the operation of
steam jet ejectors.
Motor driven mechanical vacuum pumps, such as the liquid ring type, are also popular for this
service.
Diagram of a Typical Modern Injector or Ejector
Tube Sheets
At each end of the shell, a sheet of sufficient thickness usually made of stainless steel is
provided, with holes for the tubes to be inserted and rolled. The inlet end of each tube is also bell
mouthed for streamlined entry of water. This is to avoid eddies at the inlet of each tube giving
rise to erosion, and to reduce flow friction. Some makers also recommend plastic inserts at the
entry of tubes to avoid eddies eroding the inlet end. In smaller units some manufacturers use
ferrules to seal the tube ends instead of rolling. To take care of length wise expansion of tubes
some designs have expansion joint between the shell and the tube sheet allowing the latter to
move longitudinally. In smaller units some sag is given to the tubes to take care of tube
expansion with both end water boxes fixed rigidly to the shell.
Tubes
Generally the tubes are made of stainless steel, copper alloys such as brass or bronze, cupro
nickel, or titanium depending on several selection criteria. The use of copper bearing alloys such
as brass or cupro nickel is rare in new plants, due to environmental concerns of toxic copper
alloys. Also depending on the steam cycle water treatment for the boiler, it may be desirable to
avoid tube materials containing copper. Titanium condenser tubes are usually the best technical
choice; however the use of titanium condenser tubes has been virtually eliminated by the sharp
increases in the costs for this material. The tube lengths range to about 17 m for modern power
plants, depending on the size of the condenser. The size chosen is based on transportability from
the manufacturers‟ site and ease of erection at the installation site.
Motive fluid nozzle
Converging inlet nozzle
Diverging outlet diffuser
Outlet
Diffuser throat
Inlet gas, liquid, or other
Motive fluid
Waterboxes
The tube sheet at each end with tube ends rolled, for each end of the condenser is closed by a
fabricated box cover known as a waterbox, with flanged connection to the tube sheet or
condenser shell. The waterbox is usually provided with man holes on hinged covers to allow
inspection and cleaning. These waterboxes on inlet side will also have flanged connections for
cooling water inlet butterfly valves, small vent pipe with hand valve for air venting at higher
level, and hand operated drain valve at bottom to drain the waterbox for maintenance. Similarly
on the outlet waterbox the cooling water connection will have large flanges, butterfly valves,
vent connection also at higher level and drain connections at lower level. Similarly thermometer
pockets are located at inlet and outlet pipes for local measurements of cooling water temperature.
Condensate Pumps Condensate pumps are those kinds of pumps that are used to collect and
transport condensate back into a steam system for reheating and reuse, or to remove unwanted
condensate. Condensate pumps have a tank in which condensate can accumulate. The tank size
varies depending on the application. The accumulating liquid raises a float switch which
energizes the pump. The pump then runs until the level of liquid in the tank is substantially
lowered. Some pumps contain a two-stage switch. As the liquid rises to the trigger point of the
first stage, the pump starts working. If the liquid continues to rise, the second stage will be
triggered. This stage may switch off the HVAC equipment, which is, preventing the production
of further condensate, trigger an alarm or both.
Types of Condensate Pump Boiler Feed Pump;
This pump closes the boiler, steam and condensate loop by returning the condensate back into
the system for reuse. Sump Pump This pump is installed in compartments to remove the
unwanted build-up of water. In a steam power plant, the condensate pump is normally located
adjacent to the main condenser hotwell often directly below it. This pump sends the water to a
make-up tank closer to the steam generator or boiler. If the tank is also designed to remove
dissolved oxygen from the condensate, it is known as a De aerating feed tank (DFT). The output
of the DFT supplies the feed booster pump which, in turn, supplies the feed pump (feedwater
pump) which returns the feedwater to the boiler so the cycle can start over. Two pumps in
succession are used to provide sufficient Net Positive Suction Head to prevent cavitation and the
subsequent damage associated with it. Circulating Pumps Condenser circulating pumps are used
to pump cooing water through the condenser. The source of the cooling water can be the sea,
lake, river or a cooling tower. Low speed –horizontal-double suction-volute centrifugal pumps
are used for this application. This pump has a simple but rugged design that allows ready access
to interior for examination and rapid dismantling if repairs are required. Atmospheric Relief
Valves Atmospheric relief valves provide automatic protection of costly condenser equipment.
These valves are as important as trip throttle valves, over speed governors, and other devices for
power plant protection. Atmospheric relief valves are designed and manufactured with the finest
materials and the highest quality workmanship.
Steam Power Plant Atmospheric relief valves open and close automatically.
Each valve needs to be installed vertically and properly leveled for smooth operation. Special
ring seals and a water seal is provided for zero leakage in full vacuum conditions. Each valve
opens immediately when pressure increases slightly above atmospheric pressure.
Higher than atmospheric set pressures can be provided with internally spring loaded discs.
During regular maintenance and as many times as possible, each atmospheric valve should be
opened by turning the hand wheel clockwise then closing the valves by turning counter
clockwise. This process ensures nonbinding and self-cleaning valve action. Requirements of a
Good Condensing System The requirements of ideal surface condenser used for power plants are
as follows :
(a) The steam entering the condenser should be evenly distributed over the whole cooling surface
of the condenser vessel with minimum pressure loss.
(b) The amount of cooling water being circulated in the condenser should be so regulated that
the temperature of cooling water leaving the condenser is equivalent to saturation temperature of
steam corresponding to steam pressure in the condenser. This will help in preventing under
cooling of condensate.
(c) The deposition of dirt on the outer surface of tubes should be prevented. Passing the cooling
water through the tubes and allowing the steam to flow over the tubes achieve this.
(d) There should be no air leakage into the condenser because presence of air destroys the
vacuum in the condenser and thus reduces the work obtained per kg of steam. If there is leakage
of air into the condenser air extraction pump should be used to remove air as rapidly as possible.
EVAPORATORS
Evaporators come in many different shapes and sizes. Selecting the best evaporator for a given
application can sometimes be a confusing and even intimidating task. Technical terms like
falling film, forced circulation flash, and multiple effect can add to the confusion.
The basic task of an evaporator is simple: to remove water from a solution or slurry by
evaporation. Evaporators are distinct from dryers in that the concentrate discharged from an
evaporator is always in liquid form. The discharge from a dryer is in solid form, generally a
flowable powder or meal. The feed to an evaporator is always in liquid form and remains in
liquid form even after the water is evaporated.
The physical process of evaporation requires the input of energy in the form of heat to convert a
liquid into vapor. Since all evaporators use the process of evaporation to remove water, every
evaporator requires a source of heat to operate. The heat source for almost all evaporators is
water vapor, either in the form of boiler steam or Jacket Cooling water from your Marine Engine
or waste vapor from another process.
A second requirement for all evaporators is a means to transfer heat energy from the heat source
into the evaporator liquid. Most evaporators use a tubular heater called a shell and tube heat
exchanger for this purpose. In the heat exchanger shell, water vapor condenses on the outside of
the tubes thus giving up its heat energy, called latent heat. The evaporator liquid, which is inside
the tubes, absorbs the heat given up by the water vapor. This increase in heat causes the water in
the evaporator liquid to boil. As the water in the evaporator liquid boils, it forms bubbles of
water vapor in the liquid much like a pan cooking on a stove. As these bubbles reach the surface
of the evaporator liquid and burst, the escaping water vapor carries some of the evaporator liquid
with it.
The final requirement for an evaporator, then, is a means of separating the evaporated water
vapor from the evaporator liquid. This part of an evaporator is called the vapor body.
These two main components, the heat exchanger and the vapor body, are connected together to
form an evaporator. Almost all evaporators operate in the same way.
Evaporator liquid is circulated through the heat exchanger tubes to absorb heat and then
discharged into the vapor body to give up the water vapor which is boiled off. In most
evaporators a centrifugal pump is used to circulate the evaporator liquid through the heat
exchanger and vapor body. The circulating rate of the evaporator liquid depends on the type of
evaporator and the evaporator liquid.
The pressure in the vapor body of an evaporator determines the boiling point of the water in the
evaporator liquid. If the pressure in the vapor body is atmospheric, the water will boil at 212oF.
This requires the use of boiler steam as a heat source in the heat exchanger shell to achieve
proper heat transfer. Lowering the pressure in the vapor body (by pulling a vacuum on the
evaporator) lowers the boiling point of the water. For example, at 22 inches of mercury vacuum
the water will boil at 152oF. Under these conditions waste vapor at atmospheric pressure and
212oF can be used as a heat source in the heat exchanger. For this reason evaporators are widely
used in waste heat recovery applications.
In many applications, several evaporators are connected together in series to form an evaporator
system. When this is done, the water boiled off in one evaporator is used as the heat source for
the next evaporator in line. In these systems, the individual evaporators are called effects and the
evaporator system is referred to as a multiple effect evaporator. The significant advantage of this
arrangement is that it allows the original heat input to the system to be re-used in each effect.
This greatly increases the thermal efficiency of the system.Steam condensers are devices in
which the exhaust steam from the steam turbine is condensed by
The primary difference between the various types of evaporators is the way in which the heat
exchanger and vapor body are connected together, and their physical relationship to each other.
The most common evaporators fall into three types: rising film, falling film, and forced
circulation flash.
Rising Film Evaporator
In a typical rising film evaporator, the heat exchanger is mounted vertically and the evaporator
liquid flows in an upward direction through the tubes. Water in the evaporator liquid boils as the
liquid rises in the tubes. This boiling action helps force liquid up and out of the tubes.
The liquid and vapor leave the heat exchanger together and enter the vapor body. After vapor
separation the evaporator liquid flows from the vapor body through the circulating pump to the
heat exchanger. Falling Film Evaporator
Like the rising film evaporator, the heat exchanger in a falling film evaporator is vertically
mounted. In this case, however, evaporator liquid is pumped to the top of the heat exchanger and
flows in a downward direction through the tubes. Boiling of water in the evaporator liquid
occurs as the liquid flows down through the tubes which helps to force the liquid down and out
of the tubes.
Liquid and vapor from the heat exchanger tubes enter the vapor body directly for vapor
separation. A circulating pump is required to maintain proper evaporator liquid flow.
Forced Circulation Flash Evaporator
In the forced circulation flash evaporator, the heat exchanger is mounted horizontally although
vertical mounting is possible. The evaporator liquid is pumped by the circulating pump through
the heat exchanger tubes where heat is absorbed. Unlike the rising and falling film evaporators,
this evaporator is specifically designed so that no boiling occurs while the evaporator liquid is in
the tubes. When the evaporator liquid enters the vapor body, however, some of the water in the
liquid boils and is “flashed” off. The vapor body or “flash chamber” separates the vapor and
liquid. The liquid leaving the flash chamber returns to the circulating pump.
The selection of what type of evaporator to use depends on the operating conditions and
economics of the specific application. Rising film evaporators have longer residence times and
are more sensitive to solids loading in the evaporator liquid. They are generally used for clean
liquids where short residence times are not critical.
Falling film evaporators are more commonly used than rising film types. They are compact,
usually have shorter residence times, and make use of gravity to aid the flow of liquid through
the tubes. They are less sensitive to solids loading than rising film units, but more sensitive to
liquid distribution across the tubes. In a falling film evaporator, poor liquid distribution can
cause inefficient heat transfer and tube plugging problems. Both rising and falling film
evaporators are susceptible to tube coating in the heat exchanger which reduces heat transfer and
may require periodic tube cleaning.
Forced circulation flash evaporators are used in applications where heat exchanger tube coating
is a problem. Higher liquid velocities and the lack of boiling in the heat exchanger tubes help to
prevent the tubes from coating. The circulating pump horsepower is higher than rising or falling
film units due to increased liquid pumping rates.
Most evaporator applications in rendering involve the use of waste heat, usually vapors from a
continuous cooker or dryer. Evaporators are used to concentrate wastewater streams, concentrate
water pressed from hydrolyzed feathers, dry restaurant grease, and evaporate water from raw