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Boiler
evolution.
In
the 17th century, the events of the Industrial Revolution,
primarily in England, promoted the rapid development of the steam
engine by such inventors as Thomas Newcomen and James Watt.
Watt
is credited with being the first inventor to separate the steam
engine, and the boiler, into two separate units in the latter part
of the 18th Century. In these early times, the primary use of the
boiler was to generate steam for steam driven engines. As steam
driven engines replaced the horse, as a means of motive power, it
followed that steam driven engines were rated in
‘Horsepower’.
Boiler design progressed from
what was essentially a kettle to a relatively large-diameter flue
pipe submerged in water – thus the first fire-tube boiler.
As power and pressure requirements increased, boilers became
larger and the single-flue pipe became a larger number of smaller
diameter flue tubes combined with an external, or internal,
furnace for the combustion of the fuel.
Competition
between fire-tube boiler manufacturers eventually forced
improvements in boiler design and fuel burning equipment. This,
together with a broad shift towards liquid and gaseous fuel
utilization, resulted in cleaner and more reliable combustion and
improved heat transfer within the boiler.
The basic
evolution in boiler designs were:
• Plain
Cylinder Boiler
• Cornish Boiler
• Lancashire
Boiler
• Upright Fire Tube Boilers
• Scotch
Boiler
The
Plain Cylinder Boiler
The
first advancement in boiler design came with the invention of the
Plain Cylinder Boiler.
It was a simple design and easily
constructed.As its name implies, the Plain Cylinder Boiler is a
long metal cylinder with conical (round) ends set horizontally in
a brick work. The cylinder was half filled with water and a fire
ignited in furnace at one end.
The fire and hot gasses are
first channeled from the furnace or fire box along the bottom of
the cylinder to the opposite end of the boiler. This channel is
called a "flue" and is made of brick on three sides. The
other side of the flue is the metal wall of the boiler. The flames
and hot gasses touch the bare metal and heat the water inside the
boiler.
When the hot gasses get to the end of the first
flue, they are channeled back along one side of the cylinder to
the front of the boiler. From there they are again channeled back
along the other side of the cylinder to the chimney. This would
give a boiler 40 feet long 120 feet of heating surface.
The
speed with which the fuel burned was controlled by a damper near
the chimney. Raising or lowering the damper controlled the draught
or amount of air being drawn into the furnace. More air made the
fuel burn faster and hotter making more steam. Less air saved fuel
and produced less steam.
Although this boiler design was
far more efficient than previous boilers, and had been used for
more than one-hundred years, it had two major flaws.
1. The
first was dirt. Water contained dirt and as water evaporated, this
dirt collected in the bottom of the cylinder and acted like an
insulator preventing the heat from reaching the water. Therefore,
more fuel was burnt and more cleaning was required.
2. The
second flaw was more dangerous. As the hot gasses traveled along
the 120 foot long flue, they cooled a little. The metal of the
cylinder was heated unevenly on either of the three sides causing
stress leading to frequent explosions.
The
Cornish Boiler
The invention of a workable
steam boiler and engine made mass transportation possible. However
hazardous it might have been. Economics was the driving force
behind a new design - the Cornish Boiler.
Until that time,
designers had always placed the furnace beneath the water
cylinder. Then some genius had the idea of putting the fire where
it would do the most good, in with the water. Not actually "IN"
the water but literally inside the cylinder containing the
water.
The Cornish Boiler made several design changes
1. First
the 3 feet dia furnace was placed inside a metal tube in the
boiler. This greatly increased the amount of heat transferred to
the water.
2. The
order in which the gasses moved was changed. After leaving flue #1
(the main furnace) the hot gasses were divided at the back into
two streams - flue #2 which brings the gas back to the
front.
3. At
the front of the boiler the hot gasses were directed downward into
flue #3 and traveled back beneath the boiler to the chimney. This
helped reduce the amount of mud that accumulated in the bottom of
the boiler and that increased the boilers efficiency even
more.
4. The
cylinder now had flat ends because of above design changes.
Unfortunately,
the internal tube with its furnace and fierce heat was constantly
changing length. It would bulge and then contract as the temp
dropped even slightly leading to stress. The results were the
same. An explosion.
The
Lancashire Boiler
The
need for smaller more powerful, to say nothing of safer, steam
boilers led to the Lancashire Boiler design. It had many
advantages:
1. First,
each boiler had two completely separate furnaces sitting side by
side. And each furnace had a separate flue system. This idea was
outstanding. Why?
Everything that burns contains some water.
This water must be evaporated for fuel to burn efficiently. Doing
this, the furnace cools somewhat and that in turn lowers the
amount of air being drawn into the furnace. The less air drawn in
the less heat created in the furnace. This slight cooling placed a
heavy strain on the ends of the boiler. It also slowed the heating
of the water inside the boiler reducing the amount of steam
available.
With the Lancashire Boiler each furnace is
stoked at a different time. This means that one furnace is always
producing maximum heat and that heat creates a powerful draught in
both furnaces speeding up the ignition process. Something like
blowing on a campfire. It heats things up real fast.
It
also means that more air is drawn into the system which allows
combustion (burning) of the smoke created by the "low"
burning furnace. This combustion takes place in flue #2 thereby
increasing the amount of heating of the sides of the
cylinder.
2. Like
the boilers we have already seen, the Lancashire Boiler has three
flues. But the Lancashire Boiler is designed with two separate
flue systems. Gasses from the right side furnace remain on the
right side of the boiler while hot gasses from the left furnace
remain on the left side. They do not combine until they reach the
base of the chimney connection. This system provided a very
powerful, even and constant draught in both furnaces.
3. Another
major improvement in heat transfer and fuel efficiency was the
addition of "Galloway Tubes". Hollow metal tubes which
traverse (connect both sides of ) the main flue #1. Water in the
boiler flowed through these tubes which are subject to heating by
the hottest fire and gasses which pass around them.
4. The
increased efficiency of Lancashire style boilers also allowed them
to be smaller. Commonly only seven feet in diameter (side to side)
and twenty-seven feet long. A great saving of both space and
weight.
5. There
were extensive number of internal braces designed to keep the
cylinder from rupturing. The braces, and stays kept the ends from
bulging and added much to the overall strength of the
boiler.
6. These
boilers were the first to feature the "low water safety
valve" shown in the illustration. Should the water level drop
below the top of the internal flues, the intense heat of the
furnace would quickly burn through the metal. A float rides up and
down with the water level in the boiler. As the water dropped
below a predetermined level, the valve would gradually open and
release steam pressure. The noise this valve made when it opened
would also get the attention of the engineer very, very,
quickly.
7. A
second "Pop Off Safety Valve" was also installed at the
front of the boiler. These valves operated on steam pressure
alone. When the pressure in the boiler exceeded a specified
amount, these valves suddenly "popped" wide open and
stayed wide open until they were reset or replaced.
They
can still be seen in some old paper plants. The concept of
riveting came from these boilers (in the past, the welding
quantity was not good enough for higher pressures)
 The
Upright Fire Tube Boilers
We
take a side track and explain something about the vertical
(upright) boiler.
•
The
furnace is located inside the water tank. It has a number of brass
tubes which extend through the boiler to the chimney carrying the
hot gases allowing an extremely high rate of heat transfer to the
water. In other words, the fire passed through the tubes, hence
the name, Fire Tube Boiler.
•
The
water tank or boiler is a vertical tank not a horizontal
cylinder.
•
Their
compactness and the speed with which they developed a working
steam pressure was of major importance.
•
The
relatively low pressures developed by this style of boiler made it
perfect for many jobs. From heating a home to powering small
steamboats.
Disadvantages:
•
The
extremely hot gasses made only one pass through the boiler so this
design is not as efficient as those that route the heat back and
forth. Much heat was wasted going straight up the chimney.
The
Scotch Boiler (2-pass)
Engineers
and designers of steam boilers had long understood the
relationship between the amount of heat generated in a furnace and
the ability of water to absorb that heat. Yet, unlike the
Lancashire boiler, the Scotch boiler does
• The
water tank is made from corrugated plates and does not utilize
Galloway tubes. Because of the larger water surface exposed to the
heat more heat is transferred to the water.
• The end
plates are reinforced by heavy through bolts. This combination of
through bolts and corrugated plates provided an extremely strong
boiler.
• The Scotch Boiler has “fire tubes”
arranged above the furnaces, but below the water surface.
•
At the back end of the boiler the hot gasses entered a chamber, or
Dry Back which allowed the end plate to be heated and also
directed the gasses into the fire tubes. From there the hot gasses
moved forward through the numerous tubes to the chimney.
•
The Scotch Boiler was quite versatile. Designs were built to
deliver anywhere from 6 to 300 BHP (boiler horse power).
Disadvantages:
•
Water
circulation within the boiler was poor. Cooler water was settling
at the bottom of the boiler, acting like an insulator and
decreasing the efficiency of the boiler.
•
It
also allowed mud and scales to be deposited on the outside of the
main flues.
•
The
metal tubes could not transfer heat effectively to the water.
Eventually the insulation effect would allow the metal to heat to
a point where it would bend.
Boiler
classification.
There
are two approaches in boiler design: fire tube and water tube. The
goal in all cases is to maximize the heat transfer between the
water and the hot gases heating it.
Fire-tube
boilers
The name
firetube is very descriptive. The fire, or hot flue gases from the
burner, is channeled through tubes that are surrounded by the
fluid to be heated. The
tubes in a fire-tube boiler are made of carbon steel. The
body of the boiler is the pressure vessel and contains the fluid.
In most cases this fluid is water that will be circulated for
heating purposes or converted to steam for process use.
Every
set of tubes that the flue gas travels through, before it makes a
turn, is considered a "pass". So a three-pass boiler
will have three sets of tubes with the stack outlet located on the
rear of the boiler. A 4-pass will have four sets and the stack
outlet at the front. A fire-tube boiler was more common in the
1800s.
It consists of a tank of water perforated with
pipes. The hot gases from a coal or wood fire run through the
pipes to heat the water in the tank, as shown here:
Advantages:
•
Relatively
inexpensive
•
Easy
to clean
•
Compact
in size
•
Available
in sizes from 600,000 btu/hr to 50,000,000 btu/hr
•
Easy
to replace tubes
•
Well
suited for space heating and industrial process applications
Disadvantages:
•
Not
suitable for high pressure applications 250 psig and above
•
Limitation
for high capacity steam generation
•
In
a fire-tube boiler, the entire tank is under pressure, so if the
tank bursts it creates a major explosion.
For
example, steam locomotives have fire-tube boilers, where the fire
is inside the tube and the water on the outside. These usually
take the form of a set of straight tubes passing through the
boiler through which hot combustion gases flows.
Water-tube
boilers
A Watertube design is the exact opposite of
a fire tube. Here the water flows through a rack of narrow tubes
that are encased in a furnace in which the burner fires into. The
tubes frequently have a large number of bends and sometimes fins
to maximize the surface area. These tubes are connected to a steam
drum and a mud drum. The water is heated and steam is produced in
the upper drum.
These boilers are more common today. This
type of boiler is generally preferred in high pressure
applications since the high pressure water/steam is contained
within narrow pipes which can contain the pressure with a thinner
wall.
Large steam users are also better suited for the
Water tube design. The industrial watertube boiler typically
produces steam or hot water primarily for industrial process
applications, and is used less frequently for heating
applications.
Advantages:
•
Available
in sizes that are far greater than the firetube design. Up to
several million pounds per hour of steam.
•
Able
to handle higher pressures up to 5,000 psig
•
Recover
faster than their firetube cousin
•
Have
the ability to reach very high temperatures
Disadvantages:
•
High
initial capital cost
•
Cleaning
is more difficult due to the design
•
No
commonality between tubes
•
Physical
size may be an issue
The
following simplified diagram shows you a typical layout for a
water-tube boiler:
In
a real boiler, things would be much more complicated because the
goal of the boiler is to extract every possible bit of heat from
the burning fuel to improve efficiency.
The
older fire-tube boiler design—in which the water surrounds
the heat source and the gases from combustion pass through tubes
through the water space—is a much weaker structure and is
rarely used for pressures above 350 psi (2.4 MPa). A significant
advantage of the water tube boiler is that there is less chance of
a catastrophic failure: There is not a large volume of water in
the boiler nor are there large mechanical elements subject to
failure.
Here are some other ways to classify a boiler.
Table:
Classification of boilers
Types
of fuels.
Sources
of heat for the boiler can be the combustion of fuels such as
wood, bagasse, coal, oil or natural gas. Electric boilers use
resistance or immersion type heating elements. Nuclear fission is
also used as a heat source for generating steam. Waste-heat
boilers, or HRSGs use the heat rejected from other processes such
as gas turbines.
Boilers are now specifically designed to
utilize a wide range of standard and alternative fuels.
Solids:
Coals
like Bitumen and Anthracite
Liquids:
Oils
like FO/LDO/ LSHS / HSD
Gas:
Natural
gas/ LPG
Alternative fuels: Agricultural
waste like bagasse, husk, shells / wood / shavings
FO
(Furnace Oil) This fuel is a heavy, unrefined fuel. It is less
than half the cost of diesel.
LDO (Light Diesel Oil)
LSHS
(Low Sulphur Heavy Stock means it is less polluting)
HSD (High
Speed Diesel)
Natural gas This is piped to the plant.
With
the development of fluidised bed combustion, the solid fuel
boiler's efficiencies have gone up considerably. Therefore, a lot
of plants are now looking at solid fuels.
Waste Heat There
are some application which are exothermic in nature - like, DG
sets, reactors. Their waste water is generally cooled before being
released into rivers or sewage pipes. Instead, we now use this
waste heat to pre-heat water for steam.
Fuel
Selection
Fuel is selected based on the following
parameters:
• GCV
(Gross Calorific Value)
• Availability
• Cost
•
Manageability
• Emissions
• Byproducts
•
Boiler Design
• Boiler
Efficiency (η) Fuels
GCV
(Gross Calorific Value). This tells us how much heat we get by
burning the fuel.
Availability and cost. It is important
that the fuel is available easily and at a reasonable
cost.
Manageability. Fuels like coal or bagasse, are more
difficult to transport, store and feed.ustion gases
flows.
Emmissions from boilers have become a cause for
concern in Mumbai. The eastern side of Mumbai is switching to
natural gas-fired boilers as there are residences coming up in
Mulund.
Coal has a lot of by-products, especially coal
dust. These are undesirable by-products especially for food or
pharma plants.
Boiler design. Obviously the choice of
design determines what type of boiler is used. It is the fuel
which is decided first, as the
cost of fuel used per year is
always more than the boiler cost
Efficiency
(η) based on GCV
Lancashire
– (50-60)%
2 P – 70%
3 P W B (oil) –
85%
FBC - (70 -75)%
Chain grate - (60-70)%
Fig.
Chain grate fro a coal Boiler
Modern
Boilers.
The
modern boilers are far superior to these old giants. They are
smaller, cheaper, with high efficiencies and the heat release
rates are higher, so smaller surface areas are needed for heat
transfer.
 Reverse
flame (eg : Revotherm from Thermax)
The
combustion chamber is shaped like a cylinder but narrow at the far
end. This way, when the burner fires down the centre (1st pass),
the flame doubles back on itself within the combustion chamber
(2nd pass) to come out at the front of the boiler. The flue gases
now enter the fire tubes surrounding combustion chamber, and go to
the rear (3rd pass) of the boiler and the chimney. It has an
economizer to pre-heat the boiler water where flue gases are used
to pre-heat the water.
Water-tube boilers
The
water-tube boiler was patented in 1867 by American inventors
George Herman Babcock and Stephen Wilcox. In the water-tube
boiler, water flowed through tubes heated externally by combustion
gases, and steam was collected above in a drum. Water tube boilers
are very huge and their water holding capacity is enormous. The
water-tube boiler became the standard for all large boilers as
they allowed for higher pressures than earlier boilers, higher
than 30 bar. Example, Babcock & Wilcox boiler manufactured at
Thermax Boilers Ltd., Pune.
It is a horizontal, externally
fired, stationary, high pressure, water tube boiler with a super
heater as shown below.
The
coal is fed from hopper on to the grate where it is burnt. The
flue gases are deflected by the fire brick baffles so that they
pass across the left side of the tubes in a beneficial path
transferring heat to water in the tubes and to the steam in the
super heater and finally they escape into the atmosphere through
the chimney. The drought is regulated by a damper placed at the
back chamber.
The
position of water tubes near the furnace is heated to a higher
temper than the rest. Owing to higher temperature, the density of
water decreases and hence the water rises through the uptake
header and short tube to the drum. The water at the back end,
which is at a lesser temperature now travels down through the long
tube and the downtake header. Thus, a continuous circulation of
water called as natural circulation is established between the
water tubes and the drum. The steam produced gets collected above
the water in the drum.Here, saturated steam is drawn off the top
of the drum.
Since
water droplets can severely damage turbine blades, dry steam from
the steamdrum is again heated to generate superheated steam at
730°F (390°C) or higher in order to ensure that there is
no water entrained in the steam. Cool water at the bottom of the
steam drum returns to the feedwater drum via large-bore 'downcomer
tubes', where it helps pre-heat the feedwater supply. To increase
the economy of the boiler, the exhaust gasses are also used to
pre-heat the air blown into the furnace and warm the feedwater
supply. Such water-tube boilers in thermal power station are also
called steam generating units.
NIBR Boilers
In
many small plants, the use of Non IBR Coil type boilers has
proliferated, mainly due to the following perceived benefits:
• Lower
initial cost
• Equivalent
efficiency as IBR boilers
• No
IBR approved boiler operators are required
•
No
“hassle” of annual boiler inspection by IBR
•
No
IBR piping required
Maintenance
A
coil boiler requires very stringent water quality to be
maintained. The coil requires frequent maintenance/replacements in
view of water quality normally not being maintained. Manufacturers
of coil-type non-IBR boilers do a roaring business in spare
coils.
Safety
In absence of any proper
guidelines/inspecting authority/control (such as IBR inspection),
quality of fabrication itself is suspect. Shell (IBR) boilers have
much better controls. IBR boilers have controls in their
manufacturing process which automatically bring with them a higher
level of safety.
3PWB
package
We
will study this in detail, as this is the most common boiler we
will come across.
The
3PWB packaged boiler
The
3PWB packaged boiler is the most common type of boiler in most
installations. The
main components of the 3PWB boiler are shown as under.
1. Manufacturers
name plate This has the MCR - Maximum Continuous Rating –
of the boiler mentioned on it, besides the F&A
rating.
2. Boiler
shell. This stores water for heating. It is cylindrical in
shape with both ends closed.
3. Front
and rear doors are closed using studs with lugs and brass nuts for
ease of opening and closing. These doors allow for total access to
the return tubes.
4 Burner. An
equipment which burns the fuel. Burners mix air with fuel to
provide oxygen in the combustion process. Burners are specifically
matched with the furnace diameter and lenght for
complete/efficient combustion. Burners utilise gas, light/ heavy
oil or pulverized (finely ground) coal in combination with air or
pressure atomization of the oil.
5. Air
louver, linkage and setting is visuable for ease of monitoring,
adjustment and cleaning.
6. Control
panel. This is used to control the boiler and is either boiler
mounted or on a separate skid.
7. Control
circuit junction box with terminal strips to permit checking
individual control operations.
8. Fuel
oil heating and filtering systems.
9. Structural
steel skid type base supports the boiler and protects burner. This
is bolted to the foundation.
10. Furnace.
Space in a boiler where a burner burns liquid or gas fuel. (A fire
grate burns solid fuel). The hot flue gases travel from furnace to
the chimney. A large diameter furnace will provide for complete
combustion and maximum heat transfer.
11. Wetback
turnaround eliminates the need for refractory lining, bafles,
gaskets and provides additional primary heating surface for
incrased efficiency.
12. Split/hinged
rear doors provide access to the rear tube sheet and third pass
tubes.
13. Round
rear smoke outlet.
14. Heavy
steel lifting eyes for ease of handling.
15. Mineral
fiber insulation reduces heat loss through the jacket and provides
jacket support.
16. Hard
enamel paint finish to the galvanized coated steel
jacket.
17. 3
Pass design for optimum efficiency and economical
operation.
18. Front
smoke box and doors lined with ceramic fiber blanket.
Chimney.
This is a system for venting hot gases and smoke from a boiler to
the outside atmosphere. They are typically almost vertical to
ensure the hot gases flow smoothly, drawing air into the
combustion through convection. The space inside a chimney is
called a flue. Chimneys are tall to disperse pollutants in the
exhaust over a greater area reducing the concentration of toxins
to a safe level and to increase the draw.
On
and around a boiler.
The
heart of any modern steam system is the boiler house. Here we need
to achieve
•
maximum
Safety via Boiler mountings
•
high
Efficiency and economical running via Boiler auxillaries
•
excellent
Controls on pressure and temp (see Boiler controls)
Boiler
mountings - for maximum safety
All
the following are mounted on the boile r shell and are a must for
every boiler. All of them are provided for the safe working of
boiler.
•
Feed
pumps
•
Feed
check valve
•
Main
steam stop valve(MSSV) or Crown valve
•
Mobrey
- water level indicator
•
Safety
Valve
•
Gauge
glass
•
Fusible
plug
Feed
check valve
The boiler feed check valve is
designed specifically for use on boiler feedwater systems. The
valve is opened by the boiler feedwater pressure and is closed by
its spring as soon as the flow ceases, preventing reverse flow.
The strong spring supports the head of water in an elevated
feedtank when there is no pressure in the boiler, preventing the
boiler from flooding. It is a normally a stainless steel disc
check valve to ensure tight shut-off against boiler pressure, even
under poor water conditions.
Main
Steam Stop valve (MSSV) or Crown valve
This
valve isolates the boiler and its steam pressure from the process
or plant. Always open this slowly to prevent a drastic increase in
downstream pressure. Other dangers of opening the Main steam stop
valve quickly is priming, where slugs of water may enter the
distribution system or waterhammer, where collected condensate
slugs in the distribution pipe picks up sudden velocity. It should
not be used as a pressure reducing valve to throttle steam flow,
but should be in fully open or closed mode.
Mobrey
/ Water level indicator
This is a level
switch. This is an extremely important safety mechanism which
maintains water level above the fire-tubes at all times. If the
water level falls below the tubes, heat cannot be dissipated to
water. The temperatures within the fire tube build up and the
tubes can melt.
Most
accidents happen because of a fall in water level in the boiler !!
In
fact in most cases there is a second low water level alarm
interlocked to the burner. This shuts off the burner as soon as
the water level falls below a specified limit.
When
we want to shut down a boiler, we cut off the burner and then
start the feed pumps to cool down the water in the boiler.
Safety
Valve
A safety relief valve is one of the most
critical safety devices on any boiler. It is the boiler’s
last measure of protection against overpressure. It must be
adequately sized and of the correct pressure rating for the
boiler.
But getting a safe installation is only the
beginning. The safety relief valve also must be inspected and
tested regularly. Mud and scale from the boiler can interfere with
the operation of the relief valve. Plugged discharge lines can
prevent proper operation or allow discharged water and steam to
come in contact with equipment or operating personnel.
 Lifting
the test lever while the boiler is operating will confirm its
proper operation. At no time should technicians test the valve by
increasing the pressure of the boiler to a level higher than the
safety-valve setting. They should exercise caution when testing
relief valves, as steam or hot water will be discharged through
the valve at the operating pressure of the boiler. Valves should
be tested every time a boiler is started and at the interval
recommended by the manufacturer.
A
deadweight safety valve on top of a boiler. The valve lifting
pressure is set by the movement of the weight to the left of the
arm. The further the weight from the valve, the higher the
pressure in the boiler required to release the excess steam.
Gauge
glass
 The
glass tube, or pair of flat glass plates, fitted to a water-gauge
to provide a visual indication of the water-level in a boiler or a
tank.
A
simple clear glass tube is mounted between two shut off valves
which are connected to the rear of the boiler by pipes. When both
valves are opened, the tube fills with water and steam. The
surface of the water in the tube is precisely at the same level as
the water in the boiler. You know in an instant how much water the
boiler is holding.
Feed
pumps
A pump that supplies water to a boiler. In
most modern-age boilers, these are vertical, multi-stage pumps to
provide the high pressures required.
Fusible
plug
A
hollowed threaded plug having the hollowed portion filled with a
low melting point material used, for example, in the crown of a
boiler firebox. If the water level falls below them the metal in
the plug melts and steam is dumped into the firebox to prevent
serious overheating of the plates.
Boiler
auxillaries - for high efficiency and economical running
These are used to improve the efficiency of the boiler. An
economiser, super heater and air pre-heater are the main
accessories. These are not a must for any boiler but are highly
desirable.
• Economisers
•
Super-heaters
•
Air pre-heaters
Economisers
An economiser is a device fitted to a boiler which
saves energy by using the exhaust gases from the boiler to
pre-heat the cold water used the fill it (the feed water).
Flue
gases from large boilers are typically 250 - 350°C. Stack
Economizers recover some of this heat for pre-heating water. The
water is most often used for boiler make-up water or some other
need that coincides with boiler operation. Stack Economizers
should be considered as an efficiency measure when large amounts
of make-up water are used (ie: not all condensate is returned to
the boiler or large amounts of live steam are used in the process
so there is no condensate to return.)
It consists of an
array of vertical cast iron tubes connected to a tank of water
above and below, between which the boiler's exhaust gases are
passed. This is the reverse arrangement to that of fire tubes in a
boiler itself; there the hot gases pass through tubes immersed in
water, whereas in an economiser the water passes through tubes
surrounded by hot gases. For good efficiencies, the tubes must be
free of deposits of soot.
They are often referred to as
feedwater heaters and heat the condensate from turbines before it
is pumped to the boilers.
The savings potential is based on
the existing stack temperature, the volume of make-up water
needed, and the hours of operation. Economizers are available in a
wide range of sizes, from small coil-like units to very large
waste heat recovery boilers.
Stack
Economizers should be considered as an efficiency measure when
large amounts of make-up water are used (ie: not all condensate is
returned to the boiler or large amounts of live steam is used in
the process so there is no condensate to return) or there is a
simultaneous need for large volumes of hot water.
Super-heaters
A
superheater is a device that heats the steam generated by the
boiler again, increasing its thermal energy and decreasing the
likelihood that it will condense inside the engine.
Superheat
refers to the process of increasing the temperature of steam above
saturation temperatures to produce a very "dry" steam
with absolutely no water vapor. This feature is most common in
very large power plant boilers of watertube construction. Power
plants use superheated steam to run the turbine blades. Turbine
blades are very vulnerable to water damage. Super heated steam
being absolutely dry, is much more suited for this expensive
equipment, increasing its life and reducing replacement
costs.
Air pre-heaters
An
air preheater is designed to heat air before combustion in a
boiler. The purpose of the air preheater is to recover the heat
from the flue gas from the boiler to improve boiler efficiency by
burning warm air which increases combustion efficiency, and
reducing useful heat lost from the flue. As a consequence, the
gases are also sent to the chimney or stack at a lower
temperature, allowing simplified design of the ducting and stack.
It also allows control over the temperature of gases leaving the
stack (to meet emissions regulations, for example).
It uses
waste heat to pre-heat air for combustion in boilers. Better
combustion can be achieved as the fuel can be atomized better
after pre-heating of air.
Efficiency
and losses.
A
modern steam boiler will generally operate at an efficiency of
between 80 and 85%. Some distribution losses will be incurred in
the pipework between the boiler and the process plant equipment,
but for a system insulated to current standards, this loss should
not exceed 5% of the total heat content of the steam. Heat can be
recovered from blowdown, flash steam can be used for low pressure
applications, and condensate is returned to the boiler feedtank.
If an economiser is fitted in the boiler flue, the overall
efficiency of a centralised steam plant will be around 87%.
There
are two methods to calculate boiler efficiency.
Direct
Method
Where,
Qs
= Quantity of steam (in kg/hr)
Hs = Heat contained in steam at
operating pressure (in kcal/kg)
Hw = Heat of feedwater (in
kcal/kg)
Qf = Quantity of fuel (in kg/hr)
GCV (Gross
Calorific Value) =Energy contained in fuel in kcal/kg
Indirect
Method
We
calculate boiler efficiency by the Indirect Method using the BS
845 / ASME PTC 4.1 standards.
Boiler η =100 –
losses,
=100
– (L1 + L2 + L3 + -------------+ Ln)
Here L1, L2, etc
are the various losses.
Boiler losses
Let us take
a look at these losses from and around the boiler.
• GCV
vs NCV
• Stack
loss
• Radiation
loss
GCV
Loss
There is a difference in the gross calorific
valve (GCV) - theoretical ; and Nett Calorific Valve (NCV) -
actual of fuel. This is because of two reasons.
• The
fuel contain moisture and when burnt, first the heat,makes the
moisture in fuel evaporate and then the heat is used to heat
water.
• The
fuel contains hydrogen, which also has to be burnt
In
furnace oil (FO) the difference (loss) between GCV and NCV is
6.25% (typically).
Stack Loss
The flue gases lost
to the atmosphere from chimney is called the stack loss. (The
chimney is called stack as earlier bricks were stacked to make a
chimney).
It is a function of the ΔT between the
temperature of the flue gases (Tg) and the ambient temperature
(Ta).
Stack
loss is dependent on the difference(Tg – Ta). Stack losses
can be as high as 10%, or be optimised to about 5%.
Radiation
Loss
Even though the boiler surface is insulated with 3''-
4'' of insulation material, there is still a radiation loss taking
place. In a lot of older boilers, in fact, the insulations may be
old & not working. Typically, based on boiler loading,
radiation loss is 1-4%.
Boiler
controls.
We
must have an excellent control on pressure and temperature as well
as other parameters in the boiler.
• Sequence
control
• Feedwater
level control
• Pressure
(firing) control
• Trim
control
• Blowdown
control
Sequence
control
Every boiler has a sequence of events to be
executed before start-up and after shut down. To illustrate, a
simple start-up sequence for an oil-fired boiler is shown.
Feedwater
level control
The
purpose of feedwater control system is to maintain the corect
water level in the boiler under all load conditions.
Feedwater
level control must be able to regulate water level under very
dynamic conditions when the heat rate changes in the boiler cause
the boiler level to shrink and swell, due to steam bubble volume
changes in response to iring rate changes.
It also has to
respond to momentary changes in steam demand replacing steam that
has left the boiler with feedwater.
If the feedwater
control system fails we are looking at serious problems. A high
water level can cause serious damage to the distribution system
and to m/c such as turbines. A low water level can expose the
boiler tubes allowing the high flame temperatures to weaken and
melt the boiler steel causing a catastrophic high energy release
of steam from inside the boiler.
A level control system
usually employs probes which sense the level of water in the
boiler. At a certain level, a controller will send a signal to the
feedpump which will operate to restore the water level, switching
off when a predetermined level is reached. The probe will
incorporate levels at which the pump is switched on and off, and
at which low or high level alarms are activated. Alternative
systems use floats.
Pressure (firing) control
Energy
is supplied to the boiler via a combustion process and the
combustion control system regulates the firing rate by controlling
the amount of air and fuel delivered to the burners.
Combustion
control systems are regulated to maintain the desired steam
pressure and they must be able to respond to the many fluctuating
conditions of the burner, fuel and air control sub-systems in a
co-ordinated way to maintain the steam pressure set point in spite
of varying process demands.
Trim control or, Air-fuel
ratio control
Air
is comprised of approximately 21% oxygen and 79% nitrogen. When
air is delivered for combustion, the nitrogen absorbs heat and is
carried up the stack, resulting in energy losses. If there is
excess air, the result is unused oxygen as well as even more
nitrogen to absorb heat that is carried up the stack.
Boiler
efficiency can be improved by incorporating an excess air trim
loop into the boiler controls. It is easy to detect and monitor
excess air, as oxygen not used for combustion is heated and
discharged with the exhaust gases. A stack gas oxygen analyzer can
be installed to continuously monitor excess air and adjust the
boiler fuel-to-air ratio for optimum
efficiency.
Performance/Costs: An often-stated rule of
thumb is that boiler efficiency can be increased by 1% for each
15% reduction in excess air or 20°C reduction in stack gas
temperature. An annual fuel savings of 5% is often obtained with
tighter excess air control.
You can periodically “tune”
your boiler and manually optimize fuel-to-air ratios after
measuring the oxygen in the flue gas with an inexpensive test kit.
More expensive hand held computer-based analyzers display percent
oxygen, stack gas temperature, and boiler combustion efficiency.
An automatic oxygen trim control system minimizes operating costs
through ensuring that the proper fuel-to-air mixture is maintained
at all boiler loads.
Chemistry
Excess
Air
Blowdown
control
Rather than
control blowdown manually, Continuous
blowdown, sometimes
called surface or skimmer blowdown, is more effective in
controlling the concentration in boiler water. Where continuous
blowdown systems are used, bottom blowdown is used for removal of
precipitated impurities, especially those that tend to settle in
the lower parts of the boiler.
Blowdown is also controlled
to eliminate energy wastage - which is possible if more water than
is necessary is dumped from the operating boiler - quite possible
in case of manual blowdown.
Heat exchangers can be used
with continuous blowdown, to recover energy from this expelled
boiler water.
F&A
rating.
The
boiler is rated to work at a certain pressure and at that pressure
it can generate a defined quantity of steam. This can also be
written as the F & A rating of the boiler.
This is used
as a measure of the boiler ability to produce steam. It gives us
the amount of steam (in kgs) that a boiler will produce if
supplied with water at 100ºC. eg. A 2 ton boiler gives us
2000 kgs of steam per hour. It is written as 2 TPH F&A
100.
This means that when the water inside the boiler is at
100ºC and the steam take-off is also at 100ºC, the
boiler will gives us 2 tons/hour of steam.
F&A
Rating X 540 = Actual Rating X ( hg – hfFW)
Where,
hg
= enthalpy of steam at generation pressure
hfFW = Feed water
enthalpy
Example
6.1. Take a 10 TPH boiler, @ 10.54 barg, 75°C
=
9151 kg/hr nett
So, a 10TPH boiler will never give you
10,000 kg/hr of steam.
This disparity is because the feed
is not at 100 deg C. Water at 100ºC has 100 kcal /kg latent
heat. The F&A rating assumes an ideal
condition:
+
540 kcal/kg
100 kcal/k ----------------------------> 640
kcal/kg
(water at 100°C) (+heat)
(steam's latent heat)
But
actually, the feedwater is at ambient temperature. So we need more
heat to first raise the temperature to saturation (100°C).
This is what gives rise to the difference in boiler output.
+
610 kcal/kg
30 kcal/k ----------------------------> 640
kcal/kg
(water at 30°C) (+heat)
(steam's latent heat)
FAQs.
Question
1: What is the steam space / water holding of a boiler ?
Any
boiler must be designed so that it can carry the largest volume of
water possible keeping adequate steam space.
If there is a
larger body of water at close to saturation, any sudden loading of
the boiler (switching on a new process in the plant) will not
impact the pressure. It can give off huge steam loads
intermittently. On the other hand, if there is a smaller quantity
of water in the boiler, sudden load increase could drop the steam
pressure.
A minimum steam space of 25-30% of diameter of
boiler is needed. This is because, the closer you bring water to
the top of the boiler, the greater the chances of carry over
inside a boiler as there is a lot of turbulence at the separation
surface. Bubbles of steam are rising up and bursting, so water
droplets might enter the system along with steam. That means if
the steam space is too small, dryness fraction decreases.
For
a similar reason, a larger furnace is often fitted to a boiler. It
can withstand thermal shocks which occur when there is a sudden
rise in steam demand.
Question
2: Shouldn't we do a 4th pass to get higher efficiency ?
In
fire tube boilers, the temperatures of the various passes are
:
1st Pass - 700 to 1100
2nd Pass - 400 to 700
3rd
Pass - 200 to 300
Normally, we generate steam at 10 bar
(187ºC) so in a 4th pass the water will heat the fuel.
Instead of a 4th pass, we can use the fuel gases to pre-heat the
fuel (economizer).
Question
3: Why should stack temperature rise? Is it because excess air
blows out unburnt fuel and gases?
No.
in fact, excess air will only decrease the temperature even if it
is blowing out unburnt fuel and gases.
Stack temperatures
rise mostly when the tube in the boiler cannot provie efficient
heat transfer because of soot on tubes, or oil inside the tubes.
The water cannot therefore, absorb all the heat being generated in
the firetubes and the end result is a lot of heat lost via the
stack . Hence the rise in stack temperatures. Inadequate heat
transfer area within the boilers will also result in elevated
stack temperature levels.
Water salts and scale deposits on
tube from outside also impede heat transfer. Boilers using water
from DM plants need less cleaning as compared to boilers using
well water.
Fouling of the heat transfer area also occurs
because of unclean fuel.
Question 4: Why should dampers
for excess air levels be watched ?
As the seasons
change, the air density also changes. In summer, for example, the
air is less dense as compared to winters. This is because when it
gets hot, the volume increases. So, we may need to ramp up excess
air levels during the summers as compared to winters.
Also,
the damper is a mechanical movable part. So it may lose its
calibration. A pocket O2 analyzers will help us set the damper to
a correct level. (On line O2 analyzers are also available but they
are expensive).
  
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