Boiler pressure parts

The booklet describes the Boiler Pressure parts in the power boilers of 210/250 MW TPS boilers. The booklet is a part of text book for the students of PG 1 year course in Power Plant Engineering. It describes in details about the Economiser, waterwalls,steam coils in superheaters, reheaters,etc

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BOILER PRESSURE PARTS
1.0Introduction
In a steam generator the parts through which the feed water and steam flows where the
pressure of the system is much higher than the atmospheric pressure aregenerally termed as
Boiler Pressure Parts. Most of the heat released by the fuel in the boiler is transferred to the
working fluid (Feed water / Steam) in these pressure parts. As such these form the heat
absorbing / heat recovery surface of the boiler.
The following parts of the boiler are generally termed as boiler pressure parts
1.Economiser
2.Boiler Drum
3.Water wall system
4.Superheaters
5.Reheaters
Except Boiler Drum above rest of the parts are heat absorbing/recovery surface of the boiler.
The quantum of energy absorbed in each part generally varies with the cycle pressure. The
amount of heat absorbed by feed water and steam in boiler at different pressures is generally
as illustrated in Table-1.
Component % of Heat Absorption at Cycle Pressure
140 Kg/cm
2
185 Kg/cm
2
225 Kg/cm
2
With Single R/H
140 Kg/cm
2
With Double R/H
Economiser14 17 15 10
Water Wall44 32 37 37
Superheater28 35 28 25
Re-heater 14 16 20 28
TABLE-1

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2.0Economiser
2.1Requirement
Economisers are provided in boilers to improve theefficiency of the boiler by extracting the
heat from the flue gasses and add it either sensible heat alone or sensible heat and latent heat
to the feed water before the water enters the evaporating surface of the boiler.
2.2Advantages
Provision of Economisers in a boiler has two major advantages.
i.As the econoniser recover the heat in flue gas that leaves the boiler and transfer to
working fluid there will be savings in fuel consumption.
ii.As the feed water is preheated in the Economiser and enters the boiler tubes .at an
elevated temperature (Near to saturation Temperature) the heat transfer area required for
the evaporation surface will be reduced considerably. As such the size of the boiler will be
reduced.
2.3Development
The design and use of economizers followed naturally the development of Boilers. The features
of early economizer designs are larger tube diameters and widely spaced tubes to meet the
requirement of Natural draft boilers and surface cleaning equipments and cast iron tubes
because of castiron s inherent resistance for corrosion both internal and external.
These early designs were gradually improved keeping in phase with other developments in
boiler. Deaeration of feed water prior to economizer reduced internal corrosion in economizer.
High feed water temperature at economizer inlet due to regenerative feed heating has reduced
the flue gas condensation over economizer tubes, consequently the corrosion and plugging on
the outside of the tubes. Further higher draughts due to the use of improved fans and better
soot blowers for surface cleaning are now available. These developments made it possible to
use steel for economizer tubing, most desirable tube diameters and tube spacing for
economical heat transfer and cleaning by steam or air.
The economiser in the present day Power boilers have tubes made of low carbon steel with
tube outside diameters ranging from 38mm to 52mm with spacing of about 90 to 140mm both
horizontally and vertically.
In modern Boilers the need of heat recovery by economizer is reduced a lot and economizers
are much smaller than old plants due to;

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i.Use of greater heat transfer within the superheaters and reheaters corresponding with
advances in cycle conditions thus resulting in progressively lower gas temperature at
economizer inlet.
ii.The increase in feed water temperature at economizer inlet by the regenerative feed
heaters introduced to increase the efficiency of the cycle.
iii.The need for heat recovery from the flue gas at air heater inlet for air heating for coal
drying and efficient combustion of coal which is essential for pulverized fuel fired boilers
and so necessitate to keep economizer gas outlet temperature high enough.
Using econimisers or airheater or both is decided by the total economy that will result flexibility
in operation, maintenance and selection of firing system and other related equipment. Modern
medium and high capacity boilers use both economizer and airheaters. In low capacity Boilers
especially in industrial boilers either economizer alone or airheateralone may be selected.
2.4Economiser Types
Economisers, broadly based on whether both sensible and latent heat are added or only
sensible heat is added to the feed water, are classified as;
i.Steaming Economiser
ii.Non steaming Economiser
2.4.1Steaming Economiser
When the heat available in the flue gas is sufficiently high after superheaters especially in small
capacity non reheat boilers, it would be advantageous in producing part of steam in economizer
itself. In such case sensible heat and a part of latentheat is added to the feed water at the
economizer and the economizer is termed as the steaming economizer. The evaporation to
steam in economizer is generally limited to 20% of the feed at full boiler output and less as the
load decreases.
Steaming economizer calls for treating a high percentage of the feed water to a condition which
does not cause scaling inside the tubes and the flue gas temperature required at economizer
inlet must be sufficiently higher, which is not feasible in a high capacity reheatboilers. Also
transferring a two phase fluid from economizer to boiler drum poses difficulties. Due to these
reasons steaming condensers are not preferred or provided in modern high capacity boilers.
2.4.2Non steaming Economisers
In this type of economizer only sensible heat is added to feed water by the heat transfer from
the flue gasses and the feed water leaves the economizer at a temperature lower than the
saturation temperature corresponding to the operating pressure.

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The economisers, by their tube construction can be further classified as plain tube and finned
tube economisers
2.4.2.1Plain Tube Economiser
Here the Economisers are constructed with several banks of plain tubes. The height of each
bank is not more than 2 meters for effective soot blowing. The tubes can either be in line or
staggered tube formation. Staggered formation induces more turbulence in the gasses giving
about 20% to 80% more heat transfer than in line arrangement. Though this results in
requirement of less surface for a give duty it is at the expense of higher draught loss. Inline
arrangement may need about 10% to 15% more surface but effectively cleanable with the help
of online soot blowers and also can be sassily inspected.
2.4.2.2Fin Tube arrangement
Welding of fins to the economizer tubes greatly increases the heating surface per unit length of
the tube (Factor of 2 to 8 according to the design). In comparison with plain tube economizer,
steel finned economizers occupy less space for the same thermal performance and draught
loss. The reduction in tube length for similar tube diameters and pitches is usually around 4 to
1. This results in smaller casing, less structural work to support the reduced weight, fewer
bends and fewer welds, giving a saving in the overall cost. A typical fin tube economizer is
shown as below.

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Inline arrangement is favored in this type for easy cleaning and inspection. The fins welded to
the tubes may be either continuous fin type or spiral fin types.
2.5Location and Arrangement
Location ofeconomizer will vary with the overall design of the boiler. It is usual to locate the
economizer ahead of airheaters and following the primary superheaters or reheaters in the
convective pass of the gas stream. In some cases where very low exit gas temperature and high
air temperatures are desired it may be necessary to divide the economizer and the air heater
and place the cooler section of the economizer between the air heater sections.
Generally economisers are arranged for downward flow of gas and upward flow of water. This
counter flow arrangement keeps the heating surface requirement and the draught loss
minimum for the same temperature drop in the flue gas. The upward flow of water, helps
steam if any formed during the heat transfer to move alongwith water and prevent the lockup
of steam, which will cause overheating and failure of tubes. Economiser coils are designed for
horizontal placement, which facilitate the draining of coils. The location and arrangement of
economisers in different boilers isas shown in Figure 3.3 below.

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Economiser tubes are supported in such a manner that sagging, undue deflection and
expansion will not occur at any condition of operation. A typical support system for economizer
coils with pantos is shown in Figure 3.4 .
FIGURE 3.4

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Ash hopper is provided below the economizer if the flue gas duct is taking a turn from vertical
to collect the fly ash separated from gas stream.
2.6Tube Size and Spacing
The overall heat transfer area required at economizer primarily dependson
i.Feed water inlet temperature
ii.Saturation temperature corresponding to operating pressure
iii.Feed water flow rate and
iv.Gas temperature required at Air heater inlet
On the basis of heat transfer area required, the tube diameter, length, spacing etc are decided
to ensure;
i.Water flow is uniformly distributed between tubes and resistance to flow must be as
low as possible; a low flow through a given tube or element could cause local steam
formation resulting in tube failure.
ii.The economizer fit in with the designof the preceding section of the boiler, usually the
reheater or Low Temperature Superheater.
iii.The gas side draught loss is kept minimum.
iv.Making provision for on load cleaning mechanism/equipment.
The tubes can be continuous from inlet to outlet headers withterminals rolled or welded. The
tubes can be made of any length and diameter with 38mm to 52mm size OD. The side spacing
and back spacing can be arranged for good external cleaning, absorption of heat and less
draught loss. Clear lanes of 25mm and less should be used only for clean fuels. Lanes of 38mm
to 50mm will be required for fuels liable to cause gas side deposits.

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2.7Economiser Failures
The economizer tubes get punctured due to various reasons. The economizer tube failures calls
for a boilershutdown and so reduces the plant availability. Hence greater care is required to
design, operate and maintain the economizer to avoid failures. The major causes of economizer
failure are;
i.Overheating
ii.Corrosion and
iii.Erosion
2.7.1Overheating
As the economizer tubes are generally made of carbon steel, any sustained metal temperature
above 400
0
C will be leading to tube failure. Starvation i.e. loss of water inside the tubes or
scaling inside the tubes will be the root cause for overheating.
2.7.1.1Starvation
Though the possibility of starvation of economizer tubes during normal operation of the boiler
is remote, starvation may occur during the startup of the boiler from cold conditions. There will
not be any flow through the economizer tubes till substantialamount of continuous steam
generation starts in the boiler. This leads to the water in the economizer tubes get evaporated
and move out of the tubes thereby resulting in starvation. To prevent this the boilers have a
economizer recirculation system. In this a tapping from downcomers with a NRV and an
isolating valve will be connected to economizer inlet. Keeping this line open during the lightup
will ensure the circulation through the economizer thereby avoiding starvation. This
recirculation line must be closed once normal feeding to economizer is started otherwise water
will bypass the economizer and flow directly to downcomers through recirculation and starving
the economizer.
2.7.1.2Scaling
Deposits of salts from feed water on the inner surface of the tubes reduce the heat transfer
across the tubes. The chances of scaling in the economizer tubes are high with a steaming
economizer using poor quality water. The possibility of scaling is remote when De-mineralised
water is used in Non steaming Economiser.
2.7.2Corrosion
The economizer tubes thin out due to corrosion/chemical action both from inside as well as
outside

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2.7.2.1External corrosion
External corrosion of economizer may occur when the water vapour in the flue gas condenses
on the surface of tubesand corrosion is accelerated when this happens in the presence of
products of combustion of Sulphur. The rate of corrosion increases when the metal
temperature is reduced. As the amount of sulphur in the fuel increases, the dew point
decreases and so doesthe potential rate of corrosion. In boilers having both Economisers and
Air preheaters and also having regenerative feed heaters, normally economizer is not subjected
to external corrosion.
2.7.2.2Internal Corrosion
Economisers are subjected to internalcorrosion from dissolved oxygen and low hydroxyl ion
concentration (a low pH). Oxygen corrosion can be eliminated by deaeration to zero oxygen.
Steel in economizer is attacked faster by pure water (pH-7) than by water, which has higher
hydroxyl ion concentration. It is therefore necessary to maintain a pH value between 8 & 9 for
water passing through Economiser.
2.7.3Erosion
In pulverized fuel boilers the economizer tubes are more prone to failure due to erosion than
any other above cited reasons. Especially this problem is aggravated when high abrasive ash-
coal are used. In a two pass boiler, when the economizer is located at the bottom of the second
pass the impact of ash over tubes is increasing. To minimize the adverse effect of erosion
provision of shrouding and baffles at suitable locations of the economizer tube banks are
practiced. In tower type boilers (Single Pass), the location of the economizer itself protects the
tubes from erosion.
3.0Drum and Drum Internals
3.1Requirement
In a subcritical recirculation boiler, the drum plays an important functional role. In the erection
of a power boiler, the lifting of the boiler drum is the first milestone activity. The functions of
the drum are;
i.Separation of saturated steam from the steam water mixture produced by the
evaporating tubes.
ii.Mixing of feed water from economizer and water separated from steam water
mixture, and re-circulate through the evaporating tubes.
iii.Carrying out blowdown for reduction of boiler water salt concentration.
iv.Treatment ofboiler water by chemicals.

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As the quantity of water contained in the drum below the water level is relatively small
compared to the total steam output, the function of water storage in drum is not significant.
3.1.1Steam Separation
In a recirculation typeof boiler the evaporating tubes supply only a steam water mixture to the
drum. From this, the steam of high purity acceptable to the superheaters and turbine is to be
separated. This separation must be done within a limited space in the drum, within a matter of
seconds and under a variety of velocity, pressure and other operating condition.
A number of factors influence the separation of water from steam in drum; important among
them are;
?The density of water with respect to steam.
?The amount of water in the mixture delivered to the drum (Circulation Ratio).
?The quantity of water and steam to be separated.
?Viscosity, surface tension and other such factors affected by pressure.
?Water level in the drum.
?The concentration of boiler water solids.
?The availablepressure drop for drum internal design.
There is a considerable difference between the density of water and steam at low pressures but
this difference decreases as pressure increases towards critical point. This relationship is shown
figure 3.5. the density of water at 84 kg/cm
2
is approximately 16 times that of steam whereas at
about 196 kg/cm
2
it is only 3 times that of steam. Thus as pressure increases, separating water
from steam becomes difficult.
Based on the above factors the steam separation in theboiler drum can be carried out adopting
one of the following methods.
i.Simple gravity separation.
ii.Gravity separation with baffles to supplement gravity separation.
iii.Centrifugal and gravity separation.
3.1.1.1Simple Gravity Separation

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FIGURE 3.5 GRAVITYSEPARATION
Figure 3.5 shows the steam separation by gravity only in the boiler drum. For a low rate of
steam generation (upto about 1m/sec velocity of steam leaving the water surface) there is
sufficient time for the light dense steam bubbles to separate from mixture by gravity without
being drawn into the downcomers and without carrying entrained water droplets into the
steam outlet (Fig. a). However for this same arrangement at a higher rate of steam generation
(Fig. b) the time is insufficient to attaineither of these desirable results leading to carryover of
water with steam. Hence the steam separation by gravity alone is possible if the velocity of
either the mixture or the steam bubbles within the mixture is low and the steam generated per
unit lengthof the drum must be kept low. This will be uneconomical except for low duty Boiler.
3.1.1.2 Gravity Separation Using Baffles

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Baffle plates are generally used to change or reverse a flow pattern to assist gravity separation
in the open drawn space. Figure 3.6 illustrates an example of simple baffle plates change
direction of flow of water steam mixture and act as impact plates. Water separating out on
such plates normally will drain off through or adjacent to steam flow and a controlling factor in
designand operation is the steam flow velocity through such drainage. Areas under and around
baffles must be sufficient to prevent excessive re-entrainment of spray. Limited in their impact
separating capacity, the chief purpose of plate baffles is to divert flow to make maximum use of
gravity separating capacity available from any low velocity steam space in the drum.
3.1.1.3 Centrifugal and Gravity Separation
At higher pressures water and steam are separated most efficiently in a drum utilizing
centrifugal force / radial acceleration to disengage the entrained particle. Vertical cyclones or
turbo separators with corrugated plate assembly at the outlet are installed either in a single
row or double rows internally along the length of the drum.

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FIGURE 3.7 CYCLONE SEPARATORS FOR STEAM SEPARATION
With cyclone separators provided in the drum, the water steam mixture from riser tubes is
admitted to the cylindrical section of the cyclones. The resulting centrifugal force due to such
admission causes the higher density water from a layer against the cylinder walls and the steam
moves to the core of the cylinder and then upward. (Figure 3.7). The water flows downward in
the cylinder and discharged through an annulus at the bottom below the drum water level. The
steammoving upward from the cylinder passes through a small corrugated scrubber at the top
of the cyclone for additional separation.
In the turbo separator arrangement, the water steam mixture coming through the riser tubes is
first admitted into a chamber formed between the drum wall and a baffle. The mixture sweeps
the drum shell on its path to the bottom and enters the turbo separators arranged along the
length of the drum. Spinner blades or vanes inside the separator spins the mixture as it travels
upward through the separator and thus creates a separating force. The concentrated layer of
water flowing upward along the surface of the primary tube is skimmed off and directed
downward through an outer concentrated tube for discharge below the water line with
minimum disturbance to water level. The steam and the remaining entrained water continue
upward through a steam collector nozzle and turn horizontally into the separator section
formed by corrugated plates. (Steam Dryer). The velocity at this point is lowand the water

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cannot be entrained from wetted surfaces and runs off the plates. Leaving the separator, the
steam flows upward. See Figure 3.8.
3.1.1.4Drying
Through the separators discussed above effectively separate the steam, still the steam may
contain some residual moisture or the steam may pickup moisture from the drum due to the
condition of water and boiler operating factors. To arrest this residual moisture before the
steam is allowed to leave the drum, the dryers are provided at the top of the drum. The dryers
are designed to have a large surface area on which moisture can deposit and from which it can
run back into the drum by gravity. Closely spaced corrugated or bent plates, screens or mats of
woven wire mesh can be used as dryer surface materials. The screen type dryers are generally
used.
The pressure drop across a dryer is normally low because of low flow velocities and relatively
small amounts of water involved. Dryers operate on a low velocity deposition principle and not
on a velocity separation principle. Formation of insoluble residues on the dryer from the boiler
water entrained with steam or filming action by foaming by boiler water may decrease the free
area and increase the local velocity and promote carryover.

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3.1.2Blow downs
The removal of a portion of water from the boiler drum is termed as Blowdown . Two types of
blow downs are adopted from the boiler drum;
i.Continuous Blow down and
ii.Emergency Blow down
3.1.2.1 Continuous Blow down
Staged evaporation of water in the boiler tubes of recirculation type boilers result in the
increase of concentration of impurities in the boiler water over a period of time. To keep the
concentration within limits so that no scaling occurs in the boiler tubes, it is necessary to drain a
portion of thiswater from the drum continuously and compensate the same with fresh makeup
water, which is having low amount of impurities. This process is called continuous blowdown.
The amount of water drained through CBD depends upon the total dissolved solids permitted in
the boiler water, total dissolved solids in the make up water and the percentage of make up. A
typical equation to determine the CBD amount is;
TDS in Make up water in PPM
CBD % of Feed water =--------------------------------------------X % ofMake up water flow
TDS allowed in Boiler water in PPM
Normally in High pressure utility boilers, the percentage of CBD will be maximum of 1% of the
steam generation.
3.1.2.2Emergency Blow down
Operating conditions may cause the water level rise in theboiler drum. A high level of water
above the normal level may lead to carryover of water by steam and at times separators may
be submerged in water. In such a condition to bring the water level down to normal this blow
down provision is made in the boilerdrum. Once the water level reaches a preset high level in
the drum, the boiler will be drained to the normal level through this emergency blow down.
3.1.3Chemical treatment of boiler water
The boiler water in the drum requires to be treated by dozing certain chemicals to have a fine
conditioning and maintaining the high quality. The dozing of chemicals is done to ensure;
i.Any scale forming salt in the boiler water is converted into sludge, which can be easily
removed through low point drains.

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ii.The pH valueof the boiler is maintained at the desired level to prevent corrosion as well
as to avoid silica carryover by the steam.
Though various solid alkalis can be used, the common practice is dozing of TRISODIUM
PHOSPHATE in drum water.
TRISODIUM PHOSPHATE doestwo functions. It reacts with scale forming salts like calcium
chloride, calcium sulphate etc. if any present in boiler water and convert them into sludge.
3 CaCl2+ 2 Na3PO4--------?? Ca3(PO4)2+ 6 NaCl
The resultant products are in the form ofsludge, which are collected in the bottom header from
where they can be removed through low point drains periodically. This process is called
periodical blow down. In modern boilers because of the use of highly conditioned water there is
no need of periodical blow down. Any sludge collected in the bottom headers will be negligible
and it is sufficient that they can be removed during the boiler shutdowns / overhauls. Further in
high pressure boilers there is a danger of loss of water in the drum during the periodical blow
down causing starvation of tubes.
TRISODIUM PHOSPHATE dozing improves the pH value of boiler as it reacts with water and
produces Sodium Hydroxide
Na3PO4+H2O---??NaOH+ Na2HPO4
By judicial dozing of Na3PO4 the boiler water canbe maintained at desired pH level.
As operating pressure increases the steam phase exhibits greater solvent capabilities for the
salts that may present in the water phase. This phenomena is known as vaporous carry over
and silica exhibit significant vaporous carryover. When more than 0.02 PPM of silica is carried
over by steam, it results in silica deposits in turbine blades that are difficult to be removed and
it has to be avoided.
The vaporous carryover of silica depends on
1.Silica in Boiler Water :-Increase in silica level increase the opportunity of carryover.
2.Operating Pressure:-Higher operating pressure will increase the vaporization of silica.
3.pH Value of Boiler Water:-Higher the pH Value lower the silica carryover.
Figure 3.9 indicates the relation between operating pressure and silica in boiler water at
different pH values to keep the silica carryover within 0.02 PPM.
The phosphate dozing maintains the pH value which controls the silica carryover.

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It may be noted that any excess pH value more thanrecommended results in free caustic
deposits in boiler tubes causing gauging type attack called caustic embrittlement in Boiler
Tubes.
Figure 3.9. Recommended Maximum Silica Concentration in Boiler Water at Various pH to limit
Silica in Steam to 0.02PPM.

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3.2Boiler Drum Construction and Location
The boiler drum is a very heavy vessel having hemispherical ends. Fig. 3.10. The drum will have
a hollow cylindrical centre section, which is welded at each end with hemispherical heads. The
cylindrical section is made up of two tubes plates which are rolled or pressed and welded
together longitudinally to form the cylindrical section. In some cases where the length of the
drum is to be very large the cylindrical portion will be made of more than one section called
courses and all the courses welded together.
Once the cylindrical shell is fabricated, a number of holes will be drilled or oxy-gas cut nozzles
and nipples/ stubs are weldedto these holes. Once the drum is erected at the site various
pipes/tube connections to the drum are welded to these nozzles and stubs. Also at the
hemispherical ends, holes will be cut for facilitating entry into the drum for inspection. These
holes will be covered with elliptical inspection doors.
The boiler drum is generally made of carbon steel confirming to ASME specifications SA 299 or
SA 515-70. Typical physical dimension of a boiler drum used in a 210 MW Boiler 700 T/hr flow
are;
Length- 15700 mm
Internal Diameter-1676 mm
Wall thickness- 131 mm
Weight- 123 Tonnes
Drum & Drum Internals of 201 MW Boiler
Steam and water Drum : The boiler drum forms a part of the circulation system of theboiler. The
drum serves two functions, the first and primary one being that of separating steam from the
1 2 3
HEMISPHERICAL HEADS
DRUM WITH THREE COURSES

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mixture of water and steam discharged into it. Secondly, the drumhouses all equipments used
for purification of steam after being separated from water.
This purification equipmentis commonly referred to as the Drum Internals.

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The quantity of water contained in the boiler below the water level is relatively smallcompared
to the total steam output. As a result, regardless of drum size, the matter of water storage is not
significant. Primarily, the drum sizeis determined by the space required to accommodate the
steam separating and purifying equipment. The steam space provided should be sufficient to
prevent priming&foaming. Drum diameter and length should be sufficient to provide
.accessibility for installation, inspection and servicing of the drum internals. In most cases, the
drum length generally depends on furnace width or in high capacity units it may be governed by
the space required for steam separating devices. For this purpose, the drum length is kept at
least 900 mmmore than the furnace width.
The thickness of the drum is designed taking into consideration, the operating parameters, the
diameter and location of the various holes on the drum.
Steam generated in a boiler is intimately mixed with relatively large and variableamounts of
circulating boiler water. Before the steam leaves the boiler drum and entersthe superheater
practically all of this associated boiler water must be separated fromthe steam.
Materials: The boiler drum is made of carbon steel plates. The material used shouldcomply with
the Indian Boiler Regulations. Comparing carbon steel and alloy steel as material for drum, the
carbon steel costs less per ton of material but the overall weightof drum will be higher because
of increased thickness. Fabrication and welding of carbon steel plates presents little difficulties,
whereas alloy steel fabrication has it'sproblems.
Drum Internals
Drum internals are used to separate water from steam and to direct the flow of water and steam
in a manner so as to obtain an optimum distribution of drum metaltemperature in boiler
operation. The drum internals may consists of bafflearrangements, devices which change the
direction of flow of steam and water mixture,separators employing spinning action for removing
water from steam or steam purifiersas washers and screen dryers. These devices are used in
conjunction with other to remove impurities fromthe steam leaving the boiler drum. Typical
arrangement ofsteam drum internals is shown in the figure 9.
The arrangement of drum normally consists of two or more integrated devices, each of which
may be quite different in design and operate on totally different principles. Eachstage must have
a high separation efficiency. The greater the number of stages ofseparation, the lower the
required efficiency for each stage. Thus, two stages at 99 percent efficiency, three stages at 90

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percent efficiency and sixstages at 70 percentefficiency will give similar results.
There is a considerable change in the densities of water and steam as the pressureincreases
towards the critical point. This relationship is shown in figure 10.

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Thus, with increase in pressure, the separation of water from steam by simple devices become
more difficult. It becomes necessary to use more efficient apparatus if primaryseparation is to
be achieved in a confined area.
Practically all drum internals aremade up of plate baffles, banks of screens,arrangements of
corrugated or bent plates and devices employing radial acceleration ofwater from steam.
Riser Tubes
A riser is a tube through which water and steam pass from an upper waterwall header to a
steam drum.
Superheater
Superheaters are usually classified according to the shape of the tube banks and theposition of
the header; also according to whether they receive heat by radiation orconvention, although in
some instance it may be a combination of both methods.
Types of superheaters : Depending on the firing method, fuel fired etc., the superheaters are
placed in the boiler flue passes, horizontally, vertically or combined.
Pendant type : The superheaters may be of pendant type, hanging from and supportedby their
headers.
Horizontal type : The superheaters may be of the horizontal type with tubes arranged across the
boiler. This type of superheater is self-draining which is an advantage duringlighting up and for
this reason they are now favoured by designers for the primarysection superheaters. The platen
is a plane surface receiving heat from both sides. The ratio of longitudinal pitching to transfer
pitching is very low for platen superheater.
Radiant superheater : Radiant superheater absorb heat by direct radiation from thefurnace and
are generally located at the top of the furnace. In some older designs, the superheater tubes
form part of the furnace wall and receive practically all the heat ofradiation.
Since the furnace temperature, and therefore the amount of available heat from radiation, does
not rise as rapidly as the rate of steam flow, a radiant superheater has afalling characteristic (as
shown in figure-11), the steam temperature drops as the steam flow rises.

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To minimum tube failure, high mass flow of steam through the unit is necessary, and this can be
achieved only at the expense of pressure drop. The temperaturecharacteristic of a straight
radiant superheater is shown.
Convection Superheater : Convection superheaters absorb heat mainly by theimpingement of
flow of hot gas around the tubes. A purely convection superheater hasa rising steam
temperature characteristic. The mass flow and temperature of gas entering the superheater
zone, as well as the steam flow from the boiler, increase with an increase in the firing rate. These
changes in temperature produce a greater average temperature difference between the gas and
steam, and this together with the higher gas mass flow, causes an increased rate of heat
absorption, resulting in an increased steam temperature. The temperature characteristic of a
straight convection superheater is shown in figure 12.

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Combined Superheater : A combination of the falling steam temperature characteristicof the
radiant superheater, together with the rising characteristic of the convectionsuperheater is
used in most of the installations for the purpose of maintaining constant steam temperature. It
has the advantages of providing a constant steam temperatureover a very wide range in load.
This illustrates the performance that may be expected
when the two types of surfaces are properly proportioned.
Location of radiant and convection superheaters are shown in figure 13.

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Desuperheater/Attemperator
Desuperheating or attemperation is the reduction or removal of superheat from steamto the
extent required.
As mentioned earlier, the characteristic performance of a superheater which receivesits heat lay
convection from gas flowing over it, is rising temperature with increasing output. To obtain some
degree of control, the superheater must be designed for full temperature at some partial load. As
a result, there will be excessive surface, withcorresponding excess temperatures at higher loads.
A desuperheaters may be used to reduce the steam temperature as shown in figure 14.
The preferred location of desuperheater, especially for temperature above 450 deg C isbetween
sections of superheater. In such installations, the steam is first passed througha primary
superheater where it is raised to some intermediate temperature. It is then passed through the
desuperheater and its temperature reduction is controlled so that, after continuing through the
secondary or final stage of the superheater, the requiredconstant conditions are maintained at
the outlet.
Desuperheaters are either non-contact or direct contact type.

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Reheaters
This is the part of the boiler which receives steam back from the turbine after it has given up
some of its heat energy in the high pressure section of the turbine. Thereheater raises the
temperature of this steam, usually to its original value, for furtherexpansion in the turbine. The
purpose of this reheating is to add energy to the partiallyused steam.
The arrangement and construction of a reheater is similar to that of a superheater. Inlarge
modern boiler plant, the reheat sections are mixed equally with superheatersections.
The reheat sections in modern boilers usually consists of pendant assemblies. These can be used
in combination with horizontal assemblies or a radiant wall located in the upper furnace.
The cold reheat is not cold to the senseof touch, but is the line from turbine to theboiler and is
at a temperature lower than the reheat line from boiler to the turbine called hot reheat steam.
Due to resistance of flow through the reheat section, the hot reheatsteam is at lower pressure
compared to the cold reheat steam.
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