1

STUDY OF HEAT EXCHANGERS

Submitted in partial fulfillment for the award of the degree

Of

BACHELOR OF TECHNOLOGY

IN

MECHANICAL ENGINEERING

Submitted By

B KIRAN KUMAR 11675A0316

P NIRESH KUMAR 11675A0321

Under the Guidance of

D.V.S.RAJU

Assoc.Professor

J B INSTITUTE OF ENGINEERING AND TECHNOLOGY

(Affiliated to Jawaharlal Nehru Technological University

Hyderabad)

HYDERABAD

April-2014

2

-

Department of Mechanical Engineering

Batch Certificate

Project Report submitted to the Department of Mechanical Engineering in

accordance with the partial fulfillment of academic requirement.

SUBMITTED BY

B KIRAN KUMAR (11675A0316)

P NIRESH KUMAR (11675A0321)

This is to certify that the Project work entitled STUDY OF HEAT

EXCHANGERS submitted by B KIRAN KUMAR and P NIRESH KUMAR of

3rd year 2nd semester in ME, J.B.Institute of Engineering & Technology,

Yenkapally, Moinabad in accordance with the partial fulfillment of academic

requirement, is a bonafied work carried out by them in the academic year 2013-

2014 at BHARAT HEAVY ELECTRICALS LIMITED,

RAMACHANDRAPURAM, HYDERABAD-32 under our guidance and

supervision. The project report has not been submitted to any other University or

other Institute.

INTERNAL GUIDE EXTERNAL GUIDE D.V.S.RAJU Shri. B.SRINIVAS, Assoc.Professor A.G.M-Heat Exchangers,

Engineering Dept.

BHEL, HYD-32.

HEAD OF THE DEPARTMENT Dr.C.UDAYA KIRAN

3

Study of Heat Exchangers in Thermal Power Plant application.

A MINI PROJECT REPORT SUBMITTED IN PARTIAL FULFILLMENT OF THE

ACADEMIC REQUIREMENT

SUBMITTED BY

B KIRAN KUMAR

J.B.Institute of Engineering & Technology

3rd YEAR

UNDER THE GUIDANCE OF

Shri. B.SRINIVAS

A.G.M. Heat Exchangers (Engg.)

BHEL, Hyderabad

4

ACKNOWLEDGEMENT

Success will be crowned to the people who made it a reality but the people whose

constant guidance and encouragement made it possible will be crowned first on the eve of

success.

This acknowledgement transcends the reality of formality when we would like to

express deep gratitude and respect to all those people behind the screen who guided, inspired

and helped us for the completion of our project work

We take it as a great privilege for having done this project under the guidance of our

internal guide. We consider ourselves lucky enough to get such a good project. This project

would add as an asset to our academic profile.

We express our profound thanks to Dr.K.V.J.RAO, Principal, J.B.Institute of

engineering and technology for their kind patronage

We should thanks Mr.D.V.S.RAJU Assoc. professor, ME and Mr. UDAYA KIRAN

Head of the department of mechanical engineering for scrupulous guidance and scholarly

encouragement science the dawn of this project work they extended their sagacious and sapient

advices.

We also thank our external guide Shri B.SRINIVAS and Shri

M. VENKATESWARLU for guiding us at BHEL, R.C.Puram, Hyderabad for giving us

valuable information and helping us in completing our project successively.

B KIRAN KUMAR

P NIRESH KUMAR

5

ABSTRACT

A heat exchanger is a device that is used to transfer thermal energy (enthalpy) between

two or more fluids, between a solid surface and a fluid, or between solid particulates and a fluid,

at different temperatures and in thermal contact. In heat exchangers, there are usually no

external heat and work interactions. Typical applications involve heating or cooling of a fluid

stream of concern and evaporation or condensation of single- or multicomponent fluid streams.

In other applications, the objective may be to recover or reject heat, or sterilize, pasteurize,

fractionate, distill, concentrate, crystallize, or control a process fluid.

In a few heat exchangers, the fluids exchanging heat are in direct contact. In most heat

exchangers, heat transfer between fluids takes place through a separating wall or into and out

of a wall in a transient manner. In many heat exchangers, the fluids are separated by a heat

transfer surface, and ideally they do not mix or leak. Such exchangers are referred to as direct

transfer type, or simply recuperators. In contrast, exchangers in which there is intermittent heat

exchange between the hot and cold fluidsvia thermal energy storage and release through the

exchanger surface or matrixare referred to as indirect transfer type, or simply regenerators.

Such exchangers usually have fluid leakage from one fluid stream to the other, due to pressure

differences and matrix rotation/valve switching. Common examples of heat exchangers are

shell-and tube exchangers, automobile radiators, condensers, evaporators, air preheaters, and

cooling towers.

6

TABLE OF CONTENTS

TOPIC Page

1. Introduction 07

2. Classification of heat exchangers: 07

According to construction

Indirect contact type

a. direct transfer type 08

Shell and tube

Plate

b. storage type 10

Direct contact type

3. Rankine cycle 13

4. Reheat cycle 14

5. Regenerative cycle 15

6. Condenser 16

Steam Jet air Ejector 23

Gland steam condenser 24

7. Feed water heaters 25

8. Deaerator 30

Spray type deaerator 34

Tray type deaerator 34

Spray cum tray type deaerator 35

9. Conclusion 36

7

Introduction

A heat exchanger is a component that allows the transfer of heat from one fluid (liquid

or gas) to another fluid. Reasons for heat transfer include the following:

1. To heat a cooler fluid by means of a hotter fluid

2. To reduce the temperature of a hot fluid by means of a cooler fluid

3. To boil a liquid by means of a hotter fluid

4. To condense a gaseous fluid by means of a cooler fluid

5. To boil a liquid while condensing a hotter gaseous fluid

Regardless of the function the heat exchanger fulfills, in order to transfer heat the fluids

involved must be at different temperatures and they must come into thermal contact. Heat can

flow only from the hotter to the cooler fluid.

Classification of Heat Exchanger

Although heat exchangers come in every shape and size imaginable, the construction of

most heat exchangers fall into one of two categories:

INDIRECT CONTACT

1. DIRECT TRANSFER TYPE

Shell and Tube

Plate

2. STORAGE TYPE

DIRECT CONTACT

8

Direct-Transfer Type Heat Exchangers

In this type, heat transfers continuously from the hot fluid to the cold fluid through a

dividing wall. Although a simultaneous flow of two (or more) fluids is required in the

exchanger, there is no direct mixing of the two (or more) fluids because each fluid flows in

separate fluid passages. In general, there are no moving parts in most such heat exchangers.

This type of exchanger is designated as a recuperative heat exchanger or simply as a recuperator.

{Some examples of direct transfer type heat exchangers are tubular, plate-type, and extended

surface exchangers. Note that the term recuperator is not commonly used in the process industry

for shell-and-tube and plate heat exchangers, although they are also considered as recuperators.

Recuperators are further sub classified as prime surface exchangers and extended-surface

exchangers. Prime surface exchangers do not employ fins or extended surfaces on any fluid

side. Plain tubular exchangers, shell-and-tube exchangers with plain tubes, and plate exchangers

are good examples of prime surface exchangers. Recuperators constitute a vast majority of all

heat exchangers.

Tube and Shell

The most basic and the most common type of heat exchanger construction is the tube

and shell, as shown in Figure 1. This type of heat exchanger consists of a set of tubes in a

container called a shell. The fluid flowing inside the tubes is called the tube side fluid and the

fluid flowing on the outside of the tubes is the shell side fluid. At the ends of the tubes, the tube

side fluid is separated from the shell side fluid by the tube sheet(s). The tubes are rolled and

press-fitted or welded into the tube sheet to provide a leak tight seal. In systems where the two

fluids are at vastly different pressures, the higher pressure fluid is typically directed through the

tubes and the lower pressure fluid is circulated on the shell side. This is due to economy, because

the heat exchanger tubes can be made to withstand higher pressures than the shell of the heat

exchanger for a much lower cost. The support plates shown on Figure 1 also act as baffles to

direct the flow of fluid within the shell back and forth across the tubes.

9

Plate

A plate type heat exchanger, as illustrated in Figure below, consists of plates instead of

tubes to separate the hot and cold fluids. The hot and cold fluids alternate between each of the

plates. Baffles direct the flow of fluid between plates. Because each of the plates has a very

large surface area, the plates provide each of the fluids with an extremely large heat transfer

area. Therefore a plate type heat exchanger, as compared to a similarly sized tube and shell heat

exchanger, is capable of transferring much more heat. This is due to the larger area the plates

provide over tubes. Due to the high heat transfer efficiency of the plates, plate type heat

exchangers are usually very small when compared to a tube and shell type heat exchanger with

the same heat transfer capacity. Plate type heat exchangers are not widely used because of the

inability to reliably seal the large gaskets between each of the plates. Because of this problem,

plate type heat exchangers have only been used in small, low pressure applications such as on

oil coolers for engines. However, new improvements in gasket design and overall heat

exchanger design have allowed some large scale applications of the plate type heat exchanger.

As older facilities are upgraded or newly designed facilities are built, large plate type heat

exchangers are replacing tube and shell heat exchangers and becoming more common.

10

Fig: Plate Heat Exchanger

Storage Type Exchangers

In a storage type exchanger, both fluids flow alternatively through the same flow

passages, and hence heat transfer is intermittent. The heat transfer surface (or flow passages) is

generally cellular in structure and is referred to as a matrix or it is a permeable (porous) solid

material, referred to as a packed bed. When hot gas flows over the heat transfer surface (through

flow passages), the thermal energy from the hot gas is stored in the matrix wall, and thus the

hot gas is being cooled during the matrix heating period. As cold gas flows through the same

passages later (i.e., during the matrix cooling period), the matrix wall gives up thermal energy,

which is absorbed by the cold fluid. Thus, heat is not transferred continuously through the wall

as in a direct-transfer type exchanger (recuperator), but the corresponding thermal energy is

alternately stored and released by the matrix wall. This storage type heat exchanger is also

referred to as a regenerative heat exchanger, or simply as a regenerator.{ To operate

continuously and within a desired temperature range, the gases, headers, or matrices are

switched periodically (i.e., rotated), so that the same passage is occupied periodically by hot

and cold gases.

Direct Contact Heat Exchangers

In a direct-contact exchanger, two fluid streams come into direct contact, exchange heat,

and are then separated. Common applications of a direct-contact exchanger involve mass

transfer in addition to heat transfer, such as in evaporative cooling and rectification; applications

involving only sensible heat transfer are rare. The enthalpy of phase change in such an

11

exchanger generally represents a significant portion of the total energy transfer. The phase

change generally enhances the heat transfer rate. Compared to indirect contact recuperators and

regenerators, in direct-contact heat exchangers, (1) very high heat transfer rates are achievable,

(2) the exchanger construction is relatively inexpensive, and (3) the fouling problem is generally

nonexistent, due to the absence of a heat transfer surface (wall) between the two fluids.

However, the applications are limited to those cases where a direct contact of two fluid streams

is permissible.

Classification according to Flow arrangement

Because heat exchangers come in so many shapes, sizes, makes, and models, they are

categorized according to common characteristics. One common characteristic that can be used

to categorize them is the direction of flow the two fluids have relative to each other. The three

categories are parallel flow, counter flow and cross flow.

Parallel flow, as illustrated in Figure 3, exists when both the tube side fluid and the shell

side fluid flow in the same direction. In this case, the two fluids enter the heat exchanger from

the same end with a large temperature difference. As the fluids transfer heat, hotter to cooler,

the temperatures of the two fluids approach each other. Note that the hottest cold-fluid

temperature is always less than the coldest hot-fluid temperature.

12

Counter flow, as illustrated in Figure 4, exists when the two fluids flow in opposite

directions. Each of the fluids enters the heat exchanger at opposite ends. Because the cooler

fluid exits the counter flow heat exchanger at the end where the hot fluid enters the heat

exchanger, the cooler fluid will approach the inlet temperature of the hot fluid. Counter flow

heat exchangers are the most efficient of the three types. In contrast to the parallel flow heat

exchanger, the counter flow heat exchanger can have the hottest cold- fluid temperature

greater than the coldest hot-fluid temperature.

Cross flow, as illustrated in Figure 5, exists when one fluid flows perpendicular to the

second fluid; that is, one fluid flows through tubes and the second fluid passes around the

tubes at 90 angle. Cross flow heat exchangers are usually found in applications where one of

the fluids changes state (2-phase flow). An example is a steam systems condenser, in which

the steam exiting the turbine enters the condenser shell side, and the cool water flowing in the

tubes absorbs the heat from the steam, condensing it into water. Large volumes of vapor may

be condensed using this type of heat exchanger flow.

13

RANKINE CYCLE

The Rankine cycle used in modern power plants has many more components, but the four

components are common to all power plants.

In this cycle, water is heated in the steam generator to produce high temperature and

pressure steam.

This steam is then expanded in a turbine to produce electricity from a generator that is

connected to the turbine.

14

The steam from the turbine is then condensed back into water in the condenser.

The pump then returns the water to the steam generator.

REHEAT CYCLE

For modern turbines the admissible dryness fraction of exhaust steam (at the turbine

exit) should be not less than x = 0.86 to 0.88. One of the ways to reduce the wetness of exhaust

steam at the turbine exit is to superheat the steam in the boiler. Superheating leads to an increase

in the thermal efficiency of the cycle realized, and at the same time, on the T-s diagram it shifts

the point corresponding to the conditions of exhaust steam to the right, into the region of greater

dryness fractions.

We have also found that with the same superheat temperature the use of high pressures

increases the cycle areas ratio and, consequently, the thermal efficiency of the cycle, but

simultaneously a higher pressure diminishes the dryness fraction of the exhaust steam and the

internal relative efficiency of the turbine.

15

REGENERATIVE CYCLE

As in gas-turbine plants, the thermal efficiency of a steam power plant is raised by means

of heat regeneration.

If a steam power plant is operated on a Rankine cycle without steam reheating and if

complete regeneration of heat is accomplished, then the thermal efficiency of this Rankine cycle

will be equal to the thermal efficiency of a Carnot cycle.

In actual steam power cycles regeneration is effected with the aid of surface-type or

direct-contact regenerative feed-water heaters, either of which is supplied with steam from

intermediate turbine stages The steam condenses in the regenerative feed-water heaters FWH 1

and FWH 2 heating the feed water which is delivered to the boiler.

16

TS DIAGRAM SHOWING COMBINED CYCLE(REHEAT AND REGENERATIVE)

CONDENSER AS HEAT EXCHANGER IN THERMAL POWER PLANT

CONDENSER

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:

Increased turbine output

Increased plant efficiency

17

Reduced steam flow (for a given plant output)

It is therefore very advantageous to operate the condenser at the lowest possible pressure

(highest vacuum).

FUNCTION OF CONDENSER

Function of the condenser is to create a vacuum by condensing steam,

Removing dissolved non condensable gases from the condensate

Conserving the condensate for re-use as the feed water supply to the steam generator

Providing a leak-tight barrier between the high grade condensate contained within the

shell and the untreated cooling water

Providing a leak-tight barrier against air ingress, preventing excess back pressure on the

turbine

Serving as a drain receptacle, receiving vapor and condensate from various other plant

heat exchangers, steam dumps, and turbine bleed-offs

Receptacle for adding DM makeup

CONDENSER

18

CONDENSER OPERATION

The main heat transfer mechanisms in a surface condenser are the condensing of

saturated steam on the outside of the tubes and the heating of the circulating water inside

the tubes.

Thus for a given circulating water flow rate, the water inlet temperature to the condenser

determines the operating pressure of the condenser. As this temperature is decreased,

the condenser pressure will also decrease. As described above, this decrease in the

pressure will increase the plant output and efficiency.

The non-condensable gases consist of mostly air that has leaked into the cycle.

These gases must be vented from the condenser.

REASON FOR REMOVING AIR/GAS

The gases will increase the operating pressure of the condenser. This rise in pressure

will decrease the turbine output and efficiency.

The gases will blanket the outer surface of the tubes. This will severely decrease the heat

transfer of the steam to the circulating water. Again, the pressure in the condenser will

increase.

The corrosiveness of the condensate in the condenser increases as the oxygen content

increases. Oxygen causes corrosion, mostly in the steam generator. Thus, these gases

must be removed in order to extend the life of cycle components.

CONDENSER TUBE MATERIALS

Copper based alloy (ASTM B 111, B543)

Stainless steel (ASTM A268, B268, A249, A213, A269)

Titanium (ASTM B 338 Gr 1&2)

Carbon steel (ASTM A179, A214)

19

CONDENSER IN POWER CYCLE

CONDENSER ASSOCIATED EQUIPMENTS

1 LP Turbine / condenser

2 Turbine / condenser expansion Actuated (manual) (electric joint: motor) (pneumatic)

(rubber) (stainless steel)

3 Provisions for feed water

4 LP Turbine extraction: Continuous tube cleaning system Pipes: (Lagging) (Supports)

(Expansion joints) Priming system

5 Air removal equipment: Air release valves (vacuum pumps) (steam jet air ejectors)

(hybrid pump / ejector system)

6 Pressure relief device: expansion provisions: (rupture disc) (atmospheric relief (spring

supported) (expansion valve) joint) (solid mounted)

7 Vacuum breaker valve

8 Instrumentation

20

9 Water box accessories: heaters located in transition Circulating water expansion joints

section: (rubber arch type) (Supports) (Closing plates) Gauge glasses (Lagging)

Cathodic protection

Effect of Air Ingress

For maximum thermal efficiency, corresponding to a minimum back pressure, a

vacuum is maintained in the condenser. However, this vacuum encourages air in- leakage.

Thus, to keep the concentration of non-condensable gases as low as possible, the

condenser system must be leak tight, together with any part of the condensate system that is

under vacuum. Failure to prevent or remove the non-condensable gases may cause serious

corrosion in the system, lower heat transfer properties, and/or increase plant heat rate due to the

back pressure rise associated with a high in leakage.

The cost of excess back pressure in terms of additional fuel or increased heat rate. An

adequate air-removal and monitoring system is essential.

SOURCES OF AIR LEAKAGE IN A CONDENSER

Atmospheric relief valves or vacuum breakers

Rupture disks

Drains that pass through the condenser

Turbine seals

Turbine/condenser expansion joint

Tube sheet to shell joints

Air-removal suction components

Instrumentation, sight glasses, etc.

Low-pressure feedwater heaters, associated piping,

Valve stems, piping flanges, orifice flanges

Manhole

Shell welds

Condensate pump seals

21

CIRCULATING WATER IN LEAKAGE

Circulating water in-leakage into the condenser has been the major source of impurities

introduced into the condensate and, thus, has been a major factor in boiler corrosion.

There are a number of possible causes of water in- leakage, including:

Use of tube materials, such as admiralty brass, that are susceptible to erosion/corrosion

Improperly rolled tube joints

Poor condenser design leading to tube failures.

Improperly supported tubes, which can lead to tube vibration failures

Tube manufacturing defects.

Water In-Leakage Detection Methods

Smoke

Thermography

Ultrasonics

Plastic wrap

Foam

Water Fill Leak Test

Rubber Stoppers

Individual Tube ressure/Vacuum Testing

Tracer Gas Method HELIUM

Cycle Isolation

Generating plants often suffer from power losses/heat rate due to leakages through

valves to condenser.

Check incoming drain lines, feedwater heater high level dumps, minimum flow valves,

and steam traps for leakage or improper operation which could add unexpected heat load

to the condenser.

22

To minimize leakages through valves to condenser , Select all control valves (e g

emergency drain of heaters) to condenser with leakage class v and Select all isolating

/drain valve to condenser with leakage class MSS SP 61.

CONDENSER DESIGN CRITERIA

The steam condensing plant shall be designed, manufactured and tested as per HEI

(latest edition).

The condenser(s) shall be designed for heat load corresponding to unit operation for

valves wide open (VWO) conditions, 3% make-up, design condenser pressure .

The value of design condenser pressure to be measured at 300 mm above the top row of

condenser tubes shall be guaranteed under VWO condition, 3% make-up, design CW

inlet temperature and CW flow. The condenser vacuum shall be measured with a

vacuum grid utilizing ASME basket tips.

The condenser hot well shall be sized for three (3) minute storage capacity (between

normal and low-low level).

Maximum oxygen content of condensate leaving the condenser shall be 0.015 cc per

litre over 50-100% load range.

CONDENSER ( 500 MW )

23

STANDARDS

ASME PTC 12.2 -Steam surface condensers

HEI - Standards for steam surface condensers

TEMA.

STEAM JET AIR EJECTOR

An air ejector or steam ejector is a device which uses the motion of moving fluid (Motive

Fluid) to transport another fluid (Suction fluid). It has a wide range of application in steam

ejector in boiler condenser, fresh water generator and in priming the centrifugal pump.

It works on the principle of convergent /divergent nozzle as it provides the venturi effect

at the point of diffusion as the tube gets narrows at the throat the velocity of the fluid increases

and because of the venturi affect it pressure decreases, vacuum will occur in the diffuser throat

where the suction line will be provided.

24

The operating medium of an air ejector can be either high-pressure gas or liquid. This is

passed through a nozzle and the pressure energy is converted into velocity energy. The high-

velocity fluid aspirates the air and the non-condensable gases and the mixture is projected into

a diffuser which reconverts the velocity energy into pressure energy.

GLAND STEAM CONDENSER

The gland steam condenser is utilized as a low pressure noncontact feed water heater with

the discharge drain flowing to the condenser via the condenser flash box. The gland condenser is fitted with a

gland condenser extraction fan to remove any air that accumulates in the top of the gland stream

condenser after the steam air mixture is separated.

The gland steam condenser is cooled by the condensate extracted from the main

condenser and so acting as a feed heater. The gland steam often shares its condenser with the

air ejector reducing the cost of having two units

A fan is fitted to induce a flow through the system without incurring a negative pressure

in the final pocket as this would allow the ingress of air. This is ensured by the fitting on valves

to the exhaust line from the glands so enabling the back pressure to be set.

25

FEEDWATER HEATERS IN THERMAL POWER PLANTS

FEED WATER HEATERS

A feed water heater is a power plant component used to pre-heat water delivered to a

steam generating boiler. In a steam power plant (usually modeled as a Rankine cycle), feed

water heaters allow the feed water to be brought up to the saturation temperature very gradually.

PURPOSE OF FEED WATER HEATERS

Feed water heaters serve three purposes in the power plant.

They provide efficiency gains in the steam cycle by increasing the initial water

temperature to the boiler, so there is less sensible heat addition which must occur in the

boiler,

They provide efficiency gains by reducing the heat rejected in the condenser, and they

minimize thermal effects in the boiler.

Steam is extracted from selected stages in the turbine to shell and tube heat exchangers

or to open feed water heaters where the steam and feed water are in direct contact.

FEED WATER HEATERS IN POWER CYCLES

26

FEED WATER HEATERS

In shell and tube or closed type feed water heaters the feed water flows through the tubes

and the extracted steam condenses on the shell side.

The condensed steam from each feed water heater drains successively to the next lower

pressure heater and is returned to the feed water through a heater drain pump or through

the condenser.

PRESSURE CLASSIFICATION

Low Pressure Heater: A heater located (with regard to feedwater flow) between the

condensate pump and either the boiler feed pump. It normally extracts steam from the low

pressure turbine.

27

High Pressure Heater: A heater located downstream of the boiler feed pump.

Typically, the tube side design pressure is at least 100 KG/CM2, and the steam source is the

high pressure turbine.

HEATERS TO POWER PLANT CYCLE

The heating process by means of extraction steam is referred to as being regenerative.

The feed water heaters are an integral portion of the power plant thermodynamic cycle.

Normally, there are multiple stages of feed water heating. Each stage corresponds to a

turbine extraction point.

The presence of the heaters in the cycle enhances the thermal efficiency of the power

plant.

ORIENTATION

Horizontal: Most heaters are of this configuration. These are the most stable in regard

to level control, although they occupy more floor space. Disassembly is by means of

either shell or bundle removal. Most are floor mounted, although some are mounted in

the condenser exhaust neck.

Vertical, Channel Down: Although these conserve floor space, the amount of control

area available for liquid level fluctuation is less. Disassembly is by shell removal.

Installation and removal may be more difficult than for horizontal heaters.

Vertical, Channel Up: These are the least frequently used. Disassembly is by means of

bundle removal. If a sub cooling zone is present, it must extend the full length of the

bundle, since the water must enter the bottom and exit at the top end of the heater.

ZONES

Condensing Zone: All feed waters have this zone. All of the steam is condensed in this

area, and any remaining non condensable gases must be removed. A large percentage of

the energy added by the heater occurs here.

Sub cooling Zone: The condensed steam enters this zone at the saturation temperature

and is cooled by convective heat transfer from the incoming feed water.

28

Desuperheating Zone: The incoming steam enters this zone, giving up most of its

superheat to the feed water exiting from the heater.

TUBE MATERIAL

Both copper alloys & non-ferrous alloys are used for the LP Heaters & HP Heaters tubes.

Copper alloys are used extensively in the LP Heaters tubes. These alloys have got

excellent thermal conductivity but on the other hand these alloys have problems of

copper carry over & ammonia attack, which may require a complex boiler cleaning after

short intervals.

To avoid all the above problems, the stainless steel tubes are invariably used for LP

Heaters. Stainless steel is unaffected at all operating conditions, except that, it is

susceptible to chloride induced stress corrosion.

Most common materials used for HP Heaters are carbon steel, stainless steel & Monel

metal.

FEEDWATER HEATER EFFICIENCY

Two variables are used to monitor a feed water heaters efficiency.

The heater Terminal Temperature Difference or TTD is a measure of how close the

outlet feed water temperature is to the feed water heater saturation temperature.

The heater Drain Cooler Approach or DCA is a measure of how close the heater drain

outlet temperature is to the feed water inlet temperature.

29

ITEMS AFFECT PERFORMANCE OF FEEDWATER HEATERS

Improper heater level can cause flashing in the drain cooler section and tube damage.

Check operation of automatic controls and level instrumentation.

Check for possible tube leaks in feed water heater.

Vent valves may not be set up properly.

Improper extraction line pressure drops.

Possible problem with extraction line check valve.

Tube fouling due to corrosion affects the heat transfer in the heater.

Clean tube bundles x Continuous vent orifice plugging.

Channel pass partition/gasket leak.

Efficiency Improvement as an Emissions Control Strategy

Feed water heaters are designed into the turbine cycle to improve efficiency (lower heat

rate).

As the heat rate decreases (heat rate improves), the amount of fuel for the same

generation also goes down. Of course with less fuel burned, emissions are lowered.

Safety Issue during Tube Leaks

As per HEI (14), section 6.1.2 (b), a feed water heater shell-side safety valve shall be

sized to pass the flow from a clean break in one tube resulting in the flow from two

tubes.

If a heater is not isolated promptly when a tube leak occurs, more tubes may be damaged

resulting in a higher flow into the shell than the safety can relieve. Obviously as the

heater becomes more susceptible to tube leaks, the risk of this scenario increases.

30

HEAT BALANCE SHEET

DEAERATOR

DEAERATOR PRINCIPLE

The removal of dissolved gases from boiler feed water is an essential process in a steam

system. The presence of dissolved oxygen in feed water causes rapid localized corrosion in

boiler tubes. Carbon dioxide will dissolve in water, resulting in low pH levels and the production

of corrosive carbonic acid. Low pH levels in feed water causes severe acid attack throughout

the boiler system. While dissolved gases and low pH levels in the feed water can be controlled

or removed by the addition of chemicals, it is more economical and thermally efficient to

remove these gases mechanically. This mechanical process is known as deaeration and will

increase the life of a steam system dramatically.

31

Deaeration is based on two scientific principles. The first principle can be described by

Henry's Law. Henry's Law asserts that gas solubility in a solution decreases as the gas partial

pressure above the solution decreases. The second scientific principle that governs deaeration

is the relationship between gas solubility and temperature. Easily explained, gas solubility in a

solution decreases as the temperature of the solution rises and approaches saturation

temperature. A deaerator utilizes both of these natural processes to remove dissolved oxygen,

carbon dioxide, and other non-condensable gases from boiler feed water. The feed water is

sprayed in thin films into a steam atmosphere allowing it to become quickly heated to saturation.

Spraying feed water in thin films increases the surface area of the liquid in contact with the

steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations. This

process reduces the solubility of all dissolved gases and removes it from the feed water.

The liberated gases are then vented from the deaerator.With these principles in mind,

Sterling Deaerator Company employs a two-stage system of heating and deaerating feed water.

This system reduces dissolved oxygen concentration to less than 0.005 cc/liter (7 ppb), and

completely eliminates the carbon dioxide concentration.

32

Reason to Deaerate

The most common source of corrosion in boiler systems is dissolved gas: oxygen,

carbon dioxide and ammonia. Of these, oxygen is the most aggressive. The importance of

eliminating oxygen as a source of pitting and iron deposition cannot be over-emphasized. Even

small concentrations of this gas can cause serious corrosion problems.

Makeup water introduces appreciable amounts of oxygen into the system. Oxygen can

also enter the feed water system from the condensate return system. Possible return line sources

are direct air-leakage on the suction side of pumps, systems under vacuum, the breathing action

of closed condensate receiving tanks, open condensate receiving tanks and leakage of non-

deaerated water used for condensate pump seal and/or quench water. With all of these sources,

good housekeeping is an essential part of the preventive program.

One of the most serious aspects of oxygen corrosion is that it occurs as pitting. This type

of corrosion can produce failures even though only a relatively small amount of metal has been

lost and the overall corrosion rate is relatively low. The degree of oxygen attack depends on the

concentration of dissolved oxygen, the pH and the temperature of the water.

The influence of temperature on the corrosivity of dissolved oxygen is particularly

important in closed heaters and economizers where the water temperature increases rapidly.

Elevated temperature in itself does not cause corrosion. Small concentrations of oxygen at

elevated temperatures do cause severe problems. This temperature rise provides the driving

force that accelerates the reaction so that even small quantities of dissolved oxygen can cause

serious corrosion.

Operation

Mechanical deaeration is the first step in eliminating oxygen and other corrosive gases

from the feed water. Free carbon dioxide is also removed by deaeration, while combined carbon

dioxide is released with the steam in the boiler and subsequently dissolves in the condensate.

This can cause additional corrosion problems.

Because dissolved oxygen is a constant threat to boiler tube integrity, this discussion on

the deaerator will be aimed at reducing the oxygen content of the feed water. The two major

types of deaerators are the tray type and the spray type. In both cases, the major portion of gas

removal is accomplished by spraying cold makeup water into a steam environment.

33

Tray-Type Deaerating Heaters

Tray-type deaerating heaters release dissolved gases in the incoming water by reducing

it to a fine spray as it cascades over several rows of trays. The steam that makes intimate contact

with the water droplets then scrubs the dissolved gases by its counter-current flow. The steam

heats the water to within 3-5 F of the steam saturation temperature and it should remove all

but the very last traces of oxygen. The deaerated water then falls to the storage space below,

where a steam blanket protects it from recontamination. Nozzles and trays should be inspected

regularly to insure that they are free of deposits and are in their proper position

Spray-Type Deaerating Heaters

Spray-type deaerating heaters work on the same general philosophy as the tray-type, but

differ in their operation. Spring-loaded nozzles located in the top of the unit spray the water into

a steam atmosphere that heats it. Simply stated, the steam heats the water, and at the elevated

temperature the solubility of oxygen is extremely low and most of the dissolved gases are

removed from the system by venting. The spray will reduce the dissolved oxygen content to 20-

50 ppb, while the scrubber or trays further reduce the oxygen content to approximately 7 ppb

or less.

During normal operation, the vent valve must be open to maintain a continuous plume

of vented vapors and steam at least 18 inches long. If this valve is throttled too much, air and

34

non-condensable gases will accumulate in the deaerator. This is known as air blanketing and

can be remedied by increasing the vent rate.

For optimum oxygen removal, the water in the storage section must be heated to within

5 F of the temperature of the steam at saturation conditions. From inlet to outlet, the water is

deaerated in less than 10 seconds.

The storage section is usually designed to hold enough water for 10 minutes of boiler

operation at full load.

Spray Cum Tray type Deaerator

Deaeration is based on two scientific principles.

The first principle can be described by Henry's Law. Henry's Law asserts that gas solubility

in a solution decreases as the gas partial pressure above the solution decreases.

The second scientific principle that governs deaeration is the relationship between gas

solubility and temperature. Easily explained, gas solubility in a solution decreases to almost

zero as the temperature of the solution rises and approaches saturation temperature.

A deaerator utilizes both of these natural processes to remove dissolved oxygen, carbon

dioxide, and other non-condensable gases from boiler feed water. The feed water is sprayed in

thin films into a steam atmosphere allowing it to become quickly heated to saturation

temperature.

Spraying feed water in thin films increases the surface area of the liquid in contact with the

steam, which, in turn, provides more rapid oxygen removal and lower gas concentrations. This

process reduces the solubility of all dissolved gases and removes it from the feed water. The

liberated gases are then vented from the deaerator.

35

It is of spray and trays type deaerator consist of a feed water storage tank and vapour

tank with vent condenser.

Water is sprayed from the top of vapour tank through ten spray nozzles on set of multi-

level perforated trays below it for easy and complete scrubbing with steam.

Steam is fed in storage tank through distribution header below the water level. Partial

scrubbing of steam with water takes place by raising the water temperature (Since

solubility of gases like oxygen carbon dioxide decreases with increase in water

temperature).

36

CONCLUSION

Study was conducted at BHEL, Hyderabad on Heat Exchangers used in thermal power

plant application and the modification done in the rankine cycle by regenerative and reheat

process is studied for the better performance and to increase the efficiency and output of the

power plant.