ABSTRACT

In the present world scenario everyone is aware of the importance and

economical aspects of energy conservation. Global energy production has

increased 52% over the past two decades. Oil remains the principle source of

commercial energy production. But its share has dropped from 48% to 42% since

1970, coal ranks second, gas third and primary electricity fourth. Hence increasing

the efficiency of energy service is the only way to tackle the energy crisis, not less

energy services but less energy for same service through better technology. There

is tremendous scope for energy saving of various sectors like industry, which

accounts 50% of energy use.

By keeping the attention on the present scenario, we conduct detailed study

of boilers and we are able to suggest some methods to increase the efficiency of the

boiler and hence make is more economical.

INTRODUCTION:

In the present world scenario, the consumption of energy is increasing. The

requirement of energy is increasing as each year passes. But the production is not

increased to meet the requirement. We rely a lot on fossil fuels for the production

of different kinds of energy. This leads environmental pollution. Also the rate of

fossil fuels is rising up. So energy conservation is of greater importance both in

economical and environmental means.

Among the fossil fuels, oil is the principle source of commercial energy.

Industrialized countries are responsible for 53% of growth in global energy

consumption in the past 20 years even though they accounts for only 15% of world

population. The energy demand is expected to be triple before 2025. Developing

countries will experience significant increase in the regional pollutants as

hydrocarbons, carbon monoxide and sulphur dioxide. The electrical energy

demanded in the country has been rising the annual rate of 9% where as the

generating capacity has been increased by at a rate of only 6% per annum with the

increasing demand made as power utilities, the quality of power distribution is

determining is deteriorating and has led to over widening gas supply and demand.

To conserve energy in industries, optimum utilization of energy is to be

ensured. So the equipments should be designed efficiently and efficiency

improving methods to be implemented on existing equipments. The boilers find

most application in various industries. Mostly used in steam power plants as in

plants requiring process heating. In boiler with oil fired burners, if we can reduce

the oil, considering the cost of fuel, the total cost of production can be reduced.

BRIEF PROFILE OF ORGANIZATION:

Kerala Minerals and Metals Ltd is an integrated titanium dioxide

manufacturing public sector undertaking in Kerala, India. Its operations comprise

mining, mineral separation, synthetic rutile and pigment-production plants. Apart

from producing rutile-grade titanium dioxide pigment for various types of

industries, it also produces other products like ilmenite, rutile, zircon, sillimanite,

synthetic rutile etc.

The company manufactures titanium dioxide through the chloride route. The

different grades are produced by KMML under the brand name KEMOX. KMML

has always been responsive to social and environmental causes. Some of the

initiatives taken by KMML have made a significant change to the area and its

people.

TiO2 pigment plant of KMML at Sankaramangalam has been divided into

two various small plants and auxillary plants to aid the working of main plants. We

will look at into the working of plant in brief.

KMML is having 2 separate units like mineral separation plant and TiO2

pigment plant. The raw material for the company is ileminate rich black sand

available in and around the coastal areas of Chavara. The raw material lluminate

consists of 53.83% of TiO2.

PIGMENT PRODUCTION PLANT:

Titanium dioxide is the most widely used white pigment because of its

brightness and very high refractive index, in which it is surpassed only by a few

other materials. Approximately 4.6 million tons of pigmentary TiO2 are consumed

annually worldwide, and this number is expected to increase as consumption

continues to rise. When deposited as a thin film, its refractive index and colour

make it an excellent reflective optical coating for dielectric mirrors and

some gemstones like "mystic fire topaz". TiO2 is also an effective opacifier in

powder form, where it is employed as a pigment to provide whiteness and opacity

to products such as paints, coatings, plastics, papers, inks, foods, medicines (i.e.

pills and tablets) as well as most toothpaste. In paint, it is often referred to

offhandedly as "the perfect white", "the whitest white", or other similar terms.

Opacity is improved by optimal sizing of the titanium dioxide particles.

ILMENITE BENEFICIATION PLANT:

Ilmenite is a weakly magnetic titanium-iron oxide mineral which is iron-

black or steel-gray. It is a crystalline iron titanium oxide (FeTiO3). It crystallizes in

the trigonal system. The ilmenite crystal structure is an ordered derivative of

the corundum structure; in corundum all cations are identical but in ilmenite

Fe2+ and Ti4+ ions occupy alternating layers perpendicular to the trigonal c axis.

The Ilmenite Beneficiation Plant is designed and installed based on the BCA

Cyclic Process Technology supplied by M/s. Benilite Corporation of America. The

Plant is in a single stream and is subdivided into six major sections, based on

operation:

Raw Material and Reductant handling

Roasting and Cooling 

Digestion and Filtration

Calcination and Cooling

Acidic Liquor Treatment 

Tank Farm

BOILERS:

A boiler is a closed vessel in which water or other fluid is heated. The heated

or vaporized fluid exits the boiler for use in various processes or heating

applications, including boiler-based power generation, cooking, and sanitation.

The pressure vessel in a boiler is usually made of steel (or alloy steel), or

historically of wrought iron. Stainless steel is virtually prohibited (by the ASME

Boiler Code) for use in wetted parts of modern boilers, but is used often in

superheater sections that will not be exposed to liquid boiler water. However

electrically-heated stainless steel shell boilers are allowed under the European

"Pressure Equipment Directive" for production of steam for sterilizers and

disinfectors.

In live steam models, copper or brass is often used because it is more easily

fabricated in smaller size boilers. Historically, copper was often used for fireboxes,

because of its better formability and higher thermal conductivity; however, in more

recent times, the high price of copper often makes this an uneconomic choice and

cheaper substitutes.

Cast iron may be used for the heating vessel of domestic water heaters.

Although such heaters are usually termed "boilers" in some countries, their purpose

is usually to produce hot water, not steam, and so they run at low pressure and try

to avoid actual boiling. The brittleness of cast iron makes it impractical for high

pressure steam boilers.

The source of heat for a boiler is combustion of any of several fuels, such as

wood, coal, oil, or natural gas. Electric steam boilers use resistance- or immersion-

type heating elements.

TYPES OF BOILER:

Fire Tube Boiler

In fire tube boiler, hot gases pass through the tubes and boiler feed water in

the shell side is converted into steam. Fire tube boilers are generally used for

relatively small steam capacities and low to medium steam pressures. As a

guideline, fire tube boilers are competitive for steam rates up to 12,000 kg/hour

and pressures up to 18 kg/cm2. Fire tube boilers are available for operation with

oil, gas or solid fuels. For economic reasons, most fire tube boilers are nowadays

of “packaged” construction (i.e. manufacturers shop erected) for all fuels.

Water Tube Boiler

In water tube boiler, boiler feed water flows through the tubes and enters the

boiler drum. The circulated water is heated by the combustion gases and converted

into steam at the vapour space in the drum. These boilers are selected when the

steam demand as well as steam pressure requirements are high as in the case of

process cum power boiler / power boilers.

Most modern water boiler tube designs are within the capacity range 4,500 –

120,000 kg/hour of steam, at very high pressures. Many water tube boilers

nowadays are of “packaged” construction if oil and /or gas are to be used as fuel.

Solid fuel fired water tube designs are available but packaged designs are less

common.

The features of water tube boilers are:

Forced, induced and balanced draft provisions help to improve combustion

efficiency.Less tolerance for water quality calls for water treatment plant. Higher

thermal efficiency shifts are possible.

Packaged Boiler

The packaged boiler is so called because it comes as a complete package.

Once delivered to site, it requires only the steam, water pipe work, fuel supply and

electrical connections to be made for it to become operational. Package boilers are

generally of shell type with fire tube design so as to achieve high heat transfer rates

by both radiation and convection.

The features of package boilers are:

Small combustion space and high heat release rate resulting in faster

evaporation. Large number of small diameter tubes leading to good convective

heat transfer. Forced or induced draft systems resulting in good combustion

efficiency. Number of passes resulting in better overall heat transfer. Higher

thermal efficiency levels compared with other boilers. These boilers are classified

based on the number of passes - the number of times the hot combustion gases pass

through the boiler. The combustion chamber is taken, as the first pass after which

there may be one, two or three sets of fire-tubes. The most common boiler of this

class is a three-pass unit with two sets of fire-tubes and with the exhaust gases

exiting through the rear of the boiler.

SUPERHEATED STEAM BOILERS:

Most boilers produce steam to be used at saturation temperature; that is,

saturated steam. Superheated steam boilers vaporize the water and then further heat

the steam in a super heater. This provides steam at much higher temperature, but

can decrease the overall thermal efficiency of the steam generating plant because

the higher steam temperature requires a higher flue gas exhaust temperature. There

are several ways to circumvent this problem, typically by providing an

economizer that heats the feed water, a combustion air heater in the hot flue gas.

There are advantages to superheated steam that may, and often will, increase

overall efficiency of both steam generation and its utilisation: gains in input

temperature to a turbine should outweigh any cost in additional boiler complication

and expense. There may also be practical limitations in using wet steam, as

entrained condensation droplets will damage turbine blades. Superheated steam

presents unique safety concerns because, if any system component fails and allows

steam to escape, the high pressure and temperature can cause serious,

instantaneous harm to anyone in its path. Since the escaping steam will initially be

completely superheated vapor, detection can be difficult, although the intense heat

and sound from such a leak clearly indicates its presence. These boilers are

selected when the steam demand as well as steam pressure requirements are high

as in the case of process cum power boiler / power boilers.

 Improve steam boiler efficiency:

With the rising cost of fuel prices, industries that use steam boilers for

heating or power generation are hard pressed to operate at peak efficiencies. While

steam consumption, leakages, and other heat transmission losses can contribute to

the overall energy bill, this article focuses on the heart of the steam generator - the

boiler. Controlling the boiler is of utmost importance in any steam generation

energy saving program. Below are some ways to improve boiler efficiencies:

Reducing excess air

Installing economizer

Reducing scale and deposits

Reducing blow down

Recovering waste heat from blow down

Stopping dynamic operation

Reducing boiler pressure

Operating at peak efficiency

Preheating combustion air

Switching from steam to air atomization

Switching to lower cost fuel

Reducing boiler pressure

Preheating combustion air

Reducing scale and deposits

Operating at peak efficiency

Reducing Excess Air:

The most common reason for energy inefficiencies in a boiler can be

attributed to the use of excess air during combustion at the burners. When there is

more air than is required for combustion, the extra air becomes heated up and is

finally discharged out to the atmosphere. However, there are reasons for putting in

some extra air for combustion - to compensate for imperfect burner fuel-air mixing

conditions, air density changes, control system "slop", burner maintenance, fuel

composition and viscosity variation, and imperfect atomizing steam or air controls

for burners.

Installing Economizer:

The economizer tubes may contain either circulating boiler water or

circulating feed water. Because the temperature of the exhaust gases can be quite

high, the economizer tubes may be fitted with safety valves to avoid over-pressure

damage. Also temperature control of feed water is required to prevent pump

airlock. To avoid corrosion, careful design is needed to ensure that the exhaust flue

gas temperature does not drop below the dew point.

Reducing Scale and Deposits:

The safety of the boiler is at stake. Any scale or deposits will lead to reduced

heat transfer that will eventually lead to overheating, reduction of mechanical

strength of the steel and finally to bursting.

Reducing Blow down:

Blow down of boiler water is discharging hot water into the drains.

However, blow down is necessary to maintain the boiler water concentration of

dissolved solids that are necessary for conditioning the boiler water. The dissolved

solids are necessary for preventing boiler corrosion and scaling. As steam is

generated from the evaporation of water; the remaining water in the boiler

becomes more and more concentrated. This must be drained away during blow

down.

Recovering Waste Heat from Blow down:

Since it is necessary to blow down to control the total dissolved solids in the

boiler water, methods can be adopted to recover back some of the heat from the

drained hot water. Blow down tanks, heat exchanger tubes and pumping

arrangements can be fabricated to recover some of the heat back into the boiler.

Reducing Boiler Pressure:

By reducing the boiler pressure, some of the heat losses through leakages or

transmission may be reduced slightly. However there can be problems with the

boiler with reduced pressure. The boiler circulation may be upset and the steam

lines may have insufficient capacity and flow to transport the low pressure steam.

Operating at Peak Efficiency:

When operating two or more boilers, improved efficiency can sometimes be

obtained by unequal sharing of the load so that the combined load operates at peak

efficiency.

Preheating Combustion Air:

Any heat loss from the skin of the boiler to the boiler room can be utilized

back for combustion. By preheating the intake air the combustion in the furnace

becomes more efficient.

Switching from Steam to Air Atomization:

For burners with steam atomization, switching to air atomization will

naturally result in less steam consumption overall and better boiler efficiencies.

This is only applicable for heavy fuel oil burners.

Modern boiler design benefits:

Modern boiler design offers several benefits. In the past, improper design of

boilers has caused explosions which led to loss of life and property. Modern

designs attempt to avoid such mishaps. Further, mathematical modeling can

determine how much space a boiler will need and the type of materials to be used.

When the design specifications of a boiler are determined, design engineers can

estimate a cost and time schedule for the construction.

Boiler design may be based upon:

Production of a maximum quantity of steam with minimal fuel consumption

Economic feasibility of installation

Minimal operator attention required during operation

Capability for quick starting

Conformity to safety regulations

Quality of raw water: how hard or soft the water is will determine the

material of the boiler.

Heat source - the fuel to be burned and its ash properties or the process

material from which the heat is to be recovered.

Capacity / steam output required, usually measured in tonnes per hour or

kg/sec.

Steam condition - pressure, temperature, etc.

Safety considerations

Mechanical constraints

Cost restrictions

Monetary cost

Tensile strength of material must be considered while using any joining

processes.

Feed check valve - regulates the flow of water into the boiler and prevents

the back flow of water in case of failure of the feed pump.

Steam stop valve - regulates the flow of steam that is produced in the boiler

to the steam pipe, and may also be used to stop the supply of steam from the

boiler.

Furnace:

The furnace transfers heat to the living space of the building through an

intermediary distribution system. If the distribution is through hot water (or other

fluid) or through steam, then the furnace is more commonly called a boiler. One

advantage of a boiler is that the furnace can provide hot water for bathing and

washing dishes, rather than requiring a separate water heater. One disadvantage to

this type of application is when the boiler breaks down, neither heating nor

domestic hot water are available.

Burner:

The burner in the vertical, cylindrical furnace as above, is located in the

floor and fires upward. Some furnaces have side fired burners, such as in

train locomotives. The burner tile is made of high temperature refractory and is

where the flame is contained. Air registers located below the burner and at the

outlet of the air blower are devices with movable flaps or vanes that control the

shape and pattern of the flame, whether it spreads out or even swirls around.

Flames should not spread out too much, as this will cause flame impingement.

Air registers can be classified as primary, secondary and if applicable,

tertiary, depending on when their air is introduced. The primary air register

supplies primary air, which is the first to be introduced in the burner. Secondary air

is added to supplement primary air.

Feed Pump:

A boiler feed water pump is a specific type of pump used to pump feed

water into a steam boiler. The water may be freshly supplied or returning

condensate produced as a result of the condensation of the steam produced by the

boiler. These pumps are normally high pressure units that take suction from a

condensate return system and can be of the centrifugal pump type or positive

displacement type.

Super heater:

A super heater is a device used to convert saturated steam or wet steam

into dry steam used in steam engines or in processes, such as steam reforming.

There are three types of super heaters namely:

Radiant

Convection

Separately fired

Steam Engine:

In a steam engine, the super heater re-heats the steam generated by

the boiler, increasing its thermal energy and decreasing the likelihood that it

will condense inside the engine. Super heaters increase the thermal efficiency of

the steam engine, and have been widely adopted. Steam which has been

superheated is logically known as superheated steam; non-superheated steam is

called saturated steam or wet steam.

Air Preheater:

An air preheater (APH) is a general term to describe any device designed to

heat air before another process (for example, combustion in a boiler) with the

primary objective of increasing the thermal efficiency of the process. They may be

used alone or to replace a recuperative heat system or to replace a steam coil. In

particular, this article describes the combustion air preheaters used in

large boilers found in thermal power stations producing electric power from

e.g. fossil fuels, biomasses or waste.

BOILER MOUNTINGS:

Different fittings and devices necessary for the operation and safety of a

boiler are called boiler mountings. The various boiler mountings are:

1. Water level indicator

2. Pressure gauge

3. Steam stop valve

4. Feed check valve

5. Blow-down cock

6. Fusible plug

7. Safety valve: spring loaded, dead weight, lever type.

Water Level Indicator:

The function of the water level indicator is to indicate the level of water in

the boiler constantly. Every boiler is normally fitted with two water level indicators

at its front end. It consists of three cocks and a glass tube. The steam cock I keeps

the glass tube in connection with the steam space and cock 2 puts the glass tube in

convection with the water space in the boiler. The drain cock 3 is used to drain out

the water from the glass tube at intervals to ascertain that the steam and water

cocks are clear in operation. The glass tube is generally protected with a shield.

Pressure Gauge:

The pressure gauge is used to indicate the steam pressure of the boiler. The

gauge is normally mounted in the front top of the steam drum. It consists of an

elastic metallic tube of elliptical cross-section bent in the form of circular arc. One

end of the tube is fixed and connected to the steam of the boiler and other end is

convected to a sector wheel through a link. The section remains in mesh with a

pinion fixed on a spindle. A pointer is attached to the spindle to read the pressure

on a dial gauge.

When high pressure steam enters the elliptical tube, the tube section tends to

become circular which causes the other end of the tube to move outward. The

movement of the closed end of the tube is transmitted and magnified by the link.

Steam Stop Valve:

The function of the stop valve is to regulate the flow of steam from the

boiler to the prime mover as per requirement and shut off the steam flow when not

required. It consists of main body, valve, valve seat, but and spindle, which passes

through a gland to prevent leakage of steam. The spindle is rotated by means of a

hand wheel to close or open.

Feed Check Valve:

The function of the feed check valve is to allow the supply of water to the

boiler at high pressure continuously and to prevent the hack flow of water from the

boiler when the pump pressure is less than boiler pressure. It is fitted to the shell

slightly below the normal water level of the boiler.

The lift of the non-return valve is regulated by the end position of the

spindle which is attached with the hand wheel. The spindle can be moved up or

down with the help of hand wheel which is screwed to the spindle by a nut. Under

normal conditions, the non-return valve is lifted due to the water pressure from the

pump and water is fed to the boiler. In case pump pressure falls below boiler

pressure, valve is closed automatically or when pumps tails.

Blow- Down Cock:

The function of blow-down cock is to remove sludge or sediments collected

at the bottom-most point in the water space in a boiler, while the boiler is steaming.

It is also used for complete draining of the boiler. It consists of a conical plug fitted

accurately into a similar casing. The plug has a rectangular opening which may be

brought with the line of the passage of the casing by rotating the plug. This

causes the water to be discharged from the boiler.

Fusible Plug:

The main function of the fusible plug is to put off the fire in the furnace of

the boiler when the water level in the boiler falls below an unsafe level and thus

avoid the explosion which may occur due to overheating of the tubes and shell.

The plug is generally fitted over the crown of the furnace or over the combustion

chamber.

It consists of a hollow gun metal body screwed into the fire box crown. The

body has hexagonal flange to lighten it into the shell. A gun metal plug is screwed

into the gun metal body by tightening the hexagonal flange formed into it. There is

yet another solid plug made of copper with conical top and rounded bottom.

Fusible metal holds this conical copper plug and the gun metal plug together due to

depressions provided at the mating surfaces.

Safety Valves:

The function of a safety valve is to prevent the steam pressure in the boiler

exceeding the 4esired rated pressure by automatically opening and discharging

steam to atmosphere all .Le pressure falls back to normal rated value. There are

three types of safety valves spring loaded (Rams bottom) type, dead weight type,

and lever type.

Spring Loaded Safety Valve:

The spring holds the two valves on their seats by pulling the lever down.

The lever is provided with two conical pivots, one integrally forged with the level

and the other pin connected to one end. The upper end of the spring is looked to

the lever midway between the two pivots. The lower end is hooked to the shackle

fixed to the valve chest by studs and nuts. The shackle and the lever are also

connected by two links, one end of which is pin- jointed and the other end has a

slot cut into it to allow for the pin to slide in it vertically thus allowing the lever to

be lifted.

SPECIFICATION OF THE BOILER PLANT IN KMML:

Boiler

ISGEC John Thompson two drum water tube boiler

Reg No: K464 & 465

Spreader stroke type coal and furnace oil fired. Total heating surface= 171.5 m2

Maximum working pressure= 28 kgf/cm2

A minimum flow of 4.5 T/hr super heated steam is compulsory.

Casing thickness= 5 mm

Tube outside diameter= 50.8 mm

Tube thickness= 3.251 mm

Code used= IBR 1950 & its latest amendments

Tube pitching: parallel to gas tow= 102 mm

Furnace:

Manufacturer: ISGEC John Thompson

Type: Water cooled, radiant

Combustion chamber volume= 172 m2

Tube pitching= 152 mm for front and rear walls, 90 mm for side walls.

Drum & headers Steam drum

ISGEC John Thompson

Mud drum

Internal diameter (in mm) 1219 991

Thickness (in mm) 36 32

Length (in m) 4.81 4.45

DEAERATOR:

A deaerator is a device that is widely used for the removal of oxygen and

other dissolved gases from the feed water to steam-generating boilers. In particular,

dissolved oxygen in boiler feed waters will cause serious corrosion damage in

steam systems by attaching to the walls of metal piping and other metallic

equipment and forming oxides (rust). Dissolved carbon dioxide combines with

water to form carbonic acid that causes further corrosion. Most deaerators are

designed to remove oxygen down to levels of 7 ppb by weight (0.005 cm³/L) or

less as well as essentially eliminating carbon dioxide.

The deaerators in the steam generating systems of most thermal power

plants use low pressure steam obtained from an extraction point in their steam

turbine system. However, the steam generators in many large industrial facilities

such as petroleum refineries may use whatever low-pressure steam is available.

STEAM ACCUMULATOR:

A Steam accumulator is an insulated steel pressure tank containing hot water

and steam under pressure. It is a type of energy storage device. It can be used to

smooth out peaks and troughs in demand for steam. The tank is about half-filled

with cold water and steam is blown in from a boiler via a perforated pipe near the

bottom of the drum. Some of the steam condenses and heats the water. The

remainder fills the space above the water level.

When the accumulator is fully charged the condensed steam will have raised

the water level in the drum to about three-quarters full and the temperature and

pressure will also have risen. The pressure in the drum will fall but the reduced

pressure causes more water to boil and the accumulator can go on supplying steam

for some time before it has to be re-charged.

EFFICIENCY IMPROVEMENT:

The temperature of the flue gas after economizer is 200oC which is quite

high. So an air preheater is installed in the plant can trap some amount of this heat

loss.

HEAT EXCHANGER AS AIR PREHEATER:

Heat Exchanger is equipment designed for the effective transfer of heat

energy between two fluids, a hot fluid & a coolant. The purpose may be either to

remove heat from a fluid or to give heat to a fluid. This project deals with the

design of a shell and tube heat exchanger.

REGENERATIVE HEAT EXCHANGER:

A regenerative heat exchanger, or more commonly a regenerator, is a type

of heat exchanger where the flow through the heat exchanger is cyclical and

periodically changes direction to alternately heat a thermal storage medium, such

as firebrick, then use the stored heat to heat another fluid, such as combustion air.

In thermal regenerator operation the hot fluid passes through the channels of

the packing for a length of time called the "hot period," at the end of which, the hot

fluid is switched off. A reversal now takes place when the cold fluid is admitted

into the channels of the packing, initially driving out any hot fluid still resident in

these channels, thereby purging the regenerator.

Fixed Bed Regenerators:

The most obvious technique for realizing "apparent" continuous operation is

to use two or more regenerators operating out of phase with respect to one another

so that while one regenerator is supplying heated fluid, the other regenerators is

storing heat from the heating fluid. An apparently easy way to do this is by

enclosing the set of regenerators within a system of ducts or pipes fitted with

valves to facilitate the switching of the regenerators at the end of a period of

operation.

As one set of valves close, at a reversal, so another set open: the flow of hot

gas, for example, is diverted from one regenerator to the other by the closing of

such a set of valves and the opening of the other. Simultaneously, the flow of cold

gas is switched from the other regenerator in a symmetric fashion. Such an

arrangement is called a system of fixed bed regenerators, in contrast to

the rotary regenerator which will be described shortly.

Rotary Regenerators

In the rotary regenerator, the porous packing is rotated around an axis. In its

simplest form, the packing is divided into two gas tight sections and the hot and

cold gases flow simultaneously in a direction parallel to this axis, usually in contra-

flow, through these different segments of the packing. As the packing rotates

through the hot gas stream, it stores heat, as in the hot period of a fixed bed

regenerator. This thermal energy is literally transported into the cold gas stream as

the packing is rotated. Once in the other gas stream, the heat is regenerated and is

passed to the cold gas, as in the cold period of operation of a fixed bed system.

SHELL AND TUBE HEAT EXCHANGER:

A shell and tube heat exchanger is a class of heat exchanger designs. It is the

most common type of heat exchanger in oil refineries and other large chemical

processes, and is suited for higher-pressure applications. As its name implies, this

type of heat exchanger consists of a shell (a large pressure vessel) with a bundle of

tubes inside it. One fluid runs through the tubes, and another fluid flows over the

tubes (through the shell) to transfer heat between the two fluids. The set of tubes is

called a tube bundle, and may be composed by several types of tubes: plain,

longitudinally finned, etc

Theory and Application:

Two fluids, of different starting temperatures, flow through the heat

exchanger. One flows through the tubes (the tube side) and the other flows outside

the tubes but inside the shell (the shell side). Heat is transferred from one fluid to

the other through the tube walls, either from tube side to shell side or vice versa.

The fluids can be either liquids or gases on either the shell or the tube side. In order

to transfer heat efficiently, a large heat transfer area should be used, leading to the

use of many tubes. In this way, waste heat can be put to use. This is an efficient

way to conserve energy.

Heat exchangers with only one phase (liquid or gas) on each side can be

called one- phase or single-phase heat exchangers. Two- phase heat exchangers

can be used to heat a liquid to boil it into a gas (vapour), sometimes called boilers,

or cool a vapour to condense it into a liquid (called condensers), with the phase

change usually occurring on the shell side. Boilers in steam engine locomotives are

typically large, usually cylindrically- shaped shell- and-tube heat exchangers. In

large power plants with steam- driven turbines, shell- and- tube surface condensers

are used to condense the exhaust steam exiting the turbine into condensate water

which is recycled back to be turned into steam in the steam generator.

When the rotor first passes from the hot gas to the cold gas stream, for

example, a body of hot gas in the voids of the regenerator packing, is carried by

rotation into the cold gas stream and must be purged from the regenerator, as in the

fixed bed mode of operation. In some applications, it is vital that this carryover gas

should not be permitted to contaminate the stream of cold gas being heated by the

exchanger.

Shell and tube heat exchanger design:

There can be many variations on the shell and tube design. Typically, the

ends of each tube are connected to plenums (sometimes called water boxes)

through holes in tube sheets. The tubes may be straight or bent in the shape of a U,

called U-tubes. In nuclear power plants called pressurized water reactors, large

heat exchangers called steam generators are two-phase, shell-and-tube heat

exchangers which typically have U-tubes. They are used to boil water recycled

from a surface condenser into steam to drive a turbine to produce power. Most

shell-and-tube heat exchangers are 1, 2, or 4 pass designs on the tube side. This

refers to the number of times the fluid in the tubes passes through the fluid in the

shell. In a single pass heat exchanger, the fluid goes in one end of each tube and

out the other.

Straight tube heat exchanger:

Surface condensers in power plants are often 1-pass straight-tube heat

exchangers. Two and four pass designs are common because the fluid can enter

and exit on the same side. This makes construction much simpler. There are

often baffles directing flow through the shell side so the fluid does not take a short

cut through the shell side leaving ineffective low flow volumes. These are

generally attached to the tube bundle rather than the shell in order that the bundle is

still removable for maintenance.

Counter current heat exchangers are most efficient because they allow the

highest log mean temperature difference between the hot and cold streams. Many

companies however do not use single pass heat exchangers because they can break

easily in addition to being more expensive to build. Often multiple heat exchangers

can be used to simulate the counter current flow of a single large exchanger.

The operation of regenerators at low (ambient or even lower) temperatures

permits a good deal of flexibility in the choice of packing materials. Rotary

regenerators for air conditioning applications employ a variety of packings which

include a polyethylene terephthalate film and corrugated, knitted wire mesh. Such

packings are wound round the spindle of the rotor yielding heat wheels of varying

diameters; from 1.25 to 2.5 m. corrugated aluminium sheets are sometimes used as

various honeycomb arrangements.

Selection of tube material:

To be able to transfer heat well, the tube material should have good thermal

conductivity. Because heat is transferred from a hot to a cold side through the

tubes, there is a temperature difference through the width of the tubes. Because of

the tendency of the tube material to thermally expand differently at various

temperatures, thermal stresses occur during operation. This is in addition to

any stress from high pressures from the fluids themselves.

The tube material also should be compatible with both the shell and tube

side fluids for long periods under the operating conditions (temperatures,

pressures, pH, etc.) to minimize deterioration such as corrosion. All of these

requirements call for careful selection of strong, thermally-conductive, corrosion

resistant, high quality tube materials, typically metals, including copper alloy,

stainless steel, carbon steel, non-ferrous copper alloy, Inconel, nickel, Hastelloy

and titanium. Poor choice of tube material could result in a leak through a tube

between the shell and tube sides causing fluid cross-contamination and possibly

loss of pressure.

Even higher area to volume ratios can be achieved by constructing the

regenerator of an assembly of sector shaped sections of a knitted mesh of wire of

another material, depending on the temperature and other operating conditions. For

hot gas entry temperatures of 400°C, stainless steel mesh can be employed while

for temperatures of up to 800°C ceramic or alumina fibers have been considered.

Other prefabricated heavy duty ceramic packings can be employed in regenerators

required to withstand hot gas entry temperatures of 800°C or more.

BAFFLES:

Baffles are flow-directing or obstructing vanes or panels used in some

industrial process vessels (tanks), such as shell and tube heat exchangers, chemical

reactors, and static mixers. Baffles are an integral part of the shell and tube heat

exchanger design. A baffle is designed to support tube bundles and direct the flow

of fluids for maximum efficiency.

The main roles of a baffle in a shell and tube heat exchanger are to:

Hold tubes in position (preventing sagging), both in production and

operation.

Prevent the effects of vibration, which is increased with both fluid velocity

and the length of the exchanger.

Direct shell-side fluid flow along tube field. This increases fluid velocity and

the effective heat transfer co-efficient of the exchanger.

In a static mixer, baffles are used to promote mixing. In a chemical reactor,

baffles are often attached to the interior walls to promote mixing and thus

increase heat transfer and possibly chemical reaction rates.

Types of baffles:

Implementation of baffles is decided on the basis of size, cost and their ability

to lend support to the tube bundles and direct

Longitudinal Flow Baffles (used in a two-pass shell)

Impingement Baffles (used for protecting bundle when entrance velocity is

high)

Orifice Baffles.

single segmental

double segmental

Installation of baffles:

Baffles deal with the concern of support and fluid direction in heat

exchangers. In this way it is vital that they are spaced correctly at installation. The

minimum baffle spacing is the greater of 50.8mm or one fifth of the inner shell

diameter. The maximum baffle spacing is dependent on material and size of tubes.

The Tubular Exchanger Manufacturers Association sets out guidelines. There are

also segments with a "no tubes in window" design that affects the acceptable

spacing within the design. An important design consideration is that no

recirculation zones or dead spots form – both of which are counterproductive to

effective heat transfer.

Calculation:

Quality of super heated steam= 7 T/hr

Quality of saturated steam= 16 T/hr

Quality of oil flow= 1.7 T/hr

Super Heated Temperature= 310 oC

By using steam table,

Pressure= 20 bar

hf = 908.5 kJ/kg

hfg = 1888.7 kJ/kg

x = 0.95

From steam table,

Feed water temperature at inlet= 47 oC

hfi = 196.8 kJ/kg

Enthalpy of saturated steam

hsat= hf + xhfg - hfi

hsat = 908.5 + (0.95 * 1888.5) – 196.8

hsat = 2505.965 kJ/kg

From steam table,

Pressure = 19 bar

Tsh = 310 oC

Tsat = 209.8 oC

hg = 3098.4 kJ/kg

msh = 2.5 kg/s

Cp = 2.1 kJ/kg

Calorific Value,

GCv = 10500 Kcal/kg

GCv = 4.2 * 10500

GCv = 44100 kJ/kg

Enthalpy of superheated steam

hsh = hg + (msh * Cp * (Tsh - Tsat)) – (hf + hfg)

hsh = 3098.4 + (2.5 * 2.1(310 – 209.8)) – (908.5 + 1888.5)

hsh = 827.45 kJ/kg

Efficiency of the Boiler

(m1 + m2) hsat + m2 * hsh _______________________________________________________________________________________

(Quality of oil/ hr * GCv)

(18 + 8) 2505.965 + 8 (827.45) _____________________________________________________________________________________________________

(2.3 * 44100)

ɳ = 70.7 %

*100ɳ =

ɳ = *100

PROBLEM SPECIFICATION:

The efficiency of the boiler is low when compared to the modern high

pressure boilers. By designing the air preheated we hope to increase the efficiency.

The air preheated transfers the heat from flue gases to the input air thereby;

increasing the efficiency. Thus daily fuel (furnace oil) consumption can also be

lowered.

DESIGN PROCEDURE:

The steps taken in the design process of a shell & tube heat exchanger are

1. Problem specification

2. Obtain the necessary thermo physical properties of the hot & cold fluid

streams.

3. Perform the energy balance and calculate heat transfer rate.

4. Select the tentative number of shell & tube passes and calculate LMTD.

5. Calculate the tube diameter, tube length and number of tubes.

6. Select the tube pitch and calculate the pitch distance.

7. Calculate the surface area required.

8. Calculate the flow area and linear velocity; adjust the number of tubes

according to the linear velocity. Adjust the pipe diameter.

9. Estimate the tube side and shell side heat transfer coefficients and select

proper baffle spacing.

10.Calculate the overall heat transfer coefficient (Uo) by selecting the dirt factor

(Ra). Calculate the area based on this value.

DESIGN OF AIR PREHEATER:

We want to raise the atmospheric temperature of air at 30oC to a temperature

of 80oC. So for calculation purpose, take the properties of air at 50oC.

Properties of air:

Inlet temperature = 30oC

Outlet temperature = 80oC

Density = 1.093 kg/m3

Viscosity = 19.61x10-6 Ns/m2

Specific Heat = 1005 J/kgK

Prandtl number = 0.968

Thermal conductivity = 0.0283 W/mK

Mass flow rate = 50000 kg/hr

The inlet temperature of flue gas is 200oC. So the properties of flue gas at

175oC are taken.

Propertiess of flue gas:

Inlet temperature = 190oC

Density = 0.7625 kg/m3

Viscosity = 25.65x10-6 Ns/m2

Specific Heat = 1024 J/kgK

Prandtl number = 0.681

Thermal conductivity = 0.03855 W/mK

Mass flow rate = 53000 kg/hr

Heat Transfer, (Qair) = 50000 x 1.005(80+30) = 2512500 KJ/hr

TUBE SIDE CALCULATION:

ENERGY BALANCED EQUATION:

Heat rejected from flue gas = Heat absorbed by air

i.e) Qflue gas = Qair

5300 x 1.085 x (200/Toutlet) = 2512500

Toutlet = 156oC

Select shell & tube pass:

A variety of materials including steel, copper, muntz metal, brass etc are

used for tubes. Here we used steel as our material for construction.

Logarithmic Mean Temperature Difference (LMTD):

Counter flow heat exchanger,

LMTD = (T1-T2)/ln(T1/T2)

T1 = Th1 – Tc2

T2 = Th2 – Tc1

Th1 = 200oC

Th2 = 156oC

Tc1 = 30oC

Tc2 = 80oC

Counter flow heat exchanger,

LMTD = (120 – 126)/ln(120/126)

LMTD = 123oC

Area required,

A = Q/(U x LMTD )

Overall heat transfer coefficient, Uo = 50W/m2K

Qair = 2512500 KJ/hr

1 Kcal = 4.186 kJ

A = 2512500/(50 x 4.186 x 122.9)

A = 97.98 m2

Type of pitch selection & length

For an easy cleaning purpose, we choose a square pitch. The length of the tube is

selected as 5m initially.

Outside diameter, do = 2.88 inch

Inner diameter, di = 2.469 inch

Calculation of pitch distance:

Usually pitch is selected as either 1.25 times the outer diameter (or) outer

diameter/ 6mm whichever is greater. Here the greater one is 1.25 times the outer

diameter.

Pitch, P = 1.25 x 2.88

P = 3.6 inch

Calculation of surface area of tube:

Surface area of tube,

a = pdl

a = 3.6 x 0.0254 x 5 (1 inch = 0.0254m)

a = 1.15m2

Number of tubes required,

N = A/a

N = 97.68/1.15

N = 84.93 (say 85 tubes)

Flow area,

At = N(p/4) x dl2

At = 85(3.6/4) x (2.469 x 0.0254)2

At = 0.262m2

Linear Velocity,

Linear Velocity = flow rate of air/ flow area

= mass flow rate/(density x flow area)

= 50000/(1.093 x 3600 x 0.262)

= 48.5m/s

For air, linear velocity range between 15 to 34m/s, so we increasing the number of

tubes to 100

Flow area, At = 100 x (3.6/4) (2.469 x 0.02584)2

At = 0.308m2

Linear Velocity,

Linear Velocity = flow rate of air/ flow area

= mass flow rate/(density x flow area)

= 50000/(1.093 x 3600 x 0.308)

= 41.25m/s

This is also high. So increasing the pipe diameter is necessary. Now we have

to select a new nominal pipe size. Outer diameter (do) & inner diameter (di) of tube

corresponding to the nominal size from table below,

DIMENSIONS OF STEEL PIPE:Nominal

pipe size,

IPS

(inch)

OD

(inch)

Schedule

No.

ID

(inch)

Flow area

per pipe

(inch)

Surface

per in ft,

ft2/ft.

Outside

Inside Weight

per in ft,

Ib steel

1/8 0.405 40*

80+

0.296

0.215

0.058

0.036

0.106 0.070

0.056

0.25

0.32

¼ 0.540 40*

80+

0.364

0.302

0.134

0.72

0.141 0.095

0.079

0.43

0.54

3/8 0.675 40*

80+

0.493

0.423

0.192

0.141

0.177 0.129

0.111

0.57

0.74

½ 0.840 40*

80+

0.622

0.546

0.304

0.235

0.220 0.163

0.143

0.85

1.48

¾ 1.05 40*

80+

0.824

0.742

0.534

0.432

0.275 0.216

0.194

1.13

1.48

1 1.32 40*

80+

1.049

0.957

0.864

0.718

0.344 0.274

0.250

1.68

2.17

1½ 1.90 40*

80+

1.610

1.500

2.04

1.76

0.498 0.422

0.393

2.72

3.64

2 2.38 40*

80+

2.067

1.939

3.35

2.95

0.622 0.542

0.508

3.66

5.03

2½ 2.88 40*

80+

2.469

2.323

4.79

4.23

0.753 0.647

0.609

5.80

7.67

3 3.50 40*

80+

3.068

2.900

7.38

6.61

0.917 0.804

0.760

7.58

10.3

4 4.50 40*

80+

4.026

2.900

12.7

11.5

1.178 1.055

1.002

5.80

7.67

Nominal pipe size = 3 inch

Outer diameter, do = 3.5 inch

Inner diameter, di = 3.06 inch

Flow area,

At = 100(3/4) x (3.06 x 0.0254)2

At = 0.4744m2

Linear Velocity,

Linear Velocity = flow rate of air/ flow area

= mass flow rate/(density x flow area)

= 50000/(1.093 x 3600 x 0.4744)

= 26.7m/s

New pitch (p) = 1.25 x 3.5

p = 4.375 inch

Reynolds number,

Re = (p x v x d)/ μ

Re = (1.093 x 26.7 x 0.077)/19.61 x 10-6

Re = 114589.42

Prndtl number,

Pr = 0.698

Nusselt number,

Nu = 0.023 x Re0.8 x Pr0.4

Nu = 222.12

Also Nusselt number,

Nu = (ht x di)/k

ht = (Nu x k)/di

ht = (222.12 x 0.02823)/(3.06 x 0.0254)

ht = 80.67 W/m2K

SHELL SIDE CALCULATION:

Calculation for inside diameter:

We have to enclose 10 tubes with square pitch 4.375 inch inside the shell

Inner diameter,

ds = 1.75 x do(No. of tubes)0.47

ds = 1.75 x 3.5(100)0.47

ds = 1.5m

Flow area,

As = (C x B x ds)/p

Pitch,

p = 4.375 – 3.5

p = 0.11

Clearance,

C = pitch(p) – Outer dia. of tube(do)

C = 0.11 – 3.5

C = 0.022m

Selecting 25% cue segmental baffles with spacing 0.5m

ds = 1.5m

As = (0.022 x 0.5 x 1.5)/0.11

As = 0.1513m2

Equivalent diameter for shell side

de = [4(p2 – (do2/4))]/do

de = [4(0.122 – (0.08892/4))]/0.0889

de = 0.0844m

Reynolds number,

Re = [(de x mf)/As]/ μ

Re = [(0.844 x 53000)/0.1513]/(22.43 x 10-6 x 3600)

Re = 36.61 x 105

Nusselt number,

Nu = 0.023 x Re0.8 x Pr0.3

Nu = 0.023 x 36614.370.8 x 0.680.3

Nu = 578.63

Also,

Nu = hs x de/k

hs = 578.63 x 0.0357/0.0844

hs = 244.75 W/m2K

Calculation of overall heat transfer coefficient:

1/Uo = 1/hf + Rfo +[(ro/ri) x Rfi] + (ro/rihi)

1/Uo = 1/244.75 + 0.022 + [(3.5/3.06) x 0.033] + [3.5/(3.06 x 80.67)]

1/Uo = 0.0235

Uo = 42.52 W/m2K

This is less than the assumed overall heat transfer coefficient. So it is acceptable.

Area required on the basis of Uo

Area,

A = Q/(Uo x LMTD)

A = (2512500 x 103)/(42.52 x 122.9 x 3600)

A = 139 m2

Area is almost same. So the design is acceptable.

No. of times the fluid crosses the bundle = N + 1

Number of baffles (N),

N + 1 = tube length/baffle space

N + 1 = 5/0.5

N + 1 = 1

N = 9

New Efficiency (ɳ):

Heat recovered by exchanger (Qair) = 2512500 KJ/hr

Total heat utilized = [htotal(KJ/kg) x steam produced/hour (kg/hr)]+Qair

Total heat utilized = (2642.9 x 14.5 x 103) + 2512500

Total heat utilized = 40834.55 x 103 KJ/hr

Efficiency,

ɳ = [(Total steam x htotal)/Fuel consumption x Cv] x 100

ɳ = [Total heat utilized/(Fuel consumption x Cv)] x 100

ɳ = [40834.55 x 103/(1.16 x 103 x 104004.2)] x 100

ɳ = 80.6%

If the required air preheater is installed, efficiency is found to be increased by

7.19% at present condition,

Quantity of fuel used = 1.16 T/hr

Quantity of fuel used = 1.16 x 24

Quantity of fuel used = 27.84 T/day

Let ‘X’ be the quantity of fuel used at increased efficiency.

Then,

80.6/75.6 = 27.84/X

X = 26.11 T/day

Quantity of furnace oil saved = 27.84 – 26.11

Quantity of furnace oil saved = 1.73 T/day

Price/Ton of furnace oil = Rs. 19500/-

Money Saved/day = 1.89 x 19500

Money Saved/day = Rs. 36855/-

PAYBACK PERIOD

Cost of air pre heater = Rs. 20 lakhs

Installation, transportation & other cost = Rs. 5 lakhs

Payback period = 2500000/36855

Payback period = 68 days

i.e. the amount will be back within a period of about 2 & half months.

EFFICIENCY TIPS:

The objective of a boiler is to burn the hydrogen contained in the fuel with

oxygen from the atmosphere to produce heat. Combustion efficiency analysers

exploit the fact that by knowing the fuel (and its chemical composition) and

measuring the flue gas temperature and either the oxygen or carbon dioxide level

the efficiency of the boiler can be calculated. On some boilers the settings can then

be adjusted to maximize the efficiency.

In a perfect world the maximum efficiency would be achieved with 0%

oxygen in the flue and the lowest flue gas temperature. In the real world allowance

must be made for variations and uncertainties and so 0% oxygen is not practical.

The settings on a boiler must allow for differences in fuel composition,

atmospheric pressure, wind direction, boiler demands etc. If the oxygen level is set

too low and something changes the combustion process can become ‘fuel rich’ as

there is insufficient oxygen for all the fuel to burn. This can cause high levels of

CO to be generated and in the extreme enough fuel to enter the boilers flue and

ignite (explode) outside the combustion chamber.

Typically for a natural gas boiler oxygen readings may be in the range 3% to

5%, for an oil boiler 5% to 8% and for a coal fired boiler 8% to 10%.The

efficiency of modern condensing gas boiler can be over 100% as heat is extracted

from the incoming air. A traditional brick built coal fired boiler may only be 50%

efficient. The difference between the value of Net combustion efficiency and the

value of Gross combustion efficiency for a natural gas fuelled boilers is around 8%

with the net value being higher than the gross value

Combustion Efficiency v Boiler Efficiency:

A combustion or flue gas analyser is used to measure the efficiency of the

combustion process within a boiler. This is not the same as the boiler efficiency as

it does not take account of, for example, the heat losses from the case of the boiler.

So generally the efficiency stated on the rating plate of the boiler will always be

lower than the measured efficiency of combustion.

Condensing Boilers:

Some modern boilers are now described as being condensing boilers. The

combustion efficiency calculation must be modified to properly reflect the

efficiency of the combustion process within these boilers. The practical difference

is that a condensing boiler utilises an additional heat exchanger just before it

exhausts the flue gases. This extracts additional heat from the flue gas and further

reduces energy losses. Under certain circumstances this can lead to net condensing

combustion efficiencies of greater than 100%.

Typically for a natural gas fuelled boiler the temperature of the flue gas

being exhausted needs to be less than 50oC for the condensation process to recover

additional energy. Above 50oC the normal Net combustion efficiency calculation

operates. Therefore it is usual to adjust the combustion process so that a level of

excess air is present to give a margin for safety. This level is set to account for any

likely process variable, e.g. the variability of the fuel supply, changes in

atmospheric pressure, changes in wind direction etc.

Excess air:

There is a theoretical amount of fresh air that when mixed with a fixed

amount of fuel and burnt will result in perfect combustion. In this situation all of

the fuel will have been properly burnt and all of the oxygen in the air will have

been consumed. In this circumstance there will be no excess air and combustion

efficiency will be maximized. In the real world perfect combustion is not possible.

The theoretical amount of fresh air would provide insufficient oxygen for complete

combustion and some of the carbon in the fuel would be converted into carbon

monoxide rather than carbon dioxide. A lack of air can lead to dangerous levels of

carbon monoxide being formed and smoke being produced.

Alternative options:

Although a brand new boiler can be a dramatic way of reducing the fuel bills

quickly and over a relatively short period the savings will pay back the initial

investment, there are a number of other alternative changes for heating set up that

will save money.

CONCLUSION:

The use of steam boilers in modern industries is indispensable. At the

beginning of this project we calculated the efficiency of the boiler and found it to

be low when compared to other modern high pressure boilers. But the installation

of such boilers is very high compared to the efficiency improvement methods of

existing boiler. Hence from the various efficiency improvement methods, we

suggest that the efficient utilization of steam (without wastage) could increase the

efficiency of present boiler. By improving steam efficiency we may be able to save

furnace oil, there- by contributing towards the energy conservation.