Classification & Types of Boiler

Classification & types of boilers

CLASSIFICATION & TYPES OF BOILERS

OBJECTIVE

Upon completion of this lesson, students should be able to:

Know the need for steam on a motor vessel. Know the principles involved in heat transfer and in generating steam in a boiler. Know the various ways the boilers can be classified. Know the basic constructional features of some of the popular boilers used.

LESSON OVERVIEW

This is a introductory lecture and explains the use of steam and application of various types of boilers for use on a motor vessel. Some of the popular boiler constructional design features are shown and their individual uniqueness discussed.

REFERENCES

Morton, Thomas D, Steam Engineering Knowledge for Marine Engineers (1994), Thomas Reed Publications

Flanagan, G T H, Marine Boilers Question & Answers (1995)

Operation Manuals, Aalborg Boilers

Operation Manuals, Miura Boilers

Milton, J H and Leach, Roy M, Marine Steam Boilers (1995)

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Classification & types of boilers

INTRODUCTION

The diesel engine vessels require some steam for auxiliary use. It is mainly for heating duties on conventional cargo vessels and for cargo handling in case of tanker vessels where use of electrical equipment close to the oil carrying tanks is considered dangerous. So, an oil fired auxiliary boiler is normally fitted on all motor ships. Furthermore, the main diesel engine exhaust, instead of being released to the atmosphere, has been made use of to produce steam, using an exhaust gas economiser. Steam is thus available at sea or in port, using these boilers. A small steam turbine generator is also fitted on many ships catering for the electrical needs at sea. A steam distribution system connecting oil fired auxiliary boiler with exhaust gas economiser with an open feed water circuit is now found on majority of ships.

Use of Steam on Motor vessel

Heating duties: ME Fuel oil heater, Purifier heater, Oil tank heating, Cargo heating, Air conditioning & heating plant, Calorifier, Galley supply, sea-chests, tracer lines for pipeline heating etc.

Run Turbine Generators

Run Cargo pump turbines in Tankers

Drive steam driven deck machinery like winches etc.

Operate bilge, stripping and other steam driven pumps

Drive boiler feed pump turbines

Evaporator/Fresh water generator heating media

Tank washing in tanker ships and general cleaning

For boiler Soot blowing and for the steam atomised burners

Fire fighting as used in steam smothering system

Main engine Jacket F.W. preheater and Lub oil sump and drain tanks

Use in the waste oil, incinerator, slop tanks

Use as a steam ejector media for ejector pumps and

vacuum devices

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Classification & types of boilers

Principle of heat transfer in boilers

A boiler is a closed pressure vessel in which a fluid is heated for use external to itself by the direct application of heat resulting from the combustion of fuel or by the use of electricity.

In a fuel fired boiler, chemical energy in the fuel is converted into heat in the furnace and the function of the boiler is to transfer this heat to the water in the most efficient manner. A boiler is designed to absorb the maximum amount of heat released in the process of combustion. This heat is transmitted to the water in the boiler by radiation, conduction and convection.

"Radiant" heat is heat radiated from a hot medium, i.e. the flame, to a cold body, i.e. the boiler tube, and it depends on the difference in absolute temperature to the fourth power and the colour of the body which receives heat.

"Conduction" heat is heat which passes from the hot medium, i.e. the boiler gas, to cold body, i.e. the tube by physical contact. The heat then passes from molecule to molecule of the metal with no displacement of molecules and it depends on the temperature difference across the tube thickness and the tube material.

"Convection" heat is heat transmitted from the hot medium, i.e. the boiler gas, to the cold body, i.e. the boiler tube, by movement of the hot medium and it depends on the heat transfer coefficient and the temperature difference.

In the boiler furnace, the radiant heat waves are transmitted from the hot flame and gases, to the boiler heating surfaces such as the plates and tubes. On striking these surfaces, most of the radiant heat is absorbed and passed by conduction to the water inside the tube. Convection current in the water itself complete the transfer of the heat from the burning fuel to the water and steam. Hence all three forms of heat transmission occur simultaneously. The amount and arrangement of heating surfaces

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ConvectionConduction

Radiation

Classification & types of boilers

determine the size, shape and form of the boiler. The gases can be made to flow through flues or tubes surrounded by water, hence the name fire-tube boiler, or they can be arranged to flow over the tubes with water circulating through them, hence the name water-tube boiler.

Fire-tube boilers usually consists of a shell with tubes fitted inside it, while water-tube boilers consists of a drum with tubes fitted on the outside and these tubes are also connected to a mud-drum or headers. The furnace of a fire-tube boiler can either be inside or outside the shell, while the furnace of a water-tube boiler must always be outside the drum.

Principle of Steam Generation with Natural Circulation

To remove heat from the boiler surfaces, it is necessary that adequate and positive water and steam circulation be provided (in a predetermined direction) throughout the boiler circuits. The flow of water, steam, or other fluid within the boiler is called circulation.

When heated, water decreases in density and tends to rise to the top of the vessel; conversely, cooler water tends to drop to the bottom. When water is heated to the boiling point, small steam bubbles form on the heated surface. These bubbles cling to the metal (because of surface tension) until they become large enough to overcome the tension or until they are swept away by water circulation.

Steam is much lighter than water and rises rapidly. At the surface the steam bubbles burst, releasing the steam. The movement of steam through the water creates

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Water tube conceptDrum with fire tubes

Fire tube conceptShell with fire tubes insideTank type boiler

Classification & types of boilers

turbulence and circulation. Water in a large container, such as a fire-tube boiler, has a relatively sluggish movement until circulation is established. Staggering of the tubes and proper arrangement of gas flowing through the tubes, - cause greater water turbulence and circulation. Circulation in a water-tube boiler is fast, beginning as soon as the heat is sufficient to create steam in the tubes, because the steam bubbles cause turbulence and agitation within the confines of the tubes. The action of steam and water rising in a hot tube is sufficient to cause water to flow down in another tube that is subjected to less heat.

Proper circulation is a necessity. The rate of steam release is directly related to the circulation rate of the water. Sluggish or stagnant circulation permits large bubbles to form, resulting in blistered or burned-out tubes.

The point of entry of the feedwater marks the end point as well as the beginning of boiler water circulation. The relatively cool water must return to the boiler at a point that will not interfere with natural circulation.

BOILER TYPES

Main BoilerPropulsion of the vessel

Auxiliary BoilerAids the propulsion in some way; e.g., heating of heavy fuel oil using a steam heater, necessary for propulsion would qualify the supplying boiler to be referred to as an auxiliary boiler.

Tank BoilerA boiler with large water carrying capacity where the shell is being used as the pressure vessel. Most low pressure auxiliary boilers will come into this category.

Exhaust Gas BoilerBoiler operated by hot gas from engine or other exhaust gas sources.

Drum Type BoilerWater tube boilers employing steam and water drums. They are also known as bent tube type boilers.

Package BoilerFully automatic, low capacity boilers packaged inside a box type casing, capable of quick steam production and flexible in being positioned anywhere; could be coil type or fire tube type.

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Classification & types of boilers

SUNROD™ CHS is an oil- or gas-fired water tube boiler with a thermal output upto 30 MW at 18 bar (g) and an operating temperature up to 200°C. The boiler ismanufactured in mild steel and its pin tube elements provide an extended heatingsurface.

The SUNROD™ CHS boiler for hot water has a unique pin tube extended surface. The vertical cylindrical body consists of a furnace and a convection part integrated with the drum. The furnace consists of a gas tight membrane wall. A number of downcomer tubes connecting the drum to the ring header ensuregood natural circulation at all loads through the furnace tubes. The boiler is side-fired which makes the boiler very easy to ventilate and gives an operation and maintenance friendly location of the burner and its controls. The incoming system water is mixed into the boiler drum by means of a spreader with nozzles. The hotwater generation is practically instant from start. The response from load variation is very fast.

The boiler can be inspected on the water side through the manhole in the pressure vessel and through the hand holes in the ring header. On the gas side, the boiler can be inspected from the flue gas box at the top of the boiler and from the furnace. The standard cleaning method is by water washing, which efficiently restores the original boiler performance data. After cleaning, it is possible to

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Classification & types of boilers

make a visual inspection of 100% of all the heating surfaces, a very important advantage when trying to reach highest possible efficiency during the boiler's entire lifetime.

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Classification & types of boilers

Classification of boilersClassification For Steam ships For Motor ships AdditionalCriteria InformationCapacity High Capacity Low capacity For Motor Tankers

100, 000 kg/hr and 1,000 -10,000 20,000 - 80,000over kg/hr kg/hr

Pressure High Pressure Low Pressure: Med. Pressures60 bars and above 6-15 bars used on motor

Medium Pressure tanker vessels17-30 bars

Shape Drum Type, Cylindrical, Package, TubularD-type Vertical, Tanktype Coil type

Usage Main Propulsion Auxiliary Boiler; Assist PropulsionBoiler Donkey boiler Hotel Purpose

Type of fuel Heavy fuel oil, Light Diesel, Coal, Electricity,used Gas Heavy fuel, Gas Exhaust GasWorking Water tube Firetube, Steam raisingPrinciple Watertube methodCirculation Natural Natural, Tank or Drum;Type Forced Exh Economiser,

Coil typeMedium Low Low Pressure: Low Pressure:

ExhaustPressure: pressure: Tank type gasWater tube, PackageCoil Oil Forced circulationDrum Type type, Firetube fired/Composite tubular

typeTanker All types of All types of All types of vesselsVessels vessels vesselsBabcock & Cochran Aalborg Aalborg AVWilcox Chieftain AQ3,AQ5,Mil, M11M AQ9,AQ12Foster Steambloc Sunrod Sunrod PL,PTWheeler D4 CPH,CPDBCombustion Stone-Vapour OsakaEngineering Howden-JohnsonKawasaki Clayton Hitachi Zosen HVHitachi Miura VWS Spanner SwirlyfloMitsubishi CochranMAC

Mitsubishi

Advantages of the water tube boiler over the smoke tube design

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Classification & types of boilers

Carnot EfficiencyWith the demand for higher efficiency, steam temperatures have steadily increased. That resulted in higher pressure operation using water tube boilers. To obtain maximum efficiency from the steam machinery, the steam temperature at inlet T1 should be maximum and the steam temperature at the outlet T2 is minimum where T1 and T2 are absolute temperatures, following which approximates to what happens in a heat engine.

Carnot's Eff iciency % = (T1 - T2 ) / (T1) x 100For increased Carnot efficiency. The pressure and temperature of steam should be high. The diameter of the steam and water drum should be small to limit the stress.Water tube boiler can satisfy these conditions.Efficient heat exchangeOne of the most important factor that dominates the design of boilers is the 'efficient heat exchange'. Thickness of the material reduces its conductivity but it is an important factor in limiting stresses in the material.Conduction formulae- Heat conducted = Difference in temp x area x time / thickness of material

From the above, it can be seen that the heat transfer is inversely related to the thickness of metal.

Limiting the stressFor the maximum allowable stress, as pressure increases, the diameter must decrease in order to keep the thickness within reasonable limits (to achieve better conduction of heat and to minimise thermal stresses).Thin Shell formulae- Longitudinal seam stress = Pressure x diameter / 2 x thickness of wall

Therefore, for the boiler to be an efficient heat exchanger, it should consist of a large number of small diameter tubes. The use of small diameter tubes enables high pressures to be raised. Thin tube walls, together with the greater heat exchange surface give better heat transfer - thus allowing high evaporation rates. Four tubes of 50 mm diameter have about the same cross-sectional area as one tube of 100 mm diameter. However, the four tubes have twice as much surface area per unit length.

In addition, the thin walled tubes are easier to manufacture, to bend, and to expand and bell-mouth into the drums and headers. The relatively smaller amounts of water in compared to the shell boilers also reduces the overall weight of the drum type boiler.

The high heat transfer across the small diameter tubes demands rapid and positive circulation of water through the tubes. Boilers can be designed to give sufficient natural circulation by sloping the tubes by at least 15 degrees from the horizontal, and by using unheated down comers to supply water to the lower part of the boiler. Between the fire row (screen) and generating tubes, a superheater assembly may be fitted. This consists of U-bend tube elements connected to inlet and outlet headers and placed at right angles to the screen and generating tubes. Refractory material is used on the furnace floor and front burner wall. The hot gases, which are products of combustion pass over the screen, superheater and generating tube banks. After leaving these tubes, the gases finally escape into the atmosphere through the uptake (funnel). The combustion

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Classification & types of boilers

air passes through ducting arranged in the double casing of the boiler, and then passes through air registers into the furnace.

All the generating tubes in the boiler are expanded and bell-mouthed; other tubes are normally welded into drums and headers.

D-Type Water Tube Boiler

D-type boilers are built to produce steam up to 60 bar pressure and temperature up to 510 C with evaporation rate up to 52,000 kg/hour.

The function of various components in this boiler is explained below:

1. Water Drum: Main function of the Water Drum' is to distribute the water entering it from the down comers, to the screen and generating tubes and in the same cases to the water wall headers. It also provides a space for the accumulation of suspended solids that may be precipitated from the boiler water. The blow down connection enables to remove these as required.

2. Steam Drum: Provide a reservoir for the water and a space for the separation and collection of steam. In natural circulation boilers the motive power to provide a positive circulation is achieved by the difference in density of water at different temperatures. Steam drum receives the incoming feed water and provides for distribution to water drum and headers through down comers.

3. Headers: These perform a similar duty to that of the drums, but are of smaller cross-sections, cylindrical or rectangular. Access for expanding the tubes into the headers is arranged by having numerous access plugs or doors in the headers opposite to the tube holes. Headers are constructed and tested in similar way as drums.

4. Tubes: These provide almost all the heating surface in the boiler and are manufactured to suit the requirements. Size, material and attachment to headers and drums depend upon the location, temperatures the tubes may be subjected to during operation and the fluid (water or steam) within the tube.

The small bore tubes demand pure feed condition as the thin metal thickness gives little allowance for corrosion, and care must be taken to reduce this to a minimum on both, gas and water sides of the boiler.

a) Screen tubes (Water screen): These are placed adjacent to the furnace and receive heat from the flames together with the heat from the hot gases; therefore they need a relatively large diameter to keep the ratio of steam to water low enough to prevent overheating. The main function of the screen tubes is to prevent the superheater from direct radiant heat of the furnace flame.

b) Generating tubes: These consists of numerous small diameter tubes placed in the main flow of hot gases so forming a large heat exchange surface. For a given rate of water circulation the minimum allowable tube diameter is limited, as below a certain

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Classification & types of boilers

value the ratio steam to water becomes excessive and may lead to possible overheating.

c) Waterwall tubes: These form the furnace boundary collecting heat from the furnace and a water cooled furnace needs less refractory to resist heat leakage.

In modern boilers, membrane walled construction may be used. Here, the tubes are welded together along their length by fins or strips, and this eliminates the need of refractory backing, improves efficiency and reduces boiler size.

d) Down Comer tubes: These consist of large diameter (100mm), unheated tubes placed outside the gas stream. Water from the steam drum runs down to the water drum and headers, through these tubes. Down comers replace the water rising up through the water walls and generating tubes. The water rises up through water wall and generating tubes as its density decreases upon receiving heat from the hot gases, thus providing the positive natural circulation of water.

e) Superheater tubes: These consist of small diameter tubes placed in the main

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Furnace

Screen &Generating Tubes

Water drum

Fluegases

AirHeater

Forceddraughtfan

Fresh air

Hot air tooil burners

Supply of oil fuel toburners

Steamdrum

Water tube wall

Header

Flue gasesto atmosphere

Superheated steam to steam turbines

Superheater(protected by screentubes against directradiant heat of fire.)

Floor tubes

Feed water from feed pump

Economiser

Unheated down comer

Saturatedsteam

Water tubes

Marine Water Tube Boiler

Classification & types of boilers

gas stream, after the screen tubes. Their function is to superheat the saturatedsteam to a suitable temperature. These tubes must be protected from direct radiantheat as they are liable to over-heating due to lower specific heat of steam comparedto that of water.

Advantages of water tube boiler Saving in weight-

Relevant weight ratio of a scotch boiler to a water tube boiler for the same heating surface area is 3:1 at normal water level

Higher pressures and temperatures possibleWith reference to the thin shell formula, small diameter tubes with reduced wall thickness' are able to withstand higher internal pressures without exceeding maximum stress levels

Increased plant efficiencyCarnot Cycle efficiency = T1 - T2 / T1

This means that with increased pressure within the boiler and increase temperature T1 is increased, T2 being set by the sea water temperature and hence cannot be altered, thus increasing plant efficiency.

Increase heat conductivityThe heat conducted is proportional to the temperature gradient, area and time. It is inversely proportional to the material thickness. It can be seen that the relatively thin wall numerous tubes in a water tube design offer greater conductivity and therefore allow higher firing rates per unit size.

Greater mechanical flexibilityScotch boilers are very sensitive to changing load, this is due to its poor natural circulation resulting in mechanical straining and grooving

Reduced warm up timeDue to the above scotch boilers can take a typical 20 hrs to bring on line, for a water tube boiler this is reduced to 3 to 4 hrs. slopping of the tubes by a minmum of 15o aids the natural circulation

Saving in spaceDue to the increased surface area, improved natural circulation very high evaporation rates are possible. Due to the large quantities of water required in a scotch boiler and due to the increased wall shell thickness' required for increased diameters there is a limitation to the maximum size ( thus evaporation rate ) that can be achieved. Therefore multi smoke tube unit installations are required in plants that can be served by a single watertube unit.

Increased safetyDue to the reduction in the water capacity and due to the smaller diameter tubes the possibility of serious damage via a ruptured is much reduced when compared to the catastrophic failures of scotch boilers. Overheated furnace tube failures would empty the entire contents of the boiler near instantaneously into the engine room . Limited

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Classification & types of boilers

tube diameters and protected drum surfaces mean failure of tubes releases a flow of steam dependant on the tube diameter \\

Improved furnace shapeImproved furnace shape allows for more efficient combustion of the fuel, this also allows the use of reduced excess air

Reduced tube size and wall thicknessThe reduced tube size allows the tubes to be more easily bent and expanded and bell mouthed. Thinner tube material allows for rapid raising of steam increasing the rate of heat transfer from combustion

Disadvantages

Reduced water contentThis means that a sophisticated means of controlling the water level is required, in the event of feed pump failure only a short period is allowed before the level in the drum becomes critical

Increases pressures and temperaturesThe increased temperatures means that conditioned for the reduced tube wall material are more arduous. Accurate water chemistry is necessary with even small amounts of corrosion/scaling causing large problems

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