Chapter 2 Firetube

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B o i l e r s B a s i c s

F i r e T u b e B o i l e r s

Chapter 2 : Fire Tube Boilers

2.1 Fire Tube BoilersFire tube boilers consist of a series of straight tubes that are housed inside a water-filled outer shell. The tubes are arranged so that hot combustion gases flow throughthe tubes. As hot gases flow through the tubes, they heat the water that surroundsthe tubes. The water is confined by the outer shell of the boiler that is designed as apressure vessel. To avoid the need for excessively thick materials, fire tube boilersare used for lower-pressure applications. Fire tube boilers are subdivided into threegroups.Horizontal return tubular (HRT) boilers typically have horizontal, self-containedfire tubes with a separate combustion chamber. Scotch, Scotch marine , or shellboilers have the fire tubes and combustion chamber housed within the same shell.

Firebox boilers have a water-jacketed firebox and employ, at most, three passes ofcombustion gases.Fire tube boilers are also named “Shell Boilers” & “Smoke Tube Boilers” .

2.2 Main Types of Fire Tube Boilers

Old Designs Conventional Designs

- Cornish- Lancashire- Scotch- Vertical Upright- HRT Boilers

Fire Tube Boilers

Scotch Marine- Two Pass Dry Back- Two Pass Wet Back- Three Pass Wet Back- Four Pass Wet Back- Thimble Type

Fire Box VerticalTubeless


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2.2.1 Old & Obsoletes Designs

2.2.1.1 Cornish Boiler

Cornishman Richard Trevithick (1771-1833) was the first British engineer to usehigh-pressure steam. In 1812 Trevithick implemented his ideas at Cornwall's WhealProsper mine. He used a new boiler design to supply steam at 40 psi to a single-acting condensing engine.The Cornish Boiler made several design changes. First, the furnace was placed insidea metal tube measuring three (3) feet or more in diameter (from side to side). Andthis tube was placed inside the boiler. Having all that hot metal inside the boilergreatly increased the amount of heat transferred to the water.The fire and hot gasses were still routed through three flues which run along bothsides and beneath the cylinder.After leaving flue #1 (the metal tube running through the water) the hot gasseswere divided at the aft (back) end and moved forward along flue #2 which runsalong both sides of the cylinder at the same time.At the front of the boiler the hot gasses were directed downward into flue #3 andtraveled aft beneath the boiler to the chimney. This helped reduce the amount ofmud that accumulated in the bottom of the boiler and that increased the boilersefficiency even more.The flat ends of the cylinder are another obvious design change made necessary bythe internal furnace.

Fig 2.1 A Typical Cornish Boiler

2.2.1.2 Lancashire Boiler

Lancashire boiler comprised a large steel shell usually between 5 - 9 m longthrough which passed two large-bore furnace tubes called flues. Part of each fluewas corrugated to take up the expansion when the boiler became hot, and to preventcollapse under pressure. A furnace was installed at the entrance to each flue, at thefront end of the boiler. Typically, the furnace would be arranged to burn coal, beingeither manually or automatically stoked.The hot gaseous products of combustion passed from the furnace through the large-bore corrugated flues. Heat from the hot flue gases was transferred into the watersurrounding these flues.


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The boiler was in brickwork setting which was arranged to duct the hot gasesemerging from the flues downwards and beneath the boiler, transferring heatthrough the bottom of the boiler shell, and secondly back along the sides of theboiler before exiting through the stack.These two side ducts met at the back of the boiler and fed into the chimney. Thesepasses were an attempt to extract the maximum amount of energy from the hot

product gases before they were released to atmosphere.Later, the efficiency was improved by the addition of an economizer.

Fig 2.2 A Typical Lancashire Boiler

Capacity Small LargeDimensions 5.5 m long x 2 m diameter 9 m long x 3 m diameterOutput 1 500 kg/h 6 500 kg/hPressure Up to 12 bar g up to 12 bar g

Table 2.1 Size Range of Lancashire boilers

2.2.1.3 Old Scotch Boiler

Engineers and designers of steam boilers had long understood the relationship

between the amount of heat generated in a furnace and the ability of water toabsorb that heat. Basically, the larger water surface exposed to the heat the moreheat is transferred to the water.Like the Cornish and Lancashire boilers, the Scotch Boiler utilizes internal furnaceswith the fire box and primary flue traversing the lower portion of the water cylinder.Yet unlike the Lancashire boiler, the Scotch boiler does not utilize Galloway tubes.Instead, the designers choose to manufacture the water tank from corrugated plates.The end plates are reinforced by heavy through bolts . This combination of throughbolts and corrugated plates provided an extremely strong boiler.


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The Scotch Boiler is a "Fire Tube" design. In this case a number of relatively small (31/2 inch diameter) metal tubes pass horizontally through the water cylinder andact flues. A boiler 10 feet in diameter and 20 feet long would normally contain137 individual horizontal tubes. These “fire tubes” were arranged above thefurnaces, but below the water surface.As with the previous illustrations, fire and hot gasses pass from the furnace through

the main flues which are surrounded by water. At the aft end of the boiler the hotgasses entered a chamber or Dry Back which allowed the end plate to be heatedand also directed the gasses into the fire tubes. From there the hot gasses movedforward through the numerous tubes to the chimney.The Scotch Boiler was quite versatile. Designs were built to deliver anywhere from 6to 300 BHP (boiler horse power). The largest were 10 feet in diameter, 20 feetlong and contained four furnaces. The illustration below shows a Scotch type boilerwith two furnaces.

Fig 2.3 Old Scotch type Boiler with Two Furnaces

2.2.1.4 Vertical Upright

The term Fire Tube accurately describes the basis of this boiler. The water tank orboiler is a vertical tank not a horizontal cylinder as in the other boilers alreadydescribed. Like the Cornish and Lancashire boilers the furnace is located inside thewater tank. The tank is all surrounded with water except its bottom. But take a notethere are a number of brass tubes which extend through the boiler to the chimney.Depending on the individual manufacturer, there could be as many as 100 of thesetubes.These tubes allow the products of combustion (flame, hot gasses and smoke) to pass

directly through the boiler allowing an extremely high rate of heat transfer to thewater. In other words, the fire passed through the tubes, hence the name, Fire TubeBoiler.


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Fig 2.4 A Vertical Upright Steam Boiler

2.2.1.5 Horizontal Return Tubular Boiler (HRT)

The hot gases generated by the burning coal traveled along the underside of theboiler drum and returned through the fire tubes to the boiler stack. The boiler wasencased in a brickwork setting to contain the flame, so it was externally fired boiler.The boiler was also classified as a two pass boiler.The HRT was labor intensive to construct because of the brickwork and inefficient tooperate for the same reason. Heat loss through the brick setting was too high.Although these boilers are no longer constructed, there are still some in operation.The coal firing grates have largely replaced by oil or gas burners to reduce exhaust

emissions.

Fig 2.5 HRT Fire Tube boiler


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Fig 2.6 an Old HRT Boiler Showing Steps of Construction & Brickwork

In order to reduce the extensive heat losses from the furnace walls of externally firedboilers, one designer enlarged the diameter of the return flue and put the firinggrating inside this enlarged flues. The boiler is now internally fired. The furnace wasplaced inside the shell and completely surrounded by water.However brickwork was still used to guide the hot gases, after leaving the furnace,around the outside of the shell in order to remove as much heat as possible.Horizontal return tubular (HRT) boilers typically have horizontal, self-contained firetubes with a separate combustion chamber.


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2.2.2 Conventional & Modern Designs

The modern Economic boiler has a horizontal shell and no side or sole flues. A few

earlier brick set types still exist. From one or two furnaces, hot gases pass throughthe furnace tubes to the rear and enter a steel combustion chamber lined withfirebrick. If this chamber is surrounded by water, the boiler is known as a wetback; ifit is outside the boiler shell, the boiler is known as a dry back, with a two pass boiler,gases from the combustion chamber return to the front through a bank of smoketubes below water level, enter a smoke box and pass on to the chimney. When thegases pass through a second set of smoke tubes it is a three-pass boiler.

Fig 2.7 Modern Packaged Scotch Marine Boiler

2.2.2.1 Scotch Marine Boiler

- Two Pass Dry Back

The two-pass economic boiler was only about half the size of an equivalentLancashire boiler and it had a higher thermal efficiency. It had a cylindrical outershell containing two large-bore corrugated furnace flues acting as the maincombustion chambers. The hot flue gases passed out of the two furnace flues at theback of the boiler into a brickwork setting (dry back) and were deflected through anumber of small-bore tubes arranged above the large-bore furnace flues. Thesesmall bore tubes presented a large heating surface to the water. The flue gasespassed out of the boiler at the front and into an induced draught fan, which passedthem into the chimney .


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Fig 2.8 Two-pass Dry back Boiler

Capacity Small Large

Dimensions 3 m long x 1.7 m diameter 7 m long x 4 m diameterOutput 1 000 kg/h 15 000 kg/h

Pressure Up to 17 barg up to 17 barg

Table 2.2 Size range Two-pass Dry back boilers

- Three Pass Wet Back

A further development of the economic boiler was the creation of a three-pass wetback boiler which is a standard configuration in use today, manufacturing technologyhas advanced: thinner metal tubes were introduced allowing more tubes to be

accommodated, the heat transfer rates to be improved, and the boilers themselvesto become more compact.

Fig 2.9 Three-pass Wet back Boiler


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Area of tubes Temperature Proportion of total heat1st pass 11 m2 1 600°C 65%2nd pass 43 m2 400°C 25%3rd pass 46 m2 350°C 10%

Table 2.3 Heat transfer details of a modern three pass, wet back, boiler

- Four Pass Wet Back

Four-pass units are potentially the most thermally efficient, but fuel type andoperating conditions may prevent their use. When this type of unit is fired at lowdemand with heavy fuel oil or coal, the heat transfer from the combustion gases canbe very large. As a result, the exit flue gas temperature can fall below the acid dewpoint, causing corrosion of the flues and chimney and possibly of the boiler itself. Thefour-pass boiler unit is also subject to higher thermal stresses, especially if large loadswings suddenly occur; these can lead to stress cracks or failures within the boilerstructure. For these reasons, four-pass boilers are unusual.

Fig 2.10 a Cut-away in Four-pass Wet back Boiler


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- Thimble / Reverse Type

This is a variation on conventional boiler design. The combustion chamber is in theform of a thimble, and the burner fires down the centre. The flame doubles back onitself within the combustion chamber to come to the front of the boiler. Smoke tubessurround the thimble and pass the flue gases to the rear of the boiler and thechimney.

Fig 2.11 Thimble flame Boiler

- Package Boiler

The packaged boiler is so called because it comes as a complete package withburner, level controls, feed pump and all necessary boiler fittings and mountings.Once delivered to site it requires only the steam, water, and blow down pipe work,fuel supply and electrical connections to be made for it to become operational.Development has also had a significant effect on the physical size of boilers for agiven output:- Manufacturers wanted to make the boilers as small as possible to save on materialsand hence keep their product competitive.- Efficiency is aided by making the boiler as small as it is practical; the smaller theboiler and the less its surface area, the less heat is lost to the environment.To some extent the universal awareness of the need for insulation, and the highperformance of modern insulating materials, reduces this issue.- Consumers wanted the boilers to be as small as possible to minimize the amount of

floor space needed by the boiler house, and hence increase the space available forother purposes.


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Fig 2.12 Packaged Boiler

Boiler type Fuel Length (m) Diameter(m)

Efficiency(%)

Volumetricheat release

(kW/m3)

Steam releaserate from

water Surface(kg/m2 s)

Lancashire Coal 9.0 2.75 74 340 0.07

Economic Coal 6.0 3.00 76 730 0.12Packaged Oil 3.9 2.50 82 2 330 0.20Packaged Gas 3.9 2.50 80 2 600 0.20

Table 2.4 a Comparison for 5000 kg/hr Boilers


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2.2.2.2 Fire Box

Another type of fire-tube boiler is the FIREBOX boiler that is usually used forstationary purposes. Two Split sections of a small firebox boiler are shown in figure2.10. Gases in the firebox boiler make two passes through the tubes. Firebox boilersrequire no setting except possibly an ash pit for coal fuel. As a result, they can bequickly installed and placed in service. Gases travel from the firebox through a groupof tubes to a reversing chamber. They return through a second set of tubes to theflue connection on the front of the boiler and are then discharged up the stack.The firebox boiler is typically manufactured for low pressure steam or hot waterapplications. The firebox boiler is a compact, economical unit and serves as a good fitfor seasonal use and when efficiency is not the driving factor. Sizes range from 12 to337 horsepower.

Fig 2.13 Split sections in small firebox boilers

2.2.2.4 Vertical Fire Tube Boiler

In some fire-tube boilers, the tubes run vertically, as opposed to the horizontalarrangement in the Scotch boiler. The VERTICAL-TUBE boiler sits in an uprightposition, as shown in figure 2.12. Therefore, the products of combustion (gases)make multiple passes, traveling straight up & down through the tubes and out to thestack. The vertical fire-tube boiler is similar to the horizontal fire-tube boiler in thatit is a portable, self-contained unit requiring a minimum of floor space. Handholdsare also provided for cleaning and repairing. Though self-supporting in its setting

(no brickwork or foundation being necessary), it MUST be level. The fire-tube boilerhas the same disadvantages as that of the horizontal-tube design—limited capacityand furnace volume. Before selecting a vertical fire-tube boiler, you must know howmuch overhead space is in the building where it will be used. Since this boiler sits inan upright position, a room with a high ceiling is necessary for its installation. Theblow down pipe of the vertical tire-tube boiler is attached to the lowest part of thewater leg, and the feed water inlet opens through the top of the shell.


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Fig 2.14 Vertical tubeless boiler


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2.3 Boiler Efficiency, Steam Release Rate & VolumetricHeat Release

2.3.1 Boiler Thermal Efficiency

The thermal efficiency of a particular boiler depends on many factors, including thetype of boiler, the quality of combustion, the load, the pressure and the generalcondition of the boiler. One of the main causes of inefficiency will have beeneliminated if combustion can be maintained with the minimum use of excess air andwithout unburned gases escaping to the chimney.Every boiler will lose heat by radiation and convection but the heat loss will bereduced if the boiler, its shell, or the steam and water drums are effectively laggedand the lagging is kept in good condition.Fuel efficiency can be improved by recovering heat that would otherwise be wastedand using it for pre-heating the boiler feed water. Feed water economizers recoverheat from the boiler waste gases before they are discharged but hot condensate or

exhaust steam can also be used to heat the feed water. Exhaust steam should onlybe used for this purpose if it cannot be put to better use elsewhere and live steammust not be used unless it is necessary to protect the economizer from corrosion. Airheaters placed after the economizer can, in suitable cases, recover further heat fromthe waste gases which can be used to pre-heat combustion air.

Overall Boiler efficiency can be calculated by two methods:

1- Direct Method:

100FueltheinEnergyTotal

OutputSteamtheinHeatUseful %EfficiencyBoiler ×=

)100

C.Vf mf hghsm% bη ××•

−•

= -----2.1

Where:

bη = Boiler Efficiency (%)•sm = Steam Output (KG/h)

gh = Specific enthalpy of steam at operating pressure(KJ/kG)

f h = Specific enthalpy of water at feed water temperature(KJ/kG)•f m = Fuel Consumption (KG/h)

C.V = Fuel Calorific value (kJ/KG)


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2- Indirect Method:

∑= %Losses-%100%EfficiencyBoiler ----2.2

Where Total Losses:- Heat carried out of the stack by hot flue gases (dry flue gases).- Heat carried out of the stack by hot water vapor.- Unburned fuel and products of incomplete combustion.- Heat lost by the boiler structure through its insulation (by radiation).- Heat carried away by the boiler blow down.

2.3.2 Steam Release Rate (kG/m 2 S)

This factor is calculated by dividing the amount of steam produced per second by thearea of the water plane. The lower this number, the greater the opportunity forwater particles to separate from the steam and produce dry steam.Note the modern boiler's figure is larger by a factor of almost three. This means thatthere is less opportunity for the separation of steam and water droplets.This is made much worse by water with a high TDS level, and accurate control isessential for efficiency and the production of dry steam.At times of rapidly increasing load, the boiler will experience a reduction of pressure,which, in turn, means that the density of the steam is reduced, and even highersteam release rates will occur, and progressively wetter steam is exported from theboiler.

2.3.3 Volumetric Heat Release (kW/m3)

This factor is calculated by dividing the total heat input by the volume of water in theboiler. It effectively relates the quantity of steam released under maximum load tothe amount of water in the boiler. As this number decrease, the amount of reserveenergy in the boiler increase.Note that the figure for a modern boiler relative to a Lancashire boiler is larger by afactor of almost eight, indicating a reduction in stored energy by a similar amount.

This means that a reduced amount of stored energy is available in a modern boiler.This development has been made possible by control systems which respond quicklyand with appropriate actions to safeguard the boiler and to satisfy demand.


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2.4 Boiler Ratings

Boilers ratings depend on boilers manufacturers & origins. Three types ofboiler ratings are commonly used:

- 'From and at' rating.- KW rating.- Boiler horsepower (BoHP).

'From and at' rating

The 'from and at' rating is widely used as a datum by shell boiler manufacturers togive a boiler a rating which shows the amount of steam in kg/h which the boiler cancreate 'from and at 100°C, at atmospheric pressure. Each kilogram of steam wouldthen have received 2257 kJ of heat from the boiler.Shell boilers are often operated with feed water temperatures lower than 100°C.Consequently the boiler is required to supply enthalpy to bring the water up toboiling point.Most boilers operate at pressures higher than atmospheric, because steam at anelevated pressure carries more heat energy than does steam at100°C. This calls foradditional enthalpy of saturation of water. As the boiler pressure rises, the saturationtemperature is increased, needing even more enthalpy before the feed water isbrought up to boiling temperature.Both these effects reduce the actual steam output of the boiler, for the sameconsumption of fuel. The graph in Figure 2.15 shows feed water temperaturesplotted against the percentage of the 'from and at' figure for operation at pressuresof 0, 5, 10 and 15 bar g.

Fig 2.15 “From and at” Graph


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Calculation of boiler output at certain pressure other than

atmospheric pressure

Boiler output (KG/h) at pressure (P) = evaporation factor boiler output at atmospheric pressure

Where evaporation factor can be calculated from:

temp.)waterfeed(atP) pressure(at

pressure)catmospheri(at

f hgh

fgh factornEvaporatio

- = ------2.3

Where:

h fg = Specific enthalpy of evaporation at atmospheric pressure.h g = Specific enthalpy of steam at operating pressure.h f = Specific enthalpy of water at feed water temperature.

The evaporation factor can also be obtained from fig 2.15 at feed water temp. andoperating pressure.

KW Rating

Some manufacturers will give a boiler rating in kW. This is not an evaporation rate,and is subject to the same 'from and at' factor.To establish the actual evaporation by mass, it is first necessary to know thetemperature of the feed water and the pressure of the steam produced, in order toestablish how much energy is added to each kg of water. Equation 2.3 can then beused to calculate the steam output:

(kJ/kG)addedbetoEnergy

s/h3600 (kW)RatingBoiler(kG/h)OutputSteam ×= ---2.4

- Where energy to be added = f hgh −

-

gh= Specific enthalpy of steam at operating pressure(kJ/kG) .

-

f h= Specific enthalpy of water at feed water temperature(kJ/kG) .

Boiler horsepower (BoHP)

This unit tends to be used only in the USA, Australia, and New Zealand. A boilerhorsepower is not the commonly accepted 550 ft Ibf/s and the generally acceptedconversion factor of 746 Watts = 1 horsepower does not apply.In New Zealand, boiler horsepower is a function of the heat transfer area in theboiler, and a boiler horsepower relates to 17 ft2 of heating surface,


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17 ft2

of heating surface area will produce 1 BoHP---2.5

In the USA and Australia the readily accepted definition of a boiler horsepower is theamount of energy required to evaporate 34.5 Ib of water at 212°F atmosphericconditions.

1 BoHP will evaporate 28 Ib/h to 35 Ib/h of water (at atmospheric pressure)—2.6

2.5 Advantages & Disadvantages of Fire Tube Boilers

2.5.1Advantages of Fire Tube boilers:

- The entire plant may be purchased as a complete package, only needing securingto basic foundations, and connecting to water, electricity, fuel and steam systemsbefore commissioning. This means that installation costs are minimized.- This package arrangement also means that it is simple to relocate a packaged shellboiler.- A shell boiler contains a substantial amount of water at saturation temperature,and hence has a substantial amount of stored energy which can be called upon tocope with short term, rapidly applied loads.This can also be a disadvantage in that when the energy in the stored water is used,it may take some time before the reserve is built up again.- The construction of a shell boiler is generally straight forward, which means thatmaintenance is simple.- Shell boilers often have one furnace tube and burner. This means that controlsystems are fairlysimple.- Although shell boilers may be designed and built to operate up to 27 bar. Themajority operate at 17 bar or less. This relatively low pressure means that theassociated ancillary equipment is easily available at competitive prices.

2.5.2 Disadvantages of Fire Tube boilers:

- The package principle means that approximately 27 000 kg/h is the maximumoutput of a shell boiler. If more steam is required, then several boilers need to beconnected together.- The large diameter cylinders used in the construction of shell boilers effectivelylimit their operating pressure to approximately 27 bars. If higher pressures areneeded, then a water-tube boiler is required.

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Chapter 2 Firetube

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