cmi ibo unit 2 lecture 4

8/13/2019 CMI IBO Unit 2 Lecture 4

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Caribbean Maritime Institute

School of Academic Studies

Industrial Boiler Operation

Unit 2 – Boiler Types (Fire Tube)

Lecture # 4 (1 of 2)

En . Earl S. Green Ph.D.



Reading/Reference

Kohan, A. L. Boiler Operator’s Guide .



UNIT OUTLAY

Description of fire-tube boilers, their

general construction, and P limitations.1. Lancashire boiler

2. Economic boiler (two-pass, dry back)

3. Economic boiler (three-pass, wet back)4. Packaged boiler

5. Exhaust gas boilers

Smoke/Fire tube (shell boilers)



Firetube Boilers

FTBs are containers of water which get

heated by tubes filled with hot gases from a

burning fire.



Firetube Boiler



Firetube Boilers

FTBs 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

through the tubes. As the hot gases flow through

the tubes, they heat the water surrounding the

tubes. The water is confined by the outer shell of

boiler. To avoid the need for a thick outer shell

FTBs are used for lower P applications. Generally,the heat input capacities for FTB are limited to 50

mbtu per hour or less, but in recent years the size of

FTBs has increased.



Firetube Boilers

FTBs are subdivided into 3 groups. Horizontalreturn tubular (HRT) boilers typically have

horizontal, self-contained fire tubes with a

separate combustion chamber. Scotch, Scotchmarine, or shell boilers have the fire tubes and

combustion chamber housed within the same

shell. Firebox boilers have a water-jacketedfirebox and employ at most three passes of

combustion gases.



Firetube Boilers

Most modern fire tube boilers have cylindrical outer

shells with a small round combustion chamber locatedinside the bottom of the shell. Depending on the

construction details, these boilers have tubes

configured in either 1, 2, 3, or 4 pass arrangements.Because the design of FTB boilers is simple, they are

easy to construct in a shop and can be shipped fully

assembled as a package unit.

FTBs typically have a lower initial cost, are more fuel

efficient and are easier to operate, but they are limited

generally to capacities of 25 tonnes per hour and

pressures of 17.5 kg per cm2.



Lancashire Boilers

These were first designed by Fairbairn in 1844

Although the Lancashire boiler is considered to be an

antiquated design, provided that the flue is long

enough it can be reasonably efficient. This does lead to

a bulky boiler though, particularly for its length, and

this has always limited its use to stationaryinstallations. It was the standard boiler in Greater

Manchester and Lancashire cotton mills.



Lancashire Boilers



Lancashire Boilers

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Lancashire Boilers

The Lancashire boiler is comprised of a large steelshell usually between 5 - 9 m long through which two

large-bore furnace tubes called flues are passed. Part

of each flue is corrugated to take up the expansion

when the boiler becomes hot, and to prevent collapse

under pressure. A furnace is installed at the entrance to

each flue, at the front end of the boiler.

Typically, the furnace would be arranged to burn coal,

being either manually or automatically stoked.



Lancashire Boilers

The hot gaseous products of combustion passes fromthe furnace through the large-bore corrugated flues.

Heat from the hot flue gases is transferred into the

water surrounding these flues.

The boiler is in a brickwork setting which is arranged

to duct the hot gases emerging from the flues

downwards and beneath the boiler, transferring heat

through the bottom of the boiler shell, and secondly

back along the sides of the boiler before exiting

through the stack.



Lancashire Boilers

These two side ducts meet at the back of the boiler and

feed into the chimney.

These passes are designed to extract the maximum

amount of energy from the hot product gases beforethey are released to the atmosphere.

Later, the efficiency was improved by the addition ofan economiser. The gas stream, after the third pass,

passed through the economiser into the chimney. The

economiser heats the feedwater and results in an

im rovement in thermal efficienc .



Lancashire Boilers

One of the disadvantages of the Lancashire boiler is

that repeated heating and cooling of the boiler, with

the resultant expansion and contraction that occurs,

upset the brickwork setting and ducting. This results in

the infiltration of air, which upsets the furnace

draught.

These boilers are now very expensive to produce, due

to the large amounts of material used and the labour

required to build the brick setting.



Economic Boilers (two-pass, dry back)

What is a dry back boiler and how does it

compare to a wet back boiler?

A dry back boiler has a rear wall that is

lined with a refractory, a wet back boilerhas a rear wall that is jacketed by water.




Because of this fact, wet back boilers typically

boast higher efficiencies than dry back boilers as

the heat from combustion goes directly into

heating water instead of refractory. Wet back

boilers are also more forgiving with load

changes as the reversal chamber of the boiler istotally submerged in water creating an even heat

transfer on the intermediate tube sheet to furnace

joint.



Economic Boiler (two-pass, dry back)

http://2.bp.blogspot.com/_neeqrIbq3Dw/SVWZndAxPHI/AAAAAAAAAa4/UnuRyzCEfNY/s1600-h/Fig_3_2_3.gif




The two-pass economic boiler is only

about half the size of an equivalent

Lancashire boiler but boasts a higherthermal efficiency. It has a cylindrical

outer shell containing two large-bore

corrugated furnace flues acting as the

main combustion chambers.




The hot flue gases pass out of the two furnace

flues at the back of the boiler into a brickwork

setting (dry back) and are deflected through anumber of small-bore tubes arranged above

the large-bore furnace flues. These small bore

tubes present a large heating surface to thewater. The flue gases pass out of the boiler at

the front and into an induced draught fan,

which passes them into the chimney.




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Economic Boilers (three-pass, wet back)

Standard Features

Factory packaged units with operating controls,

relief valves, burner and fuel train. Installation is

made simple in that only service connections areneeded to place in operation. Flexible burner

systems are available for firing natural gas, LP

gas, #2-6 oil or combinations. High densitymineral wool insulation assures lower radiant

heat loss. Burner system/s meet/s UL & other

requirements.




Steam Trim

• Steam pressure gauge with syphon and

testcock

• Probe type combination low water cutoff andpump control/feeder

• Water column gauge glass set and drain valve

• ASME safety relief valve(s)• Operating, high limit and modulating pressure

controls.




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Question

Why only water tube boilers and not

fire tube boilers are designed andused for power plant industries?



Questions?

cmi ibo unit 2 lecture 4

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