5a - boiler construction continued -internals - superheaters locked

7/27/2019 5a - Boiler Construction Continued -Internals - Superheaters Locked

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MainPlant

Boilers

MainPlant

Boilers

Feed water in

Economiser

Steamdrum

Waterdrum

Superheater

Downcomers

Risers

Risers

Furnace

Casing

Superheated

steam out


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Steam DrumSteam Drum


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The steam drum is designed to separate the steam from the

steam/water emulsion produced in the riser tubes.

It is important to ensure that no water is carried over into

the superheater. Separation is carried out by:-

Horizontal perforated plates acting as baffles.

A dry pipe situated at the top of the drum with slots

cut in to help prevent the passage of water. The dry

pipe carries the steam away from the drum to the

superheater.

The feed water pipe is submerged below the normal

working water level of the boiler.

Steam Drum InternalsSteam Drum Internals


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CycloneSeparators

CycloneSeparators

Used in some makes

of boiler to give

improved separation of

steam/water.


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Cyclone SeparatorsCyclone Separators


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Super-heatersSuper-heaters

The ESD III boiler

has an external

superheater that

uses the thermalenergy of the

combustion gasses

after passing

through the screen

wall


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Superheater tubes are subject to very high temperatures

(510C) and full boiler pressure (65 bar) and are, therefore,

highly stressed.

Materials: Chrome/molybdenum steel.

Tube Diameter 53mmThe superheater tubes are welded on to stub pipes on the

headers, which are external to the gas passage.

The tube bends are supported by lugs which are welded tothe screen wall riser tubes in order to remove heat.

SuperheatersSuperheaters


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The superheater is set up as three separate banks of tubes.

The two top banks (furthest from the furnace) are set up asthe primary superheater.

The lower bank (closest to the furnace) is the secondary

superheater.ALL steam flow from the boiler drum must pass through

ALL sections of the superheater continuously.

It is important to maintain maximum flow through thesuperheater at all times in order to prevent overheating.



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Steam Temperature ControlSteam Temperature Control

Air Attemporation


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ESD II

Gas Flow Control


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((AttemporationAttemporation))


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Superheat

Flow

SuperheatSuperheat

FlowFlow


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Superheat FlowSuperheat Flow

Primary Superheater Contra flow

Secondary Superheater Parallel Flow


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AttemporatorAttemporator


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AttemporatorAttemporator


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Water SprayWater Spray AttemporationAttemporation

Feed water sprayed into steam path


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Some of the superheated steam is de-superheated in order

to supply cooled saturated steam for lower quality heatingpurpose e.g.

Steam/steam generators. Evaporators. De-aerators. Air

heaters. etc.

There are two main ways of de-superheating:-

Via a cooling coil in the water drum of the boiler.

Via a water spray de-superheater.

De-superheatersDe-superheaters


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Spray De-superheaterSpray De-superheater

De-superheatedoutlet

Superheatedinlet

Drain

Feed

water inlet

Feed

water flow

control


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In order to improve the efficiency of the combustion

process and the overall efficiency of the boiler the

combustion air is pre-heated. This can be done in threeways:-

A tube type air heater using de-superheated steam.

A rotary heat exchanger (Ljungstrom)

A double casing fitted round the boiler.

Air HeaterAir Heater


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The combustion air is passed from the forced draught fan

into a double casing surrounding the furnace and

superheater.

The air is first heated by waste heat passing through the

riser tube wall and the refractory.

The air then passes through a tube type heat exchanger that

uses de-superheated steam to increase the temperature to

about 120C before the air passes to the burner registers.

Note that monowall boilers do not normally have double

casings.

Double Casing & Air HeaterDouble Casing & Air Heater


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Ljungstrom Air HeaterLjungstrom Air Heater


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5a - boiler construction continued -internals - superheaters locked

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