BOILER DRAUGHT

The draught is one of the most essential systems of thermal

power plant which supplies required quantity of air for

combustion and removes the burnt products from the system.

To move the air through the fuel bed and to produce a flow of

hot gases through the boiler, economizer, preheater and

chimney require a difference of pressure.

This difference of pressure for maintaining the constant flow of

air and discharging the gases through the chimney to

atmosphere is known as DRAUGHT.

Draught also provides velocity to flue gases and so increases

the heat transfer co-efficient in the boiler.

Importance of Draught

For the proper and the optimized heat transfer from the flue

gases to the boiler tubes draught holds a relatively high

amount of significance.

The combustion rate of the flue gases and the amount of

heat transfer to the boiler are both dependent on the

movement and motion of the flue gases.

A boiler equipped with a combustion chamber which has a

strong current of air (draught) through the fuel bed will

increase the rate of combustion.

Classification of the Draught

Natural Draught Natural draught is obtained naturally by the use of a chimney.

Chimney is a conical shape vertical tubular steel or masonry or

concrete structure having a large height.

Contd.

The exhaust gases are lighter in weight than outside air

and so lifts up naturally in the chimney and finally escape

out in the atmosphere from top of the chimney.

As flue gases flow through boiler and then to chimney,

fresh air from outside naturally enters the boiler.

So, in this way, a small pressure difference is naturally

created between the base of chimney and the air inlet

point of boiler because of density difference between hot

flue gases inside the chimney and fresh colder air outside

the chimney. This pressure difference is called natural

draught.

The Draught required for the flow of air & gas inside the

boiler is created by Chimney. It does not require any

external power for producing the draught.

The capital investment is less.

It has long life.

Chimney is also useful to safely remove the harmful

poisonous flue gases in the atmosphere at a safe height,

where they may not harm the freshness of air on earth.

Advantages of Natural Draught

Pressure :- Maximum is 15 – 20 mm of water under normal

atmospheric Temp.

The draught decreases with increase in outside air temperature

and for producing sufficient draught.

The flue gases have to be discharged at comparatively high

temperatures resulting in the loss of overall plant efficiency. And

thus maximum utilization of Heat is not possible.

Combustion is very poor.

Limitations of Natural Draught

Artificial Draught

Because of insufficient head and lack of flexibility, The use

of natural draught is limited to small capacity boilers only.

The draught required in actual power plant is sufficiently

high (300 mm of water) and to meet high draught

requirements, some other system must be used which are

known as artificial draught.

So, when the draught is produced by some external

agency i.e. mechanical fan/blower or by steam jet itself, it is

called artificial draught.

Advantages over natural draught

It is better in control and more economical than natural

draught.

The rate of combustion is high.

The air flow can be regulated according to the requirement.

It prevents the formation of smoke as complete combustion

is possible.

Limitations of Artificial Draught

The major disadvantage of the artificial draught is the high

capital cost required and high running and maintenance costs.

Classification of Artificial Draught

1. Mechanical draught: which is produced

by fan/blower

2. Steam jet draught: which is produced

by using a high velocity jet of steam.

These two are further classified as

1. Forced

2. Induced

3. Balanced

Forced Mechanical draught

It is a positive pressure draught in which fan is installed at the base of the boiler before grate.

Fan forces the outside air through fuel bed, furnace and air pre-heater and then flue gases through flue passage, economizer, Air-preheater and then to chimney.

The enclosure for the furnace has to be very tightly sealed so that gases from the furnace do not leak out in the boiler house.

Induced Mechanical draught

In this system a fan or blower is located at or near the base of

the chimney which creates a partial vacuum in the furnace and

flue passage.

Thus, the air and flue gases are drawn through the boiler due to

comparatively higher pressure of outside air.

It is convenient to produce induced draught and like in forced

draught, any type of fan/blower may be used.

Forced draught Induced draught

Pressure inside the flue passages is

slightly more than atmospheric

pressure.

Due to above reason, there is serious

danger of fire to come out and injure

someone if any leakage in boiler flue

passage take place.

It forces fresh air into the combustion

chamber, which helps in burning of fuel

and production of hot combustion

gases. Due to this force, the hot flue

gases are further pushed through the

flue passages in boiler.

Forced draught fan require less power

because it has to handle cold and dense

air, the volume per unit mass of which

is less.

Flow of flue gases through boiler is

more uniform.

Pressure inside the flue passages is

slightly less than atmospheric pressure.

Due to above reason, there is no

danger of fire to come out from boiler

flue passage in case of a leakage. Hence

Induced draught is more safe.

It sucks the hot flue gases from the

combustion chamber through flue

passages and then passes on these to

economizer, air pre heater and chimney.

Due to this suction, fresh air is also

sucked in to combustion chamber.

Induced draught fan requires more

power because it has to handle hot flue

gases, the volume per unit mass of

which is more.

Flow of flue gases is less uniform

Balanced Mechanical draught

It is a combination of forced and induced draught.

In this system, the forced draught fan overcomes the resistance

in air pre-heater and grate.

While the induced draught fan overcomes draught losses

through boiler, economizer and connecting flue passages etc.

Finally the flue gases pass on to chimney and escape to

atmosphere because of pressure created by fan.

Balanced draught is most effective and used in large boilers.

Steam Jet draught

It is a very simple and easy method of producing artificial draught

without the need of an electric motor as used in Mechanical

draught to run fan or blower.

Steam under pressure is available in the boiler. When a small

portion of steam is passed through a jet or nozzle, steam comes

out with a high velocity.

This steam jet can be used to produce draught and it is a simple

and cheap method.

Actually the steam jet is directed towards a fix direction and

carries all its energy in kinetic form.

It creates some vacuum in its surroundings.

So, it has the capacity to make flow of the flue gases either by

carrying or inducing towards chimney depending on position of

steam jet.

Induced Steam Jet draught

The jet of steam is diverted into smoke box or chimney.

The kinetic head of the steam is high but static head is low i.e. it creates a partial vaccum which draws the air through the grate, ash pit, flues and then to motor box and chimney.

Here steam jet is sucking the flue gases through boiler so it is Induced Steam Jet Draught.

This type of arrangement is employed in locomotive boilers.

Forced Steam Jet draught

Steam from the boiler after having been throttled to a

gauge pressure of 1.5 to 2 bar is supplied to the jet or

nozzles installed in ash pit.

The steam emerging out of nozzles with a great velocity

drags air along the fuel bed, furnace, flue passage and

then to the chimney.

Here steam jet is forcing the air and flue gases to flow

through boiler hence it is forced steam jet draught.

Advantages of Steam Jet draught

It is quite simple and cheap.

It has the capability of using low grade fuels.

It occupies very less space.

Initial cost is low.

Maintenance cost is low.

Exhaust steam from steam engine or turbine can be used

easily in Steam Jet Draught.

Disadvantages of Artificial Draught

• It can operate only when some steam is generated.

• Draught produced is very low.