Boiler draught calculation

1. 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.

2. 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.

3. Classification of the Draught

4. 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.

5. 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.

6.  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

7.  Pressure :- Maximum is 15 – 20 mm of water under normal
atmosphericTemp.
 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

8. 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.

9. 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.

10. 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

11. 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.

12. 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.

13. 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

14. 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.

15. 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.

16. 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.

17. 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.

18. 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.