Draught, pronounced as 'draft' is the difference of pressures, maintained above and below the fire grate. It is one of the most essential systems of a thermal power plant. The purpose of draught is to supply required quantity of air for combustion of fuel and also to remove the burnt (flue) gases from the boiler.

Definition : Boiler draught is the small difference of pressure between pressure of air outside and that of gases within a furnace.

Functions of Draught : In a. boiler, draught can be produced by a chimney, fan, air jet etc., Functions of draught are

- to force air through the fuel bed to aid proper combustion.

- to draw resulting hot gases through the system

- to remove products of combustion to the atmosphere after they have given out their heat to water being evaporated

Classification of Draught (Methods of Producing Draught) :

Draught is broadly classified into two systems, Natural Draught and Artificial Draught.

Artificial Draught is further classified as Steam Jet Draught and Mechanical Draught. Steam Jet Draught may be induced one or forced draught. Mechanical (Fan) Draught is further sub-divided as, Induced Draught, Forced Draught and Balanced Draught.

The classification of boiler draught is schematically represented as shown.


Natural Draught (Chimney Draught) :

Natural draught is produced by a chimney. A chimney is a vertical, tall tubular structure. Hot gases produced in the combustion chamber of boiler are discharged out through this chimney high into atmosphere to avoid air pollution and at the same time to produce draught in the boiler.

The draught produced by the chimney is due to the temperature difference and consequently density difference of hot gases in the chimney and cold air outside the chimney.

Before firing the boiler, atmospheric pressure at all points along the grate travel (Figure) will be the same (say pa). Pressure at point (2) (say p2) is less than pa since atmospheric pressure decreases with the attitude.

Natural Draught.jpg

Now when the boiler is fired, hot gases will fill up the chimney. Pressure pi at grate level (F) and pressure p2 at top of chimney remain unaffected. But pressure at the base of chimney, PA drops and is given by,

PA = P2 + wgH

wg = weight density of hot gases

PA < P3

H = altitude of chimney and p1 = p2 + waH

we= weight density of atmospheric air

Let Δp = pi - pA

= H (wa— wg) =

Theoretical maximum static draught

Advantages :

- Natural draught does not require any external power

- Cost involvement is less.

- Maintenance cost is almost nil

- Cleanliness is ensured

- Minimises pollution of air

- It has long life

- Best suited for small plants


- The maximum pressure available for producing natural draught is hardly 10 to 20 mm of water

- Available draught decreases with the increase of outside air temperature

- Greater amount of heat utilization from hot gases is not possible since reduction in temperature of flue gases reduce draught

- No flexibility in the system to create more draught

- Because of low velocity of air, combustion is poor and specific fuel consumption of the plant increases

- Structural constraints limit the height of chimney

Draught losses :

The losses in boiler draught may be due to the following reasons.

- The frictional resistance between flue (exhaust) gases and chimney walls

- The bends in the gas flow path

- The friction head in equipment like grate, economiser, superheater etc.,

- Draught loss in chimney is about 20% of total draught produced by it.

Artificial Draught :

Total static draught required varies from 25 to 300 mm of water in big steam power plants. It is not convenient to build a very tall chimney which could produce such a large draught. It is not economical also. Moreover natural draught is dependent on climatic conditions. Its value decreases when outside temperature is higher. An artificial draught produced by fans or blowers overcomes these drawbacks.

Advantages of Artificial Draught :

- It does not depend on climatic conditions

- Inferior quality of fuel can be economically burnt

- High temperature of flue gases as necessary for natural draught is not required. This facilitates more utilisation of hot gases in Economiser, Super heater, Air pre heater etc.

- Chimney height can be considerably reduced

- Artificial draught can be regulated according to furnace requirement

- Efficiency is much greater than that of natural draught.

Steam Jet Draught :

Steam Jet Draught is one of the simplest and cheapest methods of producing artificial draught. The exhaust steam from a non-condensing steam engine is discharged through a blast pipe placed below the chimney at high velocity. This produces a draught. It is mostly used it locomotives.

In an induced steam Jet Draught the steam jet issuing out from a nozzle is placed in the chimney. In a forced Ni swam jet draught, the jet is placed in an ashpit under the fire grate of the furnace.

Advantages : -

- simple and economical

- occupies minimum space

- requires very little attention

- low grade fuel can be used

Mechanical Draught of Fan Draught :

(A) Induced Draught : In this method a blower is placed near the chminev base (Fig). It is a centrifugal fan which sucks the burnt gases from the furnace and blows them into the chimney. This result in pressure drop in the furnace below the atmospheric pressure.

induced draught system.jpg

Hence atmospheric air flows through the furnace freely. The action of the induced draught in the furnace is similar to the action of the chimney.

The draught produced in this system is independent of the temperature of the hot gases. Hence the flue gases may be discharged as cold as possible after recovering their heat in economiser and air-preheater.

(B) Forced Draught :

In this method, a blower is placed before the grate and air is forced into the grate through the closed ash pit. This forced air passes through the furnace, economiser, air preheater and chimney.

forced draught.jpg

In this system pressure of air remains above atmospheric throughout. It is, therefore, known as positive draught system or forced draught system. A stack or chimney is also used in order to discharge out flue gases high into atmosphere, thus reducing pollution.

Induced Draught Vs Forced Draught :

The volume of gases handled by I.D. fan is much larger than forced draught fan due to high temperature of gases in the I.D. system.

The size and power required by I.D. fan is more than those of F.D. fan.

To withstand high temperature of flue gases I.D. fans are to be provided with water cooled bearings.

There is a possibility of air leakage into the furnace in I.D. systems as the pressure inside the furnace is less than the atmospheric pressure.

This affects combustion. But there is no chance of air leakage in forced draught system.

In FD. system flow of air through the grate and furnace is more uniform and hence rate of burning increases.

Balanced Draught:

In an induced draught system, when the furnace doors are opened for firing or inspection, cold air rushes into the furnace as pressure inside the furnace is below atmospheric. This reduces the effectiveness of draught and combustion is affected.

In a forced draught too, when the furnace doors are opened, the high pressure air inside the furnace will blow out suddenly and there is a possibility of extinction of lire.

To overcome these drawbacks a balanced draught is used. It is a combination of both the induced and forced draughts. (Fig)

balanced draught.jpg

Advantages of Mechanical Draught:

Following are the advantages of mechanical draught over natural draught.

- Easy control of combustion and evaporation

- Increase in evaporative capacity of boiler.

- Plant efficiency increases.

- Reduced chimney height

- Capability of consuming low grade fuel

- Prevention of dense smoke.

- Fuel consumption reduces by 15% relative to natural draught system.

- Leakage of air into boiler furnace is reduced.